SET/V3Bio.tpgs 8/21/02 11:57 AM Page 1
SCIENCE EVERYDAYOF THINGS SET/V3Bio.tpgs 8/21/02 11:57 AM Page 3
SCIENCE EVERYDAYOF THINGS
volume 3: REAL-LIFE BIOLOGY
edited by NEIL SCHLAGER written by JUDSON KNIGHT
A SCHLAGER INFORMATION GROUP BOOK
Detroit • New York • San Diego • San Francisco • Cleveland • New Haven, Conn. • Waterville, Maine • London • Munich set_v3_cpyrt 8/19/02 10:24 AM Page ii
Science of Everyday Things Volume 3: Real-Life Biology A Schlager Information Group Book Neil Schlager, Editor Written by Judson Knight
Project Editor Permissions Product Design Kimberley A. McGrath Lori Hines Michelle DiMercurio, Michael Logusz
Editorial Imaging and Multimedia Manufacturing Mark Springer Robert Duncan, Leitha Etheridge-Sims, Mary Evi Seoud, Rhonda Williams K. Grimes, Lezlie Light, Dan Newell, David G. Oblender, Robyn V. Young
© 2002 by Gale. Gale is an imprint of The For permission to use material from this While every effort has been made to ensure Gale Group, Inc., a division of Thomson product, submit your request via the Web at the reliability of the information presented in Learning, Inc. http://www.gale-edit.com/permissions, or you this publication, The Gale Group, Inc. does may download our Permissions Request form not guarantee the accuracy of the data con- Gale and Design™ and Thomson Learning ™ and submit your request by fax or mail to: tained herein. The Gale Group, Inc. accepts are trademarks used herein under license. The Gale Group, Inc., Permissions no payment for listing; and inclusion in the Department, 27500 Drake Road, Farmington publication of any organization, agency, insti- For more information contact Hills, MI 48331-3535, Permissions hotline: 248- tution, publication, service, or individual does The Gale Group, Inc. 699-8074 or 800-877-4253, ext. 8006, Fax: not imply endorsement of the editors or the 27500 Drake Rd. 248-699-8074 or 800-762-4058. publisher. Errors brought to the attention of Farmington Hills, MI 48331-3535 the publisher and verified to the satisfaction Or you can visit our Internet site at of the publisher will be corrected in future http://www.gale.com editions.
ALL RIGHTS RESERVED No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means—graphic, electronic, or mechanical, including photocopying, record- ing, taping, Web distribution, or information storage retrieval systems—without the writ- ten permission of the publisher.
LIBRARY OF CONGRESS CATALOG-IN-PUBLICATION DATA
Knight, Judson. Science of everyday things / written by Judson Knight, Neil Schlager, editor. p. cm. Includes bibliographical references and indexes. Contents: v. 1. Real-life chemistry – v. 2 Real-life physics. SBN 0-7876-5631-3 (set : hardcover) – ISBN 0-7876-5632-1 (v. 1) – ISBN 0-7876-5633-X (v. 2) 1. Science–Popular works. I. Schlager, Neil, 1966-II. Title.
Q162.K678 2001 500–dc21 2001050121
ISBN 0-7876-5631-3 (set), 0-7876-5632-1 (vol. 1), 0-7876-5633-X (vol. 2), 0-7876-5634-8 (vol. 3), 0-7876-5635-6 (vol. 4)
Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 set_fm_v3 8/19/02 10:10 AM Page iii
CONTENTS
Introduction ...... v Species ...... 204 Advisory Board ...... vii Speciation ...... 215
BIOCHEMISTRY DISEASE
Carbohydrates...... 3 Disease ...... 229 Amino Acids...... 11 Noninfectious Diseases...... 236 Proteins...... 18 Infectious Diseases ...... 244 Enzymes...... 24 IMMUNITY METABOLISM Immunity and Immunology ...... 255 Metabolism ...... 33 The Immune System...... 262 Digestion...... 44 Respiration ...... 55 INFECTION
NUTRITION Parasites and Parasitology ...... 273 Infection...... 283 Food Webs...... 67 Nutrients and Nutrition ...... 77 BRAIN AND BODY Vitamins ...... 87 Chemoreception...... 295 GENETICS Biological Rhythms...... 306
Genetics...... 99 LEARNING AND BEHAVIOR Heredity...... 110 Behavior...... 319 Genetic Engineering...... 117 Instinct and Learning ...... 327 Mutation...... 126 Migration and Navigation ...... 335 REPRODUCTION AND BIRTH THE BIOSPHERE AND ECOSYSTEMS Reproduction...... 135 142 Sexual Reproduction...... The Biosphere...... 345 151 Pregnancy and Birth ...... Ecosystems and Ecology ...... 360 370 EVOLUTION Biomes......
Evolution...... 161 BIOLOGICAL COMMUNITIES Paleontology...... 176 Symbiosis...... 383 BIODIVERSITY AND TAXONOMY Biological Communities...... 391 Succession and Climax ...... 400 Taxonomy...... 191 General Subject Index ...... 411
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY iii set_fm_v3 8/19/02 10:10 AM Page v
INTRODUCTION
Overview of the Series • How It Works: Explains the principle or theory in straightforward, step-by-step lan- Welcome to Science of Everyday Things. Our aim guage. is to explain how scientific phenomena can be understood by observing common, real-world • Real-Life Applications: Describes how the events. From luminescence to echolocation to phenomenon can be seen in everyday life. buoyancy, the series will illustrate the chief prin- • Where to Learn More: Includes books, arti- ciples that underlay these phenomena and cles, and Internet sites that contain further explore their application in everyday life. To information about the topic. encourage cross-disciplinary study, the entries In addition, each entry includes a “Key will draw on applications from a wide variety of Terms” section that defines important concepts fields and endeavors. discussed in the text. Finally, each volume includes many illustrations and photographs Science of Everyday Things initially compris- throughout. es four volumes: In addition, readers will find the comprehensive Volume 1: Real-Life Chemistry general subject index valuable in accessing the Volume 2: Real-Life Physics data. Volume 3: Real-Life Biology Volume 4: Real-Life Earth Science About the Editor, Author, Future supplements to the series will expand and Advisory Board coverage of these four areas and explore new areas, such as mathematics. Neil Schlager and Judson Knight would like to thank the members of the advisory board for Arrangement of Real-Life their assistance with this volume. The advisors Biology were instrumental in defining the list of topics, and reviewed each entry in the volume for scien- This volume contains 40 entries, each covering a tific accuracy and reading level. The advisors different scientific phenomenon or principle. include university-level academics as well as high The entries are grouped together under common school teachers; their names and affiliations are categories, with the categories arranged, in gen- listed elsewhere in the volume. eral, from the most basic to the most complex. Neil Schlager is the president of Schlager Readers searching for a specific topic should con- Information Group Inc., an editorial services sult the table of contents or the general subject company. Among his publications are When index. Technology Fails (Gale, 1994); How Products Are Within each entry, readers will find the fol- Made (Gale, 1994); the St. James Press Gay and lowing rubrics: Lesbian Almanac (St. James Press, 1998); Best • Concept: Defines the scientific principle or Literature By and About Blacks (Gale, 2000); theory around which the entry is focused. Contemporary Novelists, 7th ed. (St. James Press,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY v set_fm_v3 8/19/02 10:10 AM Page vi
Introduction 2000); Science and Its Times (7 vols., Gale, 2000- history, Knight has published Middle Ages 2001); and Science in Dispute (Gale, 2002). His Reference Library (2000), Ancient Civilizations publications have won numerous awards, includ- (1999), and a volume in U•X•L’s African ing three RUSA awards from the American American Biography series (1998). Knight’s pub- Library Association, two Reference Books lications in the realm of music include Parents Bulletin/Booklist Editors’ Choice awards, two Aren’t Supposed to Like It (2001), an overview of New York Public Library Outstanding Reference contemporary performers and genres, as well as awards, and a CHOICE award for best academic Abbey Road to Zapple Records: A Beatles book. Encyclopedia (Taylor, 1999). Judson Knight is a freelance writer, and Comments and Suggestions author of numerous books on subjects ranging from science to history to music. His work on sci- Your comments on this series and suggestions for ence includes Science, Technology, and Society, future editions are welcome. Please write: The 2000 B.C.-A.D. 1799 (U•X•L, 2002), as well as Editor, Science of Everyday Things, Gale Group, extensive contributions to Gale’s seven-volume 27500 Drake Road, Farmington Hills, MI 48331- Science and Its Times (2000-2001). As a writer on 3535.
vi VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_fm_v3 8/19/02 10:10 AM Page vii
ADVISORY BOARDTITLE
William E. Acree, Jr. Professor of Chemistry, University of North Texas Russell J. Clark Research Physicist, Carnegie Mellon University Maura C. Flannery Professor of Biology, St. John’s University, New York John Goudie Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Cheryl Hach Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Michael Sinclair Physics instructor, Kalamazoo (MI) Area Mathematics and Science Center Rashmi Venkateswaran Senior Instructor and Lab Coordinator, University of Ottawa
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY vii set_vol3_1 8/19/02 10:32 AM Page 1
SCIENCE OF EVERYDAY THINGS real-life biology BIOCHEMISTRY
CARBOHYDRATES AMINO ACIDS PROTEINS ENZYMES
1 set_vol3_1 8/19/02 10:32 AM Page 3
Carbohydrates CARBOHYDRATES
CONCEPT thorough treatment of it involves a great deal of Carbohydrates are nutrients, along with proteins technical terminology. Although we discuss the and other types of chemical compounds, but fundamentals of photosynthesis later in this they are much more than that. In addition to essay, we do so only in the most cursory fashion. sugars, of which there are many more varieties Photosynthesis involves the conversion of than ordinary sucrose, or table sugar, carbohy- carbon dioxide and water to sugars, which, along drates appear in the form of starches and cellu- with starches and cellulose, are some of the more lose. As such, they are the structural materials of well known varieties of carbohydrate. Sugars can which plants are made. Carbohydrates are pro- be defined as any of a number of water-soluble duced by one of the most complex, vital, and compounds, of varying sweetness. (What we amazing processes in the physical world: photo- think of as sugar—that is, table sugar—is actual- synthesis. Because they are an integral part of ly sucrose, discussed later.) Starches are complex plant life, it is no wonder that carbohydrates are carbohydrates without taste or odor, which are in most fruits and vegetables. And though they granular or powdery in physical form. Cellulose are not a dietary requirement in the way that is a polysaccharide, made from units of glucose, vitamins or essential amino acids are, it is diffi- that constitutes the principal part of the cell walls cult to eat without ingesting some carbohydrates, of plants and is found naturally in fibrous mate- which are excellent sources of quick-burning rials, such as cotton. Commercially, it is a raw energy. Not all carbohydrates are of equal nutri- material for such manufactured goods as paper, tional value, however: in general, the ones creat- cellophane, and rayon. ed by nature are good for the body, whereas those MONOSACCHARIDES. produced by human intervention—some forms The pre- of pasta and most varieties of bread, white rice, ceding definitions contain several words that also crackers, cookies,and so forth—are much less must be defined. Carbohydrates are made up of beneficial. building blocks called monosaccharides, the sim- plest type of carbohydrate. Found in grapes and other fruits and also in honey, they can be broken HOW IT WORKS down chemically into their constituent elements, but there is no carbohydrate more chemically What Carbohydrates Are simple than a monosaccharide. Hence, they are Carbohydrates are naturally occurring com- also known as simple sugars or simple carbohy- pounds that consist of carbon, hydrogen, and drates. oxygen, and are produced by green plants in the Examples of simple sugars include glucose, process of undergoing photosynthesis. In simple which is sweet, colorless, and water-soluble and terms, photosynthesis is the biological conver- appears widely in nature. Glucose, also known as sion of light energy (that is, electromagnetic dextrose, grape sugar, and corn sugar, is the prin- energy) from the Sun to chemical energy in cipal form in which carbohydrates are assimilat- plants. It is an extremely complex process, and a ed, or taken in, by animals. Other monosaccha-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 3 set_vol3_1 8/19/02 10:33 AM Page 4
Carbohy- Another disaccharide is lactose, or milk drates sugar, the only type of sugar that is produced from animal (i.e., mammal) rather than veg- etable sources. Maltose, a fermentable sugar typ- ically formed from starch by the action of the enzyme amylase, is also a disaccharide. Sucrose, lactose, and maltose are all isomers, with the for-
mula C12H22O11. OLIGOSACCHARIDES AND POLYSACCHARIDES. The definitions of oligosaccharide and polysaccharide are so close as to be confusing. An oligosaccharide is sometimes defined as a carbohydrate containing a known, small number of monosaccharide units, while a polysaccharide is a carbohydrate composed of two or more monosaccharides. In theory, this means practically the same thing, but in practice, an oligosaccharide contains 3-6 monosaccharide units, whereas a polysaccharide is composed of more than six. Oligosaccharides are found rarely in nature, MICROGRAPH OF PLANT CELL CHLOROPLASTS, WHERE though a few plant forms have been discovered. PHOTOSYNTHESIS, THE BIOLOGICAL CONVERSION OF LIGHT FROM THE SUN INTO CHEMICAL ENERGY, TAKES Far more common are polysaccharides (“many PLACE. HIGHER PLANTS HAVE THESE STRUCTURES, sugars”), which account for the vast majority of WHICH CONTAIN A CHEMICAL KNOWN AS CHLOROPHYLL carbohydrate types found in nature. (See Where that absorbs light and speeds up the to Learn More for the Nomenclature of Carbo- process of photosynthesis. (© Science Pictures Limit- ed/Corbis. Reproduced by permission.) hydrates Web site, operated by the Department of Chemistry at Queen Mary College, University of London. A glance at the site will suggest some- rides include fructose, or fruit sugar, and galac- thing about the many, many varieties of carbohy- tose, which is less soluble and sweet than glucose drates.) and usually appears in combination with other Polysaccharides may be very large, consist- simple sugars rather than by itself. Glucose, fruc- ing of as many as 10,000 monosaccharide units tose, and galactose are isomers, meaning that strung together. Given this vast range of sizes, it
they have the same chemical formula (C6H12O6), should not be surprising that there are hundreds but different chemical structures and therefore of polysaccharide types, which differ from one different chemical properties. another in terms of size, complexity, and chemi- cal makeup. Cellulose itself is a polysaccharide, DISACCHARIDES. When two the most common variety known, composed of monosaccharide molecules chemically bond numerous glucose units joined to one another. with each other, the result is one of three general Starch and glycogen are also glucose polysaccha- types of complex sugar: a disaccharide, oligosac- rides. The first of these polysaccharides is found charide, or polysaccharide. Disaccharides, or primarily in the stems, roots, and seeds of plants. double sugars, are composed of two monosac- As for glycogen, this is the most common form in charides. By far the most well known example of which carbohydrates are stored in animal tissues, a disaccharide is sucrose, or table sugar, which is particularly muscle and liver tissues. formed from the bonding of a glucose molecule Photosynthesis with a molecule of fructose. Sugar beets and cane sugar provide the principal natural sources of Photosynthesis, as we noted earlier, is the biolog- sucrose, which the average American is most like- ical conversion of light or electromagnetic ener- ly to encounter in refined form as white, brown, gy from the Sun into chemical energy. It occurs or powdered sugar. in green plants, algae, and some types of bacteria
4 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 5
and requires a series of biochemical reactions. being part of the reactants to the left of the arrow. Carbohy- Higher plants have structures called chloroplasts, In the present equation, neither the added ener- drates which contain a dark green or blue-black chemi- gy nor the catalyst appears on the left side, cal known as chlorophyll. Light absorption by because they are not actual physical participants chlorophyll catalyzes, or speeds up, the process of consumed in the reaction, as the carbon dioxide photosynthesis. (A catalyst is a substance that and water are. The catalyst does not participate in accelerates a chemical reaction without partici- the reaction, whereas the energy, while it is con- pating in it.) sumed in the reaction, is not a material or physi- In photosynthesis, carbon dioxide and water cal participant—that is, it is energy, not matter. react with each other in the presence of light and One might also wonder why the equation chlorophyll to produce a simple carbohydrate shows six molecules of carbon dioxide and six of and oxygen. This is one of those statements in the water. Why not one of each, for the sake of sim- realm of science that at first glance sounds a bit plicity? To produce a balanced chemical equa- dry and boring but which, in fact, encompasses tion, in which the same number of atoms appears one of life’s great mysteries—a concept far more on either side of the arrow, it is necessary to show captivating than any number of imaginary, fan- six carbon dioxide molecules reacting with six tastic, or pseudoscientific ideas one could con- water molecules to produce six oxygen molecules coct. Photosynthesis is one of the most essential and a single glucose molecule. Thus, both sides life-sustaining processes, making possible the contain six atoms of carbon, 12 of hydrogen, and nutrition of all things and the respiration of ani- 18 of oxygen. mals and other oxygen-breathing organisms. The equation gives the impression that pho- In photosynthesis, plants take a waste prod- tosynthesis is a simple, one-step process, but uct of human and animal respiration and, nothing could be further from the truth. In fact, through a series of chemical reactions, produce the process occurs one small step at a time. It also both food and oxygen. The food gives nourish- involves many, many intricacies and aspects that ment to the plant, which, unlike an animal, is require the introduction of scores of new terms capable of producing its own nutrition from its and ideas. Such a discussion is beyond the scope own body with the aid only of sunlight and a few of the present essay, and therefore the reader is chemical compounds. Later, when the plant is encouraged to consult a reliable textbook for fur- eaten by an animal or when it dies and is con- ther information on the details of photosynthesis. sumed by bacteria and other decomposers, it will pass on its carbohydrate content to other crea- tures. (See Food Webs for more about plants as REAL-LIFE autotrophs and the relationships among primary APPLICATIONS producers, consumers, and decomposers.) A carbohydrate is not the only useful prod- Fruits and Vegetables uct of the photosynthetic reaction. The reaction One of the principal ways in which people obtain produces an extremely important waste by-prod- carbohydrates from their diets is through fruits uct—waste, that is, from the viewpoint of the and vegetables. The distinctions between these plant, which has no need of oxygen. Yet the oxy- two are based not on science but on custom. Tra- gen it generates in photosynthesis makes life pos- ditionally, vegetables are plant tissues (which sible for animals and many single-cell life-forms, may be sweet, but usually are not), that are eaten which depend on oxygen for respiration. as a substantial part of a meal’s main course. By THE PHOTOSYNTHESIS EQUA- contrast, fruits are almost always sweet and are TION. The photosynthesis reaction can be rep- eaten as desserts or snacks. It so happens, too, resented thus as a chemical equation: that people are much more likely to cook vegeta- Light4 6CO2 + 6H2O C6H12O6 + 6O2 bles than they are fruits, though vegetables are Note that the arrow indicates that a chemical nutritionally best when eaten raw. reaction has taken place with the assistance of Fruits and vegetables are heavy in carbohy- light and chlorophyll. In the same way, heat from drate content, in the form of edible sugars and a Bunsen burner may be required to initiate starches but also inedible cellulose, whose role in some other chemical reaction, without actually the diet will be examined later. In a fresh veg-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 5 set_vol3_1 8/19/02 10:33 AM Page 6
Carbohy- the best part of all—the tender and fully edible drates “heart”—is enclosed beneath an intimidating shield of slender thistles. Whoever first discov- ered that an artichoke could be eaten must have been a brave person indeed, and whoever ascer- tained how to eat it was a wise one. Thanks to these adventurous souls, the world’s cuisine has an unforgettable delicacy. THE CARBOHYDRATE CON- TENT OF VEGETABLES. In terms of edible carbohydrate content, the artichoke has a low percentage. A few vegetables have a smaller percentage of carbohydrates, whereas others have vastly higher percentages, as the list shown here illustrates. In general, it seems that the carbohy- drate content of vegetables (and in each of these cases we are talking about edible carbohydrates, not cellulose) is in the range of about 5–10%, somewhere around 20%, or a very high 60–80%. There does not seem to be a great deal of varia-
CELLULOSE, SOMETIMES CALLED FIBER, IS AN IMPOR- tion in these ranges. TANT DIETARY COMPONENT THAT AIDS IN DIGESTION. IT Water, Protein, and Carbohydrate Content IS ABUNDANT IN FRUITS AND VEGETABLES, YET HUMANS LACK THE ENZYME NECESSARY TO DIGEST IT. WITH THE of Selected Vegetables: HELP OF MICROBES IN THEIR GUT, TERMITES CAN DIGEST • Artichoke: 85% water, 2.9% protein, 10.6% CELLULOSE. (© George D.Lepp/Corbis. Reproduced by permission.) carbohydrate • Beets, red: 87.3% water, 1.6% protein, 9.9% etable, for instance, water may account for about carbohydrate 70% of the volume, and proteins, fat, vitamins, • Celery: 94.1% water, 0.9% protein, 3.9% and minerals may make up a little more than 5%, carbohydrate with nearly 25% taken up either by edible sugars • Corn: 13.8% water, 8.9% protein, 72.2% and starches or by inedible cellulose fiber. carbohydrate THE EXAMPLE OF THE ARTI- • Lima bean: 10.3% water, 20.4% protein, CHOKE. Every fruit or vegetable one could 64% carbohydrate conceivably eat—and there are hundreds—con- • Potato: 79.8% water, 2.1% protein, 17.1% tains both edible carbohydrates, which are a good carbohydrate source of energy, and inedible ones, which pro- • Red pepper: 74.3% water, 3.7% protein, vide fiber. An excellent example of this edible- 18.8% carbohydrate inedible mixture is the globe, or French, arti- choke—Cynara scolymus, a member of the fami- • Summer squash: 94% water, 1.1% protein, ly Asteraceae, which includes the sunflower. The 4.2% carbohydrate globe artichoke (not to be confused with the Starches Jerusalem artichoke, or Helianthus tuberosus) appears in the form of an inflorescence, or a clus- Not all the carbohydrates in these vegetables are ter of flowers. This vegetable usually is steamed, the same. Some carbohydrates appear in the and the bracts, or leaves, are dipped in butter or form of sugar and others in the form of inedible another sauce. cellulose, discussed in the next section. In addi- Not nearly all of the bract is edible, however; tion, some vegetables are high in starch content. to consume the starchy “meat” of the artichoke, As we noted earlier, starch is white and granular, which has a distinctive, nutty flavor, one must and, unlike sugars, starches cannot be dissolved draw the leaves between the teeth. Most of the in cold water, alcohol, or other liquids that nor- artichoke’s best parts are thus hidden away, and mally act as solvents.
6 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 7
Manufactured in plants’ leaves, starch is the Carbohy- product of excess glucose produced during pho- drates tosynthesis, and it provides the plant with an emergency food supply stored in the chloroplas- ts. Vegetables high in starch content are products of plants whose starchy portions happen to be the portions we eat. For example, there is the tuber, or underground bulb, of the potato as well as the seeds of corn, wheat, and rice. Thus, all of these vegetables, and foods derived from them, are heavy in the starch form of carbohydrate. In addition to their role in the human diet, starches from corn, wheat, tapioca, and potatoes are put to numerous commercial uses. Because of its ability to thicken liquids and harden solids, starch is applied in products (e.g., cornstarch) that act as thickening agents, both for foods and nonfood items. Starch also is utilized heavily in various phases of the garment and garment-care industries to impart stiffness to fabrics. In the manufacture of paper, starch is used to increase the paper’s strength. It also is employed in the NINETEENTH-CENTURY ADVERTISEMENT FOR STARCH. IN , STARCH- production of cardboard and paper bags. ADDITION TO THEIR ROLE IN THE HUMAN DIET ES ARE PUT TO NUMEROUS COMMERCIAL USES, FOR EXAMPLE, AS THICKENING AGENTS FOR FOOD, IN THE Cellulose PRODUCTION OF CARDBOARD, AND IN VARIOUS PHASES OF THE GARMENT INDUSTRY TO IMPART STIFFNESS TO One of the aspects of fruits and vegetables to FABRICS. (© BettmannCorbis. Reproduced by permission.) which we have alluded several times is the high content of inedible material, or cellulose. (Actu- ally, it is edible—just not digestible.) A substance and insect do not have an enzyme that digests found in the cell walls of plants, cellulose is this material. Instead, they harbor microbes in chemically like starch but even more rigid, and their guts that can do the digesting for them. this property makes it an excellent substance for (This is an example of symbiotic mutualism, a imparting strength to plant bodies. Animals do mutually beneficial relationship between organ- not have rigid, walled cells, but plants do. The isms, discussed in Symbiosis.) heavy cellulose content in plants’ cell walls gives Cows are ruminants, or animals that chew them their erect, rigid form; in other words, their cud—that is, food regurgitated to be without cellulose, plants might be limp and part- chewed again. Ruminants have several stomachs, ly formless. Like human bone, plant cell walls are or several stomach compartments, that break composed of fibrils (small filaments or fibers) down plant material with the help of enzymes that include numerous polysaccharides and pro- and bacteria. The partially digested material then teins. One of these polysaccharides in cell walls is is regurgitated into the mouth, where it is chewed pectin, a substance that, when heated, forms a gel to break the material down even further. (If you and is used by cooks in making jellies and jams. have ever watched cows in a pasture, you have Some trees have a secondary cell wall over the probably observed them calmly chewing their primary one, containing yet another polysaccha- cud.) The digestion of cellulose by bacteria in the ride called lignin. Lignin makes the tree even stomachs of ruminants is anaerobic, meaning more rigid, penetrable only with sharp axes. that the process does not require oxygen. One of CELLULOSE IN DIGESTION. the by-products of this anaerobic process is As we have noted, cellulose is abundant in fruits methane gas, which is foul smelling, flammable, and vegetables, yet humans lack the enzyme nec- and toxic. Ruminants give off large amounts of essary to digest it. Termites, cows, koalas, and methane daily, which has some environmental- horses all digest cellulose, but even these animals ists alarmed, since cow-borne methane may con-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 7 set_vol3_1 8/19/02 10:33 AM Page 8
Carbohy- drates KEY TERMS
CARBOHYDRATES: Naturally occur- ENZYME: A protein material that ring compounds, consisting of carbon, speeds up chemical reactions in the bodies hydrogen, and oxygen, whose primary of plants and animals. function in the body is to supply energy. FRUCTOSE: Fruit sugar, a monosac- Included in the carbohydrate group are charide that is an isomer of glucose. sugars, starches, cellulose, and various GALACTOSE: A monosaccharide and other substances. Most carbohydrates are isomer of glucose. Less soluble and sweet produced by green plants in the process of than glucose, galactose usually appears in undergoing photosynthesis. combination with other simple sugars CATALYST: A substance that speeds up rather than by itself. a chemical reaction without participating GLUCOSE: A monosaccharide that in it. Catalysts, of which enzymes are a occurs widely in nature and is the form in good example, thus are not consumed in which animals usually receive carbohy- the reaction. drates. Also known as dextrose, grape CELLULOSE: A polysaccharide, made sugar, and corn sugar. from units of glucose, that is the principal GLYCOGEN: A white polysaccharide material in the cell walls of plants. Cellu- that is the most common form in which lose also is found in natural fibers, such as carbohydrates are stored in animal tissues, cotton, and is used as a raw material in particularly muscle and liver tissues. manufacturing such products as paper. GUT: A term that refers to all or part of COMPLEX CARBOHYDRATE: A dis- the alimentary canal, through which foods accharide, polysaccharide, or oligosaccha- pass from the mouth to the intestines and ride. Also called a complex sugar. wastes move from the intestines to the DEXTROSE: Another name for glu- anus. Although the word is considered a bit cose. crude in everyday life, physicians and bio- DISACCHARIDE: A double sugar, logical scientists concerned with this part composed of two monosaccharides. Exam- of the anatomy use it regularly. ples of disaccharides include the isomers ISOMERS: Two substances that have sucrose, maltose, and lactose. the same chemical formula but differ in
tribute to the destruction of the ozone high in the body of wastes and lowering the risk of colon Earth’s stratosphere. cancer. (See Digestion for more about the diges- Alhough cellulose is indigestible by humans, tive and excretory processes.) it is an important dietary component in that it Overall Carbohydrate aids in digestion. Sometimes called fiber or Nutrition roughage, cellulose helps give food bulk as it moves through the digestive system and aids the A diet high in cellulose content can be beneficial body in pushing out foods and wastes. This is for the reasons we have noted. Likewise, a healthy particularly important inasmuch as it helps make diet includes carbohydrate nutrients, but only possible regular bowel movements, thus ridding under certain conditions. First of all, it should be
8 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 9
Carbohy- drates KEY TERMS CONTINUED
chemical structure and therefore in chemi- dioxide and water are converted to carbo- cal properties. hydrates and oxygen.
LACTOSE: Milk sugar. A disaccharide POLYSACCHARIDE: A carbohydrate isomer of sucrose and maltose, lactose is composed of more than six monosaccha- rides. A polysaccharide sometimes is the only major type of sugar that is pro- defined as containing two or more mono- duced from animal (i.e., mammal) rather saccharides, but this definition does little than vegetable sources. to distinguish it from an oligosaccharide. MALTOSE: A fermentable sugar gener- SACCHARIDE: A sugar. ally formed from starch by the action of the SIMPLE SUGAR: A monosaccharide, enzyme amylase. Maltose is a disaccharide or simple carbohydrate. isomer of sucrose and lactose. STARCHES: Complex carbohydrates, MONOSACCHARIDE: The simplest without taste or odor, which are granular type of carbohydrate. Monosaccharides, or powdery in physical form. which cannot be broken down chemically SUCROSE: Common table sugar into simpler carbohydrates, also are known (C12H22O11), a disaccharide formed from as simple sugars. Examples of monosac- the bonding of a glucose molecule with a charides include the isomers glucose, fruc- molecule of fructose. Sugar beets and cane tose, and galactose. sugar provide the principal natural sources OLIGOSACCHARIDE: A carbohy- of sucrose, which the average American is most likely to encounter in refined form as drate containing a known, small number of white, brown, or powdered sugar. monosaccharide units, typically between SUGARS: three and six. Compare with polysaccha- One of the three principal types of carbohydrate, along with starches ride. and cellulose. Sugars can be defined as any PHOTOSYNTHESIS: The biological of various water-soluble carbohydrates of conversion of light energy (that is, electro- varying sweetness. What we think of as magnetic energy) from the Sun to chemical “sugar” (i.e., table sugar) is actually energy in plants. In this process, carbon sucrose.
understood that the human body does not have mins, minerals, proteins, and dietary fiber that an essential need for carbohydrates in and of they contain. For these healthy carbohydrates, it themselves—in other words, there are no “essen- is best to eat them in as natural a form as possi- tial” carbohydrates, as there are essential amino ble: for example, eat the whole orange, rather acids or fatty acids. than just squeezing out the juice and throwing On the other hand, it is very important to eat away the pulp. Also, raw spinach and other veg- fresh fruits and vegetables, which, as we have etables contain far more vitamins and minerals seen, are heavy in carbohydrate content. Their than the cooked versions. importance has little do with their nutritional SUGAR HIGHS AND FAT carbohydrate content, but rather with the vita- STORAGE. Carbohydrates can give people a
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 9 set_vol3_1 8/19/02 10:33 AM Page 10
Carbohy- short burst of energy, and this is why athletes times have you eaten a huge plate of nothing but drates may “bulk up on carbs” right before competition. chicken, steak, or eggs? Probably not very often, But if the carbohydrates are not quickly burned and if you have tried to eat too much of these off, they eventually will be stored as fat. This is protein-heavy foods at one time, you most likely the case even with healthy carbohydrates, but the started to get sick. situation is much worse with junk-food carbohy- The reason is that when you eat protein or drates, which offer only empty calories stripped fat, it triggers the release of a hormone called of vitamin and mineral content. One example is cholecystokinin (CCK) in the small intestine. a particular brand of candy bar that, over the CCK tells the brain, in effect, that the body is get- years, has been promoted in commercials as a ting fed, and if enough CCK is released, it signals means of obtaining a quick burst of energy. In the brain that the body has received enough fact, this and all other white-sugar-based candies food. If one continues to consume proteins or give only a quick “sugar high,” followed almost fats beyond that point, nausea is likely to follow. immediately by a much lower energy “low”—and Carbohydrates, on the other hand, do not cause a in the long run by the accumulation of fat. release of CCK; only when they enter the blood- Fat is the only form in which the body can stream do they finally send a signal to the brain store carbohydrates for the long haul, meaning that the body is satisfied. By then, most of us have that the “fat-free” stickers on many a package of piled on more mashed potatoes, which are des- cookies or cakes in the supermarket are as mean- tined to take their place in the body as fat stores. ingless as the calories themselves are empty. Car- bohydrate consumption is one of the main rea- WHERE TO LEARN MORE sons why the average American is so overweight. With an inactive lifestyle, as is typical of most Carbohydrates. Hardy Research Group, Department of adults in modern life, all those French fries, cook- Chemistry, University of Akron (Web site).
10 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 11
Amino Acids AMINO ACIDS
CONCEPT responsible for the differences in chemical prop- Amino acids are organic compounds made of erties. carbon, hydrogen, oxygen, nitrogen, and (in This explanation sounds a bit technical and some cases) sulfur bonded in characteristic for- requires a background in chemistry that is mations. Strings of amino acids make up pro- beyond the scope of this essay, but let us simplify teins, of which there are countless varieties. Of it somewhat. Imagine that the amino-acid mole- the 20 amino acids required for manufacturing cule is like the face of a compass, with a carbon the proteins the human body needs, the body atom at the center. Raying out from the center, in itself produces only 12, meaning that we have to the four directions of the compass, are lines rep- meet our requirements for the other eight resenting chemical bonds to other atoms or through nutrition. This is just one example of the groups of atoms. These directions are based on importance of amino acids in the functioning of models that typically are used to represent life. Another cautionary illustration of amino amino-acid molecules, though north, south, east, acids’ power is the gamut of diseases (most and west, as used in the following illustration, are notably, sickle cell anemia) that impair or claim simply terms to make the molecule easier to visu- the lives of those whose amino acids are out of alize. sequence or malfunctioning. Once used in dating To the south of the carbon atom (C) is a objects from the distant past, amino acids have hydrogen atom (H), which, like all the other existed on Earth for at least three billion years— atoms or groups, is joined to the carbon center by long before the appearance of the first true a chemical bond. To the north of the carbon cen-
organisms. ter is what is known as an amino group (-NH2). The hyphen at the beginning indicates that such HOW IT WORKS a group does not usually stand alone but nor- mally is attached to some other atom or group. A “Map” of Amino Acids To the east is a carboxyl group, represented as -COOH. In the amino group, two hydrogen Amino acids are organic compounds, meaning atoms are bonded to each other and then to that they contain carbon and hydrogen bonded nitrogen, whereas the carboxyl group has two to each other. In addition to those two elements, separate oxygen atoms strung between a carbon they include nitrogen, oxygen, and, in a few cases, atom and a hydrogen atom. Hence, they are not sulfur. The basic structure of an amino-acid mol- represented as O2. ecule consists of a carbon atom bonded to Finally, off to the west is the R- group, which
an amino group (-NH2), a carboxyl group can vary widely. It is as though the other portions (-COOH), a hydrogen atom, and a fourth group of the amino acid together formed a standard that differs from one amino acid to another and suffix in the English language, such as -tion. To often is referred to as the -R group or the side the front of that suffix can be attached all sorts of chain. The -R group, which can vary widely, is terms drawn from root words, such as educate or
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 11 set_vol3_1 8/19/02 10:33 AM Page 12
Amino Acids
COMPUTER-GENERATED MODEL OF A MOLECULE MADE UP OF THREE AMINO ACIDS—GLYCINE, CYSTEINE AND ALANINE. AMINO ACIDS FUNCTION AS MONOMERS, OR INDIVIDUAL UNITS, THAT JOIN TOGETHER TO FORM LARGE, CHAINLIKE MOLECULES CALLED POLYMERS; THREE AMINO ACIDS BONDED TOGETHER ARE CALLED TRIPEPTIDES. (Photo Researchers. Reproduced by permission.)
satisfy or revolt—hence, education, satisfaction, most chemically reactive parts of the molecule. and revolution. The variation in the terms Each of the common amino acids has, in addi- attached to the front end is extremely broad, yet tion to its chemical name, a more familiar name the tail end, -tion, is a single formation. Likewise and a three-letter abbreviation that frequently is the carbon, hydrogen, amino group, and car- used to identify it. In the present context, we are boxyl group in an amino acid are more or less not concerned with these abbreviations. Amino- constant. acid molecules, which contain an amino group A FEW ADDITIONAL POINTS. and a carboxyl group, do not behave like typical The name amino acid, in fact, comes from the molecules. Instead of melting at temperatures amino group and the acid group, which are the hotter than 392°F (200°C), they simply decom-
12 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 13
pose. They are quite soluble, or capable of being acids are called dipeptides, whereas three amino Amino Acids dissolved, in water but are insoluble in nonpolar acids bonded together are called tripeptides. If solvents (oil- and all oil-based products), such as there are more than 10 in a chain, they are benzene or ether. termed polypeptides, and if there are 50 or more, RIGHT-HAND AND LEFT-HAND these are known as proteins. VERSIONS. All of the amino acids in the All the millions of different proteins in living human body, except glycine, are either right- things are formed by the bonding of only 20 hand or left-hand versions of the same molecule, amino acids to make up long polymer chains. meaning that in some amino acids the positions Like the 26 letters of the alphabet that join of the carboxyl group and the R- group are together to form different words, depending on switched. Interestingly, nearly all of the amino which letters are used and in which sequence, the acids occurring in nature are the left-hand ver- 20 amino acids can join together in different sions of the molecules, or the L-forms. (There- combinations and series to form proteins. But fore, the model we have described is actually the whereas words usually have only about 10 or left-hand model, though the distinctions fewer letters, proteins typically are made from as between “right” and “left”—which involve the few as 50 to as many as 3,000 amino acids. direction in which light is polarized—are too Because each amino acid can be used many times complex to discuss here.) along the chain and because there are no restric- Right-hand versions (D-forms) are not tions on the length of the chain, the number of found in the proteins of higher organisms, but possible combinations for the formation of pro- they are present in some lower forms of life, such teins is truly enormous. There are about two as in the cell walls of bacteria. They also are quadrillion different proteins that can exist if found in some antibiotics, among them, strepto- each of the 20 amino acids present in humans is mycin, actinomycin, bacitracin, and tetracycline. used only once. Just as not all sequences of letters These antibiotics, several of which are well make sense, however, not all sequences of amino known to the public at large, can kill bacterial acids produce functioning proteins. Some other cells by interfering with the formation of pro- sequences can function and yet cause undesirable teins necessary for maintaining life and for effects, as we shall see. reproducing. Amino Acids and Proteins REAL-LIFE APPLICATIONS A chemical reaction that is characteristic of amino acids involves the formation of a bond, DNA (deoxyribonucleic acid), a molecule in all called a peptide linkage, between the carboxyl cells that contains genetic codes for inheritance, group of one amino acid and the amino group of creates encoded instructions for the synthesis of a second amino acid. Very long chains of amino amino acids. In 1986, American medical scientist acids can bond together in this way to form pro- Thaddeus R. Dryja (1940–) used amino-acid teins, which are the basic building blocks of all sequences to identify and isolate the gene for a living things. The specific properties of each kind type of cancer known as retinoblastoma, a fact of protein are largely dependent on the kind and that illustrates the importance of amino acids in sequence of the amino acids in it. Other aspects the body. of the chemical behavior of protein molecules Amino acids are also present in hormones, are due to interactions between the amino and chemicals that are essential to life. Among these the carboxyl groups or between the various R- hormones is insulin, which regulates sugar levels groups along the long chains of amino acids in in the blood and without which a person would the molecule. die. Another is adrenaline, which controls blood NUMBERS AND COMBINA- pressure and gives animals a sudden jolt of ener- TIONS. Amino acids function as monomers, gy needed in a high-stress situation—running or individual units, that join together to form from a predator in the grasslands or (to a use a large, chainlike molecules called polymers, which human example) facing a mugger in an alley or a may contain as few as two or as many as 3,000 bully on a playground. Biochemical studies of amino-acid units. Groups of only two amino amino-acid sequences in hormones have made it
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 13 set_vol3_1 8/19/02 10:33 AM Page 14
Amino Acids
NORMAL RED BLOOD CELLS (BOTTOM) AND SICKLE CELL (TOP). SICKLE CELL ANEMIA IS A FATAL DISEASE BROUGHT ABOUT BY A SINGLE MISTAKE IN AMINO ACID SEQUENCING. WHEN RED BLOOD CELLS RELEASE OXYGEN TO THE TIS- SUES, THEY FAIL TO RE-OXYGENATE NORMALLY AND INSTEAD TWIST INTO THE SHAPE THAT GIVES SICKLE CELL ANE- MIA ITS NAME, CAUSING OBSTRUCTION OF THE BLOOD VESSELS. (Photograph by Dr. Gopal Murti. National Audubon Society Col- lection/Photo Researchers, Inc. Reproduced by permission.)
possible for scientists to isolate and produce arti- released into the small intestine, from whence ficially these and other hormones, including the they pass into the bloodstream and are carried human growth hormone. throughout the organism. Each individual cell of the organism then can use these amino acids to Amino Acids and Nutrition assemble the new and different proteins required for its specific functions. Life thus is an ongoing Just as proteins form when amino acids bond cycle in which proteins are broken into individ- together in long chains, they can be broken down by a reaction called hydrolysis, the reverse of the ual amino-acid units, and new proteins are built formation of the peptide bond. That is exactly up from these amino acids. what happens in the process of digestion, when ESSENTIAL AMINO ACIDS. Out special digestive enzymes in the stomach enable of the many thousands of possible amino acids, the breaking down of the peptide linkage. humans require only 20 different kinds. Two oth- (Enzymes are a type of protein—see Enzymes.) ers appear in the bodies of some animal species, The amino acids, separated once again, are and approximately 100 others can be found in
14 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 15
plants. Considering the vast numbers of amino lysine and tryptophan. Its symptoms often are Amino Acids acids and possible combinations that exist in described as the “three Ds”: diarrhea, dermatitis nature, the number of amino acids essential to (or skin inflammation), and dementia. Thanks to life is extremely small. Yet of the 20 amino acids a greater understanding of nutrition and health, required by humans for making protein, only 12 pellagra has been largely eradicated, but there can be produced within the body, whereas the still exists a condition with almost identical other eight—isoleucine, leucine, lysine, methion- symptoms: Hartnup disease, a genetic disorder ine, phenylalanine, threonine, tryptophan, and named for a British family in the late 1950s who valine—must be obtained from the diet. (In suffered from it. addition, adults are capable of synthesizing argi- Hartnup disease is characterized by an nine and histidine, but these amino acids are inability to transport amino acids from the kid- believed to be essential to growing children, neys to the rest of the body. The symptoms at meaning that children cannot produce them on first seemed to suggest to physicians that the dis- their own.) ease, which is present in one of about 26,000 live births, was pellagra. Tests showed that sufferers A complete protein is one that contains all of did not have inadequate tryptophan levels, how- the essential amino acids in quantities sufficient ever, as would have been the case with pellagra. for growth and repair of body tissue. Most pro- On the other hand, some 14 amino acids have teins from animal sources, gelatin being the only been found in excess within the urine of Hartnup exception, contain all the essential amino acids disease sufferers, indicating that rather than and are therefore considered complete proteins. properly transporting amino acids, their bodies On the other hand, many plant proteins do not are simply excreting them. This is a potentially contain all of the essential amino acids. For very serious condition, but it can be treated with example, lysine is absent from corn, rice, and the B vitamin nicotinamide, also used to treat wheat, whereas corn also lacks tryptophan and pellagra. Supplementation of tryptophan in the rice lacks threonine. Soybeans are lacking in diet also has shown positive results with some methionine. Vegans, or vegetarians who consume patients. no animal proteins in their diets (i.e., no eggs, SICKLE CELL ANEMIA. It is also dairy products, or the like) are at risk of malnu- possible for small mistakes to occur in the trition, because they may fail to assimilate one or amino-acid sequence within the body. While more essential amino acid. these mistakes sometimes can be tolerated in nature without serious problems, at other times a Amino Acids, Health, and single misplaced amino acid in the polymer Disease chain can bring about an extremely serious con- Amino acids can be used as treatments for all dition of protein malfunctioning. An example of this is sickle cell anemia, a fatal disease ultimate- sorts of medical conditions. For example, tyro- ly caused by a single mistake in the amino acid sine may be employed in the treatment of sequence. In the bodies of sickle cell anemia suf- Alzheimer’s disease, a condition characterized by ferers, who are typically natives of sub-Saharan the onset of dementia, or mental deterioration, Africa or their descendants in the United States as well as for alcohol-withdrawal symptoms. or elsewhere, glutamic acid is replaced by valine Taurine is administered to control epileptic at the sixth position from the end of the protein seizures, treat high blood pressure and diabetes, chain in the hemoglobin molecule. (Hemoglobin and support the functioning of the liver. Numer- is an iron-containing pigment in red blood cells ous other amino acids are used in treating a wide that is responsible for transporting oxygen to the array of other diseases. Sometimes the disease tissues and removing carbon dioxide from itself involves a problem with amino-acid pro- them.) This small difference makes sickle cell duction or functioning. In the essay Vitamins, hemoglobin molecules extremely sensitive to there is a discussion of pellagra, a disease result- oxygen deficiencies. As a result, when the red ing from a deficiency of the B-group vitamin blood cells release their oxygen to the tissues, as known as niacin. Pellagra results from a diet all red blood cells do, they fail to re-oxygenate in heavy in corn, which, as we have noted, lacks a normal fashion and instead twist into the shape
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 15 set_vol3_1 8/19/02 10:33 AM Page 16
Amino Acids KEY TERMS
AMINO ACIDS: Organic compounds HORMONE: Molecules produced by liv- made of carbon, hydrogen, oxygen, nitro- ing cells, which send signals to spots remote gen, and (in some cases) sulfur bonded in from their point of origin and induce spe- characteristic formations. Strings of amino cific effects on the activities of other cells. acids make up proteins. MOLECULE: A group of atoms, usually AMINO GROUP: The chemical forma- but not always representing more than one
tion NH2, which is part of all amino acids. element, joined in a structure. Compounds BIOCHEMISTRY: The area of the bio- typically are made up of molecules. logical sciences concerned with the chemi- ORGANIC: At one time chemists used cal substances and processes in organisms. the term organic only in reference to living CARBOXYL GROUP: The formation - things. Now the word is applied to com- COOH, which is common to all amino pounds containing carbon and hydrogen. acids. PEPTIDE LINKAGE: A bond between COMPOUND: A substance in which the carboxyl group of one amino acid and atoms of more than one element are bond- the amino group of a second amino acid. ed chemically to one another. POLYMERS: Large, chainlike mole- DIPEPTIDE: A group of only two cules composed of numerous subunits amino acids. known as monomers. POLYPEPTIDE: DNA: Deoxyribonucleic acid, a mole- A group of between 10 cule in all cells and many viruses contain- and 50 amino acids. ing genetic codes for inheritance. PROTEINS: Large molecules built ENZYME: A protein material that from long chains of 50 or more amino speeds up chemical reactions in the bodies acids. Proteins serve the functions of pro- of plants and animals. moting normal growth, repairing damaged tissue, contributing to the body’s immune ESSENTIAL AMINO ACIDS: Amino system, and making enzymes. acids that cannot be manufactured by the RNA: body, and which therefore must be Ribonucleic acid, the molecule obtained from the diet. Proteins that con- translated from DNA in the cell nucleus, tain essential amino acids are known as the control center of the cell, that directs complete proteins. protein synthesis in the cytoplasm, or the space between cells. GENE: A unit of information about a SYNTHESIZE: particular heritable (capable of being To manufacture chemi- inherited) trait that is passed from parent cally, as in the body. to offspring, stored in DNA molecules TRIPEPTIDE: A group of three amino called chromosomes. acids.
that gives sickle cell anemia its name. This causes hydroxyurea in the mid-1990s, the average life obstruction of the blood vessels. Before the expectancy of a person with sickle cell anemia development of a treatment with the drug was about 45 years.
16 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 17
Amino Acids and the Distant biological history. Here, for the first time, was Amino Acids Past concrete evidence of life—at least, in a very sim- The Evolution essay discusses several types of ple chemical form—existing billions of years dating, a term referring to scientific efforts before the first true organism. The discovery of directed toward finding the age of a particular these amino-acid samples greatly influenced sci- item or phenomenon. Methods of dating are entists’ thinking about evolution, particularly the either relative (i.e., comparative and usually very early stages in which the chemical founda- based on rock strata, or layers) or absolute. tions of life were established. Whereas relative dating does not involve actual estimates of age in years, absolute dating does. WHERE TO LEARN MORE One of the first types of absolute-dating tech- “Amino Acids.” Institute of Chemistry, Department of niques developed was amino-acid racimization, Biology, Chemistry, and Pharmacy, Freie Universität, introduced in the 1960s. As noted earlier, there Berlin (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 17 set_vol3_1 8/19/02 10:33 AM Page 18
PROTEINSProteins
CONCEPT Amino acids are discussed in more depth Most of us recognize the term protein in a nutri- within the essay devoted to that topic, though, as tional context as referring to a class of foods that noted in that essay, it is impossible to treat such a includes meats, dairy products, eggs, and other subject thoroughly without going into an items. Certainly, proteins are an important part extraordinarily lengthy and technical discussion. of nutrition, and obtaining complete proteins in Such is the case with many topics in biochem- one’s diet is essential to the proper functioning of istry, the area of the biological sciences con- cerned with the chemical substances and the body. But the significance of proteins extends processes in organisms: the deeper within the far beyond the dining table. Vast molecules built structure of things one goes, and the smaller the from enormous chains of amino acids, proteins items under investigation, the more complex are are essential building blocks for living systems— the properties and interactions. hence their name, drawn from the Greek proteios, or “holding first place.” Proteins are integral to The Basics the formation of DNA, a molecule that contains genetic codes for inheritance, and of hormones. Amino acids react with each other to form a Most of the dry weight of the human body and bond, called a peptide linkage, between the car- the bodies of other animals is made of protein, as boxyl group of one amino acid (symbolized as -COOH) and the amino group (-NH ) of a sec- is a vast range of things with which we come into 2 ond amino acid. In this way they can make large, contact on a daily basis. In addition, a special chainlike molecules called polymers, which may type of protein called an enzyme has still more contain as few as two or as many as 3,000 amino- applications. acid units. If there are more than 10 units in a chain, the chain is called a polypeptide, while a HOW IT WORKS chain with 50 or more amino-acid units is known as a protein. The Complexities of All the millions of different proteins in living Biochemistry things are formed by the bonding of only 20 amino acids into long polymer chains. Because Protein is a foundational material in the struc- each amino acid can be used many times along ture of most living things, and as such it is rather the chain, and because there are no restrictions on like concrete or steel. Just as concrete is a mixture the length of the chain, the number of possible of other ingredients and steel is an alloy of iron combinations for the creation of proteins is and carbon, proteins, too, are made of something truly enormous: about two quadrillion, or more basic: amino acids. These are organic com- 2,000,000,000,000,000. Just as not all sequences of pounds made of carbon, hydrogen, oxygen, letters make sense, however, not all sequences of nitrogen, and (in some cases) sulfur bonded in amino acids produce functioning proteins. In characteristic formations. fact, the number of proteins that have significance
18 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 19
in the functioning of nature is closer to about Proteins 100,000. This number is considerably smaller than two quadrillion—about 1/2,000,000,000th of that larger number, in fact—but it is still a very large number. COMPONENTS OTHER THAN AMINO ACIDS. The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it, yet many proteins include components other than amino acids. For example, some may have sugar molecules (sugars are discussed in the essay on Carbohydrates) chemically attached. Exactly which types of sugars are attached and where on the protein chain attachment occurs vary with the specific protein. Other proteins may have lipid, or fat, molecules chemically bonded to them. Sugar and lipid molecules always are added when synthesis of the protein’s amino-acid chain is complete. Many other types of substance, including metals, also may be associated with proteins; for instance, hemoglobin, a pigment in red blood cells that is responsible for transport- SHEEP SHEARING IN NEW ZEALAND. THE ENTIRE ANI- MAL WORLD IS CONSTITUTED LARGELY OF PROTEIN, AS ing oxygen to the tissues and removing carbon ARE A WHOLE HOST OF ANIMAL PRODUCTS, INCLUDING dioxide from them, is a protein that contains an LEATHER AND WOOL. (© Adam Woolfitt/Corbis. Reproduced by iron atom. permission.) STRUCTURES AND SYNTHE- SIS. Protein structures generally are described process that we do not attempt to discuss here. at four levels: primary, secondary, tertiary, and Following synthesis, proteins fold up into an quaternary. Primary structure is simply the two- essentially compact three-dimensional shape, dimensional linear sequence of amino acids in which is their tertiary structure. the peptide chain. Secondary and tertiary struc- The steps involved in folding and the shape tures both refer to the three-dimensional shape that finally results are determined by such chem- into which a protein chain folds. The distinction ical properties as hydrogen bonds, electrical between the two is partly historical: secondary attraction between positively and negatively structures are those that were first discerned by scientists of the 1950s, using the techniques and charged side chains, and the interaction between knowledge available then, whereas an awareness polar and nonpolar molecules. Nonpolar mole- of tertiary structure emerged only later. Finally, cules are called hydrophobic, or “water-fearing,” quaternary structure indicates the way in which because they do not mix with water but instead many protein chains associate with one another. mix with oils and other substances in which the For example, hemoglobin consists of four pro- electric charges are more or less evenly distrib- tein chains (spirals, actually) of two slightly dif- uted on the molecule. Polar molecules, on the ferent types, all attached to an iron atom. other hand, are termed hydrophilic, or “water- Protein synthesis is the process whereby pro- loving,” and mix with water and water-based teins are produced, or synthesized, in living substances in which the opposing electric charges things according to “directions” given by DNA occupy separate sides, or ends, of the molecule. (deoxyribonucleic acid) and carried out by RNA Typically, hydrophobic amino-acid side chains (ribonucleic acid) and other proteins. As suggest- tend to be on the interior of a protein, while ed earlier, this is an extraordinarily complex hydrophilic ones appear on the exterior.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 19 set_vol3_1 8/19/02 10:33 AM Page 20
Proteins
U.S. WANTED POSTER FOR A WORLD WAR II NAZI SABOTEUR (JULY 1942). FINGERPRINTS ARE AN EXPRESSION OF OUR DNA, WHICH IS LINKED CLOSELY WITH THE OPERATION OF PROTEINS IN OUR BODIES. (© Bettmann/Corbis. Repro- duced by permission.)
REAL-LIFE earlier, much of our bodies’ dry weight—that is, APPLICATIONS the weight other than water, which accounts for a large percentage of the total—is protein. Our Proteins Are Everywhere bones, for instance, are about one-fourth pro- tein, and protein makes up a very high percent- Although it is very difficult to discuss the func- age of the material in our organs (including the tions of proteins in simple terms, and it is simi- skin), glands, and bodily fluids. larly challenging to explain exactly how they Humans are certainly not the only organ- function in everyday life, it is not hard at all to isms composed largely of protein: the entire ani- name quite a few areas in which these highly mal world, including the animals we eat and the important compounds are applied. As we noted microbes that enter our bodies (see Digestion
20 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 21
and Parasites and Parasitology) likewise is consti- present in each. This is only one of many key Proteins tuted largely of protein. In addition, a whole host roles that proteins play where blood is con- of animal products, including leather and wool, cerned. If certain proteins are missing, or if the are nearly pure protein. So, too, are other, less wrong proteins are present, blood will fail to clot widely used animal products, such as hormones properly, and cuts will refuse to heal. For suffer- for the treatment of certain conditions—for ers of the condition known as hemophilia, example, insulin, which keeps people with dia- caused by a lack of the proteins needed for clot- betes alive and which usually is harvested from ting, a simple cut can be fatal. the bodies of mammals. Similarly, proteins play a critical role in Proteins allow cells to detect and react to forensic science, or the application of medical hormones and toxins in their surroundings, and and biological knowledge to criminal investiga- as the chief ingredient in antibodies, which help tions. Fingerprints are an expression of our us resist infection, they play a part in protecting DNA, which is linked closely with the operation our bodies against foreign invaders. The lack of of proteins in our bodies. The presence of DNA specific proteins in the brain may be linked to in bodily fluids, such as blood, semen, sweat, and such mysterious, terrifying conditions as saliva, makes it possible to determine the identi- Alzheimer and Creutzfeldt-Jakob diseases (dis- ty of the individual who perpetrated a crime or cussed in Disease). Found in every cell and tissue of others who were present at the scene. In addi- and composing the bulk of our bodies’ structure, tion, a chemical known as luminol assists police proteins are everywhere, promoting growth and in the investigation of possible crime scenes. If repairing bone, muscles, tissues, blood, and blood has ever been shed in a particular area, organs. such as on a carpet, no matter how carefully the ENZYMES. One particularly important perpetrators try to conceal or eradicate the stain, type of protein is an enzyme, discussed in the it can be detected. The key is luminol, which essay on that topic. Enzymes make possible a reacts to hemoglobin in the blood, making it vis- host of bodily processes, in part by serving as cat- ible to investigators. This chemical, developed alysts, or substances that speed up a chemical during the 1980s, has been used to put many a reaction without actually participating in, or killer behind bars. being consumed by, that reaction. Enzymes DESIGNER PROTEINS. These are enable complex, life-sustaining reactions in the just a very few of the many applications of pro- human body—reactions that would be too slow teins, including a very familiar one, discussed in at ordinary body temperatures—and they man- more depth at the conclusion of this essay: nutri- age to do so without forcing the body to undergo tion. Given the importance and complexity of harmful increases in temperature. They also are proteins, it might be hard to imagine that they involved in fermentation, a process with applica- can be produced artificially, but, in fact, such tions in areas ranging from baking bread to production is taking place at the cutting edge of reducing the toxic content of wastewater. (For biochemistry today, in the field of “designer pro- much more on these subjects, see Enzymes.) teins.” Inside the body, enzymes and other proteins Many such designs involve making small have roles in digesting foods and turning the changes in already existing proteins: for example, nutrients in them—including proteins—into by changing three amino acids in an enzyme energy. They also move molecules around within often used to improve detergents’ cleaning our cells to serve an array of needs and allow power, commercial biochemists have doubled the healthful substances, such as oxygen, to pass enzyme’s stability in wash water. Medical appli- through cell membranes while keeping harmful cations of designer proteins seem especially ones out. Proteins in the chemical known as promising. For instance, we might one day cure chlorophyll facilitate an exceptionally important cancer by combining portions of one protein that natural process, photosynthesis, discussed briefly recognizes cancer with part of another protein in Carbohydrates. that attacks it. One of the challenges facing such PROTEINS, BLOOD, AND a development, however, is the problem of CRIME. The four blood types (A, B, AB, and designing a protein that attacks only cancer cells O) are differentiated on the basis of the proteins and not healthy ones.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 21 set_vol3_1 8/19/02 10:33 AM Page 22
Proteins KEY TERMS
AMINO ACIDS: Organic compounds HORMONE: Molecules produced by made of carbon, hydrogen, oxygen, nitro- living cells, which send signals to spots gen and (in some cases) sulfur bonded in remote from their point of origin and characteristic formations. Strings of amino induce specific effects on the activities of acids make up proteins. other cells. AMINO GROUP: The chemical forma- OMNIVORE: An organism that eats
tion -NH2, which is part of all amino acids. both plants and other animals. BIOCHEMISTRY: The area of the bio- ORGANIC: At one time chemists used logical sciences concerned with the chemi- the term organic only in reference to living cal substances and processes in organisms. things. Now the word is applied to com- CARBOXYL GROUP: The formation pounds containing carbon and hydrogen. -COOH, which is common to all amino PEPTIDE LINKAGE: A bond between acids. the carboxyl group of one amino acid and DNA: Deoxyribonucleic acid, a mole- the amino group of a second amino acid. cule in all cells, and many viruses, contain- POLYMERS: Large, chainlike mole- ing genetic codes for inheritance. cules composed of numerous subunits ENZYME: A protein material that known as monomers. speeds up chemical reactions in the bodies POLYPEPTIDE: A group of between 10 of plants and animals. and 50 amino acids. ESSENTIAL AMINO ACIDS: Amino PROTEINS: Large molecules built acids that cannot be manufactured by the from long chains of 50 or more amino body and therefore must be obtained from acids. Proteins serve the functions of pro- the diet. Proteins that contain essential moting normal growth, repairing damaged amino acids are known as complete proteins. tissue, contributing to the body’s immune HEMOGLOBIN: An iron-containing system, and making enzymes. protein in red blood cells that is responsible RNA: Ribonucleic acid, the molecule for transporting oxygen to the tissues and translated from DNA in the cell nucleus, removing carbon dioxide from them. the control center of the cell, that directs Hemoglobin is known for its deep red color. protein synthesis in the cytoplasm, or the HERBIVORE: A plant-eating organism. space between cells.
In the long term, scientists hope to design hope that one day they may be able to design proteins from scratch. This is extremely difficult proteins to meet a host of medical and industrial today and will remain so until researchers better needs. understand the rules that govern tertiary struc- ture. Nevertheless, scientists already have Proteins in the Diet designed a few small proteins whose stability or instability has enhanced our understanding of Proteins are one of the basic nutrients, along those rules. Building on these successes, scientists with carbohydrates, lipids, vitamins, and miner-
22 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 23
als (see Nutrients and Nutrition). They can be For a person who eats meat, it would be Proteins broken down and used as a source of emergency extremely difficult not to get enough protein. energy if carbohydrates or fats cannot meet According to the U.S. Food and Drug Adminis- immediate needs. The body does not use protein tration (FDA), protein should account for 10% from food directly: after ingestion, enzymes in of total calories in the diet, and since protein the digestive system break protein into smaller contains 4 calories per 0.035 oz. (1 g), that would peptide chains and eventually into separate be about 1.76 oz. (50 g) in a diet consisting of amino acids. These smaller constituents then go 2,000 calories a day. A pound (0.454 kg) of steak into the bloodstream, from whence they are or pork supplies about twice this much, and transported to the cells. The cells incorporate the though very few people sit down to a meal and amino acids and begin building proteins from eat a pound of meat, it is easy to see how a meat them. eater would consume enough protein in a day. ANIMAL AND VEGETABLE For a vegetarian, meeting the protein needs PROTEINS. The protein content in plants is may be a bit more tricky, but it can be done. By very small, since plants are made largely of cellu- combining legumes or beans and grains, it is pos- lose, a type of carbohydrate (see Carbohydrates sible to obtain a complete protein: hence, the for more on this subject); this is one reason why herbivorous animals must eat enormous quanti- longstanding popularity, with meat eaters as well ties of plants to meet their dietary requirements. as vegetarians, of such combinations as beans Humans, on the other hand, are omnivores and rice or peas and cornbread. Other excellent (unless they choose to be vegetarians) and are vegetarian combos include black beans and corn, able to assimilate proteins in abundant quantities for a Latin American touch, or the eastern Asian by eating the bodies of plant-eating animals, such combination of rice and tofu, protein derived as cows. In contrast to plants, animal bodies (as from soybeans. previously noted) are composed largely of pro- teins. When people think of protein in the diet, WHERE TO LEARN MORE some of the foods that first come to mind are “DNA and Protein Synthesis.” John Jay College of Crimi- those derived from animals: either meat or such nal Justice, City University of New York (Web site). animal products as milk, cheese, butter, and eggs.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 23 set_vol3_1 8/19/02 10:33 AM Page 24
ENZYMESEnzymes
CONCEPT Catalysts
Enzymes are biological catalysts, or chemicals In a chemical reaction, substances known as that speed up the rate of reaction between sub- reactants interact with one another to create new stances without themselves being consumed in substances, called products. Energy is an impor- the reaction. As such, they are vital to such bodi- tant component in the chemical reaction, ly functions as digestion, and they make possible because a certain threshold, termed the activa- processes that normally could not occur except at tion energy, must be crossed before a reaction temperatures so high they would threaten the can occur. To increase the rate at which a reaction well-being of the body. A type of protein, takes place and to hasten the crossing of the acti- enzymes sometimes work in tandem with non- vation energy threshold, it is necessary to do one proteins called coenzymes. Among the processes of three things. in which enzymes play a vital role is fermenta- The first two options are to increase either tion, which takes place in the production of alco- the concentration of reactants or the temperature hol or the baking of bread and also plays a part in at which the reaction takes place. It is not always numerous other natural phenomena, such as the feasible or desirable, however, to do either of purification of wastewater. these things. Many of the processes that take place in the human body, for instance, normally HOW IT WORKS would require high temperatures—temperatures, in fact, that are too high to sustain human life. Amino Acids, Proteins, and Imagine what would happen if the only way we Biochemistry had of digesting starch was to heat it to the boil- ing point inside our stomachs! Fortunately, there Amino acids are organic compounds made of is a third option: the introduction of a catalyst, a carbon, hydrogen, oxygen, nitrogen, and (in substance that speeds up a reaction without par- some cases) sulfur bonded in characteristic for- ticipating in it either as a reactant or as a product. mations. Strings of 50 or more amino acids are Catalysts thus are not consumed in the reaction. known as proteins, large molecules that serve the Enzymes, which facilitate the necessary reactions functions of promoting normal growth, repair- in our bodies without raising temperatures or ing damaged tissue, contributing to the body’s increasing the concentrations of substances, are a immune system, and making enzymes. The latter prime example of a chemical catalyst. are a type of protein that functions as a catalyst, THE DISCOVERY OF CATALY- a substance that speeds up a chemical reaction SIS. Long before chemists recognized the exis- without participating in it. Catalysts, of which tence of catalysts, ordinary people had been enzymes in the bodies of plants and animals are using the chemical process known as catalysis for a good example, thus are not consumed in the numerous purposes: making soap, fermenting reaction. wine to create vinegar, or leavening bread, for
24 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 25
instance. Early in the nineteenth century, enzyme that catalyzes the digestion of lactose, or Enzymes chemists began to take note of this phenomenon. milk sugar, and urease catalyzes the chemical In 1812 the Russian chemist Gottlieb Kirchhoff breakdown of urea, a substance in urine. (1764–1833) was studying the conversion of Enzymes bind their reactants or substrates at starches to sugar in the presence of strong acids special folds and clefts, named active sites, in the when he noticed something interesting. structure of the substrate. Because numerous When a suspension of starch (that is, parti- interactions are required in their work of cataly- cles of starch suspended in water) was boiled, sis, enzymes must have many active sites, and Kirchhoff observed, no change occurred in the therefore they are very large, having atomic mass starch. When he added a few drops of concen- figures as high as one million amu. (An atomic trated acid before boiling the suspension, how- mass unit, or amu, is approximately equal to the ever, he obtained a very different result. This mass of a proton, a positively charged particle in time, the starch broke down to form glucose, a the nucleus of an atom.) simple sugar (see Carbohydrates), whereas the Suppose a substrate molecule, such as a acid—which clearly had facilitated the reac- starch, needs to be broken apart for the purposes tion—underwent no change. In 1835 the of digestion in a living body. The energy needed Swedish chemist Jöns Berzelius (1779–1848) to break apart the substrate is quite large, larger provided a name to the process Kirchhoff had than is available in the body. An enzyme with the observed: catalysis, derived from the Greek correct molecular shape arrives on the scene and words kata (“down”) and lyein (“loosen”). Just attaches itself to the substrate molecule, forming two years earlier, in 1833, the French physiolo- a chemical bond within it. The formation of gist Anselme Payen (1795–1871) had isolated a these bonds causes the breaking apart of other material from malt that accelerated the conver- bonds within the substrate molecule, after which sion of starch to sugar, for instance, in the brew- the enzyme, its work finished, moves on to ing of beer. another uncatalyzed substrate molecule. The renowned French chemist Louis Pasteur (1822–1895), who was right about so many Coenzymes things, called these catalysts ferments and pro- nounced them separate organisms. In 1897, how- All enzymes belong to the protein family, but ever, the German biochemist Eduard Buchner many of them are unable to participate in a cat- (1860–1917) isolated the catalysts that bring alytic reaction until they link with a nonprotein about the fermentation of alcohol and deter- component called a coenzyme. This can be a mined that they were chemical substances, not medium-size molecule called a prosthetic group, organisms. By that time, the German physiologist or it can be a metal ion (an atom with a net elec- Willy Kahne had suggested the name enzyme for tric charge), in which case it is known as a cofac- these catalysts in living systems. tor. Quite often, though, coenzymes are com- posed wholly or partly of vitamins. Although Substrates and Active Sites some enzymes are attached very tightly to their Each type of enzyme is geared to interact chemi- coenzymes, others can be parted easily; in either cally with only one particular substance or type case, the parting almost always deactivates both of substance, termed a substrate. The two parts partners. fit together, according to a widely accepted theo- The first coenzyme was discovered by the ry introduced in the 1890s by the German English biochemist Sir Arthur Harden chemist Emil Fischer (1852–1919), as a key fits (1865–1940) around the turn of the nineteenth into a lock. Each type of enzyme has a specific century. Inspired by Buchner, who in 1897 had three-dimensional shape that enables it to fit detected an active enzyme in yeast juice that he with the substrate, which has a complementary had named zymase, Harden used an extract of shape. yeast in most of his studies. He soon discovered The link between enzymes and substrates is that even after boiling, which presumably so strong that enzymes often are named after the destroyed the enzymes in yeast, such deactivated substrate involved, simply by adding ase to the yeast could be reactivated. This finding led Hard- name of the substrate. For example, lactase is the en to the realization that a yeast enzyme appar-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 25 set_vol3_1 8/19/02 10:33 AM Page 26
Enzymes
THE CONVERSION OF CABBAGE TO SAUERKRAUT UTILIZES A PARTICULAR BACTERIUM THAT ASSISTS IN FERMENTATION. HERE WORKERS SPREAD SALT AND PACK CHOPPED CABBAGE IN BARRELS, WHERE IT WILL FERMENT FOR FOUR WEEKS. (© Bettmann/Corbis. Reproduced by permission.)
ently consists of two parts: a large, molecular tein. Harden, who later shared the 1929 Nobel portion that could not survive boiling and was Prize in chemistry for this research, termed the almost certainly a protein and a smaller portion nonprotein a coferment, but others began calling that had survived and was probably not a pro- it a coenzyme.
26 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 27
REAL-LIFE red blood cells are suspended. These enzymes in Enzymes APPLICATIONS the blood assist the body in everything from growth to protection against infection. The Body, Food, and Digestion One digestive enzyme that should be in the body, but is not always present, is lactase. As we Enzymes enable the many chemical reactions noted earlier, lactase works on lactose, the princi- that are taking place at any second inside the pal carbohydrate in milk, to implement its diges- body of a plant or animal. One example of an tion. If a person lacks this enzyme, consuming enzyme is cytochrome, which aids the respirato- dairy products may cause diarrhea, bloating, and ry system by catalyzing the combination of oxy- cramping. Such a person is said to be “lactose gen with hydrogen within the cells. Other intolerant,” and if he or she is to consume dairy enzymes facilitate the conversion of food to ener- products at all, they must be in forms that con- gy and make possible a variety of other necessary tain lactase. For this reason, Lactaid milk is sold biological functions. Enzymes in the human in the specialty dairy section of major supermar- body fulfill one of three basic functions. The kets, while many health-food stores sell lactaid largest of all enzyme types, sometimes called tablets. metabolic enzymes, assist in a wide range of basic Fermentation bodily processes, from breathing to thinking. Some such enzymes are devoted to maintaining Fermentation, in its broadest sense, is a process the immune system, which protects us against involving enzymes in which a compound rich in disease, and others are involved in controlling the energy is broken down into simpler substances. It effects of toxins, such as tobacco smoke, convert- also is sometimes identified as a process in which ing them to forms that the body can expel more large organic molecules (those containing hydro- easily. gen and carbon) are broken down into simpler A second category of enzyme is in the diet molecules as the result of the action of microor- and consists of enzymes in raw foods that aid in ganisms working anaerobically, or in the absence the process of digesting those foods. They of oxygen. The most familiar type of fermenta- include proteases, which implement the diges- tion is the conversion of sugars and starches to tion of protein; lipases, which help in digesting alcohol by enzymes in yeast. To distinguish this lipids or fats; and amylases, which make it possi- reaction from other kinds of fermentation, the ble to digest carbohydrates. Such enzymes set in process is sometimes termed alcoholic or ethano- motion the digestive process even when food is lic fermentation. still in the mouth. As these enzymes move with At some point in human prehistory, humans the food into the upper portion of the stomach, discovered that foods spoil, or go bad. Yet at the they continue to assist with digestion. dawn of history—that is, in ancient Sumer and The third group of enzymes also is involved Egypt—people found that sometimes the in digestion, but these enzymes are already in the “spoilage” (that is, fermentation) of products body. The digestive glands secrete juices contain- could have beneficial results. Hence the fermen- ing enzymes that break down nutrients chemi- tation of fruit juices, for example, resulted in the cally into smaller molecules that are more easily formation of primitive forms of wine. Over the absorbed by the body. Amylase in the saliva centuries that followed, people learned how to begins the process of breaking down complex make both alcoholic beverages and bread carbohydrates into simple sugars. While food is through the controlled use of fermentation. still in the mouth, the stomach begins producing ALCOHOLIC BEVERAGES. In pepsin, which, like protease, helps digest protein. fermentation, starch is converted to simple sug- Later, when food enters the small intestine, ars, such as sucrose and glucose, and through a the pancreas secretes pancreatic juice—which complex sequence of some 12 reactions, these contains three enzymes that break down carbohy- sugars then are converted to ethyl alcohol (the drates, fats, and proteins—into the duodenum, kind of alcohol that can be consumed, as which is part of the small intestine. Enzymes from opposed to methyl alcohol and other toxic food wind up among the nutrients circulated to forms) and carbon dioxide. Numerous enzymes the body through plasma, a watery liquid in which are needed to carry out this sequence of reac-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 27 set_vol3_1 8/19/02 10:33 AM Page 28
Enzymes
BAKER’S YEAST, SINGLE-CELL FUNGI THAT PRODUCE CARBON DIOXIDE IN THE OXIDATION OF SUGAR. CARBON DIOX- IDE CAUSES BREAD TO RISE WHEN YEAST IS MIXED INTO DOUGH, A RESULT OF THE FERMENTATION OF THE SUGAR BY ENZYMES IN THE YEAST. (© Lester V. Bergman/Corbis. Reproduced by permission.)
tions, the most important being zymase, which is OTHER FOODS. Of course, ethyl found in yeast cells. These enzymes are sensitive alcohol is not the only useful product of fermen- to environmental conditions, such that when the tation or even of fermentation using yeast; so, concentration of alcohol reaches about 14%, they too, are baked goods, such as bread. The carbon are deactivated. For this reason, no fermentation dioxide generated during fermentation is an product (such as wine) can have an alcoholic important component of such items. When the concentration of more than about 14%. Stronger batter for bread is mixed, a small amount of alcoholic beverages, such as whisky, are the result sugar and yeast is added. The bread then rises, of another process, distillation. which is more than just a figure of speech: it The alcoholic beverages that can be pro- actually puffs up as a result of the fermentation duced by fermentation vary widely, depending of the sugar by enzymes in the yeast, which primarily on two factors: the plant that is fer- brings about the formation of carbon dioxide mented and the enzymes used for fermentation. gas. The carbon dioxide gives the batter bulkiness Depending on the materials available to them, and texture that would be lacking without the various peoples have used grapes, berries, corn, fermentation process. Another food-related rice, wheat, honey, potatoes, barley, hops, cactus application of fermentation is the production of juice, cassava roots, and other plant materials for one processed type of food from a raw, natural fermentation to produce wines, beers, and other variety. The conversion of raw olives to the olives fermented drinks. The natural product used in sold in stores, of cucumbers to pickles, and of making the beverage usually determines the cabbage to sauerkraut utilizes a particular bac- name of the synthetic product. Thus, for terium that assists in a type of fermentation. instance, wine made with rice—a time-honored INDUSTRIAL APPLICATIONS. tradition in Japan—is known as sake, while a fer- There is even ongoing research into the creation mented beverage made from barley, hops, or of edible products from the fermentation of malt sugar has a name very familiar to Ameri- petroleum. While this may seem a bit far-fetched, cans: beer. Grapes make wine, but “wine” made it is less difficult to comprehend powering cars from honey is known as mead. with an environmentally friendly product of fer-
28 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_1 8/19/02 10:33 AM Page 29
Enzymes KEY TERMS
ACTIVATION ENERGY: A threshold ENZYME: A protein that acts as a cata- that must be crossed to facilitate a chemical lyst, a material that speeds up chemical reaction. There are three ways to reach the reactions in the bodies of plants and ani- activation energy: by increasing the con- mals without itself taking part in, or being centration of reactants, by raising their consumed by, these reactions. temperature, or by introducing a catalyst, FERMENTATION: A process involving such as an enzyme. enzymes in which a compound rich in ACTIVE SITES: Folds and clefts on the energy is broken down into simpler sub- surface of an enzyme that enable attach- stances. ment to its particular substrate. METABOLISM: The chemical process AMINO ACIDS: Organic compounds by which nutrients are broken down and made of carbon, hydrogen, oxygen, nitro- converted into energy or are used in the gen, and (in some cases) sulfur bonded in construction of new tissue or other materi- characteristic formations. Strings of amino al in the body. acids make up proteins. MOLECULE: A group of atoms, usual- BIOCHEMISTRY: The area of the bio- ly but not always representing more than logical sciences concerned with the chemi- one element, joined in a structure. Com- cal substances and processes in organisms. pounds typically are made up of mole- cules. CARBOHYDRATES: Naturally occur- ring compounds, consisting of carbon, ORGANIC: At one time, chemists used hydrogen, and oxygen, whose primary the term organic only in reference to living function in the body is to supply energy. things. Now the word is applied to com- Included in the carbohydrate group are pounds containing carbon and hydrogen. sugars, starches, cellulose, and various PROTEINS: Large molecules built other substances. Most carbohydrates are from long chains of 50 or more amino produced by green plants in the process of acids. Proteins serve the functions of pro- undergoing photosynthesis. moting normal growth, repairing damaged CATALYSIS: The act or process of cat- tissue, contributing to the body’s immune alyzing, or speeding up the rate of reaction system, and making enzymes. between substances. REACTANT: A substance that interacts CATALYST: A substance that speeds up with another substance in a chemical reac- a chemical reaction without participating tion, resulting in the formation of a chem- in it. Catalysts, of which enzymes are a ical or chemicals known as the product. good example, thus are not consumed in STARCHES: Complex carbohydrates the reaction. without taste or odor, which are granular COENZYME: A nonprotein component or powdery in physical form. sometimes required to allow an enzyme to SUBSTRATE: A reactant that typically set in motion a catalytic reaction. is paired with a particular enzyme.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 29 set_vol3_1 8/19/02 10:33 AM Page 30
Enzymes KEY TERMS CONTINUED
Enzymes often are named after their “sugar” (i.e., table sugar) is actually respective substrates by adding the suffix sucrose. ase (e.g., the enzyme lactase is paired with VITAMINS: Organic substances that, in the substrate lactose). extremely small quantities, are essential to the nutrition of most animals and some SUGARS: One of the three principal plants. In particular, vitamins work with types of carbohydrate, along with starches enzymes in regulating metabolic processes; and cellulose. Sugars can be defined as any however, they do not in themselves provide of various water-soluble carbohydrates of energy, and thus vitamins alone do not varying sweetness. What we think of as qualify as a form of nutrition.
mentation known as gasohol. Gasohol first start- converted to carbon dioxide, water, and mineral ed to make headlines in the 1970s, when an oil salts. embargo and resulting increases in gas prices, combined with growing environmental con- WHERE TO LEARN MORE cerns, raised the need for a type of fuel that Asimov, Isaac. The Chemicals of Life: Enzymes, Vitamins, would use less petroleum. A mixture of about Hormones. New York: Abelard-Schulman, 1954. 90% gasoline and 10% alcohol, gasohol burns “Enzymes: Classification, Structure, Mechanism.” Wash- more cleanly that gasoline alone and provides a ington State University Department of Chemistry promising method for using renewable resources (Web site).
30 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:34 AM Page 31
SCIENCE OF EVERYDAY THINGS real-life biology METABOLISM
METABOLISM DIGESTION RESPIRATION
31 set_vol3_2 8/19/02 10:35 AM Page 33
Metabolism METABOLISM
CONCEPT Metabolism is indeed like a furnace, in that The term metabolism refers to all of the chemical it burns energy, and that is the aspect most com- reactions by which complex molecules taken into monly associated with this concept. But metabo- an organism are broken down to produce energy lism also involves a function that a furnace does and by which energy is used to build up complex not: building new material. All metabolic reac- molecules. All metabolic reactions fall into one of tions can be divided into either catabolic or ana- two general categories: catabolic and anabolic bolic reactions. Catabolism is the process by reactions, or the processes of breaking down and which large molecules are broken down into building up, respectively. The best example of smaller ones with the release of energy, whereas metabolism from daily life occurs in the process anabolism is the process by which energy is used of taking in and digesting nutrients, but some- to build up complex molecules needed by the times these processes become altered, either body to maintain itself and develop new tissue. through a person’s choice or through outside fac- DIGESTION. One way to understand tors, and metabolic disorders follow. Such disor- the metabolic process is to follow the path of a ders range from anorexia and bulimia to obesity. typical nutrient as it passes through the body. These are all examples of an unhealthy, unnatu- The digestive process is discussed in Digestion, ral alteration to the ordinary course of metabo- while nutrients are examined in Nutrients and lism; on the other hand, hibernation allows ani- Nutrition as well as in Proteins, Amino Acids, mals to slow down their metabolic rates dramat- Enzymes, Carbohydrates, and Vitamins. Here we ically as a means of conserving energy during touch on the process only in general terms, as it times when food is scarce. relates to metabolism. The term digestion is not defined in the HOW IT WORKS essay on that subject, because it is an everyday The Body’s Furnace word whose meaning is widely known. For the present purposes, however, it is important to The term metabolism, strangely enough, is relat- identify it as the process of breaking down food ed closely to devil, with which it shares the Greek into simpler chemical compounds as a means of root ballein, meaning “to throw.” By adding dia making nutrients absorbable by the body. This is (“through” or “across”), one arrives at devil and a catabolic process, because the molecules of many related words, such as diabolical; on the which foods are made are much too large to pass other hand, the replacement of that prefix with through the lining of the digestive system and meta (“after” or “beyond”) yields the word directly into the bloodstream. Thanks to the metabolism. The connection between the two digestive process, smaller molecules are formed words has been obscured over time, but it might and enter the bloodstream, from whence they are be helpful to picture metabolism in terms of an carried to individual cells throughout a person’s image that goes with that of a devil: a furnace. body.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 33 set_vol3_2 8/19/02 10:35 AM Page 34
Metabolism The smaller molecules into which nutrients be used to put simpler molecules together to are broken down make up the metabolic pool, make more complex molecules. For example, which consists of simpler substances.The meta- suppose that a cell needs to repair a rupture in its bolic pool includes simple sugars, made by the cell membrane. To do so, it will need to produce breakdown of complex carbohydrates; glycerol new protein molecules, which are made from and fatty acids, which come from the conversion hundreds or thousands of amino-acid molecules. of lipids, or fats; and amino acids, formed by the These molecules can be obtained from the meta- breakdown of proteins. Substances in the meta- bolic pool. bolic pool provide material from which new tis- The reactions by which a compound is sue is constructed—an anabolic process. metabolized differ for various nutrients. Also, The chemical breakdown of substances in energy carriers other than ATP may play a part. the cells is a complex and wondrous process. For For example, the compound known as nicoti- instance, a cell converts a sugar molecule into namide adenine dinucleotide phosphate carbon dioxide and water over the course of (NADPH) also has a role in the catabolism and about two dozen separate chemical reactions. anabolism of various substances. The general This is what cell biologists call a metabolic path- outline described here, however, applies to all way: an orderly sequence of reactions, with par- metabolic reactions. ticular enzymes (a type of protein that speeds up chemical reactions) acting at each step along the Catabolism and Anabolism way. In this instance, each chemical reaction Energy released from organic nutrients (those makes a relatively modest change in the sugar containing carbon and hydrogen) during catabo- molecule—for example, the removal of a single lism is stored within ATP,in the form of the high- oxygen atom or a single hydrogen atom—and energy chemical bonds between the second and each is accompanied by the release of energy, a third molecules of phosphate. The cell uses ATP result of the breaking of chemical bonds between for synthesizing cell components from simple atoms. precursors, for the mechanical work of contrac- ATP and ADP tion and motion, and for transport of substances across its membrane. ATP’s energy is released Cells capture and store the energy released in when this bond is broken, turning ATP into ADP. catabolic reactions through the use of chemical The cell uses the energy derived from catabolism compounds known as energy carriers. The most to fuel anabolic reactions that synthesize cell significant example of an energy carrier is adeno- components. Although anabolism and catabo- sine triphosphate, or ATP, which is formed when lism occur simultaneously in the cell, their rates a simpler compound, adenosine diphosphate are controlled independently. Cells separate these (ADP), combines with a phosphate group. (A pathways because catabolism is a “downhill” phosphate is a chemical compound that contains process, or one in which energy is released, while oxygen bonded to phosphorus, and the term anabolism is an “uphill” process requiring the group in chemistry refers to a combination of input of energy. atoms from two or more elements that tend to Catabolism and anabolism share an impor- bond with other elements or compounds in cer- tant common sequence of reactions known col- tain characteristic ways.) lectively as the citric acid cycle, the tricarboxylic ADP will combine with a phosphate group acid cycle, or the Krebs cycle. Named after the only if energy is added to it. In cells, that energy German-born British biochemist Sir Hans Adolf comes from the catabolism of compounds in the Krebs (1900–1981), the Krebs cycle is a series of metabolic pool, including sugars, glycerol (relat- chemical reactions in which tissues use carbohy- ed to fats), and fatty acids. The ATP molecule drates, fats, and proteins to produce energy; it is formed in this manner has taken up the energy part of a larger series of enzymatic reactions previously stored in the sugar molecule, and known as oxidative phosphorylation. In the latter thereafter, whenever a cell needs energy for some reaction, glucose is broken down to release ener- process, it can obtain it from an ATP molecule. gy, which is stored in the form of ATP—a cata- The reverse of this process also takes place inside bolic sequence. At the same time, other mole- cells. That is, energy from an ATP molecule can cules produced by the Krebs cycle are used as
34 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 35
precursor molecules for reactions that build pro- acids. The attachment with other compounds Metabolism teins, fats, and carbohydrates—an anabolic greatly alters the behavior of a lipid, often mak- sequence. (A precursor is a substance, cellular ing them bipolar—that is, one end of the mole- component, or cell from which another sub- cule is water-soluble. This is important, because stance, cellular component, or cell—different in it allows lipids to move out of the intestines and kind from the precursor—is formed.) into the bloodstream. In the digestive process, lipids are made water-soluble either by being Introduction to Lipids broken down into smaller parts or through asso- ciation with another substance. The breaking As noted earlier, many practical aspects of metabolism are discussed elsewhere, particularly down usually is done via two different processes: in the essays Digestion and Nutrients and Nutri- hydrolysis, or chemical reaction with water, and tion. Also, two types of chemical compound, saponification. The latter, a reaction in which proteins and carbohydrates, are so important to a certain kinds of organic compounds are variety of metabolic processes that they are hydrolyzed to produce an alcohol and a salt, is examined in detail within entries of their own. In used in making soap. the present context, let us focus on the third major kind of nutrient, lipids or fats. REAL-LIFE Lipids are soluble in nonpolar solvents, APPLICATIONS which is the reason why a gravy stain or other grease stain is difficult to remove from clothing Putting Lipids to Use without a powerful detergent or spot remover. Water molecules are polar, because the opposing Derived from living systems of plants, animals, electric charges tend to occupy opposite sides or or humans, lipids are essential to good health, ends of the molecule. In a molecule of oil, whether not only for humans but also for other animals derived from petroleum or from animal or veg- and even plants. Seeds, for example, contain etable fat, electric charges are very small, and are lipids for the storage of energy. Because fat is a distributed evenly throughout the molecule. poor conductor of heat, lipids also can function as effective insulators, and for this reason, people Whereas water molecules tend to bond rela- living in Arctic zones seek fatty foods such as tively well, like a bunch of bar magnets attaching blubber. Some lipids function as chemical mes- to one another at their opposing poles, oil and fat sengers in the body, while others serve as storage molecules tend not to bond. (The “bond” areas for chemical energy. There is a good reason referred to here is the fairly weak one between why babies are born with “baby fat” and why molecules. Much stronger is the chemical bond children entering puberty often tend to become within molecules—a bond that, when broken, chubby: in both cases, they are building up ener- brings about a release of energy, as noted earlier.) gy reserves for the great metabolic hurdles that Their functions are as varied as their structures, but because they are all fat-soluble, lipids share in lie ahead, and within a few years, they will have the ability to approach and even to enter cells. used up those excessive fat stores. The latter have membranes that, while highly FATS AND OILS. Fats and oils are complex in structure, can be identified in simple both energy-rich compounds that are basic com- terms as containing lipids or lipoproteins (lipids ponents of the normal diet. Both have essentially attached to proteins). The behavior of lipids and the same chemical structure—a mixture of fatty lipid-like molecules, therefore, becomes very acids combined with glycerol—and are insoluble important in understanding how a substance (do not dissolve) in water. While fats remain may or may not enter a cell. Such a substance solid or at least semisolid at room temperature, may be toxic, as in the case of some pesticides, however, most oils very quickly become liquid at but if they are lipid-like, they are able to pene- increased temperatures. Animal fats and oils trate the cell’s membrane. (See Food Webs for include butter, lard, tallow, and fish oil. Numer- more about the biomagnification of DDT.) ous other oils, such as cottonseed, peanut, and In addition to lipoproteins, there are glycol- corn oils, are derived from plants. ipids, or lipids attached to sugars, as well as lipids Fats have two main functions: they provide attached to alcohols and some to phosphoric some of the raw material for synthesizing (creat-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 35 set_vol3_2 8/19/02 10:35 AM Page 36
Metabolism
CROSS-SECTION MICROGRAPH OF A BLOOD VESSEL WITH ATHEROSCLEROTIC LESION, COMPOSED OF CHOLESTEROL AND OTHER LIPIDS. HIGH LEVELS OF CERTAIN CIRCULATING FATS CAN LEAD TO A THICKENING OF THE ARTERY WALLS AND HAVE BEEN LINKED TO VARIOUS ILLNESSES, INCLUDING HEART DISEASE AND CANCER. (© Lester V. Bergman/Corbis. Reproduced by permission.)
ing) and repairing tissues, and they serve as a Bad fats, on the other hand, are usually ones concentrated source of fuel energy. Fats, in fact, that have been tampered with through a process provide humans with roughly twice as much known as hydrogenation. This is a term describ- energy, per unit weight, as carbohydrates and ing any chemical reaction in which hydrogen proteins. Fats are not only an important source of atoms are added to fill in chemical bonds day-to-day energy, but they also can be stored between carbon and other atoms, but in the case indefinitely as adipose (fat) tissue in case of of fatty foods, hydrogenation involves the satura- future need. Fats also help by transporting fat- tion of hydrocarbons, organic chemical com- soluble vitamins, such as A and D (see Vitamins), pounds whose molecules are made up of nothing but carbon and hydrogen atoms. When they are throughout the system. They cushion and form treated with hydrogen gas, they become “saturat- protective pads around delicate organs, such as ed” with hydrogen atoms. Saturated fats, as they the heart, liver and kidneys, and the layer of fat are called, are harder and more stable and stand under the skin helps insulate the body against too up better to the heat of frying, which makes them much heat loss. They even add to the flavor of more desirable for use in commercial products. foods that might otherwise be inedible. For this reason, many foods contain hydrogenat- NOT ALL FAT IS CREATED ed vegetable oil; however, saturated fats have EQUAL. Although normal amounts of certain been linked to a rise in blood cholesterol levels— kinds of fat in the diet are essential to good and to an increased risk of heart disease. health, unnecessarily high amounts (especially of Cholesterol is a variety of lipid, and, like unhealthy fats) can lead to various problems. other lipids, some of it is essential—but only Healthy fats include those from fatty fish, such as some and only of the right kind. Most cholesterol salmon, mackerel, or tuna, or from fat-contain- is transported through the blood in low-density ing vegetables, such as the avocado. In addition, lipoproteins, or LDLs, which have been nick- many vegetable oils, particularly olive oil, can be named bad cholesterol. These lipoproteins are beneficial. received by LDL receptors on the cell mem-
36 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 37
branes, but if there are more LDLs than LDL “pinch” test, is a better measure of obesity. (Being Metabolism receptors, the excess LDLs will be deposited in obese is not the same as being overweight: the the arteries. Thus, LDLs are not really “bad” muscular athlete described in the last paragraph unless there are too many of them. On the other is overweight but not obese, a term that implies an hand, “good” cholesterol (HDLs, or high-density excess of body fat.) In healthy adults, fat typical- lipoproteins) help protect against damage to the ly should account for about 18–25% of the body artery walls by carrying excess LDLs back to the weight in females and 15–20% in males. liver. The reason for the difference between men HOW MUCH IS TOO MUCH? A and women is that fat naturally accumulates in a certain amount of excess adipose tissue can be woman’s buttocks and thighs, because nature valuable during periods of illness, overactivity, or “assumes” that she will bear children, in which food shortages. Too much, however, can be case such excess fat will be useful. This is why unsightly and also can overwork the heart and women over the age of about 25 often complain put added stress on other parts of the body. High that when they and their husbands or boyfriends levels of certain circulating fats may lead to ath- embark on a fitness program together, the men erosclerosis, which is a thickening of the artery usually see results faster. The reason is that there walls, and they have been linked to various ill- is no genetic or evolutionary benefit to be gained nesses, including cancer. from a man having fat around his waist, which is With fat, as with many things where the where men usually gain. If anything—since our body is concerned, if a little is a good, this does genetic codes and makeup have changed little not mean that a lot is better. In the past, nutri- since prehistory—the well-being and propaga- tionists considered a diet that obtained 40% of its tion of the human species are best served by a calories from fats a reasonable one; today, how- lean, muscular male capable of killing animals to ever, they recommend that no more than 30% of feed and protect his family. All of this means, of all calories (and preferably an even smaller per- course, that men should not gloat if they see bet- centage) come from fat. Agreement on this point, ter results from a regular workout program; however, is far from universal. Some physicians instead, they should just recognize that nature is and scientists maintain that dietary fat does not at work in their wives’ or girlfriends’ bodies as in contribute as much to body fat as do carbohy- their own. drates. Carbohydrates are good for someone who Metabolic Disorders needs a boost of energy that can be consumed easily by the body, such as an athlete going into Enzymes, as we noted earlier, are critical partici- competition. But for inactive people—and this pants in metabolic reactions. They are like relay includes a large portion of Americans—carbohy- runners in a race, in this case a race along the drates simply are stored as fat. metabolic pathways whereby nutrients are Experts do not even agree on the answer to a turned into energy or new bodily material. question much simpler than “How much is too Therefore, if an enzyme is missing or does not much fat in the diet?”—the question “How much function as it should, it can create a serious meta- is too much fat on the body?” Some doctors clas- bolic disorder. An example is phenylketonuria sify a person as obese whose weight is at least (PKU), caused by the lack of an enzyme known 20% more than the recommended weight for his as phenylalanine hydroxylase. This enzyme is or her height, but others say that standard responsible for converting the amino acid pheny- height-and-weight charts are misleading. After lalanine to a second amino acid, tyrosine; when all, muscle weighs more than fat, and it is con- this does not happen, phenylalanine builds up in ceivable that a very muscular athlete with very the body. It is converted to a compound called little body fat might qualify as “overweight” com- phenylpyruvate, which impairs normal brain pared with the recommended weight for his or development, resulting in severe mental retarda- her height. tion. BODY FAT, THE SEXES, AND Other examples of metabolic disorders NATURE. Because of the complexity of the include alkaptonuria, thalassemia, porphyria, issue, many experts contend that the proportion Tay-Sachs disease, Hurler syndrome, Gaucher of fat to muscle, measured by the skinfold disease, galactosemia, Cushing syndrome, dia-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 37 set_vol3_2 8/19/02 10:35 AM Page 38
Metabolism The word anorexia comes from the Greek for “lack of appetite,” but the problem for people with anorexia is not that they are not hungry. On the contrary, they are starving, but unlike poor people in the Third World, they are not starving as the result of a shortage of food but because they are denying themselves nutrition. They do this because they fear gaining weight, even when they are so severely underweight that they look like skeletons. The name of a related condition, bulimia, literally means “hungry as an ox.” People with this problem go on eating binges, often gorging on junk food. Then they force their bodies to get rid of the food, either by vomiting or by taking large amounts of laxatives. A third type of eating disorder, obesity, also is characterized by uncon- trollable overeating, but in this case the person does not force the body to eject the food that has been consumed. That, at least, makes obesity more healthy than bulimia, but there is nothing DIANA, THE LATE PRINCESS OF WALES, SUFFERED healthy about accumulating vast amounts of FROM BULIMIA, A DISORDER THAT PROMPTS A PERSON TO GO ON EATING BINGES AND THEN “PURGE,” EITHER body fat, as severely obese people do. BY VOMITING OR BY TAKING LARGE AMOUNTS OF LAXA- ANOREXIA AND BULIMIA. . TIVES (© Corbis. Reproduced by permission.) Young people are more likely than older people to suffer anorexia or bulimia, conditions that betes mellitus, hyperthyroidism, and hypothy- typically become apparent at about the age of 20 roidism. Most of these conditions affect a small years. Although both men and women can expe- population; however, diabetes mellitus (dis- rience the problem, in fact, only about 5% of cussed in Noninfectious Diseases) is one of the people with these eating disorders are male. And leading killers in America. At present, no cures though anorexia and bulimia are closely relat- for metabolic disorders exist. The best approach ed—particularly inasmuch as they are psycho- is to diagnose such conditions as early as possible logical in origin but can exact a heavy biological and then arrange a person’s diet to deal as effec- toll—there are several important differences. tively as possible with that disorder. People who have anorexia or bulemia often come from families with overprotective parents Eating Disorders who have unrealistically high expectations of their children. Frequently, high expectations go Eating disorders are a different matter, because hand in hand with a wealthy background, and they are psychological rather than physiological certainly anorexia and bulimia are not condi- conditions. No one is sure what causes eating dis- tions that typically affect the poor. Anorexia and orders, but researchers think that family dynam- bulimia often seem to develop after some stress- ics, biochemical abnormalities, and modern ful experience, such as moving to a new town, American society’s preoccupation with thinness changing schools, or going through puberty. Low all may contribute. Eating disorders are virtually self-esteem, fear of losing control, and fear of unknown in parts of the world where food is growing up are common characteristics of peo- scarce, but in wealthy lands, such as the United ple with these conditions. Their need for States, problems of overeating, self-induced star- approval manifests in a quest to meet or exceed vation, or forced purging have gained consider- our culture’s idealized concept of extreme thin- able attention. ness. This quest is a part of our popular culture, Anorexia nervosa, bulimia, and obesity are promoted by waiflike models whose sunken eyes the most well known types of eating disorder. stare out of fashion magazines.
38 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 39
Like anorexia, bulimia results in starvation, The self-imposed starvation of people with Metabolism but there are behavioral, physical, and psycholog- anorexia likewise takes a heavy toll on the body. ical differences between the two. Bulimia is both The skin becomes dry and flaky, muscles begin to less and more dangerous: on the one hand, peo- waste away, bones stop growing and may become ple who have it tend to be of normal weight or brittle, and the heart weakens. Seeking to protect are overweight, and unlike those with anorexia, itself in the absence of proper insulation from fat, they are aware of the fact that they have a prob- the body sprouts downy hair on the face, back, lem. On the other hand, because the effects of and arms in response to lower body temperature. their behavior are not so readily apparent, it is In women, menstruation stops, and permanent easier for a person with bulimia to persist in the infertility may result. Muscle cramps, dizziness, pattern of bingeing and purging for much fatigue, and even brain damage as well as kidney longer. and heart failure are possible. An estimated 10% to 20% of people with anorexia die either as a Approximately one in five persons with direct result of starvation or by suicide. bulimia has a problem with drug or alcohol use, and they pursue their binges in a way not unlike To save people with anorexia, force-feeding that of a guilty addict or alcoholic hiding the may be necessary. Some 70% of anorexia patients who are treated for about six months return to spent needles or empty bottles from family normal body weight, but about 15-20% can be members. They may go from restaurant to expected to relapse. Bulimia is not as likely as restaurant to avoid being seen eating too much in anorexia to reach life-threatening stages, so hos- any one place, or they may pretend to be shop- pitalization typically is not necessary. Treatment ping for a large dinner party when, in fact, they generally calls for psychotherapy and sometimes intend to eat all the food themselves. Because of the administration of antidepressant drugs. the expense of consuming so much food, some Unlike people with anorexia, those with bulimia resort to shoplifting. usually admit they have a problem and want help During a binge, people suffering from overcoming it. bulimia favor high-carbohydrate foods, such as OBESITY. Unlike anorexia or bulimia, doughnuts, candy, ice cream, soft drinks, cookies, obesity is more of a problem among people from cereal, cake, popcorn, and bread, and they con- lower-income backgrounds. This probably sume many times the number of calories they relates to a lack of education concerning nutri- would normally consume in one day. No matter tion, combined with the fact that healthier food what their normal eating habits, they tend to eat is more expensive; by contrast, unhealthy items, quickly and messily during a binge, stuffing the such as white sugar, corn meal, and fatty cuts of food into their mouths and gulping it down, pork and other meats can fill or overfill a person’s sometimes without even tasting it. Some say they stomach inexpensively. In addition, though men get a feeling of euphoria during binges, similar to and women both tend to gain weight as they age, the “runner’s high” that some people get from women are almost twice as likely as men to be exercise. Then, when they have gorged them- obese. selves, they force the food back out, either by Some cases of obesity relate to metabolic causing themselves to vomit or by taking large problems, while others stem from compulsive quantities of laxatives. eating, which is psychologically motivated. Some Regular self-induced vomiting can cause all studies suggest that obese people are much more sorts of physical problems, such as damage to the likely than others to eat in response to stress, stomach and esophagus, chronic heartburn, loneliness, or depression. And just as emotional burst blood vessels in the eyes, throat irritation, pain can lead to obesity, obesity can lead to psy- and erosion of tooth enamel from the acid in chological scars. From childhood on, obese peo- vomit. Excessive use of laxatives can induce mus- ple are taunted and shunned, and throughout life cle cramps, stomach pains, digestive problems, they may face discrimination in school and on dehydration, and even poisoning, while bulimia, the job. in general, brings about vitamin deficiencies and Physically, obesity is a killer, especially for imbalances of critical body fluids, which, in turn, those who are morbidly obese—that is, people can lead to seizures and kidney failure. whose obesity endangers their health. Obesity is
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 39 set_vol3_2 8/19/02 10:35 AM Page 40
Metabolism
BEARS, WHICH WE THINK OF AS THE CLASSIC HIBERNATING ANIMAL, ARE ACTUALLY JUST DEEP SLEEPERS. A HIBER- NATING ANIMAL SHOWS A DRASTIC REDUCTION IN METABOLISM AND THEN AWAKES RELATIVELY SLOWLY, WHEREAS A SLEEPING ANIMAL DECREASES ITS METABOLISM ONLY SLIGHTLY AND CAN WAKE UP ALMOST INSTANTLY IF DISTURBED. (© Dan Guravich/Corbis. Reproduced by permission.)
a risk factor for diabetes, high blood pressure, nonhibernating animal would be like a business arteriosclerosis, angina pectoralis (chest pains operating at a loss—that is, using more energy due to inadequate blood flow to the heart), vari- maintaining its body temperature and searching cose veins, cirrhosis of the liver, and kidney dis- for food than it would receive from consuming ease. Obese people are about 1.5 times more like- the food. Hibernating animals use 70-100 times ly to have heart attacks than are other people, and less energy than when they are active, allowing the overall death rate among people ages 20-64 is them to survive until food is once again plentiful. 50% higher for the obese than for people of ordi- CONTRAST WITH SLEEP. Many nary weight. animals sleep more often when food is scarce, but Hibernation only a few truly hibernate. Bears, which many people think of as the classic hibernating animal, Having looked at several unnatural ways in are actually just deep sleepers. By contrast, true which people alter their metabolisms, let us close hibernation occurs only in small mammals, such with an example of a very natural way that ani- as bats and woodchucks and a few birds, among mals sometimes temporarily change theirs. This them nighthawks. Some insects also practice a is hibernation, a state of inactivity in which an form of hibernation. Hibernation differs from animal’s heart rate, body temperature, and sleep, in that a hibernating animal shows a dras- breathing rate are decreased as a way to conserve tic reduction in metabolism and then awakes rel- energy through the cold months of winter. A atively slowly, whereas a sleeping animal decreas- similar state, known as estivation, is adopted by es its metabolism only slightly and can wake up some desert animals during the dry months of almost instantly if disturbed. Also, hibernating summer. animals do not show periods of rapid eye move- Hibernation is a technique that animals have ment (REM), the stage of sleep associated with developed, as a result of natural selection over dreaming in humans. the generations (see Evolution), to adapt to harsh THE PROCESS OF HIBERNA- environmental conditions. When food is scarce, a TION. Animals prepare for hibernation in the
40 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 41
Metabolism KEY TERMS
ADIPOSE: Of or relating to animal fat. pounds as a means of making the nutrients AMINO ACIDS: Organic compounds absorbable by the body or organism. made of carbon, hydrogen, oxygen, nitro- ENZYME: A protein material that gen, and (in some cases) sulfur bonded in speeds up chemical reactions in the bodies characteristic formations. Strings of amino of plants and animals without itself taking acids make up proteins. part in, or being consumed by, these reac- ANABOLISM: The metabolic process tions. by which energy is used to build up com- GLUCOSE: A monosaccharide (sugar) plex molecules that the body needs to that occurs widely in nature and which is maintain itself and develop new material. the form in which animals usually receive ATOM: The smallest particle of an ele- carbohydrates. Also known as dextrose, ment, consisting of protons, neutrons, and grape sugar, and corn sugar. See also blood electrons. An atom can exist either alone or in sugar. combination with other atoms in a molecule. HYDROCARBON: Any organic chemi- ATP: Adenosine triphosphate, an ener- cal compound whose molecules are made gy carrier formed when a simpler com- up of nothing but carbon and hydrogen pound, adenosine diphosphate (ADP), atoms. combines with a phosphate group. LIPIDS: Fats and oils, which dissolve in BLOOD SUGAR: The glucose in the oily or fatty substances but not in water- blood. based liquids. In the body, lipids supply CARBOHYDRATES: Naturally occur- energy in slow-release doses, protect ring compounds, consisting of carbon, organs from shock and damage, and pro- hydrogen, and oxygen, whose primary vide insulation for the body, for instance function in the body is to supply energy. from toxins. Included in the carbohydrate group are METABOLIC PATHWAY: An orderly sugars, starches, cellulose, and various sequence of reactions, with particular other substances. Most carbohydrates are enzymes acting at each step along the way. produced by green plants in the process of Metabolic pathways may be either linear or undergoing photosynthesis. circular, and sometimes they are linked, CATABOLISM: The metabolic process meaning that the product of one pathway by which large molecules are broken down becomes a reactant in another. into smaller ones with the release of ener- METABOLIC POOL: A group of rela- gy. Compare with anabolism. tively simple substances (e.g., amino acids) COMPOUND: A substance in which formed by the breakdown of relatively atoms of more than one element are bond- complex nutrients. ed chemically to one another. METABOLISM: The chemical process DIGESTION: The process of breaking by which nutrients are broken down and food down into simpler chemical com- converted into energy or are used in the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 41 set_vol3_2 8/19/02 10:35 AM Page 42
Metabolism KEY TERMS CONTINUED
construction of new tissue or other materi- phate, or a chemical compound that con- al in the body. All metabolic reactions are tains oxygen bonded to phosphorus. either catabolic or anabolic. PRODUCT: A substance or substances MOLECULE: A group of atoms, usual- formed from the interaction of reactants in ly but not always representing more than a chemical reaction. one element, joined in a structure. Com- PROTEINS: Large molecules built pounds typically are made up of mole- from long chains of 50 or more amino cules. acids. Proteins serve the functions of pro- NUTRIENT: Materials essential to the moting normal growth, repairing damaged survival of organisms. They include pro- tissue, contributing to the body’s immune teins, carbohydrates, lipids (fats), vitamins, system, and making enzymes. and minerals. REACTANT: A substance that interacts NUTRITION: The series of processes by with another substance in a chemical reac- which an organism takes in nutrients and tion, resulting in the formation of a chem- makes use of them for its survival, growth, ical or chemicals known as the product(s). and development. The term nutrition also can refer to the study of nutrients, their SUGARS: One of the three principal consumption, and their use in the organ- types of carbohydrate, along with starches ism’s body. and cellulose. Sugars can be defined as any ORGANIC: At one time chemists used of various water-soluble carbohydrates of the term organic only in reference to living varying sweetness. What we think of as things. Now the word is applied to com- “sugar” (i.e., table sugar) is actually pounds containing carbon and hydrogen. sucrose; “blood sugar,” on the other hand, is glucose. PHOSPHATE GROUP: A group (that is, a combination of atoms from two or TISSUE: A group of cells, along with more elements that tend to bond with the substances that join them, that forms other elements or compounds in certain part of the structural materials in plants or characteristic ways) that includes a phos- animals.
fall by storing food; usually this storage is inter- the digestive tract is emptied completely before nal, in the form of fat reserves. A woodchuck in hibernation begins. early summer may have only about 5% body fat, Going into hibernation is a gradual process. but as fall approaches, changes in the animal’s Over a period of days, an animal’s heart rate and brain chemistry cause it to feel hungry and to eat breathing rate drop slowly, eventually reaching constantly. As a result, the woodchuck’s body fat rates of just a few beats or breaths per minute. increases to about 15% of its total weight. In Their body temperatures also drop from levels of other animals, such as the dormouse, fat may about 100°F (38°C) to about 60°F (15°C). The constitute as much as 50% of the animal’s weight lowered body temperature makes fewer demands by the time hibernation begins. A short period of on metabolism and food stores. Electric activity fasting follows the feeding frenzy, to ensure that in the brain ceases almost completely during
42 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 43
hibernation, although some areas—those that Center, Institute for Chemical Research, Kyoto Uni- Metabolism respond to external stimuli, such as light, tem- versity (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 43 set_vol3_2 8/19/02 10:35 AM Page 44
DIGESTIONDigestion
CONCEPT organic compounds made of carbon, hydrogen, Digestion is the process whereby the foods we eat oxygen, nitrogen, and (in some cases) sulfur- pass through our bodies and are directed toward bonded in characteristic formations. Proteins the purposes of either providing the body with serve the functions of promoting normal growth, energy or building new cellular material, such as repairing damaged tissue, contributing to the fat or muscle. The parts of food that the body body’s immune system, and making enzymes. cannot use, along with other wastes from the (An enzyme is a protein material that speeds up body, are eliminated in the form of excrement. chemical reactions in the bodies of plants and Aspects of digestion, particularly the production animals.) Good examples of dietary proteins of waste and intestinal gas, are not exactly topics include eggs, milk, cheese, and other dairy prod- for polite conversation, yet without these and ucts. Incomplete proteins, or ones lacking essen- tial amino acids—those amino acids that are not other digestive processes, life for humans and produced by the human body—include peas, other organisms would be impossible. The func- beans, lentils, nuts, and cereal grains. tioning of digestion itself is like that of a well- organized, cohesive sports team or even of a sym- Carbohydrates are compounds that consist phony orchestra: there are many parts and play- of carbon, hydrogen, and oxygen. Their primary ers, each with an indispensable role. function in the body is to supply energy. When a person ingests more carbohydrates than his or her body needs at the moment, the body converts HOW IT WORKS the excess into a compound known as glycogen. It then stores the glycogen in the liver and mus- Nutrients cle tissues, where it remains, a potential source of energy for the body to use in the future, though For digestion to occur, of course, it is necessary if it is not used soon, it may be stored as fat. The first to have something to digest—namely, nutri- carbohydrate group comprises sugars, starches, ents. What follows is a cursory overview of nutri- cellulose (a type of fiber), and various other ents and nutrition, subjects covered in much chemically related substances. more depth within the essay of that name. Nutri- LIPIDS, VITAMINS, AND MIN- ents include proteins, carbohydrates, fats, miner- ERALS. Lipids include all fats and oils and are als, and vitamins. In addition to these nutrients, distinguished by the fact that they are soluble animal life requires other materials, not usually (i.e., capable of being dissolved) in oily or fatty considered nutrients, which include water, oxy- substances but not in water. In the body, lipids gen, and something that greatly aids the process supply energy much as carbohydrates do, only of food digestion and elimination of wastes: much more slowly. Lipids also protect the organs fiber. from shock and damage and provide the body PROTEINS AND CARBOHY- with insulation from cold, toxins, and other DRATES. Proteins are large molecules built threats. Processed, saturated fats (fats that have from long chains of amino acids, which are been enhanced artificially to make them more
44 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 45
firm) are extremely unhealthy, and consumption for example, glandular cells in the lining of the Digestion of some types of animal fat (e.g., pork fat) is also stomach that make the hormone gastrin. inadvisable. On the other hand, vegetable fats, FROM THE MOUTH TO THE such as those in avocados and olive oil, as well as STOMACH. The first stage of digestion is the animal fats in such fish as tuna, mackerel, and ingestion, in which food is taken into the mouth salmon can be highly beneficial. and then broken down into smaller pieces by the Vitamins are organic substances that, in chewing action of the teeth. To facilitate move- extremely small quantities, are essential to the ment of the food through the mouth and along nutrition of most animals and some plants. In the tongue, it is necessary for saliva to be present. particular, they work with enzymes in regulating Usually, the sensations of sight, taste, and smell metabolic processes—that is, the chemical associated with food set in motion a series of processes by which nutrients are broken down neural responses that induce the formation of and converted into energy or used in the con- saliva by the salivary glands in the mouth. Amy- struction of new tissue or other material in the lase, an enzyme in the saliva, begins the process body. Vitamins do not in themselves provide of breaking complex carbohydrates into simple energy, however, and thus they do not qualify as sugars. (The terms simple and complex in this a form of nutrition. Much the same is true of context refer to chemical structures.) minerals, except that these are inorganic sub- By the time it is ready to be swallowed, food stances, meaning that they do not contain chem- is in the form of a soft mass known as a bolus. ical compounds made of carbon and hydrogen. The action of swallowing pulls the food down through the pharynx, or throat, and into the The Digestive System esophagus, a tube that extends from the bottom of the throat to the top of the stomach. (Note To supply the body with the materials it needs for that for the most part, we are using human energy and the building of new tissue, nutrients anatomy as a guide, but many aspects of the have to pass through the digestive system. The digestive process described here also apply to latter is composed of organs (an organ being a other higher animals, particularly mammals.) group of tissues and cells, organized into a par- The esophagus does not take part in digestion ticular structure, that performs a specific func- but rather performs the function of moving the tion within an organism) and other structures bolus into the stomach. through which nutrients move. The nutrients A wavelike muscular motion termed peri- pass first through the mouth and then through stalsis, which consists of alternating contractions the esophagus, stomach, small intestine, and and relaxations of the smooth muscles lining the large intestine, or colon. Collectively, these struc- esophagus, moves the bolus through this passage. tures are known as the alimentary canal. At the place where the esophagus meets the Nutrients advance through the alimentary stomach, a powerful muscle called the esophageal canal to the stomach and small intestine, and sphincter acts as a valve to keep food and stomach waste materials continue from the small intestine acids from flowing back into the esophagus and to the colon (large intestine) and anus. Along the mouth. (Although the most well-known sphinc- way, several glands play a role. A gland is a cell or ter muscle in the body is the one surrounding the group of cells that filters material from the blood, anus, sometimes known simply as “the sphinc- processes that material, and secretes it either for ter,” in fact, sphincter is a general term for a mus- use again in the body or to be eliminated as cle that surrounds, and is able to control the size waste. Among the glands that play a part in the of, a bodily opening.) digestive process are the salivary glands, liver, FROM THE STOMACH TO THE gallbladder, and pancreas. (The last three are SMALL INTESTINE. Chemical digestion examples of glands that are also organs.) The begins in the stomach, a large, hollow, pouchlike glands with a role in digestion secrete digestive muscular organ. While food is still in the mouth, juices containing enzymes that break down the stomach begins its production of gastric nutrients chemically into smaller molecules that juice, which contains hydrochloric acid and are absorbed more easily by the body. There are pepsin, an enzyme that digests protein. Gastric also hormones involved in digestion-there are, juice is the material that breaks down the food.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 45 set_vol3_2 8/19/02 10:35 AM Page 46
Digestion
A SIXTEENTH-CENTURY ANATOMICAL DIAGRAM OF THE INTERNAL ORGANS, SHOWING THE STOMACH, LIVER, INTESTINE, AND GALLBLADDER. (© Corbis. Reproduced by permission.)
Once nerves in the cheeks and tongue are stimu- Once the bolus touches the stomach lining, it lated by the food, they send messages to the triggers a second release of gastric juice, along brain, which, in turn, alerts nerves in the stomach with mucus that helps protect the stomach lining wall, stimulating the secretion of gastric juice from the action of the hydrochloric acid. Three before the bolus itself arrives in the stomach. layers of powerful stomach muscles churn food
46 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 47
into a thick liquid called chyme, which is pumped energy and growth. Plasma also contains waste Digestion gradually through the pyloric sphincter, which products from the breakdown of proteins, connects the stomach small intestine. including creatinine, uric acid, and ammonium THE SMALL INTESTINE. The salts. These constituents are moved to the kid- names of the small and large intestines can be neys, where they are filtered from the blood and confusing, rather like those of Upper and Lower excreted in the urine. But, of course, urine is not Egypt in ancient history. In both cases, the adjec- the only waste product excreted by the body; tives seem to refer to one thing but actually refer there is also the solid waste, processed through to something else entirely. Thus, it so happens the large intestine, or colon. that Upper Egypt was south of Lower Egypt THE LARGE INTESTINE AND (because it was “upper” in elevation, not lati- BEYOND. Like the small intestine, the large tude), while the small intestine is, in fact, much intestine is in segments. It rises up on the right longer than the large intestine. The reason is that side of the body (the ascending colon), crosses small refers to its diameter rather than its length: over to the other side underneath the stomach though it is about 23 ft. (7 m) long, the small (the transverse colon), descends on the left side, intestine is only 1 in. (2.5 cm) in diameter, while (the descending colon), and forms an S shape the large intestine, only 5 ft. (1.5 m) in length, is (the sigmoid colon) before reaching the rectum 3 in. (7.6 cm) across. and anus. In addition to its function of pumping The small intestine, which connects the solid waste, the large intestine removes water stomach and large intestine, is in three sections: from the waste products—water that, when puri- the duodenum, jejunum, and ileum. About 1 ft. fied, will be returned to the bloodstream. In (0.3 m) long, the duodenum breaks down chyme addition, millions of bacteria in the large intes- from the stomach with the aid of the pancreas tine help produce certain B vitamins and vitamin and gallbladder. The pancreas, a large gland K, which are absorbed into the bloodstream located below the stomach, secretes pancreatic along with the water. juice, which contains three enzymes that break After leaving the sigmoid colon, waste pass- down carbohydrates, fats, and proteins, into the es through the muscular rectum and then the duodenum through the pancreatic duct. The anus, the last point along the alimentary canal. In gallbladder empties bile, a yellowish or greenish all, the movement of food through the entire fluid from the liver, into the duodenum when length of the alimentary tract takes from 15 to 30 chyme enters that portion of the intestine. hours, with the majority of that time being taken Although bile does not contain enzymes, it does up by activity in the colon. Food generally spends have bile salts that help dissolve fats. about three to five hours in the stomach, another Digested carbohydrates, fats, proteins, and four to five hours in the small intestine, and most of the vitamins, minerals, and iron in food between five and 25 hours in the large intestine. are absorbed in the jejunum, which is about 4 ft. The transit time, or the amount of time it (1.2 m) long. Aiding this absorption are up to takes for food to move through the system, is a five million tiny finger-like projections called function of diet: for a vegetarian who eats a great villi, which greatly increase the surface area of the deal of fiber, it will be on the short end, while for small intestine, thus accelerating the rate at a meat eater who has just consumed a dinner of which nutrients are absorbed into the blood- prime rib, it will take close to the maximum time. stream. The remainder of the small intestine is People who eat diets heavy in red meat or junk taken up by the ileum, which is smaller in diam- foods are also likely to experience a buildup, over eter and has thinner walls than the jejunum. It is time, of partially digested material on the linings the final site for absorption of some vitamins and of their intestines. Obviously, this is not a healthy other nutrients, which enter the circulatory sys- situation, and to turn it around, a person may tem in plasma, a watery liquid in which red blood have to change his or her diet and perhaps even cells also are suspended. undergo some sort of colon-cleansing program. As it moves through the circulatory system, There is an easy way to test transit time in one’s plasma takes with it amino acids, enzymes, glyc- system: simply eat a large serving of corn or red erol (a form of alcohol found in fats), and fatty beets, and measure how long it takes for these to acids, which it directs to the body’s tissues for fully work their way through the digestive system.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 47 set_vol3_2 8/19/02 10:35 AM Page 48
Digestion REAL-LIFE the stomach. Chemically, the opposite of an acid APPLICATIONS is a base, or an alkaline substance, the classic example being sodium bicarbonate or sodium Digestive Disorders hydrogen carbonate (NaHCO3)—that is, baking soda. Baking soda alone can perform the func- It is hard to watch more than a few minutes of tion of an antacid, but the taste is rather unpleas- commercial television without seeing advertise- ments for fast foods and other varieties of junk ant, and for this reason most antacid products food or stomach-relief medicine or both. There is combine it with other chemicals to enhance the a connection, of course: a society glutted on flavor. greasy drive-through burgers and thick-crust One famous commercial stomach remedy pizzas needs something to cure the upset stom- actually uses acid. This is Alka-Seltzer, but the achs that result. presence of citric acid has more to do with mar- Indigestion is a general condition that, as its keting than with the chemistry of the stomach. name suggests, involves an inability to digest The citric acid, often used as a sweetener, imparts food properly. Heartburn, sometimes called acid a more pleasant flavor than the bitter taste of indigestion, is a specific type of indigestion that alkaline antacids, and, moreover, when Alka- occurs when the stomach produces too much Seltzer tablets are placed in water, the acid reacts hydrochloric acid. The latter is essential to diges- chemically with the sodium bicarbonate to create tion, but if a person eats a giant Polish sausage, a the product’s trademark fizz. spicy-hot bowl of jambalaya, or some other hard- Ultimately, all antacids (Alka-Seltzer includ- to-digest food (as opposed to a healthy meal of ed) work because the bases in the product react baked fish with brown rice and spinach, for with the acids in the stomach. This is a chemical instance), the stomach may produce too much of process called neutralization, in which the acid the acid. and base cancel out each other, producing water Heartburn is so named because it causes a and a salt in the process. (Table salt, or sodium sharp pain behind the breastbone, which might chloride, is just one of many salts, all of which are feel like a heart attack. It also may produce acid formed by the chemical bonding of a metal with reflux, in which the stomach acid backs up into a nonmetal—in the case of table salt, sodium and the esophagus. If you have ever experienced what chlorine, respectively.) Thanks to this process, might be called a leap of vomit, in which a burp is acid in the stomach of a heartburn sufferer is associated with the rise of burning, foul-tasting neutralized. bile through the esophagus, then you have first- ULCERS. There are some digestive dis- hand knowledge of acid reflux and heartburn. orders that cannot be cured by Alka-Seltzer or its Digestive tract diseases, such as dyspepsia, many competitors, such as Rolaids, Maalox, sometimes can cause chronic indigestion, but Mylanta, Tums, Milk of Magnesia, or Pepto-Bis- more often than not, people experience indiges- mol. Instead of just occasional indigestion or tion as a result of eating too quickly or too much, heartburn, a person may be afflicted with a sore consuming high-fat foods, or eating in a stressful in one part of the digestive tract, which may be situation. (This is why you might feel sick to your either a stomach ulcer or a duodenal ulcer. Stom- stomach when eating lunch at school on the day ach ulcers, which form in the lining of the stom- of a difficult test, a fight, or a romantic trauma, ach, are called peptic ulcers because they form such as a breakup or asking for a first date.) with the help of stomach acid and pepsin. Duo- Smoking, excessive drinking, fatigue, and the denal ulcers, which are more common, tend to be consumption of medications that irritate the smaller than stomach ulcers and heal more stomach lining also can contribute to indiges- quickly. Any ulcer, whether a small sore or a deep tion. In addition, it is a good idea not to eat too cavity, leaves a scar in the alimentary canal. soon before going to bed, since this can produce Until the early 1990s, physicians generally heartburn. maintained that personal behavior and condi- COMMERCIAL ANTACIDS. Most tions, such as stress and poor diet, were the prin- nonprescription stomach-relief medicine is in cipal factors behind ulcer. Medical researchers the form of an antacid, which, as the term sug- eventually came to believe, however, that the cul- gests, is a substance that works against acids in prit was a certain bacterium, which can live
48 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 49
Digestion
SCANNING ELECTRON MICROGRAPH OF A STOMACH ULCER. INSTEAD OF JUST OCCASIONAL INDIGESTION, A PERSON MAY BE AFFLICTED WITH AN ULCER, OR A SORE IN ONE PART OF THE DIGESTIVE TRACT. STOMACH ULCERS ARE CALLED PEPTIC ULCERS BECAUSE THEY FORM WITH THE HELP OF STOMACH ACID AND PEPSIN. (Photo Researchers. Reproduced by per- mission.)
undetected in the mucous lining of the stomach. eat only when you are really hungry and to eat This bacterium irritates and weakens the lining, slowly and chew food thoroughly. Drinking liq- making it more susceptible to damage by stom- uids with a meal is probably not a good practice; ach acids. As many as 80% of all stomach ulcers it is better to wait until you are finished, so as not may be caused by such a bacterial infection. With to interfere with the action of digestive fluids. this newfound knowledge, ulcer patients today Certainly, smoking and excessive drinking have a are more likely to be treated with antibiotics and negative impact on digestion, whereas regular antacids rather than special diets or expensive exercise has a positive influence. medicines. In the category of diet, it is a good idea to Ways to Improve Digestion minimize one’s intake of red meat, such as steak. Although most people find red meat tasty, and it There are numerous ways to improve digestion, can be a good supplier of dietary iron in limited by changing either the way one eats or the things proportions, the digestion of red meat requires one eats. In the first category, it is important to the production of much more stomach acid, and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 49 set_vol3_2 8/19/02 10:35 AM Page 50
Digestion thus it places a great burden on the digestive sys- Always Wanted to Know about Sex (But Were tem. It is also wise to eliminate as many Afraid to Ask). Just as the title of David R. processed foods as possible, including sweets and Reuben’s 1969 book inspired Woody Allen’s 1972 junk foods, and to eat as many natural foods as movie, which was very loosely based on it, the one can manage. “Everything you always wanted to know ... ” In general, one can hardly go wrong with motif inspired a whole array of imitators. Often raw vegetables, which are just about the best such titles play on the very fact that hardly any- thing a person can eat—not only because of their one wants to know, and certainly no one is afraid digestive properties but also because many of to ask, about the topic in question. A good exam- them are packed so full of vitamins and minerals. ple is an on-line article by central Asia authority (Note that vegetables are best when raw and Mark Dickens entitled “Everything You Always fresh, since cooking removes many of the nutri- Wanted to Know about Tocharian But Were ents. Canned vegetables usually are both nutri- Afraid To Ask” (http://www.oxuscom.com/ ent-poor and full of sodium or even synthetic eyawtkat.htm). The joke, of course, is that most chemicals. Frozen vegetables are much better people have never heard of Tocharian, a central than canned ones, but they still do not compare Asian language. Nonetheless, one subject ranks nutritionally with fresh vegetables.) with sex as something everyone wonders about A good diet includes a great deal of fiber, but most are afraid to ask. indigestible material that simply passes through Even the name of that topic creates prob- the system, assisting in the peristaltic action of lems, since people have so many euphemisms for the alimentary canal and in the process of elimi- it: “number two,” for instance, or BM (short for nating waste. Cellulose, found in most raw fruits bowel movement). There are baby- and child- and vegetables, is an example of fiber, also called oriented terms for this process and product, the bulk or roughage. Yogurt may be a beneficial bodily control of which can be a major problem food, because it includes “good” bacteria (a topic for a very young human being, and, of course, we discuss near the conclusion of this essay) that there is at least one grown-up term for it that will assist the digestive process. Some foods, such as not be mentioned here. People even have nick- raw bean sprouts, papaya, figs, and pineapple, names for animal dung, such as pies, patties, or contain enzymes that appear to assist the body in chips. For the sake of convenience, let us call the digesting them. process defecation, and the product human In addition, one of the greatest “foods” for waste (or excrement or feces) and admit that aiding digestion is not a food at all, but water, of everyone has wondered how something as pleas- which most people drink far too little. Some ant as food can, after passing through the ali- experts claim that a person should drink eight 8- mentary canal, turn into something as unpleas- oz. (0.24 l) glasses a day, but others maintain that ant as the final product. a person should drink half as many ounces of water as his or her weight in pounds. In other The average person excretes some 7 lb. (3.2 words, a person who weighed 100 lb. (45.36 kg) kg) of feces per day, an amount equal to a little would drink 50 oz. (1.48 l) of water a day, where- more than 1 ton (0.91 tonnes) per year. This as a person who weighed 150 lb. (68.04 kg) would waste is made up primarily of indigestible mate- drink 75 oz. (2.22 l). A good rule of thumb for rials as well as water, salts, mucus, cellular debris metric users, instead of the 2:1 pounds-to- from the intestines, bacteria, and cellulose and ounces ratio, would be 30:1 kilograms to liters. other types of fiber. Like the human body itself, Note also that tap water may contain chemicals these waste products are mostly water: about or other impurities, and therefore consuming it 75%, compared with 25% solid matter. Much of in large quantities is not advisable. A much better what goes into producing excrement has nothing alternative is bottled or filtered water. to do with what enters the digestive system, so even if a person were starving he or she would Human Waste continue to excrete feces. Many years ago, both a serious book and a COLORATION. What about the color comedic movie had the title Everything You and the smell? The color of feces comes from
50 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 51
Digestion
ESCHERICHIA COLI is a type of bacteria that lives in the intestinal tract, aiding the digestive process by suppressing the growth of harmful bacteria and synthesizing vitamins. (© 1997 Cus- tom Medical Stock Photo. Reproduced by permission.)
bilirubin, a reddish-yellow pigment found in ing bleeding in the gastrointestinal tract (the blood and bile, which passes through the liver stomach and intestines) may produce waste the and enters the small intestine via the gallbladder. color of black tar. In addition, foods with distinc- Later, as it passes through the large intestine, it is tive colors and textures also can affect the degraded by the action of bacteria, a process that appearance of stools. turns it brown and gives feces its characteristic FRIENDLY BACTERIA. color. Before addressing the smell of feces, which is the result Not surprisingly, disorders involving the red of action by bacteria in the colon, it is worth say- blood cells, liver, or gallbladder can change the color of human waste. A person with gallstones ing a few words about those single-cell organ- or hepatitis (a disease characterized by inflam- isms themselves. This is especially important in mation of the liver) is likely to excrete grayish- light of the fact that bacteria have a bad reputa- brown feces, while anemia (a condition that tion that is not entirely deserved. Without ques- involves a lack of red blood cells) may be associ- tion, there are harmful microbes in the world, ated with a yellowish stool. A person experienc- but a world completely free of these organisms
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 51 set_vol3_2 8/19/02 10:35 AM Page 52
Digestion KEY TERMS
ALIMENTARY CANAL: The entire COMPOUND: A substance in which length of tube that extends from the mouth atoms of more than one element are bond- to the anus, including the esophagus, stom- ed chemically to one another. ach, and small and large intestines. Nutrients ENZYME: A protein material that pass through the alimentary canal to the speeds up chemical reactions in the bodies stomach and small intestine, and waste mate- of plants and animals. rials from these nutrients (and from other sites in the body) pass from the small intes- FIBER: Indigestible material in food tine to the colon (large intestine) and anus. that simply passes through the digestive AMINO ACIDS: Organic compounds system, assisting in the peristaltic action of made of carbon, hydrogen, oxygen, nitro- the alimentary canal and in the processing gen, and (in some cases) sulfur bonded in of waste. Examples of fiber, also called bulk characteristic formations. Strings of amino or roughage, include cellulose. acids make up proteins. GASTROINTESTINAL TRACT: The BILE: A yellowish or greenish digestive stomach and intestines. fluid excreted by the liver. GLAND: A cell or group of cells that fil- BOLUS: A term for a chewed mass of ters material from the blood, processes that food making its way through the initial material, and secretes it either for use again portions of the alimentary canal. in the body or to be eliminated as waste. CARBOHYDRATES: Naturally occur- GLUCOSE: A type of sugar that occurs ring compounds, consisting of carbon, widely in nature. Glucose is the form in hydrogen, and oxygen, whose primary which animals usually receive carbohy- function in the body is to supply energy. drates. Included in the carbohydrate group are GLYCOGEN: sugars, starches, cellulose, and various A white polysaccharide other substances. that is the most common form in which carbohydrates are stored in animal tissues, CELLULOSE: A polysaccharide that is particularly muscle and liver tissues. the principal material in the cell walls of plants. Cellulose also is found in such natural GUT: A term that refers to all or part of fibers as cotton and is used as a raw material the alimentary canal. Although the word is in manufacturing such products as paper. considered a bit crude in everyday life, COLON: The large intestine, through physicians and biological scientists con- which waste materials pass on their way to cerned with this part of the anatomy use it excretion through the anus. regularly.
would be one in which humans and other ani- in our alimentary canals, particularly in the mals would be unable to live. In fact, we have a colon. mutually beneficial relationship, a type of sym- Bacteria live in the guts—a term that refers to biosis (see Symbiosis) with the microorganisms all or part of the alimentary canal—of most ani-
52 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 53
Digestion KEY TERMS CONTINUED
HEMOGLOBIN: An iron-containing POLYSACCHARIDE: A complex pigment in red blood cells that is responsi- sugar, in which the molecules are com- ble for transporting oxygen to the tissues posed of many glucose subunits arranged and removing carbon dioxide from them. in a chain. Polysaccharides can be broken LIPIDS: Fats and oils, which dissolve in down chemically to produce simple sugars, oily or fatty substances but not in water- or monosaccharides. based liquids. In the body, lipids supply PROTEINS: Large molecules built energy in slow-release doses, protect from long chains of amino acids. Proteins organs from shock and damage, and pro- serve the functions of promoting normal vide insulation for the body, for instance, growth, repairing damaged tissue, con- from toxins. tributing to the body’s immune system, METABOLISM: The chemical process and making enzymes. by which nutrients are broken down and SPHINCTER: A general term for a converted into energy or used in the con- muscle that surrounds and is able to con- struction of new tissue or other material in trol the size of a bodily opening. the body. SYMBIOSIS: A biological relationship MINERALS: Inorganic substances that, in which (usually) two species live in close in a nutritional context, serve a function proximity to each other and interact regu- similar to that of vitamins. Minerals may larly in such a way as to benefit one or both include chemical elements, particularly metallic ones, such as calcium or iron, as of the organisms. well as some compounds. TISSUE: A group of cells, along with ORGAN: A group of tissues and cells, the substances that join them, that forms organized into a particular structure, that part of the structural materials in plants or performs a specific function within an animals. organism. VITAMINS: Organic substances that, in ORGANIC: At one time, chemists used extremely small quantities, are essential to the term organic only in reference to living the nutrition of most animals and some things. Now the word is applied to com- plants. In particular, vitamins work with pounds containing carbon and hydrogen. enzymes in regulating metabolic processes; PERISTALSIS: A series of involuntary they do not in themselves provide energy, muscle contractions that force bolus, and however, and thus vitamins alone do not later waste, through the alimentary canal. qualify as a form of nutrition.
mals, where they assist in such difficult digestive nants) have stomachs with several compartments. activities as the processing of chewed grasses. The The first of these compartments is called the latter is heavy in cellulose, and to digest it, cows, rumen, and it serves as home to millions of bacte- sheep, deer, and other grass eaters (known as rumi- ria, which assist in breaking down the heavy fibers.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 53 set_vol3_2 8/19/02 10:35 AM Page 54
Digestion Humans’ bacterial symbiotic partners (actu- Hydrogen sulfide is just one of many ally, this is a type of symbiosis known as mutual- unpleasant-smelling chemical products that ism, in which both creatures benefit) include bac- result from bacterial action on solids in the gut. teria of the species Escherichia coli, or E. coli. The Others include indole, skatole, ammonia, and name is no doubt familiar to most readers from mercaptans, though the most distinctive- its appearance in the news in connection with smelling of all are indole and skatole, which horror stories involving E. coli poisoning in food come primarily from the digestion of an amino or local water supplies. Certainly, E. coli can be acid known as tryptophan. In addition, the par- extremely harmful when it is outside the human ticular foods a person eats, as well as the specific gut, but inside the gut it is humans’ friend. bacterial residents (some harmful) of his or her E. coli is a coprophile (literally, “excrement gut, can affect the odor of intestinal gas. lover”), meaning that it depends on feces for sur- When gases pass outside the rectum, the vival. Fecal matter itself can contain all manner result is flatulence (of course, there are other, less of harmful substances associated with the polite words for it), which is the subject of much decomposition of foods or with the body’s efforts schoolboy humor. Even inside the body, intestin- to rid itself of toxins (including pathogens, or al gas can make noise and cause embarrassment, disease-carrying parasites—see Parasites and in the form of borborygmus—intestinal rum- Parasitology), so anything associated with feces is bling caused by moving gas. As for the flamma- dirty and potentially dangerous. It is for this rea- bility of intestinal gas, it probably results from son that E. coli can cause serious illness or death the high proportion of hydrogen, an extremely if it gets into other parts of the body. flammable gas. As long as it stays where it belongs, however, E. coli not only aids in the digestive process but WHERE TO LEARN MORE also provides the body with vitamin K, essential Avraham, Regina. The Digestive System. New York: for proper blood clotting, as well as vitamin B12, Chelsea House Publishers, 1989. thiamine, and riboflavin. Every person carries Ballard, Carol. The Stomach and Digestive System. Austin, millions and millions of these helpful fellow trav- TX: Raintree Steck-Vaughn, 1997. elers; even though a single bacterium weighs Digestive System Diseases. Karolinska Institutet (Web almost nothing in human terms, the combined site).
54 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 55
Respiration RESPIRATION
CONCEPT cells with oxygen and receives carbon dioxide to Respiration is much more than just breathing; in transfer to the environment. fact, the term refers to two separate processes, This is just one meaning—albeit a more only one of which is the intake and outflow of familiar one—of the word respiration. Respira- breath. At least cellular respiration, the process by tion also can mean cellular respiration, a series of which organisms convert food into chemical chemical reactions within cells whereby food is energy, requires oxygen; on the other hand, some “burned” in the presence of oxygen and convert- forms of respiration are anaerobic, meaning that ed into carbon dioxide and water. This type of they require no oxygen. Such is the case, for respiration is the reverse of photosynthesis, the instance, with some bacteria, such as those that process by which plants convert dioxide and convert ethyl alcohol to vinegar. Likewise, an water, with the aid of solar energy, into complex anaerobic process can take place in human mus- organic compounds known as carbohydrates. cle tissue, producing lactic acid—something so (For more about carbohydrates and photosyn- painful that it feels as though vinegar itself were thesis, see Carbohydrates.) being poured on an open sore. How Gases Move Through the Body HOW IT WORKS Later in this essay, we discuss some of the ways in Forms of Respiration which various life-forms breathe, but suffice it to say for the moment—hardly a surprising revela- Respiration can be defined as the process by tion!—that the human lungs and respiratory sys- which an organism takes in oxygen and releases tem are among the more complex mechanisms carbon dioxide, one in which the circulating for breathing in the animal world. In humans medium of the organism (e.g., the blood) comes and other animals with relatively complex into contact with air or dissolved gases. Either breathing mechanisms (i.e., lungs or gills), oxy- way, this means more or less the same thing as gen passes through the breathing apparatus, is breathing. In some cases, this meaning of the absorbed by the bloodstream, and then is con- term is extended to the transfer of oxygen from verted into an unstable chemical compound (i.e., the lungs to the bloodstream and, eventually, into one that is broken down easily) and carried to cells or the release of carbon dioxide from cells cells. When the compound reaches a cell, it is into the bloodstream and thence to the lungs, broken down and releases its oxygen, which pass- from whence it is expelled to the environment. es into the cell. Sometimes a distinction is made between exter- On the “return trip”—that is, the reverse nal respiration, or an exchange of gases with the process, which we experience as exhalation— external environment, and internal respiration, cells release carbon dioxide into the bloodstream, an exchange of gases between the body’s cells and where it is used to form another unstable chemi- the blood, in which the blood itself “bathes” the cal compound. This compound is carried by the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 55 set_vol3_2 8/19/02 10:35 AM Page 56
Respiration bloodstream back to the gills or lungs, and, at the Glucose is a simple sugar produced in cells end of the journey, it breaks down and releases by the breakdown of more complex carbohy- the carbon dioxide to the surrounding environ- drates, including starch, cellulose, and such com- ment. Clearly, the one process is a mirror image plex sugars as sucrose (cane or beet sugar) and of the other, with the principal difference being fructose (fruit sugar). In cellular respiration, an the fact that oxygen is the key chemical compo- organism oxidizes glucose (i.e., combines it with nent in the intake process, while carbon dioxide oxygen) so as to form the energy-rich compound plays the same role in the process of outflow. known as adenosine triphosphate (ATP). ATP, HEMOGLOBIN AND OTHER critical to metabolism (the breakdown of nutri- ents to provide energy or form new material), is COMPOUNDS. In humans the compound the compound used by cells to carry out most of used to transport oxygen is known by the name their ordinary functions. Among those functions hemoglobin. Hemoglobin is an iron-containing are the production of new cell parts and chemi- protein in red blood cells that is responsible for cals, the movement of compounds through cells transporting oxygen to the tissues and removing and the body as a whole, and growth. carbon dioxide from them. In the lungs, hemo- In cellular respiration, six molecules of glu- globin, known for its deep red color, reacts with cose (C H O ) react with six molecules of oxy- oxygen to form oxyhemoglobin. Oxyhemoglobin 6 12 6 gen (O ) to form six molecules of carbon dioxide travels through the bloodstream to cells, where it 2 (CO ), six molecules of water (H O), and 36 breaks down to form hemoglobin and oxygen, 2 2 molecules of ATP. This can be represented by the and the oxygen then passes into cells. On the following chemical equation: return trip, hemoglobin combines with carbon 4 dioxide to form carbaminohemoglobin, an 6C6H12O6 + 6 O2 6 CO2 + 6 H2O + 36 ATP unstable compound that, once again, breaks The process is much more complicated than down—only this time it is carbon dioxide that it this equation makes it appear: some two dozen releases, in this case to the surrounding environ- separate chemical reactions are involved in the ment rather than to the cells. overall conversion of glucose to carbon dioxide, water, and ATP. In other species, compounds other than hemoglobin perform a similar function. For The Mechanics of Breathing example, some types of annelids, or segmented worms, carry a green blood protein called All animals have some mechanism for removing chlorocruorin that functions in the same way as oxygen from the air and transmitting it into the hemoglobin does in humans. And whereas bloodstream, and this same mechanism typically hemoglobin is a molecule with an iron atom at is used to expel carbon dioxide from the blood- the center, the blood of lobsters and other large stream into the surrounding environment. Types crustaceans contains hemocyanin, in which cop- of animal respiration, in order of complexity, per occupies the central position. Whatever the include direct diffusion, diffusion into blood, substance, the compound it forms with oxygen tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, and carbon dioxide must be unstable, so that it fungi, and plants all obtain the oxygen they use can break down easily to release oxygen to the for cellular respiration directly from the environ- cells or carbon dioxide to the environment. ment, meaning that there are no intermediate Cellular Respiration organs or bodily chemicals, such as lungs or blood. More complex organisms, such as Both forms of respiration involve oxygen, but sponges, jellyfish, and terrestrial (land) flat- cellular respiration also involves a type of nutri- worms, all of which have blood, also breathe ent—materials that supply energy, or the materi- through direct diffusion. The latter term als for forming new tissue. Among the key nutri- describes an exchange of oxygen and carbon ents are carbohydrates, naturally occurring com- dioxide directly between an organism, or its pounds that consist of carbon, hydrogen, and bloodstream, and the surrounding environment. oxygen. Included in the carbohydrate group are More complex is the method of diffusion sugars, starches, cellulose, and various other sub- into blood whereby oxygen passes through a stances. moist layer of cells on the body surface and then
56 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 57
Respiration
ONE FORM OF RESPIRATION IS DIFFUSION INTO BLOOD, WHEREBY OXYGEN PASSES THROUGH A MOIST LAYER OF CELLS ON THE BODY SURFACE, THEN THROUGH CAPILLARY WALLS AND INTO THE BLOODSTREAM, WHERE IT MOVES ON TO TISSUES AND CELLS. AMONG THE ORGANISMS THAT RELY ON DIFFUSION INTO BLOOD ARE ANNELIDS, A GROUP THAT INCLUDES LEECHES. (© Lester V. Bergman/Corbis. Reproduced by permission.)
through capillary walls (capillaries are small humans and other higher animals. A remnant of blood vessels that form a network throughout this chapter from humans’ evolutionary history the body) and into the bloodstream. Once oxy- can be seen in the way that an embryo breathes in gen is in the blood, it moves throughout the body its mother’s womb, not by drawing in oxygen to different tissues and cells. Among the organ- through its lungs but through gill-like mecha- isms that rely on diffusion into blood are nisms that disappear as the embryo develops. annelids, a group that includes earthworms, var- LUNGS. Lungs are composed of many ious marine worms, and leeches. small chambers or air sacs surrounded by blood In tracheal respiration air moves through capillaries. Thus, they work with the circulatory openings in the body surface called spiracles. It system, which transports oxygen from inhaled air then passes into special breathing tubes called to all tissues of the body and also transports car- tracheae that extend into the body. The tracheae bon dioxide from body cells to the lungs to be divide into many small branches that are in con- exhaled. After air enters the lungs, oxygen moves tact with muscles and organs. In small insects, air into the bloodstream through the walls of these simply moves into the tracheae, while in large capillaries. It then passes from the lung capillar- insects, body movements assist tracheal air ies to the different muscles and organs of the movement. Insects and terrestrial arthropods body. (land-based organisms with external skeletons) Although they are common to amphibians, use this method of respiration. reptiles, birds, and mammals, lungs differ enor- Much more complicated than tracheae, gills mously throughout the animal kingdom. Frogs, are specialized tissues with many infoldings. Each for instance, have balloon-like lungs that do not gill is covered by a thin layer of cells and filled have a very large surface area. By contrast, if the with blood capillaries. These capillaries take up entire surface of an adult male human’s lungs oxygen dissolved in water and expel carbon diox- were spread flat, it would cover about 750 sq. ft. ide dissolved in blood. Fish and other aquatic (70 m2), approximately the size of a handball animals use gills, as did the early ancestors of court. The reason is that humans have about 300
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 57 set_vol3_2 8/19/02 10:35 AM Page 58
Respiration million gas-filled alveoli, tiny protrusions inside holding their breath. A buildup of carbon diox- the lungs that greatly expand the surface area for ide and hydrogen ions (electrically charged gas exchange. atoms) in the bloodstream stimulates the breath- Birds have specialized lungs that use a mech- ing centers to become active, no matter what we anism called crosscurrent exchange, which allows try to do. On the other hand, if a person were air to flow in one direction only, making for more underwater, the lungs would draw in water efficient oxygen exchange. They have some eight instead of air, and though water contains air, the thin-walled air sacs attached to their lungs, and drowning person would suffocate. when they inhale, air passes through a tube called ANAEROBIC BACTERIA. Some the bronchus and enters posterior air sacs—that creatures, however, do not need to breathe air but is, sacs located toward the rear. At the same time, instead survive by anaerobic respiration. This is air in the lungs moves forward to anterior air true primarily of some forms of bacteria, and sacs, or ones located near the bird’s front. When indeed scientists believe that the first organisms the bird exhales, air from the rear air sacs moves to appear on Earth’s surface were anaerobic. to the outside environment, while air from the Those organisms arose when Earth’s atmosphere front moves into the lungs. This efficient system contained very little oxygen, and as the composi- moves air forward through the lungs when the tion of the atmosphere began to incorporate bird inhales and exhales and makes it possible for more oxygen over the course of many millions of birds to fly at high altitudes, where the air has a years, new organisms evolved that were adapted low oxygen content. to that condition. Humans and other mammals have lungs in The essay on paleontology discusses Earth’s which air moves in and out through the same early history, including the existence of anaero- pathway. This is true even of dolphins and bic life before the formation of oxygen in the whales, though they differ from humans in that atmosphere. The appearance of oxygen is a they do not take in nutrition through the same result of plant life, which produces it as a by- opening. In fact, terrestrial mammals, such as the product of the conversion of carbon dioxide that human, horse, or dog, are some of the only crea- takes place in photosynthesis. Plants, therefore, tures that possess two large respiratory openings: are technically anaerobic life-forms, though that one purely for breathing and smelling and the term usually refers to types of bacteria that nei- other for the intake of nutrients as well as air ther inhale nor exhale oxygen. Anaerobic bacte- (i.e., oxygen in and carbon dioxide out). ria still exist on Earth and serve humans in many ways. Some play a part in the production of REAL-LIFE foods, as in the process of fermentation. Other APPLICATIONS anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill Anaerobic Respiration most creatures—and not just because of the lack of oxygen—they consume waste materials, Activity that involves oxygen is called aerobic; breaking them down chemically into simpler hence the term aerobic exercise, which refers to compounds. running, calisthenics, biking, or any other form HUMANS AND ANAEROBIC of activity that increases the heart rate and RESPIRATION. Even in creatures, such as breathing. Activity that does not involve oxygen humans, that depend on aerobic respiration, intake is called anaerobic. Weightlifting, for anaerobic respiration can take place. Most cells instance, will increase the heart rate and rate of are able to switch from aerobic to anaerobic res- breathing if it is done intensely, but that is not its piration when necessary, but they generally are purpose and it does not depend on the intake not able to continue producing energy by this and outflow of breath. For that reason, it is called process for very long. For example, a person who an anaerobic exercise—though, obviously, a per- exercises vigorously may be burning up glucose son has to keep breathing while doing it. faster than oxygen is being pumped to the cells, In fact, a person cannot consciously stop meaning that cellular respiration cannot take breathing for a prolonged period, and for this place quickly enough to supply all the energy the reason, people cannot kill themselves simply by body needs. In that case, cells switch over to
58 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 59
Respiration
COMPUTERIZED TOMOGRAPHIC VIEW OF THE LUNGS, SHOWING THE TRACHEA (CENTER) SPLITTING TO FORM THE TWO MAIN BRONCHI, WHICH LEAD TO THE LUNGS. INSIDE THE LUNGS, MANY BRANCHING BRONCHI TERMINATE IN ALVEOLI, AIR SACS WHERE GAS EXCHANGE TAKES PLACE. (© BSIP/Gems Europe/Photo Researchers. Reproduced by permission.)
anaerobic respiration, which results in the pro- often have sore muscles from lactic acid buildup,
duction of lactic acid, or C3H6O3. One advantage even though they may not exercise. Lactic acid of anaerobic respiration is that it can take place also can lead to a buildup of uric acid crystals in very quickly and in short bursts, as opposed to the joints, in turn causing gout, a very painful aerobic respiration, which is designed for slower disease. and steadier use of muscles. The disadvantage is LACTIC ACID IN FOOD AND that anaerobic respiration produces lactic acid, INDUSTRY. Lactic acid is certainly not with- which, when it builds up in muscles that are out its uses, and it is found throughout nature. overworked, causes soreness and may even lead When lactose, or milk sugar, is fermented by the to cramps. action of certain bacteria, it causes milk to sour. LACTIC ACID IN THE BODY. The same process is used in the manufacture of Eventually, the buildup of lactic acid is carried yogurt, but the reaction is controlled carefully to away in the bloodstream, and the lactic acid is ensure the production of a consumable product. converted to carbon dioxide and water vapor, Lactic acid also is applied by the dairy industry in both of which are exhaled. But if lactic acid levels making cheese. Molasses contains lactic acid, a in the bloodstream rise faster than the body can product of the digestion of sugars by various neutralize them, a state known as lactic acidosis species of bacteria, and lactic acid also is used in may ensue. Lactic acidosis rarely happens in making pickles and sauerkraut, foods for which a healthy people and, more often than not, is a sour taste is desired. result of the body’s inability to obtain sufficient A compound made from lactic acid is used oxygen, as occurs in heart attacks or carbon as a food preservative, but the applications of lac- monoxide or cyanide poisoning or in the context tic acid extend far beyond food production. Lac- of diseases such as diabetes. tic acid is important as a starting material for The ability of the body to metabolize lactic making drugs in the pharmaceutical industry. acid is diminished significantly by alcohol, which Additionally, it is involved in the manufacturing impairs the liver’s ability to carry out normal of lacquers and inks; is used as a humectant, or metabolic reactions. For this reason, alcoholics moisturizer, in some cosmetics; is applied as a
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 59 set_vol3_2 8/19/02 10:35 AM Page 60
Respiration mordant, or a chemical that helps fabrics accept Respiratory ailments often take the form of dyes, to textiles; and is employed in tanning allergies such as hay fever, symptoms of which leather. include sneezing, runny nose, swollen nasal tis- sue, headaches, blocked sinuses, fever, and Respiratory Disorders watery, irritated eyes. Hay fever is usually aggra- vated by the presence of pollen or ragweed in In almost any bodily system, there are bound to the air, as is common in the springtime. Other be disorders, or at least the chance that disorders allergy-related respiratory conditions may be may occur. This is particularly the case with aggravated by dust in the air, and particularly something as complex as the respiratory system, by the feces of dust mites that live on dust par- because the more complex the system, the more ticles. things that can go wrong. Among the respiratory BRONCHIAL AILMENTS. disorders that affect humans is a whole range of Allergic ailments from the common cold to emphysema, reactions can be treated by antihistamines (see and from the flu to cystic fibrosis. The Immune System for more about allergies), but simple treatments are not available for such THE COMMON COLD. Colds are complex respiratory disorders as asthma, chron- among the most common conditions that affect ic bronchitis, and emphysema. All three are char- the respiratory system, though what we call the acterized by an involuntary constriction in the common cold is actually an invasion by one of walls of the bronchial tubes (the two divisions of some 200 different types of virus. Thus, it is real- the trachea or windpipe that lead to the right and ly not one ailment but 200, though these are vir- left lungs), which causes the tubes to close in tually identical, but the large number of viral such a way that it becomes difficult to breathe. causative agents has made curing the cold an Emphysema can be brought on by cigarette insurmountable task. smoking, and indeed some heavy smokers die When you get a cold, viruses establish them- from that ailment rather than from lung cancer. selves on the mucus membrane that coats the On the other hand, a person can contract a respiratory passages that bring air to your lungs. bronchial illness without engaging in smoking or If your immune system is unsuccessful in ward- any other activity for which the sufferer could ing off this viral infection, the nasal passages ultimately be blamed. Indeed, small children may become inflamed, swollen, and congested, mak- have asthma. One treatment for such disorders is ing it difficult to breathe. the use of a bronchodilator, a medicine used to Coughing is a reflex action whereby the relax the muscles of the bronchial tubes. This body attempts to expel infected mucus or may be administered as a mist through an phlegm. It is essential to removing infected secre- inhaler, or given orally like other medicine. tions from the body, but of course it plays no role TUBERCULOSIS AND PNEU- in actually bringing a cold to an end. Nor do MONIA. More severe is tuberculosis, an infec- antibiotics, which are effective against bacteria tious disease of the lungs caused by bacteria. but not viruses (see Infection). Only when the Tuberculosis attacks the lungs, leading to a body builds up its own defense to the cold— chronic infection with such symptoms as fatigue, assuming the sufferer has a normally functioning loss of weight, night fevers and chills, and per- immune system—is the infection driven away. sistent coughing that brings up blood. Without INFLUENZA AND ALLERGIES. treatment, it is likely to be fatal. Indeed, it was a Influenza, a group of viral infections that can significant cause of death until the introduction include swine flu, Asian flu, Hong Kong flu, and of antibiotics in the 1940s, and it has remained a Victoria flu, is often far more serious than the problem in underdeveloped nations. Additional- common cold. A disease of the lungs, it is highly ly, thanks to mutation in the bacteria themselves, contagious, and can bring about fever, chills, strains of the disease are emerging that are high- weakness, and aches. In addition, influenza can ly resistant to antibiotics. be fatal: a flu epidemic in the aftermath of World Another life-threatening respiratory disease War I, spread to far corners of the globe by is pneumonia, an infection or inflammation of returning soldiers, killed an estimated 20 million the lungs caused by bacteria, viruses, mycoplas- people. ma (microorganisms that show similarities to
60 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 61
Respiration KEY TERMS
AEROBIC: Oxygen-breathing. the form in which animals usually receive ANAEROBIC: Non-oxygen-breathing. carbohydrates. Also known as dextrose, grape sugar, and corn sugar. ATP: Adenosine triphosphate, an ener- gy carrier formed when a simpler com- HEMOGLOBIN: An iron-containing pound, adenosine diphosphate (ADP), protein in human red blood cells that is combines with a phosphate group. responsible for transporting oxygen to the tissues and removing carbon dioxide from CAPILLARY: A very small blood vessel. them. Hemoglobin is known for its deep Capillaries form networks throughout the red color. body. LYMPH: The portion of the blood that CARBOHYDRATES: Naturally occur- includes white blood cells and plasma but ring compounds, consisting of carbon, not red blood cells. hydrogen, and oxygen, whose primary function in the body is to supply energy. LYMPH NODES: Masses of tissue, at Included in the carbohydrate group are certain places in the body, that act as filters sugars, starches, cellulose, and various for blood. other substances. Most carbohydrates are METABOLISM: The chemical process produced by green plants in the process of by which nutrients are broken down and undergoing photosynthesis. converted into energy or are used in the CELLULAR RESPIRATION: A construction of new tissue or other materi- process that, when it takes place in the al in the body. All metabolic reactions are presence of oxygen, involves the intake of either catabolic or anabolic. organic substances, which are broken MONOSACCHARIDE: The simplest down into carbon dioxide and water, with type of carbohydrate. Monosaccharides, the release of considerable energy. which cannot be broken down chemically CIRCULATORY SYSTEM: The parts into simpler carbohydrates, also are known of the body that work together to move as simple sugars. blood and lymph. They include the heart, NUTRIENT: Materials that supply blood vessels, blood, and the lymphatic energy or the materials to form new tissue glands, such as the lymph nodes. for organisms. They include proteins, car- COMPOUND: A substance in which bohydrates, lipids (fats), vitamins, and atoms of more than one element are bond- minerals. ed chemically to one another. PHOSPHATE GROUP: A group (that FERMENTATION: A process, involving is, a combination of atoms from two or enzymes, in which a compound rich in more elements that tend to bond with energy is broken down into simpler sub- other elements or compounds in certain stances. characteristic ways) involving a phosphate, GLUCOSE: A monosaccharide (sugar) or a chemical compound that contains that occurs widely in nature and which is oxygen bonded to phosphorus.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 61 set_vol3_2 8/19/02 10:35 AM Page 62
Respiration KEY TERMS CONTINUED
PHOTOSYNTHESIS: The biological tinction is made between external respira- conversion of light energy (that is, electro- tion, or an exchange of gases with the magnetic energy) from the Sun to chemical external environment, and internal respi- energy in plants. In this process carbon ration, an exchange of gases between the dioxide and water are converted to carbo- body’s cells and the blood. hydrates and oxygen. SIMPLE SUGAR: A monosaccharide, RESPIRATION: A term that can refer or simple carbohydrate. either to cellular respiration (see defini- TISSUE: A group of cells, along with tion) or, more commonly, to the process by the substances that join them, that forms which an organism takes in oxygen and part of the structural materials in plants or releases carbon dioxide. Sometimes a dis- animals.
both viruses and bacteria), and fungi, as well as annually. No cure for cystic fibrosis exists, and such inorganic agents as inhaled dust or gases. the disease is invariably fatal, with only about Symptoms include pleurisy (chest pain), high 50% of sufferers surviving into their thirties. fever, chills, severe coughing that brings up small Lung complications are the leading cause of amounts of mucus, sweating, blood in the spu- death from cystic fibrosis, and most symptoms of tum (saliva and mucus expelled from the lungs), the disease are related to the sticky mucus that and labored breathing. clogs the lungs and pancreas. People with cystic In 1936, pneumonia was the principal cause fibrosis have trouble breathing, and are highly of death in the United States. Since then, it has susceptible to bacterial infections of the lungs. been controlled by antibiotics, but as with tuber- Coughing, while it may be irritating and painful culosis, resistant strains of bacteria have devel- oped, and therefore the number of cases has if you have a cold, is necessary for the expulsion increased. Today, pneumonia and influenza com- of infected mucus, but mucus in the lungs of a bined are among the most significant causes of cystic fibrosis is too thick to be moved. This death in the United States (see Diseases). makes it easy for bacteria to inhabit the lungs and LUNG CANCER AND CYSTIC cause infection. FIBROSIS. Respiratory ailments may also take the form of lung cancer, which may or may WHERE TO LEARN MORE not be a result of smoking. Cigarette smoking Bryan, Jenny. Breathing: The Respiratory System. New and air pollution are considered to among the York: Dillon Press, 1993. most significant causes of lung cancer, yet people Cellular Metabolism and Fermentation. Estrella Mountain have been known to die of the disease without Community College (Web site). being smokers or having been exposed to signifi-
62 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_2 8/19/02 10:35 AM Page 63
Llamas, Andreu. Respiration and Circulation. Milwaukee: Roca, Núria, and Marta Serrano. The Respiratory System, Respiration Gareth Stevens, 1998. the Breath of Life. Illus. Antonio Tenllado. New York: Paustian, Timothy. Anaerobic Respiration. Department of Chelsea House Publishers, 1995. Bacteriology, University of Wisconsin–Madison (Web Silverstein, Alvin, and Virginia B. Silverstein. The Respi- site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 63 set_vol3_3 8/19/02 10:40 AM Page 65
SCIENCE OF EVERYDAY THINGS real-life biology NUTRITION
FOOD WEBS NUTRIENTS AND NUTRITION VITAMINS
65 set_vol3_3 8/19/02 10:40 AM Page 67
Food Webs FOOD WEBS
CONCEPT whereas the term biota typically refers only to The idea of a food chain is common in everyday plant and animal life within a biological commu- life, so much so that it has become a metaphor nity or ecosystem. (For more on these subjects, applied in many situations. A high achiever in see Ecosystems and Ecology and Biological Com- business or other endeavors is said to be “at the munities.) top of the food chain,”and images of big fish eat- Trophic Levels ing little fish abound in cartoons. Yet in the study of the biological sciences, the concept of food The organisms in a biological community are chains is part of the much larger idea of a food linked in their need to obtain energy from food, web. Whereas a food chain is a linear series of which derives from the Sun through plant life. organisms dependent on each other for food, a (There are, however, some communities, in areas food web is an interconnected set of food chains such as deep-ocean rifts, that are not dependent in the same ecosystem. Food webs make possible on sunlight at all.) The Sun’s energy is electro- the transfer of energy from plants through herbi- magnetic and travels in the form of radiation, vores to carnivores and omnivores, and ultimate- which Earth receives as light and heat. Plants, ly to the detritivores and decomposers that known as primary producers, convert this elec- enrich the soil with organic waste. Just as a food tromagnetic energy into chemical energy web can transfer materials essential to the life of through the process of photosynthesis. organisms, it is also a devastatingly efficient con- The plants are eaten by herbivores (plant- duit for the transfer of poisons. eating animals), known also as primary con- sumers, examples of which include squirrels, HOW IT WORKS rabbits, mice, deer, cows, horses, sheep, and seed- eating birds. These creatures, in turn, are eaten by Food Webs in Context secondary consumers, which are either carni- vores, which are creatures that eat only meat, or An ecosystem is a community of independent omnivores—creatures, such as humans, that eat organisms along with the inorganic components meat and plants. (chiefly soil, water, air, and rocks) that make up There may even be tertiary, or third-level, their environment. A biome is a large ecosystem, consumers. These are animals that eat secondary characterized by its dominant life-forms—for consumers; examples are mountain lions and example, the Amazonian rain forest. hawks, both of which eat such second-order con- That portion of an ecosystem composed sumers as snakes and owls. Human societies that only of living things, as opposed to the formerly eat dogs or cats, as well as those that engage in living or never living components, is known as a cannibalism, also behave as tertiary consumers. biological community. This community includes (See Biological Communities for a biological creatures from all five kingdoms of living organ- explanation of what otherwise is considered an isms (including bacteria, algae, and fungi), abhorrent and immoral practice—not to men-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 67 set_vol3_3 8/19/02 10:40 AM Page 68
Food Webs tion a dangerous one, due to the risk of such dis- however, break down the nutrients in decayed eases as kuru, a type of spongiform encephalopa- organic matter to a far greater extent than do thy.) In any case, the further along in the chain of detritivores. trophic levels or stages of the food web, the fewer Typically, decomposers are microorganisms, consumers there are. including bacteria and fungi, and they process ENERGY AND TROPHIC LEV- materials in such a way that complex compounds ELS. It is fairly obvious that when a creature is undergo the chemical reaction of decomposi- “higher on the food chain” (to use the common tion. Through decomposition, compounds are expression), it has fewer natural predators. The broken down into simpler forms, or even into reason for this is that at each successive trophic their constituent elements, which provide the level, there are simply fewer organisms; this, in environment with nutrients necessary to the turn, is due to the fact that the energy available to growth of more plant life. each level is progressively smaller, and the organ- CATEGORIZING THE TROPHIC isms themselves progressively larger. This, in LEVELS. The organisms in the food web can turn, stems from one of the most intriguing, be viewed in three groups: producers (plants), maddening concepts in the entire universe: the consumers (primary- and secondary-consuming second law of thermodynamics, which we discuss animals, whether herbivores, carnivores, or shortly. omnivores), and decomposers (that is, both Because of the diminishing number of detritivores and true decomposers). Producers organisms at each trophic level, the food web and consumers are part of a larger structure often is depicted as a pyramid, a concept we known as the grazing food web, in which food is explore further later in this essay. The number of “on its way up the food chain,” as it were. organisms begins to increase again at the next Decomposers and detritivores make up the trophic level beyond secondary or tertiary con- decomposer food web, which brings food back sumers, that of decomposers. Large omnivores “down” to the soil. and carnivores may not be prey for other crea- Producers also are called autotrophs, from tures in life, but everything dies eventually, and Greek roots meaning “self-feeders,” because they anything that has ever lived is food for detriti- are not dependent on other organisms as a vores, or organisms that feed on waste matter. source of energy. Beyond the level of the primary DETRITIVORES AND DECOM- producers, all consumers are known as het- POSERS. Detritivores, which range in size erotrophs, or “other-feeders.”These creatures feed and complexity from maggots to vultures, may on other organisms to obtain their energy and not be the most appealing creatures on Earth, but are classified according to the types of food they without them life itself would suffer. By consum- eat—herbivores, carnivores, omnivores, as we ing the remains of formerly living things, they already have discussed. Detritivores and decom- break organic material down into inorganic sub- posers also are considered heterotrophs. stances. In other words, their internal systems chemically process compounds containing the Organisms and Energy element carbon in characteristic structures. They then release that carbon into the atmosphere and Rather than depending on other organisms for soil in such a way that what remains is inorganic energy, autotrophs obtain energy from the Sun material that enriches the soil for the growth of and carbon dioxide from the atmosphere. From new plant life. these components, they build the large organic molecules that they need to survive. Green plants But detritivores are not the last stop on the do this through the process of photosynthesis, a food web. The final trophic level, before the cycle chemical reaction that can be represented as fol- comes back around to plants, contains the largest lows: number of organisms in the entire food web— perhaps billions and billions, even in a space solar energy + carbon dioxide (CO2) + water 4 smaller than a coffee cup. These are decom- (H2O) glucose (sugar: C6H12O6) + oxygen posers, or organisms that, as with detritivores, (O2) obtain their energy from the chemical break- Actually, in order to produce what chemists down of dead organisms. The decomposers, call a balanced equation, it would be necessary to
68 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 69
show this equation as a reaction between the pated in the form of heat and sound, as a natural Food Webs energy and six molecules each of carbon dioxide by-product of operating the engine. Even with- and water, which would produce a glucose mole- out running an air conditioner or other energy- cule and six oxygen molecules. In any case, what consuming device, only about 30% of the energy we have described here is an amazing thing and from the gas goes to turning the wheels. one of the great wonders of nature. Sunlight aids THE ECOLOGICAL PYRAMID. plants in converting carbon dioxide, which they What this means for the food web is that there is receive from the respiration of animals, along bound to be a loss of energy in the transfer from with water (which also may come from animal one trophic level to another. Organisms never respiration, though this is not necessarily the manage to retrieve 100% of the energy from the case), into a sugar molecule for the plant’s suste- materials they eat; in fact, the figure is more like nance. Furthermore, oxygen, essential to the life 10%. A rabbit that eats a carrot gets only about of virtually all animals, also is produced—yet 10% of the energy in it, and an owl that eats the from the standpoint of the plant, it is simply a rabbit gets only about 10% of the energy from waste by-product! the rabbit, or 1% of the energy in the carrot. THE SECOND LAW OF THER- Because of these diminishing returns, there are MODYNAMICS. The productivity of always fewer organisms at each successive troph- plants, which is measured in terms of biomass ic level on the grazing food web. This fact is (the combined mass of all organisms at a partic- expressed in a model known as the ecological ular trophic level in a food web), determines the pyramid, or energy pyramid, which shows that as amount of “fixed,” or usable, energy available to the amount of total energy decreases with each other trophic levels on the food web. The amount trophic level, so does the biomass. As a result, it of energy available always will be less for each may take 1,000 carrots to support 100 rabbits, 10 successive trophic level, through the point where owls, and one hawk. consumers end and decomposers begin—that is, The picture changes as the shift is made through the level of the secondary or perhaps from the grazing web to the decomposer web. tertiary consumer. Detritivores and decomposers are extraordinari- If there is any scientific equivalent of the ly efficient feeders, reworking detritus over and curse in the Garden of Eden (the punishment for over and extracting more fixed energy as they do. the sins of Adam and Eve, according to Judeo- Eventually, they break the waste down into sim- Christian belief), it is the second law of thermo- ple inorganic chemicals, which, as we have noted, dynamics. Just as the expulsion from Paradise in then may be reused by the primary producers. the biblical story ensured that life would be much The number of organisms in the decomposing more difficult for humans than it would have food web dwarfs that of all others combined, been in Eden, so the second law thwarts all ambi- though decomposers themselves are very small, tions toward transcending the limits of physical and their combined population takes up very lit- reality. tle physical space. The first law of thermodynamics states that it is impossible to obtain more energy from a sys- REAL-LIFE tem than is put into it. Thus, for instance, a car will go only as far as is allowed by the amount of APPLICATIONS energy that is pumped into its tank. The first law, Keystone Species discovered in the mid–nineteenth century, effec- tively ruled out any hopes of a perpetual-motion The keystone in an archway is a wedge-shaped machine, but the second law, derived a few stone at the top of the arch. It’s position is decades later, delivered even worse news. extraordinarily important: if the keystone is Though it can be stated in a number of ways, removed, the arch will collapse. Thus, the key- the second law essentially means that it is impos- stone has become an often-used metaphor in sible to extract as much energy from a system as other circumstances, as, for instance, in the nick- one puts into it. Thus, in the case of an automo- name of Pennsylvania, the “Keystone State.” In bile, most of the energy contained in the gas does the realm of ecology, the term keystone species not go toward moving the car; rather, it is dissi- refers to those organisms that, like a keystone in
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 69 set_vol3_3 8/19/02 10:40 AM Page 70
Food Webs
THE TERM KEYSTONE SPECIES REFERS TO AN ORGANISM THAT PLAYS A CRITICAL ROLE IN ITS ENVIRONMENT, ONE THAT MAY BECOME APPARENT ONLY ONCE IT IS REMOVED FROM AN ECOSYSTEM. ON THE WEST COAST OF NORTH AMERICA, FOR INSTANCE, REMOVAL OF A CERTAIN SPECIES OF STARFISH CAUSED A RAPID GROWTH IN THE NUMBERS AND BIOMASS OF THE MUSSEL UPON WHICH THE STARFISH FED. (© Stuart Westmorland/Corbis. Reproduced by permission.)
architecture or engineering, play a critical role in far out of proportion to its relative biomass or their environments. productivity. Typically, a keystone species is a top Within a food web, for instance, a keystone predator (that is, a large secondary or even terti- species can have a powerful influence, one that is ary consumer), though occasionally an herbivore
70 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 71
can occupy the keystone position. Often, the role aspect of the character or quality of the environ- Food Webs of the keystone species becomes apparent only ment. For instance, in an ecosystem affected by once it is removed, either experimentally or by pollution, examination of indicator plant species natural forces, from an ecosystem. may reveal the pollution patterns. By their pres- STARFISH AND MUSSELS. In ence, indicator species also may serve to show the temperate ecosystems on the west coast of North quality or integrity of an ecosystem. Such is the America, for instance, removal of a certain case, for instance, with the spotted owl, or Strix species of starfish (Pisaster ochraceous) was found occidentalis, and other species that depend on to cause a rapid growth in the numbers and bio- old-growth forests. (See Succession and Climax mass of the mussel Mytilus californianus. The lat- for more on this subject.) Because the needs of ter then forced out other species and proceeded these species are so particular, their presence or to dominate the biological community. As it absence can illustrate the health or lack thereof of turned out, the starfish acted as a keystone pred- the biome in question. ator by consuming these mussels. Other indicator plants also can be used to Specifically, the starfish prevented the mus- determine the presence of valuable mineral sel from gaining dominance that it otherwise deposits in the soil, because those minerals make would have gained, owing to its competitive their way into the tissues of the plants them- superiority in relation to other species within this selves. Nickel concentrations as great as 10% have particular coastal ecosystem. Yet the starfish been found in the tissues of Russian plants from could not eliminate the mussel, because it was the mustard family, and a mintlike species called incapable of feeding on larger individuals of that Becium homblei has proved useful for locating species. The result was that the community copper deposits in parts of Africa. Since the plant enjoyed a much greater degree of diversity and can tolerate more than 7% copper in soil (a great complexity than it would have if the mussel had amount and many times the percentage of cop- been allowed to dominate. per in the human body, for instance), it can and SEA OTTERS AND KELP does live near enormous copper deposits. FORESTS. Another keystone species in a INDICATING TOXINS. Some plants geographic area close to that of the starfish we can serve as indicators of serpentine minerals, have just described is the sea otter, native to west- varieties of compounds that can be toxic in large ern North America. Its principal food source is concentrations. In California, for instance, where the sea urchin, an herbivore that, in turn, sur- serpentine soils are not uncommon, there plant vives by consuming kelp, a large form of algae. By species unique to specific ecosystems high in ser- controlling the numbers and densities of sea pentine mineral content. Elsewhere, there are urchins, sea otters allowed kelp to retain a rela- types of lichens that are sensitive to toxic gases, tively large biomass within the community, thus such as sulfur dioxide, and thus these lichens can facilitating the growth of “kelp forests.” be monitored as a way of keeping tabs on air pol- When humans began hunting sea otters for lution. their fur during the late eighteenth and into the In semiarid regions where soils contain large nineteenth centuries, however, the ecological quantities of the element selenium, plants can effect soon was felt in the form of declining kelp accumulate such large concentrations of the ele- forests. Fortunately, hunting did not render the ment that they become poisonous to primary species extinct, and since the 1930s, sea otters consumers (for example, rabbits) who eat them. have been colonizing many of their former habi- The result may be temporary or even permanent tats. This colonization has resulted in a corre- blindness. In such situations, legumes of the sponding increase in the density of surrounding genus Astragalus, which can accumulate as much kelp forests. as 15,000 ppm (parts per million)—a compara- tively enormous concentration—serve as indica- Indicator Species tors. Their heavy selenium concentration gives A concept similar to that of the keystone species them a noticeably unpleasant smell. is the idea of an indicator species: plants or ani- Aquatic invertebrates and fish often have mals that, by their presence, abundance, or been surveyed for what they can show as to the chemical composition, demonstrate a distinctive quality of water and the health of aquatic ecosys-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 71 set_vol3_3 8/19/02 10:40 AM Page 72
Food Webs
BEACHGOERS ARE SPRAYED WITH DDT IN THE FIRST PUBLIC TEST OF A NEW MACHINE FOR DISTRIBUTING THE INSEC- TICIDE IN 1945. ONCE PESTICIDES SUCH AS DDT HAVE BEEN SPRAYED IN A REGION, RAIN CAN WASH THEM INTO CREEKS, LAKES, AND OTHER BODIES OF WATER, WHERE THEY ARE ABSORBED BY CREATURES THAT EITHER DRINK OR SWIM IN THE WATER AND THUS ENTER THE FOOD WEB. (© Bettmann/Corbis. Reproduced by permission.)
tems. The presence of a creature known by the buildup of toxins, and particularly chemical pol- disgusting name sewage worm, or tubificid lutants, in the tissues of individual organisms. worm (Tubificidae), indicates the degradation of Part of the danger in these toxins is the fact that water quality by the introduction of sewage or the organism cannot easily process them either other outside organic matter that consumes oxy- by metabolizing them (i.e., incorporating them gen. Tubificid worms are virtually anaerobic, or into the metabolic system, as one does food or non-oxygen-breathing, unlike the animals that water) or by excreting them through urine or would occupy a similar niche in a healthy aquat- other substances produced by the body. ic environment. The only way for the organism to release Bioaccumulation and toxins, in fact, is by passing them on to other Biomagnification members of the food web. Because organisms at each successive trophic level must consume more The subject of indicator species leads us, natural- biomass to meet their energy requirements, they ly enough, to the topic of pollution in the food experience an increase in contamination, a phe- web, which can be seen on a small scale in the nomenon known as biomagnification. The fol- phenomenon of bioaccumulation and which lowing example illustrates how toxins enter the manifests on a much broader scale as biomagni- food web and gradually make their way down the fication. One of the key concepts in ecological line, growing in proportion as they do. studies is the idea that a disturbance in one area Particles of pollutant may stick to algae, for can lead to serious consequences elsewhere. The instance, which are so small that the toxin does interconnectedness of components in the envi- little damage at this level of the food web. But ronment thus makes it impossible for any event even a small herbivore, such as a zooplankton, or phenomenon to be truly isolated. takes in larger quantities of the pollutant when it Nowhere is this principle better illustrated consumes the algae, and so begins the cycle of that in the processes of bioaccumulation and biomagnification. By the time the toxin has biomagnification. The first of these is the passed from a zooplankton to a small fish, the
72 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 73
amount of pollutant in a single organism might THE HUMAN FACTOR. Because the Food Webs be 100 times what it was at the level of the algae. species of bird affected were not ones that people The reason, again, is that the fish can consume 10 normally consume for food, DDT biomagnifica- zooplankton that each has consumed 10 algae. tion did not have a wide-ranging effect on These particular numbers, of course, are human populations. However, tests showed that used simply for the sake of convenience, as were some DDT had made its way into the fat deposits those cited earlier in relation to the ecological of some members of the population. In any case, pyramid. Note, incidentally, the similarity of the bioaccumulation and biomagnification have relationships between this “pyramid of poison” threatened humans, for instance in the late 1940s and the ecological pyramid, whereby energy, and early 1950s, when cows fed on grass that had which is beneficial, passes between trophic levels. been exposed to nuclear radiation, and this The higher the trophic level, the smaller the radioactive material found its way into milk. amounts of energy that the organism extracts Traces of the radioactive isotope strontium- from its food—but the higher the amount of 90, a by-product of nuclear weapons testing in toxic content. By the time the toxins have passed the atmosphere from the late 1940s onward, fell on to a few more levels in the food web, they to earth in a fine powder that coated the grass. might be appearing in concentrations as great as Later the cows ate the grass, and strontium-90 10,000 times their original amount. wound up in the milk they produced. Because of DDT BIOMAGNIFICATION. Among the its similarities to calcium, the isotope became most prominent examples of chemical pollutants incorporated into the teeth and gums of children that are bioaccumulated are such pesticides as who drank the milk, posing health concerns that DDT (dichlorodiphenyltrichloroethane). DDT is helped bring an end to atmospheric testing in the an insecticide of the hydrocarbon family, a large early 1960s. group of chemical compounds of which the Humans themselves can serve as repositories many varieties of petroleum are examples. for contaminants, a particular concern for a Because it is based in hydrocarbons, DDT is mother nursing her baby. Assuming the mother’s hydrophobic (“water-fearing”) and instead mixes own system has been contaminated by toxins, it with oils—including the fat of organisms. is likely that her milk contains traces of the In the twenty or so years leading up to 1972, harmful chemical, which will be passed on to her Americans used vast amounts of DDT for the child. Obviously, this is a very serious matter. purpose of controlling mosquitoes and other Nursing mothers, babies, and their loved ones are pests. DDT appeared to be a remarkably success- not the only people affected by bioaccumulation ful killer, and in fact it turned out to be a little too and the storing of toxins in fatty tissues. In fact, successful. As it found its way into water sources, some physicians and nutritionists maintain that DDT entered the bodies of fish, and then those of one of the reasons for the buildup of fat on a per- predatory birds such as osprey, peregrine falcons, son’s body (though certainly not the only reason) brown pelicans, and even the bald eagle. may be as a response to toxins, the idea being that The detrimental effect of DDT on America’s the body produces fat cells as a means of keeping national symbol, a bird protected by law since the toxins away from the bloodstream. 1940, aptly illustrates the ravages exacted by this Another case of large-scale bioaccumulation powerful insecticide. DDT levels in birds became occurred during the 1970s and 1980s, when fish so high that the birds’ eggshells were abnormally such as tuna were found to contain bioaccumu- thin, and adult birds sitting on the nest would lated levels of mercury. In the face of such con- accidentally break the shells of unhatched chicks. cerns, governments have intervened in several As a result, baby birds died, and populations of ways, including the banning of DDT spraying, as these species dwindled. Environmentalists in the mentioned earlier. The U.S. federal government late 1960s and early 1970s raised public aware- and the governments of some states have issued ness of this phenomenon, and this led to the ban- warnings against the consumption of certain ning of DDT spraying in 1972. The period since types of fish, owing to bioaccumulated levels of that time has seen dramatic increases in bird toxic pollutants. Bioaccumulation is particularly populations. serious in the case of species that live a long time,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 73 set_vol3_3 8/19/02 10:40 AM Page 74
Food Webs KEY TERMS
AUTOTROPHS: Primary producers. insects, viruses, single-cell organisms, and Autotroph means “self-feeder,” and these so on—as well as all formerly living things organisms are distinguished by the fact that that have not yet decomposed. they do not depend on other organisms as BIOTA: A combination of all flora and a source of energy. Instead, they obtain fauna (plant and animal life, respectively) energy from the Sun and carbon dioxide in a region. from the atmosphere, and from these con- CARNIVORE: A meat-eating organ- stituents they build the large organic mole- ism, or an organism that eats only meat (as cules that they need to survive. distinguished from an omnivore). BIOACCUMULATION: The buildup of DECOMPOSER FOOD WEB: That toxic chemical pollutants in the fatty tis- portion of the food web occupied by detri- sues of organisms. tivores and decomposers. (Compare with BIOGEOCHEMICAL CYCLES: The grazing food web.) changes that particular elements undergo DECOMPOSERS: Organisms that as they pass back and forth through the obtain their energy from the chemical various earth systems (e.g., the biosphere) breakdown of dead organisms as well as and particularly between living and non- from animal and plant waste products. The living matter. The elements involved in principal forms of decomposer are bacteria biogeochemical cycles are hydrogen, oxy- and fungi. gen, carbon, nitrogen, phosphorus, and sulfur. DECOMPOSITION REACTION: A chemical reaction in which a compound is BIOLOGICAL COMMUNITY: The liv- broken down into simpler compounds, or ing components of an ecosystem. into its constituent elements. In the bios- BIOMAGNIFICATION: The increase in phere, this often is achieved through the bioaccumulated contamination at higher help of detritivores and decomposers. levels of the food web. Biomagnification DETRITIVORES: Organisms that feed results from the fact that larger organisms on waste matter, breaking organic material consume larger quantities of food—and, down into inorganic substances that then hence, in the case of polluted materials, can become available to the biosphere in more toxins. the form of nutrients for plants. Their BIOMASS: The combined mass of all function is similar to that of decomposers; organisms at a particular trophic level in a however, unlike decomposers—which tend food web. to be bacteria or fungi—detritivores are BIOME: A large ecosystem, character- relatively complex organisms, such as ized by its dominant life-forms. earthworms or maggots. BIOSPHERE: A combination of all liv- ECOLOGY: The study of the relation- ing things on Earth—plants, mammals, ships between organisms and their envi- birds, amphibians, reptiles, aquatic life, ronments.
74 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 75
Food Webs KEY TERMS CONTINUED
ECOSYSTEM: A community of inter- types of food they eat. Thus, they are dependent organisms along with the inor- known as herbivores, carnivores, and so ganic components of their environment. on. ENERGY TRANSFER: The flow of INDICATOR SPECIES: A plant or energy between organisms in a food web. animal that, by its presence, abundance, or FIRST LAW OF THERMODYNAMICS: chemical composition, demonstrates a A law of physics stating that the amount of particular aspect of the character or quali- energy in a system remains constant, and ty of the environment. therefore it is impossible to perform work KEYSTONE SPECIES: A species that that results in an energy output greater plays a crucial role in the functioning of its than the energy input. ecosystem or that has a disproportionate FOOD CHAIN: A series of singular influence on the structure of its ecosystem. organisms in which each plant or animal NICHE: A term referring to the role depends on the organism that precedes it. that a particular organism plays within its Food chains rarely exist in nature; there- biological community. fore, scientists prefer the term food web. OMNIVORE: An organism that eats FOOD WEB: A term describing the both plants and other animals. interaction of plants, herbivores, carni- ORGANIC: At one time chemists used vores, omnivores, decomposers, and detri- the term organic only in reference to living tivores in an ecosystem. Each of these things. Now the word is applied to most organisms consumes nutrients and passes compounds containing carbon, with the them along to other organisms (or, in the exception of carbonates (which are miner- case of the decomposer food web, to the als) and oxides, such as carbon dioxide. soil and environment). The food web may PHOTOSYNTHESIS: The biological be thought of as a bundle or network of conversion of light energy (that is, electro- food chains, but since the latter rarely exist separately, scientists prefer the concept of a magnetic energy) from the Sun to chemical food web to that of a food chain. energy in plants. PRIMARY CONSUMERS: GRAZING FOOD WEB: That portion Animals of the food web occupied by autotrophs, that eat green plants. (Compare with sec- herbivores, carnivores, and omnivores. ondary consumers.) (Compare with decomposer food web.) PRIMARY PRODUCERS: Green HERBIVORE: A plant-eating organ- plants that depend on photosynthesis for ism. their nourishment. SECONDARY CONSUMERS: HETEROTROPHS: Secondary con- Ani- sumers, or “other-feeders.” These creatures mals that eat other animals. feed on other organisms to obtain their SECOND LAW OF THERMODYNAM- energy and are classified according to the ICS: A law of physics, which can be stat-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 75 set_vol3_3 8/19/02 10:40 AM Page 76
Food Webs KEY TERMS CONTINUED
ed in several ways, all of which mean the ble to extract from a system the same same thing. According to one version of the amount of energy that was put into it. second law, it is impossible to transfer ener- TROPHIC LEVELS: Various stages gy with perfect efficiency, because some within a food web. For instance, plants are energy always will be lost in the transfer. on one trophic level, herbivores on anoth- This is the same as saying that it is impossi- er, and so on.
because a longer life allows for much longer peri- Extension Toxicology Network (EXTOXNET): Toxicology ods of bioaccumulation. For this reason, some Information Briefs (Web site).
76 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 77
NUTRIENTS AND
Nutrients and Nutrition NUTRITION
CONCEPT use of them for its survival, growth, and develop- In the modern world people are accustomed to ment. The term nutrition also can refer to the hearing a great deal about nutrients and nutri- study of nutrients, their consumption, and their tion. Words such as protein, carbohydrate, vita- processing in the bodies of organisms. Here the mins, minerals, and fats are a regular part of daily general term organism has been used, but for the life, yet few people who talk about these nutrients most part the present essay is concerned with really know what they are. In fact, these are the animal nutrition, or at least the nutrition of pri- basic building blocks of nutrition, whereby ani- mary consumers (animals that eat plants) and mal life is sustained. Whereas plants can get their secondary consumers (animals that eat other energy directly from the Sun and the atmos- animals). phere, animals (including humans) depend on AUTOTROPHS AND THEIR other organisms to provide them with nutrition. NUTRIENTS. By contrast, plants and a few These other organisms include plants, which other types of organism are autotrophs, or pri- generate carbohydrates as a result of photosyn- mary producers in the food web. Autotroph thesis, as well as other animals that eat plants and means “self-feeder,” and these organisms are dis- thereby build proteins and fats. Plants also may tinguished by the fact that they do not depend on contain proteins and fats, and both plants and other organisms as a source of energy. Instead, animals contain vitamins and minerals. These plants obtain energy from the Sun and carbon nutrients, consumed in the proper forms and dioxide from the atmosphere, and from these proportions, sustain life and prevent the miseries materials they build the large organic molecules of malnutrition—a condition that can involve that they need to survive. either undernourishment or overnourishment. Though plants are the most obvious exam- ple of an autotroph, they are not the only ones. In HOW IT WORKS the deep oceans, far from any plant life, primary consumers depend on phytoplankton, which are Nutrients and Nutrition microscopic organisms that encompass a range of bacteria and algae. Nonplant autotrophs may In order to live, animals must consume nutrients, use means different from those employed by of which there are five major classes: carbohy- plants in generating their own food. For example, drates, proteins, lipids or fats, vitamins, and min- there are certain nonplant autotrophic organisms erals. In addition to these constituents, of course, that live in the deep oceans near hydrothermal animal life requires other materials for its suste- vents, which are cracks in the ocean floor caused nance-water, oxygen, and fiber, which aids in the by volcanic activity. These organisms, unlike digestive processing of foods-but these compo- most autotrophs, do not need sunlight to survive. nents usually are not regarded as nutrients. Instead, they build their own nutrients in a sun- Nutrition itself is the series of processes by less world, using sulfur compounds found near which an organism takes in nutrients and makes the vents.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 77 set_vol3_3 8/19/02 10:40 AM Page 78
Chemical Elements and Nutrients biogeochemical cycles, whose names are itali- and Nutrition cized: nitrogen (3%), calcium (1.4%), phosphorus Nutrition An element is a chemical substance made of only (1.0%), magnesium (0.50%), potassium one kind of atom, whereas in a compound, atoms (0.34%), sulfur (0.26%), sodium (0.14%), chlo- of more than one element are chemically bonded rine (0.14%), iron (0.004%), and zinc (0.003%). to one another. Unlike compounds, elements Note that many of these elements are found in cannot be broken chemically into other sub- vitamin and mineral supplements that people stances. There are approximately 90 elements might take on a daily basis to augment the essen- that occur in nature, and many of these ele- tial nutrients in their bodies. There are excep- ments—but not nearly all—are important to tions, however, such as sodium, of which most nutrition. people already ingest too much in the form of ELEMENTS IN THE HUMAN salt. BODY AND BIOGEOCHEMICAL TRACE ELEMENTS. Generally CYCLES. Even when we rule out obviously speaking, it is safe to assume that any element harmful elements, such as lead or uranium, there that appears naturally in the human body is are still numerous chemical elements that play a healthful as a nutritional component. This rule of part in the nutrition of living things. This can be thumb goes only so far, however: chlorine, for illustrated by a glance at the abundance of vari- instance, is poisonous in large quantities, where- ous chemical elements in the human body, which as in the very small proportions found in the include oxygen, carbon, and hydrogen. Oxygen human body, it can be essential to health and alone accounts for a whopping 65% of the well-being. It is certainly possible to ingest some human body’s mass, and carbon (18%), hydro- elements in unhealthy quantities, a fact that is gen (10%), and oxygen together make up 93% of particularly true of trace elements. the mass in the human body. Copper is an example of a trace element, so A great deal of oxygen and hydrogen, of named because only traces of them are present in course, is found in that most useful of all chemi- the human body. In tiny quantities, copper is cal compounds, water. In this vein, it should be beneficial to human health, but if that small noted that all the elements that take part in bio- amount is exceeded, the effects can run the geochemical cycles, which are essential to the gamut from sneezing to diarrhea. In the proper functioning of Earth, appear in relatively large proportions, however, trace elements are essen- proportions within the human body. These ele- tial: without enough iodine, for instance, goiter, a ments are hydrogen, oxygen, carbon, nitrogen, large swelling of the thyroid gland in the neck phosphorus, and sulfur. (For more about biogeo- area, can develop. Chromium helps the body chemical cycles and the elements involved in metabolize sugars, which is why people con- them, including their proportion within the cerned with losing weight or toning their bodies human body’s mass, see The Biosphere.) through exercise may take a chromium supple- Carbon is present in all living things, and its ment. Even arsenic, which is lethal in large quan- presence in certain forms is key to distinguishing tities, is a trace element in the human body, and organic from inorganic substances. Contrary to medicines for treating such illnesses as “sleeping popular belief, organic substances are not just sickness” contain tiny amounts of arsenic. Other living things, their parts, and their products. trace elements include cobalt, fluorine, man- Something that has never been living still can be ganese, molybdenum, nickel, selenium, silicon, considered organic, provided that it contains and vanadium. compounds that include carbon. (The only exceptions would be carbonates and carbon REAL-LIFE oxides, two groups of carbon-based compounds that are excluded from the ranks of organic sub- APPLICATIONS stances.) As we shall see, carbon, along with oxy- Nutrients gen and hydrogen, plays a key role in nutrition. Most of the remaining 7% of the body’s If you glance at the side of a cereal box, or virtu- mass is composed of ten other elements. Among ally any other food product manufactured in the these elements are the other three involved in United States, chances are that you will see a table
78 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 79
Nutrients and Nutrition
WHEN A PERSON INGESTS MORE CARBOHYDRATES THAN THE BODY NEEDS, THE BODY CONVERTS THE EXCESS INTO A COMPOUND KNOWN AS GLYCOGEN. IT THEN STORES THE GLYCOGEN IN THE LIVER (SHOWN HERE) AND MUSCLE TIS- SUES, WHERE IT REMAINS, A POTENTIAL SOURCE OF ENERGY FOR THE FUTURE. (© Lester V. Bergman/Corbis. Reproduced by permission.)
listing proportions of nutrients per serving. This have introduced offerings that represent a nod to is the “Nutrition Facts” label, which replaced the nutritional concerns. These include products old “Nutrition Information Per Serving” label in that are fat- or sodium-free, or are otherwise the early 1990s. Both forms of information label geared toward greater health consciousness. were administered by the United States Food and Clearly, diet is a significant concern to Drug Administration (FDA), with the newer Americans, the most well-fed group of people labeling format being far more extensive in terms that has ever existed, and terms from the Nutri- of the information it provides. tion Facts label—proteins, carbohydrates, fats, Usually these tables show the amount of minerals, and vitamins—are household words, nutrients both in terms of mass (usually ren- known to almost everyone but understood by dered in metric components, such as grams or only a few. In much of the remainder of this milligrams) and as a proportion of recommend- essay, we explore these concepts, discussing what ed daily value according to the FDA. These list- they mean in very basic scientific terms, as well as ings must include information about some com- in terms of their significance in the diets of ponents such as calories, fat, sodium, sugars, cer- humans and other animals. tain vitamins, and so on. In addition to these PROTEINS. Proteins are large mole- mandatory listings, labels may contain informa- cules built from long chains of amino acids, tion that the manufacturer chooses to provide which are organic compounds made of carbon, concerning other food components, such as hydrogen, oxygen, nitrogen, and (in some cases) potassium or insoluble fiber. sulfur bonded in characteristic formations. Pro- Today, even fast-food restaurants such as teins serve the functions of promoting normal McDonald’s, which is probably not the first name growth, repairing damaged tissue, contributing that comes to mind when one thinks of healthy to the body’s immune system, and making eating, provide extensive nutritional information enzymes. (An enzyme is a protein material that to customers. Additionally, makers of fast food or speeds up chemical reactions in the bodies of supermarket “junk food” such as potato chips plants and animals.)
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 79 set_vol3_3 8/19/02 10:40 AM Page 80
Nutrients Proteins in the human body contain about tend to drift to opposite ends of the molecule. As and 20 different amino acids, of which the body is a result, oil has the slippery texture for which it is Nutrition able to manufacture 12 from the foods we eat. known. The other eight, which the body requires for pro- With their affinity for nonpolar molecules, tein production but is unable to manufacture on lipids are soluble, or capable of dissolving, in oily its own, are known as the essential amino acids. or fatty substances but not in water. In the body, When a protein contains all of the essential lipids, like carbohydrates, supply energy, only in amino acids, it is known as a complete protein. different ways and on a different timetable. When Among the best forms of complete protein are burned, a gram of lipid actually produces about fish, red meat, and poultry as well as eggs, milk, three times as much energy as a gram of carbo- cheese, and other dairy products. Fittingly, a pro- hydrate, but this energy release takes place much tein that lacks at least one of the essential amino more slowly. Among the other functions per- acids is known as an incomplete protein. Exam- formed by lipids in the body are protection of ples include peas, beans, lentils, nuts, and cereal organs from shock and damage and the provi- grains. These can, however, be combined in such sion of insulation for the body, for instance, from a way as to make a complete protein, beans and toxins. (It is for this reason that toxins often are rice being a good example. stored in fat cells. See Food Webs for more about CARBOHYDRATES. Carbohydrates DDT bioaccumulation in the fat cells of ani- are natural compounds that consist of carbon, mals.) hydrogen, and oxygen and whose primary func- VITAMINS AND MINERALS. Vit- tion in the body is to supply energy. When a per- amins are organic substances that, in extremely son ingests more carbohydrates than his or her small quantities, are essential to the nutrition of body needs at the moment, the body converts the most animals and some plants. In particular, they excess into a compound known as glycogen. It work with enzymes in regulating metabolic then stores the glycogen in the liver and muscle processes, but they do not in themselves provide tissues, where it remains, a potential source of energy; thus, vitamins alone do not qualify as a energy for the body to use in the future. form of nutrition. Much the same is true of min- Sugars, starches, cellulose, and various other erals, except that they are inorganic substances. chemically related substances are part of the car- And whereas vitamins are usually chemically bohydrate group. Most carbohydrates are pro- complex (the formula for vitamin A, for instance, duced by green plants in the process of undergo- is C20H29OH), minerals may be as simple as a sin- ing photosynthesis. Nutritionally, the carbohy- gle element—for instance, iron or calcium. drates include sugar in its various forms as well Though the body can produce some vita- as another class of food that people do not always mins, in general, vitamins and minerals are sub- think of as carbohydrates: fruits. Additionally, stances that the body is incapable of making for such starchy foods as potatoes, rice, and wheat itself. Therefore, for optimal health, it is neces- products (bread, pasta, and so on) rank as sary to include them in the diet on a regular, if important carbohydrates, while cereal grains and not daily, basis. They also have in common the corn are examples of foods that contain both fact that the body needs them only in very small starchy carbohydrates and proteins. quantities, for which reason they are sometimes LIPIDS. All fats and oils are lipids; these known as micronutrients. substances are distinguished by the fact that they Vitamin A, for example, is a substance nec- are soluble only in compounds made of nonpo- essary to the functioning of the eye’s retina in lar molecules. Water is an example of a polar adjusting to light, and thus proper vitamin A lev- molecule, because the oxygen and hydrogen els are essential for night vision. Without vitamin atoms tend to occupy opposite “ends” of the mol- A, a person can be afflicted with a condition ecule, with one end exerting a negative electric known as night blindness, as well as with dryness charge and the other end a positive one. There- of the skin. Vitamin A is also essential to bone fore, water molecules tend to stick closely togeth- growth. This vitamin occurs naturally in such er. On the other hand, oil molecules, which con- foods as green and yellow vegetables, eggs, fruits, sist of carbon and hydrogen, are nonpolar, and liver and particularly in fish liver oils, such as because the atoms of the two elements do not cod liver oil.
80 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 81
Calcium, a mineral, helps build strong bones however, physicians, nutritionists, dieticians, and Nutrients and teeth. It also has a role in the normal func- other specialists had begun to question the and Nutrition tioning of nerve and muscle activity. Ninety-nine emphasis on carbohydrates in the USDA food percent of the body’s calcium is stored in the pyramid and other mainstream diets. For a young skeleton and teeth, while the remainder circulates person, whose body is still growing, the food in the bloodstream, where it helps make possible pyramid is a good dietary plan. But for a person muscle contractions. Bones are 70% calcium by past the early twenties, particularly those who are weight, which gives them their strength and overweight or suffering from a condition such as rigidity. Calcium, which is even more prevalent diabetes, other approaches may be needed. than iron, is the most abundant metallic element The average American adult is considerably in the human body. Good dietary sources of cal- overweight for his or her height and age group, a cium include milk and milk products, eggs, such fact for which a number of practices can be leafy green vegetables as spinach, and sardines. blamed. Among these practices are inactivity. The U.S. Department of Agri- Things have changed a great deal since our ances- culture Food Pyramid tors spent their whole lives in a flurry of physical activity, hunting animals for food and remaining The U.S. Department of Agriculture (USDA) has ever on the move. Our bodies themselves—a developed a diagram, called the food pyramid, to product of natural selection that took place over illustrate the components needed in a healthy countless generations (see Evolution)—know diet. The bottom and widest level of the pyramid nothing about this change. They are still pro- contains the cereal foods, such as breads, pastas, grammed to perform as they did 10,000 years and rice. Primarily carbohydrates, these foods are ago, storing fat for use in lean times. a major source of energy, and therefore the Thus, inactivity breeds obesity, a condition USDA recommends 6-11 servings of 1-2 oz that cannot be addressed successfully by diets (30–60 g) from this food group. As to the exact aimed simply at reducing consumption. In such number of servings, this is a function of such a situation, the body simply holds on to its fat variables as age, gender, weight, and degree of more fiercely, and this is one reason why a star- regular physical activity. vation diet is less than useless as a means of The second level of the food pyramid, which bringing about healthy weight loss. Starving one- is smaller than the first, consists of fruits and self also reduces lean muscle mass, which further vegetables. These foods, which are also primarily slows the metabolism and makes it still harder to carbohydrates, are especially important in sup- burn fat. In fact, one of the best ways to lose plying vitamins and minerals. A secondary func- weight is by combining resistance exercise (i.e., tion is the delivery of indigestible fiber, which weight lifting) with proper eating. improves the functioning and health of the large Then, of course, there are the things Ameri- intestine, or colon. From this group, the USDA cans eat: junk food and fried foods, for instance. recommends 5-9 servings a day. Eating junk food, pumped full of chemicals and At the third level of the pyramid are pro- white sugar, is like dumping garbage into a gas teins, including meats, eggs, beans, nuts, and tank. As for fried foods, an American seldom milk products. According to the USDA, the per- realizes how much is in his or her diet until mak- centage of these foods in one’s diet should be ing a visit overseas. In Germany, for instance, vir- much smaller than the percentage of carbohy- tually nothing is fried, and though people eat drates. Smaller still is the quantity of servings at hearty meals with plenty of sausage, potatoes, the top level, which contains the lipids. The small bread, and beer, obesity is far less of a problem in space allotted to this food emphasizes the fact Germany than in America. (Furthermore, the that fats and oils should be consumed in small American traveler is likely to have far better quantities for optimum health. bowel movements on the high-fiber German diet than on the greasy, fatty, salty American diet.) Fat and the American Diet The Case for Proteins What we have just described is the orthodox view of nutrition in the United States as the nation As healthy as a diet based on the food pyramid is, entered the twenty-first century. By that time, an overweight adult who stuck religiously to it
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 81 set_vol3_3 8/19/02 10:40 AM Page 82
Nutrients and Nutrition
THE AVERAGE AMERICAN ADULT IS CONSIDERABLY OVERWEIGHT, A RESULT OF INACTIVITY COUPLED WITH POOR DIET. THINGS HAVE CHANGED A GREAT DEAL SINCE OUR ANCESTORS SPENT THEIR LIVES IN A FLURRY OF PHYSICAL ACTIV- ITY, HUNTING ANIMALS FOR FOOD AND EVER ON THE MOVE, BUT THE HUMAN BODY IS STILL PROGRAMMED TO PER- FORM AS IT DID 10,000 YEARS AGO, STORING FAT FOR USE IN LEAN TIMES. (© Bettmann/Corbis. Reproduced by permission.)
most likely would remain overweight, even if he Malnutrition or she combined it with a regular program of As we noted earlier, corn has both protein and aerobic exercise (e.g., jogging). Thus, by the late carbohydrate components, but this does not twentieth century, numerous experts across a mean that a diet heavy in corn and corn products wide spectrum of health professions began to is a healthy one. Such a diet was not uncommon challenge the old orthodoxy that held up carbo- among poor people in the American South dur- hydrates as the central component of a healthy ing the late nineteenth and the early twentieth diet. Instead, a growing body of opinion favored centuries and among even poorer people in Mex- diets that rely more on proteins and even healthy ico and other parts of Latin America to the pres- fats, such as those in broiled or grilled salmon or ent day. With Southern foods, such as grits, olive oil. hominy, and cornbread, and Latin American Whereas carbohydrate consumption can foods, such as corn tortillas and polenta, it is help an athlete gain a burst of energy, for most quite possible to eat cheaply from a diet that people carbohydrates are simply the raw materi- relies primarily on corn. Yet someone who does als for fat, which the body will store when it dis- so is at risk of serious health problems, because covers that it does not need the carbohydrates corn is lacking in two essential amino acids, themselves as an immediate source of energy. lysine and tryptophan. Furthermore, the brain has a mechanism for sig- This is just one example of malnutrition, a naling the body that it has consumed enough condition that develops when the body does not protein, whereas there is no such mechanism obtain the right amount of the vitamins, miner- where carbohydrates are concerned. To test this, als, and other nutrients it needs to maintain try eating a meal of just protein: chances are that healthy tissues and organ function. Malnutrition you will feel you have had enough fairly quickly. occurs in people who are either undernourished On the other hand, try eating a meal of just or overnourished. Undernourishment is a conse- starches; you will find that you can eat and eat quence of consuming too few essential nutrients and eat, piling on calories as you do. or using or excreting them more rapidly than
82 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 83
Nutrients and KEY TERMS Nutrition
AMINO ACIDS: Organic compounds elements cannot be broken down chemi- composed of carbon, hydrogen, oxygen, cally into simpler substances. nitrogen, and (in some cases) sulfur bond- ENZYME: A protein material that ed in characteristic formations. Strings of speeds up chemical reactions in the bodies amino acids make up proteins. of plants and animals. AUTOTROPHS: Primary producers. ESSENTIAL AMINO ACIDS: Amino Autotroph means “self-feeder,” and these acids that cannot be manufactured by the organisms are distinguished by the fact body and which therefore must be that they do not depend on other organ- obtained from the diet. Proteins that con- isms as a source of energy. Instead, they tain essential amino acids are known as need only sunlight, water, and a few simple complete proteins. chemical compounds, from which they FOOD WEB: A term describing the build the large organic molecules that they interaction of plants, herbivores, carni- need to survive. vores, omnivores, decomposers, and detri- BIOGEOCHEMICAL CYCLES: The tivores in an ecosystem. Each of these changes that particular elements undergo organisms consumes nutrients and passes as they pass back and forth through the them along to other organisms (or, in the various earth systems (e.g., the biosphere) case of the decomposer food web, to the and particularly between living and nonliv- soil and environment). The food web may ing matter. The elements involved in bio- be thought of as a bundle or network of geochemical cycles are hydrogen, oxygen, food chains, but since the latter rarely exist carbon, nitrogen, phosphorus, and sulfur. separately, scientists prefer the concept of a food web to that of a food chain. CARBOHYDRATES: Naturally occur- ring compounds, consisting of carbon, LIPIDS: Fats and oils. With their affini- hydrogen, and oxygen, whose primary ty for nonpolar molecules, lipids dissolve function in the body is to supply energy. in oily or fatty substances but not in water- Included in the carbohydrate group are based liquids. In the body, lipids supply sugars, starches, cellulose, and various energy in slow-release doses, protect other substances. Most carbohydrates are organs from shock and damage, and pro- produced by green plants in the process of vide insulation for the body, for instance, undergoing photosynthesis. from toxins. MALNUTRITION: Any one of several COMPLETE PROTEIN: A protein that conditions that develop when the body includes all eight essential amino acids. does not obtain the right amount of vita- COMPOUND: A substance in which mins, minerals, and other nutrients it atoms of more than one element are bond- needs to maintain healthy tissues and ed chemically to one another. organ function. Malnutrition may result ELEMENT: A substance made up of from undernourishment or overnourish- only one kind of atom. Unlike compounds, ment.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 83 set_vol3_3 8/19/02 10:40 AM Page 84
Nutrients and Nutrition KEY TERMS CONTINUED
MINERALS: Inorganic substances that, PRIMARY CONSUMERS: Animals in a nutritional context, serve a function that eat green plants. Compare with sec- similar to that of vitamins. Minerals may ondary consumers. include chemical elements—particularly PRIMARY PRODUCERS: Green metallic ones, such as calcium or iron—as plants that depend on photosynthesis for well as some compounds. their nourishment. NUTRIENT: Materials essential to the PROTEINS: Large molecules built survival of organisms. They include pro- from long chains of amino acids. Proteins teins, carbohydrates, lipids (fats), vitamins, serve the functions of promoting normal and minerals. growth, repairing damaged tissue, con- tributing to the body’s immune system, NUTRITION: The series of processes by and making enzymes. which an organism takes in nutrients and SECONDARY CONSUMERS: makes use of them for its survival, growth, Ani- mals that eat other animals. and development. The term nutrition also can refer to the study of nutrients, their TRACE ELEMENT: A chemical element consumption, and their use in the organ- that appears within an organism or other ism’s body. natural system in very small quantities, or traces. In the human body, for instance, ORGANIC: At one time chemists used such trace elements as copper or iodine are the term organic only in reference to living essential to health, though they comprise things. Now the word is applied to most far less than 1% of the body’s mass. compounds containing carbon, with the VITAMINS: Organic substances that, in exception of carbonates (which are miner- extremely small quantities, are essential to als), and oxides, such as carbon dioxide. the nutrition of most animals and some PHOTOSYNTHESIS: The biological plants. In particular, vitamins work with conversion of light energy (that is, electro- enzymes in regulating metabolic processes; magnetic energy) from the Sun to chemical however, they do not in themselves provide energy in plants. In this process carbon energy, and thus vitamins alone do not dioxide and water are converted to sugars. qualify as a form of nutrition.
they can be replaced. This brings about the all- the average American. In America even the poor- too-familiar scenarios most of us associate with est of people are reasonably well fed, certainly malnutrition: scenes of starving children in the compared with the poor of Africa, Asia, or Latin third world, their bellies distended from kwashi- America. But that does not mean that malnutri- orkor (which we discuss next) and thus abnor- tion is not a problem in the affluent Western mally large, far out of proportion to their bony world. In the United States undernourishment arms and legs. may not be nearly as much of a problem, but Such scenes of horror, and the virtually dietary imbalances and excesses are, and they unimaginable misery of which they provide a have become associated with many of the leading hint, are almost beyond the comprehension of causes of death and disability. Overnutrition
84 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 85
results from eating too much, eating too many of outward manifestation of the condition is, of Nutrients the wrong things (including junk foods, especial- course, the swollen, bloated abdomen, brought and Nutrition ly those containing white sugar), not exercising about by a decrease in the amounts of the protein enough, or taking too many vitamins or other albumin (also found in egg whites) in the blood- dietary replacements. As with many things, stream. Skin discoloration may occur, along with where vitamins are concerned, it is not the case severe diarrhea, an enlarged liver, and atrophy, or that if a little is good, a lot is better. Vitamins can withering, of muscles and glands. Kwashiorkor only be absorbed by the body if ingested on a full can bring about retarded mental and physical stomach, and an excess of fat-soluble vitamins development as well, but at least there is a treat- such as A or D could actually be toxic. (See Vita- ment: adding proteins to the diet. For this reason, mins for more on this subject.) aid organizations supply powdered milk, which, KWASHIORKOR. For any nutrient if administered to these undernourished children that the body requires, there is a corresponding in time, can save them from further damage to disease, ailment, or condition (or many of them) their bodies and minds. that will develop if a person’s body is deprived of MALNUTRITION IN THE UNIT- that nutrient. For instance, people whose diets ED STATES. Malnutrition exists even in the lack protein may become susceptible to a condi- United States and not only in the form of tion known as kwashiorkor, characterized by apa- overnourishment. Particularly among the thy, exhaustion, fatigue, wasting of muscle tissue, nation’s poor, undernourishment is serious, and edema (swelling, a result of water collecting though it is not necessarily because food is in the body.) unavailable to poor families. Rather, the right Kwashiorkor, in fact, is the condition that food may be unavailable, because the health causes swelling in the stomachs of children suf- foods described at the conclusion of this essay fering from malnutrition. When people eat a diet tend to be expensive. Furthermore, lack of edu- consisting mainly of starchy vegetables, as is cation and information ensures that poor fami- common in parts of Africa, Latin America, and lies maintain destructive dietary practices that southern Asia, they consume an appropriate leave their bodies deprived of essential nutrients. amount of calories, but they do not get certain Low-cost, high-bulk items, such as sugar, white essential amino acids that are important for flour, and corn meal, may provide immediate sat- growth. This is particularly serious for children. isfaction to the stomach, but they leave the body Nursing babies usually do not suffer from kwa- undernourished in the long run. shiorkor, because they receive adequate protein Nevertheless, the most prevalent form of from their mother’s milk. A child who is weaned, malnutrition in America is still overnourish- however, is at much greater risk. ment. Implied in the term overnourishment, of In much of the third world, where human course, is the idea that the person so afflicted is populations are high relative to the number of not eating the right kinds of things, because it is animals raised for meat, meat itself is a luxury. A hard to be overnourished with a proper diet. child of a poor family is unlikely to have meat in Junk foods, such as one finds in the candy aisle of his or her diet, and without the mother’s milk the a convenience store or in a fast-food restaurant, child is left virtually without sources of protein. contain carbohydrates and some proteins (not to This situation may have informed the choice of mention plenty of fats); usually, these nutrients the word kwashiorkor to describe this condition. are presented in such a way as to maximize taste Derived from the Ga language of coastal Ghana and minimize nutritional value. The result is in Africa, the word is based on kwàsìokó, a Ga “empty calories” that simply go to the waistline, term for “the influence a child is said to be under buttocks, and abdomen of the consumer. when a second child comes.” In regions where America is awash in food, but very little of it birth control is unavailable or simply is not used, is the kind of food that sustains the health of the it is easy to imagine how an older child would be body or prolongs life. People have been condi- forced to give up the mother’s milk to make tioned by advertising to believe that white bread room for a second child. is the only kind of bread (even though the The results are devastating for the child process of refining white flour leaches out most deprived of protein sources. The most shocking of the nutrients); that it is possible to have “zero
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 85 set_vol3_3 8/19/02 10:40 AM Page 86
Nutrients fat” cookies sweetened with white sugar (which Everything Else We Eat and Drink. New York: and simply turns to fat as soon as the body digests it); William Morrow, 1995. Nutrition and that a diet of burgers, fries, and soft drinks is Food and Nutrition Information Center, National Agricul- tural Library, U.S. Department of Agriculture (Web “normal.” The result is an overnourished, over- site).
86 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 87
Vitamins VITAMINS
CONCEPT about vitamins per se until that time, folk wis- Most of us have been told to take our vitamins, dom certainly had taken account of the fact that but few people know why, and despite all the talk certain foods are essential to the health and well- about them in modern culture, vitamins remain being of humans and animals. something of a mystery. Vitamins are organic Vitamins may be defined as organic sub- substances, essential for maintaining life func- stances, found in food, that are essential in very tions and preventing disease among humans and small quantities for the health of most animals animals and even some plants. They are found in and some plants. Organic substances, discussed very small quantities in food; certain health spe- in The Biosphere, are compounds (substances in cialists recommend taking vitamin supplements which atoms of more than one element are to augment the supplies in food, while others chemically bonded to one another) containing insist that a well-balanced diet provides all the hydrogen and carbon. Primarily, vitamins work vitamins that an ordinary person needs. Some with enzymes (protein materials that speed up vitamins, such as vitamin C and the B complex, chemical reactions in the bodies of plants and are water-soluble, which means that they are animals) in regulating metabolic processes—that excreted easily and must be ingested every day. is, processes that convert food to energy. They do Others, such as vitamins A, D, E, and K, are fat- not in themselves provide energy, however, and soluble and therefore are retained in the body’s thus vitamins alone do not qualify as a form of fatty tissues. With such vitamins, there may be a nutrition. danger of taking too much, but in the case of most vitamins, the greatest harm comes from not Organisms require vitamins only in very receiving enough. Vitamin deficiencies can be the small amounts: the total amount of vitamin mass cause of rickets, pellagra, and other diseases that a person needs in one day, for instance, is only have plagued the poor in the Western world and about 0.0011 lb. (0.5 g). Yet vitamins are the third world in the past and in the present. absolutely essential to the maintenance of health and for disease prevention, and most animals are not capable of synthesizing or manufacturing HOW IT WORKS vitamins on their own. Nonetheless, most ani- mals can produce vitamin C, though there are An Introduction to Vitamins exceptions—humans included. Once they were called vitamines, but for reasons Animals depend on plants for their nutri- that we address later, the “e” was dropped, and tion, either directly or indirectly (i.e., either by they became known as vitamins. There is also a consuming the plant or by consuming an animal reason for the strange alphabet of vitamins (A, B, that has consumed the plant). Plants, on the C, D, E, K), which, like the change in spelling, other hand, are autotrophs, meaning that they came out of the early days of scientific research can meet their nutritional needs with only sun- into the subject during the first third of the twen- light, water, and a few chemical compounds. tieth century. Though people did not know Among the nutrients plants produce are vita-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 87 set_vol3_3 8/19/02 10:40 AM Page 88
Vitamins mins, which they pass on to animals that con- today the only alphabetical vitamin names are A, sume them directly or indirectly. (See Food Webs the B complex, C, D, E, and K. for more about autotrophs and the relationship of animal consumers to them.) REAL-LIFE Classifying Vitamins APPLICATIONS Numerous vitamin groups are necessary for the Fat-Soluble Vitamins nutritional needs of humans, and though only minute amounts of each are required to achieve We are accustomed in modern life to being told their purpose, without them life could not be that fat is bad for us, but to quote a much-cited maintained. Some vitamins, including A, D, E, line from the American composer George Gersh- and K, are fat-soluble, meaning that they are win’s opera Porgy and Bess, “It ain’t necessarily found in fattier foods and in body fat. Thus, they so.” Fat is not inherently bad for people; in fact, a can be stored in the body; for this reason, it is not certain amount in the diet is essential. The prob- necessary to include them in the diet every day. lem in America today is the type of fat that peo- In fact, it could be dangerous to do so, since it is ple consume. There is a big difference between possible that they would build up to toxic levels the healthy, natural, unsaturated fats one might in the tissues. Other vitamins, the most notable find, say, in fresh salmon, and the highly of which are vitamin C and the many vitamins in processed and saturated fats in a bag of potato what is known as the “B complex,” are water-sol- chips. (The term saturated means that every gap uble. They are found in the watery parts of food in which a hydrogen atom could fit in a string of and body tissue, and because they are excreted carbon and hydrogen atoms has been filled. This regularly in the urine, they cannot be stored by helps make fats firm, for use in such products as the body. Instead, they must be consumed on a shortening.) daily basis. This difference in solubility is extremely important to the way the vitamins Such fat is extremely harmful, because the function within an organism and in the ways and body is not able to process it; even so, a certain amounts in which they are consumed. amount of natural fat in the diet can be highly beneficial. This is true in large part because fat THE NAMES OF VITAMINS. Vit- can serve as a medium for the fat-soluble vita- amins originally were classified in terms of their mins A, D, E, and K, which are deposited in the solubility in water or in fat, and these distinctions body’s fat cells. But as we noted earlier, it is remain important for the reasons outlined above. important not to overdose on fat-soluble vita- Today, vitamins are known primarily by letters of mins, because then what is inherently healthy can the alphabet, a fact that harks back to a naming become extremely unhealthy. system developed as more and more vitamins were discovered in the early years of the twenti- VITAMIN A. In 1596 the Dutch explor- eth century. er Willem Barents (1550–1597) and his ship- As scientists detected the existence of more wrecked crew spent a grueling winter on the vitamins, or what they thought were vitamins, island of Novaya Zemlya in the Arctic Ocean they assigned to them successive letters of the north of Russia. They had sailed from Holland in alphabet: A, B, C, and so on. Eventually, however, search of the Northeast Passage, which, like the they discovered that some substances originally more famous Northwest Passage above Canada, thought to be vitamins were not vitamins, and offered the prospect of a short, relatively direct they removed them from the roster. For example, sea route from Europe to Asia and the Americas. what used to be called vitamin F is simply an The problem was that the ice made sailing the essential fatty acid, a necessary component of the northern seas virtually impossible. It would be diet of a mammal but not the same thing as a almost three centuries before a crew managed to vitamin. In other cases, what were once believed negotiate the Northeast Passage, by which time to be individual vitamins later were subsumed the European powers had long since given up all into the B complex. Among these substances are hopes of using it as a viable sailing route. (The riboflavin, formerly termed vitamin G, and same was true of the Northwest Passage, which biotin, once called vitamin H. The result is that was not traversed until 1906.)
88 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:40 AM Page 89
Vitamins
THE DUTCH EXPLORER WILLEM BARENTS AND HIS SHIPWRECKED CREW EXPERIENCED VITAMIN A POISONING WHEN THEY ATE POLAR BEARS TO STAY ALIVE ON THE ISLAND OF NOVAYA ZEMLYA IN THE ARCTIC OCEAN. POLAR BEAR LIVER CONTAINS ABOUT 450 TIMES THE RECOMMENDED DAILY DOSE OF VITAMIN A; SYMPTOMS OF POISONING INCLUDE PAINFUL JOINTS, BONE THICKENING, PEELING OF THE SKIN, AND LIVER DISEASE. (© Bettmann/Corbis. Reproduced by per- mission.)
Barents and his men knew none of that, nor ents included—never lived to see Holland again, would they have cared in that miserable winter of in part because the side effects of vitamin A tox- 1596–1597. All they cared about was survival, the icity had weakened them. chances for which seemed slim—and not just So why take vitamin A at all? Because it is because of the almost inhuman cold or the fact necessary for proper growth of bones and teeth, that their ship had been cracked to pieces by the for the maintenance and functioning of skin and ice. Men were dying of scurvy, a vitamin-defi- mucous membranes, and for the ability to see in ciency disease we discuss later in the context of dim light. There is some evidence that it can help vitamin C, as well as from the cold. Yet there were prevent cataracts (a clouding of the lens in the a few blessings, mainly in the form of available eye) and cardiovascular disease, a condition of wood for fuel and animals for food. The men the heart and circulatory system. Furthermore, killed polar bears and ate their meat, and no doubt they were thankful just to stay alive. They when taken at the onset of a cold, vitamin A can could not have guessed, however, that they were ward off the illness and fight its symptoms. actually killing themselves with an overdose of One of the first signs of vitamin A deficien- vitamin A. cy is “night blindness,” in which the rods of the Just 1 lb. (0.454 kg) of polar bear liver con- eye (necessary for night vision) fail to function tains about 450 times the recommended daily normally. Extreme cases of vitamin A deficiency dose of vitamin A, and the men in Barents’s expe- can lead to total blindness. Other symptoms dition were absorbing far more of the vitamin include dry and scaly skin, problems with the than they should have. In time, they began to mucous linings of the digestive tract and urinary experience the effects of vitamin A poisoning: system, and abnormal growth of teeth and bones. painful joints, bone thickening, peeling of the The bodies of healthy adults who have an ade- skin over the entire body, and chronic liver dis- quate diet can store several years’ supply of this ease. When spring came, the men managed to vitamin, but young children, who have not had make it off the island, but many of them—Bar- time to build up such a large reserve, suffer from
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 89 set_vol3_3 8/19/02 10:41 AM Page 90
Vitamins
DURING THE GREAT DEPRESSION, A CHILD IN THE SOUTHERN UNITED STATES SHOWS SIGNS OF RICKETS, STEM- MING FROM VITAMIN D DEFICIENCY. UNDER THE INFLUENCE OF THIS DEBILITATING AND DISFIGURING DISEASE, THE LEGS BECOME BOWED BY THE WEIGHT OF THE BODY, AND THE WRISTS AND ANKLES THICKEN. (© Marion Post Wolcott/Cor- bis. Reproduced by permission.)
deprivation much more quickly if they do not nearly impossible to ingest beta-carotene in toxic consume enough. amounts, unlike vitamin A from animal sources, Vitamin A is present in meats (mainly liver), since the body will not convert excess amounts to fish oil, egg yolks, butter, and cheese. Although toxic levels of vitamin A. plants do not have vitamin A, dark green leafy VITAMIN D. Vitamin D is actually two
vegetables and yellow fruits and vegetables (e.g., different substances, D2 and D3. (There was no carrots, sweet potatoes, cantaloupe, corn, and D1, since the substance designated thus at one peaches) contain a substance called beta- time turned out to be a mixture of several com-
carotene, which is converted to vitamin A in the pounds, including calciferol, or D2) Both forms intestine and then absorbed by the body. It is of vitamin D are activated, or made effective, by
90 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:41 AM Page 91
sunlight, and for this reason vitamin D often is When an apple turns brown a few minutes after Vitamins called the sunshine vitamin. It is hard to suffer a you have cut it open, it is the result of oxidation. vitamin D deficiency if one gets enough sunshine Oxidation also may be linked to the effects in combination with consuming such foods as of aging in humans as well as to other conditions, eggs (specifically, the yolk), such fatty fish as such as cancer, hardening of the arteries, and salmon, and enriched milk. (Milk does not natu- rheumatoid arthritis. It appears that oxygen mol- rally contain vitamin D, but the vitamin is some- ecules, which draw electrons to them, extract times included as an additive.) these electrons from the membranes in human Vitamin D lets the body utilize calcium and cells. Over time, this can cause a gradual break- phosphorus in bone and tooth formation, and a down in the body’s immune system. Antioxi- deficiency causes a bone disease called rickets. dants, such as vitamin E or beta carotene, there- Under the influence of this physically debilitating fore may be important in preserving human and disfiguring disease, legs become bowed by the health and well-being. weight of the body, and the wrists and ankles Vitamin E is particularly important for thicken. The teeth are badly affected and, for a counteracting oxidation in fats. When they are young child, take much longer to mature. Infants oxidized, fats form a highly reactive substance and children are most likely to suffer the effects of called peroxide, which is often very damaging to rickets, but since all milk and infant formulas have cells. Vitamin E is more reactive (i.e., more likely vitamin D added to them, the condition is seen to form or break chemical bonds) than the fatty rarely in the industrialized world today. In the bru- acid molecule, and, therefore, the vitamin reacts tal early days of the Industrial Revolution, howev- instead of the fat. Because cell membranes are er (i.e., in England ca. 1760–1830), crowded slum composed partly of fat molecules, vitamin E is conditions in areas where there was little or no vitally important in maintaining the nervous, sunlight made possible many cases of rickets. circulatory, and reproductive systems and in pro- Whereas rickets primarily affects children, tecting the kidneys, lungs, and liver. adults may suffer from a disease called osteoma- Because vitamin E is so common in foods, it lacia, caused by a deficiency of vitamin D, calci- is very difficult to suffer from a deficiency of this um, and phosphorous. Sometimes seen in the vitamin unless a person avoids consuming fats Middle East and other parts of Asia, osteomalacia altogether—another example of why a no-fat brings with it rheumatic pain and causes the diet is not a healthy one. The effects of vitamin E bones to become soft and deformed. As with deficiency, all of which are apparently linked to rickets, the treatment for osteomalacia is a com- the loss of its antioxidant protection, include bination of calcium, phosphorous, and vitamin cramping in the legs, fibrocystic breast disease (a D. On the other hand, as with all fat-soluble vita- condition that involves the formation of lumps mins, a person may take in excessive amounts of and cysts in the breasts), and even muscular dys- vitamin D, which has its own ill effects: nausea, trophy. The seriousness of the latter two diseases diarrhea, weight loss, and pain in the bones and only serves to highlight the importance of vita- joints. Damage to the kidneys and blood vessels min E to the body. also can occur as calcium deposits build up in VITAMIN K. Like vitamin D, vitamin K these tissues. is composed of two groups of compounds, vita-
VITAMIN E. Composed of at least seven mins K1 and K2. There is also a substance called similar chemicals called the tocopherols, vitamin K3, but this vitamin is actually menadione, a syn- E is found in green leafy vegetables, wheat germ thetic compound from which the other forms of and other plant oils, egg yolks, and meat. The K are derived. You can find vitamin K in many main function of this vitamin is to act as an plants, especially green leafy ones such as antioxidant, to counteract the harmful effects spinach, and in liver. Vitamin K is also made by oxygen can have on tissues. It may seem strange the bacteria that live in the intestine—the “good” to speak of oxygen causing harm, since it is bacteria that help make possible the processing of essential to life, but oxidation is an extremely food through the body. powerful chemical reaction that, under various Vitamin K appears to be critical to blood conditions, can manifest as rotting or putrefac- clotting, thanks to its role in assisting the forma- tion, rusting, or even combustion and explosion. tion of a chemical called prothrombin in the liver.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 91 set_vol3_3 8/19/02 10:41 AM Page 92
Vitamins names,” while biotin and folate, or folic acid, are not known by “B names” at all.
Vitamin B1, present in whole grains, nuts, legumes (e.g., peas), pork, and liver, helps the body release energy from carbohydrates. More than 4,000 years ago, the Chinese described a dis- ease we know today as beriberi, which affects the nervous and gastrointestinal systems and causes nausea, fatigue, and mental confusion. The cause
of beriberi is a deficiency of thiamine, or B1, found in the husks or bran of rice and grains. White rice, which most people find more pleas- ing to the palate than brown rice, is the result of a milling and polishing process in which the husks—and along with them, this important nutrient—are removed. Manufacturers today produce “enriched” rice, flour, and other grain products by adding thiamine back in, but until scientists discovered the importance of thiamin in grain husks, many people, especially in the Far East, suffered the effects of beriberi. (Early research on beriberi will be discussed later.) Vitamin B helps the body release energy THE AMERICAN CHEMIST LINUS PAULING (SHOWN 2 TOSSING AN ORANGE), WINNER OF THE NOBEL PRIZES from fats, proteins, and carbohydrates. It can be IN CHEMISTRY AND IN PEACE, HELPED POPULARIZE VITA- obtained from whole grains, organ meats (e.g., MIN C, ALSO KNOWN AS ASCORBIC ACID. (© Roger Ress- liver), and green leafy vegetables. Lack of this meyer/ Corbis. Reproduced by permission.) vitamin causes severe skin problems. Vitamin B6 is important in the building of body tissue as well Deficiencies of this vitamin rarely occur as the as in protein metabolism and the synthesis of result of an incomplete diet; instead, it is usually hemoglobin (an iron-containing pigment in red a consequence of liver damage and the blood’s blood cells that is responsible for transporting inability to process the vitamin. The deficiency oxygen to the tissues and removing carbon diox- manifests in unusual bleeding or large bruises ide). A deficiency can cause depression, nausea, under the skin or in the muscles. Adults in the and vomiting. Vitamin B12 is necessary for the West seldom experience vitamin K deficiencies, proper functioning of the nervous system and in but newborn infants have been known to suffer the formation of red blood cells. It can be from brain hemorrhage owing to a lack of this obtained from meat, fish, and dairy products. vitamin. Anemia (a lack of red blood cells, which pro- duces a lethargic condition), nervousness, Water-Soluble Vitamins fatigue, and even brain degeneration, can result from vitamin B12 deficiency. B VITAMINS. The two water-soluble Niacin is also highly important to human vitamins, as we shall see, have played a major part health, as we explain later in the context of the in medical history. Actually, there are more than disease pellagra. Pantothenic acid helps release two water-soluble vitamins, because vitamin B is energy from fats and carbohydrates and is found really a complex of about a dozen vitamins— in large quantities in egg yolks, liver, eggs, nuts, hence, the name B complex. Among them are and whole grains. Deficiency of this vitamin vitamin B1 (thiamine), vitamin B2 (riboflavin), causes anemia. Biotin, widely available from vitamin B6 (pyridoxine), and vitamin B12 (cobal- grains, legumes, and liver, plays a part in the amin). A few others—for example, niacin (vita- release of energy from carbohydrates and in the
min B7) and pantothenic acid (vitamin B3), are formation of fatty acids. A lack of biotin causes known better by names other than their “B dermatitis, or skin inflammation.
92 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:41 AM Page 93
VITAMIN C. The American chemist and vitamins was unknown, and sailors at sea contin- Vitamins peace activist Linus Pauling (1901–1994), winner ued to live on a diet that consisted primarily of of the Nobel Prize in chemistry (1954) and peace salted meats and hard biscuits—items that could (1962), helped popularize vitamin C, also known be stored easily without spoilage in an era before as ascorbic acid. It was Pauling who originated refrigeration. the idea, now widespread in society, that massive In 1746, Lind, a ship’s doctor, observed that doses of vitamin C can ward off the common 80 of 350 seamen aboard his ship came down cold. Pauling went further, by maintaining that with scurvy during a 10-week cruise. Conducting vitamin C offers protection against some forms a controlled experiment, he took 12 of the sailors of cancer. While scientific studies have been in whom scurvy had developed and divided them unable to prove this theory, they do suggest that into six groups. He gave each pair different sub- the vitamin can at least reduce the severity of the stances, such as nutmeg, cider, seawater, and symptoms associated with colds. vinegar; the final pair was given lemons or Most animals can synthesize this vitamin in oranges. The two men given the oranges and the liver, where glucose (a type of sugar that lemons both completely recovered in about a occurs widely in nature) is converted to ascorbic week. Not only was this a milestone in the histo- acid. This is not the case with at least four types ry of vitamin research, but it also was the first of animal: monkeys, guinea pigs, Indian fruit example of a clinical trial, or the testing of a med- bats, and humans, all of which must obtain vita- ication by careful and well-documented experi- min C from their diets. Citrus fruits, berries, and mentation in which other variables or factors are some vegetables (e.g., tomatoes and peppers) kept unchanged. are good sources of vitamin C. It is a fragile vita- min, one that is oxidized or destroyed easily. It would be another half-century before the Food storage or food processing can render it British navy adopted Lind’s techniques. Another ineffective; so, too, can soaking vitamin C–con- Scottish physician, Sir Gilbert Blane (1749– taining fruits and vegetables in water for long 1834), had long fought for the adoption of Lind’s periods. methods, and finally, in 1796, he persuaded the navy to give each sailor a daily ration of lemons. Vitamin Deficiencies and At that time, the term lime was common for both History lemons and limes, and, as a result, British sailors became known as limeys. Eventually, the treat- Most of the early history in the study of vitamins ment spread to the population as a whole, but centered around what are now known as the outbreaks of scurvy continued until after World water-soluble vitamins. Although vitamins as War I, when doctors isolated vitamin C as the such were not discovered until early in the twen- controlling factor in scurvy prevention. tieth century, it was common knowledge long before that time that substances in certain foods BERIBERI IN THE DUTCH were necessary for good health. An important EAST INDIES. In 1897 the Dutch govern- turning point came in the mid–eighteenth centu- ment sent the physician Christiaan Eijkman ry, with the work of the Scottish physician James (1858–1930) as part of a government commis- Lind (1716–1794) on a vitamin deficiency condi- sion to the Dutch East Indies (now Indonesia), tion that jeopardized England’s vast merchant which had been long afflicted by a condition and military navies. known as beriberi. There are various forms of At a time when England had emerged as the this disease, including infant beriberi, which can world’s leading sea power, even Her Majesty’s kill a breast-feeding baby after the fifth month, as sailing crews were at the mercy of a condition well as various juvenile and adult forms. In the known as scurvy. Common among crews who childhood and adult versions of the disease, there had been at sea too long, scurvy could result in is a preliminary condition of fatigue, loss of swollen joints, bleeding gums, loose teeth, and an appetite, and a numb, tingling feeling in the legs. inability to recover from wounds. Scientists today This condition can lead to either wet beriberi, recognize scurvy as resulting from a deficiency of characterized by the accumulation of fluid vitamin C, available in such citrus fruits as throughout the body and a rapid heart rate that oranges. At the time, however, the concept of can bring about sudden death, or dry beriberi,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 93 set_vol3_3 8/19/02 10:41 AM Page 94
Vitamins KEY TERMS
AMINO ACIDS: Organic compounds METABOLISM: The chemical process made of carbon, hydrogen, oxygen, nitro- by which nutrients are broken down and gen, and (in some cases) sulfur bonded in converted into energy or used in the con- characteristic formations. Strings of amino struction of new tissue or other material in acids make up proteins. the body. ANEMIA: A condition marked by a lack MINERALS: Inorganic substances that, of red blood cells or hemoglobin or a in a nutritional context, serve a function shortage in total blood volume, any one of similar to that of vitamins. Minerals may which can produce a lethargic condition. include chemical elements, particularly ANTIOXIDANT: An enzyme, or some metallic ones, such as calcium or iron, as other organic substance, that is capable of well as some compounds. counteracting the negative impact of oxygen ORGANIC: At one time chemists used (which draws electrons to it) on living tissue. the term organic only in reference to living CARBOHYDRATES: Naturally occur- things. Now the word is applied to com- ring compounds, consisting of carbon, pounds containing carbon and hydrogen. hydrogen, and oxygen, whose primary PROTEINS: function in the body is to supply energy. Large molecules built Included in the carbohydrate group are from long chains of amino acids. Proteins sugars, starches, cellulose, and various serve the functions of promoting normal other substances. growth, repairing damaged tissue, con- tributing to the body’s immune system, COMPOUND: A substance in which atoms of more than one element are bond- and making enzymes. ed chemically to one another. TISSUE: A group of cells, along with ENZYME: A protein material that the substances that join them, that form speeds up chemical reactions in the bodies part of the structural materials in plants or of plants and animals. animals. GLUCOSE: A type of sugar that occurs VITAMINS: Organic substances that, in widely in nature. Glucose is the form in extremely small quantities, are essential to which animals usually receive carbohy- the nutrition of most animals and some drates. plants. In particular, vitamins work with HEMOGLOBIN: An iron-containing enzymes in regulating metabolic processes; pigment in red blood cells that is responsi- however, they do not in themselves provide ble for transporting oxygen to the tissues energy, and thus vitamins alone do not and removing carbon dioxide. qualify as a form of nutrition.
which is marked by a loss of sensation and weak- Experimenting with birds, Eijkman noticed ness in the legs. The patient first needs to walk that some of the fowl experienced paralysis and with the aid of a stick and then becomes bedrid- polyneuritis (a disorder affecting several nerves at den and easy prey to infectious diseases. once), as in the dry form of beriberi. The director
94 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_3 8/19/02 10:41 AM Page 95
of the hospital had told Eijkman that he could happened that corn products constituted a major Vitamins not feed the birds with table scraps from the din- part of the diet in areas suffering from high rates ing hall, where the diet was heavy in polished rice. of pellagra. The poor of Spain and Latin Ameri- The doctor thus was forced to feed his birds ca subsisted on a diet heavy in corn products, whole rice, and something amazing happened: such as tortillas and polenta, while their counter- the birds began to regain their ability to move parts in the southern United States survived on and experienced no recurrence of paralysis. cornbread, grits, hominy, and other variants of Eijkman’s colleagues rejected his claim that corn. Although such foods made it possible to fill the birds had contracted some form of beriberi, the stomach cheaply, this diet was killing people though, in fact, he was correct. He was incorrect, in large numbers because it was not delivering however, in his supposition that the polished rice the essential B vitamin niacin. contained a toxin that was missing from the MODERN KNOWLEDGE OF VIT- whole, unpolished rice. After Eijkman and the AMINS. As it turned out, corn, in fact, does rest of the medical commission left the East contain niacin, but to release the niacin from the Indies, another Dutch physician, Gerrit Grijns large, fibrous molecules in corn, it is necessary to (1865–1944), stayed on to study the disease. He treat the corn with an alkaline solution, such as discovered that when the chickens were taken off limewater. This is just one example of the vast the rice diet completely and fed meat instead, knowledge that has accumulated since the time they did not show signs of the characteristic when the Polish-American biochemist Casimir paralysis; if the meat was overcooked, however, Funk (1884–1967) coined the term vitamine in the condition reappeared. In 1901, Grijns showed 1912. The first half of the word came from the that beriberi could be cured by putting the rice Latin vita, or “life,” and the second half reflected polishings back into the rice. As it turned out, the Funk’s belief that all these substances belonged to husks and the meat contained vitamin B , also 1 a group of chemicals known as amines. Scientists present in wheat germ, whole grain and enriched bread, legumes, peanuts, and nuts. later dropped the “e” when they discovered that not all vitamins contain an amine group. PELLAGRA IN THE AMERICAS. A vitamin-deficiency disease often associated In the director Stephen Spielberg’s with poverty, pellagra produces symptoms acclaimed 1987 film Empire of the Sun, one of the known as the “three Ds”: diarrhea, dermatitis, characters asks another, “Do you believe in vita- and dementia, or mental deterioration. It was mins?” The question reflects the relative newness first identified in 1762 by the Spanish physician of vitamins as an idea at the time when the movie Gaspar Casal (ca. 1680–1759), who wrote about was set, during World War II. After the war, inter- the “mal de la rosa”—so called because of the est in commercially produced vitamin supple- reddened dermatitis that appeared around the ments exploded, particularly among America’s back of the victim’s neck—that afflicted sufferers middle classes. Pauling’s promotion of vitamin in one particular region of Spain. C, coming in the 1950s and 1960s, found a will- Casal was far ahead of his time in maintain- ing audience. ing that inadequate nutrition caused pellagra, Although vitamin supplements remain a big but for many centuries the belief persisted that business today, opinions vary as to the impor- the disease resulted from infection. The break- tance of enhancing one’s diet with them. Many through came with the work of the American nutritionists insist that eating a well-balanced physician Joseph Goldberger (1874–1929), a diet, consisting of the major food substances, is member of the U.S. Public Health Service who an effective and economical way to obtain nutri- studied the high numbers of pellagra cases ents for health. On the other hand, advocates of among poor blacks and whites in the southern health foods and alternative medicine (medical United States. Goldberger established that pella- practices that are not recognized officially by gra stems from insufficient niacin, which is groups of university-trained and state-licensed required to release energy from glucose. medical doctors) insist that the recommended Niacin is present in whole grains, meat, fish, daily allowances established by the U.S. Food and and dairy products. One of the foods from which Drug Administration do not provide sufficient niacin is not easily available is corn, and it so vitamins for an average person.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 95 set_vol3_3 8/19/02 10:41 AM Page 96
Vitamins The American Dietetic Association (ADA) Kiple, Kenneth F., and Kriemhild Coneè Ornelas. The recommends that nutrient needs come from a Cambridge World History of Food.New York:Cam- variety of foods taken from different dietary bridge University Press, 2000. sources rather than from self-prescribed vitamin Nardo, Don. Vitamins and Minerals. New York: Chelsea supplements. The organization makes House Publishers, 1994. allowances, however, for supplement usage by Reference Guide for Vitamins (Web site). people who need extra doses of key vitamins and
96 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 97
SCIENCE OF EVERYDAY THINGS real-life biology GENETICS
GENETICS HEREDITY GENETIC ENGINEERING MUTATION
97 set_vol3_4 8/19/02 10:45 AM Page 99
Genetics GENETICS
CONCEPT much as possible. For this reason, the Heredity Genetics is the area of biological study concerned essay is concerned with such issues as how traits with heredity and with the variations between are passed on and why they appear in a particular organisms that result from it. It demands an generation but not another. That essay addresses understanding of numerous terms, such as DNA the topics of alleles, dominant and recessive genes, (deoxyribonucleic acid), a molecule in all cells and so on. It also briefly discusses the history of that contains blueprints for genetic inheritance; studies in areas that encompass genetics, heredity, genes, units of information about particular her- and the mechanics thereof. In general, the Hered- itable traits, which are made from DNA; and ity essay is concerned with the larger patterns of chromosomes, DNA-containing bodies, located inheritance over the generations, while the pres- in the cells of most living things, that hold most ent one examines inheritance at a level smaller of the organism’s genes. The vocabulary of genet- than the microscopic—that is, from the molecular ics goes far beyond these three terms, as we shall or biochemical level. see, but these are the core concepts. Among the areas in which genetics is applied is forensic sci- Somatic and Germ Cells ence, or the application of science to matters of Heredity begins with the cell, the smallest basic law—specifically, through “DNA fingerprinting,” whereby samples of skin, blood, semen, and unit of all life. The information for heredity is other materials can be used to prove or disprove carried within the cell nucleus, which is the con- a suspect’s innocence. Another fascinating appli- trol center not only in physical terms (it is usual- cation of genetics is the Human Genome Project, ly located near the middle of the cell) but also an effort whose goals include the location and because it contains the chromosomes. Within identification of every gene in the human body. these threadlike structures is the genetic infor- mation organized in DNA molecules. HOW IT WORKS There are two basic types of cell in a multi- cellular organism: somatic, or body, cells, and Genetics and Heredity germ, or reproductive, cells. The somatic cells are the primary components of most organisms, Genetics and heredity, the subject of another essay making up everything except some of the the in this book, are closely related ideas. Whereas cells in reproductive organs. The somatic cells of heredity is the transmission of genetic character- humans have 23 pairs of chromosomes, or 46 istics from ancestor to descendant through the chromosomes overall, and are thus known as genes, genetics is concerned with hereditary traits diploid cells. As the cells grow, they reproduce passed down from one generation to the next. It is themselves by a process called mitosis, whereby a very hard, if not impossible, to separate the two diploid cell splits to produce new diploids, each concepts completely, yet the entire body of knowl- of which is a replica of the original. Thus cells edge encompassed by these topics is so large and grow and are replaced, making possible the for- so complex that it is best to separate them as mation of specific tissues and organs, such as
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 99 set_vol3_4 8/19/02 10:45 AM Page 100
Genetics muscles and nerves. Without mitosis, an organ- the DNA molecule is made up of a combination ism’s cells would not regenerate, resulting not of two of these letters. only in cell death but possibly even the death of DNA SEQUENCES. In this genetic the entire organism. Mitosis is also the means of code A always combines with T and C with G, to reproduction for organisms that reproduce asex- form what is called a base pair. Specific ually (see Reproduction). sequences of these base pairs make up the genes. A germ cell, by contrast, undergoes a process Although a four-letter alphabet may seem rather of cell division known as meiosis, whereby it small for constructing the extensive vocabulary becomes a haploid cell—a cell with half the basic that defines the myriad life-forms on Earth, in number of chromosomes, which for a human practice, the sequences of these base pairs make would be 23 unpaired chromosomes. The sperm for almost limitless combinations. For any cells in a male and the egg cells in a female are sequence, there are four possibilities as to the first both haploid germ cells: each contains only 23 two letters (AT, TA, CG, or GC) and four more chromosomes, and each is prepared to form a possibilities for the second two letters. Thus, just new diploid by fusion with another haploid. for a four-letter sequence, there are 16 possibili- Sperm cells and egg cells are known as gametes, ties, and for each pair of letters added to the mature male or female germ cells that possess a sequence, the total is multiplied by four. Given haploid set of chromosomes and are prepared to the long strings of base pairs that form DNA form a new diploid by undergoing fusion with a sequences, the numbers can be extremely large. haploid gamete of the opposite sex. The more complex an organism, from bacte- When egg and sperm fuse, they form a ria to humans, the more rungs, or genetic zygote, in which the diploid chromosome num- sequences, appear on the ladder. The entire ber is restored, with the zygote possessing the genetic makeup of a human, for example, may same chromosomes as both the sperm and the contain three billion base pairs, with the average egg. This cell carries all the genetic information gene unit being 2,000-200,000 base pairs long. needed to grow into an embryo and eventually a Each one of these combinations has a different full-grown human, with the specific traits and meaning, providing the code not only for the attributes passed on by the parents. Not all off- type of organism but also for specific traits, such spring of the same parents are the same, of as brown hair and blue eyes, dimples, detached course, and this is because the sperm cells and earlobes, and so on and on. Except for identical egg cells vary in their genetic codes—that is, in twins, no two humans have exactly the same their DNA blueprints. genetic information. The DNA Blueprint DNA Replication, Protein Synthesis, and RNA To understand genes and their biological func- tion in heredity, it is necessary to understand the Genetic information is duplicated during the chemical makeup and structure of DNA. The process of DNA replication, which is initiated by complete DNA molecule often is referred to as proteins in the cells. To produce identical genetic the blueprint for life, because it carries all the information during cell mitosis, the DNA hydro- instructions, in the form of genes, for the growth gen bonds between the two strands arebroken, and functioning of organisms. This fundamental splitting the DNA in half lengthwise. This process molecule is similar in appearance to a spiral stair- begins a few hours before the initiation of cell case, which also is called a double helix. The sides mitosis, and once it is completed, each half of the of the DNA ladder are made up of alternate sugar DNA ladder is capable of forming a new DNA and phosphate molecules, like links in a chain. molecule with an identical genetic code. It can do The rungs, or steps, of DNA are made from a this because of specific chemical catalysts (a sub- combination of four different chemical bases. stance that enables a chemical reaction without Two of these, adenine and guanine, are known as taking part in it) that help synthesize the com- purines, and the other two, cytosine and plementary strand. thymine, are pyrimidines. The four letters desig- Catalysts formed from proteins are known nating these bases—A, G, C, and T—are the as enzymes, and the functioning of specific cells alphabet of the genetic code, and each rung of and organisms is conducted by enzymes synthe-
100 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 101
sized by the cells. Cells contain hundreds of dif- because there are so many strands of DNA in the Genetics ferent proteins, complex molecules that make up world, and so much material in the strands, more than half of all solid body tissues and con- mutation is bound to happen eventually—and, to trol most biological processes within and among an extent, at least, this is a good thing. Mutation these tissues. A cell functions in accordance with is the engine that drives evolution, and a certain the particular protein—one of thousands of dif- amount of genetic variation is necessary if species ferent types—it contains. It is the genetic base- are to adapt by natural selection to a changing pair sequence in DNA that determines, or “codes environment. If it were not for mutation, neither for,” the specific arrangement of amino acids to humans nor the many millions of other species build particular proteins. that exist would ever have appeared. Since the sites of protein production lie out- Mutation often occurs when chromosome side the cell nucleus, coded messages pass from the segments from two parents physically exchange DNA in the nucleus to the cytoplasm, the materi- places with each other during the process of al inside the cell that is external to the nucleus. meiosis. This is known as genetic recombination. This transfer of messages is achieved by RNA, or Genes also can change by mutations in the DNA ribonucleic acid, and specifically by messenger molecule, which take place when a mutagen RNA, or mRNA. Other types of RNA molecules alters the chemical or physical makeup of DNA. are involved in linking the amino-acids together in The mutations that result are of two types, corre- a sequence form to shape the protein. (For more sponding to the two basic varieties of cell: somat- about amino acids, proteins, and enzymes, see the ic mutations, which occur solely within the respective essays devoted to each subject.) affected individual, and germinal mutations, which happen in the DNA of germ cells, produc- Mutation ing altered genes that may be passed on to the next generation. Once a protein has been created for a specific The odd thing about mutations is that while function, it cannot be changed. This is why the most of them are harmful, the few that are bene- theory of acquired characteristics (the idea that ficial are, as we have noted, the driving force changes in an organism’s overall anatomy, as behind the evolution of life-forms that success- opposed to changes in its DNA, can be passed on fully adapt to their environments. Thus, while to offspring) is a fallacy. People may have genes most germinal mutations bring about congenital that make it easier for them to acquire certain disorders (birth defects) ranging from physical traits, such as larger muscles or the ability to play abnormalities to deficiencies in body or mind to the piano through exercise or practice, respec- diseases, every once in a while a germinal muta- tively, but the traits themselves, if they are tion results in an improvement, such as a change acquired during the life of the individual and are in body coloring that acts as camouflage. If the not encoded in the DNA, are not heritable. trait improves an individual organism’s chances There is only one way in which changes that for survival within a particular environment, it take place during the life of an organism can be may become a permanent trait of the species, passed on to its offspring, and that is if those because the offspring with this gene have a changes are encoded in the organism’s DNA. This greater chance of survival and thus will pass on is known as mutation. Suppose lung cancer the trait to succeeding generations. (For more develops in a man as a result of smoking; unless about mutation, see the essay by that title. See a tendency to cancer is already a part of his genet- also Evolution for a discussion of the role played ic makeup, he cannot genetically pass the disease by mutation and natural selection in the evolu- on to his unborn children. But if the tobacco has tion of species.) acted as a mutagen, a substance that brings about mutation, it is possible that his DNA can be REAL-LIFE altered in such a way as to pass on either the ten- dency toward lung cancer or some other charac- APPLICATIONS teristic. The Genetics Revolution Because DNA is extremely stable chemically, it rarely mutates, or experiences an alteration in In the modern world genetics plays a part in its physical structure, during replication. But more dramatic breakthroughs than any other
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 101 set_vol3_4 8/19/02 10:45 AM Page 102
Genetics
A SCIENTIST STUDIES THE ARRANGEMENT OF CHROMOSOME PAIRS, THE THREADLIKE DNA-CONTAINING BODIES IN CELLS THAT CARRY GENETIC INFORMATION. THE HUMAN GENOME PROJECT IS WORKING TO COMPLETE A MAP OUT- LINING THE LOCATION AND FUNCTION OF THE GENES IN HUMAN CELLS. (© BSIP/V & L /Photo Researchers. Reproduced by per- mission.)
field of biological study. These breakthroughs midst of a revolution, and at the center of this have an impact in a wide variety of areas, from exciting (and, to some minds, terrifying) phe- curing diseases to growing better vegetables to nomenon is the realm of genetic engineering: the catching criminals. The field of genetics is in the alteration of genetic material by direct interven-
102 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 103
tion in genetic processes. In agriculture, for many inherited or genetic diseases are caused by Genetics instance, genes are transplanted from one organ- the lack of an enzyme or protein, scientists hope ism to another to produce what are known as one day to treat the unborn child by inserting transgenic animals or plants. This approach has genes to provide the missing enzyme. (For more been used to reduce the amount of fat in cattle about inherited disorders, see the essays Disease, raised for meat or to increase proteins in the milk Noninfectious Diseases, and Mutation.) produced by dairy cattle. Fruits and vegetables THE HUMAN GENOME PROJECT. also have been genetically engineered so that they One of the most exciting developments in genet- do not bruise easily or have a longer shelf life. ics is the initiation of the Human Genome Pro- Not all of the work in genetics is genetic ject, designed to provide a complete genetic map engineering per se; in the realm of law, for outlining the location and function of the 40,000 instance, the most important application of or so genes that are found in human cells. (A genetics is genetic fingerprinting. A genetic fin- genome is all of the genetic material in the chro- gerprint is a sample of a person’s DNA that is mosomes of a particular organism.) With the detailed enough to distinguish it from the DNA completion of this map, genetic researchers will of all others. The genetic fingerprint can be used have easy access to specific genes, to study how to identify whether a man is the father of a par- the human body works and to develop therapies ticular child (i.e., to determine paternity), and it for diseases. Gene maps for other species of ani- can be applied in the solving of crimes. If biolog- mals also are being developed. ical samples can be obtained from a crime The project had its origins in the 1990s, with scene—for example, skin under the fingernails of the efforts of the United States Department of a murder victim, presumably the result of fight- Energy (DOE) and the National Institutes of ing against the assailant in the last few moments Health (NIH). The NIH connection is probably of life—it is possible to determine with a high clear enough, but the DOE’s involvement at first degree of accuracy whether that sample came might seem strange. What, exactly, does genetics from a particular suspect. The use of DNA in have to do with electricity, petroleum, and other forensic science is discussed near the conclusion concerns of the DOE? The answer is that the of this essay. DOE grew out of agencies, among them the THE REVOLUTION IN MEDI- Atomic Energy Commission (AEC), established CINE. Some of the biggest strides in genetic soon after the explosion of the two atomic engineering and related fields are taking place, bombs over Japan in 1945. Even at that early not surprisingly, in the realm of medicine. date, educated nonscientists understood that the Genetic engineering in the area of health is radioactive fallout produced from nuclear aimed at understanding the causes of disease and weaponry can act as a mutagen; therefore, Con- developing treatments for them: for example, gress instructed the AEC to undertake a broad recombinant DNA (a DNA sequence from one study of genetics and mutation and the possible species that is combined with the DNA of anoth- consequences of exposure to radiation and the er species) is being used to develop antibiotics, chemical by-products of energy production. hormones, and other disease-preventing agents. Eventually, scientists in the AEC and, later, Vaccines also have been genetically re-engineered the DOE recognized that the best way to under- to trigger an immune response that will protect take such a study was to analyze the entire scope against specific diseases. One approach is to of the human genome. The project formally remove genetic material from a diseased organ- commenced on October 1, 1990, and is sched- ism, thus making the material weaker and initiat- uled for completion in the middle of the first ing an immune response without causing the dis- decade of the twenty-first century. Upon comple- ease. (See Immunity and Immunology for more tion, the Human Genome Project will provide a about how vaccines work.) vast store of knowledge and no doubt will lead to Gene therapy is another outgrowth of genet- the curing of many diseases. ics. The idea behind gene therapy is to introduce Still, there are many who question the specific genes into the body either to correct a Human Genome Project in particular, and genet- genetic defect or to enhance the body’s capabili- ic engineering in general, on ethical grounds, ties to fight off disease and repair itself. Since fearing that it could give scientists or govern-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 103 set_vol3_4 8/19/02 10:45 AM Page 104
Genetics
THE EXPLOSION OF AN ATOMIC BOMB OVER NAGASAKI, JAPAN, DURING WORLD WAR II (SEPTEMBER 1, 1945). THE RADIOACTIVE FALLOUT PRODUCED FROM NUCLEAR WEAPONRY CAN ACT AS A MUTAGEN, ALTERING THE CHEMICAL OR PHYSICAL MAKEUP OF DNA. (© Bettmann/Corbis. Reproduced by permission.)
ments too much power, unleash a Nazi-style engineering, and their opponents, see Genetic eugenics (selective breeding) program, or result Engineering. in horrible errors, such as the creation of deadly Genetics in Forensic Science new diseases. In fact, it is impossible to search “genetic engineering” on the World Wide Web Forensic science, as we noted earlier, is the appli- without coming across the Web sites of literally cation of science to matters of law. It is based on dozens and dozens of agencies, activist groups, the idea that a criminal always leaves behind and individuals opposed to genetic engineering some kind of material evidence that, through and the mapping of the human genome. For careful analysis, can be used to determine the more about the Human Genome Project, genetic identity of the perpetrator—and to exonerate
104 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 105
someone falsely accused. Among those forms of Genetics material evidence of interest to forensic scientists working in the field of genetics are blood, semen, hair, saliva, and skin, all of which contain DNA that can be analyzed. In addition, there are areas of forensic science that rely on biological study, though not in the area of genetics: blood typing as well as the analysis of fingerprints or bite marks, both of which have patterns that are as unique to a single individual as DNA is. One of the first detectives to use science, including biology and medicine, in solving crimes was a fictional character: Sherlock Holmes, whose creator, the British writer Sir Arthur Conan Doyle (1859–1930), happened to be a physician as well. The first full-fledged (and real) police practitioner of forensic science was the French police official Alphonse Bertillon (1853–1914), who developed an identification system that consisted of a photograph and 11 body measurements, including dimensions of the head, arms, legs, feet, hands, and so on, for each individual. Bertillon claimed that the likeli- hood of two people having the same measure- O. J. SIMPSON REACTS TO THE JURY’S VERDICT. DESPITE DNA EVIDENCE THAT LINKED BLOOD AT THE ments for all 11 traits was less than one in 250 CRIME SCENE WITH BLOOD IN HIS CAR, THE JURY million. In 1894 fingerprints, which were easier FOUND HIM NOT GUILTY OF DOUBLE MURDER. (AP/WIde to use and more unique than body measure- World Photos. Reproduced by permission.) ments, were added to the Bertillon system. Fingerprints, unlike DNA, are unique to the the appearance of his prints at the scene of her individual; indeed, identical twins have the same murder in her Los Angeles home could be DNA but different fingerprints. Mark Twain explained away easily, even though she had taken (1835–1910) could not have known this in 1894, out a restraining order against her former hus- when he published The Tragedy of Pudd’nhead band (who she had accused of spousal abuse) Wilson, and the Comedy of Those Extraordinary some time before the murder. Rather than fin- Twins. Nonetheless, the story involves a murder gerprints, the prosecution in his murder trial committed by one man and blamed on his twin, used DNA evidence connecting blood at the who eventually is exonerated on the basis of fin- crime scene with blood found in Simpson’s vehi- gerprint evidence—still a new concept at the cle. (Some of this blood was apparently his own, time. In some situations, however, fingerprint since he had mysterious cuts on his hands that he evidence may be unavailable, and though law- could not explain to police officers.) enforcement agencies have developed extraordi- A jury found Simpson not guilty on October nary techniques for analyzing nearly invisible 3, 1995, and jurors later claimed that the prose- (i.e., latent) prints, sometimes this is still not cution had failed to make a strong case using enough. DNA evidence. Furthermore, they cited police THE SIMPSON CASE AND THE contamination of the DNA evidence, which had CONTROVERSY OVER DNA EVI- been established in their minds by Simpson’s DENCE. For example, in the infamous mur- defense team, as a cause for reasonable doubt der of Nicole Brown Simpson and Ron Goldman concerning Simpson’s guilt. In fact, assuming on June 12, 1994, fingerprint evidence would that the defense was fully justified in this claim, have been ineffective in the case against the sus- that would have meant only that the DNA sam- pect, the former football star and actor O. J. ples would have been less (not more) likely to Simpson. Since Nicole Simpson was his ex-wife, convict Simpson.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 105 set_vol3_4 8/19/02 10:45 AM Page 106
Genetics KEY TERMS
ACQUIRED CHARACTERISTICS: CYTOPLASM: The material inside a Sometimes known as acquired characters or cell that is external to the nucleus. Lamarckism, after one of its leading propo- DIPLOID: A term for a cell that has the nents, the French natural philosopher Jean basic number of doubled chromosome Baptiste de Lamarck (1744–1829), the the- cells. In humans, somatic cells, which are ory of acquired characteristics is a fallacy diploid cells, have 23 pairs of chromo- that should not be confused with muta- somes, for a total of 46 chromosomes. tion. Acquired characteristics theory main- DNA: tains that changes that occur in an organ- Deoxyribonucleic acid, a mole- ism’s overall anatomy (as opposed to cule in all cells, and many viruses, that con- changes in its DNA) can be passed on to tains genetic codes for inheritance. offspring. DOMINANT: In genetics, a term for a AMINO ACIDS: Organic compounds trait that can manifest in the offspring made of carbon, hydrogen, oxygen, nitro- when inherited from only one parent. Its gen, and (in some cases) sulfur bonded in opposite is recessive. characteristic formations. Strings of amino ENZYME: A protein material that acids make up proteins. speeds up chemical reactions in the bodies BASE PAIR: A pair of chemicals that of plants and animals without itself taking form the “rungs” on a DNA molecule, part in or being consumed by those reac- which has the shape of a spiral staircase. A tions. base pair always consists of a type of chem- FORENSIC SCIENCE: The applica- ical called a purine on one side and a tion of science to matters of law and legal chemical termed a pyrimidine on the or police procedure. other. This means that DNA base pairs GAMETE: A mature male or female always consist of adenine linked with germ cell that possesses a haploid set of thymine and guanine with cytosine. chromosomes and is prepared to form a BIOCHEMISTRY: The area of the bio- new diploid by undergoing fusion with a logical sciences concerned with the chemi- haploid gamete of the opposite sex. cal substances and processes in organisms. GENE: A unit of information about a BODY CELL: See somatic cell. particular heritable trait. Usually stored on CHROMOSOME: A DNA-containing chromosomes, genes contain specifications body, located in the cells of most living for the structure of a particular polypep- things, that holds most of the organism’s tide or protein. genes. GENETIC ENGINEERING: The alter- CONGENITAL DISORDER: An ation of genetic material by direct inter- abnormality of structure or function or a vention in genetic processes. disease that is present at birth. Congenital GENETIC FINGERPRINT: A sample disorders also are called birth defects. of a person’s DNA that is detailed enough
106 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 107
Genetics KEY TERMS CONTINUED
to distinguish it from all other people’s MITOSIS: A process of cell division DNA. that produces diploid cells. Compare with GENETIC RECOMBINATION: A meiosis. process whereby chromosome segments MRNA: Messenger ribonucleic acid, a from two parents physically exchange molecule of RNA that carries the genetic places with each other during the process information for producing proteins. of meiosis. This is one of the ways that MUTAGEN: A chemical or physical fac- mutation occurs. tor that increases the rate of mutation. GENETICS: The area of biological MUTATION: Alteration in the physical study concerned with heredity, with hered- structure of an organism’s DNA, resulting itary traits passed down from one genera- in a genetic change that can be inherited. tion to the next through the genes, and NATURAL SELECTION: with the variations between organisms that The process result from heredity. whereby some organisms thrive and others perish, depending on their degree of adap- GENOME: All of the genetic material in tation to a particular environment. the chromosomes of a particular organism. NUCLEIC ACIDS: Acids, including GERM CELL: One of two basic types DNA and RNA, that are made up of of cells in a multicellular organism. In con- nucleotide chains. trast to somatic, or body, cells, germ cells are involved in reproduction. NUCLEOTIDE: A compound formed from one of several types of sugar joined GERMINAL MUTATION: A mutation that occurs in the germ cells, meaning that with a base of purine or pyrimidine (see the mutation can be passed on to the base pair) and a phosphate group. organism’s offspring. Nucleotides are the basis for nucleic acids. NUCLEUS: HAPLOID: A term for a cell that has The control center of a half the number of chromosome cells that cell, where DNA is stored. appear in a diploid, or somatic, cell. In POLYPEPTIDE: A group of between 10 humans, germ cells, which are haploid and 50 amino acids. cells, have 23 unpaired chromosomes, as PROTEINS: Large molecules built opposed to the 23 paired chromosomes (46 from long chains of 50 or more amino overall) that appear in a somatic cell. acids. Proteins serve the functions of pro- HEREDITY: The transmission of genet- moting normal growth, repairing damaged ic characteristics from ancestor to descen- tissue, contributing to the body’s immune dant through the genes. system, and making enzymes. HERITABLE: Capable of being inherited. RECESSIVE: In genetics, a term for a MEIOSIS: The process of cell division trait that can manifest in the offspring only that produces haploid genetic material. if it is inherited from both parents. Its Compare with mitosis. opposite is dominant.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 107 set_vol3_4 8/19/02 10:45 AM Page 108
Genetics KEY TERMS CONTINUED
REPRODUCTIVE CELL: See germ cell. SOMATIC MUTATION: A mutation RNA: Ribonucleic acid, a molecule that occurs in cells other than the repro- translated from DNA in the cell nucleus ductive, or sex, cells. These mutations, as contrasted with germinal mutations, can- that directs protein synthesis in the cyto- not be transmitted to the next generation. plasm. See also mRNA. SYNTHESIZE: To manufacture chemi- SOMATIC CELL: One of two basic types cally, as in the body. of cells in a multicellular organism. In con- trast to germ cells, somatic cells (also known TRANSLOCATION: A mutation in as body cells) do not play a part in reproduc- which chromosomes exchange parts. tion; rather, they make up the tissues, organs, ZYGOTE: A diploid cell formed by the and other parts of the organism. fusion of two gametes.
At the same time, a number of legitimate men. One of them, named Colin Pitchfork, paid concerns regarding the use of DNA evidence another man to provide a sample in his place. were raised by experts for the defense in the This attracted the attention of the police, who Simpson trial. Samples can become contaminat- tested his DNA and found their man. ed and thus difficult to read; small samples are Since that time, DNA evidence has been difficult for analysts to work with effectively; and used in more than 24,000 cases and has aided in results are often open to interpretation. Further- the conviction of about 700 suspects. The DNA more, the outcome of the Simpson case illus- in such cases is not always obtained from a trates the fact that findings based on DNA evi- human subject. In the investigation of the May dence are not readily understood by non-special- 1992 murder of Denise Johnson in Arizona, a ists, and may not make the best basis for a case- homicide detective found two seed pods from a particularly in one so fraught with controversy. paloverde tree in the bed of a pickup truck The prosecution based its case almost entirely on owned by the suspect, Mark Bogan. The accused extremely technical material, explained in excru- man admitted having known the victim but ciating detail by experts who had devoted their denied ever having been near the site where her lives to studying areas that are far beyond the body was found. It so happened that there was a understanding of the average person. Attempting paloverde tree at the site, and testing showed that to wow the jurors with science, the prosecution the DNA in the pods on his truck bed matched instead seemed to create the impression that that of the tree itself. Bogan became the first sus- DNA evidence was some sort of hocus-pocus pect ever convicted by a plant. invented to frame an innocent man. Simpson On the other hand, in some cases, DNA evi- went free, though the jury in a 1996 civil trial dence has cleared a suspect falsely accused. Such (which took a much simpler approach, eschew- was the case with Kerry Kotler, convicted in 1981 ing complicated DNA testimony) found him for rape, robbery, and burglary and sentenced to guilty. 25–50 years in jail. In 1988, Kotler began peti- DNA EVIDENCE SUCCESS tioning for DNA analysis, which subsequently STORIES. Because of the Simpson case, the showed that his DNA did not match that of the use of DNA evidence gained something of a bad rapist, who had left a semen sample in the vic- name. Nonetheless, it has been successful in less tim’s underwear. Kotler was released in Decem- high profile cases, beginning in 1986, when Eng- ber 1992 and in March 1996 was awarded $1.5 lish police tracked down a rapist and murderer million in damages for his wrongful imprison- by collecting blood samples from some 2,000 ment. The story does not end there, however.
108 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 109
Kotler’s case turned out to be one of the more Genetics Education Center, University of Kansas Medical Genetics bizarre in the annals of forensic DNA testing. Center (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 109 set_vol3_4 8/19/02 10:45 AM Page 110
HEREDITYHeredity
CONCEPT whose importance in the study of genetics is Heredity is the transmission of genetic character- comparable to that of Charles Darwin (1809– istics from ancestor to descendant through the 1882) in the realm of evolutionary studies—was genes. As a subject, it is tied closely to genetics, the Austrian monk and botanist Gregor Mendel the area of biological study concerned with (1822–1884). hereditary traits. The study of heritable traits GENES. For thousands of years, people helps scientists discern which are dominant and have had a general understanding of genetic therefore are likely to be passed on from one par- inheritance—that certain traits can be, and ent to the next generation. On the other hand, a sometimes are, passed along from one generation recessive trait will be passed on only if both par- to the next—but this knowledge was primarily ents possess it. Among the possible heritable anecdotal and derived from casual observation traits are genetic disorders, but study in this area rather than from scientific study. The first major is ongoing, and may yield many surprises. scientific breakthrough in this area came in 1866, when Mendel published the results of a study on HOW IT WORKS the hybridization of plants in which he crossed pea plants of the same species that differed in Heredity and Genetics only one trait. Mendel bred these plants over the course of As discussed at the beginning of the essay on several successive generations and observed the genetics, the subjects of genetics and heredity are characteristics of each individual. He found that inseparable from each other, but there are so certain traits appeared in regular patterns, and many details that it is extremely difficult to wrap from these observations he deduced that the one’s mind around the entire concept. It is advis- plants inherited specific biological units from able, then, to break up the overall topic into more each parent. These units, which he called factors, digestible bits. One way to do this is to study the today are known as genes, or units of information biochemical foundations of genetics as a subject about a particular heritable trait. From his find- in itself, as is done in Genetics, and then to inves- ings, Mendel formed a distinction between geno- tigate the impact of genetic characteristics on type and phenotype that is still applied by scien- inheritance in a separate context, as we do here. tists studying genetics. Genotype may be defined Also included in the present essay is a brief as the sum of all genetic input to a particular history of genetic study, which reveals something individual or group, while phenotype is the actu- about the way in which these many highly com- al observable properties of that organism. We plex ideas fit together. Many brilliant minds have return to the subjects of genotype and phenotype contributed to the modern understanding of later in this essay. genetics and heredity; unfortunately, within the MUTATION AND DNA. Although present context, space permits the opportunity to Mendel’s theories were revolutionary, the scien- discuss only a few key figures. The first—a man tific establishment of his time treated these new
110 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 111
DNA MAKES ITS APPEAR- ideas with disinterest, and Mendel died in obscu- Heredity ANCE. rity. Then, in 1900, the Dutch botanist Hugo De All this time, scientists knew about the Vries (1848–1935) discovered Mendel’s writings, existence of DNA without guessing its function. became convinced that his predecessor had made Then, in the 1940s, a research team consisting of an important discovery, and proceeded to take the Canadian-born American bacteriologist Mendel’s theories much further. Unlike the Aus- Oswald Avery (1877–1955), the American bacte- trian monk, De Vries believed that genetic riologist Maclyn McCarty (1911–), and the changes occur in big jumps rather than arising Canadian-born American microbiologist Colin from gradual or transitional steps. In 1901 he Munro MacLeod (1909–1972) discovered the gave a name to these big jumps: mutations. blueprint function of DNA. By taking DNA from Today a mutation is defined as an alteration of a one type of bacteria and inserting it into anoth- er, they found that the second form of bacteria gene, which contains something neither De Vries took on certain traits of the first. nor Mendel understood: deoxyribonucleic acid, or DNA. The final proof that DNA was the specific molecule that carries genetic information came Actually, DNA, a molecule that contains in 1952, when the American microbiologists genetic codes for inheritance, had been discov- Alfred Hershey (1908–1997) and Martha Chase ered just four years after Mendel presented his (1927–) showed that transferring DNA from a theory of factors. In 1869 the Swiss biochemist virus to an animal organ resulted in an infection, Johann Friedrich Miescher (1844–1895) isolated just as if an entire virus had been inserted. But a substance from the remnants of cells in pus. perhaps the most famous DNA discovery The substance, which contained both nitrogen occurred a year later, when the American bio- and phosphorus, separated into a protein and an chemist James D. Watson (1928–) and the Eng- acid molecule and came to be known as nucleic lish biochemist Francis Crick (1916–) solved the acid. A year later he discovered DNA itself in the mystery of the exact structure of DNA. Their goal nucleic acid, but more than 70 years would pass was to develop a DNA model that would explain before a scientist discerned its purpose. the blueprint, or language, by which the molecule THE DISCOVERY OF CHROMO- provides necessary instructions at critical SOMES. In the meantime, another major moments in the course of cell division and step in the history of genetics was taken just two growth. To this end, Watson and Crick focused years after De Vries outlined his mutation theory. on the relationships between the known chemi- In 1903 the American surgeon and geneticist cal groups that compose DNA. This led them to Walter S. Sutton (1877–1916) discovered chro- propose a double helix, or spiral staircase, model, mosomes, threadlike structures that split and which linked the chemical bases in definite pairs. then pair off as a cell divides in sexual reproduc- Using this twisted-ladder model, they were able tion. Today we know that chromosomes contain to explain how the DNA molecule could dupli- DNA and hold most of the genes in an organism, cate itself, since each side of the ladder contains a but that knowledge still lay in the future at the compound that fits with a compound on the time of Sutton’s discovery. opposite side. If separated, each would serve as In 1910 the American geneticist Thomas the template for the formation of its mirror Hunt Morgan (1866–1945) confirmed the rela- image. tionship between chromosomes and heredity Autosomes and Sex through experiments with fruit flies. He also dis- Chromosomes covered a unique pair of chromosomes called the sex chromosomes, which determine the sex of Genetic information is organized into chromo- offspring. From his observation that a sex-specif- somes in the nucleus, or control center, of the ic chromosome was always present in flies that cell. Human cells have 46 chromosomes each, had white eyes, Morgan deduced that specific except for germ, or reproductive, cells (i.e., sperm genes reside on chromosomes. A later discovery cells in males and egg cells in females), which showed that chromosomes could mutate, or each have 23 chromosomes. Each person receives change structurally, resulting in a change of char- 23 chromosomes from the mother’s egg and 23 acteristics that could be passed on to the next chromosomes from the father’s sperm. Of these generation. 23 chromosomes, 22 are called autosomes, or
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 111 set_vol3_4 8/19/02 10:45 AM Page 112
Heredity might say “unattached earlobe,” indicating a lobe that is not fully attached and therefore can be flapped. DOMINANT AND RECESSIVE ALLELES. Each person has two alleles of the same gene—the genotype for a single locus. These can be written as uppercase or lowercase letters of the alphabet, with capital letters defin- ing dominant traits and lowercase letters indicat- ing recessive traits. A dominant trait is one that can manifest in the offspring when inherited from only one parent, whereas a recessive trait must be inherited from both parents in order to manifest. For instance, brown eyes are dominant and thus would be represented in shorthand with a capital B, whereas blue eyes, which are recessive, would be represented with a lowercase b. Geno- types are either homozygous (having two identi- cal alleles, such as BB or bb) or heterozygous (having different alleles, such as Bb). The pheno- type, however—that is, the actual eye color— IF TWO GROUPS OF THE SAME SPECIES ARE SEPARATED must be one or the other, because both sets of FOR A LONG TIME, GENETIC DRIFT MAY LEAD TO THE FORMATION OF DISTINCT SPECIES, AS WHEN THE COL- genes cannot be expressed together. ORADO RIVER CUT OPEN THE GRAND CANYON AND ISO- Unless there is some highly unusual muta- KAIBAB SQUIRREL OF THE NORTH RIM FROM LATED THE tion, a child will not have one brown eye and one THE ABERT SQUIRREL (SHOWN HERE) OF THE SOUTH. (© W. Perry Conway/Corbis. Reproduced by permission.) blue eye; instead, the dominant trait will over- power the recessive one and determine the eye color of the child. If an individual’s genotype is non-sex chromosomes, meaning that they do not BB or Bb, that person definitely will have brown determine gender. The remaining chromosome, eyes; the only way for the individual to have blue the sex chromosome, is either an X or a Y. eyes is if the genotype is bb—meaning that both Females have two Xs (XX), and males have one of parents have blue eyes. Oddly, two parents with each (XY), meaning that females can pass only an brown eyes could produce a child with blue eyes. X to their offspring, whereas males can pass How is that possible? Suppose both the mother either an X or a Y. (This, in turn, means that the and the father had the heterozygous alleles Bb— sperm of the father determines the gender of the a dominant brown and a recessive blue. There is offspring.) then a 25% chance that the child could inherit Alleles both parents’ recessive genes, for a bb geno- type—and a blue-eyed phenotype. The 44 autosomes have parallel coded informa- LEARNING FROM HEREDI- tion on each of the two sets of 22 autosomes, and TARY LAW. What we have just described is this coding is organized into genes, which pro- called genetic dominance, or the ability of a single vide instructions for the synthesis (manufacture) allele to control phenotype. This principle of of specific proteins. Each gene has a set locus, or classical Mendelian genetics does not explain position, on a particular chromosome, and for everything. For example, where height is con- each locus, there are two slightly different forms cerned, there is not necessarily a dominant or of a gene. These differing forms, known as alleles, recessive trait; rather, offspring typically have a each represent slightly different codes for the height between that of the parents, because same trait. One allele, for instance, might say height also is determined by such factors as diet. “attached earlobe,” meaning that the bottom of (Also, more than one pair of genes is involved.) the lobe is fully attached to the side of the head Hereditary law does, however, help us predict and cannot be flapped. Another allele, however, everything from hair and eye color to genetic dis-
112 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 113
orders. As with the blue-eyed child of brown- Heredity eyed parents, it is possible that neither parent will show signs of a genetic disorder and yet pass on a double-recessive combination to their children. Again, however, other factors—including genetic ones—may come into play. For example, Down syndrome (discussed in Mutation) is caused by abnormalities in the number of chromosomes, with the offspring possessing 47 chromosomes instead of the normal 46.
REAL-LIFE APPLICATIONS Population Genetics
Studies in heredity and genetics can be applied not only to an individual or family but also to a whole population. By studying the gene pool (the sum of all the genes shared by a population) for a given group, scientists working in the field of population genetics seek to explain and under- stand specific characteristics of that group. Among the phenomena of interest to population THERE ARE SEVERAL THOUSAND GENETIC DISORDERS, geneticists is genetic drift, a natural mechanism CLASSIFIED INTO AUTOSOMAL DOMINANT, AUTOSOMAL RECESSIVE, SEX-LINKED, AND MULTIFACTORIAL TYPES. for genetic change in which specific traits coded DWARFISM, FOR EXAMPLE, CAN BE CAUSED BY ACHON- in alleles change by chance over time, especially DROPLASIA, TRANSMITTED BY A GENE INHERITED FROM in small populations, as when organisms are iso- ONE PARENT (DOMINANT), OR BY A GROWTH HORMONE lated on an island. If two groups of the same DEFICIENCY, TRANSMITTED BY GENES INHERITED FROM BOTH PARENTS (RECESSIVE). species are separated for a long time, genetic drift (© Photo Researchers. Reproduced by permission.) may lead even to the formation of distinct species from what once was a single life-form. When the Colorado River cut open the Grand Canyon, it separated groups of squirrels that lived in the of peoples from Siberia to North America in high-altitude pine forest. Over time, populations about 11,000 B.C. took place in two distinct ceased to interbreed, and today the Kaibab squir- waves. rel of the north rim and the Abert squirrel of the south are different species, no more capable of Genetic Disorders interbreeding than humans and apes. There are several thousand genetic disorders, Where humans are concerned, population which can be classified into one of several genetics can aid, for instance, in the study of groups: autosomal dominant disorders, which genetic disorders. As discussed in Mutation, cer- tain groups are susceptible to particular condi- are transmitted by genes inherited from only one tions: thus, cystic fibrosis is most common parent; autosomal recessive disorders, which are among people of northern European descent, transmitted by genes inherited from both par- sickle cell anemia among those of African and ents; sex-linked disorders, or ones associated Mediterranean ancestry, and Tay-Sachs disease with the X (female) and Y (male) chromosome; among Ashkenazim, or Jews whose ancestors and multifactorial genetic disorders. If one par- lived in eastern Europe. Studies in population ent has an autosomal dominant disorder, the off- genetics also can supply information about pre- spring have a 50% chance of inheriting that dis- historic events. As a result of studying the DNA ease. Approximately 2,000 autosomal dominant in fossil records, for example, some scientists disorders have been identified, among them have reached the conclusion that the migration Huntington disease, achondroplasia (a type of
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 113 set_vol3_4 8/19/02 10:45 AM Page 114
Heredity KEY TERMS
ALLELE: For any locus, one of two (or GERM CELL: One of two basic types more) slightly different forms of a gene. of cells in a multicellular organism. In con- These differing forms mean that alleles trast to somatic, or body, cells, germ cells code for different versions of the same trait. are involved in reproduction. AUTOSOMES: The 22 non-sex chro- HEREDITY: The transmission of genet- mosomes. ic characteristics from ancestor to descen- CHROMOSOME: A DNA-containing dant through the genes. body, located in the cells of most living HETEROZYGOUS: Having two differ- things, that holds most of the organism’s ent alleles—for example, Bb. genes. HOMOZYGOUS: Having two identical DNA: Deoxyribonucleic acid, a mole- alleles, such as BB or bb. cule in all cells, and many viruses, that con- LOCUS: The position of a particular tains genetic codes for inheritance. gene on a specific chromosome. DOMINANT: In genetics, a term for a MUTATION: Alteration in the physical trait that can manifest in the offspring structure of an organism’s DNA, resulting when inherited from only one parent. Its in a genetic change that can be inherited. opposite is recessive. NUCLEUS: GENE: A unit of information about a The control center of a particular heritable trait. Usually stored on cell, where DNA is stored. chromosomes, genes contain specifications PHENOTYPE: The actual observable for the structure of a particular polypep- properties of an organism, as opposed to tide or protein. its genotype. GENE POOL: The sum of all the genes RECESSIVE: In genetics, a term for a shared by a population, such as that of a trait that can manifest in the offspring only species. if it is inherited from both parents. Its GENETIC DISORDER: A condition, opposite is dominant. such as a hereditary disease, that can be SEX CHROMOSOMES: Chromo- traced to an individual’s genetic makeup. somes that determine gender. Human GENETIC DOMINANCE: The ability females have two X chromosomes (XX), of a single allele to control phenotype. and males have an X and a Y (XY). GENOTYPE: The sum of all genetic SYNTHESIZE: To manufacture chemi- input to a particular individual or group. cally, as in the body.
dwarfism), Marfan syndrome (extra-long limbs), The first two are discussed in Mutation. polydactyly (extra toes or fingers), some forms of Marfan syndrome, or arachnodactyly (“spider glaucoma (a vision disorder), and hypercholes- arms”), is historically significant because it is terolemia (high levels of cholesterol in the believed that Abraham Lincoln suffered from blood). that condition. Some scientists even maintain
114 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 115
that his case of Marfan, a disease sometimes them from germs in the air. From this sad fact Heredity accompanied by eye and heart problems, was so derives the title of an early John Travolta movie, severe that he probably would have died six The Boy in the Plastic Bubble (1976), based on the months or a year after the time of his actual true story of the SCID victim Tod Lubitch. (The death by assassination at age 56 in April 1865. ending, in which Travolta, as Tod, leaves his bub- RECESSIVE GENE DISOR- ble and literally rides off into the sunset with his DERS. Just as a person has a 25% chance of beautiful neighbor Gina, is more Hollywood fic- inheriting two recessive alleles, so two parents tion than fact. Lubitch actually died in his early who each have a recessive gene for a genetic dis- teens, shortly after receiving a bone marrow order stand a 25% chance of conceiving a child transplant.) with that disorder. Among the approximately MULTIFACTORIAL GENETIC 1,000 known recessive genetic disorders are cys- DISORDERS. Scientists often find it diffi- tic fibrosis, sickle cell anemia, Tay-Sachs disease, cult to determine the relative roles of heredity galactosemia, phenylketonuria, adenosine deam- and environment in certain medical disorders, inase deficiency, growth hormone deficiency, and one way to answer this question is with sta- Werner syndrome (juvenile muscular dystro- tistical and twin studies. Identical and fraternal phy), albinism (lack of skin pigment), and twins who have been raised in different and iden- autism. Several of these conditions are discussed tical homes are evaluated for multifactorial briefly elsewhere, and albinism is treated at genetic disorders. Multifactorial genetic disor- length in Mutation. Note that all of the disorders ders include medical conditions associated with mentioned earlier, in the context of population diet and metabolism, among them obesity, dia- genetics, are recessive gene disorders. Phenylke- betes, alcoholism, rickets, and high blood pres- tonuria (see Metabolism) and galactosemia are sure. Other such multifactorial conditions are a examples of metabolic recessive gene disorders, tendency toward certain infectious diseases, such in which a person’s body is unable to carry out as measles, scarlet fever, and tuberculosis; schizo- essential chemical reactions. For example, people phrenia and some other psychological illnesses; with galactosemia lack an enzyme needed to clubfoot and cleft lip; and various forms of can- metabolize galactose, a simple sugar that is found cer. The tendency of a particular person to be in lactose, or milk sugar. If they are given milk susceptible to any one of these disorders is a and other foods containing galactose early in life, function of that person’s genetic makeup, as well they eventually will suffer mental retardation. as environmental factors. SEX-LINKED GENETIC DISOR- DERS. Dominant sex-linked genetic disorders Breeding within the Family affect females, are usually fatal, and—fortunate- If there is one thing that most people know about ly—are rather rare. An example is Albright heredity and breeding, it is that a person should hereditary osteodystrophy, which brings with it never marry or conceive offspring with close rel- seizures, mental retardation, and stunted growth. atives. Aside from moral restrictions, there is the On the other hand, several recessive sex-linked fear of the genetic defects that would result from genetic disorders are well known, though at least close interbreeding. How close is too close? Cer- one of them, color blindness, is relatively harm- tainly, first cousins are off-limits as potential less. Among the more dangerous varieties of these disorders, which are passed on to sons mates, though second or third cousins (people through their mothers, the best known is hemo- who share the same great-grandparents and the philia, discussed in Noninfectious Diseases. same great-great-grandparents, respectively) are Many recessive sex-linked genetic disorders affect probably far enough apart. Hence, the phrase the immune, muscular, and nervous systems and “kissin’ cousins,” meaning a relative who is a dis- are typically fatal. An example is severe combined tant enough to be considered a potential partner. immune deficiency syndrome (SCID), which is What kind of defects? Hemophilia, men- characterized by a very poor ability to combat tioned earlier, is popularly associated with royal- infection. The only known cure for SCID is bone ty because several members of European ruling marrow transplantation from a close relative. houses around the turn of the nineteenth centu- Short of a cure, patients may be forced to live ry had it. Common wisdom maintains that the enclosed in a large plastic bubble that protects tendency toward the disease resulted from the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 115 set_vol3_4 8/19/02 10:45 AM Page 116
Heredity fact that royalty were apt to marry close relatives. win, who fathered many healthy children with In fact, hemophilia has nothing to do with royal- his cousin, Emma Wedgwood. ty per se and certainly bears no relation to mar- riages between close relatives. Research findings WHERE TO LEARN MORE gathered over the course of more than three Ackerman, Jennifer. Chance in the House of Fate: A Nat- decades, beginning in 1965, indicate that many ural History of Heredity. Boston: Houghton Mifflin, views about first cousins marrying may be more 2001. a matter of tradition than of scientific fact. Center for the Study of Multiple Birth (Web site). According to information published in the Jour-
116 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 117
Genetic Engineering GENETIC ENGINEERING
CONCEPT Although it is central to the latest advances Genetic engineering is the alteration of genetic in modern genetic research, DNA was discovered material by direct intervention in genetic more than 130 years ago. In 1869 the Swiss bio- processes with the purpose of producing new chemist Johann Friedrich Miescher (1844–1895) substances or improving functions of existing isolated a substance, containing both nitrogen organisms. It is a very young, exciting, and con- and phosphorus, that separated into a protein troversial branch of the biological sciences. On and an acid molecule. He called it nucleic acid, the one hand, it offers the possibility of cures for and in this material he discovered DNA. Some 74 years would pass, however, before scientists rec- diseases and countless material improvements to ognized the function of the nucleic acid Miesch- daily life. Hopes for the benefits of genetic engi- er had discovered. Then, in 1944, a research team neering are symbolized by the Human Genome led by the Canadian-born American bacteriolo- Project, a vast international effort to categorize gist Oswald Avery (1877–1955) found that by all the genes in the human species. On the other taking DNA from one type of bacterium and hand, genetic engineering frightens many with inserting it into another, the second bacterium its potential for misuse, either in Nazi-style took on certain traits of the first. This experi- schemes for population control or through sim- ment, along with other experiments and research, ple bungling that might produce a biological proved that DNA serves as a blueprint for the holocaust caused by a man-made virus. Symbol- characteristics and functions of organisms. ic of the alarming possibilities is the furor THE DOUBLE HELIX. Nine years inspired by a single concept on the cutting edge later, in 1953, the American biochemist James D. of genetic engineering: cloning. Watson (1928–) and the English biochemist Francis Crick (1916–) solved the mystery of HOW IT WORKS DNA’s structure and explained the means by which it provides necessary instructions at criti- DNA cal moments in the course of cell division and growth. They proposed a double helix, or spiral Any discussion of genetics makes reference to staircase, model, which linked the chemical bases DNA (deoxyribonucleic acid), a molecule that of DNA in definite pairs. Using this twisted-lad- contains genetic codes for inheritance. DNA der model, they were able to explain how the resides in chromosomes, threadlike structures DNA molecule could duplicate itself, since each found in the nucleus, or control center, of every side of the ladder is identical to the other; if sep- cell in every living thing. Chromosomes them- arated, each would serve as the template for the selves are made up of genes, which carry codes formation of its mirror image. for the production of proteins. The latter, of The sides of the DNA ladder are composed which there are many thousands of different of alternating sugar and phosphate molecules, varieties, make up the majority of the human like links in a chain, and consist of four different body’s dry weight. chemical bases: adenine, guanine, cytosine, and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 117 set_vol3_4 8/19/02 10:45 AM Page 118
Genetic To see where this might lead, imagine that you Engineering started with a penny and tried to quadruple your funds every day. The first day there would not be a dramatic increase, since you would have to earn only $0.04, and even by day 4 you would need only $2.56 to meet your goal. But as the quadrupling process continued, day by day the sums of money would get bigger ($655.36 on day 8) and bigger ($16,772.16 on day 12) and bigger ($687,194,767.36 on day 18). Given the fact that the human body contains an almost unfathomable number of genes, each of which may be between 2,000 and 200,000 base pairs long, one can begin to imagine just how large the number of possibilities would become. Each one of these combinations has a differ- ent meaning, providing the code for all manner of specific traits, such as brown hair and blue eyes, dimples, unattached earlobes, and so on. Except for identical twins, no two humans have exactly the same genetic information. What fol- lows are just a few facts about the human genome—that is, all of the genetic material in the chromosomes of the human organism: COMPUTER MODEL OF DNA, SHOWING ITS DOUBLE Some Facts About the Human Genome HELIX, OR SPIRAL STAIRCASE, FORM, WHICH LINKS THE CHEMICAL BASES OF DNA IN PAIRS. EACH SIDE OF THE • The human body contains about 100 tril- LADDER IS IDENTICAL TO THE OTHER; IF SEPARATED, lion cells. EACH WOULD SERVE AS THE TEMPLATE FOR THE FORMA- • Each cell has a DNA code consisting of TION OF ITS MIRROR IMAGE. (© Kenneth Eward/Grafz/Science Source/Photo Researchers. Reproduced by permission.) some 1.5 billion base pairs. • The DNA in each cell, if stretched to its full length, would be 6 ft. (1.8 m) long—yet it thymine. The four letters designating these fits into a space about 0.0004 in. (0.0001 bases—A, G, C, and T—are the alphabet of the cm) across, smaller than the head of a pin. genetic code, and each rung of the DNA mole- • If all of the DNA in the human body were cule is made up of a combination of two of these stretched end to end, it would reach to the letters. Owing to specific chemical affinities, A Sun and back more than 600 times. always combines with T and C with G, to form • If a person attempted to recite the entire what is called a base pair. Specific sequences of human genome, with all its base pairs, at the these base pairs, which are bonded to each other rate of one letter per second, 24 hours a day, by atoms of hydrogen, constitute the genes. it would take a century. ENDLESS COMBINATIONS. A • Every second scientists working on the four-letter alphabet may seem rather small for Human Genome Project are decoding some constructing the extensive vocabulary that 12,000 letters of DNA. defines the myriad life-forms on Earth. If one • Our DNA is 98% identical to that of chim- stops to consider the exponential operations panzees. involved, however, it is easy to understand how • Only 0.2% of all human DNA differs large the range of possibilities can become. For between individuals; in other words, people any sequence, there are four possibilities for the are 99.8% the same, and all the vast differ- first two letters (AT, TA, CG, or GC) and four ences between people are a product of just more possibilities for the second two letters. 1/500th of the total DNA. Thus, just for a four-letter sequence, there are 16 • Despite all that scientists know about DNA, possibilities, and for each pair of letters added to a staggering 97% of all human DNA has no the sequence, the total is multiplied by four. known function.
118 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:45 AM Page 119
Principles of Genetic American biochemists Stanley Cohen (1922–) at Genetic Engineering Stanford University, and Herbert Boyer (1936–) Engineering Just as DNA is at the core of studies in genetics, at the University of California at San Francisco recombinant DNA (rDNA)—that is, DNA that discovered an enzyme that greatly increased the has been genetically altered through a process efficiency of the Berg procedure. The gene-trans- known as gene splicing—is the focal point of fer technique developed by Berg, Boyer, and genetic engineering. In gene splicing, a DNA Cohen formed the basis for much of the ensuing strand is cut in half lengthwise and joined with a progress in genetic engineering. strand from another organism or perhaps even another species. Use of gene splicing makes pos- REAL-LIFE sible two other highly significant techniques. APPLICATIONS Gene transfer, or incorporation of new DNA into an organism’s cells, usually is carried out with the Big Business in DNA help of a microorganism that serves as a vector, or carrier. Gene therapy is the introduction of Ever since the breakthrough discoveries of Wat- normal or genetically altered genes to cells, gen- son, Crick, and others in the 1950s made genetic erally to replace defective genes involved in engineering a possibility, the new field has prom- genetic disorders. ised increasingly bigger payoffs. These payoffs DNA also can be cut into shorter fragments take the form of improvements to human life through the use of restriction enzymes. (An and profits to those who facilitate those improve- enzyme is a type of protein that speeds up chem- ments. The possible applications of genetic engi- ical reactions.) The ends of these fragments have neering are virtually limitless—as are the profits an affinity for complementary ends on other to be made from genetic engineering as a busi- DNA fragments and will seek those out in the ness. As early as the 1970s, entrepreneurs (inde- target DNA. By looking at the size of the frag- pendent businesspeople) recognized the com- ment created by a restriction enzyme, investiga- mercial potential of genetically engineered prod- tors can determine whether the gene has the ucts, which promised to revolutionize life, tech- proper genetic code. This technique has been nology, and commerce as computers also were used to analyze genetic structures in fetal cells doing. Thus was born one of the great buzzwords and to diagnose certain blood disorders, such as of the late twentieth century: biotechnology, or sickle cell anemia. the use of genetic engineering for commercial GENE TRANSFER. Suppose that a purposes. particular base-pair sequence carries the instruc- Several early biotechnology firms were tion “make insulin”; if a way could be found to founded by scientists involved in fundamental insert that base sequence into the DNA of bacte- research: Boyer, for example, teamed up with the ria, for example, those bacteria would be capable venture capitalist Robert Swanson in 1976 to of manufacturing insulin. This, in turn, would form Genentech (Genetic Engineering Technolo- greatly improve the lives of people with type 1 gy). Other pioneering companies, including diabetes, who depend on insulin shots to aid Cetus, Biogen, and Genex, likewise were founded their bodies in processing blood sugar. (See Non- through the collaboration of scientists and busi- infectious Diseases for more about diabetes.) nesspeople. Today biotechnology promises a rev- Although the concept of gene transfer is rel- olution in numerous areas, such as agriculture. atively simple, its execution presents considerable Recombinant DNA techniques enable scientists technical obstacles. The first person to surmount to produce plants that are resistant to freezing these obstacles was the American biochemist temperatures, that will take longer to ripen, that Paul Berg (1926–), often referred to as the “father will develop their own resistance to pests, and so of genetic engineering.”In 1973 Berg developed a on. By 1988 scientists had tested more than two method for joining the DNA from two different dozen kinds of plants engineered to have special organisms, a monkey virus known as SV40 and a properties such as these. Yet no field of biotech- virus called lambda phage. Although the accom- nology and genetic engineering is as significant plishment was clearly a breakthrough, Berg’s as the applications to health and the cures for method was difficult. Then, later that year, the diseases.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 119 set_vol3_4 8/19/02 10:45 AM Page 120
Genetic MEDICINES AND CURES. The U.S. Department of Energy, to study the genetic Engineering use of rDNA allows scientists to produce many effects of radioactive nuclear fallout. In 1990 the products that were previously available only in Department of Energy in cooperation with the limited quantities: for example, insulin, which we National Institutes of Health (NIH), launched referred to earlier. Until the 1980s the only source the project. At about the same time, the govern- of insulin for people with diabetes came from ments of the United Kingdom, Japan, Russia, animals slaughtered for meat and other purpos- France, and Italy initiated their own, similar es. The supply was never high enough to meet undertakings, which are coordinated with Amer- demand, and this drove up prices. Then, in 1982, ican efforts. the U.S. Food and Drug Administration (FDA) The purpose of the project is to locate each approved the sale of insulin produced by geneti- human gene and determine its specific structure cally altered organisms—the first such product and function. Such knowledge will provide the to become available. Since 1982 several addition- framework for studies in health, disease, biology, al products, such as human growth hormone, and medicine during the twenty-first century have been made with rDNA techniques. and no doubt will make possible the cures for One of the most exciting potential applica- countless diseases. Although great strides have tions of genetic engineering is the treatment of been made in gene therapy in a relatively short genetic disorders, which are discussed in Heredi- time, its potential usefulness has been limited by ty, through the use of gene therapy. Among the lack of scientific data concerning the multitude more than 3,000 such disorders, quite a few of of functions that genes control in the human which are quite serious or even fatal, many are the body. For gene therapy to advance to its full result of relatively minor errors in DNA sequenc- potential, scientists must discover the biological ing. Genetic engineering offers the potential to role of each of these genes and locate each base provide individuals with correct copies of a gene, pair of which they are comprised. which could make possible a cure for that condi- A PROGRESS REPORT. Scientists tion. In the 1980s scientists began clinical trials of participating in the project have identified an a procedure known as human gene therapy to average of one new gene a day, and this rate of replace defective genes. The technique, still very discovery has increased. At the time of its estab- much in the developmental stage, offers the hope lishment in 1990 (under the leadership of James of cures for diseases that medicine has long been D. Watson, who served as director until 1992), powerless to combat. HGP was expected to reach completion by 2005. In 2001 scientists at the Weizmann Institute In 2002, however, the project’s leadership pre- in Israel brought together two of the most excit- dicted completion by some time in the following ing fields of research, biotechnology and com- year. Along the way, they had discovered that the puters, to produce the DNA-processing human genome, originally believed to include nanocomputer. It is an actual computer, but it is 100,000 to as many as 150,000 genes, actually so small that a trillion of them would fit in a test consists of about 30,000 to 40,000 genes. tube. It consists of DNA and DNA-processing Both HGP and a private firm, Celera enzymes, both dissolved in liquid; thus its input, Genomics (founded 1998), had undertaken the output, and software are all in the form of DNA study of the human genome, and in June 2000 molecules. The purpose of the nanocomputer is the entities jointly reported that they had fin- to analyze DNA, detecting abnormalities in the ished the initial sequencing of the three billion- human body and creating remedies for them. odd base pairs in the human genome. By that The Human Genome Project point, researchers also had completed thorough DNA sequences for many other organisms. The At the center of genetic studies, with vast poten- basis for the latter undertaking is that humans tial applications to genetic engineering, is the share many genes with other life-forms. With the Human Genome Project (HGP), an internation- completion of initial sequencing, scientists work- al effort to analyze and map the DNA of humans ing on the HGP undertook the effort of deter- and several other organisms. As discussed in the mining the exact sequence of the base pairs that essay Genetics, the HGP began with efforts by the make them up all human genes. Long before Atomic Energy Commission, a predecessor to the completion, the project had yielded some infor-
120 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 122
Genetic body of opposition to genetic engineering, Engineering biotechnology, and the HGP. And no aspect of the larger subject is more upsetting to certain individuals, as well as special interest groups, as that of cloning. Cloning
A clone is a cell, group of cells, or organism that contains genetic information identical to that of the parent cell or organism. It is a form of asexu- al reproduction (see Reproduction), and as such it is not as new as it seems; what is new, however, is humans’ ability to manipulate cloning at the genetic level. The first clones produced by humans as long as 2,000 years ago were plants developed from grafts and stem cuttings. By cloning—a process that calls into play complex laboratory techniques and the use of DNA repli- cation—people usually mean a relatively recent scientific advance. Among these techniques is the ability to isolate and copy (that is, to clone) indi- vidual genes that direct an organism’s develop- ment. ON FEBRUARY 24, 1997, THE FIRST MAMMAL CLONED FROM AN ADULT CELL, A LAMB NAMED DOLLY (SHOWN THE PROMISE OF CLONING. HERE AS AN ADULT), WAS BORN IN EDINBURGH, SCOT- The cloning of specific genes can provide large LAND. (Photograph by Jeff Mitchell. Archive Photos. Reproduced by numbers of copies of that gene for use in genetic permission.) and taxonomic research as well as in the practical areas of medicine and farming. In the latter field, aging reproduction among those with less desir- the goal is to clone plants with specific traits that able traits, eugenics was at one time a main- make them superior to naturally occurring stream movement whose adherents included the organisms. For example, in 1985 scientists con- distinguished U.S. Supreme Court justice Oliver ducted field tests using clones of plants whose Wendell Holmes, Jr. (1841–1935). genes had been altered in the laboratory to gen- OTHER CONCERNS. The specter of erate resistance to insects, viruses, and bacteria. eugenics raises the threat that a single human, or New strains of plants resulting from cloning a group of humans, could “play God” with the could produce crops that can grow in poor soil or lives of others. Another dramatic fear associated even underwater and fruits and vegetables with with genetic engineering is the threat that a improved nutritional qualities and longer shelf genetically re-engineered virus could turn out to lives. A cloning technique known as twinning be extremely virulent, or deadly, and spread. could induce livestock to give birth to twins or There are other, more mundane questions of even triplets, and on the environmental front ethics: for instance, is it appropriate for scientists cloning might help save endangered species from to establish private, for-profit corporations to extinction. benefit from discoveries they made while work- In the realm of medicine and health, cloning ing for public-sponsored research institutions? has been used to make vaccines and hormones. It No wonder, then, that the budget for the HGP in has become possible, by combining two different the United States includes a small allocation (3% kinds of cells (such as mouse and human cancer of its total) toward study of the ethical, legal, and cells), to produce large quantities of specific anti- social implications (ELSI) of the project. The bodies, via the immune system, to fight off dis- ELSI Working Group is charged with studying ease. When injected into the bloodstream, these the issues of fairness, privacy, delivery of health cloned antibodies seek out and attack disease- care, and education. Meanwhile, there is a vast causing cells anywhere in the body. By attaching
122 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 123
a tracer element to the cloned antibodies, scien- however, did not develop to a stage that was suit- Genetic tists can locate hidden cancers, and by attaching able for transplantation into a human uterus. Engineering specific cancer-fighting drugs, the treatment dose Then, on October 13, 2001, scientists at can be transported directly to the cancer cells. Advanced Cell Technology in Worcester, Massa- EXPERIMENTS IN CLONING. chusetts, successfully cloned a human embryo. The modern era of laboratory cloning began in They had not created human life, as it might 1958 when the British plant physiologist F. C. sound; what they had developed instead was a Steward (1904–1993) cloned carrot plants from source for nerve and other tissues that could be mature single cells placed in a nutrient culture harvested for use in medicine and research. Still, containing hormones. The first cloning of animal the news—overshadowed though it was in Amer- cells took place in 1964, when the British molec- ica, where people were still reeling from the Sep- ular biologist John B. Gurdon (1933–1989) took tember 11 terrorist attacks—was earth-shatter- nuclei from intestinal cells of toad tadpoles and ing. Human cells had been reproduced, and once injected them into unfertilized eggs. The cell again it appeared that the production of human nuclei in the eggs had been destroyed with ultra- clones might be possible. violet light, but when the eggs were incubated, It is easy to understand how people might Gurdon found that 1–2% of the eggs developed respond with alarm to such frightening news into fertile, adult toads. with alarm. Such fears have a great deal more to The first successful cloning of mammals do with Hollywood than they do with science. In occurred nearly 20 years later, when scientists in fact, the accomplishment of the Massachusetts Switzerland and the United States successfully firm, while impressive from a scientific stand- cloned mice using a method similar to Gurdon’s point, was fairly modest compared with the approach. Their method required one extra step, Frankenstein-like image presented by however: after taking the nuclei from the anti–genetic engineering scaremongers. “Cloned embryos of one type of mouse, they transferred an embryo” actually sounds a great deal more them into the embryos of another type of mouse. dramatic than what the Massachusetts scientists The latter served as a surrogate, or replacement, achieved, with just one embryo reaching the size mother. The cloning of cattle livestock was tried of six cells before the cells stopped dividing. This first in 1988, when embryos from prize cows is hardly the beginnings of a clone army. were transplanted to unfertilized cow eggs whose At any rate, the cloning practiced at the own nuclei had been removed. An even greater Massachusetts firm was therapeutic cloning, breakthrough transpired on February 24, 1997, involving the production of genetic material for with the birth of a lamb named Dolly in Edin- the treatment of specific conditions. It is a far cry burgh, Scotland. Dolly was no ordinary sheep: from reproductive cloning, which entails she was the first mammal born from the cloning implanting a cloned embryo in a uterus—and of an adult cell. Thus, she had been produced by even that is still a long way from the clichéd asexual reproduction in the form of genetically image of clones produced in a test tube without engineered cloning rather than by anything any parents other than the biological material resembling a normal process. Nonetheless, she used to create them. proved her own ability to reproduce the old-fash- Such ideas are related much more closely to ioned way when, on April 23, 1998, she gave birth those highlighted in Aldous Huxley’s 1932 novel to a daughter named Bonnie. Brave New World than they are to scientific reali- ARE HUMANS NEXT? Though ties. And even if humans wanted to develop such Dolly’s and Bonnie’s births excited hopes, they technology, it would be many, many years in the also inspired fears. If large mammals could be future. As for “creating life,” to do so is probably cloned, could humans? As early as 1993 an not even possible; if it is, such an achievement is attempt had been made at cloning human about as far off as travel to another solar system. embryos as part of studies on in vitro (out of the This is not to say that all fears of cloning and body) fertilization. The purpose was to develop genetic engineering are unwarranted; on the con- fertilized eggs in test tubes and then to implant trary, it is good to have a healthy level of skepti- them into the wombs of women having difficul- cism. But it is also good to be an equal-opportu- ty becoming pregnant. These fertilized eggs, nity skeptic and therefore to question ideas in the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 123 set_vol3_4 8/19/02 10:46 AM Page 124
Genetic Engineering KEY TERMS
AMINO ACIDS: Organic compounds DNA strand in half lengthwise and joining made of carbon, hydrogen, oxygen, nitro- it with a strand from another organism or gen, and (in some cases) sulfur bonded in perhaps even another species. characteristic formations. Strings of amino GENE THERAPY: The introduction of acids make up proteins. normal or genetically altered genes to cells, BASE PAIR: A pair of chemicals that typically to replace defective genes involved form the “rungs” on a DNA molecule, in genetic disorders. which has the shape of a spiral staircase. GENE TRANSFER: Incorporation of BIOTECHNOLOGY: A name for the new DNA into an organism’s cells, usually industry built around the application of with the help of a microorganism that genetic engineering techniques. serves as a vector. CHROMOSOME: A DNA-containing GENETIC DISORDER: A condition, body, located in the cells of most living such as a hereditary disease, that can be things, that holds most of the organism’s traced to an individual’s genetic makeup. genes. GENETIC ENGINEERING: The alter- CLONE: A cell, group of cells, or organ- ation of genetic material by direct inter- ism that contains genetic information vention in genetic processes with the pur- identical to that of its parent cell or organ- pose of producing new substances or ism. improving functions of existing organisms. CLONING: A specialized genetic NUCLEUS: The control center of a process whereby clones are produced. cell, where DNA is stored. Cloning is a form of asexual reproduction. PROTEINS: Large molecules built DNA: Deoxyribonucleic acid, a mole- from long chains of 50 or more amino cule in all cells, and many viruses, that con- acids. Proteins serve the functions of pro- tains genetic codes for inheritance. moting normal growth, repairing damaged ENZYME: A protein material that tissue, contributing to the body’s immune speeds up chemical reactions in the bodies system, and making enzymes. of plants and animals without itself taking rDNA: Recombinant DNA, or DNA part in or being consumed by those reac- that has been genetically altered through tions. gene splicing. GENE: A unit of information about a RESTRICTION ENZYMES: Enzymes particular heritable trait. Usually stored on that break DNA into fragments at particu- chromosomes, genes contain specifications lar sites. for the structure of a particular polypep- VECTOR: In the context of genetics, a tide or protein. vector is a microorganism or virus that is GENE SPLICING: A process whereby used to transfer DNA from one organism recombinant DNA is formed by cutting a to another.
124 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 125
popular culture—including opposition to genet- Genetics Education Center, University of Kansas Medical Genetic ic engineering. Center (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 125 set_vol3_4 8/19/02 10:46 AM Page 126
MUTATIONMutation
CONCEPT chromosomes. Stretches of DNA that hold coded A word familiar to all fans of science fiction, instructions for the manufacture of specific pro- mutation refers to any sudden change in DNA— teins are known as genes, of which the human deoxyribonucleic acid, the genetic blueprint for race has approximately 40,000 varieties. If the an organism—that creates a change in an organ- DNA of a particular gene is altered, that gene ism’s appearance, behavior, or health. Unlike in may become defective, and the protein for which the sci-fi movies, however, scientists typically use it codes also may be missing or defective. Just one the word mutant as an adjective rather than as a missing or abnormal protein can have an enor- noun, as, for example, in the phrase “a mutant mous effect on the entire body: albinism, for strain.” Mutation is a phenomenon significant to instance, is the result of one missing protein. many aspects of life on Earth and is one of the Mutations also can be errors in all or part of principal means by which evolutionary change a chromosome. Humans normally have 23 pairs takes place. It is also the cause of numerous con- of chromosomes, and an extra chromosome can ditions, ranging from albinism to cystic fibrosis have a tremendous negative impact. For example, to dwarfism. Mutation indicates a response to an there should be two of chromosome 21, as with outside factor, and the nature of that factor can all other chromosomes, but if there are three, the vary greatly, from environmental influences to result is Down syndrome. People with Down drugsto high-energy radiation. syndrome have a unique physical appearance and are developmentally disabled. Nor is an extra HOW IT WORKS chromosome the only chromosomal abnormali- ty that causes problems: if chromosomes 9 and DNA, Chromosomes, and 22 exchange materials, a phenomenon known as Mutations translocation, the result can be a certain type of leukemia. Down syndrome also results from Deoxyribonucleic acid, or DNA, is a molecule in translocation. the cells of all life-forms that contains genetic Germinal mutations are those that occur in codes for inheritance. DNA, discussed elsewhere the egg or sperm cells and therefore can be in this book, is as complex in structure as it is passed on to the organism’s offspring. Somatic critically important in shaping the characteristics mutations are those that happen in cells other of the organism to which it belongs, and there- than the sex cells, and they cannot be transmitted fore it is not surprising that a subtle alteration in to the next generation. This is an important dis- DNA can produce significant results. Alterations tinction to keep in mind in terms of both the to DNA are called mutations, and they can result causes and the effects of mutation. If only the in the formation of new characteristics that are somatic cells of the organism are affected, the heritable, or capable of being inherited. mutation will not appear in the next generation; Every cell in the body of every living organ- on the other hand, if a germinal mutation is ism contains DNA in threadlike structures called involved, what was once an abnormality may
126 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 127
become so common in certain populations that it shared tendency toward a particular disease. Cys- Mutation emerges as the norm. tic fibrosis, for instance, is most common in peo- ple of northern European descent, while sickle The Role of Mutation in cell anemia (see Amino Acids) occurs frequently Evolution in those of African and Mediterranean ancestry. A fatal disorder known as Tay-Sachs is found pri- Most of the forms of mutation we discuss in this marily in Jewish people whose ancestors came essay appear suddenly (i.e., in a single genera- from Eastern Europe. In many cases, the particu- tion) and affect just a few generations. Yet even lar mutation, while harmful in one regard, such seemingly “normal” characteristics as our proved to be a useful one for that population. We ten fingers and ten toes or our two eyes or our rel- know, for example, that while two copies of the atively hairless skin (compared with that of apes) mutant sickle cell anemia gene cause illness, one are ultimately the product of mutations that took copy confers resistance to malaria—a very useful shape over the many hundreds of millions of trait to people living in the tropics, where malar- years during which animal life has been evolving. ia is common. Evolution, in fact, is driven by mutation, along with natural selection (see Evolution). THE PIMA “FAT-STORAGE MUTATION.” Researchers have noted a high Over the eons, advantageous mutations, incidence of obesity among the Pima, a Native examples of which we look at later, have allowed American tribe whose ancestral homeland is life to develop and diversify from primitive cells along the Gila and Salt rivers in Arizona. The into the multitude of species—including Homo Pima tend to eat a diet that is no more fatty than sapiens—that exist on Earth today. If DNA repli- that of the average American—which, of course, cated perfectly every time, without errors, the only means that it is plenty fatty, complete with chips, life-forms existing now would be those that exist- bologna, ice cream, and all the other high-calo- ed about three billion years ago: single-cell organ- rie, low-nutrient foods that most Americans con- isms. Mutations, therefore, are critical to the sume. But whereas the average American is over- development of diverse life-forms, a phenomenon weight, the average Pima is more dramatically so. known as speciation (see Speciation). Mutations This suggests that long ago, when the ancestors of that allow an organism to survive and reproduce the Pima had to face repeated periods of famine better than other members of its species are in the dry lands of the American Southwest, sur- always beneficial, though a mutation that may be vival favored the individual or individuals who beneficial in some circumstances can be harmful had a mutation for fat storage. It so happens that in others. Mutations become especially important today, there is more than enough food at the local when an organism’s environment is changing— supermarket, but by now the Pima as a group has something that has happened often over the the fat-storage gene. Therefore, many members course of evolutionary history. And though we of the tribe have to undergo strict dietary and cannot watch evolution taking place, we can see exercise regimens so as not to become grossly how mutations are used among domesticated overweight and susceptible to heart disease and plants and animals, as discussed later. other ailments. REAL-LIFE Favorable Mutations APPLICATIONS As with other mutations relating to ethnic Ethnicity and Mutation groups, scientists have hypothesized that some advantage must be conferred upon people with Every single human trait—blue eyes, red hair, single copies of the cystic fibrosis gene or the Tay- cystic fibrosis, a second toe longer than the big Sachs disease gene. Though many mutations are toe, and so on—is the result of some genetic harmful, others prove to be beneficial to a species mutation somewhere back down the line. Traits by helping it adapt to a particular environmental that are shared by all people must have arisen influence. Useful mutations, in fact, are the driv- long ago, while other traits occur only in certain ing force behind evolution. populations of people. Traits may be as innocu- The processes of evolution are usually much ous as eye color or hair texture or as grave as a too slow for people to discern, but it is possible to
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 127 set_vol3_4 8/19/02 10:46 AM Page 128
Mutation It produces shaking and a range of other symp- toms, including depression, irritability, and apa- thy, and is usually fatal. The gene associated with Huntington’s is dominant. The horrible degenerative brain condition known as Creutzfeldt-Jakob disease, discussed in Diseases, is usually caused by another mutation. (Though it can be caused by infection, most cases of the disease are the result of heredity.) As with some of the other conditions we have mentioned, this one seems to affect particular groups more than others. Whereas the worldwide incidence of this rare condition is about one in one million, among Libyan Jews the rate is higher. The disease is a type of spongiform encephalopathy, so named because it produces characteristic spon- gelike patterns on the surface of the brain. Spongiform encephalopathies are caused by the appearance of a prion, a deviant form of protein whose production typically is caused by a muta- tion. PYGMIES, A GROUP OF PEOPLE IN SOUTHERN AFRICA, Most hereditary diseases are, by definition, APPEAR TO BE MIDGETS THROUGH A GERMINAL MUTA- TION, BUT IN MOST POPULATIONS THE MUTATION IS linked with a mutation. Such is the case with SOMATIC, OCCURRING ONLY OCCASIONALLY IN FAMILIES hemophilia, for instance (see Noninfectious Dis- WHOSE OTHER MEMBERS ARE OF ORDINARY SIZE. eases), and with cystic fibrosis, a lethal disorder (© Bettmann/Corbis. Reproduced by permission.) that clogs the lungs with mucus and typically kills the patient before the age of 30 years. Cystic observe the effects of selective breeding when fibrosis, like Huntington, occurs when a person applied to domesticated animals and plants. The inherits two copies of a mutated gene. In 1989 artificial selection of pigeons by breeders, in fact, researchers found the source of cystic fibrosis on provided the English naturalist Charles Darwin chromosome 7, where an infinitesimal change in (1809–1882) with a model for his theory of nat- the DNA sequence leads to the production of an ural selection, discussed in Evolution. Likewise, aberrant protein. animal and plant breeders use mutations to pro- Congenital Disorders duce new or improved strains of crops and live- stock. Careful breeding in this manner has In the past, all manner of superstitions arose to spawned the many different breeds of dogs, cats, explain why a child was born, for instance, with a and horses—each with their characteristic color- cleft palate, a situation in which the two sides of ing, size, temperament, and so on—that we know the roof of the mouth fail to meet, causing a today. It also has resulted in crops that are resist- speech disorder that may be mild or severe. Once ant to drought or insects or which have a high known as a harelip, the cleft palate was said to yield per acre. Likewise, goldfish, yellow roses, have formed as a result of the mother’s being and Concord grapes are all descendants of ances- frightened by a hare while she was carrying the tors with specific mutations. child. In fact, it is just one example of a congeni- Diseases and Mutation tal disorder, an abnormality of structure or func- tion or a disease that is present at birth. Congen- The majority of mutations, however, are less than ital disorders, which also are called birth defects, favorable, and this is illustrated by the relation- may be the result of several different factors, ship between mutation and certain hereditary mutation being one of the most significant. diseases. An example is Huntington disease, a Among the many examples of congenital disor- condition that strikes people in their forties or der are the hereditary diseases we have already fifties and slowly disables their nervous systems. mentioned, as well as dwarfism, Down syn-
128 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 129
Mutation
AN EXTRA CHROMOSOME 21 CAUSES DOWN SYNDROME. PEOPLE WITH THIS CONGENITAL DISORDER HAVE UNIQUE PHYSICAL FEATURES, SUCH AS A WIDE, FLAT FACE AND SLANTED EYES, AND ARE DEVELOPMENTALLY DISABLED. (© Laura Dwight/Corbis. Reproduced by permission.)
drome, albinism, and numerous other condi- itary dwarfism carry with them no ill effect on tions. the mental capacity. For example, people with the DWARVES AND MIDGETS. The type of dwarfism known as achondroplasia have term dwarf has many associations from fairy short limbs and unusually large heads, but the tales—an example of the combined fascination life span and intelligence of someone with this and revulsion with which people with congenital condition are quite normal. In the case of dias- disorders have long been treated—but it also is trophic dwarfism, the brain is fine, but the skele- used to describe persons of abnormally short ton is deformed, and the risk of death from res- stature. A dwarf is distinguished from a midget in piratory failure is high in infancy. Persons with a number of ways, all of which indicate that the diastrophic dwarfism who survive early child- features of a midget are less removed from the hood, however, are likely to enjoy a normal life norm. Midgets, while small, have bodies with span. proportions in the ordinary range. Likewise, the DOWN SYNDROME. Like people intelligence and sexual development of an adult with many other congenital disorders, those with midget are similar to those of other adults, and a Down syndrome used to be called by a name that midget or midget couple typically produces chil- now is considered crude and insensitive: mon- dren of ordinary size. Pygmies, a group of people goloid. The term, when used with a capital M, in southern Africa, appear to be midgets through refers to people of east Asian descent and is anal- a germinal mutation, but in many populations ogous to other broad racial groupings: Cauca- the mutation is somatic, occurring only occa- soid, Negroid, and Australoid. In the case of peo- sionally in families whose other members are of ple with Down syndrome, mongoloid referred to ordinary size. the unusual facial features that mark someone Dwarfs, by contrast, have several different with that condition. disorders. One variety of dwarfism, known in the A person with Down syndrome (caused by past as cretinism, is characterized by a small, an extra chromosome in the 21st chromosomal abnormally proportioned body and an impaired pair) is likely to have a wide, flat face and eyes mind. On the other hand, several forms of hered- that are slanted, sometimes with what is known
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 129 set_vol3_4 8/19/02 10:46 AM Page 130
Mutation
AN ALBINO NORTHERN FUR SEAL. A CONDITION THAT RESULTS FROM AN INHERITED DEFECT IN MELANIN METABOLISM (MELANIN IS RESPONSIBLE FOR THE COLORING OF SKIN), ALBINISM IS MARKED BY AN ABSENCE OF PIGMENT FROM THE HAIR, SKIN, AND EYES. (© John Francis/Corbis. Reproduced by permission.)
as an inner epicanthal fold—all facial characteris- skin, and eyes. The hair of an albino tends to be tics common among people who are racially whitish blond, the skin an extremely pale white, Mongoloid. Numerous other facial features iden- and the eyes pinkish. Albinism occurs among tify a person with Down syndrome as someone other animals: hence the white rats, rabbits, and who suffers from a specific congenital disorder, mice almost everyone has seen. Domestic white including a short neck, ears that are set low, a chickens, geese, and horses are partial albinos small nose, large tongue and lips, and a chin that that retain pigment in their eyes, legs, and feet. As slopes. People with Down syndrome are apt to was once true of people with other congenital have poor muscle tone and possess abnormal disorders, human albinos once inspired fear and ridge patterns on their palms and fingers and the awe. Sometimes they were killed at birth, and in soles of their feet. Heart and kidney problems are the mid–nineteenth century, albinos were exhib- common with Down syndrome as well, but one ited in carnival sideshows. In these cruel specta- feature is most common of all: mental retarda- cles, sometimes whole families were put on dis- tion. The condition occurs in about one of 1,000 play, touted as a unique race of “night people” live births among women under age 40 but about who lived underground and came out only when one in 40 live births to older women. Overall, the the light was dim enough not to hurt their eyes. incidence is about one in 800 live births. As noted On the other hand, some ethnic groups earlier, the cause of Down syndrome is transloca- experience enough albino births that another tion, but the reason translocation occurs is not one causes no excitement. For example, among known. the San Blas Indians of Panama, one in approxi- ALBINISM. Compared with dwarfism mately 130 births is an albino, compared with or Down syndrome, albinism is not nearly as one in 17,000 for humans as a whole. Albinism severe in terms of its effect on a person’s func- comes about when melanocytes (melanin-pro- tioning. A condition that results from an inherit- ducing cells) fail to produce melanin. In tyrosi- ed defect in melanin metabolism (melanin is nase-negative albinism, the most common form, responsible for the coloring of skin), albinism is the enzyme tyrosinase (a catalyst in the conver- marked by an absence of pigment from the hair, sion of tyrosine to melanin) is missing from the
130 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_4 8/19/02 10:46 AM Page 131
Mutation KEY TERMS
AMINO ACIDS: Organic compounds NATURAL SELECTION: The process made of carbon, hydrogen, oxygen, nitro- whereby some organisms thrive and others gen, and (in some cases) sulfur bonded in perish, depending on their degree of adap- characteristic formations. Strings of amino tation to a particular environment. acids make up proteins. ORGANIC: At one time chemists used CHROMOSOME: DNA-containing the term organic only in reference to living bodies, located in the cells of most living things. Now the word is applied to com- things, that hold most of the organism’s pounds containing carbon and hydrogen. genes. POLYPEPTIDE: A group of between 10 CONGENITAL DISORDER: An abnor- and 50 amino acids. mality of structure or function or a disease PROTEINS: that is present at birth. Congenital disorders Large molecules built also are called birth defects. from long chains of 50 or more amino acids. Proteins serve the functions of pro- DNA: Deoxyribonucleic acid, a mole- moting normal growth, repairing damaged cule in all cells, and many viruses, contain- tissue, contributing to the body’s immune ing genetic codes for inheritance. system, and making enzymes. GENE: A unit of information about a RNA: particular heritable trait. Usually stored on Ribonucleic acid, the molecule chromosomes, genes contain specifications translated from DNA in the cell nucleus, for the structure of a particular polypep- the control center of the cell, that directs tide or protein. protein synthesis in the cytoplasm, or the space between cells. GERMINAL MUTATION: A mutation that occurs in the egg or sperm cells, which SOMATIC MUTATION: A mutation therefore can be passed on to the organ- that occurs in cells other than the repro- ism’s offspring. ductive, or sex, cells. These mutations, as HERITABLE: Capable of being inherit- contrasted with germinal mutations, can- ed. not be transmitted to the next generation. MUTAGEN: A chemical or physical fac- SPECIATION: The divergence of evo- tor that increases the rate of mutation. lutionary lineages and creation of new species. MUTATION: Alteration in the physical structure of an organism’s DNA, resulting TRANSLOCATION: A mutation in in a genetic change that can be inherited. which chromosomes exchange parts.
melanocytes. When the enzyme is missing, no tyrosinase-negative albinism. It is equally com- melanin is produced. In tyrosinase-positive mon among blacks and whites, while more blacks albinism, a defect in the body’s tyrosine transport than whites are affected by tyrosinase-positive system impairs melanin production. One in albinism. Native Americans have a particularly every 34,000 persons in the United States has high incidence of both forms of albinism.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 131 set_vol3_4 8/19/02 10:46 AM Page 132
Mutagens and Other Causes Mutation were linked with the chimney sweep’s cancer of As might be expected, cells that divide many, late eighteenth-century England, discussed in many times in a lifetime are more at risk of errors Noninfectious Diseases. In fact, cancer itself is a and mutations than cells that divide less fre- kind of mutation, involving uncontrolled cell quently. In a human female, egg cells are fully growth. Other environmental factors that are formed at birth, and they never divide thereafter. known to bring about mutations include expo- By contrast, sperm cells are being produced con- sure to pesticides, asbestos, and some food addi- stantly, and the older a man is, the more fre- tives, many of which have been banned. quently his sperm-producing cells have divided. By age 20 they will have divided 200 times and by WHERE TO LEARN MORE age 45 about 770 times. This has led scientists to “Are Mutations Harmful?” Talk. Origins (Web site). hypothesize that when a baby is born with a con-
132 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 133
SCIENCE OF EVERYDAY THINGS real-life biology REPRODUCTION AND BIRTH
REPRODUCTION SEXUAL REPRODUCTION PREGNANCY AND BIRTH
133 set_vol3_5 8/19/02 11:06 AM Page 135
Reproduction REPRODUCTION
CONCEPT much more so, freighted as it is with degrees of The term reproduction encompasses the entire meaning that go far beyond mere biology—asex- variety of means by which plants and animals ual reproduction is a fairly simple, cellular produce offspring. Reproductive processes fall process. Of course, nothing in nature is really into two broad groupings: sexual and asexual, the simple, and, in fact, the dividing and replication latter being the means by which bacteria and of DNA (deoxyribonucleic acid, the genetic blue- algae reproduce. Many plants reproduce sexually print material found in each cell) is a complicat- by means of pollination, and some plants alter- ed subject; however, that subject, too, is discussed nate between sexual and asexual forms of repro- in the essay Genetics. DNA is located at the cell duction. Other creatures, such as bees and ants, nucleus, which is the cell’s control center, and the reproduce through a form of reproduction called nucleus is the first part of the cell to divide in parthenogenesis, which is neither fully sexual nor asexual reproduction. After the nucleus splits, the asexual. cytoplasm, or the cellular material external to the nucleus, then divides. The result is the formation of two new daughter cells whose nuclei have the HOW IT WORKS same number and kind of chromosomes as the parent. Asexual Reproduction The adaptive advantage of asexual reproduc- Asexual reproduction involves only one organ- tion is that organisms can reproduce quickly and ism, as opposed to two in sexual reproduction. It by doing so colonize favorable environments rap- occurs when a single cell divides to form two idly. (See Evolution for more about the impor- daughter cells that are genetically identical to the tance of adaptation and environment in shaping parent cell. This process is known as fission, and species.) For example, some bacteria can double it may take the form either of binary fission, in their numbers every 20 minutes. In addition to which two new cells are produced, or multiple bacteria, which are discussed in more detail in fission, which results in the creation of many new Infection, other life-forms that reproduce asexu- cells. Since there is no fusion of two different ally include protozoa (varieties of which are cells, the daughter cells produced by asexual examined in Parasites and Parasitology), blue- reproduction are genetically identical to the par- green algae, yeast, dandelions, and flatworms. ent cell. Asexual reproduction usually takes place Sexual Reproduction by mitosis, a process during which the chromo- somes in a cell’s nucleus are duplicated before cell Sexual reproduction involves the union of two division. (Mitosis, chromosomes, and many organisms rather than the splitting of one. Like other topics referred to in this essay are discussed asexual reproduction, it is a process that takes in considerably more detail in Genetics.) place at the cellular level. In sexual reproduction Whereas sexual reproduction is extremely it is not binary fission that occurs, but the fusion complex—and human sexual reproduction is of two cells. Nor are the two cells identical;
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 135 set_vol3_5 8/19/02 11:06 AM Page 136
VEGETATIVE PROPAGATION. Reproduc- rather, the cells—known as gametes—can be tion identified as either male or female according to Strawberries reproduce by forming growths the makeup of their chromosomes. The male called runners, which grow horizontally and gen- gamete is called a sperm cell, and the female erate new stalks. At some point, the runner gamete is termed an egg cell. In sexual reproduc- decomposes, leaving a new plant that is a clone of tion, the sperm cell fuses, or bonds, with the egg the original. This is an example of vegetative propagation, a term for a number of processes by cell to produce a cell that is genetically different which crop plants are produced asexually. Vege- from either of the parent cells. This process of tative propagation is used for such crops as pota- fusion is known as fertilization, and the fertilized toes, bananas, raspberries, pineapples, and some egg is called a zygote. Gametes are produced in flowering plants. Its advantage to farmers is that the male testes and female ovaries by a splitting the crops will be more uniform than those grown process called meiosis. (Meiosis and other terms from seed. Furthermore, some plants are difficult mentioned briefly in these paragraphs are dis- to cultivate from seed, and the vegetative propa- cussed in much more detail in Genetics.) gation of those plants makes it possible to grow Meiosis produces haploid cells, or ones that crops that otherwise would not be available for have half the number of chromosomes as are in a commercial marketing. normal cell for that species. When the haploid In reproducing potatoes through vegetative sperm and egg cells fuse at fertilization, however, propagation, farmers plant the so-called eyes to the chromosomes from both combine, so that the produce duplicates of the parent. With banana normal number of chromosomes appears in the plants, the farmer separates the suckers that grow zygote. The shuffling of the parents’ genetic mate- from the root of the plant and plants them. The rial that happens during meiosis allows for new farmer raising raspberry bushes bends the gene combinations in offspring that account for branches and covers them with soil, whereupon a variations between offspring (which is why you process not unlike that of the runner growth of don’t look just like your siblings) and which, over mosses takes place: the branches eventually grow time, can improve a species’ chances of survival. into a separate plant, with their own root system, and ultimately can be detached from the parent REAL-LIFE plant. APPLICATIONS Between Asexual and Sexual
Examples of Asexual The example of vegetative propagation suggests Reproduction that there is not a sharp dividing line between sexual and asexual reproduction—that is, that As we noted earlier, bacteria, blue-green algae, many organisms can reproduce either way. This most protozoa, yeast, and flatworms all repro- is true even of humans, who, in theory, could be duce asexually, as do mosses and starfish. (The cloned, though the technology to do so—let last actually reproduce both sexually and asexu- alone resolution of the ethical issues of the pro- ally by means of alternation of generations, dis- cedure—lies in the far distant future. (See Genet- cussed later.) The products of asexual reproduc- ic Engineering for more on this subject.) Even tion are known as clones—an example of the humans, however, can use external fertilization, fact, discussed in Genetic Engineering, that which is sexual reproduction without sexual cloning and the concept of clones are not as new intercourse (see Sexual Reproduction). as one might imagine. (See that essay for much Plants go through a process known as alter- more about artificial cloning.) A starfish can nation of generations, in which they alternate as regenerate and eventually produce a whole new sexual and asexual reproducers, or gametophytes organism from a single severed appendage, while and sporophytes, respectively. In the asexual flatworms divide in two and regenerate to form stage, the sporophyte produces diploid reproduc- two new flatworms. This formation of a separate tive cells called spores, which develop into game- organism is obviously much more complex than tophytes. These gametophytes produce haploid the simple splitting of single bacteria cells, but it gametes, which then unite sexually to form a is still a form of asexual reproduction. diploid zygote that grows into a sporophyte. In
136 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 137
Reproduc- tion
A PINEAPPLE TOP PLACED ON SOIL CAN ROOT AND DEVELOP INTO A PLANT. NUMEROUS CROP PLANTS, AMONG THEM, POTATOES, BANANAS, RASPBERRIES, AND PINEAPPLES, ARE PRODUCED ASEXUALLY, A PROCESS CALLED VEGETATIVE PROPAGATION. (© Corbis. Reproduced by permission.)
plain English, this means that the asexual “grand- spores, which it releases into the air. These tiny parent” generates a sexually reproducing “child,” spores, carried by the wind, float away from their which in turn produces a “grandchild” that is point of origin until they come to rest, and soon asexual, like its grandparent. the cycle begins once again. At one phase in the alternation of generation PARTHENOGENESIS. There are for mosses, for instance, male and female moss also organisms, including bees, ants, wasps, and plants grow from spores. Male moss plants pro- other insects, that reproduce in a way that is nei- duce sperm cells, which, when the moss receives ther fully sexual nor asexual. This is partheno- rainfall, are able to propagate because they have a genesis, a type of reproduction in which a gamete medium (water) in which to move. They fertilize develops without fertilization. In other words, a the female plants, producing zygotes. The zygote sex cell is reproduced without actual intercourse grows on top of the female moss plant, which between male and female. The gamete is almost helps to store moisture and thus provides a hos- always female—a fact indicated in the name pitable environment in which the zygote can itself, which comes from parthenos, Greek for develop. The zygote eventually produces haploid “maiden.”
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 137 set_vol3_5 8/19/02 11:06 AM Page 138
Reproduc- plants. Actually, cross-pollination, or the transfer tion of pollen from one plant to another, would per- haps be analogous to sexual intercourse in ani- mals. Pollination occurs in seed-bearing plants, as opposed to the more primitive spore-produc- ing plants, such as ferns and mosses. Gym- nosperms, such as pines, firs, and spruces, pro- duce male and female cones, whereas angiosperms produce flowers containing a male organ called the stamen and a female organ called the pistil. Both types of plants rely on insects and other creatures to aid in the pollen transfer. DARWIN’S MOTH. The German physician and botanist Rudolf Jakob Camerarius (1665–1721) was the first scientist to demon- strate that plants reproduce sexually, and he pio- neered the study of pollination. One of the scien- tists influenced by his work was the English nat- uralist Charles Darwin (1809–1882), who dis- cussed the subject in The Various Contrivances by THE ACORN WORM UNDERGOES ASEXUAL REPRODUC- which Orchids Are Fertilized by Insects (1862). TION BY BUDDING; SMALL PIECES FRAGMENT FROM THE Darwin wrote this book partly to support the TRUNK, AND EACH GROWS INTO A NEW WORM. (© Lester V. Bergman/Corbis. Reproduced by permission.) ideas on evolution presented in his much more well known book Origin of Species (1859). In Var- ious Contrivances, he suggested that orchids and The Parthenon in Athens, like the city itself, their insect pollinators evolved by interacting is named after the goddess Athena (also called with one another over many generations. Minerva), who was known by the nickname As an example, he discussed Angraecum Parthenos. She is said to have been born fully sesquipedale, an orchid native to Madagascar. formed, having sprung from the head of her Darwin had not seen the plant in its native habi- father, Zeus, dressed in armor and ready for bat- tat, however; he had looked only at its dried tle. Thus, her own birth was a form of partheno- leaves. The white flower of this orchid has a foot- genesis, a word whose second half (a name well long (30 cm) tubular spur with a small drop of known from the Bible) means “beginning.” nectar at its base, and from observing this, he Pollen and Pollination hypothesized that the orchid had been pollinated by an insect with a foot-long tongue. This Pollen is a fine, powdery substance consisting of hypothesis, he wrote, “has been ridiculed by microscopic grains containing the male gameto- some entomologists,” or scientists who study phyte of certain plants that reproduce sexually. insects. After all, no such creature had been These plants include angiosperms, a type of plant found in Madagascar. But then, around the turn that produces flowers during sexual reproduc- of the nineteenth century—some two decades tion, and gymnosperms, which reproduce sexu- after Darwin’s death—it was found. A Madagas- ally through the use of seeds that are exposed and can moth was discovered that had a foot-long not hidden in an ovary, as with an angiosperm. tongue that uncoils to sip the nectar of A. Pollen is designed for long-distance dispersal sesquipedale as it cross-pollinated the flowers. from the parent plant, so that fertilization can PLANTS AND THEIR POLLINA- occur. Pollination is the transfer of pollen from TORS. Angiosperms and gymnosperms are the male reproductive organs to the female discussed in Ecosystems and Ecology, where each reproductive organs of a plant, and it precedes is compared in terms of its degree of adaptation fertilization. In other words, pollination is the to its environment. Angiosperms seem to be the equivalent of sexual intercourse for seed-bearing hands-down winner: by enlisting the aid of
138 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 139
Reproduc- KEY TERMS tion
ALTERNATION OF GENERATIONS: ENZYME: A protein material that A process whereby plant generations alter- speeds up chemical reactions in the bodies nate as sexual and asexual reproducers— of plants and animals without itself taking gametophytes and sporophytes, respectively. part in or being consumed by those reac- ASEXUAL REPRODUCTION: One of tions. the two major varieties of reproduction FERTILIZATION: The process of cellu- (along with sexual reproduction), In con- lar fusion that takes place in sexual repro- trast to sexual reproduction, which duction. The nucleus of a male reproduc- involves two organisms, asexual reproduc- tive cell, or gamete, fuses with the nucleus tion involves only one. Asexual reproduc- of a female gamete to produce a zygote. tion occurs when a single cell divides through mitosis to form two daughter GAMETE: A reproductive cell—that is, cells, which are genetically identical to the a mature male or female germ cell that pos- parent cell. sesses a haploid set of chromosomes and is prepared to form a new diploid by under- BINARY FISSION: The process in going fusion with a haploid gamete of the asexual reproduction whereby a single cell opposite sex. Sperm and egg cells are, divides to form two daughter cells that are genetically identical to the parent cell. respectively, male and female gametes. GAMETOPHYTE: CLONE: A cell, group of cells, or organ- In alternation of ism that contains genetic information iden- generations, a gametophyte is a plant that tical to that of its parent cell or organism. reproduces sexually. CLONING: A specialized genetic GERM CELL: One of two basic types process whereby clones are produced. of cells in a multicellular organism. In con- Cloning is a form of asexual reproduction. trast to somatic or body cells, germ cells play a part in reproduction. CHROMOSOME: A DNA-containing body, located in the cells of most living HAPLOID: A term for a cell that has things, that holds most of the organism’s half the number of chromosomes that genes. appear in a diploid or somatic cell. CROSS-POLLINATION: The transfer MEIOSIS: The process of cell division of pollen from one plant to another. that produces haploid genetic material. CYTOPLASM: The material inside a Compare with mitosis. cell that is external to the nucleus. MITOSIS: A process of cell division DIPLOID: A term for a cell that has the that produces diploid cells, as in asexual basic number of doubled chromosomes. reproduction. Compare with meiosis. DNA: Deoxyribonucleic acid, a mole- NUCLEUS: The control center of a cule in all cells, and many viruses, that con- cell, where DNA is stored. tains genetic codes for inheritance. OVARY: Female reproductive organ EGG CELL: A female gamete. that contains the eggs.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 139 set_vol3_5 8/19/02 11:06 AM Page 140
Reproduc- tion KEY TERMS CONTINUED
OVULE: Female haploid gametophyte SEXUAL REPRODUCTION: One of of seed plants, which develops into a seed the two major varieties of reproduction upon fertilization by a pollen grain. (along with asexual reproduction). In con- PARTHENOGENESIS: A type of trast to asexual reproduction, which reproduction that involves the develop- involves a single organism, sexual repro- ment of a gamete without fertilization. In duction involves two. Sexual reproduction other words, a sex cell (usually female) is occurs when male and female gametes reproduced without actual intercourse undergo fusion, a process known as fertil- between male and female. ization, and produce cells that are geneti- POLLEN: Male haploid gametophyte cally different from those of either parent. of seed plants (including angiosperms and SOMATIC CELL: One of two basic gymnosperms), which unites with the types of cells in a multicellular organism. ovule to form a seed. Pollen is a fine, pow- In contrast to germ cells, somatic cells (also dery substance consisting of microscopic known as body cells) are not involved in grains. reproduction; rather, they make up the tis- POLLINATION: The transfer of pollen sues, organs, and other parts of the organ- from the male reproductive organs to the ism. female reproductive organs of a plant. Pol- SPERM CELL: A male gamete. lination precedes fertilization. See also cross-pollination. SPOROPHYTE: In alternation of gen- erations, a sporophyte is a plant that repro- REGENERATION: A biological process among some lower animals where- duces asexually. by a severed body part is restored by the ZYGOTE: A diploid cell formed by the growth of a new one. fusion of two gametes.
insects and other pollinators, they manage to For example, some flowers are pure red, or pollinate much more efficiently than gym- nearly pure red, and have very little odor. In most nosperms, which have to produce vast quantities such situations, the pollinator is a bird species, of pollen for each grain that reaches its target. since birds have excellent vision in the red region Typically, pollination benefits the animal pollina- of the spectrum but a rather undeveloped sense tor by supplying it with sweet nectar and, of of smell. It so happens that Europe, which has no course, benefits the plant by providing direct pure red native flowers, also has no bird-pollinat- transfer of pollen from one plant to the pistil of ed native flower species. Not all bird-pollinated another plant. For this reason, specific plant and flowers are red, but they are all characterized by animal species have developed a relationship of striking, and sometimes contrasting, colors that mutualism, a form of symbiosis in which each readily catch the eye. Examples of plants polli- participant reaps benefits (see Symbiosis). In nated by birds include the cardinal flower, the red many cases, plant and pollinator have evolved columbine, the hibiscus, the eucalyptus, and together, and it is possible to determine which animal pollinates a certain flower species simply varieties of orchid, cactus, and pineapple. by studying the morphologic features (shapes), Some flowering plants have a very strong color, and odor of the flower. odor but are very dark, or at least drab, in color.
140 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 141
These flowers and plants—examples include the Elia, Irene. The Female Animal. New York: Henry Holt, Reproduc- saguaro cactus, century plant, or cup-and-saucer 1988. tion vine—are often pollinated by bats, which have “Flowering Plant Reproduction.” Estrella Mountain Com- very poor vision, are typically active during the munity College (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 141 set_vol3_5 8/19/02 11:06 AM Page 142
SEXUALSexual Reproduction REPRODUCTION
CONCEPT ductive system, a group of organized structures Sexual reproduction is one of the two major that can be subdivided into male and female ways, along with asexual reproduction, that reproductive systems. During puberty, which plants and animals create offspring and thus typically occurs between the ages of 10 and 14 propagate the species. Critical to sexual repro- years, the reproductive systems of both sexes duction is the process of fertilization, whereby mature. This phase is marked in part by the the male and female sex cells fuse, or bond. Fer- release of eggs (female sex cells) in the female tilization may be of two types, either internal or ovary and the formation of sperm (male sex external, and though humans normally fertilize cells) in the male testes. Reproduction can take by the first of those means, they may use the sec- place only when a sperm unites with an egg, a ond, in the form of in vitro fertilization. Humans, process called fertilization. of course, have by far the most complicated THE MALE REPRODUCTIVE reproductive process, inasmuch as it is surround- SYSTEM. The testes are the pair of male ed by a vast societal, interpersonal, and moral reproductive glands located in the scrotum, a framework that is not a factor in animal repro- skin-covered sac that hangs from the groin. Each duction. Part of that framework is the activity testis produces sperm cells, while the testes as a that precedes sexual reproduction: attraction, whole secrete testosterone. Testosterone is a hor- courtship, and so forth. Although no other ani- mone—a type of molecule that sends signals to mal’s courtship rituals rival those of humans for spots remote from its point of origin to induce sophistication, some of them are quite impressive specific effects on the activities of other cells. in their complexity. Testosterone is associated with masculinity, though females secrete it in much smaller quan- HOW IT WORKS tities as well. In males, testosterone secretion is critical to the development of secondary sexual The Reproductive System characteristics—those unique traits that mark a person as a male or female, though they do not The contrast between sexual and asexual repro- occur in the sexual organs themselves. A deep- duction is examined in Reproduction, an essay ened voice is an example of a male secondary sex that also provides examples of plant reproduc- characteristic evident at puberty. tion through pollination. The present essay is Sperm cells produced in the testes move to concerned primarily with human sexual repro- the epididymis, a coiled tube at the base of the duction and secondarily with animal sexual penis where they are stored and matured. During reproduction. Some technical aspects of repro- ejaculation, or the ejection of sperm from the duction at the cellular level require consultation penis during orgasm, sperm travel from the epi- of processes explained in Genetics; here we con- didymis through a long tube called the vas defer- fine our technical discussion to reproduction at ens to the urethra. This single tube, which the level of organs, fluids, and other bodily com- extends from the bladder to the tip of the penis, ponents. Reproduction is facilitated by the repro- is also the means by which urine passes out of the
142 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 143
body. Liquid secretions from various glands month by thickening, but if fertilization does not Sexual combine with sperm (itself a gooey substance take place, the endometrium is shed during men- Reproduc- tion that is barely liquid) to form the semen, or semi- struation. nal fluid. Ejaculated semen may contain as many as 400 million sperm. Fertilization The Female Reproductive During sexual intercourse, a man releases System approximately 300 million sperm into a woman’s vagina, but only one of the sperm can fertilize the The female system is much more complicated ovum. The successful sperm cell must enter the than the male version and has a role in all stages uterus, swim up the fallopian tube (a trumpet- of reproduction. Whereas the male system pri- shaped passageway between the ovary and the marily delivers semen to the vagina, the female uterus) to meet the ovum, and then pass through system plays a critical part from fertilization until a thick coating, known as the zona pellucida, that long after the birth of offspring. It produces ova, surrounds the egg. The head of the sperm cell or eggs, receives sperm from the penis, houses contains enzymes (a type of protein that speeds and provides nutrients to the developing zygote up chemical reactions—see Enzymes) that break (fertilized egg) and later the embryo and fetus, through the zona pellucida and allow the sperm gives birth to offspring, and feeds those offspring to penetrate the egg. Once the head of the sperm after birth. is inside the egg, the tail falls off, and the outside The visible part of the female reproductive of the egg thickens to prevent another sperm system, which, of course, is not even half of the from entering. Many variables affect whether fer- entire picture, includes the opening of the vagina tilization occurs after intercourse among and the external genital organs, or vulva. The humans. One factor is a woman’s ovulatory, or vagina, a muscular tube extending from the menstrual, cycle. Human eggs can be fertilized uterus to the outside of the body, is the recepta- for only a few days after ovulation, which typical- cle for sperm ejaculated during sexual inter- ly occurs only once every 28 days. (To learn about course and also forms part of the birth canal that what happens after fertilization, see Pregnancy will be used later, when the offspring comes to and Birth.) term. The external genital organs, known collec- tively as the vulva, include the labia, folds of skin REAL-LIFE on both sides of the openings to the vagina and APPLICATIONS urethra; the clitoris, a small, sensitive organ that is comparable to the male penis inasmuch as it External Fertilization swells when stimulated; and the mons pubis, a mound of fatty tissue above the clitoris. Most land animals use some form of internal fer- THE OVARIES AND MENSTRU- tilization similar to that which we have described ATION. Eggs are produced in the ovaries, for humans. External fertilization, on the other oval-shaped organs in the groin that also gener- hand, is more common among aquatic animals, ate sex hormones. At birth, a female’s ovaries who simply dump their sperm and eggs into the contain hundreds of thousands of undeveloped water and let currents mix the two male and eggs, each surrounded by a group of cells to form female cells together. The sea urchin is a typical a follicle, or sac; however, only about 360-480 fol- example: a male sea urchin releases several billion licles reach full maturity. During puberty the sperm into the water, and these sperm then swim action of hormones causes several follicles to toward eggs released in the same area. Fertiliza- develop each month. Normally, just one follicle tion occurs within seconds when sperm come fully matures, rupturing and releasing an ovum into contact and fuse with eggs. As noted in through the ovary wall in a process called ovula- Reproduction, external fertilization is essentially tion. The mature egg enters one of the paired fal- sexual reproduction without sexual intercourse. lopian tubes, where it may be fertilized by a For humans the process of reproduction by sperm and move on to the uterus to develop into external means may lack the intimacy of internal a fetus. The lining of the uterus, called the reproduction, but since 1978 a form of external endometrium, prepares for pregnancy each fertilization has offered the opportunity of con-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 143 set_vol3_5 8/19/02 11:06 AM Page 144
Sexual Reproduc- tion
DURING IN VITRO fertilization, eggs are removed surgically from a female’s reproductive tract and fertilized in a test tube or petri dish with sperm (shown here). After the fertilized eggs have divided twice and an embryo is starting to form, they are reintroduced to the female’s body. (© Owen Franken/Corbis. Reproduced by permission.)
ceiving children to couples who otherwise might ation, they added male sperm to the egg in the have remained childless. petri dish, where fertilization took place, and IN VITRO Fertilization. This then implanted the eight-celled embryo in the form of external fertilization is known as in vitro, mother’s body. The result of this extraordinary or “in glass”—that is, in a glass test tube or petri operation was a healthy baby named Louise dish. The term is contrasted with in vivo, mean- Brown. ing “in a living organism,” or in utero, “in the A WORLD WITHOUT SEX? The uterus.” During in vitro fertilization, eggs are birth of Louise, whose twentieth birthday was removed surgically from a female’s reproductive celebrated in Britain with great fanfare, excited tract. They then can be fertilized in a test tube or fears and controversy similar to those surround- petri dish by sperm taken from the woman’s ing cloning (see Genetic Engineering). As with partner or another male. After the fertilized eggs cloning, the fear was that the technology of in have divided twice, indicating that the operation vitro fertilization would lead to the depersonal- has “taken” and an embryo is starting to form, ized manufacturing of human beings associated they are reintroduced to the female’s body. If all with some nightmarish future society, but this goes well, the embryo and fetus eventually result has not come to pass for several reasons. One is in a normal birth. that in vitro fertilization is successful only about In vitro fertilization has been performed suc- 15% of the time. Another, much more significant cessfully on a variety of domestic animals since reason is that there are few women capable of the 1950s but on humans only since the late producing a child through internal fertilization 1970s. Two English physicians, Patrick Steptoe who would want to conceive without the intima- (1913–1988) and Robert G. Edwards (1925–), cy of sexual intercourse. developed a method for stimulating ovulation There are exceptions, of course, in real life with hormone treatment and then retrieving the (two women in a same-sex relationship who nearly mature eggs and placing them in a petri want a child, for example) as well as in fiction. In dish to mature. In their breakthrough 1978 oper- the latter category, the character of Jenny Fields
144 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 145
in the American novelist John Irving’s World Sexual According to Garp—published, ironically, in the Reproduc- tion same year as the birth of Louise Brown—would undoubtedly have conceived a child by in vitro means if those means had been available to her. As it was, Jenny, who conceived the book’s title character during World War II, manages to do so by having intercourse purely for the purpose of producing offspring. She does this by choosing a wounded, brain-damaged airman who will never remember having had sex with her and who dies shortly thereafter. Courtship
There is something far greater than mere bio- chemistry involved in sexual reproduction, which is why humans enjoy the intimacy of pro- ducing a baby. This is also why courtship and mating rituals are a significant part of human life, providing a means by which a male and female join as partners. Such is true even in mod- ern America and the West in general, where the aftermath of the 1960s sexual revolution has left AMONG ANIMALS, COURTSHIP IS A COMPLEX SET OF behind a world largely stripped of its former BEHAVIORS THAT LEADS TO MATING. HERE MALE mystery. For better or worse, sex before marriage FRIGATE BIRDS DISPLAY THEIR INFLATED SCARLET is a common part of life today in a way that it was POUCHES TO ATTRACT FEMALES. (© Wolfgang Kaehler/Corbis. not before the 1960s, with the advent of birth Reproduced by permission.) control and “free love,” and today there is little stigma attached to the conception of a child out- animals use rituals, a series of behaviors for com- side wedlock. That much has changed—and munication that is performed the same way by all changed dramatically—but humans still practice the males or females in a species. They are gov- courtship. erned by fixed-action patterns (FAPs), which are Males still have to prove to females that they virtually identical with instinct (see Instinct and are suitable mates, usually by displaying their Learning). In courtship, some animals leap and physical prowess or some other attribute associ- dance, others sing, and still others ruffle their ated with masculinity. Females still do most of feathers or puff up pouches. The male peacock the choosing, and despite all the changes in views displays his glorious plumage to the female, and toward male and female roles, the ideas of basic humpback whales advertise their presence under male and female differences seem to be hard- the sea by singing a song that can be heard hun- wired into the minds of most people. This is par- dreds of miles away. Courtship behavior enables ticularly so of people who are heterosexual and an animal to find, identify, attract, and arouse a especially those who either have children or mate. Animals use signals, such as the release of intend to have them. Such attitudes are not sur- pheromones, or scent signals, as well as visual prising, since humans, while being something displays to claim a particular mate or a territory. more than animals, are still animals as well. CHOOSING A MATE. Usually, the ANIMAL COURTSHIP. Among ani- females do the choosing. In some species of mals, courtship is a complex set of behaviors that birds, males display themselves in a small com- leads to mating. Courtship behavior communi- munal area called a lek, where females select a cates to each potential mate that the other is not mate from the parading males. Across the animal a threat and serves to reveal to each that the kingdom, males generally compete with one species, gender, and physical condition of the another for mates, either by fighting or by ritual- other are suitable for mating. During courtship, ized displays, and females pick the best quality of
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 145 set_vol3_5 8/19/02 11:06 AM Page 147
young. Among the long-jawed long-horned bee- to conform. In any case, the American ideal of a Sexual tles that live in the Arizona desert, males battle thin female is far from universal, in terms of Reproduc- tion each other for saguaro cactus fruit, and the either place or time. Many traditional cultures females mate in exchange for access to the fruit. favor a more amply formed female body, as did Genetic fitness, or the ability to survive— Western civilization in the past—a fact exempli- and to advance the survival of the species—is fied by the paintings of seventeenth-century another important factor in mate selection. This artists, such as Peter Paul Rubens, famous for his is a large part of the reason why males may fight fleshy female subjects. This is an ideal much clos- each other for a female—to show her that they er to nature, since our evolutionary lineage has are the most fit. Of course, the female animal tended to favor women with large hips who are does not know that she is choosing on this basis, capable of bearing many children. but she is, and she is likely to select a partner with EVOLUTION AND THE SENSI- a striking appearance, capable of energetic dis- TIVE MAN. As with much else about sexual plays—both of which are signs of good health. reproduction and the courtship rituals associated with it, the idea of beauty—itself an expression Secondary Sex Characteris- of secondary sex characteristics—is much sim- tics, Evolution, and the pler for men. As we noted earlier, physical fitness Model of an Ideal Mate in men almost always is regarded as attractive, but women often value men for other traits, most Humans, too, have deeply ingrained ideas regard- ing what makes an attractive potential mate, notably intelligence and sense of humor. Where- though to some extent, those ideas are cultural. as many men place an emphasis on physical Most societies regard a muscular male as attrac- appearance in choosing a mate, women’s expec- tive, for obvious evolutionary reasons: a physical- tations are much more complex and they are also ly fit male can provide for his mate and offspring, likely to be much more forgiving toward mem- both by acquiring food and other resources and bers of the male population who do not look like by protecting the nest. On the other hand, the Tom Cruise or some other Hollywood image of modern American ideal of feminine beauty is attractiveness. more removed from nature. For one thing, this In defense of modern men, many have com- image of an attractive female is a thin body but plained that modern women do not always want large breasts—two things that seldom go togeth- what they say they want. For example, from the er in nature but which are possible in the modern early 1970s onward, it often was said in public world through breast implantation surgery. That discussions of sexuality on TV or in magazines is, the achievement of such an ideal is available to that women wanted men to be more sensitive. a woman with a naturally thin body who also That is, they wanted men who were more verbal possesses the financial resources (and, of course, and given to talking about their feelings and who the desire) to undergo such surgery. were more aware of the woman and her needs. It A woman naturally gifted with large breasts is not surprising that men were failing to meet is apt also to have large buttocks, hips, and these expectations, since the idea of the “strong, thighs, and while some men may find such silent type” is a masculine ideal in many cultures. anatomical features (particularly the first) desir- Furthermore, not only civilization but also evo- able, this more natural version of the female lution has tended to favor men who are given body is not the image usually promoted in adver- more to actions than to words—men who can tising or other media. Therefore, if a woman with make war, either on the battlefield or in the busi- a full figure wishes to meet societal expectations ness world. regarding beauty, she must endure something Such aggressive characteristics are all well much more strenuous than a mere operation—a and good among animals or in a human society strict low-fat diet and a great deal of exercise. just struggling to survive. But in the modern Given these often unnatural expectations from West, where most material needs are easily met, society, it is no wonder that a great many women women have sought and expressed a desire for express frustration with their attempts to con- something more from men. Such was the situa- form to them, nor is it any wonder that many tion that emerged in the wake of the sexual revo- women in the modern world simply stop trying lution, when people became more open not only
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 147 set_vol3_5 8/19/02 11:06 AM Page 148
Sexual Reproduc- tion KEY TERMS
CHROMOSOME: A DNA-containing of its kind. An unborn human usually is body, located in the cells of most living called a fetus during the period from three things, that holds most of the organism’s months after fertilization to the time of genes. birth. DIPLOID: A term for a cell that has the GAMETE: A reproductive cell—that is, basic number of doubled chromosomes. a mature male or female germ cell that pos- EGG CELL: A female gamete. sesses a haploid set of chromosomes and is prepared to form a new diploid by under- EMBRYO: The stage of animal develop- going fusion with a haploid gamete of the ment in the uterus before the animal is considered a fetus. In humans this is equiv- opposite sex. Sperm and egg cells are, alent to the first three months. respectively, male and female gametes. GENITALIA: FALLOPIAN TUBES: A set of trum- The sex organs, which pet-like tubes that carries a fertilized egg include the male penis and the female from the ovary to the uterus. vagina. FAPS: Fixed-action patterns of behav- GESTATION: The period between fer- ior, or strong responses on the part of an tilization and birth during which the animal to particular stimuli. FAP is virtual- unborn offspring develops in the uterus. ly synonymous with instinct. HAPLOID: A term for a cell that has FERTILIZATION: The process of cellu- half the number of chromosomes that lar fusion that takes place in sexual repro- appear in a diploid or somatic cell. duction. The nucleus of a male reproduc- HORMONE: Molecules produced by tive cell, or gamete, fuses with the nucleus living cells, which send signals to spots of an female gamete to produce a zygote. remote from their point of origin and FETUS: An unborn or unhatched ver- induce specific effects on the activities of tebrate that has taken on the shape typical other cells.
about sex itself but about their feelings as well. they did, in books, in movies such as those of Although the sexual revolution yielded a number Woody Allen, and in therapist-led encounter of negative effects, including the loss of mystery groups or men’s groups. Gradually, however, a associated with sex and an increase in out-of- surprising fact emerged: women did not want wedlock pregnancies and cases of sexually trans- men to be too sensitive and certainly not too mitted disease, it also made it possible for people nonmasculine. They wanted a man who would to be frank in a way that they had never been talk about his feelings but not a man who wore before. One outgrowth of this was the revelation, his heart on his sleeve. They wanted to be treated in many women’s magazines, that women were as equals in the workplace, but if there was a no longer satisfied or impressed with strong, strange noise in the house at night, they wanted silent men. the man to go see what it was. They wanted a During the 1970s the model of the sensitive man who would respect them—but not a man man emerged, and for the first time it became who was so polite and predictable that he pos- possible for men to talk about their feelings. And sessed no air of mystery.
148 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 149
Sexual Reproduc- KEY TERMS CONTINUED tion
MENSTRUATION: Sloughing off of SEXUAL REPRODUCTION: One of the lining of the uterus, which occurs the two major varieties of reproduction, monthly in nonpregnant females who have along with asexual reproduction. In con- not reached menopause (the point at trast to asexual reproduction, which which menstrual cycles cease) and which is involves a single organism, sexual repro- manifested as a discharge of blood. duction involves two. Sexual reproduc- OVARY: Female reproductive organ that contains the eggs. tion occurs when male and female gametes undergo fusion, a process known OVUM: An egg cell. as fertilization, and produce cells that are PUBERTY: A stage in the maturation of genetically different from those of either humans and higher primates wherein the person first becomes capable of sexual parent. reproduction. It is marked by the develop- SPERM CELL: A male gamete. ment of secondary sex characteristics, the maturation of the genital organs, and the TESTES: The pair of male reproduc- beginning of menstruation in females. tive glands located in the scrotum, a skin- Puberty typically takes place somewhere covered sac that hangs from the groin. between the ages of about 10 and 14 years. UTERUS: A reproductive organ, found SECONDARY SEX CHARACTERIS- in most female mammals, in which an TICS: Those unique traits that mark an individual as a male or female but which embryo and later a fetus grow and develop. are not manifested in the sexual organs VAGINA: A passage from the uterus to themselves. Facial hair and a deep voice in the outside of the body. males or breast development as well as hip and buttocks development in females are ZYGOTE: A diploid cell formed by the examples. fusion of two gametes.
All of this reveals a great deal about humans sex, the sexes, and the need to propagate the and sexual reproduction. First of all, mating, or species through mating and reproduction. the interaction between males and females, is about far more than reproduction or even sex. It WHERE TO LEARN MORE is an interaction that involves the whole person. Second, human sexuality is surrounded by webs Avraham, Regina. The Reproductive System. Philadelphia: Chelsea House Publishers, 2001. of psychological, social, and spiritual complexity that make it a phenomenon quite different from Cool Nurse (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 149 set_vol3_5 8/19/02 11:06 AM Page 150
Sexual Kimball, Jim. “Sexual Reproduction in Humans.” Kim- “Puberty Information for Boys and Girls.”American Acad- Reproduc- ball’s Biology Pages (Web site). emy of Pediatrics (Web site).
150 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 151
PregnancyPREGNANCY and Birth AND BIRTH
CONCEPT Oviparous animals may fertilize their eggs One of the greatest dramas in the world of living either externally or internally, though all animals things is that which takes place in pregnancy and that fertilize their eggs externally in nature are birth. Pregnancy forms a bond between mother oviparous. (See Sexual Reproduction for more and offspring that, in humans at least, lasts about internal and external fertilization.) In cases of internal fertilization, male animals somehow throughout life. Humans and many other ani- pass their sperm into the female: for example, mals are viviparous, meaning that offspring male salamanders deposit a sperm packet, or develop inside the mother’s body and are deliv- spermatophore, onto the bottom of their breed- ered live. By contrast, birds and some other vari- ing pond and then induce an egg-bearing (or eties of animal are oviparous, meaning that they gravid) female to walk over it. The female picks deliver offspring in eggs that must develop fur- up the spermatophore and retains it inside her ther before hatching. In the modern world, body, where the eggs become fertilized. These human females experience birth in several fertilized eggs later are laid and develop external- ways—vaginal or cesarean, with anesthetics and ly. Oviparous offspring undergoing development without, at home or in a hospital—but just a few before birth obtain all their nourishment from hundred years ago, there was little variety in an the yolk and the protein-rich albumen, or experience that was almost always painful and “white,” rather than from direct contact with the dangerous. mother. Ovoviviparity is common in a wide range of animals, including certain insects, fish, lizards, HOW IT WORKS and snakes, but it is much less typical than oviparity. Ovoviviparous insects do not supply Oviparity, Viviparity, and oxygen or nourishment to their developing eggs; Ovoviviparity they merely give them a safe brooding chamber for development. Nonetheless, species of ovovi- The birth of live offspring is a reproductive fea- viparous fish, lizards, and snakes appear to pro- ture shared by mammals, some fishes, and select- vide some nutrition and oxygen to their growing ed invertebrates, such as scorpions, as well as var- offspring. Because nutrition is provided in these ious reptiles and amphibians. Animals who give instances, some zoologists consider them exam- birth to live offspring are called viviparous, ples of true live birth, or viviparity. meaning “live birth.” In contrast to viviparous VIVIPARITY. Viviparity is the type of birth animals, other animals—called oviparous, mean- process that takes place in most mammals and ing “egg birth”—give birth to eggs that must many other species. Viviparous animals give birth develop before hatching. Finally, there are ovovi- to living young that have been nourished in close viparous animals, or ones that produce eggs but contact with their mothers’ bodies. The offspring retain them inside the female body until hatching of both viviparous and oviparous animals devel- occurs, so that “live” offspring are born. op from fertilized eggs, but the eggs of viviparous
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 151 set_vol3_5 8/19/02 11:06 AM Page 152
Pregnancy and Birth
OVIPAROUS ANIMALS GIVE BIRTH TO EGGS THAT MUST DEVELOP BEFORE HATCHING. THE SURINAM TOAD, FOR EXAM- PLE, CARRIES HER EGGS ATTACHED TO HER BACK WHILE THEY MATURE. (© David A. Northcott/Corbis. Reproduced by permission.)
animals lack a hard outer covering, or shell. embryo and later, when it begins to assume the Viviparous young grow in the adult female until shape typical of its species, a fetus. In the uterus, they are able to survive on their own outside her the unborn offspring receives nutrients and oxy- body. In many cases, the developing fetuses of gen during the period known as gestation, which viviparous animals are connected to a placenta, a extends from fertilization to birth. (In humans special membranous organ with a rich blood the gestation period is nine months.) supply that lines the uterus in pregnant mam- The zygote forms in one of the mother’s fal- mals. It provides nourishment to the fetus lopian tubes, the tubes that connect the ovaries through a supply line called an umbilical cord. with the uterus. It then travels to the uterus, All mammals, except for the platypus and where it becomes affixed to the uterine lining. the echidnas, are viviparous; only these two Along the way, the zygote divides a number of unusual mammals, called montremes, lay eggs. times, such that by the time it reaches the uterus (See Speciation for more about mammal it consists of about 100 cells and is called an species.) Some snakes, such as the garter snake, embryoblast. The exact day on which the embry- are viviparous, as are certain lizards and even a oblast implants on the uterine wall varies, but it few insects. Ocean perch, some sharks, and a few is usually about the sixth day after fertilization. popular aquarium fish are also viviparous. Even By the end of the first week, a protective sac, certain plants, such as the mangrove and the tiger known as the amniotic cavity, begins to form lily, are described as viviparous because they pro- around the embryoblast. duce seeds that germinate, or sprout, before they EMBRYO AND FETUS. Changes become detached from the parent plant. then begin to take place at a rapid rate. As each From Zygote to Fetus week passes, the embryo takes on more and more necessary and distinctive features, such as blood The essays on Reproduction and Sexual Repro- vessels in week 3, internal organs in week 5, and duction discuss the basics of the reproductive finger and thumb buds on the hands in week 7. process through the point of fertilization. A fer- Unfortunately, miscarriages are not uncommon tilized egg is called a zygote, but once it begins to in the early weeks of pregnancy. The mother’s develop in the uterus or womb, it is known as an immune system (see Immune System) may react
152 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 153
to cells from the embryo that it classifies as “for- moves into the birth canal and is expelled from Pregnancy eign” and begin to attack those cells. The embryo the vagina. The uterus continues to contract even and Birth may die and be expelled. The first three months after the placenta is delivered, and it is thought of embryonic development are known as the first that these contractions serve to control bleeding. trimester, or the first three-month period of After the baby is born, the umbilical cord growth. At the end of the first trimester, the that has attached the fetus to the placenta is embryo is about 3 in. (7.5 cm) long and looks clamped. The clamping cuts off the circulation of like a tiny version of an adult human. Thereafter, the cord, which eventually stops pulsing owing to the growing organism is no longer an embryo, the interruption of its blood supply. The baby but a fetus. Fetal development continues through now must breathe air through its own lungs, the second and third trimesters until the baby is whereas before it has been breathing, fishlike, in ready for birth at the end of ninth months. the warm, wet environment of the mother’s Preparing for Birth amniotic fluid. The process of labor described here in a very cursory fashion (it is actually much At the end of the gestation period, the mother’s more complicated) can take from less than one uterus begins to contract rhythmically, a process hour to 48 hours, but typically the entire birth called labor. This is accompanied by the release of process takes about 16 hours. hormones, most notably oxytocin. From the time of fertilization, quantities of the hormone prog- REAL-LIFE esterone, which keeps the uterus from contract- APPLICATIONS ing, are high; but during the last weeks of gesta- tion, maternal progesterone levels begin to drop, Changing Views on while levels of the female hormone estrogen rise. Childbirth When progesterone levels drop to very low levels and estrogen levels are highest, the uterus begins Before modern times, the realm of childbirth was to contract. a world exclusive to women, and few men ever Meanwhile, as birth approaches, the brain’s entered the birth chamber. It was a place of pituitary gland releases oxytocin, a hormone that excruciating pain and serious danger to the stimulates uterine contractions and controls the mother giving birth, so filled with blood and production of milk in the mammary glands (a screaming that few men would have dared enter process called lactation). Synthetic oxytocin even if they had wanted to do so. Women had to sometimes is given to women to induce labor. give birth without anesthesia and any number of Scientists believe that the pressure of the fetus’s other amenities of modern medical care, includ- head against the cervix, the opening of the ing sophisticated diagnostic techniques and uterus, ultimately initiates the secretion of oxy- equipment, such as ultrasound, as well as anti- tocin. As the fetus’s head presses against the septic environments and surgical techniques, cervix, the uterus stretches and relays a message such as cesarean section. along nerves to the pituitary gland, which In those days, birthing assistance was the responds by releasing oxytocin. The more the work of midwives, women who lacked formal uterus stretches, the more oxytocin is released. schooling in medicine (which was unavailable to LABOR AND DELIVERY. Rhyth- most women in any case) but made up in experi- mic contractions dilate the cervix, causing the ence for what they lacked in education. By about fetus to move down the birth canal and to be 1500, however, as medicine began to progress expelled together with the placenta, which has after many centuries of stagnation, male doctors supplied the developing fetus with nutrients increasingly forced midwives out of a job. In from the mother during the gestation period. 1540 the European Guild of Surgeons declared Before delivery, the placenta separates from the that “no carpenter, smith, weaver, or woman shall wall of the uterus. Since the placenta contains practice surgery.” A major turning point in the many blood vessels, its separation from the wall male takeover of birthing assistance duties came of the uterus causes bleeding. This bleeding is with the invention of the forceps, tong-like normal, assuming that it is not excessive. After instruments that could be used for extracting a the placenta separates from the uterine wall, it baby during difficult births. The inventor was the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 153 set_vol3_5 8/19/02 11:06 AM Page 154
Pregnancy English obstetrician (a physician concerned with many years, the screams of women giving birth and Birth childbirth) Peter Chamberlen the Elder “naturally” once again filled the halls of hospital (1560–1631), and he and his descendants for a maternity wards and home birthing rooms. century closely guarded the design of the bril- liant invention. Even the mothers on whom it Modern Childbirth was used never saw the instrument, and mid- The last few paragraphs represent an extreme wives were prohibited from using forceps to assist during childbirth. reaction—a view not shared by many women, who have been more than happy to avail them- OBSTETRICIANS TAKE OVER. selves of the benefits of childbirth in the modern By the eighteenth century, however, Cham- world. Such benefits include an epidural, a type berlen’s descendants had released their exclusive of anesthetic procedure that serves to alleviate claim over the forceps, and use of the instrument the pain of parturition, or childbirth, while mak- spread to other medical professionals. This gave ing it possible for the mother to remain con- male obstetricians a great technological advan- scious. Still, even for women who have no inter- tage over female midwives and further ensured est in giving birth at home or without the aid of the separation of the midwives from the medical profession. By 1750 numerous physicians and drugs, much has changed in the world of child- surgeons had gained the status of “man-mid- birth. Women may choose a happy medium wives,” and the growth of university courses on between the medical establishment and more obstetrics established it as a distinct medical spe- traditional methods, for instance, by opting to cialty. By the latter part of the 1700s, most consult with an obstetrician and a midwife. women of the upper classes had come to rely on Today, many obstetricians are women. This professionally trained doctors rather than mid- has had an incalculable effect on making child- wives, yet in America, where doctors were scarcer birth psychologically easier for many women: than in Europe, the profession of midwife con- though some are happy to retain a male obstetri- tinued to flourish into the 1800s. Still, by the cian, many others find themselves much more early twentieth century, childbirth had moved comfortable being cared for by a physician who, out of the home and into the hospital, and at in all likelihood, has given birth herself. The mid-century it had become a completely medical increasingly important role of the female obste- process, attended by physicians and managed trician, along with other factors, serves to sym- with medical equipment and procedures, such as bolize the fact that the world has progressed fetal monitors, anesthesia, and surgical interven- beyond the old false dilemma between medical tions. care from a male or a female, between medicine THE REACTION IN THE LATE and nature, between hospital and home. TWENTIETH CENTURY. Many women A HOSPITAL AS HOME. Hospital of the late twentieth century found themselves rooms, in fact, are starting to resemble rooms at dissatisfied with this clinical approach to child- home. Everywhere one looks in the modern birth. Some believed that the medical establish- maternity environment, there is evidence that ment had taken control of a natural biological much has changed, not only from the very old process, and women who wanted more com- mand over labor and delivery helped popularize days, when male doctors were not involved in new ideas on childbirth that sought to reduce or childbirth at all, but also from the more recent eliminate medical interventions. Today, some past, when males took over the process entirely. women choose to deliver with the help of a In a brilliant innovation, many hospitals have nurse-midwife, who, like her premodern coun- created a situation in which the woman gives terparts, is trained to deliver babies but is not a birth in her own hospital room, which is outfit- doctor. There are women who even choose home ted with couches, cabinets, curtains, and rocking birth, attended by a doctor or midwife or some- chairs to make it look like a home rather than a times both. There are even brave souls who, in hospital. To emphasize the smooth transition the face of increasing concern about the effect of between home life and the delivery room, fathers, anesthesia on the fetus, refuse artificial means of once banished from the labor and delivery cham- controlling pain and instead rely on breathing bers, now are welcomed as partners in the birth and relaxation techniques. For the first time in process.
154 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 155
Pregnancy and Birth
ULTRASOUND IMAGE OF A 30-WEEK-OLD FETUS. BY THE EIGHTEENTH WEEK OF PREGNANCY, ULTRASOUND TECHNOL- OGY CAN DETECT MANY STRUCTURAL ABNORMALITIES, SUCH AS SPINA BIFIDA, HEART AND KIDNEY DEFECTS, AND HARELIP. (© BSIP/Kretz Technik/Photo Researchers. Reproduced by permission.)
A father may even cut the umbilical cord, By the eighteenth week of pregnancy, ultra- and he is certainly likely to be in the delivery sound technology can detect many structural room with a video camera, recording the event abnormalities, such as spina bifida (various for posterity—yet another change from the past. defects of the spine), hydrocephaly (water on the Fathers are not the only ones filming in the deliv- brain), anencephaly (no brain), heart and kidney ery room. Today, cable television networks, such defects, and harelip (in which the upper lip is as the Learning Channel, provide programming divided into two or more parts). On a less dire that offers a frank view of the delivery process, and much more pleasant note, it can also give complete with candid footage that sometimes future parents an opportunity to gain their first can be as dramatic as it is revealing. The mater- glimpse of their child, and an experienced ultra- nity ward, once a closed place, has increasingly sound technician usually can tell them the baby’s become an open book. sex if they choose to learn it before the birth. PRENATAL TESTING. Saving and Improving Lives Chorionic villi sampling is the most sophisticated modern Many a mother and father alike can breathe a technique used to assess possible inherited genet- prayer (or at least a sigh) of thanks for all the ic defects. This test typically is performed innovations that today make birth much safer between the sixth and eighth week of embryonic than it once was. Among them are a variety of development. During the test, a narrow tube is techniques for embryo and fetal diagnosis, which passed through the vagina or the abdomen, and a help make parents aware of possible problems in sample of the chorionic villi (small hairlike pro- the growing embryo. Ultrasound diagnosis, a jections on the covering of the embryonic sac) is technique similar to that applied on submarines removed while the physician views the baby via for locating underwater structures, uses high- ultrasound. pitched sounds that cannot be heard by the Chorionic villi are rich in both embryonic human ear. These sounds are bounced off the and maternal blood cells. By studying them, embryo, and the echoes received are used to genetic counselors can determine whether the identify embryonic size. baby will have any of several defects, including
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 155 set_vol3_5 8/19/02 11:06 AM Page 156
Pregnancy and Birth KEY TERMS
EMBRYO: The stage of animal develop- remote from their point of origin and ment in the uterus before the point at induce specific effects on the activities of which the animal is considered a fetus. In other cells. humans this is equivalent to the first three OVARY: Female reproductive organ months. that contains the eggs. FALLOPIAN TUBES: A set of trum- OVIPAROUS: A term for an animal pet-like tubes that carries a fertilized egg that gives birth to eggs that must develop from the ovary to the uterus. before hatching. Compare with viviparous. FERTILIZATION: The process of cellu- OVOVIVIPAROUS: A term for an ani- lar fusion that takes place in sexual repro- mal that produces eggs but retains them duction. The nucleus of a male reproduc- inside the body until hatching occurs, so tive cell, or gamete, fuses with the nucleus that “live” offspring are born. Compare of a female gamete to produce a zygote. with oviparous and viviparous. FETUS: An unborn or unhatched ver- OVUM: An egg cell. tebrate that has taken on the shape typical of its kind. An unborn human usually is UTERUS: A reproductive organ, found called a fetus during the period from three in most female mammals, in which months after fertilization to the time of an embryo and, later, a fetus grows and birth. develops. GESTATION: The time between fertil- VAGINA: A passage from the uterus to ization and birth, during which the unborn the outside of the body. offspring develops in the uterus. VIVIPAROUS: A term for an animal HORMONE: Molecules produced by that gives birth to live offspring. Compare living cells, which send signals to spots with oviparous.
Down syndrome (characterized by mental retar- ease, cystic fibrosis, and Down syndrome. How- dation, short stature, and a broadened face), cys- ever, amniocentesis involves the risk of fetal loss tic fibrosis (which affects the digestive and respi- as a result of disruption of the placenta. Chori- ratory systems), and the blood diseases hemo- onic villi sampling is even more risky, with an philia, sickle cell anemia, and thalassemia. (Sev- even higher possibility of fetal loss than amnio- eral of these disorders are discussed in different centesis, probably because it is conducted at an essays throughout this book; for instance, Down earlier stage. syndrome is examined in Mutation.) As with In alpha-fetoprotein screening, which takes ultrasound, it also can show the baby’s gender. place somewhere between the 16th and 18th Another important form of prenatal (before weeks, proteins from the amniotic sac and the the birth of the child) testing is amniocentesis, fetal liver are taken as a means of screening for performed around week 16, in which amniotic specific defects. Because of uncertainties involved fluid is drawn from the uterus by means of a nee- in interpretation of the results, alpha-fetoprotein dle inserted through the abdomen. Amniocente- screening is not a common procedure. sis, too, can reveal the sex of the child, as well as CESAREAN SECTION. Another a host of genetic disorders such as Tay-Sachs dis- extremely important technique that has saved the
156 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_5 8/19/02 11:06 AM Page 157
life of many babies and mothers is cesarean sec- tical. Today the C-section, as it is called, has Pregnancy tion. The normal position for a baby in delivery become a routine procedure—one that has saved and Birth is head first; when a baby is in the breech posi- literally hundreds of thousands, perhaps mil- tion, with its bottom first, it poses grave dangers lions, of lives. to both the mother and the child. Not only could the baby fail to emerge in time to begin breathing WHERE TO LEARN MORE normally, thus running the risk of brain damage, Assisted Reproduction Foundation (Web site). but it also can become stuck, endangering the life
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 157 set_vol3_6 8/19/02 11:09 AM Page 159
SCIENCE OF EVERYDAY THINGS real-life biology EVOLUTION
EVOLUTION PALEONTOLOGY
159 set_vol3_6 8/19/02 11:09 AM Page 161
Evolution EVOLUTION
CONCEPT HOW IT WORKS Among the dominant concepts of the modern The “Watch Analogy” and world in general, and biology in particular, few What It (Unintentionally) are as powerful—or as misunderstood—as evo- Teaches About Evolution lution. Even the name is something of a mis- nomer, since it almost implies some sort of striv- Throughout this essay, we discuss misconcep- ing to reach a goal, as though the “purpose” of tions relating to evolution. Such misconceptions evolution were to produce the most intelligent have had such a strong impact on modern civi- species, human beings. In fact, what drives evo- lization that it is important to begin by setting lution is not a quest for biological greatness but aside a few misguided ideas that strike at the very something much more down to earth: the need heart of the evolutionary process. Many of these for organisms to survive in their environments. misconceptions are embodied in a popular Closely tied to evolution are two processes, “argument” against evolution that goes some- mutation and natural selection. Natural selec- thing like this: Suppose you took a watch apart and laid the pieces on the ground. If you came tion is a process whereby survival is related back in a billion years, would you really expect directly to the ability of an organism to fit in the watch to have assembled itself? with its environment, while mutation involves changes in the genetic instructions encoded in This argument is a virtual museum of all the organisms. fallacies associated with evolution. First of all, a watch (or any of the other variations used in sim- Although the English naturalist Charles ilar arguments) is mechanical, not organic or Darwin (1809–1882) often is regarded as the biological, which is the class of objects under dis- father of evolutionary theory, he was not the first cussion within the framework of evolution. In thinker to suggest the idea of evolution as such; that sense, the answer to this question is easy however, by positing natural selection as a mech- enough: No, a watch probably never would anism for evolution, he provided by far the most assemble itself, because it is not made of living convincing theory of evolutionary biological material and it has no need for survival. change up to his time. In the years since Darwin, Another problem with the watch argument evolutionary theory has evolved, but the essential is that it starts with impossibly large pieces. Let us idea remains a sound one, and it is a “theory” assume that the watch is a living being; even so, only in the sense that it is impossible to subject it one would not expect its dials and gears to to all possible tests. The idea that evolution is assemble themselves. But evolution does not somehow still open to question is another perva- make such claims: there is nothing in the theory sive misconception, and it often appears hand in of evolution to lead one to believe that a collec- hand with the most pervasive misconception of tion of organs lying around on a beach eventual- all—that evolution is in some way anti-Christ- ly would piece themselves together to make a ian, anti-religion, or anti-God. whale.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 161 set_vol3_6 8/19/02 11:09 AM Page 162
Evolution be called the “fallacy of intention,” or the idea that evolution is driven by some overall purpose. THE “FALLACY OF INTEN- TION.” Hidden in the watch analogy is the idea of the watch itself, the finished product, as a “goal.” By the same analogy, the single-cell eukaryotes of a billion or two billion years ago were forming themselves for the purpose of later becoming pine trees or raccoons or people. This is not a valid supposition, as can be illustrated by analogies to human history. The history of human beings, of course, has taken place over a much, much shorter span than evolutionary history. (The Paleontology essay contains several comparisons between the span of human life on Earth and Earth’s entire exis- tence.) Moreover, unlike cells, people do form goals and act on intentions, so if there were any good example of change with a goal in mind, it would have to come from human beings. Yet even in the few thousand years that humans have CHARLES DARWIN (The Library of Congress.) existed in organized societies, most trends have occurred not as part of a major plan but as a According to what paleontologists (see Pale- means of adapting to conditions. ontology) and other scientists can deduce, over Consider the situation of a group of nomads the course of three billion years life-forms who lived in what is now southern Russia about evolved from extremely simple self-replicating 5,000 years ago. At some point, this vast collec- carbon-based molecules to single-cell organisms. tion of tribes began to migrate outward, some This is hardly what one would call breakneck moving into an area that is now central Asia and speed. The more visible or “exciting” part of evo- the Indian subcontinent and others migrating lution, with the proliferation of species that pro- westward. No sane person would argue that the duced the dinosaurs and (much later) humans, westward-traveling members of this group knew took place in the past billion years. In fact, the that in moving to the geographically advanta- pace of change was still very, very slow until geous territory of Europe, they were putting in about half a billion years ago, and it has been place conditions that would help give their accelerating ever since. For the vast majority of descendants dominance over most of the planet evolutionary history, however, change has been some 4,500 years later. Rather, they were proba- so slow that, by contrast, watching paint dry bly just trying to find better land for grazing their would be like playing a high-speed video game. horses. Ironically, for the watch scenario to be truly We cannot say what the Indo-Europeans, as analogous to anything in evolution, one would they are known to history, were looking for. Our have to start with atoms and molecules not whole only evidence that they existed is the similarities gears and dials. Opponents of evolutionary theo- between the languages of Europe, India, and ry might take this fact as being favorable to their Iran, first noted by the German philologist and cause, but if the watch were made of living, folklorist Jacob Grimm (1785–1863) at about the organic material rather than metal, it is possible same time that Darwin was formulating his the- that the molecules would have some reason to ory of evolution. Grimm, in fact, used methods join in the formation of organelles and, later, not unlike those of Darwin, but instead of fossils cells. Or perhaps they would not. Therein lies he studied words and linguistic structures. Along another problem with the watch analogy and, the way, he found remarkable links, such as the indeed, with many of the attempts to argue Sanskrit word agni, cousin to the Latin term ignis against evolution on a religious basis. This might and such modern English words as ignite.
162 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 163
Natural Selection In contrast to the Indo-Europeans, we know Evolution a great deal about another group of westward- As we noted earlier, one of the principal mecha- moving nomads, the Huns of around A.D. 300, nisms of evolutionary processes is natural selec- who were indeed looking for better grazing tion. This in itself illustrates the lack of intention, lands. Dislocated from their native areas by the or “goal orientation,”in evolution. Like the name building of China’s Great Wall, the Huns crossed evolution itself, the term natural selection can be the Danube River, displacing the Ostrogoths. The deceptive, implying that nature selects certain Ostrogoths, in turn, moved westward, and this organisms to survive and condemns others to migration set in motion a domino effect that extinction. In fact, something quite different is at would bring an end to the Western Roman work. Empire in A.D. 476. Species tend to overproduce, meaning that Did the Huns intend to destroy the Roman the number of field mice, for instance, born in Empire and bring about the Middle Ages? No any year is so large that this entire population reasonable person would adopt such a conspira- cannot possibly survive. The reason is that there is torial view of history. Even more absurd, did the never enough of everything—food, water, or liv- Chinese build the Great Wall with the idea of ing space—for all members of the population to precipitating this entire chain of events? Again, receive what they need. Therefore, only those best no one would assert such a premise. If those adapted to the environment are likely to survive. trends in the evolution of societies were not goal- directed, why would we assume that cells and FASTER, FURRIER MICE. Sup- organisms would have to be striving toward a pose, for instance, that the climate in the area particular end to obtain certain results? where two field mice live is very cold, and sup- CONFUSING EVOLUTION pose that some of the field mice have more pro- WITH GOD. In fact, there is no driving “pur- tective fur than others; obviously, they are more pose” to evolution—no scientifically based sub- likely to live. If there are many speedy predators stitute for God operating from behind the scenes around, judging purely on the basis of that factor and manipulating the evolutionary process to alone, it would be easy to predict that the swiftest achieve its ultimate aims. Evolution is not guided of the field mice would survive. Thus, faster-run- by any one large aim but by a million or a billion ning, furrier mice would be “selected” over the small aims—the need for a particular species of slower or less furry mice. mollusk to survive, for instance. Natural selection is not simply a matter of As we discuss in the course of this essay, the one particular mouse surviving in an environ- idea of an underlying conflict between evolution ment. Instead, it involves the survival of specific and Christianity (or any other religion, for that strains, or lines of descent, that are more suited to matter) is almost entirely without merit. On the the environment in question. Individuals adapt- other hand, it is theoretically possible that all the ed to an environment are more likely to live and processes of evolution took place without a cre- reproduce and then pass on their genes to the ator—but this still should not pose a threat to next generation, while those less adapted are less anyone’s idea of God. likely to reproduce and pass on their traits. The genetic strains that survive are not “better” than There is nothing in evolution that would those that do not—they are only better adapted. lead to the conclusion that there is no God, that the universe is not God’s handiwork, or that God The process of natural selection is ongoing. does not continue to engage in a personal rela- For example, in generation A, the furrier field tionship with each human. Neither is there any- mice survive and pass on their “furriness” gene to thing in evolution that would lead to the conclu- their offspring. Some of the offspring may still sion that God does exist. Rather, the matter of not be furry, and these mice will be less likely to God is simply not relevant to the questions survive and reproduce. In addition, since there addressed by evolution. In other words, evolu- are almost always several survival factors affect- tion leaves spiritual belief where it should be (at ing natural selection, it is likely that other traits least, according to Christianity): in the realm of also will determine the survivability of certain individual choice. individual mice and their genes.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 163 set_vol3_6 8/19/02 11:09 AM Page 164
Evolution For instance, there may be furry but slow In fact, “survival of the fittest,” in a more mice in generation B, which despite their adapta- accurate interpretation, means that individuals tion to temperature conditions are simply not that “fit,” or “fit in with,” their environments are fast enough to get away from predators. There- those most likely to survive. This is a far cry from fore, the mice in generation C are likely to be fur- any implication of strength or superiority. Imag- rier and faster than their ancestors. Additional ine a group of soldiers in combat: Which type of survival factors may come into the picture, to soldier is most likely to survive? Is it the one who ensure that the average member of generation D scores highest on physical training tests, looks the has sharper teeth in addition to swifter feet and a finest in a uniform, comes from a more socially furrier body. upper-class home, and has the most advanced education? Or is it the one who keeps his head Although this illustration depicts evolution- low, acts prudently, does not rush into dangerous ary changes as taking place over the course of situations without proper reconnaissance, and four generations, they are more likely to occur obeys instruction from qualified leaders? over the span of 400 or 4,000 or four million gen- erations. In addition, the process is vastly more Clearly, the second set of characteristics has complicated than it has been portrayed here, much more to do with survival, even though because numerous factors are likely to play a these qualities may seem less “noble” than the first set. Yet it is by adapting, or proving his or her part. The essential mechanism outlined here, adaptability, to the environment of war that a however, prevails: certain traits are “naturally soldier survives—not by displays of strength or selected” because individuals possessing those other types of “fitness” that simply appear traits are more capable of survival. impressive. In the same way, the fitness of a THE “SURVIVAL OF THE species does not necessarily have anything to do FITTEST.” The concept of natural selection with strength: after all, the lion, the “king of sometimes is rendered popularly as the “survival beasts,” would die out in a polar climate or a of the fittest.” Scientists are less likely to use this desert or an aquatic environment. phrase for several reasons, including the fact that it has been associated with distasteful social Mutation philosophies or murderous political ideologies— Although natural selection is of principal impor- for example, Nazism. Additionally, the word tance in evolution, mutation also plays a pivotal fittest is a bit confusing, because it implies “fit- role. Mutation is the process whereby changes ness,” or the quality of being physically fit. take place in the genetic blueprint for an organ- This implication, in turn, might lead a per- ism as a result of alterations in the physical struc- son to believe that natural selection entails the ture of an organism’s DNA (deoxyribonucleic survival of the strongest, which is not the case. Yet acid). DNA is a molecule in all cells and in many this is precisely what proponents of a loosely viruses that contains genetic codes for inheri- defined philosophy known as social Darwinism tance. DNA carries genetic information that is claimed. Popular among a wide range of groups transmitted from parent to offspring; when a and people in the late nineteenth and early twen- mutation occurs, this new genetic information— tieth centuries, social Darwinism could be used often quite different from the genetic code in the service of almost any belief. Industrialists received by the parent from the grandparent—is and men of wealth asserted that those who suc- passed on instead. ceeded financially did so because they were the Under normal conditions of reproduction, a fittest, while Marxists claimed that the working copy of the DNA from the parent is replicated class ultimately would triumph for the same rea- and transmitted to the offspring. The DNA from son. Across the political spectrum, social Dar- the parent normally is copied exactly, but every winism confused the meaning of “fittest” with once in a while errors arise during replication. that of other concepts: “strongest,” “most These errors usually originate in noncoding advanced,” or even “most moral.” All of this, it regions of the DNA and therefore have little need hardly be said, is misguided, not least effect on the observable traits of the offspring. because evolutionary theory has nothing to do On the other hand, some mutations may be with race, ethnicity, or social class. lethal, and thus the offspring does not survive for
164 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 165
the mutation to become apparent. In a very few bacteria. In a fraction of those who survive, how- Evolution cases, however, offspring with a slightly modified ever, a mutation may develop that makes them genetic makeup manage to survive. resistant to the medication. Eventually, these CONTRAST WITH ACQUIRED mutant bacteria will reproduce, creating more CHARACTERISTICS. Mutation is not to mutants and in time yielding an entire popula- be confused with the inheritance of acquired tion resistant to the antibiotic. characteristics, a fallacious doctrine that had its This is the reason why antibiotics can lose adherents when Darwin was a young man. If their effectiveness over time: bacteria with acquired characteristics were taken to an mutant genes will render every antibiotic useless extreme, a lumberjack who loses his arm cutting eventually. The same often can happen with down a tree and later conceives a child with his insect sprays, as roaches and other pests develop wife would most likely father a child who is miss- into mutant strains that are capable of surviving ing an arm. This notion is absurd, and attempts exposure to these pesticides. Such species, with to put forward a workable theory of acquired their short cycles of birth, reproduction, and characteristics in the late eighteenth and early death, are extremely well equipped for survival as nineteenth centuries involved much greater sub- a group, which explains why many an unpleasant tlety. Still, the idea is misguided. “bug” (whether a bacterium or an insect) has The French natural philosopher Jean Bap- long been with us. (See Mutation for more on tiste de Lamarck (1744–1829), one of the leading this subject.) proponents of acquired characteristics, main- tained that giraffes had gained their long necks REAL-LIFE from the need to stretch and reach leaves at the APPLICATIONS top of tall trees. In other words, if a giraffe parent had to stretch its neck, a giraffe baby would be “Proving” Evolution born with a stretched neck as well. Later, Dar- win’s natural selection provided a much more Later in this essay, we look at examples of evolu- plausible explanation for how the giraffe might tion in action and other phenomena that support have acquired its long neck: assuming that the the ideas of evolutionary theory. But before nutrients it needed were at the highest levels of examining these many “proofs” of evolution, a the local trees, the traits of tallness, long necks, few words should be said about the very fact that and the ability to stretch would be selected natu- evolution seems to require so much more proof rally among the giraffe population. than most other scientific theories. MUTATIONS AND SURVIVAL. All scientific ideas must be capable of being Unlike the idea of acquired characteristics, muta- proved or disproved, of course, but the demand tion does not entail the inheritance of anatomi- for proof in the case of evolution goes far beyond cal traits acquired in the course of an organism’s the usual rigors of science. In fact, at this point, life; rather, it is changes in the DNA that are the people demanding proof are not scientists passed on. For example, when mind-altering but certain sectors of the population as a drugs became popular among young people in whole—in particular, religious groups or indi- the 1960s, concerns were raised that the offspring viduals who fear evolution as a challenge to their of drug takers might suffer birth defects as a beliefs. result of alterations in their DNA. For the most QUANTUM MECHANICS: A part, this did not happen. Conditions such as MUCH MORE DIFFICULT IDEA. By Huntington disease and cystic fibrosis, however, contrast, quantum mechanics, though it encom- are the result of mutations in DNA; so, too, is passes ideas completely opposed to common albinism, which eliminates skin pigment. sense, has not sustained anything approaching Although mutations often are regarded as the same challenge or the demand for proof that undesirable because they can affect the health of evolution has encountered from nonscientists. A individuals adversely, they also can have positive theory in physics and chemistry that details the effects for the population in question. Suppose a characteristics of energy and matter at a sub- group of bacteria is exposed to an antibiotic, atomic level, quantum mechanics goes against which rapidly kills off the vast majority of the such common assumptions as the idea that we
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 165 set_vol3_6 8/19/02 11:09 AM Page 166
Evolution can know both the location and the speed of an discuss later, unfold within a short enough peri- object. It is as though science had proved that od of time that humans can observe them. In down was up and up was down. If there were ever general, however, evolutionary processes take a “dangerous” theory, inasmuch as it undermines place over such extraordinarily long spans of all our assumptions about the world, it is quan- time that it would be impossible to subject them tum mechanics not evolution, which is a fairly to direct observation. straightforward idea by comparison. None of this, however, does anything to dis- Quantum mechanics has gone virtually credit evolutionary theory. For that matter, the unchallenged (at least on a social or moral, as idea that the entire physical world is made of opposed to a scientific, basis), whereas even atoms is still technically a theory, though there is today there are many people who refuse to accept no significant movement of people attempting to the idea of evolution. Granted, quantum discredit it. The reason, of course, is that atomic mechanics is a much younger idea, having origi- theory does not seem to contradict anyone’s idea nated only in the 1920s, and it is vastly more dif- of God. (This was not always the case, however. ficult to understand. But the real reason why evo- Almost 2,500 years ago, a Greek philosopher lution has come under so much more challenge, named Democritus developed the first atomic of course, has to do with the fact that it is per- theory, but because his ideas were associated with ceived (mistakenly) as challenging the primacy of atheism, atomic theory was largely rejected for God. more than two millennia.) JUST A THEORY? One of the FACING THE FACTS. If people real- aspects of evolution often cited by opponents is ly understood the word theory, they would give it the fact that it is, after all, the theory of evolution. a great deal more respect. Unfortunately, the The implication is that if it is still just a theory, it word so often is misused and applied to anything must be open to question. In a sense, this is accu- that has not been proved that it has begun to rate: for scientific progress to continue, ideas seem almost like an insult to call evolution a the- should never be accepted as absolute, unassail- ory. After all, in the present essay, we refer to able truths. But this is not what opponents of acquired characteristics as a theory, and in every- evolution are getting at when they cite its status day life one often hears much less respectable as a “mere” theory. In fact, their use of this point ideas given the status of theory. For this reason, it as a basis for attack only serves to illustrate a mis- is worth taking note of the process, from obser- understanding with regard to the nature of scien- vation to hypothesis to the formulation of gener- tific knowledge. al statements, that goes into the development of The word theory in “theory of evolution” a truly scientific theory. simply means that evolutionary ideas have not In forming his theory of evolution, Darwin been and, indeed, cannot be tested in every pos- began with several observations about the natu- sible circumstance. Most ideas in science are sim- ral world. Among the things he observed is the ply theories rather than laws because in few cases fact, which we noted earlier, that for a particular is it possible to say with absolute certainty that species, more individuals are born than can pos- something always will be the case. One of the few sibly survive with available resources. On the actual scientific laws is the conservation of ener- basis of this observation, he formed a hypothesis, gy, which holds that for all natural systems the or inference. His inference was that because pop- total amount of energy remains the same, though ulations are greater than resources, the members transformations of energy from one form to of a population must compete for resources. another take place. This has been tested in such a A theory is made up of many hypotheses, wide variety of settings and circumstances that but to proceed from a collection of hypotheses to there is no reason to believe that would it ever not a true theory, these inferences must be subjected be the case. to rigorous testing. Thus, Darwin, in effect, said By contrast, there probably never will be to himself, “Is what I have said true? Are there enough tests on evolution to advance it to the more individuals of a species than there are avail- status of a law. The reason is quite simply that able resources?” Then he began looking for evolution takes a long time. Some examples, such examples, and like a true scientist, he did so with as the instances of industrial melanism that we the attitude that if he found examples that con-
166 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 167
tradicted his hypothesis, he would reject the are somehow linked, in this case using scientific Evolution hypothesis and not the facts. facts as a basis for rejecting religion. As it turns out, of course, there are always One such scientist was Darwin himself, who more members of a population than there are embraced agnosticism because his own findings resources. This can be illustrated in a small way had proved that the biblical account of creation by observing a litter of puppies or piglets strug- cannot be literally true. In this religious choice, gling to obtain milk from their mother. Chances he was following in a family tradition: his grand- are that the mother will not have enough teats for father, the physiologist Erasmus Darwin all her babies, and the “runt,” unless it is able to (1731–1802), belonged to the mechanist school, force its way through the others to the milk a muddle of atheism, bad theory, and genuine source, may die. Only after testing this hypothe- science. sis and other hypotheses, such as that of natural The mechanists claimed that humans were selection, did Darwin formulate his theory. mere machines whose activities could be under- Evolution and Religion stood purely in terms of physical and chemical processes. Claims such as these ultimately led to The fact that some puppies or piglets die for lack the discrediting of their movement, whose ideas of milk is not a nice or pleasant thought, but it is failed to explain such biological processes as the truth. Again, like a true scientist, Darwin growth. At the same time, such mechanist philoso- accepted reality, without attempting to mold it to phers as the French physician and philosopher fit his personal beliefs about how things should be. Julien de La Mettrie (1709–1751) went far beyond As a great thinker from the generation that the territory of science, teaching that atheism was preceded Darwin’s, the Scottish philosopher the only road to happiness and that the purpose of David Hume (1711–1776), wrote in his Enquiry human life was to experience pleasure. Concerning Human Understanding: “There is no The thinker who perhaps did the most to method of reasoning more common, and yet confuse science and atheism was one of Darwin’s more blamable, than, in philosophical disputes, most significant early followers, the German nat- to endeavor the refutation of a hypothesis, by a ural scientist and philosopher Ernst Haeckel pretense of its dangerous consequences to reli- (1834–1919). It was Haeckel, not Darwin, who gion and morality.” In other words, there is an first proposed an evolutionary explanation for understandable, but nonetheless inexcusable, the origin of human beings, which, of course, human tendency to evaluate ideas not on the was a major step beyond even Darwin’s claim basis of whether they are true but rather on the that all of life had evolved over millions of years. basis of whether they fit with our ideas about the In the course of developing this idea, Haeck- world. el, who was a practicing Christian until he read A scientist may be a Christian, or an adher- Darwin’s On the Origin of Species by Means of ent of some other religion, and still approach the Natural Selection, renounced his faith and adopt- topic of evolution scientifically—as long as he or ed a belief system he called monism, which is she does not allow religious convictions to influ- based on the idea that there is only a physical ence acceptance or nonacceptance of facts. The realm and no spiritual one. Technically, Haeckel scientist should start with no preconceived was not an atheist but a pantheist, since his phi- notions and no allegiance to anything other than losophy included the idea of a single spirit that the truth. If that person’s religious conviction is lives in all things, both living and nonliving. strong enough, it can weather any new scientific Whatever the case, Haeckel’s monism is no more idea. scientific than Christianity. CONFUSING ATHEISM WITH HUMANS AND “MONKEYS.” It is inter- SCIENCE. This brings up an important esting that the man who put forward the notori- point regarding the alleged conflict between reli- ous idea that humans and apes are related also gion and science. Not all the blame for this would attempt to turn evolution into a sort of belongs with religious groups or individuals who “proof” of atheism. In fact, the evolutionary con- shut their minds to scientific knowledge. Many nection between humans and lower primates, or scientists over the years likewise have adopted the “monkeys,”has long been the most powerful point fallacy of maintaining that religion and science of contention between religion and evolution.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 167 set_vol3_6 8/19/02 11:09 AM Page 168
Evolution This, in fact, remains one of the most chal- Creationism, which sometimes goes by the lenging aspects of evolutionary theory—not name of creation science, is based on the belief because it is hard to see how the human body is that God created the universe and did so in a very similar to an ape’s body but because there is such short period of time. This claim, creationists a vast difference between a human mind and that maintain, can be supported by scientific evi- of an ape. Whereas our physical similarity to pri- dence. Scientific evidence, however, is not really mates is easy to establish, the fact is that no other what drives creationism, which is based on a lit- animal—ape, dolphin, pig, or dog—comes close eral reading of the first two chapters of the Book to humans in terms of reasoning ability. Nor is it of Genesis. Taken to an extreme, this means that reasoning ability alone that separates humans God created the universe about 6,000 years ago in from other animals. Humans possesses a propen- six days of 24 hours each. sity for conceptualization and a level of self- Adherents of creationism begin with the awareness that sets them completely apart from premise of a six-day Creation (or at least, a very other creatures, so much so that the brains of young Earth) and then look for facts to support apes, cats, birds, and even frogs seem more or less the premise—exactly the opposite of the alike compared with that of a human. approach taken by true science. The findings of creationists do not change much over the years, Animals are concerned with a few things: eat- unlike evolutionary science, which has continued ing, sleeping, eliminating waste, and procreating. to develop with new discoveries. Some mammals have the ability to engage in play, but there is still no comparison between even the Sometimes creationists attempt to use the most advanced mammalian brains and that of a findings of evolutionary science against it. For human. Other primates have the ability to use instance, they may interpret industrial melanism sticks or stones as tools, but only humans—prac- (the adaptation of moths to discoloration in the tically from the beginning of the species 2.5 mil- environment caused by pollution, discussed later lion years ago—have the ability to fashion tools. in this essay) as proof that organisms can change Only humans are gifted, or cursed, with restless very quickly. This, of course, does not take into minds ever in search of new knowledge. account the fact that moths have very short life spans compared with humans, for whom evolu- Does any of this disprove evolution? It does tionary change takes much longer. Creationists not. Does it pose a significant challenge to the also point to areas of evolutionary theory where idea that humans and other primates evolved all scientists are not in agreement, citing these as from a common ancestor? Not as it has been stat- “proof” that the whole theory is unsound. ed here. All that has been said in the preceding INTELLIGENT DESIGN THEO- paragraphs is simply a matter of everyday obser- RY AND THE COURT BATTLE. In vation, but it is not a scientific hypothesis, let contrast to creationism, intelligent design theory alone a theory. Clearly, there are some questions is not based on any particular religious position. still to be answered as to why and how humans Instead, it begins with an observation that would developed brains so radically different from find a great deal of agreement among many peo- those of other primates, but the place for such ple, including those who support evolutionary questioning is within the realm of science not theory. The idea is that evolution alone does not outside it. explain fully how life on Earth came to exist as it CREATIONISM. Another thing we can does, with all its complexity and order. According say about the human mind is that it has a ten- to intelligent design theory, there must have been dency to mold ideas toward its own preconcep- some intelligence behind the formation of the tions as to how things should be. As Hume universe. observed, there is a great temptation, in the There is another contrast between intelligent minds of all people, to demand that scientific design theory and creationism. Whereas it is hard facts conform to a particular set of religious or to imagine a genuine scientist embracing cre- political beliefs. Such is the case with creationism ationism, it is not difficult at all to picture a sci- and “intelligent design theory,” two scientific entific thinker adopting the viewpoint of intelli- belief systems whose adherents have attempted gent design. In fact, this has happened, though to challenge evolutionary theory. long before the “movement” had a name.
168 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 169
Darwin’s contemporary, the English natural- came to the forefront, favored for survival by Evolution ist Alfred Russel Wallace (1823–1913), who pub- nature. lished his own theory of evolution at about the Back in England, he already had seen such a same time as Darwin’s Origin of Species, parted mechanism at work in the artificial breeding of ways with Darwin because he maintained that pigeons, whereby breeders favored certain gene there must be a spiritual force guiding evolution. pools—for instance, white-tailed birds—over Only such a force, he maintained, could explain others. (Breeders of dogs and other animals the human soul. From a philosophical and theo- today still employ artificial-selection techniques logical standpoint, this idea has a great deal of to produce desirable strains.) Darwin posited a merit, but because it cannot be tested, it cannot similar process of selection in nature, only this truly be regarded as science. one was not artificial, directed by a goal-oriented Neither creationism nor intelligent design human intelligence, but natural and guided by has received any support in the scientific com- the need for survival. munity—nor, during court battles over the teach- THE SPREAD OF SPECIES. ing of creationism in the public schools during Among the phenomena Darwin observed in the the 1980s, did that idea receive the support of the Galápagos was the differentiation among the 13 United States justice system. Creationism, the varieties of finch (a type of bird) on the islands as courts ruled, is a religious and not a scientific well as the contrasts among these finches and doctrine. Evolutionary theory is based on an ever their counterparts on the mainland. As Darwin increasing body of evidence that is both observ- began to discover, they shared many characteris- able and reproducible. To teach these other doc- tics, but each variety had its own specific traits trines alongside evolution in the public schools (for instance, the ability to crack tough seeds for would convey the impression that creationism food) that allowed it to fill a particular niche in and intelligent design had been subjected to the its own environment. same kinds of rigorous tests that have been From the beginning Darwin was influenced applied to evolution, and this is clearly not the by the recent findings in geology, a newly emerg- case. ing science whose leading figures maintained Evidence for Evolution that Earth was very, very old. (These scientists included the Scottish geologist Charles Lyell A great deal of evidence for evolution appeared [1797–1875], whose Principles of Geology, pub- in the seminal text of evolutionary theory (men- lished between 1830 and 1833, Darwin read tioned previously), On the Origin of Species by aboard the Beagle) The relationship between Means of Natural Selection, which Darwin pub- geology and evolution has persisted, and findings lished in 1859. In fact, he had collected much of in the earth sciences continue to support evolu- the evidence he discusses in this volume nearly tionary theory. three decades earlier, from 1831 to 1836, aboard Among the leading ideas in geology and a scientific research vessel off the coast of South other geosciences since the mid–twentieth centu- America. (He delayed publication because he ry is plate tectonics, which indicates (among rightly feared the controversy that would ensue other things) that the continents of Earth are and resolved to present his ideas only when he constantly moving. (See Paleontology for further learned that Wallace had developed his own the- discussion of this topic.) This idea of continental ory of evolution.) drift provided a mechanism for species differen- Just 22 years old, Darwin traveled on the tiation of the kind Darwin had observed. HMS Beagle, from which he collected samples of It appears that in the past, when the land- marine life. His most significant work was done masses were joined, organisms spread over all on the Galápagos Islands some 563 mi. (900 km) available land. Later, this land moved apart, and west of Ecuador. As he studied organisms there, the organisms became isolated. Eventually, dif- Darwin found that they resembled species in ferent forms evolved, and in time these distinct other parts of the world, but they were also organisms became incapable of interbreeding. unique and incapable of interbreeding with sim- This is what occurred, for instance, when the ilar species on the mainland. He began to suspect Colorado River cut open the Grand Canyon, sep- that for any particular environment, certain traits arating groups of squirrels who lived in the high-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 169 set_vol3_6 8/19/02 11:09 AM Page 170
Evolution
VAMPYRUM BAT. WIDELY DIVERGENT ORGANISMS SOMETIMES POSSESS A COMMON STRUCTURE, ADAPTED TO THEIR INDIVIDUAL NEEDS OVER COUNTLESS GENERATIONS YET REFLECTIVE OF A SHARED ANCESTOR. THE CAT’SPAW, THE DOLPHIN’S FLIPPER, THE BAT’S WING, AND THE HUMAN HAND ARE ALL VERSIONS OF THE SAME ORIGINAL FIVE-DIGIT APPENDAGE, CALLED THE PENDTADACTYL LIMB. (© Gary Braasch/Corbis. Reproduced by permission.)
altitude pine forest. Eventually, populations an indication of a common four-footed ancestor ceased to interbreed, and today the Kaibab squir- that likewise had limbs with five digits at the end. rel of the northern rim and the Abert squirrel of The embryonic forms of animals also reflect the south are separate species. common traits and shared evolutionary fore- COMMON ANCESTRY. Darwin rec- bears. This is why most mammals look remark- ognized that some of the best evidence for evolu- ably similar in early stages of development. In tion lies hidden within the bodies of living crea- some cases animals in fetal form will manifest tures. If organisms have a history, he reasoned, vestigial features reflective of what were once then vestiges of that history will linger in their functional traits of their ancestors. Thus, fetal whales, while still in their mothers’ wombs, pro- bodies—as studies in comparative anatomy duce teeth after the manner of all vertebrates show. An example is a phenomenon that sounds (creatures with an internal framework of bones), as if it is made up, but it is very real: snake hips. only to reabsorb those teeth, which they will not Though their ancestors ceased to walk on four need in a lifetime spent filtering plankton legs many millions of years ago, snakes still pos- through their jaws. sess vestigial hind limbs as well as reduced hip and thigh bones. The molecular “language” of DNA also pro- vides evidence of shared evolutionary lineage. In some cases widely divergent organisms When one studies the DNA of humans and possess a common structure, adapted to their chimpanzees, very close similarities rapidly individual needs over countless generations yet become apparent. Likewise, there are common reflective of a shared ancestor. A fascinating structures in the hemoglobin, or red blood cells, example of this is the pentadactyl limb, a five- of different types of organisms. Comparisons of digit appendage common to mammals and hemoglobin make it possible to pinpoint the date found, in modified form, among birds. The cat’s of the last common ancestor of differing species. paw, the dolphin’s flipper, the bat’s wing, and the For example, hemoglobin analysis reveals an human hand are all versions of the same original, ancestor common to humans and frogs dating
170 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 171
Evolution
LOBSTER FOSSIL FROM THE LATE JURASSIC PERIOD. THE PRESERVED REMAINS OF SUCH PREHISTORIC LIFE-FORMS APPEAR IN THE ORDER OF THEIR EVOLUTION IN THE STRATA, OR LAYERS, OF EARTH’S SURFACE, WHICH GEOLOGISTS ARE ABLE TO DATE: THE AGE OF A STRATUM ALWAYS CORRELATES WITH THE FOSSILS DISCOVERED THERE. (© Layne Kennedy/Corbis. Reproduced by permission.)
back 330 million years, whereas the common ally, fossilization involves changes in the hard human and mouse ancestor lived 80 million portions, including bones, teeth, and shells. This years ago, and the ancestor we share with the rhe- series of changes, in which minerals are replaced sus monkey walked the earth “only” 26 million by different minerals, is known as mineralization. years ago. Fossilized remains of single-cell organisms THE FOSSIL RECORD. The fossil have been found in rock samples as old as 3.5 bil- record also provides an amazing amount of evi- lion years, and animal fossils have been located in dence concerning common ancestors. Fossilized rocks that date to the latter part of Precambrian remains of invertebrates (animals without an time, as long ago as one billion years. Certain fos- internal skeleton), vertebrates, and plants appear sil types, known as index fossils or indicator in the strata or layers of Earth’s surface in the species, have been associated strongly with par- same order that the complexities of their anato- ticular intervals of geologic time. An example is my suggest. The more evolutionarily distant the ammonoid, a mollusk that proliferated for organisms lie deeper, in the older layers, beneath about 350 million years, from the late Devonian the remains of the more recent organisms. Geol- to the early Cretaceous periods, before experi- ogists are able to date rock strata with reasonable encing mass extinction. accuracy, and the age of a layer always correlates The fossil record is far from an open book, with the fossils discovered there. In other words, however, and interpreting fossil evidence there would never be a stratum dating back 400 requires a great deal of judgment. All manner of million years that contained fossils of natural phenomena such as earthquakes can mastodons, which evolved much later. destroy fossil beds, rendering the evidence A fossil is the remains of any prehistoric life- unreadable or at least unreliable. Nor is it a fore- form, especially those preserved in rock before gone conclusion that the animals who left behind the end of the last ice age, about 10,000 years ago. fossils are fully representative of the species exist- The process by which a once living thing ing at a given time. Fossils are far more likely to becomes a fossil is known as fossilization. Gener- be preserved in certain kinds of protected aquat-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 171 set_vol3_6 8/19/02 11:09 AM Page 172
Evolution ic environments, for instance, than on land (par- and radioactive isotopes are ones that sponta- ticularly at higher elevations, where erosion is a neously eject various high-energy particles over significant factor), and therefore paleontologists’ time. Because chemists know how long it takes knowledge of life forms in the distant past is for half the isotopes in a given sample to stabilize heavily weighted toward marine creatures. (a half-life), they can judge the age of such a sam- FAUNAL SUCCESSION AND ple by examining the ratio of stable to radioactive OTHER FORMS OF DATING. Key to isotopes. In the case of uranium, one isotopic the demonstration of evolution is the age of form, uranium 238, has a half-life of 4,470 mil- samples and the idea that many of the processes lion years, which is very close to the age of Earth described took place a long, long time ago. This itself. raises the question of how scientists know the age Evolution at Work of things. In fact, they have at their disposal sev- eral techniques, both relative and absolute, for Every creature that exists today is the result of an dating objects. incredibly complex, lengthy series of changes One of the earliest ideas of dating in geology brought about by mutation and natural selec- was faunal dating, or the use of bones from ani- tion, changes that influenced the evolution of mals (fauna) to determine age. This was the that life-form. Take for instance the horse, whose brainchild of the English engineer and geologist evolutionary background is as well-documented William Smith (1769–1839), whose work is an as that of any creature. example of the fact that evolutionary ideas were The horse family, or Equidae, had its origins “in the air” long before Darwin. While excavating at the beginnings of the Eocene epoch about 54 land for a set of canals near London, Smith dis- million years ago. This first ancestor, known as covered that any given stratum contains the same Hyracotherium or eohippus (“dawn horse”) was types of fossils, and therefore strata in two differ- extremely small—only about the size of a dog. In ent areas can be correlated. Smith stated this in addition, it had four hooves on its front feet and what became known as the law of faunal succes- three on each rear foot, with all of its feet being sion: all samples of any given fossil species were padded, which is quite a contrast with the four deposited on Earth, regardless of location, at unpadded, single-hoofed feet of the modern more or less the same time. As a result, if a geol- horse. These and other features, such as head size ogist finds a stratum in one area that contains a and shape, constitute such a marked difference particular fossil and another in a distant area from what we know about horses today that having the same fossil, it is possible to conclude many scientists have questioned the status of that the strata are the same. eohippus as an equine ancestor. However, com- Faunal succession is relative, meaning that it parison with fossils from later, also extinct, hors- does not provide clues as to the actual age in es shows a clear line of descent marked by an years of a particular sample. Since the mid–twen- increase in body size, a decrease in the number of tieth century, however, scientists have had at their hooves, an elimination of foot pads, lengthening disposal several means for absolute dating, which of the legs and fusion of the bones within, devel- make it possible to determine the rough age of opment of new teeth suited for eating grass, an samples in years. Most of these mechanisms for increase in the length of the muzzle, and a growth dating are based on the fact that over time, a par- in both the size and development of the brain. ticular substance converts to another, mirror Of course, this was not a clear-cut, neat, and substance. By comparing the ratios between steadily unfolding process, and some features them, it is possible to arrive at an estimate as to appeared abruptly; still, the progression is there the amount of time that has elapsed since the to be observed in the fossil record. Over the organism died. course of the many millions of years since eohip- Chief among the techniques for absolute pus, species have emerged that were distin- dating is radiometric dating, which uses ratios guished by a particular feature—for example, between two different kinds of atoms for a given teeth size and shape—only to disappear if condi- element: stable and radioactive isotopes. Isotopes tions favored species with other traits. Evolution- are atoms that differ in their number of neu- ary lines have branched off, with some dead-end- trons, or neutrally charged subatomic particles, ing, and others continuing.
172 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 173
Thus, during the Miocene epoch, which last- As the Industrial Revolution got into full Evolution ed from about 26 million to 7 million years ago, swing during the 1800s, factory smokestacks put various evolutionary branches competed for a so much soot into the air in some parts of Eng- time until the emergence of Parahippus. This land that it killed the lichen on the trees, and by species had teeth adapted for eating grass, in con- the 1950s, most pepper moths were dark-col- trast to those of earlier horse ancestors, which ored. It was at that point that Bernard Kettlewell grazed on leaves and other types of vegetation (1907–1979), a British geneticist and entomolo- that did not require strong teeth. After Parahip- gist (a scientist who studies insects), formed the pus came Merychippus, which resembled a mod- hypothesis that the pepper moths’ coloration ern pony, and from which came numerous late- protected them from predators, namely birds. Miocene evolutionary lines. Most of these were Kettlewell therefore reasoned that, before three-toed, but Pliohippus had one toe per foot, pollution appeared in mass quantities, light-col- and it was from this form that the genus Equus ored moths had been the ones best equipped to (which today includes horses, donkeys, and protect themselves because they were camou- zebras) began to emerge in the late Pliocene flaged against the lichen on the trees. After the epoch about 3 million years ago. beginnings of the Industrial Revolution, howev- INDUSTRIAL MELANISM AND er, the presence of soot on the trees meant that light-colored moths would stand out, and there- THE PEPPER MOTH. Despite the stag- fore it was best for a moth to be dark in color. gering spans of time involved in evolution, one This in turn meant that natural selection had need not look back billions of years to see evolu- favored the dark moths. tion at work. Both natural selection and muta- tion play a role in industrial melanism, a phe- In making his hypothesis, Kettlewell predict- nomenon whereby the processes of evolution can ed that he would find more dark moths than be witnessed within the scale of a human life- light moths in polluted areas, and more light time. Industrial melanism is the high level of than dark ones in places that were unpolluted by occurrence of dark, or melanic, individuals from factory soot. As it turned out, dark moths out- a particular species (usually insects) within a numbered light moths two-to-one in industrial- ized areas, while the ratios were reversed in geographic region noted for its high levels of unpolluted regions, confirming his predictions. dark-colored industrial pollution. To further test his hypothesis, Kettlewell set up With so much pollution in the air, trees tend hidden cameras pointed at trees in both polluted to be darkened, and thus a dark moth stands a and unpolluted areas. The resulting films showed much greater chance of surviving, because pred- birds preying on light moths in the polluted ators will be less able to see it. At the same time, region, and dark moths in the unpolluted one— there is a mutation that produces dark-colored again, fitting Kettlewell’s predictions. moths, and in this particular situation, these ANGIOSPERMS AND GYM- melanic varieties are selected naturally. On the NOSPERMS. A final interesting example of other hand, in a relatively unpolluted region, the natural selection at work lies in the comparative lighter-colored individuals of the same species success rates of angiosperms and gymnosperms. tend to have the advantage, and therefore natural An angiosperm is a type of plant that produces selection does not favor the mutation. flowers during sexual reproduction, whereas a The best-known example of industrial gymnosperm reproduces sexually through the melanism occurred in a species known as the use of seeds that are exposed, for instance in a pepper moth, or Biston betularia, which usually cone. Angiosperms are a beautiful example of lives on trees covered with lichen. (An example of how a particular group of organisms can adapt to a lichen is reindeer “moss”; see Symbiosis.) Prior its environment and do so in a much more effi- to the beginnings of the Industrial Revolution in cient way than that of its evolutionary forebears. England during the late eighteenth century, the On the other hand, gymnosperms, with their proportion of light-colored pepper moths was much less efficient form of reproduction, per- much higher than that of dark-colored ones, haps one day will go the way of the dinosaur. both of which were members of the same species Flowering plants evolved only about 130 differentiated only by appearance. million years ago, by which time Earth long since
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 173 set_vol3_6 8/19/02 11:09 AM Page 174
Evolution KEY TERMS
ACQUIRED CHARACTERISTICS: FOSSIL: The mineralized remains of Sometimes known as acquired characters or any prehistoric life-form, especially those Lamarckism after one of its leading propo- preserved in rock before the end of the last nents, the French natural philosopher Jean ice age. Baptiste de Lamarck (1744–1829), acquired characteristics is a fallacy that FOSSILIZATION: The process by should not be confused with mutation. which a once living organism becomes a Acquired characteristics theory maintains fossil. Generally, fossilization involves min- that changes that occur in an organism’s eralization of the organism’s hard portions, overall anatomy (as opposed to changes in such as bones, teeth, and shells. its DNA) can be passed on to offspring. GENE: A unit of information about a DATING: Any effort directed toward particular heritable (capable of being finding the age of a particular item or phe- inherited) trait that is passed from parent nomenon. Methods of dating are either to offspring, stored in DNA molecules relative (that is, comparative and usually called chromosomes. based on rock strata, or layers) or absolute. The latter, based on methods such as the GEOLOGIC TIME: The vast stretch of study of radioactive isotopes, typically is time over which Earth’s geologic develop- rendered in terms of actual years or mil- ment has occurred. This span (about 4.6 lions of years. billion years) dwarfs the history of human DNA: Deoxyribonucleic acid, a mole- existence, which is only about 2.5 million cule in all cells and in many viruses that years. Much smaller still is the span of contains genetic codes for inheritance. human civilization, only about 5,500 years.
had been dominated by another variety of seed- angiosperm, by contrast, the seeds are tucked producing plant, the gymnosperm, of which safely away inside the ovary. Furthermore, the pines and firs are an example. Yet in a relatively evolution of the flower not only has added a great short period of time, geologically speaking, deal of beauty to the world but also has provided angiosperms have become the dominant plants a highly successful mechanism for sexual repro- in the world. In fact, about 80% of all living plant duction. This sexual reproduction makes it pos- species are flowering plants. Why did this hap- sible for new genetic variations to develop, as pen? It happened because angiosperms devel- genetic material from two individuals of differing oped a means whereby they coexist more favor- ancestry come together to produce new off- ably than gymnosperms with the insect and ani- spring. (For more about angiosperms and gym- nosperms, see Ecosystems and Ecology.) mal life in their environments.
Gymnosperms produce their seeds on the WHERE TO LEARN MORE surface of leaflike structures, and this makes the seeds vulnerable to physical damage and drying Campbell, Neil A., Lawrence G. Mitchell, and Jane B. Reece. Biology: Concepts and Connections. 2nd ed. as the wind whips the branches back and forth. Menlo Park, CA: Benjamin/Cummings, 1997. Furthermore, insects and other animals view Darwin, Charles, and Richard E. Leakey. The Illustrated gymnosperm seeds as a source of nutrition. In an Origin of Species. New York: Hill and Wang, 1979.
174 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 175
Evolution KEY TERMS CONTINUED
HYPOTHESIS: An unproven state- MUTATION: Alteration in the physical ment regarding an observed phenomenon. structure of an organism’s DNA, resulting in a genetic change that can be inherited. INDUSTRIAL MELANISM: The high level of occurrence of dark, or melanic, NATURAL SELECTION: The process individuals from a particular species (usu- whereby some organisms thrive and others perish, depending on their degree of adap- ally insects) within a geographic region tation to a particular environment. noted for its high levels of dark-colored PALEONTOLOGY: industrial pollution. The study of life- forms from the distant past, primarily as INVERTEBRATE: An animal without revealed through the fossilized remains of an internal skeleton. plants and animals. LAW: A scientific principle that is SCIENTIFIC METHOD: A set of prin- shown always to be the case and for which ciples and procedures for systematic study no exceptions are deemed possible. that includes observation; the formation of MINERALIZATION: A series of hypotheses, theories, and laws; and contin- changes experienced by a once living ual testing and reexamination. organism during fossilization. In mineral- THEORY: A general statement derived ization, minerals in the organism are from a hypothesis that has withstood suffi- replaced or augmented by different miner- cient testing. als or the hard portions of the organism VERTEBRATE: An animal with an dissolve completely. internal skeleton.
Dennett, Daniel Clement. Darwin’s Dangerous Idea: Evo- Evolution. Public Broadcasting System (Web site). lution and the Meanings of Life. New York: Simon and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 175 set_vol3_6 8/19/02 11:09 AM Page 176
PALEONTOLOGYPaleontology
CONCEPT ple are familiar with fossils, a term that probably Paleontology is the study of life-forms from the calls to mind an image of a flat piece of rock with distant past, as revealed primarily through the a shadowy imprint of a leaf or animal on it. In record of fossils left on and in the earth. It is a fact, a fossil is the preserved remains of a once subdiscipline of both the biological and the earth living organism that has undergone a process sciences, one that brings to bear the techniques of known as mineralization, in which the organic geologic study and several areas of biology, materials in hard parts of the organism (for including botany and zoology. But the term pale- example, teeth or bones) are replaced by miner- ontology, as used in the present context, also pro- als, which are inorganic. vides a convenient means of encompassing, in a The difference between the organic and the single word, the study of the distant biological inorganic, which we discuss later, is more than a past. Such study, of course, is related intimately difference between the living and nonliving, or to the investigation of evolutionary processes even the living and formerly living. For now, and phenomena. In the present context, however, though, it will suffice to say that all living and our concern is not so much with the theory and formerly living things, as well as their parts and principles of evolution, discussed elsewhere in products, are organic. The stress on formerly liv- this book, but rather with a relatively short ing things is important, since paleontology by overview of biological history. This area of bio- definition encompasses plants, animals, and logical investigation calls upon such concepts as microscopic life-forms that lived a long time ago. mass extinction and fossilization as well as the (As for just how long “a long time ago” really is, study of plant and animal forms that scientists that subject, too, is discussed later in this essay.) know only from scientific reconstruction of the past rather than from direct experience. Chief Among the fields related, or subordinate, to among these life-forms are the dinosaurs, which paleontology are paleozoology, which focuses on dominated Earth for a period of more than 100 the study of prehistoric animal life; paleobotany, million years, ending about 65 million years ago. the study of past plant life; and paleoecology, the This span of time, impressive as it seems on the study of the relationships between prehistoric human scale, is minuscule compared with the plants and animals and their environments. entire history of life on Earth. Despite the emphasis on past life-forms, howev- er, paleontologists also avail themselves of evi- dence gleaned from observation of animals living HOW IT WORKS today. In this approach, paleontology shows its relationship to geology, which, along with biolo- Paleontology Among the gy, is one of its two “parents.” Sciences GEOLOGY AND PALEONTOL- Paleontology is the investigation of life-forms OGY. One of the key concepts in geology is the from the distant past, primarily through the principle of uniformitarianism, which arose study of fossilized plants and animals. Most peo- from the early history of the geologic sciences. At
176 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 177
a time when the nascent science was embroiled in Paleontol- a debate over Earth’s origins and the age of the ogy planet, the Scottish geologist James Hutton (1726–1797) transcended this debate by focusing on the processes at work on Earth in the present day. (In terms of geologic time or even the evolu- tion of species, the eighteenth century and twen- ty-first century might as well be a few seconds apart, so the expression present day applies to Hutton’s time as much as to ours.) Rather than simply speculate as to how Earth had come into being, Hutton maintained that scientists could understand the processes that had formed it by studying the geologic phenomena they could see around them. The reason this approach works is that natural laws do not change over time, nor do the processes that are at work within nature. On the other hand, particular processes may not be in operation at all times, nor can it be assumed that the rate at which the processes take place is the same. The two fore- going sentences encompass some of the observa- tions made by the modern American paleontolo- gist Stephen Jay Gould (1941–2002) with his for- A MUSEUM REPRODUCTION OF NEANDERTHAL MAN, OR HOMO SAPIENS NEANDERTHALENSIS. The span of mulation of the “four types of uniformity” in Ever time since the first appearance of the genus Since Darwin: Reflections in Natural History (1977). Homo (to which humans, or Homo sapiens, It is certainly appropriate that paleontology and belong) is minuscule: 2.5 million years com- geology interface in this concept of uniform pared with 4.6 billion years, or about 0.04% of the planet’s history. change, because both are concerned with piecing (© Bettmann/Corbis. Repro- duced by permission.) together the distant past from the materials avail- able in the present. prising to discover the large proportion of that In fact, from the standpoint of the earth sci- time during which life existed on Earth; it is equal- ences, paleontology belongs to a branch of geol- ly amazing to learn what a small portion of the ogy known as historical geology, or the study of biological history of the planet involved anything Earth’s physical history. (This field of study is contrasted to the other principal branch of the that modern humans would recognize as “life.” discipline, physical geology, the study of the For the moment, let us go even further back, material components of Earth and of the forces to the very beginning—an almost inconceivably that have shaped the planet.) Other subdisci- long time ago. Scientists believe that the universe plines of historical geology are stratigraphy, the began between 10 billion and 20 billion years study of rock layers, or strata, beneath Earth’s ago, with an explosion nicknamed the “big bang,” surface; geochronology, the study of Earth’s age a cataclysm so powerful that it sent galaxies and the dating of specific formations in terms of careening outward on a course they maintain geologic time; and sedimentology, the study and even today. Among those galaxies moving out- interpretation of sediments, including sedimen- ward from the universal point of origin was our tary processes and formations. Milky Way, which, about six billion years ago, began to develop a rotating cloud of cosmic gas Before Life somewhere between its center and its rim. That Later in this essay, we consider just what is meant was the beginning of our solar system. by “geologic time,” the grand sweep of several bil- THE SUN AND EARTH. The center lion years during which Earth evolved from a of the cloud, where the greatest amount of gases cloud of gases to its present form. It may be sur- gathered, was naturally the densest and most
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 177 set_vol3_6 8/19/02 11:09 AM Page 178
Paleontol- massive portion as well as the hottest. There, nings of life made their appearance, in the form of ogy hydrogen—the lightest of all elements—experi- self-replicating molecules of carbon-based matter. enced extraordinary amounts of compression owing to the density of the clouded gases around Life Begins it, and underwent nuclear fusion, or the bonding One famous cliché of science-fiction movies is of atomic nuclei. the use of the phrase “carbon-based life-forms” This hot center became the Sun about five to describe humans. In fact, all living things con- billion years ago, but there remained a vast neb- tain carbon, and if we ever do find life, intelligent ula of gas surrounding it. As the fringes of this or otherwise, on other planets, chances are nebula began to cool, the gases condensed, form- extremely high that it, too, will be “carbon- ing solids around which particles began to accu- based.” Carbon, in fact, is almost synonymous mulate. These were the future planets, including with life, and hence the word organic, in its sci- ours. The process of planetary formation took entific meaning, refers to all substances that con- place over a span of about 500 million years. The tain carbon. The only exceptions are the elemen- planets contained various chemical elements, tal carbon in diamonds or graphite, the carbon- formed by nuclear fusion on the Sun—a fasci- ate forms that make up many of Earth’s rocks, nating aspect of life on Earth, since it means that and such oxides as carbon dioxide and monox- the particles that make up the human body came ide, all of which are considered inorganic. from nuclear reactions on the stars. It may sound as though a huge portion of THE EARLY ATMOSPHERE. The carbon’s possible forms already have been elimi- brand-new Earth possessed an atmosphere that nated from the list of organic substances, but, in consisted primarily of elemental hydrogen, nitro- fact, carbon is capable of forming an almost lim- gen, carbon monoxide, and carbon dioxide. This itless array of compounds with other elements, “air” would have been unbreathable to all but a particularly hydrogen. A class of molecule known very small portion of the life-forms that exist on as hydrocarbons, which are nothing but strings Earth today. Yet for the development of life, it was of carbon and hydrogen molecules bonded absolutely essential that no oxygen be present in together, is the basis for literally millions of Earth’s atmosphere at that very early time. organic compounds, from petroleum to polymer The reason is that oxygen is an extremely plastics. It may sound odd to hear plastics reactive element, which is why it is involved in an referred to as organic,but this only highlights the array of chemical reactions (including combus- difference between the popular and scientific tion and rusting) known collectively as oxida- meanings of that term. tion-reduction reactions. If oxygen had been pres- ORGANIC MATERIALS. At one ent at that time, it likely would have reacted with time, organic referred only to living things, things other elements immediately, rather than permit- that were once living, and materials produced by ting the formation of what eventually became living things (for example, sap, blood, and urine). organic materials. As recently as the early nineteenth century, scien- WATER. Water appeared as the result of tists believed that organic substances contained a meteorite bombardment from space, which took supernatural “life force,” but in 1828 the German place in the first half-billion years of Earth’s exis- chemist Friedrich Wöhler (1800–1882) made an tence. It might seem strange to learn that water, a amazing discovery. compound essential to life on Earth, came from By heating a sample of ammonium cyanate, the void of space—where, as far as we know, a material from a nonliving source, Wöhler con- there are no other life-forms. But this is not as verted it to urea, a waste product in the urine of strange as it sounds: water itself is inorganic, and mammals. As he later observed, “without benefit even today frozen water exists on several planets of a kidney, a bladder, or a dog,”he had turned an in our solar system. inorganic substance into an organic one. It was In any case, water eventually accumulated on almost as though he had created life. Actually, Earth’s surface in quantities sufficient for its con- what he had discovered was the distinction densation, with the result that clouds formed and between organic and inorganic material, which rain fell on Earth more or less continually for results from the way in which the carbon chains many millions of years. It was then that the begin- are arranged.
178 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 179
Geologic Time Marches On This explanation of the difference between Paleontol- organic and inorganic is pivotal to understand- ogy By about 2.5 billion years ago, bacteria had begun ing how the beginnings of life first formed on to undergo a form of photosynthesis, as plants do Earth. It is an almost inconceivably large step today. As a result, oxygen started to accumulate from the nonliving to the living but not nearly so in Earth’s atmosphere, and this had two interest- much of a jump from the inorganic to the organ- ing implications for life on Earth. One of the ic. In fact, it appears that what happened on results of oxygen formation was that formation Earth in its distant past was that organic (but not of “new” cells—that is, spontaneously formed living) substances underwent chemical reactions cells that did not come from already living mat- with inorganic ones to produce the rudiments of ter—ceased altogether, because they were killed life. The American chemist Stanley Miller off by reactions with oxygen. Second, aerobic res- (1930–) illustrated this with an experiment that piration thereafter became the dominant means involved a mixture of hydrogen, methane (CH ), 4 for releasing energy among living organisms. ammonia (NH3), and water. Subjected to a dis- charge of electric sparks intended to simulate The real history of life-forms on Earth—the lightning in Earth’s early atmosphere, the mix- portion of that history about which paleontolo- ture eventually yielded amino acids, which are gists can learn a great deal from observation of among the chief components of proteins. fossils and other materials—dates from the beginning of the Cambrian period, about 550 EARLY FORMS OF LIFE. The course that early forms of life followed during million years ago. At this point, about 88% of the first 800 million years of Earth’s existence was Earth’s history already had passed, yet the a lengthy one, and if a person could have remaining 12% contains virtually all the really glimpsed Earth at any interval of a few million dramatic events in the formation of life on Earth. years during this time, it might have seemed as Later in this essay, we examine a few analogies though nothing at all was happening. In fact, that help put these time periods into perspective however, life-forms were undergoing the most as well as the concept of geologic time itself. profound changes imaginable. PHASES IN EARTH’S HISTO- RY. That span of 0.8 billion years saw a transi- In the course of discussing these topics, it is tion from elemental carbon to organic com- sometimes necessary to make reference to geo- pounds and from organic compounds to logic time divisions—eons, eras, periods, and organelles, which are discrete components of epochs. These are not units of a specific length in cells, and finally to cells themselves—the build- years, like a century or a millennium; instead, ing blocks of life. The processes by which this they are distinct phases in Earth’s history that happened were exceedingly complicated, and historical geologists (including paleontologists) modern scientists have little to go on in forming have pieced together from fossils and other mate- their suppositions as to how these transitions rials. Their names usually refer to locations came about. Among many key pieces of informa- where fossils relevant to that phase of geologic tion missing from the picture, for instance, is the time were found: for example, the Jurassic peri- matter of how and when DNA first appeared in od, whose name became a household word after cells. (For more on DNA, see Cells.) the release of the 1993 blockbuster movie Jurassic Park, is named after the Jura Mountains of The first cells to form were known as prokary- Switzerland and France. otic cells, or cells without a nucleus. (These cells, too, are discussed in the essay Cells.) Prokaryotic The historical juncture mentioned in the cells may have been little more than sacs of DNA paragraph before last—the beginning of the that were capable of self-replication—much like Cambrian period, some 550 million years ago, bacteria today, which are themselves prokaryotic. was also the end of the Proterozoic eon, the third These early forms of bacteria, which dominated of three eons in what is known as Precambrian Earth for many millions of years, were apparently time. The fourth and present eon is the Protero- anaerobic and eventually split into three branches: zoic, which has included three eras: Paleozoic archaebacteria, eubacteria, and eukaryotes. Out of (about 550–240 million years ago), Mesozoic the last group grew all other forms of life, includ- (about 240–65 million years ago), and Cenozoic ing fungi, plants, and animals. (about 65 million years ago to the present).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 179 set_vol3_6 8/19/02 11:09 AM Page 180
CATACLYSMS AND CONTINEN- Paleontol- Human civilization has existed for about ogy TAL DRIFT. The divisions between these 5,500 years, the blink of an eye in geologic terms. phases have not been drawn arbitrarily; rather, Even the span of time since the first appearance of they are based on evidence suggesting that those the genus Homo (to which humans, or Homo sapi- points in Earth’s history were marked by violent, ens, belong), is minuscule: 2.5 million years com- cataclysmic events. Once life had come into the pared with 4.6 billion years, or about 0.04% of the picture, these cataclysms brought about death on planet’s history. A couple of analogies, one to a a vast scale, which we discuss later, in the context shorter span of time and another to a measure- of mass extinctions. A number of phenomena ment of space, will help us understand the scope caused mass extinctions at various points; most of time encompassed in the evolution of species. notable among them was the impact of mete- LIFE APPEARS IN JUNE—SO orites. Also significant was continental drift, TO SPEAK. Suppose that the entire history which, as its name implies, is the movement of of Earth were likened to a single year of 365.25 the continents from distant origins. days, starting with the formation of the planet Just as evolutionary theory informs much of from a cloud of dust and ending with the present. our modern thinking about biology, the theory At what point in the year would the first aerobic of plate tectonics holds a dominant position in life-forms appear? We are not talking about any- geology and related earth sciences. Plate tectonics thing approaching a human or even a clam or a involves the movement of large segments in the fern or a piece of algae—just a single-cell organ- crust and the upper mantle of Earth, and one of ism that depends on oxygen for energy. the outcomes of such movement is continental One might guess that such life-forms drift. appeared sometime in January, or at least by the Even today, the continents we know are end of winter in late March. In fact, the first aer- moving slowly away from or toward one another, obic single-cell life-forms arose around June 15— but that movement is so slow that it would take nearly halfway through a year. Even at about the millions of years for any change to be perceptible. time of Thanksgiving, the most complex organ- At one time, however, the continents were dis- isms would be fish and a few early amphibians. tributed quite differently than they are today. For We tend to associate the dinosaurs with the example, at the end of the Permian period and early phases of Earth’s history, but this only illus- the beginning of the Triassic, when the dinosaurs trates our distorted view of geologic time. In fact, began to appear on the scene, all of Earth’s land- the Jurassic period would be analogous to a peri- masses were joined in a single continent, Pangea, od of about five days from December 15–20. By that stretched between the North and South Christmas Day, all remaining dinosaurs would Poles and was surrounded by a vast ocean. have been headed toward extinction, their dead bodies eventually forming the fossil fuels that REAL-LIFE power present-day civilization. APPLICATIONS By this point we are within a few days of the year’s end, and yet nothing remotely resembling Our Place in the Grand a human has appeared. Our own genus would Scheme of Things not come on the scene until around 8:00 P.M.on December 31. The New Year’s Eve countdown All areas of historical geology—including pale- would be nearing by the time human civilization ontology—are concerned with geologic time, a began, at about 42 seconds before midnight. term that refers to the great sweep of Earth’s his- Christ’s birth would have occurred at about 14 tory. This is a timescale that dwarfs the span of seconds before midnight, and the final 10-second human existence, and to study the history of life countdown would begin about the time the on Earth, a mental adjustment of monumental Roman Empire fell. The life span of the average proportions is required. One must discard all person would correspond to approximately half a notions used in studying the history of human second or less. civilization, including such concepts as modern, ANOTHER ANALOGY: LOS medieval, and ancient, which are essentially use- ANGELES TO NEW YORK. To use less when discussing geologic time. another analogy, suppose we are driving from
180 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 181
Los Angeles to New York City, and that this dis- ably long, when one considers the diversity of Paleontol- tance corresponds to Earth’s history. Once we forms that have evolved in the past half-billion ogy reach western Nebraska, with about 46% of the years. What follows is an extremely brief distance behind us, we would come to the begin- overview, touching on a few points in the devel- ning of the Proterozoic era and the origins of aer- opment of life on Earth. obic life-forms. One might wonder why “noth- The discussion here is far from comprehen- ing” happened on all those long miles from L.A. sive, the purpose being not to provide a detailed through the deserts, mountains, and plains of the overview of evolutionary processes but to illus- western United States, but as we have seen, a trate the evolution of life-forms by a few exam- great deal happened: Earth formed from a cloud ples. The reader is strongly encouraged to consult of gas, was pounded by meteors, and gradually a textbook or other reliable information source became the home to oceans. for greater illumination of these particular top- The end of the Proterozoic era (about 545 ics. Along with this overview, we look at some of million years ago) would be at about 88% of the the more dramatic aspects of paleontologic distance from Los Angeles to New York—some- study: dinosaurs and mass extinction. where around Pittsburgh, Pennsylvania. By this The fossilized history of life on Earth really point, the continental plates have formed, oxygen began in earnest only with the Cambrian period, has entered the atmosphere, and soft-bodied which saw an explosion of invertebrate (without organisms have appeared. We are a long way an internal skeleton) marine forms. These domi- from Los Angeles, and yet almost the entire his- nated from about 550 to about 435 million years tory of life on Earth, at least in terms of relative- ago. At the latter point, the boundary between ly complex organisms, lies ahead of us. the Ordovician and Silurian periods of the Pale- If we skip ahead by about 339 million ozoic era, there occurred the first of five major years—a huge leap not only in time but also in mass extinctions, as the “supercontinent” of biological development—we come to the point Gondwana crossed the South Pole, freezing most when the dinosaurs appeared. We are now 95% life-forms that were then alive. As always hap- of the way from the beginning of Earth’s history pened with these mass extinctions, some life- to the present, and if it is measured against the forms survived, and within a few million years distance from Los Angeles to New York, we life again was thriving. would be somewhere around Scranton, in east- COMING OUT OF THE WATER. ern Pennsylvania. Another 89 mi. (142 km) from One of the favorite subjects of modern cartoon- the Scranton area would put us at a point about ists is the migration of creatures from the water 65 million years ago, or the time when the to the land. For example, following the enact- dinosaurs became extinct. We then would have ment of stiff airport security guidelines in the less than 40 mi. (64 km) to drive to reach the wake of the September 11 terrorist attacks, a New period where genus Homo appeared, by which Yorker cartoon in late 2001 showed this evolution time we would be in the middle of Manhattan. from fish to amphibian (a creature that can live Compared with the distance from L.A. to both in the water and on the ground) and, ulti- New York, the span of time that Homo sapiens has mately, to the human being. Then, in the last evo- existed would be much shorter than the drive lutionary sequence, Homo sapiens is depicted in a from Central Park to the Empire State Building. business suit, going through an airport security The entire sweep of human civilization and his- scanner. Most cartoons based on this event in tory, from about a thousand years before the paleontologic history follow a common theme, building of the pyramids to the beginning of the which can be summed up thus: “We went third millennium A.D., would be smaller than a through all that evolution just for this?” city block. It is much smaller, in fact—close to the Certainly, the transition from water to solid size of a modest storefront, or 15.54 ft. (5 m). ground was a massive step—one of almost incon- How Did We Get Here? ceivable importance. This invasion of the land, which probably took place about 400 million On the one hand, the history of relatively com- years ago, perhaps started when fishes of the plex life-forms is relatively short compared with Devonian period (named after Devonshire in Earth’s history; on the other hand, it is unbeliev- England) began breathing oxygen near the sur-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 181 set_vol3_6 8/19/02 11:09 AM Page 182
Paleontol- face of shallow waters. In time they made their has misappropriated for the purpose of insisting ogy way onto the land, and as the number of species that one particular ethnic or social group is natu- grew, their lobed fins developed in different ways rally superior to another (see Evolution). for different groups of fish, at length becoming Why does mass extinction occur? As we the many limbs and appendages specific to large noted earlier, one possible cause of mass extinc- groups of species, such as birds or mammals. This tion is a sudden and dramatic change in ocean may seem a bit far-fetched, but even today catfish levels. Other causes include volcanic eruptions or are known to pull themselves out of the water the effects of events or objects from space—the and onto land if a specific need to do so arises. explosion of a star, perhaps, or the impact of a Why did the first fish leave the water? Per- meteorite on Earth. Although scientists have a haps because the water levels were receding, as reasonable idea of the immediate causes of mass they have done periodically in the course of extinction in some cases, their understanding of Earth’s history as a result of massive freezing of the root causes still is limited. This fact was global water supplies—that is, ice ages. Whatever expressed by the University of Chicago paleobiol- the reason, we can be sure of the fact that the fish ogist David M. Raup, who wrote in Extinction: were not in any way attempting to reach some Bad Genes or Bad Luck?: “The disturbing reality is “higher” state of development, nor was any force that for none of the thousands of well-document- pushing them to “evolve.” Therein lies one of the ed extinctions in the geologic past do we have a major misconceptions about evolution, even solid explanation of why the extinction occurred.” among many of its adherents. The very name THE GREAT FIVE MASS evolution is somewhat unfortunate, because it EXTINCTIONS. The five largest known suggests that life-forms are moving gradually mass extinctions occurred at intervals of between toward a state of heightened development. 50 million and 100 million years over a span of Instead, they simply are adapting to circum- time from about 435 million to 65 million years stances that confront them; hence, the entire ago. The first of them we have mentioned as per- phenomenon might well be called by the much haps being associated with the first migrations of less dramatic-sounding name adaptation. organisms to land. This was at the end of the Mass Extinction Ordovician period, about 435 million years ago, when a drop in the ocean level wiped out one- Let us now pause in our narrative to discuss fourth of all marine families. No wonder some of examples of a phenomenon that has been men- the fish adapted to life on land! tioned several times already: mass extinction. One Changes in sea level along with climate of the truly amazing things about the history of changes also appear to have brought about the life on Earth is the way in which forces have seem- second great mass extinction, which we also have ingly conspired to wipe out virtually all living mentioned, near the end of the Devonian period, things, not just once but at least five major times, about 370 million years ago. But no biological along with a number of more limited instances. cataclysm in Earth’s history can compare to the Over the course of Earth’s history—even its mass extinction that attended the close of the very recent history—numerous species have Permian period some 240 million years ago—an become extinct, usually as a result of their inabil- event so traumatic to life on Earth that it has ity to adapt to changes in their natural environ- been dubbed the great dying. ment. In the extremely recent past (recent geo- Imagine if 96% of all people alive today were logically, that is—since the end of the last ice age killed—a staggering and terrifying concept. This about 10,000 years ago), some extinctions or would be far more fatalities than the combined endangerments of species have been attributed to death toll of both world wars; the Black Death of human activities, including hunting and the dis- 1347–1351 and the influenza epidemic of ruption of natural habitats. For the most part, 1918–1920; the various Nazi, Stalinist, and however, extinction is simply a part of Earth’s his- Maoist acts of genocide; and all major earth- tory, a result of the fact that organisms incapable quakes and other natural disasters in history. of adaptation soon die out. This is the infamous Now imagine that along with all those people, “survival of the fittest,” an aspect of evolutionary 96% of all other life-forms—plants and trees, theory that many a social or political ideologue birds and fishes, single-cell organisms and the
182 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 183
Paleontol- ogy
DINOSAURS DOMINATED EARTH FOR A PERIOD OF MORE THAN 100 MILLION YEARS, ENDING ABOUT 65 MILLION YEARS AGO. THE FLYING TRICERATOPS, a creature of the late Cretaceous period, may have been a link between dinosaurs and birds. (© Kevin Schafer/Corbis. Reproduced by permission.)
like, which vastly outnumber the relatively small jawless fishes. Plant life on land consisted of ferns human population of Earth—all ceased to exist and mosses. Then came the late-Devonian mass as well. Such was the almost incomprehensible extinction we have mentioned, which removed scale of the so-called great dying, an event whose many of the newly evolved invertebrates, includ- cause has not been determined, though it may ing most fishes. By the end of the Devonian, have resulted from a volcanic eruption in Siberia. about 360 million years ago, fish had evolved jaws, and life was thriving on land. Among these The Age of the Dinosaurs life-forms were reptiles in the animal kingdom, and gymnosperms, or plants that reproduce sex- Now that we have mentioned only three of the five ually by spreading exposed seeds—a primary great mass extinctions, it is appropriate to discuss example from our own time being pine trees. the creatures that died in the fourth and fifth: the dinosaurs, or “terrible lizards.” Though they were MONSTERS OF THE MESO- the most well-known creatures of the Mesozoic ZOIC. Although the existence of dinosaurs has era, which lasted from about 240–65 million years been an established fact for as long as anyone ago, dinosaurs were far from the only notable alive can remember, we have had knowledge of species on Earth in that phase of geologic history. them only since the mid–nineteenth century. It In fact, it was during the first period of the Meso- was then that early paleontologists began to piece zoic era, the Triassic, that a group of creatures des- together evidence from fossils, which revealed tined to make a greater impact on Earth—mam- the record of a time when enormous reptiles had mals—first appeared. During this time, botanical walked the earth. Not all of these dinosaurs were life included grasses, flowering plants, and trees of so large, however. Some were as small as chick- both the deciduous (leaf-shedding) and conifer- ens, while others—the sauropods, the largest ter- ous (cone-bearing) varieties. restrial animals of all time—were the equivalent Backtracking just a bit, at the beginning of of half a dozen stories tall. the Devonian period, about 410 million years Some dinosaurs were awesomely fierce pred- ago, the first vertebrates (animals with an internal ators, while others were mild-mannered plant skeleton) made their appearance in the form of eaters. Dinosaurs fell into two groups based on the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 183 set_vol3_6 8/19/02 11:09 AM Page 184
Paleontol- shape of their hips, which were either lizardlike or world. Moving on its powerful hind legs with a ogy birdlike. Though the lizardlike Saurischia emerged gait that was at once swift and lumbering, it used first, they lived alongside the birdlike Ornithischia its tail as a counterbalance and stabilizer. Once it throughout the late Triassic, Jurassic, and Creta- had come abreast of its prey, T. rex probably ceous periods. Ornithischia were all herbivores, or attacked its victim with powerful head butts and plant eaters, whereas Saurischia included both then tore the animal apart with its massive jaws herbivores and carnivores, or meat eaters. Natural- before ripping into it with some 60 dagger- ly, the most fierce of the dinosaurs were carni- shaped teeth as much as 6 in. (15 cm) long. vores, a group that included the largest carnivore Of course, T. rex is notable precisely because ever to walk the Earth, Tyrannosaurus rex. it was so unusual among dinosaurs in size and, Most dinosaurs had long tails and necks and for the most part, power. Deinonychus, for exam- walked on four legs, though some were bipedal, ple, was much smaller—only about 10 ft. (3 m) meaning either that they had only two legs or and 220 lb. (100 kg)—but these “running lizards” that they used only their two rear legs for move- were still fearsome predators, noted for their ment or locomotion. Again, the shape of the sickle-like claws. The phrase “sickle-like claws” creature correlated to some degree with its might call to mind another creature, this one habits: all four-legged dinosaurs were relatively made famous by Jurassic Park: Velociraptor, or gentle, slow-moving herbivores, whereas most of “swift plunderer,” a walking death machine that the bipedal ones were fast-moving predators. for all the terror it inspired in moviegoers (and, Their teeth also reflected their diet: not surpris- no doubt, in many a Mesozoic creature) attained ingly, carnivores had much sharper cutting sur- a length of only about 6 ft. (2 m). faces shaped like serrated knives, which made it Among the many lessons about dinosaurs to easy to rip their prey into smaller pieces before be learned from Jurassic Park was the striking swallowing the pieces whole. contrast between herbivores and carnivores, a As recently as the 1970s, scientists tended to fact emphasized by the scene in which the two believe that dinosaurs were cold-blooded crea- children take refuge with a herd of sauropods. tures markedly lacking in brain power. This view These gentle giants possessed four legs the size of is reflected in a number of popular-culture large pillars and had extraordinarily long necks sources, such as the 1983 song by the Police, and tails. Most famous among the sauropods was “Walking in Your Footsteps,” which suggests that the Apatosaurus, which achieved a length of 65 ft. if humans were to destroy themselves by nuclear (20 m) and a weight of 30 tons (27 metric tons). warfare, they would seem even more “dumb” Apatosaurus’s appearance in the scene with the than the dinosaurs. By contrast, the film Jurassic children was not the great creature’s first time in Park, made a decade later, reflects changes in the limelight of popular culture: known as Bron- paleontologists’ attitudes toward dinosaurs, tosaurus until the early 1990s, this dinosaur was a gained as a result of continued research. As it popular fixture of the Flintstones cartoon. turns out, dinosaurs were far from dumb, and In addition to land-based dinosaurs, there they may even have been warm-blooded—unlike were such creatures as the flying Triceratops, most reptiles but like birds, to whom they may which may have been a link between dinosaurs have been related. and birds. (The name raptor, incidentally, usual- SOME TYPES OF DINOSAUR. ly refers not to a type of dinosaur, but to a bird of Few creatures have ever captured the imagina- prey.) It should be noted that Triceratops and tion of humans as much as Tyrannosaurus rex— Velociraptor were creatures of the late Cretaceous a particularly impressive feat, given the fact that period, during which about a hundred different the last T. rex died tens of millions of years before dinosaur genera (plural of genus) flourished. humans ever came on the scene. With a name This is an important point, because many books that means “absolute ruler lizard” in Greek, this picture all manner of dinosaurs coexisting, when, terrifying creature reached a maximum length of in fact, various ones existed at different times 45 ft. (14 m) and may have weighed as much as 9 over a period of about 180 million years. Also, it tons (8 metric tons). is worth noting that the major nonhuman “star” T. rex must have made a truly awesome sight of Jurassic Park did not even live during the running through the forests of its Mesozoic Jurassic period!
184 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 185
Later Mass Extinctions—and Around the world, geologists have found traces Paleontol- New Life of iridium deposited at a layer equivalent to the ogy Today, of course, the dinosaurs are long gone— boundary between the Cretaceous and Tertiary so distant in time, in fact, that the remains of periods, the Tertiary being the beginning of the many millions have become petroleum. This has present Cenozoic era. This is significant, because happened in situations where specific conditions iridium seldom appears on Earth’s surface—but prevail: for example, the remains could not be it is found in asteroids. allowed to rot, and decay had to be anaerobic and THE ASTEROID HITS. It appears that had to take place within certain types of rock. Yet the asteroid smashed into what is now the north- oil reserves (for as long as they last) are not the ern tip of Mexico’s Yucatán peninsula. Today that only reason why the dinosaurs’ disappearance area is home to a crater some 6 mi. (9.6 km) deep was beneficial from the human standpoint. At and 186 mi. (300 km) in diameter. Note, howev- the time when dinosaurs controlled the world, er, that 65 million years ago, the Yucatán was not mammals were small and hid from predators exactly where it is now. It had begun to drift in such as T. rex. Without the mass extinction of the the direction of its present location, but the map dinosaurs, this class of creatures might never of the future continent of North America was have come to the forefront, and human beings quite different from what it is today and included might not have evolved. large submerged areas. Among them was the Actually, two mass extinctions rocked the region of impact, which only later rose to the sur- world of the dinosaurs. The first (fourth among face as a result of tectonic activity. the list of five major mass extinctions) happened The nearest large landmass at the time was a at the end of the Triassic, the first period of the good distance away—equivalent to northern Mesozoic era. This was about 205 million years Louisiana. But it hardly mattered that there were ago, when an asteroid may have hit Earth. What- no dinosaurs at the point of impact. Traveling at ever the cause, the result was that creatures in the more than 100,000 MPH (160,000 km/h), which seas suffered major mass extinction. So, too, did is almost four times as fast as the fastest space- those on the land, but many species of dinosaurs craft built by human beings, the asteroid proba- and mammals managed to survive the event, bly brought about an explosion as intense as which marked the transition between the Triassic many thousands of hydrogen bombs. It may not and Jurassic periods. have raised a mushroom cloud per se, but it THEORIES REGARDING THE probably produced an even more dramatic for- DINOSAURS’ DEMISE. The dinosaurs mation as it sent more than 48,800 cu. mi. continued to flourish for another 140 million (200,000 km3) of debris and gases into the years, and their descendants might still be walk- atmosphere. This would be enough dust to cover ing the earth were it not for the last great mass the state of Mississippi to a depth of 1 mi. (1.6 extinction, which took place about 65 million km) years ago. What killed the dinosaurs? Paleontolo- The sound of the impact must have been gists and other scientists have proposed several ear-shattering, even far away on what would one theories: a rapid climate change; the emergence day become North America. Then came the tidal of new poisonous botanical species, eaten by her- waves, some as tall as 394 ft. (120 m), with earth- bivorous dinosaurs, that resulted in the passing quakes soon to follow. But while these tidal waves of toxins along the food web; an inability to com- undoubtedly caused massive localized death, or pete successfully with the rapidly evolving mam- mass mortality, what really brought about the mals; and even an epidemic disease to which the mass extinction of the dinosaurs was the after- dinosaurs possessed no immunity. math of impact. All that dust in the atmosphere Interesting as many of these theories are, effectively blocked out the Sun’s light, bringing none has gained anything like the widespread about a dramatic cooling on Earth’s surface. As a acceptance achieved by another scenario. result, plants died, thus depriving herbivorous According to this highly credible theory, an aster- dinosaurs of an energy source. The herbivores oid hit Earth, hurtling vast quantities of debris died and then, in a domino effect brought about into the atmosphere, blocking out the sunlight, by the interdependence of components in the and greatly lowering Earth’s surface temperature. food web, the carnivores soon followed.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 185 set_vol3_6 8/19/02 11:09 AM Page 186
Paleontol- ogy
ASTEROID APPROACHING EARTH. ACCORDING TO ONE THEORY, THE MASS EXTINCTION OF THE DINOSAURS RESULT- ED FROM THE IMPACT OF AN ASTEROID HITTING EARTH, HURTLING VAST QUANTITIES OF DEBRIS INTO THE ATMOS- PHERE, BLOCKING OUT THE SUNLIGHT, AND GREATLY LOWERING EARTH’S SURFACE TEMPERATURE. (© D. Hardy. Photo Researchers. Reproduced by permission.)
MAMMALS, HUMANS, AND a few seconds ago, in geologic terms), we emerged MASS EXTINCTION. When that asteroid from the cold, ready to take over the world. hit, it brought an end to the dinosaurs and their Near the end of the last ice age, peoples from Mesozoic world, ushering in the Cenozoic era, Siberia crossed a land bridge spanning what is which saw the rise of mammals. Some examples today the Bering Strait between Russia and Alas- of the creatures that proliferated in the course of ka. Their descendants, of course, are the Native the past 65 million years are tiny prehistoric Americans, and native is a fitting term, since they horses with four toes as well as a giant rhinocer- arrived in the Americas about 12,000 years ago, or os-like herbivore, both of which lived in the about 7,000 years before the Europeans’ ancestors Eocene epoch about 40 million years ago. arrived in Europe. What they found, as they As time went on, the range of species—not poured into the Americas, was a range of species only animals and plants but also the much less quite different from those known today. There complex organisms of the other three kingdoms were mammoth and mastodons; giant bears, (Protista, including protozoa and algae; Monera beaver, and bison; and even saber-toothed “tigers” such as bacteria; and Fungi)—grew larger and (which were not directly related to modern-day larger. An ice age struck the planet about 1.65 tigers), camels, and lions. Prehistoric America was million years ago, bringing about much smaller also home to horses, but these creatures and many cases of mass extinction than the ones we have others were wiped out by hunting. Horses did not discussed. But other instances of mass extinction reappear in the New World until Europeans would occur at the hands of a creature that brought them after A.D. 1500, when they would appeared about 2.5 million years ago: Homo sapi- prove an indispensable aid in European efforts to ens. The ice age—which was only one of many conquer the Native Americans’ lands. and thus is referred to often as the “last ice age”— would prove a major turning point in the history WHERE TO LEARN MORE of the human species. At the beginning, we were Cadbury, Deborah. Terrible Lizard: The First Dinosaur just embarking on the beginnings of the Stone Hunters and the Birth of a New Science. New York: Age, whereas at the end, just 10,000 years ago (or Holt, 2001.
186 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_6 8/19/02 11:09 AM Page 187
Paleontol- KEY TERMS ogy
AEROBIC: Oxygen-breathing. be thought of as a bundle or network of AMPHIBIAN: A creature capable of liv- food chains, but since the latter rarely exist ing either in the water or on solid ground. separately, scientists prefer the concept of a food web to that of a food chain. ANAEROBIC: Non-oxygen-breathing. FOSSIL: The mineralized remains of CARNIVORE: A meat-eating organ- any prehistoric life-form, especially those ism. preserved in rock before the end of the last EON: The longest phase of geologic ice age. time. Earth’s history has consisted of four FOSSILIZATION: The process by eons, the Hadean, Archaean, Proterozoic, which a once living organism becomes a and Phanerozoic. The next-smallest subdi- fossil. Generally, fossilization involves min- vision of geologic time is the era. eralization of the organism’s hard portions, EPOCH: The fourth-longest phase of such as bones, teeth, and shells. geologic time, shorter than an era. The cur- GEOLOGIC TIME: The vast stretch of rent epoch is the Holocene, which began time over which Earth’s geologic develop- about 10,000 years ago. ment has occurred. This span (about 4.6 ERA: The second-longest phase of geo- billion years) dwarfs the history of human logic time, after an eon. The current eon, existence, which is only about 2.5 million the Phanerozoic, has had three eras, the years. Much smaller still is the span of Paleozoic, Mesozoic, and Cenozoic, which human civilization, only about 5,500 years. is the current era. The next-smallest subdi- GYMNOSPERM: A type of plant that vision of geologic time is the period. reproduces sexually through the use of EUKARYOTE: A cell that has a nucleus seeds that are exposed, not hidden in an and organelles (sections of the cell that ovary as with an angiosperm. perform specific functions) bound by HERBIVORE: A plant-eating organ- membranes. ism. FOOD CHAIN: A series of singular ICE AGE: A period of massive and organisms in which each plant or animal widespread glaciation (the spread of gla- depends on the organism that precedes it. Food chains rarely exist in nature; there- ciers). Ice ages usually occur in series over fore, scientists prefer the term food web. stretches of several million or even several hundred million years and have taken place FOOD WEB: A term describing the periodically—often in conjunction with interaction of plants, herbivores, carni- mass extinctions—throughout Earth’s his- vores, omnivores, decomposers, and detri- tory. tivores in an ecosystem. Each of these INVERTEBRATE: organisms consumes nutrients and passes An animal without them along to other organisms (or, in the an internal skeleton. case of the decomposer food web, to the MASS EXTINCTION: A phenomenon soil and environment). The food web may in which numerous species cease to exist at
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 187 set_vol3_6 8/19/02 11:09 AM Page 188
Paleontol- ogy KEY TERMS CONTINUED
or about the same time, usually as the ships between prehistoric plants and ani- result of a natural calamity. mals and their environments. MINERAL: A naturally occurring, typi- PALEONTOLOGY: The study of life- cally inorganic substance with a specific forms from the distant past, primarily as chemical composition and a crystalline revealed through the fossilized remains of structure (that is, a structure in which the plants and animals. constituent parts have a simple and defi- PALEOZOOLOGY: An area of paleon- nite geometric arrangement that is repeat- tology devoted to the study of prehistoric ed in all directions). animal life. MINERALIZATION: A series of changes PERIOD: The third-longest phase of experienced by a once living organism dur- geologic time, after an era. The current ing fossilization. In mineralization, minerals eon, the Phanerozoic, has had 11 periods, in the organism are replaced or augmented and the current era, the Cenozoic, has con- by different minerals, or the hard portions sisted of three periods, of which the most of the organism dissolve completely. recent is the Quaternary. The next-smallest ORGANIC: At one time chemists used subdivision of geologic time is the epoch. the term organic only in reference to living PHOTOSYNTHESIS: things. Now the word is applied to most The biological compounds containing carbon, with the conversion of light energy (that is, electro- exception of carbonates (which are miner- magnetic energy) from the Sun to chemical als) and oxides, such as carbon dioxide. energy in plants. PALEOBOTANY: An area of paleontol- PROKARYOTE: A cell without a nucle- ogy involving the study of past plant life. us or organelles bound by membranes. PALEOECOLOGY: An area of paleon- VERTEBRATE: An animal with an tology devoted to studying the relation- internal skeleton.
Gould, Stephen Jay. Ever Since Darwin: Reflections in Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, Natural History. New York: W. W. Norton, 1977. MD: Discovery Communications, 1999. K–12: Paleontology—Dinos (Web site). Raup, David M. Extinction: Bad Genes or Bad Luck? New
188 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:11 AM Page 189
SCIENCE OF EVERYDAY THINGS real-life biology BIODIVERSITY AND TAXONOMY
TAXONOMY SPECIES SPECIATION
189 set_vol3_7 8/19/02 11:11 AM Page 191
Taxonomy TAXONOMY
CONCEPT logical sciences, that simply refers to the act of Taxonomy is the area of the biological sciences systematically arranging ideas or objects into cat- devoted to the identification, naming, and classi- egories according to specific criteria. fication of living things according to apparent While its meaning is narrower than that of common characteristics. It is far from a simple classification, even taxonomy still has broader subject, particularly owing to many disputes over applications than the way in which it is used in the rules for classifying plants and animals. In the biological sciences. In a general sense, taxon- terms of real-life application, taxonomy, on the omy refers to the study of classification or to one hand, is related to the entire world of life on methods of classification—for example, “politi- Earth, but on the other hand, it might seem an cal taxonomy,” as we used it in the introduction ivory-tower discipline that it has nothing to do to this essay. Literary critics sometimes refer to a with the lives of ordinary people. Nonetheless, to writer’s taxonomy of characters. Within the bio- understand the very science of life, which is biol- logical sciences, however, the term designates ogy, it is essential to understand taxonomy. Each specifically a subdiscipline involving the process discipline has its own form of taxonomy: people and study of the identification, naming, and clas- cannot really grasp politics, for instance, without sification of organisms according to apparent knowing such basics of political classification as common characteristics. the difference between a dictatorship and a PHYLOGENY AND NOMENCLA- democracy or a representative government and TURE. one with an absolute ruler. In the biological sci- Two other terms that one is likely to ences, before one can begin to appreciate the run across in the study of taxonomy are phyloge- many varieties of organisms on Earth, it is essen- ny and nomenclature. Phylogeny is the evolu- tial to comprehend the fundamental ideas about tionary history of organisms, particularly as that how those organisms are related—or, in areas of history refers to the relationships between life- dispute, may be related—to one another. forms and the broad lines of descent that unite them. Taxonomy is less fundamental a concept than phylogeny. Whereas taxonomy is a human HOW IT WORKS effort to give order to all the data, phylogeny is the true evolutionary relationship between living Taxonomy in Context organisms. Some scientists call phylogeny the tree The term taxonomy is actually just one of sever- of life, meaning that it represents the underlying al related words describing various aspects of hierarchical structure by which life-forms classification in the biological sciences. In keep- evolved and are related to one another. ing with the spirit of order and intellectual tidi- The word naming was used earlier in the ness that governs all efforts to classify, let us start definition of taxonomy because it is a familiar, with the most general concept, which happens to easily understandable word. However, a more be classification itself. Classification is a very accurate term, and one that helps illuminate the broad term, with applications far beyond the bio- distinction between taxonomy and systematics, is
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 191 set_vol3_7 8/19/02 11:11 AM Page 192
Taxonomy form, among birds. This is a homologous feature, indicating a common ancestor that likewise had limbs with five digits at the end. By contrast, there is no indication of a close evolutionary relation- ship in the fact that birds, butterflies, and bats all have wings that are similar in shape. Rather, the laws of physics require that a wing be of a certain shape in order to hold an object aloft, which is why the contour of an airplane wing, when viewed from the side, is remarkably like that of a bird’s wing where it joins the animal’s body. Cladistics and Numerical Taxonomy
Cladistics is a system of taxonomy that dis- tinguishes taxonomic groups or entities on the basis of shared derived characteristics, hypothe- sizing evolutionary relationships to arrange them in a treelike, branching hierarchy. The expression derived characteristics in this definition means that the characteristics that unite two types of THE GREEK PHILOSOPHER ARISTOTLE IS REGARDED AS organism are not necessarily present in a shared THE FATHER OF TAXONOMY. THE ARISTOTELIAN PRINCI- PLES OF CLASSIFICATION WERE GOVERNED BY THE IDEA evolutionary ancestor. Rather, they have devel- THAT THERE ARE CONSTANT, UNCHANGING “ESSENCES” oped over the course of evolutionary history THAT UNITE CLASSES OF ORGANISMS, WHICH IS COM- since the time of that shared ancestor. PLETELY AT ODDS WITH THE EMPIRICAL MENTALITY THAT GOVERNS TAXONOMY TODAY. (The Library of Congress.) In explaining cladistics to the ordinary human being, the vast majority of science writers seem to be at a loss as to how to make the topic nomenclature. The latter can be defined as the comprehensible. Thus, such terms as derived act or process of naming as a well as a system of characteristics and its opposite, primitive charac- names, particularly one used in a specific science teristics, usually are left undefined. A welcome or discipline. exception is Paul Willis, who, in an on-line article Homologous and Analogous for the Australian Broadcasting Corporation (see Features Where to Learn More) gave a wonderful illustra- tion that was an attempt to analyze the relation- Before going on to discuss methods of classifica- ships between a mouse, a lizard, and a fish. tion, it is important to note just which character- “They’ve all got backbones,”Willis wrote,“so istics of an organism’s morphological aspect (i.e., the feature ‘backbone’ is useless [as an indication structure or form) are important to scientists of evolutionary branching]; it’s a ‘primitive’ char- working in the field of taxonomy. In theorizing acter that tells you nothing. But the [derived] fea- relationships between species, taxonomists are ture ‘four legs’ is useful because it’s an evolution- not interested in what are known as analogous ary novelty shared only between the lizard and features, those characteristics that are superficial- the mouse. This implies that the lizard and ly similar but not as a result of any common evo- mouse are more closely related to each other than lutionary origin. Rather, they are interested in either is to the fish. Put another way, the lizard homologous features, or features that have a and the mouse share a common ancestor that common evolutionary origin, even though they had four legs.” Willis went on to note that “the may differ in terms of morphological form. more evolutionary novelties we can find that One example of a shared evolutionary char- support a particular relationship, the greater our acteristic, discussed briefly in the essay Evolution, confidence that the relationship is correct. ‘Air is the pentadactyl limb, a five-digit appendage breathing,’ ‘neck’ and ‘amniotic egg’ are another common to mammals and found, in modified three evolutionary novelties that tie the lizard
192 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:11 AM Page 193
and the mouse together and leave the fish as a To put it in colloquial terms, by referring to Taxonomy more distant relative.” a taxonomic key, a taxonomist may determine NUMERICAL TAXONOMY. Cladis- that if an organism “looks like a duck and quacks tics, the most widely applied approach to taxon- like a duck, it must be a duck”—only, in this omy, has undergone considerable change since it instance, the taxa being compared are much was introduced by the German zoologist Willi more specific than the common term duck and Hennig (1913–1976) in the 1950s. Particularly the characteristics much more precisely important has been the marriage of cladistics described. (For one thing, there are several dozen with another taxonomic idea born in the species in the genus Anas, which includes all mid–twentieth century, phenetics, or numerical “proper” ducks, and many more species in the taxonomy. Introduced by the Austrian biologist family Anatidae, or waterfowl, that are common- Robert Reuven Sokal (1926–) and the English ly called by “duck names”—including such microbiologist Peter Henry Andrews Sneath amusingly named species as the ruddy duck, lack (1923–), numerical taxonomy is an approach in duck, freckled duck, and comb duck.) If there is no already established “duck” that the species in which specific morphological characteristics of question resembles, the taxonomist may have an organism are measured and assigned numeri- discovered an entirely new genus, family, order, cal value, so that similarities between taxa (taxo- class, or even phylum. nomic groups or entities) can be compared mathematically. These mathematical compar- A taxonomist may use what is called a isons are performed through the use of algo- dichotomous key, which presents series of alter- rithms, or specific step-by-step mathematical natives much like a flow chart. For example, if the procedures for computing the answer to a partic- flowers of a sample in question are white and the ular problem. The aim of numerical taxonomy is stem is woody, then (depending on additional to remove all subjectivity (such as the taxono- alternatives) it could be either species A or mist’s “intuition”) from the process of classifica- species B. If the flowers are not white and the tion. Initially, many traditional taxonomists stem is herbaceous (non-woody), then, presented rejected numerical taxonomy, because its results with another set of additional alternatives, it is sometimes contradicted their own decades-long possible that the plant is either species C or studies of comparative morphological features. species D. Nearly all modern taxonomists apply numerical CLASSIFICATION. In discussing methods in taxonomy, although there is often cladistics and phenetics, we touched briefly on heated debate as to which particular algorithms the process of classification. Suffice it to say that should be used. this process is far more complex and technically elaborate than these few paragraphs can begin to Identification, Classification, suggest. We return later to specifics of classifica- and Nomenclature tion as they relate to systems and innovations introduced by the Greek philosopher Aristotle Earlier, taxonomy was defined in terms of its (384–322 B.C.), the Swedish botanist Carolus Lin- relationship to the identification, classification, naeus (1707–1778), and the English naturalist and nomenclature of taxa. Let us now briefly Charles Darwin (1809–1882), the three most consider each in turn, with the understanding important men in the history of taxonomy that they are exceedingly complex, technical sub- before the twentieth century. For the present, our jects that can be treated here in the most cursory focus is on the overall ranking system. fashion. The process of identification is a partic- ularly complex one. When an apparently new There are many possible ranks of classifica- taxon is discovered, a taxonomist prepares an tion but only seven that are part of what is organized written description of the characteris- known as the obligatory taxonomy, or obligatory tics of similar species, which are referred to as a hierarchy. These ranks are kingdom, phylum, taxonomic key. Instead of using pictures, which class, order, family, genus, and species. Listed often poorly convey the natural variations in here are all possible ranks, with obligatory ranks morphological features, taxonomists prefer to in italics. use a taxonomic key in written form, which pro- • Kingdom vides much more detail and exactitude. • Subkingdom
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 193 set_vol3_7 8/19/02 11:11 AM Page 194
Taxonomy • Phylum species name is like a personal name. The differ- •Subphylum ence is that whereas there may be thousands of • Superclass boys and men named John Smith, there is only • Class one species called Homo sapiens. Beyond the • Subclass species name, there may be subspecies names: • Infraclass humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. • Cohort Additional rules govern the inclusion of a name • Superorder or an abbreviation, at the end of the species or • Order subspecies name, to recognize the individual who • Suborder first identified it. • Superfamily • Family • Subfamily REAL-LIFE •Tribe APPLICATIONS • Genus The Urge to Classify • Subgenus • Species One might ask what all the fuss is about. Why is • Subspecies classification so important? We attempt to The reader occasionally may come across answer that question from a few angles, includ- nonobligatory ranks, most notably subphylum, ing a brief look at the lengthy historical quest to but for the most part the only ranks referred to in develop a workable taxonomic system. But what this book are the obligatory ones. was the original impulse that motivated that NOMENCLATURE. In accordance quest? One clue can be found in the Greek roots with a tradition established by Linnaeus, all of the word taxonomy: taxis, or “arrangement,” group names are in Latin, thus facilitating ease of and nomos, or “law.” The search for a taxonomic communication. There are some rules concern- system represents humankind’s desire to make ing names of groups: for instance, those of fami- order out of the complexities with which nature lies use the suffix -idae. In the world of taxono- presents us. When it comes to the organization of my, however, few rules are accepted universally. ideas (including ideas about the varieties of life- Even as basic a term as phylum is not universal, forms), this desire for order is more than a mere since botanists prefer the word division. preference. It is a necessity. The proper name of any ranking more gen- THE ANALOGY TO A LIBRARY. eral than species is capitalized (e.g., phylum Imagine a library without any organizational sys- Chordata), with species and subspecies names in tem, with books simply crammed willy-nilly on lowercase. Genus, species, and subspecies names the shelves. Such a place would be totally chaotic, are rendered in italics (e.g., Homo sapiens, or and if one happened to find a book one was look- “man the wise”), while proper names of the more ing for, it would be a case of pure luck. The odds general groupings are presented in ordinary type would be weighted heavily against such luck, (e.g., class Mammalia). If the same name appears especially in a university library or a large munic- a second time in the same article, the genus name ipal or regional one. Just as a good-size universi- usually is abbreviated: thus, H. sapiens. ty library has upward of a million volumes, and Just as most people (with such rare excep- many large university libraries have several mil- tions as Cher and Madonna) are identified by lion, so there are at least a couple of million iden- two names, a personal and a family name, taxon- tified species, and the total may be much larger. omy makes use of a system called binomial Some entomologists (scientists who study nomenclature, in which each type of plant or insects) speculate that there may be ten million animal is given a two-word name, with the first species of insect alone. name identifying the genus and the second the THE LURE OF A NEW SPECIES. species. In binomial nomenclature, the genus When a zoologist or botanist discovers what he or name is analogous to the family name, inasmuch she believes to be a new species, the taxonomic as there are many species within a genus, and the system provides a standard against which to check
194 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:11 AM Page 195
it—rather as you would do if you thought you stitute a form of classification, there is a great dif- Taxonomy had discovered a book that was not in the library. ference between this and scientific taxonomy. If the “new” species matches an established one, SCIENCE VERSUS “COMMON that may be the end of the story—unless the sci- SENSE.” Taxonomy is tied closely to evolu- entist has discovered a new aspect of the species tionary study, and Darwin’s theory of evolution or a new subspecies. And if there is no match in was a turning point in the history of scientific the taxonomic “library,” the scientist has discov- classification. Thus, taxonomists are concerned ered an entirely new life-form, with all the grand more with the evolutionary patterns that link and terrifying ramifications that may ensue. organisms than they are with what may be only The new species might be an herb from superficial similarities. Habitat, for instance, is which a cure can be synthesized for a devastating significant in studying biomes, but it seldom disease, or it could be a parasite that carries a new plays a role in taxonomy. Nor is the ability to fly, and previously unknown malady. Whatever it is, as we have noted, necessarily an indicator of tax- it is better to know about it than not to know, onomic similarities. and though the vast majority of “new” species are A striking example of the difference between not nearly as exciting as the preceding paragraph scientific taxonomy and “common sense” classifi- would imply, each has its part to play in the over- cation is the fact that whales and dolphins are all balance of life. Discovery of new species is grouped along with other mammals (class Mam- particularly important when those species are malia) rather than with fish and other creatures endangered or might be in the process of disap- that most readily come to mind when thinking of pearing even as they are identified. aquatic organisms. In fact, whales and dolphins Nonscientific Taxonomy share not only a wide array of primitive charac- teristics with mammals (for example, the pen- Without knowing anything about scientific tax- tadactyl limb described earlier) but also the onomy, almost anyone can begin to classify ani- derived characteristic that defines mammal: the mals and perhaps plants. If we limit the discus- secreting of milk from mammary glands, by sion purely to animals, there are many basic which a mother feeds her young. Not only is it parameters according to which we could classify impossible to get milk from a fish (even family them, just off the tops of our heads, as it were. For Chanidae, known by the common name “milk- example, there are aquatic and terrestrial animals, fish”), but fish lack even that primitive character- and these general groupings can be broken down istic, the pentadactyl limb, that links mammals, further according to biome or habitat (see Bio- at least distantly, with nonmammalian creatures, mes). There are animals that walk, fly, swim, slith- such as birds (class Aves). er, or move by some other means. Animals can be COMMON TERMS AND FOLK divided according to their forms of reproduction, TAXONOMY. For the sake of convenience, in whether asexual or sexual, oviparous or vivipa- many places throughout this book, common rous (expelling or retaining a fertilized egg, terms such as bird, horse, fish, and so forth are respectively), and so on. As discussed in Food used. But common terms are far from adequate Webs, animals may be classified as herbivores, in a scientific context, because such terminology carnivores, omnivores, or detritivores or as pri- can be deceptive, as exemplified by the nonduck mary, secondary, or tertiary consumers. They may “ducks” mentioned earlier. Likewise, shellfish be endothermic or ectothermic (warm-blooded and starfish are not “fish” as that term is usually or cold-blooded), and they may be covered with understood. But while common terminology can scales, feathers, fur, or skin. (In the last case, that be misleading, sometimes correlations with sci- skin may be protected by either mucus or hair.) entific taxonomy can be found in what is known On and on go the categories, and if one is as folk taxonomy. The latter is a term for the tax- inclined toward a classifying mind, this kind of onomic systems applied in relatively isolated mental exercise can be fun. Certainly, little chil- non-Western societies. For example, the folk tax- dren enjoy it, and many educational programs onomy of native peoples in New Guinea identi- and games call on the child to group animals thus. fied 136 bird species in the mountains of that Although these kinds of groupings, and the efforts island, a figure that came amazingly close to the to place animals into one group or another, con- 137 species identified by the German-born
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 195 set_vol3_7 8/19/02 11:11 AM Page 196
Taxonomy
ILLUSTRATION OF A SEA MONSTER FROM “DE NATURA RERUM” by Albertus Magnus. During the Middle Ages, taxonomic writings consisted primarily of books of imaginary creatures; the first signs of scientific reawakening in taxonomy came with plant and animal catalogues by such medieval scholars as Magnus. (© Gianni Dagli Orti/Corbis. Reproduced by permission.)
American evolutionary biologist Ernst Mayr from listing the champions at the Pythian Games (1904–) when he studied New Guinea’s birds (a festival like the Olympics) to classifying the using scientific methods. constitutions of various Greek city-states to ana- lyzing the body parts of animals. Aristotle him- Aristotle, Linnaeus, Darwin, self dissected hundreds of animals to understand and Beyond what made them tick, and he proved to be some 2,000 years ahead of his time in recognizing that Among his many other accomplishments as a the dolphin is a mammal and not a fish. His sys- thinker, Aristotle is regarded as the father of the tem of classification, however, was a far cry from biological sciences and of taxonomy. Among the the ideas that developed in nineteenth-century dominant ideas in his work as a philosopher are taxonomy; rather than searching for evolution- the concepts of hierarchy and classification, and ary lines of descent, he ranked animals in order thus he took readily to the idea of classifying of their physical complexity. things. At his school in Athens, he put his stu- In most aspects of his other work, Aristotle dents to work on all sorts of taxonomic pursuits, established sharp distinctions between his own
196 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:11 AM Page 197
ideas and those of his teacher, Plato (427?–347 Carl von Linné but adopted the Latinized name Taxonomy B.C.). For example, Aristotle rejected Plato’s posi- Carolus Linnaeus. Even that late in scientific his- tion that every idea we can conceive is but a dim tory, scholars still wrote chiefly in Latin, not reflection of an essential concept—for example, because they were trying to adhere to tradition that our idea of “red” is only a shadowy copy of but because it remained a common language the perfect notion of “redness.”Yet in his taxono- between educated people of different countries. my, Aristotle seemed to hark back to his days as Thus, Linnaeus’s great work, which he first pub- Plato’s star pupil. The Aristotelian principles of lished in 1737 but revised numerous times, was classification were governed by the idea that named Systema naturae, or “The Natural Sys- there are constant, unchanging “essences” that tem.” Thanks to Linnaeus, Latin became unite classes of organisms. This idea of essences enshrined permanently as the language of taxon- is completely at odds with the empirical (experi- omy the world over, but this was far from his only ence-based) mentality that governs taxonomy accomplishment. today. Nonetheless, for two millennia, Aris- It was Linnaeus who introduced binomial totelian ideas represented the cutting edge in tax- nomenclature, in a 1758 revision of his Systema, onomy and much else. and also Linnaeus who established several of the THE MIDDLE AGES AND obligatory rankings. Moreover, he instituted the RENAISSANCE. After Aristotle and his first taxonomic keys, and his system, first applied brilliant student Theophrastus (371?–287? B.C.), in botany, became accepted in the zoological the father of botany, there would be no Western community as well. Others, including Baron biological theorists of remotely comparable Georges Cuvier (1769–1832), Michel Adanson stature until the time of the Renaissance. In the (1727–1806), and Comte Georges Buffon meantime, taxonomy, as with so many other (1707–1788), refined Linnaeus’s system, but he areas of learning in Europe, declined badly. Dur- stands as a towering figure in the discipline. ing the Middle Ages, what passed for taxonomic Later, the French natural philosopher Jean writings consisted primarily of bestiaries, books Baptiste de Lamarck (1744–1829) proposed a full of fanciful and imaginary creatures, such as distinction between vertebrates, or animals with the unicorn. The first signs of scientific reawak- spinal columns, and invertebrates. Today this dis- ening in the biological sciences in general, and tinction is not considered as useful as it once was, taxonomy in particular, came with plant and ani- since it is lopsided—that is, there are nine times mal catalogues by such great medieval scholars as as many invertebrates as vertebrates in the ani- Peter Abelard (1079–1142) and Albertus Magnus mal kingdom—but at the time, it represented an (ca. 1200–1280). Even so, their work consisted advancement. Less questionable were the distinc- primarily of summations of existing Aristotelian tions introduced in 1866 by the German biologist knowledge rather than new contributions. Ernst Haeckel (1834–1919) between plants, ani- In the sixteenth century, the Swiss scientist mals, and single-cell organisms. As Haeckel rea- Konrad von Gessner (1516–1565) wrote Historia soned, at the level of unicellular organisms, dis- animalium (1551–1558), a groundbreaking work tinctions between plant and animal really make that included descriptions of many animals no sense. never before seen by most Europeans. Gesner DARWIN AND THE TWENTIETH also denounced the practice of including ficti- CENTURY. By far the most influential figure tious animals in bestiaries. Around the same in taxonomy during the nineteenth century was time, the discoveries of new plant and animal the man also recognized as the most influential species in the New World began to point up the figure in all of biology during that era: Darwin. need for a taxonomy that went beyond Aristo- Whereas Linnaeus had retained the Aristotelian tle’s. The first scholar of the modern era to attack focus on the “essence” of the animal’s features, this problem was the Italian botanist Andrea Darwin swept away such notions and, in his Ori- Cesalpino (1519–1603), but nearly two centuries gin of Species (1859), proposed that the “commu- would pass before the development of a workable nity of descent” is “the one known cause of close classification system. similarity in organic beings” and therefore the LINNAEUS AND OTHERS. The only reasonable basis for taxonomic classification man who revolutionized taxonomy was born systems. As result of Darwin’s work, taxonomists
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 197 set_vol3_7 8/19/02 11:11 AM Page 198
Taxonomy became much more oriented toward the repre- isms, however, are eukaryotes, or cells with a sentation of phylogeny in their classification sys- nucleus as well as organelles (sections of the cell tems. Therefore, instead of simply naming and that perform specific functions). cataloguing species, modern taxonomists also try Fungi: fungi, molds, mushrooms, yeasts, to construct evolutionary trees showing the rela- mildews, and smuts (a type of fungus that afflicts tionships between different species. certain plants). Fungi are multicellular, consist- Since Darwin’s time, taxonomy has seen ing of specialized eukaryotic cells arranged in a numerous innovations, including the introduc- filamentous form (that is, a long, thin series of tion of cladistics by Hennig and of numerical cells attached either to one another or to a long, taxonomy by Sokal and Sneath. Taxonomists thin cylindrical cell). There are some 100,000 today make use of something unknown at the varieties of fungi. time of Darwin: DNA (deoxyribonucleic acid, a Plantae: plants, of which there are upward of molecule that contains genetic codes for inheri- 250,000 species. Although plant is a common tance), which provides a wealth of evidence term, there is no universally accepted definition showing relationships between creatures. For that includes all plants and excludes all nonplants. example, a comparison of human and chim- One of the most important characteristics of panzee DNA reveals that we share more than plants is the fact that they receive their nutrition 98% of the same genetic material, indicating that almost purely through photosynthesis. Beyond the two lines of descent are related more closely the plant kingdom, this is true only of a few than either is to apes. protests and bacteria. (For the most part, the The Five Kingdoms three lower kingdoms obtain nutrition through absorption.) Other characteristics of plants There are several taxonomic systems, distin- include the fact that they are incapable of loco- guished in part by the number of different king- motion; have cells that contain a form of carbo- doms that each system recognizes. The system hydrate called cellulose, making their cell walls used in this book is that of five kingdoms, listed more or less rigid; are capable of nearly unlimited here, which is the result of modifications by the growth at certain localized regions (unlike most American biologists Lynn Margulis (1938–) and animals, which have set numbers of limbs and so Karlene V. Schwartz (1936–) to the work of earli- forth); and have no sensory or nervous system. er taxonomists. (It should be noted that biolo- Animalia: animals, of which there are more gists are increasingly using a system of six king- than 1,000,000 species. Like plants, animals are doms under three domains: eubacteria, arachaea, characterized by specialized eukaryotic cells, but and eukaryotes. For the sake of simplicity, how- also like plants, the comprehensive definition of ever, the five-kingdom system is used here.) animal is not as obvious as one might imagine. These five kingdoms are as follows: Mobility, or a means of locomotion, is not a Monera: bacteria, blue-green algae, and defining characteristic, since sponges and corals spirochetes (spiral-shaped, undulating bacteria). are considered animals. The principal difference Members of this kingdom, consisting of some between animals and plants is at the cellular 10,000 or more known species, are single-cell level: animals either lack cells walls entirely or prokaryotes, meaning that the cell has no distinct have highly permeable walls, unlike the cellulose nucleus. Some researchers have divided Monera cell walls in plants. Another defining characteris- into Eubacteria, or “true” bacteria, and Archae- tic of animal is that they obtain nutrition by bacteria, which are bacteria-like organisms capa- feeding on other organisms. Additionally, ani- ble of living in extremely harsh and sometimes mals usually have more or less fixed morpholog- anaerobic (oxygen-lacking) environments, such ical characteristics and possess a nervous system. as in acids, saltwater, or sewage. The fact that most animals are mobile helps Protista (or Protoctista): protozoans, slime account for the large number of animal species molds (which resemble fungi), and algae other compared with those of other kingdoms; over the than the blue-green variety. Made up of more course of evolutionary history, mobility brought than 250,000 species, this kingdom is distin- about the introduction of animals to a wide guished by the fact that its members are single- range of environments, which required a wide cell organisms, like the Monera. These organ- range of adaptations.
198 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:11 AM Page 199
Taxonomy
A DIVER INSIDE A BARREL SPONGE ALONG A CORAL REEF IN THE CARIBBEAN. MOBILITY, OR A MEANS OF LOCOMO- TION, IS NOT A DEFINING CHARACTERISTIC OF KINGDOM ANIMALIA; INDEED, SPONGES AND CORALS ARE CONSIDERED ANIMALS. (© Jeffrey L. Rotman/Corbis. Reproduced by permission.)
Space does not permit a discussion of the most phyla, apart from higher animals and some various phyla, let alone the smaller divisions, in plants, makes for rather dry reading to a nonsci- anything like the detail we have accorded to king- entist. These divisions are discussed in further doms. Furthermore, the distinctions among detail, however, within the essays Species and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 199 set_vol3_7 8/19/02 11:12 AM Page 200
Taxonomy Speciation. The latter essays also address the def- time in 87 years, taxonomists identified an inition of species, a great and continuing chal- entirely new insect order. Actually, the order con- lenge that faces taxonomists. sists of a single known species, but this one is so different from others that it must be grouped Taxonomy in Action separately. Discovered in Namibia, in southwest- ern Africa, the creature was given the nickname Two stories reported in National Geographic “the gladiator” in honor of the Academy Award- News online (see “Where to Learn More”) in winning 2000 film of that name. 2001 and 2002 illustrate the fact that scientific classification is an ongoing process, and that the Entomologist Oliver Zompro of the Max world of taxonomy is frequently home to contro- Planck Institute of Limnology in Plön, Germany, versies and surprises. Lee R. Berger of the Geo- described the creature as “a cross between a stick graphic reported the first story, on December 17, insect, a mantid, and a grasshopper,”according to 2001, under the heading “How Do You Miss a the Geographic. Because its first body segment is Whole Elephant Species?” As it turns out, there the largest, it is distinguished from a stick insect, are not just two species of elephant, as had long whereas it differs from a mantid inasmuch as it been believed, but three. uses both fore and mid-legs to capture prey. And while it looks like a grasshopper, “the gladiator” In addition to the Asian elephant (Elephas cannot jump. maximus) scientists had long recognized the African savanna elephant, or Loxodonta africana, Measuring as much as 1.6 in. (4 cm) long, as a second species. However, DNA testing (see the insect, whose order is designated as Man- Genetics and Genetic Engineering) in 2001 tophasmatodea, is a carnivorous, nocturnal crea- revealed a second African variety, Loxodonta ture. Its discovery raised the number of known cyclotisare or the African forest elephant, former- insect orders to 31, a discovery that Piotr ly believed to constitute merely a subspecies. Naskrecki, director of the Conservation Interna- tional Invertebrate Diversity Initiative, compared The news was not entirely new: as early as a to finding a mastodon or saber-toothed tiger. century prior to the announcement of the “new” Colorado State University ecologist Diana Wall species, zoologists had begun to suspect that the described the discovery as “tremendously excit- forest elephant was a separate grouping distin- ing” and told the Geographic, “This new order guished by a number of characteristics. For could be a missing link to determining relation- example, the forest elephant is physically smaller, ships between insects and other groups. ... Every with males seldom measuring more than 8 ft. textbook discussing the orders of insects will now (2.5 m) at the shoulder, as compared to 13 ft. (4 need to be rewritten.” m) for a large savanna male. Additionally, ivory samples confiscated from poachers or illegal hunters have revealed that the material in the WHERE TO LEARN MORE tusks of the forest variety is pinker and harder Classification—The Dinosaur FAQ (Web site).
200 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 201
Taxonomy KEY TERMS
ALGORITHM: A specific set of step-by- CLASSIFICATION: A very broad step procedures for computing answers to term, with application far beyond the bio- a mathematical problem. logical sciences, that refers to the act of sys- ANALOGOUS FEATURES: Morpho- tematically arranging ideas or objects into logical characteristics of two or more taxa categories according to specific criteria. A that are superficially similar but not as a more specific term is taxonomy. result of any common evolutionary origin. EUKARYOTE: A cell that has a nucleus For example, birds, bats, and butterflies all as well as organelles (sections of the cell have wings, but this is not because they are that perform specific functions) bound by closely related. Compare with homologous membranes. features. FAMILY: The third most specific of the BINOMIAL NOMENCLATURE: A seven obligatory ranks in taxonomy, after system of nomenclature in biological tax- order but before genus. onomy whereby each type of plant or ani- FUNGI: One of the five kingdoms of mal is given a two-word name, with the living things, consisting of multicellular first name identifying the genus and the eukaryotic cells arranged in a filamentous second the species. Genus name is always form (that is, a long, thin series of cells capitalized and abbreviated after the first attached either to one another or to a long, use, and species name is lowercased. Both thin cylindrical cell.) Fungi include “true” are always shown in italics; thus, Homo fungi, molds, mushrooms, yeasts, mildews, sapiens and, later in the same document, H. and smuts (a type of fungus that afflicts sapiens. certain plants). BIOME: A large ecosystem (community GENUS: of interdependent organisms and their The second most specific of inorganic environment) characterized by the obligatory ranks in taxonomy, after its dominant life-forms. There are two family but before species. basic varieties of biome: terrestrial, or HOMOLOGOUS FEATURES: Mor- land-based, and aquatic. phological characteristics of two or more CLADISTICS: A system of taxonomy taxa that indicate a common evolutionary that distinguishes taxonomic groups or origin, even though the organisms may dif- entities (i.e., taxa) on the basis of shared fer in terms of other morphological fea- derived characteristics, hypothesizing evo- tures. An example is the pentadactyl limb, lutionary relationships to arrange these in common to many birds and most mam- a treelike, branching hierarchy. Cladistics is mals (e.g., the human’s four fingers and one of several competing approaches to thumb), which indicates a common ances- taxonomic study. tor. Compare with analogous features. CLASS: The third most general obliga- KINGDOM: The highest or most gener- tory of the taxonomic classification ranks, al ranking in the obligatory taxonomic sys- after phylum but before order. tem. In the system used in this book, there
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 201 set_vol3_7 8/19/02 11:12 AM Page 202
Taxonomy KEY TERMS CONTINUED
are five kingdoms: Monera, Protista, Fungi, PHOTOSYNTHESIS: The biological Plantae, and Animalia. conversion of light energy (that is, electro- magnetic energy) from the Sun to chemical MONERA: One of the five kingdoms of energy in plants. In this process carbon living things, consisting of single-cell dioxide and water are converted to sugars. prokaryotes, including bacteria, blue-green algae, and spirochetes (spiral-shaped PHYLOGENY: The evolutionary histo- undulating bacteria that may cause such ry of organisms, particularly as that histo- diseases as syphilis). ry refers to the relationships between life- forms, and the broad lines of descent that MORPHOLOGY: Structure or form, or unite them. the study thereof. PHYLUM: The second most general of NOMENCLATURE: The act or process the obligatory taxonomic classification of naming, or a system of names—particu- ranks, after kingdom and before class. larly one used in a specific science or disci- PROKARYOTE: A cell without a nucle- pline. See also binomial nomenclature. us. NUMERICAL TAXONOMY: An PROTISTA (OR PROTOCTISTA): approach to taxonomy in which specific One of the five kingdoms of living things, morphological characteristics of an organ- consisting of single-cell eukaryotes. Pro- ism are measured and assigned numerical tista include protozoans, slime molds value, so that similarities between two types (which resemble fungi), and algae other of organism can be compared mathemati- than the blue-green variety. cally by means of an algorithm. Numerical SPECIES: The most specific of the taxonomy also is called phenetics. seven obligatory ranks in taxonomy. OBLIGATORY TAXONOMY (OR SYSTEMATICS: The science of classi- OBLIGATORY HIERARCHY): The fying and studying organisms with regard seven taxonomic ranks by which all species to their natural relationships. must be identified, whether or not they TAXON: A taxonomic group or entity. also are identified according to nonobliga- TAXONOMY: tory categories, such as subphylum, cohort, The area of the biological or tribe. These ranks are kingdom, phylum, sciences devoted to the identification, class, order, family, genus, and species. nomenclature, and classification of organ- isms according to apparent common char- ORDER: The middle of the seven oblig- acteristics. The word taxonomy also can be atory ranks in taxonomy, more specific used more generally to refer to the study of than class but more general than family. classification or to methods of classifica- PHENETICS: Another name for tion (e.g.,“the taxonomy of Dickens’s char- numerical taxonomy. acters.”)
202 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 203
Simpson, George Gaylord. Principles of Animal Taxono- Tudge, Colin. The Variety of Life: A Survey and a Celebra- Taxonomy my. New York: Columbia University Press, 1961. tion of All the Creatures That Have Ever Lived. Lon- don: Oxford University Press, 2000. Taxonomy Browser, National Center for Biotechnology Whyman, Kathryn. The Animal Kingdom: A Guide to Ver- Information, National Library of Medicine, National tebrate Classification and Biodiversity. Austin, TX: Institutes of Health (Web site). Raintree Steck-Vaughn, 1999.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 203 set_vol3_7 8/19/02 11:12 AM Page 204
SPECIESSpecies
CONCEPT helpful to have at least some knowledge of the One of the challenges that faces a student of the larger subject. At one time taxonomists were con- biological sciences is the seemingly endless array cerned most with the morphological characteris- of unfamiliar terms that one must learn. It is a tics (i.e., the structure or form) of organisms as a relief to come across a relatively familiar one, such basis for classifying many species within a larger as species. Although it has a scientific sound to it, grouping. Today, however, shared evolutionary the word has entered everyday language, such that lineage is much more important than morpho- when people use it, most everyone understands logical features in determining whether taxa what is meant. Or do they? As it turns out, there (plural of taxon, meaning a taxonomic group or is no hard and fast definition for the word. entity) can be classed together. Organisms may be Nonetheless, it is easy enough to find examples of linked closely in terms of evolutionary lines of species, since there are many millions of them in descent but differ in a particular morphological five kingdoms of living things—a product of a aspect as a result of the adaptive changes that phenomenon know as speciation, whereby evolu- accompany natural selection. The latter, a key tionary lines of descent diverge and new species concept in the theory of evolution put forward by are created. In the world today, there are many the English naturalist Charles Darwin interesting groups of species, distinguished nei- (1809–1882), is a process whereby some organ- ther by evolutionary line nor by taxonomy but isms thrive and others perish, depending on their instead by the ways in which they interact with degree of adaptation to a particular environment. their environments. Among these groups are It is therefore possible for organisms in a endangered species, of whose existence most peo- particular environment to develop a common ple are aware, owing to the spread of the environ- adaptive mechanism through generations of nat- mentalist message through media and entertain- ural selection, even though those organisms ment outlets since the early 1970s. Less familiar is themselves are not related to fish closely in terms another broad group that in many cases threaten of evolutionary line of descent. Thus, whales and humans: introduced or invasive species. dolphins, mammals that live underwater, evolved the ability to swim just as well as fish, but that HOW IT WORKS does not mean they are connected closely. Con- versely, organisms may be close, or relatively Taxonomy in Brief close, in terms of evolutionary lines of descent yet differ in significant morphological features. The concept of species falls under the heading of To use the whale and dolphin example again, taxonomy, the area of the biological sciences these creatures are classified as mammals owing devoted to the identification, naming, and classi- to certain particulars (discussed later), but they fication of living things according to apparent differ from the vast majority of mammals in that common characteristics. Taxonomy is discussed they have no legs. They do, however, have four in detail within the essay on that subject, but to appendages, just like the rest of the mammalian appreciate the topic of species in context, it is class; as a result of natural selection, however,
204 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 205
theirs ceased to operate as legs (an encumbrance specifics play no significant role in categorizing Species for life in the water) a long time ago, and today them within any of the more specific groupings they function instead as fins. to which they belong. Furthermore, the generic definitions of the categories—for example, class Obligatory Ranks as opposed to class Mammalia, class Insecta, or some other class in the taxonomic system—are The classification system used today is an out- purely relative. In other words, class is simply the growth of a system developed by the Swedish obligatory rank that is more specific than phy- botanist Carolus Linnaeus (1707–1778) in the lum but more general than order. 1730s. The realms of zoology and botany, areas of biology devoted to the study of animal and plant DESIGNATING A SINGLE SPECIES life, respectively, differ somewhat with regard to WITHIN THE RANKS. When preparing an their classification systems, but both use interna- outline for a paper, students are taught that no tional codes of nomenclature with roots in the topic should have only one subheading; instead, Linnaean system. There are many possible ranks that solitary subheading should be moved up one of classification, but only seven are obligatory, level. Such rules do not apply in taxonomy, and it meaning that all species must be assigned a place is not necessary that there be more than one sub- in these groupings. The obligatory ranks are list- group within a larger group. For example, there ed here. The entire list of rankings, including ver- might be only one class in a phylum. Taxono- sions of obligatory ranks with such prefixes as mists use detailed definitions to single out partic- sub-, super-, and infra-, as well as such addition- ular groups, such as class Mammalia. The follow- al ranks as cohort or tribe, are given in Taxono- ing list shows the placement of humans within my. Note the difference between the zoological the larger taxonomic universe, along with brief and botanical names for the second rank. explanations of a few (though far from all) char- acteristics that define each group. Obligatory Taxonomic Ranks • Kingdom Animalia: Multicell eukaryotic • Kingdom (that is, possessing cells with a nucleus and • Phylum (Division in botany) specialized compartments called organelles) • Class organisms that obtain their nutrition solely •Order by feeding on other organisms. (Other • Family defining characteristics of Animalia are dis- •Genus cussed in Taxonomy.) • Species • Phylum Chordata: Animals whose bodies, at As discussed in Taxonomy, this book uses a least at some point in their life cycles, system of five kingdoms, whose characteristics include a rudimentary internal skeleton are defined in that essay. Even at the level of with a stiff supporting rod known as a noto- kingdom, not everything is delineated precisely chord. All chordates at some point also (see the discussion in Taxonomy), and there are breathe through gills (in the case of a significant areas of dispute. For example, some human, while still in the womb). Other taxonomic systems include viruses. Because characteristics set apart chordates, includ- viruses are not cellular in structure and are not ing a tail or the remnants of one. Humans universally regarded as true organisms, however, belong to the subphylum Vertebrata, or they are not included in the five-kingdom system chordates with a spinal column. used here. • Class Mammalia: Vertebrates that feed their Below the level of kingdom, definitions young from special milk-secreting glands, become even more difficult. Organisms are known as mammae, located on the mother’s grouped into phyla on the basis of body plan or body. Mammals have other distinguishing organization, but there is no regular pattern for characteristics, such as a hinged lower jaw grouping within the smaller categories. For attached to the skull. example (as noted later herein), humans are • Order Primates: A group of mammals placed within their particular phylum and sub- whose characteristics may include some phylum on the basis of their spinal columns and version of an opposable digit (e.g., the overall internal bone structure, but those human thumb) and other features that,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 205 set_vol3_7 8/19/02 11:12 AM Page 206
Species while they are prevalent among most pri- proper full name of our species is Homo sapiens mates, are not universal to them. Not every sapiens Linneaus, 1758. one of these traits is exclusive to primates, a group that includes prosimians (e.g., The Mystery of Species lemurs), monkeys, apes, and humans. If one studies the delineation of humans’ place • Family Hominidae: Primates noted for their in the overall taxonomic structure, one may erect posture, large brains, rounded skulls, notice that for several groupings, the defining small teeth, bipedal locomotion (i.e., they characteristics are a bit “fuzzy around the edges.” walk on two legs), and tendency to use lan- This is true even of the animal kingdom, as guage for communication. Humans are the noted in Taxonomy: mobility and locomotion, only surviving species in the family, but seemingly so integral to the definition of animal, extinct hominids include Homo habilis are not prevalent among all animal species. (about 1.6 million years ago) and H. erectus Given the many gray areas and areas of dispute (about two million years ago) as well as the in the larger taxonomic categories, it should more distant Australopithecus (about eight come as no surprise that the smallest of the million years ago). obligatory rankings, that of species, lacks a pre- • Genus Homo: Hominids with especially cise definition. large skulls as well as the features that char- acterize family Hominidae. Members of this The most widely accepted definition of genus, which included H. erectus and H. species is the one put forward by the German- habilis as well as H. sapiens, also are known born American evolutionary biologist Ernst for their ability to fashion precise tools. Mayr (1904–) in the 1940s. Mayr’s idea, known • Species Homo sapiens: Members of the as the biological species concept, defines a species genus Homo (“man”) noted for, among as a population of individual organisms capable other things, the ability to use symbols and of mating with one another and producing fertile writing. This category includes modern offspring in a natural setting. Members of two humans and the extinct Cro-Magnon and different, but closely related species in some cases Neanderthal man. can mate with one another to produce infertile offspring, the most well-known example being Note that the proper name of any ranking the mule, a sterile hybrid produced by the union more general than species is capitalized (e.g., of a male donkey and a female horse. phylum Chordata), with species (and subspecies) names in lowercase. Genus, species, and sub- The definition offered by the biological species names are rendered in italics (e.g., Homo species concept requires qualification. While sapiens, or “man the wise”), whereas proper many plants and animals reproduce sexually, names of the more general groupings are pre- many more do not; no single-cell life-forms sented in ordinary type (e.g., class Mammalia). If reproduce in this way, yet there are certainly the same name appears a second time in the same many different and distinct species of bacteria article, the genus name usually is abbreviated: and protozoa. Thus, a further qualification typi- thus, H. sapiens. Another important abbreviation cally is added to the definition of species: mem- is spp., implying several species within a genus— bers of the same species share a gene pool, or a for example, Quercus spp. refers to more than total sum of genes. Genes carry information one species of oak. about heritable traits, which are passed from par- Taxonomy makes use of a system called ent to offspring. Whereas the gene pool is shared binomial nomenclature, in which each species is by members of a species, nonmembers of that identified by a two-word name, designating species have genes that do not belong to that gene genus and species proper. Beyond the species pool. To use a rudimentary example, let’s say that name, there may be subspecies names: humans there is a gene pool containing genes x, y, and z. are subspecies sapiens, so our full species name Individuals that have these genes fit within the with subspecies is Homo sapiens sapiens. Addi- gene pool, but an individual with gene w does tional rules govern the inclusion of a name or an not. abbreviation at the end of the species or sub- The definition of species remains challeng- species name, to recognize the person who first ing, with special problems raised in the area of identified it—in this case, Linnaeus. Hence the botany. It is also sometimes possible to confuse
206 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 207
Species
INUIT VILLAGERS BUTCHER A WHALE. AMONG ENDANGERED SPECIES IS THE RIGHT WHALE, SO CALLED BECAUSE WHALERS OF THE NINETEENTH CENTURY CONSIDERED IT THE “RIGHT” WHALE TO HUNT: IT SWIMS SLOWLY AND CLOSE TO SHORE AND CAN BE FOUND AND SLAUGHTERED EASILY. (© Lowell Georgia/Corbis. Reproduced by permission.)
species and race, a grouping that applies not only The number of endangered species world- in the world of humans but also that of other wide is not known. In the United States—a coun- animal and even plant species. Race is different try that, unlike most, expends considerable effort from species inasmuch as races are not isolated on keeping track of its endangered species— genetically from one another; in other words, there were more than 750 species and subspecies there are no biological barriers to interbreeding listed by the late 1990s as endangered under the between races. (See Speciation for a discussion of federal Endangered Species Act. Additional the process whereby single species develop over endangered species are being added at a rate of about 50 per year, and there is a “waiting list” of time into more than one reproductively isolated an estimated 3,500 candidate species. species.) Efforts at monitoring endangered species in the United States have directed a disproportion- REAL-LIFE ate amount of attention toward larger organisms; APPLICATIONS consequently, smaller endangered species from such groups as arthropods, mosses, and lichens Endangered Species have received insufficient attention. The regions of the United States with the largest numbers of An endangered species is any plant, animal, or endangered species are in the humid southeast microorganism that is at risk of becoming extinct and the arid southwest. These areas tend to have or at least of disappearing from a particular local the unfortunate combination of unique ecologi- habitat. Over the course of Earth’s geological his- cal communities alongside runaway urbaniza- tory, species have become extinct naturally— tion and resource development. sometimes in large proportions, as discussed in Overdevelopment and destruction of habi- the context of mass extinction in Paleontology. In tats is perhaps the most well-known ways that modern times, however, species and their natural humans endanger the survival of species. For communities are threatened mostly by human example, the habitat of the northern spotted owl activities. is under threat from loggers in the Pacific North-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 207 set_vol3_7 8/19/02 11:12 AM Page 208
Species west (see Succession and Climax). Another threat As for the blue whale, it occurs virtually is the introduction of new species, particularly worldwide, and with a typical weight of 150 tons predators, to an area that is not their natural (136 tonnes) and a length of 100 ft. (30 m), it is habitat—a topic we discuss in more depth later the largest animal ever to have lived on Earth. in this essay. Because it is such a fast swimmer, it could not be HUNTING THE ESKIMO hunted effectively by whalers in sailing ships. Once steam-powered ships were invented, how- CURLEW. Another way humans threaten ever, these whales were taken in tremendous species is by excessive hunting. An example of a numbers and became endangered. Because of its species thus threatened is the Eskimo curlew precarious status, this species has not been hunt- (Numenius borealis), a sandpiper (a type of bird) ed for several decades, but it remains rare and that was still abundant in North America during endangered. the nineteenth century. A large, friendly creature, it was hunted in vast numbers during its season- The Fate of the Dodo al migrations over the prairies and coasts of Canada and the United States and during its win- When a species becomes extinct, it is gone forev- ter seasons in South America. (See Migration and er. It is like a family whose last member has died Navigation for more about birds’ winter migra- without leaving an heir, but in this case the tions.) The Eskimo curlew became very rare by impact is potentially much more profound. Sev- the end of the nineteenth century, and the last eral thousand species have become extinct as the time an Eskimo curlew nest was seen (1866), the result of human activities, mostly hunting, in the past few hundred years, and of these species per- guns of the Civil War were practically still smok- haps none is more well known than Raphus ing. The last time a scientific team collected an cucullatus, or the dodo. Eskimo curlew specimen was in 1922. It might seem that the bird is extinct, but this is not the Long before the application of the term clue- case. Although it is extremely rare, there have less in the 1990s, a person out of touch or out of been a few reliable sightings of individuals and step was called a dodo. How did the bird’s name small flocks of this species, mostly during migra- come to be a synonym for stupidity? Perhaps it is tion in Texas and elsewhere but also in its breed- just the funny sound of the name, or perhaps it is ing habitat in the Canadian Arctic. Once abun- the fact that the dodo looked a bit like a turkey, dant, the Eskimo curlew now hangs on by a another bird name used for someone of less than exemplary capabilities. Or perhaps the applica- thread. tion of the name dodo in this way carries a hint of RIGHT WHALES AND BLUE blaming the victim—the implication that the WHALES. More familiar is the endanger- dodo somehow played a part in its own extinc- ment of whales, a cause made popular by many a tion. “Save the Whales” bumper sticker. Among In fact, the dodo’s only shortcoming was its endangered animals of this group are the blue inability to overcome the threat posed by an whale (Balaenoptera musculus) and various extremely dangerous predator: the human. A species from the genus Balaena, or right whale. member of the dove or pigeon family, the dodo The latter species gained its common name was flightless and lacked natural enemies until because whalers of the nineteenth century con- humans discovered its homeland, the Indian sidered it the “right” whale to hunt: it swims Ocean island of Mauritius, in the early sixteenth slowly and close to shore and so could be found century. First came the Portuguese and then, in and slaughtered easily. In addition, it yields a 1598, the Dutch, who made the island a colony in large amount of oil, used for lighting lamps in 1644. By 1681 the dodo had ceased to exist. Not the era when Herman Melville’s Moby Dick only did sailors collect the birds for food, but (1851) was written. The estimated world popula- introduced species, including dogs, cats, pigs, tion of right whales is currently about 2,000 indi- monkeys, and rats, also preyed on dodos. They viduals, much depleted from the historical high were subjected to regular slaughter by sailors, but numbers; though it is now protected from whal- the species managed to breed and survive on the ing, it suffers an excessive mortality rate from remote areas of the island for a time. After the ship collisions. establishment of their colony, however, Dutch
208 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 209
Species
A MEMBER OF THE DOVE OR PIGEON FAMILY, THE DODO WAS FLIGHTLESS AND LACKED NATURAL ENEMIES UNTIL HUMANS DISCOVERED ITS HOMELAND, THE ISLAND OF MAURITIUS, IN THE EARLY SIXTEENTH CENTURY. BY1681, THROUGH THE EFFECTS OF PREDATION, THE DODO HAD CEASED TO EXIST. (© Bettmann/Corbis. Reproduced by permission.)
settlers launched what amounted to an extermi- have been introduced to a region or continent, nation campaign. usually but not always through human activity. No part of Earth’s living environment can be In the case of species introduced by humans, removed without repercussions, and the destruc- some were introduced deliberately and were tion of the dodo illustrates the ripple effect that intended to improve conditions for some human occurs when one species is eliminated. As it activity (for example, in agriculture) or to turned out, the bird had a symbiotic relationship achieve desired aesthetic results—for example, (see Symbiosis) with the dodo tree, or Calvaria when colonists wanted to plant a flower or tree major, whose fruit it ate, thus releasing the seeds that reminded them of home. Other introduc- to germinate. With the dodo gone, the dodo tree tions have been accidental, as when plants were stopped being able to reproduce. Fortunately, it is brought with soil transported as ballast in ships a species with a long life, and some specimens of or insects were conveyed with timber or food. C. major continue to survive after some 300 BENEFICIAL AND HARMFUL years; when those die, however, this species, too, INTRODUCTIONS. Some introduced will be extinct. species have been wildly successful. In fact, most agricultural plants and animals are introduced Exotic, Introduced, and Inva- species: for example, wheat (Triticum aestivum) sive Species was originally native only to a small region of the Middle East, but it now grows virtually anywhere An introduced species is one that has been spread conditions are suitable for its cultivation. Like- to a new environment or habitat as a result of wise, corn, or maize (Zea mays), has spread far human activity. An invasive species may or may beyond its home in Central America. The domes- not have been spread by humans (the ones we tic cow (Bos taurus) once lived only in Eurasia discuss were), but as the name suggests, it threat- and the turkey (Meleagris gallopavo) only in ens an aspect of the habitat to which it has been North America, but today these species can be introduced. Both introduced and invasive species found throughout the world. If all introduced are examples of exotic species, or species that species were like cows and corn, or turkeys and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 209 set_vol3_7 8/19/02 11:12 AM Page 210
Species wheat, there would not be much cause for alarm. the gypsy moth (Lymantria dispar), which has Many introduced species are invasive, however, defoliated many trees since its introduction to and pose a wide variety of threats—threats to North America from Europe in 1869. their environments or, in some cases, to human Similarly, the introduced elm bark beetle well-being. All manner of weeds and pests are (Scolytus multistriatus) has helped spread Dutch among the nefarious roll-call of invasive species, elm disease, itself caused by an introduced fun- a broad grouping that ranges from nuisances to gus, Ceratocystis ulmi. It would be interesting to serious dangers. note the irony inherent in this affliction, which at ACCIDENTAL AND DELIBER- first glance seems to involve another apparently ATE INTRODUCTIONS. There are more introduced species, the “Dutch elm.” There is no than 30,000 introduced species in the United such tree, however; the name refers to the fact States, and most of them enhance rather than that the disease arrived in America from Hol- diminish the quality of life. For example, there land, probably some time after World War I. Its are the many species introduced by colonists to principal victim is the American elm, or Ulmus make them feel more comfortable in their new americana. homes, among them, the Norway maple (Acer DELIBERATE AND HARMFUL platanoides), linden (Tilia cordata), horse chest- INTRODUCTIONS. Not all harmful intro- nut (Aesculus hippocastanum), and other trees as duced species were introduced accidentally. Set- well as many exotic species of shrubs and herba- tlers from Europe deliberately brought pets, such ceous plants. The European settlers also intro- as the domestic dog (Canis familiaris) and cat duced some species of birds and other animals (Felis catus); while these pets may add greatly to with which they were familiar, such as the starling the quality of human life, they can cause prob- (Sturnus vulgaris), house sparrow (Passer domes- lems, because they are wide-feeding predators. ticus), and pigeon, or rock dove (Columba livia). Such creatures threaten vulnerable animals in These are all deliberate introductions; on the many places, especially isolated oceanic islands. other hand, accidental introductions are more Among other predators are mongooses (family likely to be undesirable. When cargo ships from Viverridae), often introduced to get rid of snakes, Europe did not have a full load of goods, they as well as omnivores, such as pigs (Sus scrofa) and had to carry other heavy material as ballast, to rats (Rattus spp.) Meat-eating animals are not help the vessel maintain its stability on the ocean. the only threat: herbivores such as sheep (Ovis Early ships to the New World often used soil as aries) and goats (Capra hircus) also endanger ballast, and upon arriving, sailors dumped this plant life in some areas as a result of overgrazing. soil near the port. In this way, many European A particularly striking example of harmful, weeds and other soil-dwelling organisms arrived deliberate species introduction is the Nile perch in the Americas. In addition, ships have used (Lates niloticus). First introduced to Africa’s Lake water as ballast since the late nineteenth century, Victoria in the 1950s, it has proved an economi- and many aquatic species have become widely cally important food source, with a large world- distributed by this practice. This is how two wide market. The problem is that the Nile perch major pests, the zebra mussel (Dreissena poly- is an extraordinarily active predator and has morpha, discussed later) and the spiny water flea brought about a tragic mass extinction of native (Bythothrepes cederstroemii) were introduced to fishes. Until the 1980s, Lake Victoria supported the Great Lakes from European waters. an extremely diverse community of more than Several European weeds are toxic to cattle if 400 species of fish, mostly cichlids (family Cich- eaten in large quantities, and when these plants lidae), with 90% of those species being endemic, become abundant in pastures, they represent a meaning that they exist only in one area. About significant potential problem. Some examples of one-half of the endemic species are now extinct toxic introduced weeds in the pastures of North in Lake Victoria because of predation by the Nile America include common Saint-John’s-wort perch, although some species survive in captivity, (Hypericum perforatum), ragwort (Senecio and a few are still in the lake. jacobaea), and common milkweed (Asclepias syr- KILLER BEES, ZEBRA MUS- iaca). Several introduced insects have become SELS, AND KUDZU. Three notable troublesome pests in forests, as is the case with examples of invasive species in America are
210 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 211
Species
THE AFRICANIZED, OR “KILLER,” BEE IS ONE EXAMPLE OF AN INVASIVE SPECIES. THESE BEES HAVE NO MORE VENOM THAN DOMESTICATED HONEYBEES, BUT THEY ATTACK MORE QUICKLY AND IN GREAT NUMBERS AND HAVE SPREAD THROUGHOUT MUCH OF SOUTH AND CENTRAL AMERICA, TEXAS, AND CALIFORNIA. (© S. Camazine/Photo Researchers. Repro- duced by permission.)
Africanized honeybees (Apis mellifera scutellata), cate native clams, and destroy the breeding habi- better known as “killer bees”; the zebra mussel tats of other aquatic animals. These invaders have (Dreissena polymorpha); and kudzu (Pueraria placed a great burden not only on the environ- lobata). The first “killer” bees were released acci- ment but also on the economy of the Great Lakes dentally by a Brazilian bee breeder in 1957. These region: area industries spend hundreds of mil- aggressive insects have no more venom than lions of dollars annually to unclog pipes and domesticated honeybees (another A. mellifera equipment. subspecies, which is also an Old World import), Kudzu is an integral part of culture in the but they attack more quickly and in great num- southern United States, but it originated in Japan bers. Interbreeding with resident bees and some- and did not arrive on American shores until times traveling with cargo shipments, African- 1876. In that year, numerous foreign govern- ized bees have spread at a rate of up to 200 mi. ments sent exhibits to the Centennial Exposition, (320 km) a year and now threaten humans, fruit held in Philadelphia to honor the country’s 100th orchards, and domestic bees throughout much of birthday. Two generations later, during the Great South and Central America and north to Texas Depression, the U.S. Soil Conservation Service and California. began promoting the use of kudzu for erosion The zebra mussel was introduced to the control. Great Lakes in about 1985 in ballast water At a time when work was scarce, young men dumped by a ship or ships arriving from Europe. in the government-sponsored Civilian Conserva- It colonizes any hard surface, including rocks, tion Corps (CCC) earned a living by planting wharves, industrial water-intake pipes, and the kudzu throughout the South. The federal gov- shells of native bivalve mollusks. A bean-sized ernment paid farmers as much as $8.00 an acre— female zebra mussel can produce 50,000 larvae a fabulous sum at the time—to plant kudzu (an immature form of an animal) in a single year. fields. Before another generation had passed, in Growing in masses with up to 70,000 individuals 1953, the federal government stopped promoting per square foot, zebra mussels clog pipes, suffo- the use of kudzu. In 1972, just four years shy of a
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 211 set_vol3_7 8/19/02 11:12 AM Page 212
Species KEY TERMS
BINOMIAL NOMENCLATURE: A EXOTIC SPECIES: Species that have system of nomenclature in biological tax- been introduced to a region or continent, onomy whereby each type of plant or ani- usually but not always through human mal is given a two-word name, with the activity. See also introduced species and first name identifying the genus and the invasive species. second the species. The genus name is EXTINCTION: A condition in which all always capitalized and abbreviated after the members of a taxon have ceased to exist. first use, and the species name is lower- FAMILY: The third most specific of the cased. Both are always shown in italics— seven obligatory ranks in taxonomy, after thus, Homo sapiens and, later in the same order but before genus. document, H. sapiens. GENE: A unit of information about a CLASS: The third most general of the particular heritable (capable of being obligatory taxonomic classification ranks, inherited) trait that is passed from parent after phylum but before order. to offspring, stored in DNA molecules called chromosomes. DNA: Deoxyribonucleic acid, a mole- cule in all cells, and many viruses, contain- GENE POOL: The sum of all the genes ing genetic codes for inheritance. shared by a population, such as that of a species. ENDANGERED SPECIES: Any GENUS: plant, animal, or microorganism that is at The second most specific of the obligatory ranks in taxonomy, after risk of becoming extinct or at least of dis- family but before species. appearing from a particular local habitat. HYBRID: The product of sexual union ENDEMIC SPECIES: Species that between members of two species or other exist in only one geographic region. smaller and less genetically separate EUKARYOTE: A cell that has a nucleus groups, such as two races. In the case of as well as organelles (sections of the cell species hybrids, the process of hybridiza- that perform specific functions) bound by tion involves genetic abnormalities that membranes. lead in most cases to sterility.
century after its first introduction, kudzu was well as the cover of Murmur, the music group officially declared a weed by the U.S. Department R.E.M.’s 1983 debut, which features a photo- of Agriculture. graph of a kudzu-covered railroad trestle near Obviously, something had gone wrong. The the group’s hometown of Athens. problem was that kudzu grew too well—so fast, Kudzu covered more than railroad tracks, in fact, that in the minds of many southerners, it and in the mid–twentieth century, it began to began to possess some sort of mystical signifi- seem as though it would cover the entire South cance. This preoccupation with kudzu is reflect- with its tangled vines. The plant is capable of ed in the work of several Georgians, whose state growing by as much as 1 ft. (0.3 m) per day dur- has been particularly afflicted with the vine. ing the summer and can cover virtually anything There is the poem “Kudzu” by James Dickey as that is not moving. Over the course of a good
212 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 213
Species KEY TERMS CONTINUED
INTRODUCED SPECIES: A species PHYLUM: The second most general of that has been spread to a new environment the obligatory taxonomic classification or habitat, whether deliberately or acciden- ranks, after kingdom and before class. tally, as a result of human activity. Intro- PROKARYOTE: A cell without a nucle- duced species, like invasive species, are us. considered exotic species. RNA: Ribonucleic acid, the molecule INVASIVE SPECIES: An exotic translated from DNA in the cell nucleus, species that threatens some aspect of the the control center of the cell, that directs habitat to which it has been introduced. protein synthesis in the cytoplasm, or the KINGDOM: The highest or most gener- space between cells. al ranking in the obligatory taxonomic sys- SPECIATION: The divergence of evo- tem. In the system used in this book there are five kingdoms: Monera, Protista, Fungi, lutionary lineages and creation of new Plantae, and Animalia. species. SPECIES: MORPHOLOGY: Structure or form, or The most specific of the the study thereof. seven obligatory ranks in taxonomy. Species often are defined as a population of NATURAL SELECTION: The process individual organisms capable of mating whereby some organisms thrive and others perish, depending on their degree of adap- with one another and producing fertile off- tation to a particular environment. spring in a natural setting. Also, members of the same species share a gene pool. NOMENCLATURE: The act or process of naming or a system of names—particu- TAXON: A taxonomic group or entity. larly one used in a specific science or disci- TAXONOMY: The area of the biological pline. See also binomial nomenclature. sciences devoted to the identification, ORDER: The middle of the seven oblig- nomenclature, and classification of organ- atory ranks in taxonomy, more specific isms according to apparent common char- than class but more general than family. acteristics.
year, kudzu can grow by as much as 60 ft. (20 m), root, called kuru, has long been used in foods and and it has proved impervious to many herbicides. medications in China and Japan. One herbicide used in Auburn, Alabama, actually One might wonder why Japan is not covered made it grow better! Thanks to the development in kudzu and why kudzu is not crawling up the of better chemical treatments, and the use of Great Wall of China. The answer is more than a lit- tle interesting from a biological standpoint. When grazing animals, such as goats, kudzu no longer is kudzu was transplanted to America, it was taken perceived as such a great threat. Additionally, var- out of its native environment and thus away from ious entrepreneurs and scientists have set out to the local insects that threatened its growth. In its make use of the vine in weaving baskets or in new home there were no threats to its spread, and preparing foods and medicines. Ground kudzu with no obstacles in its way, it began to take over
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 213 set_vol3_7 8/19/02 11:12 AM Page 214
Species the South. (For more about the development of Levy, Charles K. Evolutionary Wars: A Three-Billion-Year species, see Speciation. See also the discussion of Arms Race—The Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999. keystone and indicator species in Food Webs.) Schilthuizen, Menno. Frogs, Flies, and Dandelions: Speci- WHERE TO LEARN MORE ation—The Evolution of New Species. New York: Oxford University Press, 2001. All Species Foundation (Web site).
Integrated Taxonomic Information System (ITIS), United Van Driesche, Jason, and Roy Van Driesche. Nature out of States Department of Agriculture (Web site). Place: Biological Invasions in the Global Age. Washing-
214 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 215
Speciation SPECIATION
CONCEPT biology students. These seven, known as the One of the defining characteristics of a species is obligatory hierarchy, are kingdom, phylum, class, its reproductive isolation: the fact that among order, family, genus, and species. In the case of animals and plants that reproduce sexually, it is mammals, it is also useful to refer to subphylum, impossible for members of two different species which in this case is Vertebrata (see the classifica- to mate and produce fertile offspring. Speciation tion of humans in Species), but for the most part is the process whereby a single species develops it is enough for the beginning student to attain at over time into two distinct, reproductively isolat- least some mastery of the obligatory ranks. ed species. It is one of the key evolutionary Note that species is the most specific of these processes and is responsible for the diversity of ranks, which is fitting, because species and specif- life that exists on Earth. In the following essay we ic come from the same Latin root, specie, or explore not only the basic facts of speciation and “kind.” Nonetheless, it is difficult to define biological diversity but also an example of adap- species beyond a reference to its place among the tive radiation, in the form of the wide range of categories of the obligatory taxonomy. According species within the mammalian order. to the biological species concept, discussed briefly in Species, a species is any population of HOW IT WORKS individual organisms capable of mating with one another and producing fertile offspring in a nat- Species and Speciation ural setting. This is far from the only definition, however. The concept of species, discussed in the article INTERSPECIFIC MATING. Occa- devoted to that subject, is an extraordinarily sionally, it is possible to produce an infertile complex one. Owing to limitations of space, that hybrid, such as a mule, which is created by the essay only hints at the many details, the compet- mating of a male donkey and a female horse, or a ing schools of thought, and the varying defini- hinny, the product of the less common union tions of species. Likewise, in the present context, between a male horse and a female donkey. The it is possible to examine the concept of speciation infertility is due to genetic disorders that arise only in the most cursory fashion. In addition to when mating takes place between distinct consulting the essay on Species for more infor- species, and even this imperfect product is possi- mation, the reader is encouraged to review the ble only by mating two species that are very article on Taxonomy. closely related. Donkeys and horses, for instance, Taxonomy is the area of the biological sci- both belong to family Equidae, which makes ences devoted to the identification, nomencla- them very closely connected. ture, and classification of organisms according to In the taxonomic ranking of humans, this apparent common characteristics. It uses a wide would be equivalent to a human mating with a array of specialized rankings for grouping ani- fellow hominid, or member of family Hominidae. mals, but only seven of them are essential to most If the long-extinct genus Australopithecus were
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 215 set_vol3_7 8/19/02 11:12 AM Page 216
Speciation group, along with all other mammalian orders, later in this essay.) THE PROBLEM OF DEFINING SPECIES. Although the biological species concept is accepted widely, it has its shortcom- ings, not least of which is the fact that not all species reproduce sexually. Although sexual reproduction is the case with a wide array of ani- mals and even plants, quite a few organisms reproduce by some asexual means: for example, single-cell organisms reproduce by splitting. Among the competing definitions of species is the phenetic (or morphological) species con- cept, which relies in part on common sense. According to the phenetic species concept, a species is the smallest possible population of organisms that consistently and continually remains distinct and distinguishable by ordinary methods of observation. There are also a variety of definitions that fall under the heading “phylo- genetic species concepts,” all of which maintain in one way or another that taxonomic classifica-
A SPECIES IS A POPULATION OF INDIVIDUAL ORGANISMS tions should incorporate the most widely recog- CAPABLE OF MATING WITH ONE ANOTHER AND PRODUC- nized hypotheses regarding the evolutionary ING FERTILE OFFSPRING. OCCASIONALLY, IT IS POSSI- lines of descent that produced the organisms in BLE TO PRODUCE AN INFERTILE HYBRID, SUCH AS A MULE, THROUGH THE MATING OF A MALE DONKEY AND question. A FEMALE HORSE. (© D. Robert & Lori Franz/Corbis. Reproduced by permission.) The Process of Speciation
Clearly, there is no hard and fast definition of still around, it is not inconceivable that humans species, but in general terms, everyone who has could mate with them and produce at least sterile some familiarity with the concept has at least a offspring. Of course, it is unlikely that many basic knowledge of what does and does not qual- humans would want to mate with Australopithe- ify as a species. We will leave finer distinctions to cus, the most famous example of which was trained taxonomists and other biologists and named “Lucy” after the Beatles’ song “Lucy in the move on to a fact regarding which there is no dis- Sky with Diamonds.” Standing about 3.5-5 ft. agreement: a wide array of species exists in the (1–1.5 m) tall, Australopithecus was very close in world today. Some estimates calculate the num- appearance to a modern ape who lived about four ber of species in the five kingdoms—animals, million years ago. plants, monerans, protista, and fungi (see Taxon- All of humans’ close relatives are extinct, and omy for a very brief identification of each)—at today our nearest relatives are members of the about 1.5 million. order Primates: apes, monkeys, and marsupials. This is only the number of identified species, It is impossible to imagine a human mating with however. Other figures, based on the probable one of these animals and producing offspring of numbers of unidentified species in the world, put any kind. Likewise, it is extremely unlikely that a the sum total in the tens of millions. Whatever horse or donkey could mate with a tapir or rhi- the case, it is obvious that over the course of evo- noceros, which are about as distant in relation to lutionary history (discussed in Evolution and them as other primates are to us. (These species Paleontology), there has been a widespread adap- all belong to the order Perissodactyla, herbivo- tive radiation—that is, a diversification of species rous mammals possessing either one or three as a result of specialized adaptations by particu- hoofed toes on each hind foot. We discuss this lar populations of organisms.
216 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 217
Speciation events are described as either situation of competition for resources that Speciation allopatric or sympatric. Allopatric (“different favored stickleback species at either extreme of places”) speciation occurs when a population of size, as opposed to those of medium size and organisms is divided by a geographic barrier, a medium-sized mouths. great example being the division of squirrel RATE OF EVOLUTIONARY species caused by the formation of the Grand CHANGE. Closely tied to speciation is the Canyon (see Evolution). Another example is the rate of evolutionary change, or the speed at speciation of the black-throated green warbler, which new species arise. This is a long process, which today consists of one species in the eastern one that is usually not observable within a United States, along with three others in the human lifetime or even the span of many life- western part of the country. Some scientists spec- times, though bacteria at least have shown some ulate that there may once have been a single evolutionary change in their growing resistance species of black-throated green warbler, whose to antiobiotics (see Infection). DNA analysis (see population was split by the formation of a glacier Genetics and Genetic Engineering for more during the Pleistocene epoch. The latter was the about DNA) has been used to examine the rate of period of the last ice age, which ended about evolutionary change. To perform such analysis, it 10,000 years ago, but the end of the ice age was a is necessary first to determine the percentage of slow process. It may be that glaciers, formed in similarity between the organisms under study: the latter part of that time, helped to separate the greater the similarity, the more recently the what became three different western species. organisms probably diverged from a common Species share the same gene pool, or the sum stock. Data obtained in this manner then must be of all genetic codes possessed by members of that corroborated by information obtained from species. The isolation of two populations slowly other sources, such as the fossil record and com- results in differences between gene pools, until parative anatomy studies. the two populations are unable to interbreed At certain times the rate of evolutionary either because of changes in mating behavior or change can be very rapid, leaving little fossil evi- because of incompatibility of the DNA between the two populations. (Deoxyribonucleic acid, or dence of intermediate forms, a phenomenon DNA, contains genetic codes for inheritance. See known as punctuated equilibrium. This is con- Genetics for more on this subject.) More rare trasted with phyletic (that is, evolutionary) grad- than allopatric speciation, sympatric (“same ualism. Of course, the term rapid in this context place”) speciation happens when a group of indi- is relative, since we are talking about vast spans of viduals becomes reproductively isolated from the time. Life on Earth has existed for about 3,000 larger population of the original species. This million years, and the fossil record goes back type of speciation typically results from muta- some 1,000 million years. This is the case, in part, tion, or alterations in DNA that result in a genet- because to leave fossilized remains, an organism ic change. must have “hard parts” that can become mineral- ized to turn into fossils. (See Paleontology for Studies of three-spined sticklebacks, a vari- more on these subjects.) ety of freshwater fish, in British Columbia have revealed what appears to be a fascinating exam- ple of sympatric speciation. Evolutionary biolo- REAL-LIFE gist Dolph Schluter and others have discovered APPLICATIONS that the region contains two species of stickle- back, one with a large mouth that feeds on large The Diversity of Mammals prey close to shore, the other with a small mouth that feeds on plankton in open water. Both One of the most interesting examples of specia- species jointly inhabit five different lakes. tion is that which has produced the vast array of Through DNA analysis, scientists have deter- species, including humans, that fall within the mined that the lakes were colonized independ- mammalian class. Mammals began evolving ently by common marine ancestors, meaning before the dawn of the Cenozoic era about that the process of sympatric speciation between 65 million years ago. The Cenozoic era, which the two varieties had to have occurred independ- started with a catastrophic event that brought ently at least five times. This seems to indicate a about the mass extinction of the dinosaurs and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 217 set_vol3_7 8/19/02 11:12 AM Page 218
Speciation the end of the Mesozoic era (see Paleontology), is combined aspects of both reptiles and mammals truly the age of the mammal. Just as dinosaurs walked the earth. dominated the Mesozoic, today the world belongs to mammals as to no other class of creature. Mammalian Orders Since its humble beginnings in the shadow of The listing of the 20 orders of living mammals the dinosaurs, class Mammalia has undergone a that follows is arranged not alphabetically but in massive radiation to the point that today some the probable order in which these groups 4,625 species of mammal, in about 125 families evolved. (This is not to imply that the process and 24 orders, are recognized. (That number is was orderly or linear; it was not.) Very few dates changing, as noted later in the context of ele- are given, simply because there is much dispute phants.) This diversity is tied closely to mammals’ in most cases. Numbers of species within each enormous mobility, which facilitated their spread order are also a subject of debate among taxono- throughout the world. Aside from much less mists, and therefore these numbers are not complex life-forms, such as arachnids and insects always precise. (see Parasites and Parasitology), mammals are In the essay, Species, there is a taxonomic believed to be distributed more widely through- listing of the obligatory ranks for humans; out the world than any other comparable taxo- included within that listing is a short description nomic grouping. Insects may be the most diverse of the kingdom (Animalia), phylum (Chordata), of all animal classes, with numbers of species that and subphylum (Vertebrata) to which mammals may be many times greater than the number of belong. Mammal itself is defined as a vertebrate mammals, but considering mammals’ much- (an animal with a spinal column) that feeds its greater level of physical development and com- young from special milk-secreting glands, termed plexity, the diversity of their species is astounding. mammae, located on the mother’s body. Mam- MAMMALS’ EARLY EVOLU- mals are warm-blooded or endothermic, mean- TION. In the next section we list the orders of ing that their internal temperatures remain rela- mammals and give very brief descriptions of tively stable, and their bodies usually are covered each. The purpose here is not to provide any- with hair. They have other distinguishing charac- thing like a comprehensive discussion but rather teristics as well, such as a relatively large cranium to illustrate the enormous range of species in a (skull) with a hinged lower jaw attached to it. class that includes anteaters, dolphins, humans, MONOTREMES. Order Monotrema elephants, and bats. The fact that all these diverse consists of primitive, egg-laying mammals spread creatures, and many more, emerged from a com- throughout parts of the region known as Ocea- mon evolutionary lineage is almost as amazing as nia, which includes Australia, New Zealand, and the fact that this common ancestor was a reptile. islands of the southeastern Pacific. The habitat of Mammals are believed to have come from this order lies specifically in Australia, Tasmania, the reptilian order Therapsida, which emerged and New Guinea. Monotremes, as they are called, during the Triassic period (from about 245 to are distinguished further by the fact that their 208 million years ago) in the early part of the mammary glands are without nipples, that teeth Mesozoic era. Over the course of many millions are present only in the young, and that adults of years, these creatures began to develop a num- have horny beaks. ber of mammal-like qualities—in particular, The monotremes illustrate the fact that to be endothermy, or the ability to maintain internal constituted as an order or family, a taxon, or tax- temperature regardless of environmental condi- onomic group, need not have large numbers. The tions. In other words, these cold-blooded crea- entire order consists of a single existing species, tures became warm-blooded. This evolutionary the duck-billed platypus, which constitutes a process was as complex as it was lengthy. Nor was family of its own, and two species of echidnas, there a clean break with the past—no moment creatures that look like a cross between a platy- when the therapsids faded away or when it would pus and a porcupine. (The “porcupine” look have been clear that mammals had taken the comes from the fact that their bodies are covered place of their reptilian ancestors. Rather, in what in spines, or spiky protrusions.) must have been a fascinating taxonomic situa- MARSUPIALS. The marsupials, or tion, for many millions of years, species that order Marsupialia, include two other famous ani-
218 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 219
mal citizens of Oceania: the kangaroo and its Speciation close relative, the wallaby. Marsupials’ young are poorly developed at birth and must continue to grow while attached to their mothers’ nipples. For this reason, they must remain close to the mother, and therefore natural selection for mar- supials favored those strains in which females possess a pouch bearing four teats. Immediately after birth, the young marsupi- al (a kangaroo baby is called a joey) installs itself in the mother’s pouch. Given this situation, mar- supials can support only one offspring a year and thus are not given to the large litters that charac- terize another order, Carnivora, which we discuss later. Kangaroo offspring remain in the pouch until the age of about 7-10 months, by which time the mother has conceived again; the female kangaroo goes into heat just a few days after giv- ing birth. The embryonic kangaroo remains in a state of dormancy, or arrested development, until the older sibling has left the pouch. The marsupial order (some authorities call it a superorder, with numerous subordinate orders) MARSUPIALS’ YOUNG ARE POORLY DEVELOPED AT BIRTH consists of some 240 species. The greatest num- AND MUST CONTINUE TO GROW WHILE ATTACHED TO ber of these, including many species of kangaroo, THEIR MOTHERS’ NIPPLES. IMMEDIATELY AFTER BIRTH, wallaby, wombat, and koala, are found exclusive- THE YOUNG KANGAROO (CALLED A JOEY) INSTALLS ly in Australia. Some 70 additional species are ITSELF IN THE MOTHER’S POUCH AND REMAINS THERE UNTIL THE AGE OF 7-10 MONTHS. (© Michael S. scattered across parts of Oceania, including Aus- Yamashita/Corbis. Reproduced by permission.) tralia, Tasmania, New Guinea, and smaller islands. There are an additional 70 species in the Americas, including four species of the genus either lack teeth or have very small ones. Evolu- Didelphis—the large American opossum, better tionary development has adapted the forward known in the southern United States as possums. limbs of these creatures for digging or for hold- Why the preponderance of marsupials in ing on to the branches of trees. Included in this Australia and Oceania as a whole? The reason lies order are some 30 species of sloth, anteater, and in Earth’s geologic history, which has seen regu- armadillo. Sloths are herbivores, armadillos are lar collisions and divisions of the continents, omnivores (i.e., they eat plants and small ani- which are even today shifting slowly under our mals), and anteaters, as their name would sug- feet. It appears that prior to about 70 million gest, are hard-core insectivores. years ago, at a time when most of Earth was unit- The term insectivore can refer to any organ- ed in a single supercontinent called Pangaea, ism that lives by eating insects, but it is also the marsupials originated in what is now North America and migrated to the land masses that name for members of the order Insectivora, became Australia and Oceania. Because the bulk which includes shrews, hedgehogs, moles, and of marsupial species remained on Australia and various other, less well known groups. Some 400 nearby areas when Pangaea began to break apart, species, of which about 300 are shrews in a single marsupials underwent much greater speciation family (Soricidae), make up this order. Not only there than in North America. their diet but also their pointed snouts and XENARTHRANS, INSECTI- rodent-like appearance distinguishes this group. VORES, SCANDENTIA, AND DER- Many, but not all, are diggers, like the MOPTERA. Known variously as xenarthrans xenarthrans. Like all mammals, they have the and edentates, members of order Xenarthra pentadactyl limb (an appendage with five digits,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 219 set_vol3_7 8/19/02 11:12 AM Page 220
Speciation like the human arm and hand)—in their case, a from the southwestern United States to the foot with five toes. northwestern third of South America. As one Order Scandentia, which is identical with would suspect of a creature named vampire, they the family Tupaiidae, or tree shrews, is sometimes live off blood, which they suck at night from birds grouped with order Insectivora. Despite their or other mammals, including humans. These name, tree shrews, of which there are five genera creatures are livestock pests and strike fear in and between 15 and 19 species, may live either on humans both because of the imaginary associa- the ground or in the trees. Squirrel-like in tion with vampires and for the quite real threat of appearance, they have strong claws on all their contracting rabies from them. The vast majority toes and are excellent climbers. of bats, however, are creatures that cause no harm to humans and often are unfairly persecuted as Another very small order of tree-dwellers is the result of human prejudices. Even in the case Dermoptera, which consists of just two species of of the vampire bat, there is a strong possibility flying lemur. Found in Indonesia and the Philip- that it may one day help save human lives. Scien- pines, these creatures are equipped with skin tists have discovered that the saliva of Desmodus flaps adapted for gliding. This aspect of their morphological makeup calls to mind the “flying” rotundus is better than any other known sub- squirrel, but their nocturnal habits are more like stance for keeping blood from clotting; therefore, those of lemurs (discussed later, with other pri- vampire bat saliva may one day be adapted for mates); hence their common name. use in treating heart attacks and strokes. PRIMATES. CHIROPTERANS. Among the most The order of humans, Pri- fascinating of mammalian orders is Chiroptera, mates, falls approximately in the middle of the better known as bats. This order, which consists mammalian class in terms of evolutionary order. of about 900 species in some 175 genera, is the This is an interesting aspect of speciation, evolu- only group of truly flying mammals, as opposed tion, and taxonomy: even though humans them- to the “flying” lemurs we just discussed. Yet they, selves are the most advanced of all creatures, it is too, have the pentadactyl limb, only in their case not a logical necessity that we should come from the forelimb has been adapted as a wing. Among the most recently evolved order. In fact, the oppo- the intriguing features of bats is their use of site would seem to be the case. To produce a species acoustic orientation, or echolocation, to find whose intelligence dwarfs that of all other animals, their way through the dark caves and nocturnal the line of descent should be a long one. Where pri- exteriors that make up their world. Contrary to mates are concerned, that is certainly the case. The popular belief, they are not blind, but they do oldest primate samples date back some 75 million have very small eyes, simply because vision is not years, or long before the end of the Mesozoic. important for bat navigation. (See Migration and Because it is from primates that humans Navigation for more about this subject.) draw their lineage, more has been written about As befits an order with such a wide array of primate evolution than on that of all other mam- species, bats run the gamut with respect to their malian orders combined. The subject is such a eating habits. The majority of bat species are vast one that we will not attempt to approach it insectivores that consume many thousands times here, except to encourage the reader to study in their weight in insects each year. Many others are more detail elsewhere the process by which the fruit bats, important members of the ecosystems human lineage emerged from order Primates, they occupy, because they consume fruit and family Hominidae, and genus Homo. spread seeds, helping assist in seed dispersal. Primates consist of two broad groups, subor- (Some bats also aid in pollination; see Reproduc- ders Prosimii and Anthropoidea. The first, the tion). Then there are the three species of vampire prosimians, includes five families (or six, since bat, which are largely responsible for bats’ unfor- tree shrews are sometimes included) of lemurs, tunate reputation with humans. lorises, and tarsiers. The other suborder, known Members of subfamily Desmodontinae, as the higher primates, encompasses another six species of vampire bat include Diaemus youngi families: marmosets and tamarins; South Ameri- and Diphylla ecaudata. However, the best-known can monkeys other than marmosets; African and variety is the common vampire bat, Desmodus Asian monkeys; lesser apes, or siamangs and gib- rotundus, which is native to an area that stretches bons; great apes, or orangutans, gorillas, and
220 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 221
chimpanzees; and humans, both living and • Hyaenidae (hyenas) Speciation extinct. (Most orders contain extinct members, • Mustelidae (skunks, mink, weasels, badgers, but for the most part they are not discussed here.) and otters) Most primates are tree dwellers, and among • Otariidae (eared seals) the approximately 230 species, there is enormous • Odobenidae (walrus) variation in eating habits. Many lemurs are insec- • Phocidae (earless seals) tivores, while great apes tend to be fruit eaters. • Procyonidae (raccoons) Quite a few are omnivores, though no primate • Ursidae (bears) other than humans is known for eating large • Viverridae (mongooses and civets). mammals, such as cows, sheep, and pigs. The Note that Felidae is a particularly varied pentadactyl limb (an appendage with five digits) is a significant feature for primates, which alone family of some 36 species: lions, lynxes, tigers, have the advantage of the opposable thumb for leopards, and even ordinary domesticated cats. grasping. Humans and a few other primate Thirty-five species belong to a single subfamily, species are also the only animals with four limbs Felinae, which is native to most parts of the who are not only capable of standing upright but world other than Australia, Madagascar, most also function best in this way. oceanic islands, and, of course, Antarctica. The last species, the cheetah (Acinonyx jubatus), is CARNIVORES. As with insectivore, segregated into another subfamily, Acinonychi- carnivore is a name both for an eating prefer- nae, primarily because this cat, native to Africa ence—in this case, meat—and for members of a and southwest Asia, is a daytime hunter, unlike its primate order, Carnivora. Most members of this nocturnal cousins. extraordinarily varied group eat meat, including, in some cases, the “meat” of insects. Bears and CETACEANS AND SIRENIANS. some other species are omnivorous, meaning Orders Cetacea and Sirenia include the majority that they also eat plants, and hyenas and jackals of aquatic mammals, as opposed to the many are classic examples of detritivores, or animals amphibious mammals, such as seals, sea lions, who feed on the remains of other creatures. sea elephants, and walruses, that belong various- The distinction between detritivore and ly to families Otariidae, Odobenidae, and Phoci- carnivore relates not to the materials each con- dae of the order Carnivora. Cetaceans include sumes but to their place in the food web. Rather whales, dolphins, and porpoises, while sirenians than consume live creatures, hyenas and jackals are made up of just three species of manatee and feed on the carcasses of dead ones. Usually these one of dugong. Sirenians are large, friendly crea- creatures are artiodactyls (discussed later), such tures that inhabit the Atlantic coast and tributary as antelopes, which have been killed by other rivers (manatee) or the Indian and Pacific coast- carnivores—big cats, such as the lion or chee- lines (dugong), but cetaceans are much more tah. After the big cats have fed on the fleshy familiar. parts of the prey, hyenas come to consume the With cetaceans, two questions, one specific flesh that remains, and they are followed by and one general, often arise. The answer to the jackals and vultures, swoop in to pick the bones. first of these, “What is the difference between a These detritivores help process the remains of dolphin and a porpoise?,” is that a porpoise is formerly living things, which ultimately return smaller and more chubby and has a blunt snout, to the soil. (See Food Webs for more on this whereas a dolphin has a beaklike snout. Some subject.) taxonomists and marine biologists put porpoises Clearly, all members of order Carnivora eat in the same family as dolphins, whereas others meat, though in different ways and sometimes in treat them as two different families. The more combination with fruit or other vegetation. Nat- general, and much more important, question is ural selection has equipped them for this pur- “Why are these mammals living in the water?” In pose with sharp claws and teeth. Carnivora fact, life itself first appeared in the sea, so perhaps includes some 270 species grouped into ten fam- the question should be “Why or how did any- ilies, listed here: thing start living on land?” The transition from • Canidae (dogs, wolves, jackals, and foxes) water to land took place long before the age of • Felidae (cats) the dinosaurs, much less the emergence of mam-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 221 set_vol3_7 8/19/02 11:12 AM Page 222
Speciation mals, but later, some mammals began to return one or three hoofed toes. Included among peris- to the water, probably about 70 million years ago. sodactyls is another large animal from the grass- Certainly, there is no question that cetaceans lands of Africa and tropical Asia: the rhinoceros. are mammals, a fact first recognized by Aristotle The group also encompasses donkeys, zebras, (384–322 B.C.), a Greek thinker who is noted not and tapirs, but by far the most important in only as one of the greatest philosophers of all human terms is Equus caballus, the domesticated time but also as the father of the biological sci- horse. Described by the French zoologist Comte ences. (See Taxonomy for more about Aristotle’s Georges de Buffon (1707–1788) as “the proudest contributions.) As Aristotle observed, whales and conquest of man,” the horse was domesticated dolphins bear live young and suckle them with (adapted so as to be useful and advantageous for milk-producing glands; their bodies have hair, humans) some 6,000 years ago. Nonetheless, feral albeit only very small strands; and they possess or wild horses remain an important subspecies. lungs, breathing air through a blowhole. Order Hyracoidea also consists of hoofed As evidence of their terrestrial, or land- mammals: seven species of hyrax, a primarily her- based, origins, consider that a whale fetus pos- bivorous creature native to Africa and the far south- sesses the remnants of four limbs, each with five western extremities of Asia. Hyraxes are sometimes fingers (the pentadactyl limb), like any land lumped in with pikas under the term rock rabbit, mammal. Adult whales and dolphins have the but, in fact, pikas are lagomorphs, a group we dis- streamlined, fishlike morphological appearance cuss later. To further the confusion, hyraxes are that is necessary for life underwater, but their probably the animal called a coney in the Bible, resemblance to fishes is of the superficial, analo- even though there is an animal called a cony (no gous variety discussed in Taxonomy. They have “e”) that is actually a lagomorph. This is just one of maintained and modified key terrestrial features; many examples of confusion resulting from the for example, a blowhole atop the head—one in complexities of the animal world and humans’ dolphins, two in whales—replaces the nostrils, attempts to name and classify its members. and thus the passageways for food and air are TUBULIDENTATES. For most completely separate. This differs from the situa- orders, the lowercase adjectival name (e.g., pri- tion with most terrestrial mammals, which take mates, carnivores, insectivores, and so on) is in food and air through the same opening. commonly used. On the other hand, the names PROBOSCIDEANS, PERISSO- hyracoidean and tubulidentate are seldom used DACTYLS, AND HYRAXES. Moving for more obscure groups, such as Hyracoidea and from the largest aquatic mammals, the whales, to Tubulidentata. If any order of mammal is the largest terrestrial variety, we come to the obscure, it is Tubulidentata, which emerged some order Proboscidea, which includes elephants. 60 million years ago and which consists of a sin- Our discussions of most orders in class Mam- gle species: the African ant bear (Orycteropus malia have illustrated particular aspects of taxon- afer). The latter creature is better known by the omy and speciation, and so it is with pro- name aardvark, which in the Afrikaans language boscideans, which give evidence of the many means “earth pig.” species from the past that are gone forever. The Aardvarks at first glance might seem to order is a large one, with three suborders and belong with anteaters, sloths, and armadillos in some 300 species, but anyone who searches for the order Edentata, and that is what taxonomists most of those species will search in vain. All but thought for a long time. The latter half of the three species are extinct. These three are the name tubulidentate, however, suggests the area of Asian elephant (Elephas maximus) and two vari- differentiation: the teeth. Aardvarks’ teeth are eties of African elephant (Loxodonta africana or unique among those of all mammals. Viewed African savanna elephant and Loxodonta cyclotis- from the top, the aardvark’s jawbone is V-shaped, are, the African forest elephant). Until 2001, tax- with the teeth midway along either side of the V. onomists believed that there were only two living The teeth themselves are not fixed to the jaw but species of elephant. (See Taxonomy for more on rest in the flesh attached to it, and instead of this subject.) being covered with enamel, they are protected Another 16 species belong to the order Peris- with a cementlike substance. The substance sodactyla, herbivores whose hind feet bear either comes from tubules that run under the teeth.
222 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 223
Speciation
ORDER PHOLIDOTA CONSISTS OF SEVEN SPECIES OF SCALY ANTEATER, OR PANGOLIN. THE WORD PANGOLIN comes from a Malay term meaning “rolling over,” a reference to the fact that when it is threatened, the animal curls into a little ball. (© Keren Su/Corbis. Reproduced by permission.)
ARTIODACTYLS. Like many other are threatened; fewer than 1,000 remain. As the mammalian orders we have discussed, members BBC reported, this makes them more endangered of order Artiodactyla are ungulates, or hoofed than the more well known giant panda. The crea- animals. Whereas perissodactyls have odd-num- tures survived nuclear testing in the area, which bered toes, artiodactyls have even-numbered ceased in 1996, but they continue to be threat- toes—either two or four—on each foot. This ened by much less spectacular varieties of explo- large group, consisting of some 220 species, com- sive: dynamite and land mines, planted by hun- prises a wide variety of well-known species in gry locals. John Hare of the Wild Camel Protec- nine families. Among them are cows, pigs, sheep, tion Foundation told the BBC, “We found land goats, deer, antelope, bison, camels, giraffes, hip- mines put by the saltwater springs. So when the popotamuses, and numerous less well known camels come to drink, they step on them. Bang! varieties, such as okapi, pronghorn, peccaries, They are blown to pieces and picked up as meat.” and deerlike chevrotains. As to whether the camels constitute a sepa- The camel family is particularly widespread rate species, the molecular geneticist Olivier geographically, including as it does many vari- Hanotte told the BBC: “There are two possibili- eties—the llama, alpaca, and vicuña of South ties here. One is that the domestic camel was bred America—whose home is far from the habitats in from these wild ones some time back in history. the Near East that are associated with the camel. The second is that the domestic camel we see In this family is what may be a previously undis- today was bred from another species that has dis- covered species, whose existence the British appeared. This would mean that these wild Broadcasting Corporation (BBC) reported in camels are a totally separate species.” As of early early 2001. Living on a former nuclear weapons 2002 the camels’ fate, both practically and taxo- testing range in a remote region of Chinese cen- nomically, remained undecided. tral Asia, these creatures drink saltwater, which in PHOLIDOTA/PANGOLINS. Order itself is an unusual characteristic. Pholidota consists of seven species of scaly Although it is extraordinarily hardy, even by anteater, or pangolin. Members of this order are the standards of camels, the central Asian camels normally called pangolins, rather than an adjecti-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 223 set_vol3_7 8/19/02 11:12 AM Page 224
Speciation KEY TERMS
ADAPTIVE RADIATION: A diversifi- DOMESTICATE: To adapt an organ- cation of species over time as a result of ism, whether plant or animal, so as to be specialized adaptations by particular pop- useful and advantageous for humans. ulations of organisms. FAMILY: The third most specific of the ALLOPATRIC SPECIATION: A type seven obligatory ranks in taxonomy, after of speciation that occurs when a popula- order but before genus. tion of organisms is divided by a geo- GENE: A unit of information about a graphic barrier. particular heritable (capable of being inherited) trait that is passed from parent ANALOGOUS FEATURES: Morpho- to offspring, stored in DNA molecules logic characteristics of two or more taxa called chromosomes. that are superficially similar but not as a GENE POOL: result of any common evolutionary origin. The sum of all the genes shared by a population, such as that of a CARNIVORE: A meat-eating organ- species. ism, or an organism that eats only meat (as GENUS: The second most specific of distinguished from an omnivore). the obligatory ranks in taxonomy, after CLASS: The third most general of the family but before species. obligatory taxonomic classification ranks, HERBIVORE: A plant-eating organism. after phylum but before order. HYBRID: The product of sexual union DETRITIVORES: Organisms that feed between members of two species or other on waste matter, breaking organic material smaller and less genetically separate down into inorganic substances that then groups, such as two races. In the case of can become available to the biosphere in species hybrids, the process of hybridiza- the form of nutrients for plants. Their tion involves genetic abnormalities that function is similar to that of decomposers; lead in most cases to sterility. however, unlike decomposers—which tend INSECTIVORE: An insect-eating to be bacteria or fungi—detritivores are organism. relatively complex organisms, such as KINGDOM: The highest or most gener- earthworms or maggots. al ranking in the obligatory taxonomic sys- DNA: Deoxyribonucleic acid, a mole- tem. In the system used in this book there cule in all cells, and many viruses, contain- are five kingdoms: Monera, Protista, Fungi, ing genetic codes for inheritance. Plantae, and Animalia.
val form of Pholidota. The word pangolin comes order. Pangolins are also like aardvarks in the from a Malay term meaning “rolling over,” a ref- sense that their evolutionary relationship to erence to the fact that when it is threatened, the other mammals is not clear. animal curls into a little ball. As with the aard- vark, members of this order once were grouped RODENTS, LAGOMORPHS, AND with Edentata but now are considered a separate MACROSCELIDEANS. Rodents, or mem-
224 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_7 8/19/02 11:12 AM Page 225
Speciation KEY TERMS CONTINUED
MORPHOLOGY: Structure or form, or forms, and the broad lines of descent that the study thereof. unite them.
MUTATION: Alteration in the physical PHYLUM: The second most general of structure of an organism’s DNA, resulting the obligatory taxonomic classification in a genetic change that can be inherited. ranks, after kingdom and before class. NATURAL SELECTION: The process SPECIATION: The divergence of evo- whereby some organisms thrive and others lutionary lineages and creation of new perish, depending on their degree of adap- species. See allopatric speciation and sym- tation to a particular environment. patric speciation. NUMERICAL TAXONOMY: An ap- SPECIES: proach to taxonomy in which specific mor- The most specific of the phological characteristics of an organism seven obligatory ranks in taxonomy. are measured and assigned numerical Species often are defined as a population of value, so that similarities between two types individual organisms capable of mating of organism can be compared mathemati- with one another and producing fertile off- cally by means of an algorithm. Numerical spring in a natural setting. Also, members taxonomy also is called phenetics. of the same species share a gene pool. OMNIVORE: An organism that eats SYMPATRIC SPECIATION: A type of both plants and other animals. speciation that occurs when a group of ORDER: The middle of the seven oblig- individuals becomes reproductively isolat- atory ranks in taxonomy, more specific ed from the larger population of the origi- than class but more general than family. nal species. This type of speciation typical- PENTADACTYL LIMB: An appendage ly results from mutation. with five digits, like the human arm and TAXON: A taxonomic group or entity. hand. This appendage is common to all mammals, though it may take very different TAXONOMY: The area of the biological forms—for example, the dolphin’s flipper. sciences devoted to the identification, PHENETICS: Another name for nomenclature, and classification of organ- numerical taxonomy. isms according to apparent common char- acteristics. PHYLOGENY: The evolutionary histo- ry of organisms, particularly as that histo- VERTEBRATE: An animal with a spinal ry refers to the relationships between life- column.
bers of order Rodentia, are familiar to us as both rodents. The group consists of some 2,205 pests and pets as well as aids to research through species, among them mice, rats, squirrels, their use as test subjects in laboratories. They are beavers, gophers, and porcupines. The name also the most abundant of all mammalian orders: rodent comes from the Latin rodere, meaning “to about one-fourth of all families, 35% of all gen- gnaw,” and, indeed, the defining characteristic of era, and 50% of all living species of mammal are rodents is their chisel-like upper front teeth.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 225 set_vol3_7 8/19/02 11:12 AM Page 226
Speciation Whereas rats typically are despised crea- others place them in another order, Mentophyla, tures, mice (distinguished from rats simply with tree shrews. The latter often have been because they are smaller) often are considered placed variously in orders Scandentia, Insectivo- cute—that is, if the mouse in question is a pet or ra, or Primates, indicating that many areas of a laboratory mouse, rather than a pest chewing mammalian taxonomy remain in dispute. up the insulation or electrical wiring in some- one’s house. The fact that rodents are so often WHERE TO LEARN MORE pests and pets arises in part from rodents’ close association with humans. This is a distinction in Boxhorn, Joseph. “Observed Instances of Speciation.” Talk. Origins (Web site).
226 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 227
SCIENCE OF EVERYDAY THINGS real-life biology DISEASE
DISEASE NONINFECTIOUS DISEASES INFECTIOUS DISEASES
227 set_vol3_8 8/19/02 11:14 AM Page 229
Disease DISEASE
CONCEPT infection through which a virus, bacterium, or Disease is a term for any condition that impairs other parasite enters the body. Infectious dis- the normal functioning of an organism or body. eases, infections, and the immune system that Although plants and animals also contract dis- usually protects us against them are discussed eases, by far the most significant disease-related elsewhere in this book. Our attention in the pres- areas of interest are those conditions that afflict ent context will be devoted to the other two human beings. They can be divided into three broad categories—noninfectious, or intrinsic, categories: intrinsic, or coming from within the diseases and diseases of unknown origin. body; extrinsic, or emerging from outside it; and CLASSIFYING INTRINSIC DIS- of unknown origin. Until the twentieth century EASES. There are several basic varieties of brought changes in the living standards and intrinsic disease, or conditions that are neither health care of industrialized societies, extrinsic contagious nor communicable. These varieties diseases were the greater threat; today, however, are listed in the next few paragraphs. The essay diseases of intrinsic origin are much more famil- Noninfectious Diseases includes a discussion of iar. Among them are stress-related diseases, other systems for classifying diseases of either the autoimmune disorders, cancers, hereditary dis- intrinsic or the extrinsic variety. eases, glandular conditions, and conditions resulting from malnutrition. There are also ill- Hereditary diseases: diseases that are genetic, nesses, such as Alzheimer’s disease, whose causes meaning that they are passed down from genera- remain essentially unknown. tion to generation. An example, discussed in Noninfectious Diseases, is hemophilia. Heredity is not a “cause,” and some of the diseases of HOW IT WORKS unknown origin may be transmitted from parent to offspring. Some forms of cancer are hereditary Classifying Diseases as well, as are other conditions discussed else- Any condition that impairs the normal function- where in this book. (See Nonifectious Diseases, ing of an organism can be called a disease. In the Mutation, and Heredity.) human organism, as in all others, there are cer- Glandular diseases: Conditions involving a tain basic requirements, which in the human gland—that is, a cell or group of cells that filters body include the need for a certain proper material from the blood, processes that material, amount of oxygen, acidity, salinity (salt content), and secretes it either for use again in the body or nutrients, and so on. These conditions must all to be eliminated as waste. Examples include dia- be maintained within a very narrow range, and betes mellitus, examined in Noninfectious Dis- any deviation can bring about disease. eases, as well as various kidney and liver diseases, Diseases can be classified into three general among them, hepatitis and jaundice. Goiter, a groups. There are conditions that are infectious, swelling in the neck area caused by a diet poor in or extrinsic, meaning that they are caused by an iodine, is both a glandular and a dietary condi-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 229 set_vol3_8 8/19/02 11:14 AM Page 230
Disease tion, a fact that illustrates the overlap between and Alzheimer’s disease, are discussed near the disease types. conclusion of this essay. Dietary diseases: These are all illnesses that relate to nutrient deficiencies—either an overall REAL-LIFE lack of adequate nutrition (i.e., malnutrition) or APPLICATIONS the absence of a key nutrient. Examples include pellagra, scurvy, and rickets, all of which are vita- A Changing Threat min deficiencies, as well as kwashiorkor, which brings about a swollen belly and is caused by a At one time the diseases that posed the greatest lack of protein. Vitamin deficiencies are dis- threat to human survival were infectious ones, cussed in Vitamins, and kwashiorkor and other such as the Black Death (actually a combination varieties of malnutrition are examined in Nutri- of bubonic and pneumonic plague), which killed ents and Nutrition. about a third of Europe’s population during the Cancers: Cancer is not just one disease but period from 1347 to 1351. Plagues or epidemics, some 100 conditions. Its two main characteristics in fact, are among the persistent themes in histo- are uncontrolled growth of diseased cells in the ry, punctuating the fall of empires and the rise of human body and migration of the disease from others. the original site to distant sites within the body. If A plague that struck the eastern Roman the spread is not controlled, cancer can result in (Byzantine) Empire in the sixth century, for death. (See Noninfectious Diseases for more.) instance, brought an end to a plan by the great Stress-related diseases: Some heart condi- Justinian I to reconquer the Italian peninsula and tions are hereditary or glandular, but quite a few restore Roman rule in western Europe. It also diseases of the heart and circulatory system are spelled the beginning of the end of Byzantine exacerbated by stress. Examples include heart glory (though the empire hung on until 1453) murmurs, hardening of the arteries, and varicose and opened the way for the rise of Islam and veins. We will examine heart disease and the gen- Muslim influence over the Mediterranean. Thus, eral effects of stress shortly. the course of history up to the present day, Autoimmune diseases: This is a particularly including the events of the European Middle terrifying category of disease, because it involves Ages, the Crusades, and even the modern-day a rejection of the body itself by the body’s own conflict between the West and Islamic terrorists, immune system. Autoimmune diseases, examples can be traced in part back to a plague in about of which include lupus and rheumatoid arthritis, A.D. 540. are discussed in The Immune System. Wherever people have gathered in large DISEASES OF UNKNOWN ORI- numbers, infectious diseases have arisen. Small- GIN. Finally, there are diseases for which there pox and chicken pox, cholera and malaria, diph- is no known cause. In some cases, it is possible theria and scarlet fever, influenza and polio— that heredity, diet, or some other aspect of these and many other diseases have threatened human existence has a role, but it is not certain. the very survival of whole populations, bringing And even if, say, heredity plays a part, the exact about a collective death toll that dwarfs that of hereditary factors are not established. In any case, twentieth-century wars and genocide. Yet it was many of the categories of disease we have listed in the twentieth century—ironically, the era do not amount to “causes,”but rather are types of when humans discovered the capacity to kill disease. Moreover, some diseases classifiable in themselves in truly frightening numbers through one of the listed categories also belong in the world wars, nuclear weaponry, and totalitarian ranks of the diseases with unknown causes. For social experiments—that the threat of infectious instance, many autoimmune diseases are myste- diseases began to recede. rious to scientists. Likewise, chronic fatigue syn- Thanks to successful vaccination programs, drome, considered a disease of unknown origin, many infectious diseases are largely a thing of the is obviously a stress-related disorder, while past. This is true even of smallpox, a scourge that fibromyalgia, characterized by sore muscles and effectively ended in 1978 thanks to a United tissues, may be stress-related as well. Two brain Nations inoculation program, but which diseases of unknown origin, Creutzfeldt-Jakob reemerged as a potential threat of biological ter-
230 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 232
Stress and Heart Disease Disease researchers have determined that long-term Stress, simply put, is a condition of mental or stress causes the accumulation of fat, starch, cal- cium, and other substances in the linings of the physical tension brought about by internal or blood vessels. This condition ultimately results in external pressures. Many events can cause stress: heart disease. something as simple as taking a test or driving through rush-hour traffic or as traumatic as the HEART DISEASES. The human death of a loved one or contracting a serious ill- heart weighs just 10.5 oz. (300 g), but it contracts ness. Stress may be short-lived, as when facing a more than 100,000 times a day to drive blood particular deadline, or it may be the ongoing, through about 60,000 mi. (96,000 km) of vessels. crippling stress related to a job that is slowly An average heart will pump about 1,800 gal. killing the victim. (6,800 l) of blood each day. With exercise, that amount may increase as much as six times. In an People who experience severe traumas, such average lifetime the heart will pump about 100 as soldiers in combat, may experience a condi- million gal. (380 million l) of blood. The heart is tion called post-traumatic stress disorder divided into four chambers: the two upper atria (PTSD). This condition first came to public and the two lower ventricles. The wall that attention after World War I, a war that complete- divides the right and left sides of the heart is the ly dwarfed all preceding conflicts in its intensity septum. Movement of blood between chambers and brutality. Formerly bright-eyed, optimistic and in and out of the heart is controlled by valves youths came home behaving like madmen or that allow transit in only one direction. nervous wrecks, and soon the condition gained the nickname shell shock. (Actually, shell shock Given its importance to human life, it fol- dated back more than 50 years, to what might be lows that heart disease is an extremely serious regarded as the first modern war in the West— condition. Among the many illnesses that fall the first “total war” involving relatively sophisti- under the general heading of heart disease is con- cated weaponry and a fully engaged citizenry: genital heart disease, a term for any defect in the America’s Civil War, from which combatants heart that is present at birth. About one of every returned home with a condition known as “sol- 100 infants is born with some sort of heart dier’s heart.”) abnormality, the most common form being the atrial septal defect, in which an opening in the EFFECTS OF STRESS. Whereas septum allows blood from the right and left atria PTSD has a distinct psychological dimension, in to mix. many stress-related diseases there is not as obvi- ous a link between mental states and bodily dis- Coronary heart disease, also known as coro- orders. Nonetheless, it is clear that stress kills. nary artery disease, is the most common form of Some of the physical signs of stress are a dry heart disease. A condition termed arteriosclero- mouth and throat, headaches, indigestion, sis, in which there is a thickening of the artery tremors, muscle tics, insomnia, and a tightness of walls, or a variety of arteriosclerosis known as the muscles in the shoulders, neck, and back. atherosclerosis results when fatty material, such Emotional signs of stress include tension, anxiety, as cholesterol, accumulates on an artery wall. and depression. During stress, heart rate quick- This forms plaque, which obstructs blood flow. ens, blood pressure increases, and the body releas- When the obstruction occurs in one of the main es the hormone adrenaline, which speeds up the arteries leading to the heart, the heart does not body’s metabolism. Stress may disrupt homeosta- receive enough blood and oxygen, and its muscle sis, an internal bodily system of checks and bal- cells begin to die. ances, leading to a weakening of immunity. Creutzfeldt-Jakob Disease Diseases and conditions associated with stress include adult-onset diabetes (see Nonin- A particularly frightening category of unexplained fectious Diseases), ulcers, high blood pressure, diseases includes those that attack and destroy the asthma, migraine headaches, cancer, and even brain. Among them are two conditions named the common cold. The last, of course, is an infec- after German scientists: the psychiatrists Alfons tious illness, but because stress impairs the Maria Jakob (1884–1931) and Hans Gerhard immune system, it can leave a person highly sus- Creutzfeldt (1885–1964) and the neurologist Alois ceptible to infection. Furthermore, medical Alzheimer (1864–1915). Creutzfeldt-Jakob dis-
232 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 233
Disease
COMPUTER GRAPHIC OF THE BRAIN OF AN ALZHEIMER PATIENT (LEFT) COMPARED WITH A NORMAL BRAIN (RIGHT). ALZHEIMER’S DISEASE SHRINKS THE BRAIN, WHICH SHOWS SIGNS OF THE DEGENERATION OF NERVE CELLS, TAN- GLED PROTEIN FILAMENTS, AND LESIONS CAUSED BY ACCUMULATION OF BETA-AMYLOID PROTEIN. (Photograph by Alfred Pasieka. Photo Researchers, Inc. Reproduced by permission.)
ease, fortunately, is a rare condition. The disease, tainted with by-products of other cows, and as a first described by the two doctors in the 1920s, ini- result of the outbreak, Great Britain issued wide- tially shows itself with the loss of memory, and ranging controls prohibiting the production of within a few weeks it progresses to visual prob- feed containing any materials from cows. (These lems, loss of coordination, and seizure-like mus- particular feed-production practices were never cular jerking. Death usually follows within a year. common in the United States.) It appears that Creutzfeldt-Jakob disease ensues when a certain protein in the brain, Alzheimer’s Disease known as prion protein, changes into an abnor- Whereas Creutzfeldt-Jakob disease is a little- mal form. As to what causes that change, scien- known condition, Alzheimer’s disease is all too tists remain in the dark. The disease attacks about familiar to the families of the more than four one of a million people worldwide, and victims million sufferers in America today. Note the ref- are typically about 50-75 years of age. During the erence to the families rather than the victims 1990s something strange happened: the disease began affecting relatively large numbers of young themselves: one of the most devastating aspects people in the United Kingdom. A 1996 report of of Alzheimer’s disease is the patient’s progressive British medical experts, however, linked the surge loss of contact with reality, such that a patient in in Creutzfeldt-Jakob cases to what might be con- an advanced stage does not even know that he or sidered a dietary condition: bovine spongiform she has the disease. A progressive brain disease encephalopathy, or mad cow disease, contracted that brings about mental deterioration, from eating cattle with a form of prion disease. Alzheimer’s disease is signaled by symptoms that The only way to contract such a condition, how- include increasingly poor memory, personality ever, is by eating the brain or spinal cord of an changes, and a loss of concentration and judg- affected cow, something that could only happen ment. Although most victims are older 65 years, in the case of hamburger or sausage, in which one Alzheimer’s is not a normal result of aging. Up does not always know what one is getting. The until the 1970s people assumed that physical and cows themselves got the disease from eating feed mental decline were normal and unavoidable
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 233 set_vol3_8 8/19/02 11:14 AM Page 234
Disease THE IMPACT OF ALZHEIMER’S KEY TERMS DISEASE. A slight decline in short-term memory (as opposed to long-term memories of childhood and the like) is typical even in healthy DISEASE: A general term for any con- elderly adults, but the memory loss seen in dition that impairs the normal functioning Alzheimer’s disease is much more severe. As years of an organism. pass, memory loss becomes greater, and person- ality and behavioral changes occur. Later symp- EXTRINSIC: A term for a disease that toms include disorientation, confusion, speech is communicable or contagious and comes impairment, restlessness, irritability, and the from outside the body. Compare with inability to care for oneself. Although victims intrinsic. may remain physically healthy for years, the pro- gressive decline of their mental faculties is ulti- GLAND: A cell or group of cells that fil- mately fatal: eventually, the brain loses the ability ters material from the blood, processes that to control basic physical functions, such as swal- material, and secretes it either for use again lowing. Persons with Alzheimer’s disease typical- in the body or to be eliminated as waste. ly live between five and ten years after diagnosis, although improvements in health care in recent INTRINSIC: A term for a disease that is years have enabled some victims to survive for 15 not communicable or contagious and years or even longer. comes from inside the body. Compare with Improvements in health care also may help extrinsic. explain the fact that the numbers of Alzheimer victims are growing. Medical discoveries of the twentieth century served to prolong life greatly, such that there are far more people alive today features of old age and dismissed such cases of who are 65 years of age or older than there were deterioration as “senility.” Yet as early as 1906, in 1900. More accurate reporting no doubt plays Alzheimer himself discovered evidence that a part as well. Whereas about 2.5 million cases pointed in a different direction. were reported throughout the 1970s, by the end of the twentieth century there were some four In that year Alzheimer was studying a 51- million living Alzheimer victims, and by the mid- year-old woman whose personality and mental twenty-first century that number is expected to abilities were obviously deteriorating. She forgot climb to the range of 13 million if physicians do things, became paranoid, acted strangely, and not find a cure. Meanwhile, Alzheimer’s causes just over four years after he began working with the deaths of more than 100,000 American adults her, she died. Following an autopsy, Alzheimer each year and costs $80–90 billion annually in examined sections of her brain under a micro- health-care expenses. scope and noted deposits of an unusual sub- UNDERSTANDING stance in her cerebral cortex—the outer, wrin- ALZHEIMER. It is not a simple procedure to kled layer of the brain, where many of the higher diagnose Alzheimer’s disease, and despite all the brain functions, such as memory, speech, and medical progress since the time of Alois thought, originate. The substance Alzheimer saw Alzheimer, the “best” method for determining under the microscope is now known to be a pro- whether someone has the condition is hardly a tein called beta-amyloid. About 75 years later sci- good one. The only possible physical procedure entists and physicians began to recognize a for definitively diagnosing Alzheimer’s disease is strong link between “senility” and the condition to open the skull and remove a sample of brain Alzheimer had identified. Since then, the public tissue for microscopic examination. This is rarely has become more aware of the disease, especially done, of course, because brain surgery is far too since Alzheimer’s disease has stricken such well- drastic a procedure for simply obtaining a sam- known figures as the former president Ronald ple of tissue. Reagan (1911–) and the actress Rita Hayworth The immediate cause of Alzheimer’s is the (1918–1987). death of brain cells and a decrease in the connec-
234 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 235
WHERE TO LEARN MORE tions between those cells that survive. But what Disease causes that? Many scientists today believe that the Centers for Disease Control and Prevention (Web site). presence of beta-amyloid protein is a cause in
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 235 set_vol3_8 8/19/02 11:14 AM Page 236
NONINFECTIOUS
DISEASESNoninfectious Diseases
CONCEPT infectious diseases: bacteria, viruses, and other In contrast to infectious, or extrinsic, diseases, parasites. Infectious diseases, discussed elsewhere noninfectious, or intrinsic, conditions are neither in this book (see Infection, Infectious Diseases, contagious nor communicable. They arise from and Parasites and Parasitology), continue to pose inside the body as a result of hereditary condi- a threat to life in underdeveloped nations. In tions or other causes, such as dietary deficiencies. North America, Europe, the emerging capitalist Although infectious forms historically have been democracies of eastern Asia, and a handful of the most life-threatening varieties of disease and other materially and technologically advanced remain so even today in much of the world, non- lands, however, infectious diseases have taken a infectious disease is a far more serious concern in back seat to noninfectious ones as well as to other industrialized nations, such as the United States. threats. Some categories of intrinsic diseases include CAUSES OF DEATH IN AMERI- stress-related, dietary, and autoimmune condi- CA. In the United States today, the four leading tions as well as diseases of unknown origin. causes of death (see list in Disease) are noninfec- Additionally, there are several other categories we tious conditions: heart disease, cancer, stroke, examine in this essay: hereditary diseases, such as and chronic obstructive lung diseases. The first of hemophilia; glandular conditions, of which dia- these conditions is discussed in Disease, and the betes mellitus is a powerful example; and cancer, second is examined later in this essay. Stroke, of which there are approximately 100 different which occurs when a clot obstructs the flow of varieties. blood to the brain, may be classified along with heart disease as a stress-related ailment of the HOW IT WORKS circulatory system. Chronic obstructive lung dis- eases include emphysema, cystic fibrosis, and The Threat of Noninfectious other conditions with widely differing causes Diseases that nonetheless all affect the same organ in the same way. The world has long suffered under the threat of The remainder of the list includes two non- infectious diseases, some of which include small- disease-related conditions (accidents at no. 5 and pox, chicken pox, cholera, malaria, diphtheria, suicide at no. 8), three more noninfectious dis- scarlet fever, influenza, polio, pneumonia, and eases (diabetes mellitus at no. 7, kidney disease at even the common cold. Except for the last one, no. 9, and chronic liver disease and cirrhosis at which is rarely fatal, such conditions have racked no. 10); and just one infectious-disease-related up a considerable death toll. For centuries people set of causes. The latter, no. 6, consists of two dis- attributed these diseases to all manner of false eases, pneumonia and influenza, which may be causes, ranging from divine curses to an imbal- the result of another type of infection, the only ance of bodily fluids. Only with the development infectious condition that poses a serious threat in of the microscope in the 1600s did scientists the Western world today: AIDS, or acquired begin to identify the real cause behind most immunodeficiency syndrome.
236 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 237
Noninfec- tious Diseases
MAGNETIC RESONANCE IMAGING IS ONE TYPE OF DIAGNOSTIC TEST USED TO LOCATE TUMORS. (© Neal Preston/Corbis. Reproduced by permission.)
Varieties of Disease tors and scientists use for grouping diseases are topographic; anatomic; physiological; patholog- The essay Disease classifies noninfectious diseases ic; etiologic, or causal; and epidemiological clas- as follows: hereditary or genetic diseases (e.g., sifications. Topographic classification refers to hemophilia, discussed later in this essay); glandular bodily region or system: for instance, the circula- diseases, or conditions involving a group of cells that filters material from the blood (e.g., diabetes tory system, the neurological system, and so on. mellitus, also included in the present essay); dietary The second method, anatomic (by organ or tis- diseases (see the essays Vitamins and Nutrients and sue), also uses parts of the body as a criterion for Nutrition); cancers (discussed here); stress-related classification. The designation of heart and lung diseases (see Disease); autoimmune diseases (see diseases, used earlier in discussing leading causes Immunity and Immunology); and diseases of of mortality in the United States, is an example of unknown origin (see Immunity and Immunology). this system. This is one way of dividing up ailments, and Physiological classifications divide diseases it happens to be the method applied in the essays in terms of function or effect (for example, meta- of this book that deal with diseases. This method bolic disorders, some of which are discussed in has the advantage of illustrating the wide varia- Metabolism), while pathologic classifications tion in noninfectious diseases, but it would not separate diseases by the nature of the process that necessarily be the best model to use for an in- the disease takes: for example, inflammatory dis- depth professional study of disease. Scientists eases. Etiologic, or causal, classifications are used who study illnesses typically use one of several most commonly in discussing infectious dis- methods of classification that, while less broadly eases, where broad types of causes can include based than the one used here—and perhaps less viruses, bacteria, or other types of parasites. Like- interesting as well—are more efficient, because wise, epidemiological classifications usually refer they group all diseases according to the same to infectious diseases. Epidemiology is an area of characteristics. the medical sciences devoted to the study of dis- SOME OTHER CLASSIFICA- ease, including its incidence, distribution, and TION SYSTEMS. Among the systems doc- control within a population.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 237 set_vol3_8 8/19/02 11:14 AM Page 238
Noninfec- REAL-LIFE CAUSES AND TYPES. A majority tious APPLICATIONS of cancers are caused by changes in the cell’s Diseases DNA because of damage from the environment. Cancer Environmental factors that are responsible for the initial mutation in DNA are called carcino- Although we are accustomed to hearing of “can- cer” as though it were one disease, it is actually gens, and there are many types, which we discuss many diseases, close to 100 in number. Some of shortly. Additionally, some cancers have a genet- the most common varieties include skin, lung, ic basis: in other words, a person can inherit and colon cancer, as well as breast cancer in faulty DNA from his or her parents, which can women and prostate cancer in men. Blood and predispose the patient to cancer of one kind or lymph node cancers, known as leukemias and another. While there is scientific evidence that lymphomas, respectively, are widespread, whereas both factors (environmental and genetic) play a cancer of the kidneys, ovaries, uterus, pancreas, role, less than 10% of all cancers are purely bladder, and rectum are included among the can- hereditary. cers that most often affect Americans. There are several different types of cancers. As this listing suggests, most cancers attack In addition to leukemias and lymphomas, men- either body parts or systems and therefore are tioned earlier, there are carcinomas, or cancers often classified anatomically or topographically; that arise in the epithelium (the layers of cells yet several characteristics unite these conditions. covering the body’s surface and lining the inter- Cancer strikes the genes, which are carriers of nal organs and various glands). These types genetic information that make up part of DNA alone account for about 90% of all cancers. Some (deoxyribonucleic acid), a molecule that appears forms of skin cancer are melanomas, which typi- in all cells. By gaining control at this level, the cally originate in the pigment cells. Other forms cancer is like a terrorist who has established a of cancer include sarcomas (cancers of the sup- grip on all the communication or transportation porting tissues of the body, e.g., bone, muscle, systems in a country. and blood vessels), and gliomas, or cancers of the nerve tissue. Many genes produce proteins that play a part in controlling the processes of cell growth and DIAGNOSING CANCER. Many division. An alteration, or mutation, to the DNA signs indicate the onset of cancer, among them, molecule can disrupt the genes and produce changes in the size, color, or shape of a wart or a faulty proteins, causing the cells to become mole; a sore that does not heal; or persistent abnormal and multiply. The abnormal cell begins cough, hoarseness, or sore throat. Many other to divide uncontrollably and eventually forms a diseases can produce similar symptoms, however, new growth, known as a tumor, or neoplasm. In a and for this reason it is important for a person to healthy person, the immune system can recognize visit a doctor for regular checkups and diagnosis. the neoplastic cells and destroy them before they Usually, diagnosis calls for fairly routine physical have a chance to divide. Some mutant cells may examination, though in the case of cancers of the escape immune detection, however, and survive reproductive organs, “routine” can still be plenty to become tumors or cancers. (The immune sys- invasive. tem is discussed in Immunity and Immunology.) Doctors examining women for cancers of Tumors can be either benign or malignant. A the ovaries, uterus, cervix, and vagina must pal- benign tumor is slow growing, does not spread or pate the internal organs—that is, examine them invade surrounding tissue, and, once removed, by touch. For males, inspection of the rectum usually does not recur. A malignant tumor, on and the prostate is included in the physical exam- the other hand, invades surrounding tissue and ination. The doctor inserts a gloved finger into spreads to other parts of the body. Therefore, the rectum and rotates it slowly to feel for any even if the malignant tumor is removed, if the growths, tumors, or other abnormalities. The cancer cells have spread to the surrounding tis- doctor also palpates the testicles to identify any sues, cancer will return. If the cancer cells are lumps, thickening, or differences in the size, allowed to keep growing in number, migrating weight, or firmness. Such examinations, as well as from the site of origin and spreading throughout diagnoses for certain other types of cancer in pri- the body, they eventually will kill the patient. vate parts (namely, colon cancer), can be less
238 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 239
than pleasant, but they are certainly preferable to highly effective, chemotherapy may be physically Noninfec- an early and painful death. difficult for the patient and may have side effects, tious Diseases If the patient has an abnormality that could including temporary hair loss. be indicative of cancer, the doctor may order Immunotherapy uses the body’s own diagnostic tests. These tests may include laborato- immune system to destroy cancer cells. The vari- ry studies of sputum or saliva, blood, urine, and ous immunological agents being tested include stool (feces). To locate tumors, such imaging tests substances produced by the body as well as vac- as computerized tomography (CT) scans, mag- cines. Unlike traditional vaccines, cancer vaccines netic resonance imaging (MRI), ultrasound, or do not prevent cancer; instead, they are designed fiber-optic scope examinations may be used. The to train the immune system of the cancer most definitive diagnostic test, however, is the patient’s body to attack and destroy cancer cells. biopsy, in which a piece of tissue is surgically Hormone therapy is standard treatment for some removed for examination under a microscope. types of cancers that are hormone-dependent Besides confirming whether a patient has cancer, and which grow faster in the presence of particu- the biopsy also provides information about the lar hormones. Among them are cancers of the type of cancer, the stage it has reached, the aggres- prostate, breast, and uterus, and the therapy is siveness of the cancer, and the extent of its spread. designed to block the production or action of the Screening examinations, conducted regular- hormones involved. ly by health care professionals, can result in the A particularly aggressive form of treatment detection of cancers at an early stage. In addition, is bone-marrow transplantation, which involves advances in molecular biology (an area of biolo- taking tissue from within a donor’s bone cavities, gy concerned with the physical and chemical where blood-forming cells are located, and trans- basis of living matter) and cancer genetics have planting it into the patient. In addition, cancer led to the development of several tests for assess- patients may use massage, reflexology, herbal ing one’s risk of developing cancers. These new remedies, and other forms of treatment dubbed techniques include genetic testing, in which “alternative,” meaning that they usually are not molecular probes are used to identify mutations recognized by the mainstream of the medical in certain genes that have been linked to particu- profession. (Lack of official recognition does not lar cancers. At present, however, there are limita- necessarily mean anything: many people have tions to genetic testing, a fact that emphasizes the benefited from alternative cancer treatments.) need for better strategies of early detection. Who Is at Risk? TREATING CANCER. Although there is as yet no cure for cancer, there are treat- One of every four deaths in the United States is ments designed to remove as much of the tumor from cancer, and each year more than a million or tumors as possible and to prevent the recur- Americans are diagnosed with some form of the rence or spread of the cancer. Cancer treatment disease. Of these people, about half will die of the can take many different forms, including surgery, disease. Cancer can attack anyone, even children, radiation, chemotherapy, immunotherapy, hor- though, fortunately, cases of cancer in very young mone therapy, and bone-marrow transplanta- patients are the exception. Most cases are seen in tion. Physicians recommend specific treatments middle-aged or older adults. Although scientists based on the needs, condition, and illness of the are a long way from being able to predict who particular patient. will get cancer (much less effectively prevent it), Surgery, the most frequently used form of they have identified numerous risk factors. A risk cancer treatment, involves the removal of the vis- factor is not necessarily a cause per se (though it ible tumor. It is most effective when a cancer is may be); rather, if there seems to be a link small and confined to one area of the body. Radi- between a particular behavior and a specific dis- ation, which kills tumor cells by bombarding ease, that behavior is referred to as a risk factor them with high-energy waves or particles, may be for that disease. Major risk factors for cancer are used alone in cases where a tumor is unsuitable tobacco use, alcohol consumption, diet, certain for surgery. More often, however, it is used in types of sexual and reproductive behavior, infec- conjunction with surgery and chemotherapy or tious agents, family history, occupation, environ- with drugs to kill cancer cells. While it can be ment, and pollution.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 239 set_vol3_8 8/19/02 11:14 AM Page 240
SPECIFIC RISK FACTORS. Noninfec- nificant group. They account for 4% of all cancer tious Approximately two-fifths of all cancer deaths deaths, and there is a certain poignancy in the Diseases annually are linked to tobacco use and excessive fact that these cancer victims contracted their ill- alcohol consumption. In addition to the relation- ness not by drinking or smoking or bad eating ship between smoking and lung cancer (80–90% but simply by earning a living. Nowhere was this of all lung cancer patients are smokers), smoking poignancy more evident than in some of the first also has been shown to be a contributory factor to documented cases of cancer arising from occu- a whole host of other cancers. Moreover, scientists pational causes. have established that second-hand smoke (or pas- In 1775 the English surgeon Percivall Pott sive smoking) can increase one’s risk of cancer. (1714–1788) described the high incidence of can- Nor is “smokeless tobacco” a safe alternative to cer of the scrotum among former chimney cigarettes: snuff and chewing tobacco have been sweeps, most of them men in their twenties. This associated with countless cases of mouth cancer. was bad enough, but the fact that they had con- Excessive drinking is a risk factor in liver tracted the cancer much earlier hinted at a reality cancer and other illnesses, and the deadly combi- even more grim. In the harsh early days of the nation of tobacco and excessive alcohol use sig- Industrial Revolution, before child labor laws had nificantly increases the chances that a person will even been imagined, boys as young as four— contract mouth, pharynx, larynx, or esophageal orphans or children of desperately poor fami- cancer. Tobacco and alcohol are not the only can- lies—were put to work cleaning the insides of cer-related agents that people take into their bod- chimneys. The chimneys were so narrow that ies: about one-third of all cancer deaths annually only children could clean them, and because of are related to the things people eat. For example, the tight fit, even small boys were unable to wear immoderate intake of fat, leading to obesity, has any clothes while doing their job. As a result, soot been associated with cancers of the breast, colon, became embedded in their skin, and, since few rectum, pancreas, prostate, gallbladder, ovaries, people bathed, it stayed there, a silent killer whose and uterus. effects became apparent only many years later. There are even varieties of cancer that are In addition to helping make English society linked to contagious diseases. Since the mid- aware of the injustices put upon members of its 1970s, scientists have obtained evidence showing lowest classes, Pott’s research introduced the that approximately 15% of all cancer deaths medical world to the concept of occupational worldwide can be traced to viruses, bacteria, or health. Still, until the last third of the twentieth parasites. One such pathogen, the human papil- century, large numbers of industrial workers in loma virus, is sexually transmitted. Having too the West labored at occupations with built-in many sex partners and becoming sexually active cancer hazards. Thus, asbestos workers contract- too early have been shown to increase one’s ed lung cancer at a high rate, and a link became chances of contracting cancer of the cervix. (On apparent between bladder cancer and dye and the other hand, women who do not have children rubber industries. Likewise, connections were or those who have them late in life have a higher established between skin or lung cancer and the risk of both ovarian and breast cancer.) jobs of smelters, gold miners, and arsenic work- Certain cancers, including those of the ers; between leukemia and glue and varnish breast, colon, ovaries, and uterus, recur genera- workers’ occupations; between liver cancer and tion after generation in some families, and there- the business of PVC (polyvinyl chloride) manu- fore family history and genetics cannot be ruled facturers; and between lung, bone, and bone out as risk factors. In addition, less well-known marrow cancer and the work of radiologists and cancers, such as the eye condition known as uranium miners. retinoblastoma, have been traced to certain genes Radiation itself is an environmental hazard that can be tracked within a family. Thus, it is that affects not only workers in those specialized possible that inheriting particular genes makes a industries just named but also anyone who has person susceptible to certain cancers. been exposed to radioactive materials. Fortu- OCCUPATION, ENVIRONMENT, nately, this is less of a hazard today, thanks to AND POLLUTION. Cancers arising from numerous bans on nuclear testing; nonetheless, occupational hazards make up a particularly sig- radiation—including ultraviolet radiation from
240 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 241
Noninfec- tious Diseases
PORTRAIT OF YOUNG CHIMNEY SWEEPERS FROM THE 1890S. BEFORE CHILD LABOR LAWS, BOYS AS YOUNG AS FOUR WERE PUT TO WORK CLEANING THE INSIDES OF CHIMNEYS; MANY LATER CONTRACTED CANCER OF THE SCROTUM FROM EXPOSURE TO SOOT. (© Bettmann/Corbis. Reproduced by permission.)
the Sun—causes 1–2% of all cancer deaths. Addi- eral sons with hemophilia who died before they tionally, it has been estimated that 1% of cancer had the opportunity to become king. Her deaths are due to air, land, and water pollution, nephew, Russia’s Czar Nicholas II, had a son who particularly as a result of chemical dumping in was hemophiliac as well. The boy’s affliction water supplies. caused his mother to seek the help of the charis- matic “healer” Grigori Rasputin, whose close Hemophilia involvement with the royal family fed Russian In contrast to the boys Pott treated, sufferers discontent and helped contribute to the over- from hemophilia in the past often came from the throw of the czar in 1917. (Ironically, the boy highest echelons of society. A hereditary disease died not from hemophilia but perished, along that primarily affects males, hemophilia was with his family, before a Bolshevik firing squad.) passed down through many royal lines, with the Thus, once again (as noted in Disease), disease females acting as carriers and some of the males affected the course of history. in the bloodline becoming victims of the disease. Its name taken from Greek and Latin words England’s Queen Victoria (1819–1901) had sev- that together mean “love of bleeding,” hemophilia
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 241 set_vol3_8 8/19/02 11:14 AM Page 242
Noninfec- betes mellitus, a serious condition caused by an tious absence of, or an insufficient amount of, insulin. Diseases Insulin, a hormone produced by the pancreas in varying amounts, helps maintain a normal con- centration of glucose, or blood sugar (see Carbo- hydrates). Diabetes in general is a glandular dis- ease, the gland in this case being the pancreas, but type 1 diabetes also is considered an autoim- mune disorder because the body’s own immune system destroys its insulin-producing cells. (See Immunity and Immunology and The Immune System for more about autoimmune disorders.) Diabetes prevents the body from putting glucose to use, and instead large amounts of it are excreted in the urine. The word diabetes means “siphon,” a reference to one of its major symp- toms: frequent urination in an attempt to expel glucose. The urine itself is full of sugar; hence the term mellitus, meaning “honey.”(In this vein, it is worth mentioning Chen Chuan, an eighth-cen- tury Chinese physician who surely qualifies as ALEXIS, SON OF CZAR NICHOLAS II OF RUSSIA, HAD one of history’s most dedicated scientists. He was HEMOPHILIA, A HEREDITARY DISEASE THAT IMPAIRS THE the first to describe the sweetness of urine in CLOTTING OF BLOOD. THE DISEASE PRIMARILY AFFECTS patients suffering from diabetes, presumably as MALES AND HAS BEEN PASSED DOWN THROUGH MANY ROYAL BLOODLINES. (© Bettmann/Corbis. Reproduced by permis- the result of firsthand research that went above sion.) and beyond the call of duty.) EXTENT AND TYPES OF DIA- BETES. More than 12 million Americans, is caused by a genetic defect that prevents the and some 100 million people worldwide, are body from developing proteins needed to help affected by diabetes. That number is increasing the blood clot. What would be a minor bruise or by 5-6% annually, primarily the result of the scratch for an ordinary person is therefore a life- population’s increased longevity, combined with threatening situation to a hemophiliac, who runs other factors such as increasing obesity and con- a severe risk of bleeding to death from even the sumption of rich, processed, and carbohydrate- most minor cut. A hemophiliac must therefore loaded foods (and drinks such as beer). Approx- live in a state of constant fearfulness: as a young- imately 300,000 deaths each year are attributed to ster, for instance, he cannot run and jump and get diabetes, which is of two principal varieties. Type into mischief like other boys, for fear that he 1, or insulin-dependent diabetes, is present at might skin his knee—a minor pain for most boys birth, is characterized by insulin deficiency, and but a serious injury to him. For hemophiliacs normally is treated by taking insulin injections. today, there is good news and bad news. The good Type 2, or non-insulin-dependent diabetes, aris- news is that the condition can be treated with es not at birth, but somewhat later (though it can transfusions containing the necessary proteins, occur in childhood) and then among people who and this has extended the life expectancy of some have normal insulin levels. (With type 2 diabetes, victims. The bad news, however, is that in this day the problem is the body’s inability to use its and age the treatment itself might kill them: the insulin efficiently.) Type 2, which typically stems transfusions originate from donated blood, from dietary causes—is preventable, but it is not which may contain the virus that causes AIDS, as treatable by insulin injections. Type 2 diabetes well as pathogens linked to other diseases. also may temporarily affect pregnant women, Diabetes Mellitus who may experience heightened glucose levels in a condition known as gestational diabetes. Much more prevalent than the genetic disorder (There is another kind of diabetes altogether, a hemophilia, or any one form of cancer, is dia- rare condition known as diabetes insipidus,
242 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 243
which involves inadequate production of anoth- Noninfec- er hormone, vasopressin.) tious KEY TERMS Diseases EFFECTS OF DIABETES. One might wonder why problems with blood sugar CARCINOGEN: A substance or agent could be so serious as to kill a million people every three years and to qualify diabetes as one of that induces the development of cancer. the leading killers in America. The reason is that DISEASE: A general term for any con- the body depends on glucose as a source of dition that impairs the normal functioning immediate energy, and in the absence of usable glucose, it begins instead to use its fat cells. This of an organism. rapid burning of fat produces a surplus of organ- DNA: Deoxyribonucleic acid, a mole- ic compounds known as ketones, and ketone cule in all cells, and many viruses, contain- accumulation brings about an accumulation of acids in the blood, a condition known as ketoaci- ing genetic codes for inheritance. dosis. Severe ketoacidosis can cause nausea, vom- EXTRINSIC: A term for a disease that iting, and a loss of consciousness, or diabetic is communicable or contagious and comes coma. If the patient does not receive a shot of from outside the body. Compare with insulin, he or she can die. Thus, the symptoms displayed by Julia Roberts’s diabetic character in intrinsic. her first major movie, Steel Magnolias (1989), GENE: A unit of information about a were not overdone. Diabetes can also bring about particular heritable (capable of being other conditions, including blindness, kidney diseases, and long-term organ damage. inherited) trait that is passed from parent THE TRIUMPH OF BANTING to offspring and stored in DNA molecules AND BEST. As difficult as life is for diabet- called chromosomes. ics today, it is infinitely better than it was before GENETICS: The study of hereditary 1921. That was the year when the Canadians traits passed down from one generation to Frederick Banting (1891–1941), a surgeon, and Charles Herbert Best (1899–1978), a physiolo- the next through the genes. gist, isolated insulin. Thanks to their work, and GLAND: A cell or group of cells that fil- the subsequent development of insulin thera- ters material from the blood, processes that py—typically using insulin harvested from cows or pigs—deaths from ketoacidosis and diabetic material, and secretes it either for use again coma declined. A person with type 1 diabetes, in the body or to be eliminated as waste. formerly consigned to a dramatically shortened INTRINSIC: A term for a disease that is life of misery, could hope to have something not communicable or contagious and approaching a normal existence. Today diabetes remains a life-shortening illness, and doctors and comes from inside the body. Compare with scientists continue to search for a cure, but ever extrinsic. since 1921, the lot of those with diabetes has been improving steadily. (Banting was later knighted and shared the 1923 Nobel Prize in physiology and medicine for his achievement. Best, however, American Institute for Cancer Research (Web site). in one of history’s great snubs, received neither
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 243 set_vol3_8 8/19/02 11:14 AM Page 244
Infectious Diseases INFECTIOUS DISEASES
CONCEPT disease and the means of fighting these microor- The history of the human species, it has been ganisms are discussed in Infection. Among the said, is the history of infectious disease. Over the leading figures in that history were the French centuries, humans have been exposed to a vast chemist and microbiologist Louis Pasteur amount and array of contagious conditions, (1822–1895) and the German bacteriologist including the Black Death and other forms of Robert Koch (1843–1910), who contributed plague, typhoid fever, cholera, malaria, influenza, greatly to what is known today as germ theory— and the acquired immunodeficiency syndrome, the idea that infection and infectious diseases are or AIDS. Only in the past few hundred years have brought about by microorganisms. In most cases, scientists begun to have any sort of accurate idea the organisms are too small to be seen with the concerning the origin of such diseases, through naked eye. They include varieties of amoeba and the action of microorganisms and other para- worm, discussed in the essay Parasites and Para- sites. Such understanding has led to the develop- sitology, as well as viruses and some forms of ment of vaccines and methods of inoculation, yet bacteria and fungi, which together are known as even before they made these great strides in med- pathogens, or disease-carrying parasites. Other icine, humans had an unseen protector: their terms related to infectious diseases, their agents, own immune systems. and the prevention and study of them are defined in the essay Infection. HOW IT WORKS IMMUNE MECHANISMS. The human body has numerous mechanisms for pro- Infection and Immunity tecting itself from infectious disease, the first line of defense being the skin. Skin shields us all the There are two basic types of disease: ones that are time from unseen attackers and generally is able infectious, or extrinsic, meaning that they are to prevent pathogens from entering the body; contagious or communicable and can be spread however, any break in the skin, such as a cut or by contact between people, and ones that are scrape, provides an opening for microorganisms intrinsic, or not infectious. Diseases in general to invade the body. Germs that normally would and noninfectious diseases in particular are dis- be prevented from entering the body are able to cussed in essays devoted to those subjects. So, invade the bloodstream through such openings. too, is infection itself, a subject separate from This is why it is so very important, in any situa- infectious diseases: a person can get an infection, tion involving potential contact with infection, to such as tetanus or salmonella, without necessari- protect the skin. With the advent of AIDS, doc- ly having a disease that can be passed on through tors and members of other professions who are contact with others in the same way that colds, likely to touch people carrying diseases—includ- malaria, or syphilis is spread. ing officers arresting addicts or prostitutes—are The background on scientists’ progressive much more likely to do their work wearing heavy understanding of the microorganisms that cause plastic gloves.
244 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 245
Suppose that a microorganism makes it Cholera, caused by a bacterium found in Infectious through the barrier of skin, thanks to a cut or dirty wells and rivers from India to England (in Diseases other opening; if so, the body puts into action a the 1800s, at least), is an example of a waterborne second defensive mechanism, the immune sys- disease. Many foodborne pathogens tend to tem. This system is a network of organs, glands, bring about what would be more commonly and tissues that protects the body from foreign thought of as an illness than a disease, since in substances. Without a properly functioning everyday language the latter term implies a long- immune system, a person could die simply by term affliction, whereas food poisoning usually walking out the door in the morning and coming lasts for a week or so. (Still, some forms of food into contact with an airborne infectant. Even in poisoning can be fatal.) Bacterial contamination relatively healthy people, the immune system may may occur when food is not cooked thoroughly, be unable to react adequately to an invasion of is left unrefrigerated, is prepared by an infected microorganisms. In such cases, disease develops. food handler, or otherwise is handled in an unsanitary or improper fashion. (The case of Transmission of Diseases Typhoid Mary, discussed near the conclusion of this essay, is an extreme example of this form of Infectious diseases, by definition, are transmitted transmission.) easily from one person to another. We have all Additionally, diseases may be transferred by been told, for instance, not to drink after some- vectors—animals (usually insects) that carry one who has a cold. On a much more serious microorganisms from one person to another. Vec- level, persons who are sexually active or poten- tors may spread a disease either by mechanical or tially sexually active, but not settled in a monog- by biological means. Mechanical transmission amous (one-partner) relationship, are advised to occurs, for example, when flies transfer the germs avoid unprotected sexual contact so as not to for typhoid fever from the feces (stool) of infect- contract AIDS or some other sexually transmit- ed people to food eaten by healthy people. Bio- ted disease (STD). In these and many other cases, logical transmission takes place when an insect microorganisms travel from the carrier of the bites a person and takes infected blood into its disease to the uninfected person. (Actually, in the own system. Once inside the insect’s gut, the dis- case of AIDS, the pathogen is a virus, which is ease-causing organisms may reproduce, increas- not, strictly speaking, an organism or even a liv- ing the number of parasites that can be transmit- ing thing; however, viruses usually are lumped in ted to the next victim. This is how the Anopheles with bacteria, amoeba, and some fungi as mosquito vector, for instance, transfers malaria. microorganisms.) Pathogens can be spread by many methods REAL-LIFE other than direct contact, including through APPLICATIONS water, food, air, and bodily fluids—blood, semen, saliva, and so on. For instance, any time a person A Tour of Diseases with an infection coughs or sneezes, they may be transmitting illness. This is how diseases such as The range of infectious diseases, from conditions measles and tuberculosis are passed from person that merely cause discomfort to those that bring to person. AIDS and various STDs, as well as about death, is truly staggering. Some have many other conditions, such as hepatitis, are brought about vast epidemics that have wiped transferred when one person comes into contact out huge populations, and many have changed with the bodily fluids of another. This is the case the course of history, while others are hardly not only with sexual intercourse but also with known to anyone outside the ranks of epidemiol- blood transfusions and any number of other ogists and the victims of the disease. Some, such interactions, including possibly drinking after as smallpox, have been eradicated or largely erad- someone. (Contrary to rumors that circulated in icated through inoculation campaigns, while the early 1980s, when AIDS first made itself others, most notably AIDS, continue to elude known, that particular syndrome cannot be efforts to defeat them. transferred by saliva, but the common cold and Diseases can be classified according to the other viral infections can be.) systems or body parts affected. Some of those
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 245 set_vol3_8 8/19/02 11:14 AM Page 246
Infectious systems and parts, with examples of diseases in Byzantium (Constantinople) this realm Diseases relating to each, include the following. became known to history as the Byzantine (East- • Upper respiratory tract: common cold, ern Roman), Empire, though its citizens saw sinusitis, croup themselves simply as “Romans” and thus as the • Lower respiratory tract: pneumonia, bron- inheritors of Roman civilization. Italy itself had chitis fallen under the control of nomadic invaders, the • Cardiovascular system: rheumatic fever Visigoths, but Emperor Justinian I (483–565) undertook a vast and costly campaign to wrest • Central nervous system: meningitis, control of the Italian peninsula from the barbar- encephalitis ians. Had he succeeded, the entire course of • Genitourinary tract: sexually transmitted medieval history in Western Europe might have diseases (i.e., venereal diseases, such as been different; he did not, largely because of a syphilis, gonorrhea, and the herpes simplex plague that swept Constantinople in 541. viral infection) • Gastrointestinal tract: cholera, salmonella, Through a series of interconnected events, hepatitis the plague permanently weakened Byzantium • Bones and joints: septic arthritis and left the Mediterranean world ripe for con- • Skin: warts, candida quest by a new power: Islam. Both directly and • Eyes: conjunctivitis (pink eye) indirectly, the plague of 541 served to divide Eastern and Western Europe. Not only was the Another way to classify diseases is according Roman Empire never truly reunited, meaning to the types of organism that cause them: bacte- that the two halves of the continent grew increas- ria, viruses, or other forms of parasite, particu- ingly separate, but the rise of Islam made possi- larly worms, amoeba, and insects. The first two ble the Crusades (1095–1291). The latter sowed groups are discussed in further detail within Infection and the other varieties of parasite in further discord between the East and the West, Parasites and Parasitology. owing to the fact that Western European cru- saders overran Byzantium and incited trouble Bacterial infections include anthrax, botu- between the Byzantines and Arabs. Ultimately, lism, tetanus (lockjaw), leprosy, tuberculosis, the split between Eastern and Western Europe, diphtheria, whooping cough, plague, and a vari- which became particularly pronounced during ety of pneumococcal, staphylococcal, and strep- the years of Communism and the Iron Curtain tococcal illnesses. Among viral illnesses and dis- (1945–1990), can be traced to the plague of 541. eases are the common cold, influenza, infectious mononucleosis, smallpox, chicken pox, measles, The Black Death mumps, rubella (or German measles), yellow fever, poliomyelitis (i.e., polio), rabies, herpes The Byzantine plagues (there were several, occur- simplex, and AIDS. Diseases related to other ring at intervals of a few generations), killed mil- varieties of parasite include malaria, Rocky lions of people, yet for sheer scope of destruc- Mountain spotted fever, trichinosis, scabies, and tion—and, perhaps, historical impact—they river blindness. Nonmicroscopic parasites, par- were dwarfed by the plague that devastated ticularly such worms as hookworm and pin- Europe in the years 1347–1351. This one became worm, bring about disease-like forms of parasitic known as the Plague (with a capital P) or by infestation within the body. another name that gave some hint of the terror that was as much a part of the epidemic as the Plagues ghastly physical symptoms it brought on: the From earliest times infectious diseases have Black Death. wreaked havoc on the human species, and this It began in Asia and quickly made its way to was particularly so with the various plagues that the shores of the Black Sea, where it erupted in struck Europe in ancient and medieval times. As September 1346. The first outbreak in Western noted in Infection, a plague in the fifth century Europe occurred 13 months later, in October B.C. helped bring an end to the golden age of 1347, at the Sicilian port of Messina, from Greek civilization. A thousand years later, anoth- whence it was an easy jump to the Italian main- er plague befell Greece, which by then dominat- land. By the following April all of Italy was infect- ed what remained of the Roman Empire. Based ed; meanwhile, the Plague had reached Paris in
246 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:14 AM Page 247
Infectious Diseases
A POLITICAL CARTOON FROM ABOUT 1870 ILLUSTRATES THE UNSANITARY CONDITIONS IN NEW YORK CITY, WITH THE POLITICIAN AND PUBLIC WORKS COMMISSIONER BOSS TWEED WELCOMING A CHOLERA EPIDEMIC. (© Bettmann/Corbis. Reproduced by permission.)
January 1348, and within a year, 800 people a day black from respiratory failure—hence the name were dying in that city alone. Quickly it penetrat- Black Death. ed the entire European continent and beyond, SOCIAL IMPACT OF THE from North Africa to Scandinavia and from Eng- PLAGUE. Lacking any modern concept of land to the hinterlands of Russia. By 1351 it had what causes disease, people looked for spiritual spread so far and wide that sailors arriving in explanations. Some believed that the world was Greenland found its ports deserted. coming to an end, while others joined sects of fla- gellants, religious enthusiasts who wandered the The only merciful thing about the Black countryside, beating themselves with lashes as a Death was that death came quickly. Victims typi- way of doing penance. The flagellants were tied cally died within four days—a hundred hours of closely tied to a rising trend toward anti-Semi- agony. If they caught a strain of bubonic plague, tism: searching for someone to blame, Europeans their lymph glands swelled; if it was pneumonic found a convenient scapegoat in the Jews, who, plague, the lungs succumbed first. Either way, as they claimed, had started the Plague by poison- the end approached, the victim turned purplish- ing the wells of Europe.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 247 set_vol3_8 8/19/02 11:14 AM Page 248
Infectious The Black Death aptly illustrates how infec- touching lepers, it is not very contagious. A scene Diseases tious diseases can have an impact on history in in the 1973 blockbuster Papillon illustrates this ways both big and small. In just five years the dis- fact. The title character, a prison escapee played ease killed about 30% of Europe’s population, by Steve McQueen, takes a drag from a cigar which had been 100 million in 1300 but which offered to him by a leper, who then asks him if he would not reach that level again until 1500. All knew that leprosy is not contagious. Papillon says over the continent, farms were emptied and vil- no, indicating that he simply intended to build a lages abandoned, leading to scarcity and higher sense of shared risk with someone who he hoped prices. In the short run, these economic condi- would aid his escape. tions spurred peasant revolts, but in the long run, The example of leprosy shows something the shortage of workers brought about higher about the many curiosities involved in diseases wages and contributed to the emergence of the and their study: for example, the fact that a dis- working and middle classes. Neither popes nor ease can be infectious without being significantly priests, neither kings nor noblemen, were any contagious. Leprosy is by definition infectious, more equipped than the common people to con- inasmuch as it is caused by a pathogen known as front the fearsome disease, and this, too, helped Mycobacterium leprae, but the latter is unusual provoke the rise of competing classes and new for a number of reasons, including the fact that it centers of power in European society. is extremely slow in dividing, unlike most bacte- THE ETIOLOGY OF THE ria. After years of study, researchers are still not PLAGUE. The Black Death, in short, may be clear as to how leprosy is transmitted, and many regarded as the beginning of the end of the Mid- believe that genetics may play a role. Thanks to dle Ages—a hideously painful event that never- increased understanding of the disease, the stig- theless carried positive consequences, which ma that used to go with leprosy—including the might hardly have been achieved without it. The reference to people with the disease as “lepers”— irony was that the force at the center of all this has largely been lifted. Yet places such as the lep- devastation and change was too small to be seen rosy facilities at Carville, Louisiana, and Molokai, by the naked eye. Although the disease was car- Hawaii, continued to exist for many years, if only ried by rats, the cause of the Black Death was because the disfigurement associated with the actually a bacillus known today as Pastuerella disease influenced the separation of leprosy suf- pestis or Yersinia pestis, which uses fleas as a vec- ferers from the rest of society. In 1998, with only tor. Modern medicines such as streptomycin, a about 6,000 victims of the disease left in the variety of antibiotic developed after World War entire country, the federal government closed the II, would have stopped the Plague, but such con- facilities at Carville and Molokai. cepts were a long time in coming. Although the Leprosy remains a threat, with some two worst phase of the epidemic ended in 1351, it million cases of the disease worldwide, primarily continued to spread, reaching Moscow by 1353; in nations of Asia, Africa, and Latin America that the next five centuries saw occasional outbreaks are both underdeveloped and located in tropical of the disease. As late as 1894 a strain of plague zones. It has, however, ceased to be the worldwide killed more than six million people in Asia over danger that it once was, and as such it joins ranks the course of 14 years. with numerous other afflictions that formerly The Changing Face of held all of humankind in the grip of terror. For Disease example, tuberculosis, caused by a bacillus that attacks the lungs, afflicted a huge population in The many biblical passages dealing with leprosy the nineteenth century, bringing an end to the illustrate the role that infectious disease has careers of figures that ranged from the great Eng- played in human life from the earliest times. The lish poet John Keats to the American gunslinger fact that leprosy causes the victim’s skin to turn Doc Holliday. Holliday, in fact, traveled to Tomb- ghostly white and brings about a gradual wither- stone, Arizona, where he and Wyatt Earp partici- ing away of body parts must certainly have pated in the infamous shootout at the O.K. Cor- seemed like a curse from God. In fact, leprosy, ral, because he thought the climate would help also known as Hansen disease, is caused by the his condition. Their story has been portrayed in bacillus Mycobacterium leprae, and despite the countless films; for example, in Tombstone many fears throughout the ages associated with (1993), Val Kilmer gives an extremely convincing
248 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:15 AM Page 249
Infectious Diseases
LEPROSY, CAUSED BY THE BACILLUS MYCOBACTERIUM LEPRAE, brings about a gradual withering away of body parts. There are some two million cases of the disease worldwide, primarily in the underdeveloped nations of Asia, Africa, and Latin America. (© Paul A. Souders/Corbis. Reproduced by permis- sion.)
portrayal of the debilitating effects that Holli- which once posed an enormous threat to Ameri- day’s tuberculosis (aggravated by his lifestyle) can children and crippled one of America’s great- must have had on him. Today, tuberculosis is not est leaders, President Franklin D. Roosevelt, is an nearly the scourge that it once was, though it artifact of history, thanks to vaccines developed remains a problem, particularly because of after World War II. patients’ increasing resistance to the antibiotics Yet some killers never really die. For instance, used to treat it. (See Infection for more about malaria, caused by a protozoan parasitic genus antibiotics.) known as Plasmodium and spread by mosquito VACCINATION AND CONTINU- biological vectors, infects from 300 to 500 million ING THREATS. When Europeans invaded people annually and kills up to 2.7 million people the lands of Native Americans, they brought with every year. Although the substance known as qui- them a host of microorganisms to which they nine showed some promise as a treatment during had developed an immunity but to which the most of the nineteenth and twentieth centuries, Indians were completely vulnerable. Although Plasmodium has become increasingly resistant to Europeans and their descendants had developed it. In the search for a cure for what has been called immunities to various diseases, thanks to gener- “the most devastating disease in history,” some ations of exposure to pathogens, they and the rest 100,000 drugs have been tested. of the world remained vulnerable to a host of contagious disease, including cholera, smallpox, Some Other Killers chicken pox, measles, mumps, yellow fever, polio, malaria, and many others. Today, vaccines have The twentieth century saw its own version of the virtually eradicated many of these contagious Plague, in the form of the 1918–1920 influenza diseases and keep others at bay. (Anyone who has epidemic. Carried to all corners of the globe by ever had a cholera vaccine, which causes the soldiers returning from World War I, “the patient’s body to become miserably sore, achy, Influenza,” as it came to be known (again with a and tender for about 48 hours, has some idea of capital letter to distinguish it as the greatest out- just how awful the disease itself must be.) Polio, break of a particular disease), killed 20 million
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 249 set_vol3_8 8/19/02 11:15 AM Page 250
Infectious AIDS. (For more about AIDS, see Immunity and Diseases Immunology.) THE EBOLA VIRUS. AIDS was not the only infectious condition to come out of cen- tral Africa and terrorize the world in the late twentieth century. Beginning in about 1975, numerous viruses, previously unknown and ter- rifyingly lethal, emerged from tropical regions of Africa, South America, and Asia. So great was the rise of new infectious diseases that some epi- demiologists believed this was tied with econom- ic development: as humans cultivated previously undeveloped lands and delved into more isolated parts of the world, they might be exposing new viruses. Few of these inspired as much terror as the Ebola virus, and the fear is understandable, given the effects of the disease. Three to nine days after the illness enters the body, the victim begins to experience fever and other flu-like symptoms, sudden exhaustion, sore throat, muscle pain, and FOLLOWING THE SEPTEMBER 11, 2001, TERRORIST headache. Vomiting and diarrhea soon follow, ATTACKS ON THE UNITED STATES, A SERIES OF LETTERS CONTAINING ANTHRAX SPORES SHOWED UP AROUND and the vomit and stools are black with blood. THE COUNTRY, AND EXPOSURE TO THE DISEASE LED TO Soon hemorrhaging occurs, with blood flowing A HANDFUL OF DEATHS. IN ONE INCIDENT HAZARDOUS from the nose, ears, and even the eyes. Internal MATERIALS EXPERTS WERE CALLED TO CAPITOL HILL TO organs begin to liquefy, and within three weeks of INVESTIGATE. (© AFP/Corbis. Reproduced by permission.) contracting the virus, the victim is usually dead. An almost unbelievably hideous condition, people—more than the war itself. Then there is Ebola might seem at first glance a great deal like the greatest epidemic of the latter part of the the Black Death. Why, then, has it not ravaged twentieth century and the early twenty-first cen- whole populations the way the Plague did? It is tury: AIDS. This disease is linked to the human certainly not because scientists have a cure for immunodeficiency virus (HIV), a retrovirus (see Ebola; the best doctors can hope to do, if they Infection for an explanation of retrovirus) that detect the disease early enough, is to provide sup- causes a gradual breakdown of the victim’s portive care, such as blood transfusions, that may immune system. save the patient’s life. Yet even the worst out- People do not die of AIDS per se but of the breaks of the disease have not occurred on any- illnesses—particularly pneumonia or Kaposi’s thing like the scale of the Plague: the worst sarcoma, a cancer of the tissues—to which AIDS known outbreak of Ebola, in Uganda in makes them susceptible. The disease is transmit- 2000–2001, killed 425 people. ted primarily by sexual contact and intravenous Part of the reason Ebola is not capable of drug use. A smaller number of particularly trag- spreading rapidly is, ironically, because it is such ic cases result from no actions on the part of the an efficient killer: it kills its human victims before victim, who in this case is either the recipient of they have a chance to spread it to many other vic- infected blood or the child of a mother with tims. Other than nonfatal incidents in laborato- AIDS. Since the disease first came to public atten- ries in the United States, England, and Italy, as tion in 1981, 21.8 million people worldwide (and well as one case in a monkey export facility in the about 750,000 in the United States) have died Philippines—various primates are carriers—all from it. The vast majority of deaths have been in Ebola cases and outbreaks have been in Africa, sub-Saharan Africa, and 90% of all AIDS cases primarily in Zaire (now Democratic Republic of are in developing countries. Worldwide, approx- the Congo), Sudan, and Gabon. Many times, imately 36.1 million people have either HIV or local conditions, situations, and practices have
250 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_8 8/19/02 11:15 AM Page 251
exacerbated the spread of the disease. For exam- Infectious ple, in 1996, a group of people in Gabon found a KEY TERMS Diseases dead chimpanzee in the forest and ate it; as a result, 37 people died. The Uganda outbreak became much worse than it might have been EXTRINSIC: A term for a disease that because locals, lacking education as to antiseptic is communicable or contagious and comes procedures, failed to take proper precautions. from outside the body. Compare with Many died as a result of attending funerals of intrinsic. Ebola victims at which bodies were not disposed of properly. GERM THEORY: A theory in medi- TYPHOID MARY. Sometimes a single cine, widely accepted today, that infections, person can be a walking epidemic, as in the case contagious diseases, and other conditions of the Irish cook Mary Mallon (1869–1938), bet- are caused by the actions of microorgan- ter known as “Typhoid Mary.” Mallon was an isms. example of the fact that some people, because of genetic characteristics or other specifics, can act IMMUNE SYSTEM: A network of as carriers of a disease without ever contracting it organs, glands, and tissues that protects the themselves. Even though Typhoid Mary had Sal- body from foreign substances. monella typhosa bacteria in her system, she did IMMUNITY: The condition of being not get sick; still, she was highly contagious, and her profession as cook made her particularly able to resist a specific disease, particularly dangerous. At least three deaths and 53 cases of through means that prevent the growth typhoid fever were linked directly to her, with and development or counteract the effects thousands of other probable cases of infection of pathogens. indirectly caused by this human vector. INFECTION: A state or condition in Part of what made her so notorious—hence her nickname, given to her by the press—was the which parasitic organisms attach them- fact that Mallon did not seem to care how many selves to the body or to the inside of the people she infected. In the first decade of the body of another organism, causing con- twentieth century, authorities tracked her down tamination and disease in the host. as the cause of, or at least a contributing factor in, INTRINSIC: an outbreak of typhoid in the New York City A term for a disease that is area. Instead of cooperating with officials, Mal- not communicable or contagious and lon repeatedly escaped before being caught and comes from inside the body. Compare with confined to Riverside Hospital on New York’s extrinsic. North Brother Island in 1910. She served three years in isolation there before her release, after PATHOGEN: A disease-carrying para- which she promptly went back to work as a site, usually a microorganism. cook—despite explicit orders not to do so. It was STD: Sexually transmitted disease. this (and an outbreak of typhoid fever at her place of work, which happened to be a hospital) VECTOR: An organism, such as an that earned her the nickname by which she insect, that transmits a pathogen to the became known to history. She was caught again body of a host. in 1915 and spent the remainder of her life on North Brother Island. THE THREAT OF BIOLOGICAL WARFARE. Infinitely more despicable than other grazing animals. Under the right circum- Typhoid Mary are terrorists and rogue nations stances, anthrax can kill a human in about 36 that would willingly unleash infectious disease hours, though a number of antibiotic treatments on large, unsuspecting civilian populations. One are effective in the early stages of the disease. such pathogen is Bacillus anthracis, the cause of During the late twentieth century, the Unit- anthrax, a deadly bacterial disease of cattle and ed States and Soviet Union experimented with
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 251 set_vol3_8 8/19/02 11:15 AM Page 252
Infectious the use of anthrax in biological warfare, and an rikyo attempted unsuccessfully to launch botu- Diseases accidental release of anthrax spores at a Soviet lism attacks in Tokyo on three occasions in 1995. lab in 1979 led to some 68 deaths. Following the The Japanese government itself—that is, the Axis September 11, 2001, terrorist attacks on the Japanese government of World War II—experi- World Trade Center in New York City and on the mented with another biological agent, tularemia, Pentagon, a series of letters containing anthrax or Francisella tularensis. The pathogen, which spores showed up around the United States, and causes lung inflammation and death, is consid- exposure to the disease led to a handful of deaths. ered one of the most dangerous forms of biolog- Although the attacks were linked initially to ical weapon, because it is extremely efficient and Osama bin Laden and his al-Qaeda organization, easy to spread. America’s military, borrowing an authorities increasingly began to suspect that idea from its former enemy, developed its own F. home-grown terrorists were simply exploiting tularensis strain in the late 1960s but destroyed its the September 11 attacks as cover for their own stockpile in 1973. deeds. Still, there was little doubt that bin Laden, WHERE TO LEARN MORE the Iraqi dictator Saddam Hussein, or North Centers for Disease Control and Prevention (Web site). Korea’s ruling clique would use biological agents
252 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 253
SCIENCE OF EVERYDAY THINGS real-life biology IMMUNITY
IMMUNITY AND IMMUNOLOGY THE IMMUNE SYSTEM
253 set_vol3_9 8/19/02 11:17 AM Page 255
IMMUNITY AND
Immunity and Immunology IMMUNOLOGY
CONCEPT type of white blood cell. The principal types of Immunity is the condition of being able to resist lymphocyte are B cells and T cells. These cells a specific disease, particularly through means recognize random antigens, or substances capa- that prevent the growth and development of dis- ble of requiring an immune response. ease-carrying organisms or counteract their Certain researchers believe that while some effects. It is regulated by the immune system, a B cells and T cells are directed toward fighting an network of organs, glands, and tissues that pro- infection, others remain in the bloodstream for tects the body from foreign substances. months or even years, primed to respond to Immunology is the study of the immune system, another invasion of the body. Such “memory” immunity, and immune responses. Progress in cells may be the basis for immunities that allow immunology over the past two centuries has humans to survive such plagues as the Black made inoculation—the prevention of a disease Death of 1347–1351 (see Infectious Diseases). by the introduction to the body, in small quanti- Other immunologists, however, maintain that ties, of the virus or other microorganism that trace amounts of a pathogen persist in the body causes the disease—widely accepted and prac- and that their continued presence keeps the ticed. Despite such progress, however, some dis- immune response strong over time. eases evade human efforts to counteract them through medicine or other forms of treatment. Immunology This is particularly the case with a disease in which the immune system shuts down entirely: a Immunology is the study of how the body condition known as acquired immunodeficiency responds to foreign substances and fights off syndrome, or AIDS. infection and other disease-causing agents. Immunologists are concerned with the parts of the body that participate in this response, and HOW IT WORKS this investigation takes them beyond looking merely at tissues and organs to studying specific Immunity and the Immune types of cells or even molecules. System From ancient times, humans have recog- The functioning of the immune system is consid- nized that some people survive epidemics, when ered in a separate essay, along with the means by the majority are dying. About 1,500 years ago in which that system responds to foreign invasion. India, physicians even practiced a form of inocu- Also included in that essay is a discussion of lation, as we discuss later. The modern science of allergies, which arise when the body responds to immunology, however, had its beginnings only in ordinary substances as though they were 1798, when the English physician Edward Jenner pathogens, or disease-carrying parasites. The (1749–1823) published a paper in which he body cannot know in advance what a pathogen maintained that people could be protected from will look like and how to fight it, so it creates mil- the deadly disease smallpox by the prick of a nee- lions and millions of different lymphocytes, a dle dipped in the pus from a cowpox boil. (Cow-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 255 set_vol3_9 8/19/02 11:17 AM Page 256
Immunity pox is a related, less-lethal disease that, as its all was their method of treating smallpox, which and name suggests, primarily affects cattle.) remained one of the world’s most deadly diseases Immunology Later, the great French biologist and chemist until its eradication in the late 1970s. Indian Louis Pasteur (1822–1895) theorized that inocu- physicians apparently took pus or scabs from the lation protects people against disease by exposing sores of a mildly infected patient and rubbed the them to a version of the pathogen that is harm- material into a small cut made in the skin of a less enough not to kill them but sufficiently like healthy person. The Indians’ method was risky, the disease-causing organism that the immune and there was always a chance that the patient system learns to fight it. Modern vaccines against would become deathly ill, but the idea survived such diseases as measles, polio, and chicken pox and gradually made its way west over the ensuing are based on this principle. centuries. HUMORAL AND CELLULAR SMALLPOX VACCINATION. IMMUNITY. In the late nineteenth century, a Smallpox, or variola, is carried by a virus that scientific debate raged between the German causes the victim’s body to break out in erupting, physician Paul Ehrlich (1854–1915) and the pus-filled sores. Eventually, these sores dry up, Russian zoologist Élie Metchnikoff (1845–1916) leaving behind scars that may alter the appear- concerning the means by which the body pro- ance of the victim permanently, depending on tects against diseases. Ehrlich and his followers the intensity of the disease. Such was the case maintained that proteins in the blood, called with Lady Mary Wortley Montagu (1689–1762), antibodies, eliminate pathogens by sticking to a celebrated English writer and noblewoman. them. This phenomenon and the theory sur- Known for her passionate relationships, roman- rounding it became known as humoral immuni- tic and otherwise, Lady Montagu had been ty. Metchnikoff and his students, on the other scarred from youth by smallpox, and no doubt hand, had noted that certain white blood cells this experience gave her heightened concern for could swallow and digest foreign materials. This the victims of the disease. While she was in cellular immunity, they claimed, was the real way Turkey with her husband, Edward, an ambassa- that the body fights infection. In fact, as modern dor, she became aware of an inoculation method, immunologists have shown, both the humoral probably based on the Hindu practice of many and cellular responses identified by Ehrlich and centuries before, used by local women. Lady Metchnikoff, respectively, play a role in fighting Montagu arranged for her three-year-old son to disease. be inoculated against smallpox in 1717, and after returning home, initiated smallpox inoculations in England. REAL-LIFE APPLICATIONS Nonetheless, the problem remained that the inoculated person contracted a serious case of Inoculation and Vaccines the disease and died, at least some of the time. More than 80 years later, in 1796, during a small- Inoculation is the prevention of a disease by the pox epidemic, Jenner decided to test a piece of introduction to the body, in small quantities, of folk wisdom to the effect that anyone who con- the virus or other microorganism that causes tracted cowpox became immune to human that particular ailment. It is a brilliant idea, yet smallpox. He took cowpox fluid from the sores of one that seems to go against common sense. For a milkmaid named Sarah Nelmes and rubbed it that reason, it was a long time in coming: not into cuts on the arm of an eight-year-old boy, until the time of Jenner, in about 1800, did the James Phipps, who promptly came down with a concept of inoculation become widely accepted mild case of cowpox. Soon, however, James in the West. Nonetheless, it had been applied recovered, and six weeks later, when Jenner more than 13 centuries earlier in India. injected him with samples of the smallpox virus, In the period between about 500 B.C. and the boy was unaffected. A.D. 500, Hindu physicians made extraordinary Jenner, who published his findings after con- strides in a number of areas, pioneering such ducting additional tests, coined a new term for techniques as plastic surgery and the use of the type of inoculation he had used: vaccination, tourniquets to stop bleeding. Most impressive of from the Latin word for cowpox, vaccinia. (The
256 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 257
Immunity and Immunology
THE MODERN SCIENCE OF IMMUNOLOGY HAD ITS BEGINNINGS IN 1798, WHEN THE ENGLISH PHYSICIAN EDWARD JEN- NER PUBLISHED A PAPER IN WHICH HE MAINTAINED THAT PEOPLE COULD BE PROTECTED FROM THE DEADLY DISEASE SMALLPOX BY THE PRICK OF A NEEDLE DIPPED IN THE PUS FROM A COWPOX BOIL. (© Bettmann/Corbis. Reproduced by per- mission.)
latter term comes from the Latin vacca, or “cow,” for poliomyelitis (more commonly known as the source of such terms as the French vache.) polio), in which the skeletal muscles waste away With the success of his vaccine, Jenner was and paralysis and often permanent disability and awarded a sum of money to continue his work, deformity ensue. Although polio had been and he soon oversaw the vaccination of thou- known for ages, the first half of the twentieth sands of English citizens, including the royal century had seen an enormous epidemic in the family. The practice spread to Germany and Rus- United States. sia and then to the United States. In Lady Mon- The most famous victim of this scourge was tagu’s time, the American clergyman Cotton the future president Franklin D. Roosevelt Mather (1663–1728) had been an advocate of (1882–1945), who contracted it while on vaca- vaccination, and now President Thomas Jeffer- tion in 1921. Throughout the 1930s and 1940s, son (1743–1826) became an ardent proponent of polio remained a threat, especially to children; at Jenner’s methods. the peak of the epidemic, in 1952, it killed some RABIES AND POLIO INOCULA- 3,000 Americans in one year, while 58,000 new cases were reported. At the same time, Salk was TION. The next advancement in the study of working on his vaccine, which finally was vaccines came almost 100 years after Jenner’s dis- declared safe after massive testing on school- covery. In 1885 Pasteur saved the life of Joseph children. In 1961 an oral polio vaccine developed Meister, a nine-year-old boy who had been by the Polish-born American virologist Albert attacked by a rabid dog, by using a series of Sabin (1906–1993) was licensed in the United experimental rabies vaccinations. Pasteur’s rabies States. Whereas the Salk vaccine contained the vaccine, the first human vaccine created in a lab- killed versions of the three types of poliovirus oratory, was made from a version of the live virus that had been identified in the 1940s, the Sabin that had been weakened by drying it over potash vaccine used weakened live poliovirus. Because it (sodium carbonate—burnt wood ashes). was taken by mouth, the Sabin vaccine proved Exactly 70 years later, the American microbi- more convenient and less expensive to adminis- ologist Jonas Salk (1914–1995) created a vaccine ter than the Salk vaccine, and it soon overtook
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 257 set_vol3_9 8/19/02 11:17 AM Page 258
Immunity the latter in popularity. By the early 1990s health even worse, babies whose AIDS-infected mothers and organizations reported that polio was close to have passed the disease on to them. Immunology extinction in the Western Hemisphere. Within two to four weeks of being infected TRIUMPHS AND CONTINUING with the virus that causes AIDS (HIV, human CHALLENGES. Thanks to these and other immunodeficiency virus), a patient will experi- vaccines, many life-threatening infectious dis- ence what at first seems like flu: high fever, eases have been forced into retreat. In the United headaches, sore throat, muscle and joint pains, States, children starting kindergarten typically nausea and vomiting, open ulcers in the mouth, immunized against polio, diphtheria, tetanus, swollen lymph nodes, and perhaps a rash. As the measles, and several other diseases. Other vacci- immune system begins to fight the invasion, nations are used only by people who are at risk of some cells produce antibodies to neutralize the contracting a disease, are exposed to a disease, or viruses that are floating free in the bloodstream. are traveling to an area (usually in the Third Killer T cells destroy many other cells infected World) where particular diseases are common. with the AIDS virus, and the patient enters a Such vaccinations include those for influenza, phase of the disease in which no symptoms are yellow fever, typhoid, cholera, and hepatitis A. evident. Internationally, 80% of the world’s children Although at this point it seems as though the had been inoculated as of 1990 for six of the pri- worst is over, in fact, the AIDS virus is at work on mary infectious diseases: polio, whooping cough, the immune system, quietly destroying the measles, tetanus, diphtheria, and tuberculosis. body’s protection by infecting those T cells that Smallpox was no longer on the list, because would protect it. With an immune system that efforts against it had proved overwhelmingly gradually becomes more and more unresponsive, successful. (See Infectious Diseases for more on the patient is made vulnerable to any number of the threat, or nonthreat, of smallpox as a form of infections. Normally, the body would be able to biological warfare.) Despite these successes, how- fight off these attacks with ease, but with the ever, each year more than two million children immune system itself no longer functioning who have not received any vaccinations die of properly, infectious diseases and cancers are free infectious diseases. Even polio has continued to to take over. The result is a long period of be a threat in some parts of the world: as many as increasing misery and suffering, sometimes 120,000 cases are reported around the world each accompanied by dementia or mental deteriora- year, most in developing regions. And as if the tion caused by the ravaging of the brain by dis- threat from age-old diseases were not enough, in ease. Whatever the course it takes, the end result the last quarter of the twentieth century a new of AIDS is always the same: not just death but a killer entered the fray: AIDS. miserable, excruciatingly painful death. BIRTH OF A KILLER. AIDS Believed to have originated in Africa, where the majority of A viral disease that is almost invariably fatal, AIDS cases still are found (see Infectious Dis- AIDS destroys the immune systems of its victims, eases for statistics on AIDS), the disease first leaving them vulnerable to a variety of illnesses. appeared in the United States in 1981. In that No cure has been found and no vaccine ever year two patients were diagnosed with an unusu- developed. The virus that causes AIDS has al form of pneumonia and with Kaposi’s sarco- proved to be one of the most elusive pathogens in ma, a type of cancer that previously had struck history, and so far the only effective way not to only people of Mediterranean origin aged 60 contract the disease is to avoid sharing bodily years and older. The appearance of that condi- fluid with anyone who has it. This means not tion in younger persons of non-Mediterranean having sex without condoms (and, to be on the origin prompted an investigation by the United truly safe side, not having sex outside a commit- States Centers for Disease Control and Preven- ted, fully monogamous relationship) and not tion (CDC). engaging in intravenous drug use. But there are Through the efforts of physicians both some people who have contracted the AIDS virus inside and outside the CDC, understanding of through no actions or fault of their own: people AIDS—the name and acronym appeared in who have received it in blood transfusions or, 1982—gradually emerged. In 1983 scientists at
258 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 259
the Pasteur Institute in Paris, as well as a separate Immunity team in the United States, identified the virus and Immunology that causes AIDS, a pathogen that in 1986 was given the name human immunodeficiency virus (HIV). Further research showed that HIV, a retrovirus (see Infectious Diseases for an expla- nation of retrovirus), is subdivided into two types: HIV-1 and HIV-2. In people who have HIV-2, AIDS seems to take longer to develop; however, neither form of HIV carries with it a guarantee that a person will contract the disease. At first it was believed that if someone were HIV- positive, meaning that the person had the virus, it was a virtual death sentence. Therefore in 1991, when the basketball superstar Earvin “Magic” Johnson (1959–) announced that he was HIV- positive, it was an extremely melancholy event. Fans and admirers all over the world assumed that Johnson shortly would contract AIDS and begin to wither away in the process of suffering an exceedingly panful, dehumanizing death. The fact that Johnson was alive and healthy A VIRAL DISEASE THAT IS ALMOST INVARIABLY FATAL, AIDS DESTROYS THE IMMUNE SYSTEMS OF ITS VICTIMS, more than ten years after the diagnosis of his LEAVING THEM VULNERABLE TO A VARIETY OF ILLNESS- infection with HIV serves to indicate that there is ES, AMONG THEM, KAPOSI’S SARCOMA (SHOWN HERE), a great deal of difference between being HIV- A TYPE OF CANCER THAT BEFORE THE 1980S STRUCK MEDITERRANEAN ORIGIN AGED 60 positive and having AIDS. It also says much ONLY PEOPLE OF YEARS AND OLDER. (© Roger Ressmeyer/Corbis. Reproduced by about people’s emerging understanding of the disease and the virus that causes it. So, too, does Johnson’s experience as he attempted, twice, to small amount of satisfaction, that AIDS was make a return to the court after retiring in the God’s punishment for homosexuality. wake of his HIV announcement. Before examin- Then, during the mid-1980s, AIDS began ing his experiences, let us look at the social cli- spreading throughout much of society: to het- mate engendered by this politically volatile erosexuals, hemophiliacs (see Noninfectious Dis- immunodeficiency syndrome. eases) and others who received blood, and even CHANGING VIEWS ON AIDS. babies. The fact that AIDS could be transferred AIDS first was associated almost exclusively with through heterosexual intercourse proved that it the male homosexual community, which con- was not just a disease of homosexuals. Nor were tracted the disease in large numbers. This had a all homosexuals necessarily susceptible to it. great deal to do with the fact that male homosex- In fact, the safest of all sexual groups was homo- uals were apt to have far more sexual partners sexual women, who often tended toward than their heterosexual counterparts and because monogamy and whose form of sexual contact anal intercourse is more likely to involve bleeding was least invasive. and hence penetration of the skin shield that As AIDS spread throughout society, so did protects the body from infection. The association paranoia. Rumors circulated that a person could of AIDS with homosexuality led many who con- catch the disease from a mosquito bite or from sidered themselves part of the societal main- any contact with the bodily fluids of another per- stream to dismiss AIDS as a “gay disease,”and the son—not just semen or blood but even sweat or fact that intravenous drug users also contracted saliva. People with AIDS began to acquire the sta- the disease seemed only to confirm the prejudice tus lepers once had held (see Infectious Dis- that AIDS had nothing to do with heterosexual eases). By the mid-1990s views had changed con- non-junkies. Some so-called Christian ministers siderably, and society as a whole had a much even went so far as to assert, sometimes with no more realistic view of AIDS. This came about to
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 259 set_vol3_9 8/19/02 11:17 AM Page 260
Immunity and Immunology KEY TERMS
ALLERGY: A change in bodily reactivi- INOCULATION: The prevention of a ty to an antigen as a result of a first expo- disease by the introduction to the body, in sure. Allergies bring about an exaggerated small quantities, of the virus or other reaction to substances or physical states microorganism that causes the disease. that normally would have little significant LYMPHOCYTE: A type of white blood effect on a healthy person. cell, varieties of which include B cells and T ANTIBODIES: Proteins in the human cells, or B lymphocytes and T lymphocytes. immune system that help the body fight MACROPHAGE: A type of phagocytic foreign invaders, especially pathogens and cell derived from monocytes. toxins. MONOCYTE: A type of white blood cell ANTIGEN: A substance capable of stim- that phagocytizes (engulfs and digests) for- ulating an immune response or reaction. eign microorganisms. APC: An antigen-presenting cell—a MONOGAMOUS: macrophage that has ingested a foreign cell Having only one and displays the antigen on its surface. mate. B CELL: A type of white blood cell that PATHOGEN: A disease-carrying para- gives rise to antibodies. Also known as a B site, usually a microorganism. lymphocyte. PHAGOCYTE: A cell that engulfs and EPIDEMIC: Affecting or potentially digests another cell. affecting a large proportion of a popula- T CELL: A type of lymphocyte, also tion (adj.) or an epidemic disease (n.) known as a T lymphocyte, that plays a key HUMORAL: Of or relating to the anti- role in the immune response. T cells bodies secreted by B cells that circulate in include cytotoxic T cells, which destroy bodily fluids. virus-infected cells in the cell-mediated IMMUNE SYSTEM: A network of immune response; helper T cells, which are organs, glands, and tissues that protects the key participants in specific immune body from foreign substances. responses that bind to APCs, activating IMMUNITY: The condition of being both the antibody and cell-mediated able to resist a particular disease, particu- immune responses; and suppressor T cells, larly through means that prevent the which deactivate T cells and B cells. growth and development or counteract the VACCINE: A preparation containing effects of pathogens. microorganisms, usually either weakened IMMUNOLOGY: The study of the or dead, which are administered as a means immune system, immunity, and immune of increasing immunity to the disease responses. caused by those microorganisms.
some extent because of increased education and Johnson, who was by far the most widely known awareness—and in no small part because of and admired HIV-positive celebrity.
260 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 261
MAGIC COMES BACK. After play- your door with $1 million than you have of Immunity ing on the United States “Dream Team” that catching AIDS in a basketball game.” and Immunology trounced all opponents at the 1992 Summer Olympics in Barcelona, Spain, Johnson attempt- WHERE TO LEARN MORE ed a comeback with the Lakers the following Aaseng, Nathan. Autoimmune Diseases. New York: year. Owing to fears on the part of many other Franklin Watts, 1995. players that they might contract AIDS by coming American Autoimmune Related Diseases Association, Inc. into close contact with him on the court, howev- (AARDA) (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 261 set_vol3_9 8/19/02 11:17 AM Page 262
THEThe Immune System IMMUNE SYSTEM
CONCEPT pathogens or toxins or by some other outside The immune system is a network of organs, threat. Second, the immune response must be glands, and tissues that protects the body from activated quickly, before the invaders destroy foreign substances. These substances include many body tissue cells. For the immune system bacteria, viruses, and other infection-causing to respond effectively, several conditions must be parasites and pathogens. Usually, the immune in place, including the proper interaction of non- system is extremely effective in performing its specific and specific defenses. The nonspecific work of defending the body, but sometimes an defenses on the skin do not identify the antigen error occurs in this highly complex system, and it (a substance capable of stimulating an immune can lead to terrible mistakes. The result can be an response or reaction) that is attacking or poten- allergic reaction, which can be as simple as a case tially attacking the body; instead, these defenses simply react to the presence of what it identifies of the sniffles and as serious as a fatal condition. as something foreign. Often, the nonspecific Or the error can manifest as an autoimmune dis- defenses effectively destroy microorganisms, but order, such as lupus, in which the body rejects its if these defenses prove ineffective and the own constituents as foreign invaders. microorganisms manage to infect tissues, the specific defenses go into action. The specific HOW IT WORKS defenses function by detecting the antigen in question and mounting a response that targets it The Immune System in for destruction. General THE MAJOR HISTOCOMPATI- BILITY COMPLEX. How does the specific The human body is under near constant attack system “know” what is foreign and what is part of from pathogens, or disease-carrying parasites, of the body? The cell membrane of every cell is the type discussed in Infection, Infectious Dis- studded with various proteins, which together eases, and Parasites and Parasitology. No human are known as the major histocompatibility com- would live very long without the immune system, plex, or MHC. The MHC is a kind of pass code, which includes two levels or layers of protection, since all cells in the body must possess an identi- the nonspecific and the specific defenses. The cal pattern so that the body will identify those nonspecific defenses, including the skin and cells as belonging to the “self.” An invading mucous membranes, serve as a first defensive line microorganism, such as a bacterium, does not for preventing pathogens from entering the body. have the same MHC, and when the immune sys- The specific defenses are activated when these tem encounters it, it alerts the body that it has microorganisms get past the nonspecific defens- been invaded by a foreign cell. es and invade the body. Every person has his or her nearly unique For the immune system to work properly, MHC, and the response of the immune system to two things must happen: first, the body must rec- foreign MHC can pose a problem where organ ognize that it has been invaded, either by transplants are concerned. Because the immune
262 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 263
The Immune System
MARROW, THE SOFT TISSUE AT THE CORE OF BONES, IS A KEY PRODUCER BOTH OF LYMPHOCYTES AND OF ANOTH- ER COMPONENT OF BLOOD, THE HEMOGLOBIN-CONTAINING RED BLOOD CELLS. (© Lester V. Bergman/Corbis. Reproduced by per- mission.)
system interprets the transplanted organ, with its bacteria that might enter the body via the nose foreign MHC, as an invader, the body may reject and mouth. Peyer’s patches, scattered throughout the transplant, and therefore organ recipients the small intestine and appendix, are lymphatic usually take immunosuppressant drugs to quell tissues that perform this same function in the the immune response. Furthermore, doctors digestive system. The thymus gland, located often attempt transplants only between close rel- within the upper chest region, is another site of atives, who are likely to have genetically similar lymphocyte production, though it is most active MHCs, or try to find organs that match in the during childhood. The thymus gland continues major histocompatibility antigens. to grow until puberty, protecting a child through the critical years of early development, but in Parts of the Immune System adulthood it shrinks almost to the point of van- The organs of the immune system include the ishing. lymphatic vessels, lymph nodes, tonsils, thymus, Marrow, the soft tissue at the core of bones, Peyer’s patch, and spleen. Each of these organs is a key producer both of lymphocytes and of either produces the cells that participate in the another component of blood, the hemoglobin- immune response or serves as a site for immune containing red blood cells. Because of its critical function. Lymphocytes, a type of white blood role in the immune system, it is a very serious cell, are concentrated in the lymph nodes, which decision to allow marrow to be extracted (itself are masses of tissue that act as filters for blood at an extremely serious operation, of course) for various places throughout the body-most use in a cancer treatment, as described in Nonin- notably the neck, under the arms, and in the fectious Diseases. The spleen, in addition to con- groin. As the lymph (white blood cells plus plas- taining lymphatic tissue and producing lympho- ma) filters through the lymph nodes, foreign cells cytes, acts as a reservoir for blood and destroys are detected and overpowered. worn-out red blood cells. The tonsils, located at the back of the throat ANTIBODIES, B CELLS, AND T and under the tongue, contain large numbers of CELLS. The functioning of the immune sys- lymphocytes and filter out potentially harmful tem also calls into play a wide array of sub-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 263 set_vol3_9 8/19/02 11:17 AM Page 264
The Immune stances, most notably antibodies and the two sig- skin is broken, it creates a pathway for invasion. System nificant varieties of lymphocyte: B cells and T Even a minor cut on a finger can serve as an cells. Antibodies, the most well known of the opening for a microorganism that, once inside three, are proteins in the human immune system the body, will flourish in the body’s warm, blood- that help fight foreign invaders. B cells (B lym- washed interior. When it is established, the bac- phocytes) are a type of white blood cell that gives terium begins to divide rapidly, but already the rise to antibodies, whereas T cells (T lympho- specific immune system has begun to mount its cytes), are a type of white blood cell that plays an resistance, and sometimes evidence of the battle important role in the immune response. T cells can be seen on the outside—for example, in the are a key component in the cell-mediated form of a red, pus-exuding welt. In the blood- response, the specific immune response that uti- stream, lymphocytes engulf bacteria and carry lizes T cells to neutralize cells that have been them toward the lymph nodes. For this reason, infected with viruses and certain bacteria. There when the body is under attack, it begins produc- are three types of T cells: cytotoxic, helper, and ing white blood cells at an accelerated rate, and suppressor T cells. Cytotoxic T cells destroy for this reason doctors sometimes measure a virus-infected cells in the cell-mediated immune patient’s white blood cell count. If the number is response, whereas helper T cells play a part in high, the physician knows that an infection is activating both the antibody and the cell-mediat- active somewhere in the patient’s body. ed immune responses. Suppressor T cells deacti- Killer blood cells, known by the generic vate T cells and B cells when needed, and thus name phagocyte, engulf the bacteria and digest- prevent the immune response from becoming them, but even as this is occurring, the rapid too intense. reproduction of the bacterium provides a chal- The intricacies of the immune system’s func- lenge to the immune system. If the infectious tioning are far beyond the scope of this essay. The agent reproduces at a rate beyond the control of reader interested in a more in-depth review of the immune system, the physician may provide the substances, organs, glands, and processes is help in the form of an antibiotic. Alternatively, he encouraged to seek clarification from a textbook. or she may lance (cut open) a superficial infec- On the other hand, a very basic and nontechnical tion to allow it to drain and to provide access for example of how the body resists infection can an antiseptic agent. If the bacterial invasion is help clarify, in general terms, how the immune minor, the immune system soon dispatches the system does its work. invader, and the system returns to normal. Often, some of the white blood cells form REAL-LIFE antibodies against such invading bacteria, so that APPLICATIONS the immune system will be better armed to com- bat any future invasions by the same microor- Protecting the Body ganism. The white blood cell count returns to its normal level, but still with the capability of As discussed in Infection, not all bacteria are bad; mobilizing the immune defense on short notice. in fact, many are helpful or even essential to It is this response that is the basis for inocula- humans. When the word bacteria is mentioned, tions against certain infections, a topic discussed however, most of us think of the “bad” bacteria, in Immunity. Sometimes, however, something which is understandable, since there are so many goes wrong in the production of antibodies, and of them and their effects can be so dramatic. instead of properly protecting the body against Suppose such a bacterium enters the body, which invaders, the immune system creates an allergy. is an easy situation to imagine—it happens all Allergies the time. Indeed, even as you are reading these words, literally trillions of bacteria the world over An allergy is a change in bodily reactivity to an are attempting to invade human bodies, includ- antigen as a result of a first exposure. Allergies ing your own. Their chances of success are deter- bring about an exaggerated reaction to sub- mined by the immune system and response. stances or physical states that would normally Most of the time, the skin provides us with have little significant effect on a healthy person. sufficient protection from invaders, but if the Although the immune system behaves as if it is
264 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 265
The Immune System
ELECTRON MICROGRAPH OF A SINGLE DUST MITE. THEIR DEAD BODIES AND EXCREMENT CAUSE ASTHMA AND ALLER- GIC REACTIONS TO HOUSEHOLD DUST. (© Eye of Science/Photo Researchers. Reproduced by permission.)
fighting off a pathogen, in fact, it is launching a may be accompanied by a number of stressful complex series of reactions against an irritant. symptoms, ranging from mild reactions, such as The irritant, or allergen, may well be an other- hives (the formation of red, swollen areas on the wise innocuous substance that hardly bothers a surface of the skin) to a life-threatening situation person without the allergy. It could even be known as anaphylactic shock. The latter, a condi- something that other people enjoy—for exam- tion characterized by a sudden drop in blood ple, peanuts or bananas—or at least something, pressure and difficulty in breathing, can be such as animal hair, that does not typically cause accompanied by acute skin irritation in the form people undue discomfort. Allergies also may of angry red boils all over the body. Collapse or involve a substance, such as venom from a bee coma can ensue and may result in death. sting, that most people consider far from pleas- CAUSES OF ALLERGY. Pollens ant but which does not pose a serious threat to from grasses, trees, and weeds produce such someone who is not allergic to it. allergic reactions as sneezing, runny nose, In extreme cases of allergic reaction, the sit- swollen nasal tissues, headaches, blocked sinuses, uation that follows exposure to an allergen truly and watery, irritated eyes. Of the 46 million aller- is one of life and death. The immune response gy sufferers in the United States, about 25 million
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 265 set_vol3_9 8/19/02 11:17 AM Page 266
The Immune System KEY TERMS
ALLERGY: A change in bodily reactivi- cells to neutralize cells that have been ty to an antigen as a result of a first expo- infected with viruses and certain bacteria. sure. Allergies bring about an exaggerated GLAND: A cell or group of cells that fil- reaction to substances or physical states ters material from the blood, processes that that normally would have little significant material, and secretes it either for use again effect on a healthy person. in the body or to be eliminated as waste. ANTIBODIES: Proteins in the human HEMOGLOBIN: An iron-containing immune system that help fight foreign invaders, especially pathogens and toxins. protein in red blood cells that is responsi- ble for transporting oxygen to the tissues ANTIGEN: A substance capable of and removing carbon dioxide from them. stimulating an immune response or reac- Hemoglobin is known for its deep red tion. color. ANTISEPSIS: The practice of inhibit- HUMORAL: ing the growth and multiplication of Of or relating to the anti- microorganisms, usually by ensuring the bodies secreted by B cells that circulate in cleanliness of the environment. bodily fluids. APC: An antigen-presenting cell—a IMMUNE SYSTEM: A network of macrophage that has ingested a foreign cell organs, glands, and tissues that protects the and displays the antigen on its surface. body from foreign substances. B CELL: A type of white blood cell that IMMUNITY: The condition of being gives rise to antibodies. Also known as a B able to resist a particular disease, particu- lymphocyte. larly through means that prevent the CELL-MEDIATED RESPONSE: The growth and development or counteract the specific immune response that utilizes T effects of pathogens.
have this form of allergy, known to scientists as an allergen first enters the body, the lymphocytes rhinitis but to the populace as hay fever. Other make what are known as E antibodies. These common allergens are dust and dust mites, pet antibodies attach to mast cells, large cells that are hair and fur, insect bites, certain foods or drugs, found in connective tissue and contain hista- and skin contact with specific chemical sub- mines. The histamines are chemicals released by stances. About 12 million Americans are allergic basophils, a type of lymphocyte, during the to a variety of chemicals. inflammatory response. Some people are allergic to a wide range of The second time a given allergen enters the substances, while others are affected by only a body of a person who has an allergy, it becomes few or none. Why the difference? The reasons can attached to the E antibodies. They stimulate the be found in the makeup of an individual’s mast cells to discharge their histamines and other immune system, which may produce several anti-allergen substances. One type of histamine chemical agents that cause allergic reactions. The travels to various receptor sites in the nasal pas- main immune system substances responsible for sages, respiratory system, and skin, dilating the symptoms of allergy are the histamines that smaller blood vessels and constricting airways. are produced after exposure to an allergen. When The results include some of the reactions associ-
266 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 267
The Immune System KEY TERMS CONTINUED
IMMUNOLOGY: The study of the PATHOGEN: A disease-carrying para- immune system, immunity, and immune site, usually a microorganism. responses. PHAGOCYTE: A cell that engulfs and LYMPH: That portion of the blood that digests another cell. includes white blood cells and plasma but T CELL: A type of white blood cell, also not red blood cells. known as a T lymphocyte, that plays a key LYMPH NODES: Masses of tissue at role in the immune response. T cells certain places in the body that act as filters include cytotoxic T cells, which destroy for blood. virus-infected cells in the cell-mediated LYMPHOCYTE: A type of white blood immune response; helper T cells, which are cell, varieties of which include B cells and T key participants in specific immune cells, or B lymphocytes and T lymphocytes. responses that bind to APCs, activating MACROPHAGE: A type of phagocytic both the antibody and cell-mediated cell derived from monocytes. immune responses; and suppressor T cells, MHC: Major histocompatibility com- which deactivate T cells and B cells. plex, a group of proteins found on the TISSUE: A group of cells, along with membrane of each cell in an organism’s the substances that join them, which form body. Since all cells in a particular body part of the structural materials in plants or have the same MHC pattern, the MHC is a kind of pass code, identifying cells in the animals. body as belonging to the “self.” WHITE BLOOD CELLS: Blood cells MONOCYTE: A type of white blood cell that are colorless, lack hemoglobin, have a that phagocytizes (engulfs and digests) for- nucleus, and include the lymphocytes and eign microorganisms. other varieties.
ated with allergies, for instance, sneezing or the of the histamines that cause the allergic reac- formation of hives. Another type of histamine tions. The allergens are still there, but the body’s constricts the larger blood vessels and travels to allergic reactions are suspended for the period of the receptor sites found in the salivary and tear time that the antihistamines are active. Antihist- glands and in the stomach’s mucosal lining. amines, sold both in prescription and over-the- These histamines stimulate the release of stom- counter forms, also constrict the smaller blood ach acid, thus creating a stomach ulcer condition. vessels and capillaries, thereby removing excess TREATMENTS. There are many treat- fluids. Decongestants can bring relief as well, but ments for allergy, including (obviously) avoid- they can be used for only a short time, since their ance of the substance to which the patient is continued use can irritate and intensify the aller- allergic. Among these treatments are the admin- gic reaction. istration of antihistamines, which either inhibit In cases of extreme allergic reaction leading the production of histamine or block histamines to anaphylactic shock, the patient may require an at receptor sites. After the administration of anti- injection of epinephrine (also sometimes called histamines, E antibody receptor sites on the mast adrenaline), a hormone that the body produces cells are blocked, thereby preventing the release for responding to situations of fear and danger.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 267 set_vol3_9 8/19/02 11:17 AM Page 268
SOME AUTOIMMUNE DIS- The Immune In the case of anaphylactic shock, which involves EASES. System such severe constriction of the breathing pas- Examples of autoimmune disorders sages that the patient runs a risk of suffocation, include lupus, rheumatoid arthritis, autoim- epinephrine causes the passages to open, making mune hemolytic anemia, pernicious anemia, and type 1 diabetes mellitus. (The last of these dis- it possible to breathe again. It also constricts the eases is discussed in Noninfectious Diseases.) blood vessels, increasing the pressure and making Lupus, or systemic lupus erythematosus, is seen the blood move more rapidly throughout the mainly in young and middle-aged women, and body. The body’s own supply of epinephrine is its symptoms include fever, chills, fatigue, weight not enough to counteract anaphylactic shock, loss, skin rashes (especially a “butterfly” rash on however, and therefore a person experiencing the face), patchy hair loss, sores in the mouth or that condition must receive an emergency injec- nose, enlargement of the lymph nodes, stomach tion containing many times the amount of the problems, and irregular menstrual cycles. Lupus hormone naturally supplied by the body. It may also may induce problems in the cardiopul- be administered at a hospital, though doctors monary, urinary, and central nervous systems usually advise people with severe allergies to keep and can cause seizures, depression, and psy- an emergency supply on hand. chosis. Autoimmune Disorders Rheumatoid arthritis, as its name suggests, is a type of both rheumatism and arthritis, which Allergies are one example of an immune system are general names for diseases associated with gone awry, and though they can be fatal, they inflammation of connective tissue. Rheumatoid typically are a reaction to only one or two sub- arthritis occurs when the immune system attacks stances. An autoimmune disorder, on the other and destroys the tissues that line bone joints and hand, is an entirely different class of phenome- cartilage. The disease can affect any part of the non: it a condition in which a person’s body actu- body, although some joints may be more suscep- ally rejects itself. This condition comes about tible than others are. As it progresses, joint func- when the ability of the immune system to read tion diminishes sharply, and deformities arise. MHCs becomes scrambled, such that it fails to Like rheumatism and arthritis, anemia is a recognize cells from within the body and instead general term for several conditions. Forms of it rejects them as though they came from outside. are marked either by a lack of red blood cells As a result, the body sets in motion the same (hemoglobin) or by a shortage in total blood vol- destructive operation against its own cells that it ume, and these deficiencies can produce effects normally would carry out against bacteria, virus- that range from lethargy or sluggishness to death. es, and other such harmful invaders. Autoimmune hemolytic anemia occurs when the body makes antibodies that coat red blood cells. The reasons why the immune system Patients have been known to experience a variety becomes dysfunctional are not well understood, of symptoms, including jaundice, characterized but most researchers agree that a combination of by a yellowish coloration, before dying—some- genetic, environmental, and hormonal factors times just a few weeks after showing the first plays into autoimmunity. They also speculate signs of the disease. that certain mechanisms may trigger it. First, a Pernicious anemia was so named at a time substance normally restricted to one part of the when it, too, was almost always fatal (pernicious body, and therefore not usually exposed to the means “deadly”), though treatments developed immune system, is released into other areas, in the twentieth century have changed that situa- where it is attacked. Second, the immune system tion. A disorder in which the immune system may mistake a component of the body for a sim- attacks the lining of the stomach in such a way ilar foreign component. Third, cells of the body that the body cannot metabolize vitamin B12 (see may be altered in some way, by drugs, infection, Vitamins), pernicious anemia manifests symp- or some other environmental factor, so that they toms that include weakness, sore tongue, bleed- are no longer recognizable as “self” to the ing gums, and tingling in the extremities. immune system. Fourth, the immune system Because the disease leads to a decrease in stom- itself may be dysfunctional, for instance, because ach acid, nausea, vomiting, loss of appetite, of a genetic mutation. weight loss, diarrhea, and constipation are also
268 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_9 8/19/02 11:17 AM Page 269
possible. Furthermore, since B12 is essential to the Deane, Peter M. G., and Robert H. Schwartz. Coping with The Immune functioning of the nervous system, a deficiency Allergies. New York: Rosen Publishing Group, 1999. System can result in a host of neurological problems, Focus on Allergies (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 269 set_vol3_10 8/19/02 11:18 AM Page 271
SCIENCE OF EVERYDAY THINGS real-life biology INFECTION
PARASITES AND PARASITOLOGY INFECTION
271 set_vol3_10 8/19/02 11:18 AM Page 273
PARASITES AND
Parasites and Parasitology PARASITOLOGY
CONCEPT Organisms engaging in symbiotic relationships When people hear the word parasite, one of the are called symbionts. first ideas or images that probably comes to mind There are three basic types of symbiosis, dif- is that of disease. Though many parasites do ferentiated as to how the benefits (and the detri- carry diseases, including some extremely deadly ments, if any) are distributed. In the case of ones, “disease-carrying” is not necessarily a parasitism, of course, the arrangement benefits defining characteristic of a parasite. Rather, a only one of the two: the parasite, an organism parasite can be identified as any organism that that obtains nourishment or other life support at depends on another organism, the host, for food, the expense of the host. Though predation (the shelter, or some other benefit and which receives relationship of predator to prey) is technically a these benefits in such a way that the host experi- form of symbiosis, it usually is not considered in ences detrimental effects as a consequence. The- the context of symbiotic relationships; therefore, oretically, organisms from all across the king- parasitism is really the only variety of symbiosis doms of living things can be characterized as par- that is detrimental to one of the organisms. asites; in practice, however, the realm of organ- Unlike the predator killing its prey, however, the isms studied by parasitologists is confined to parasite allows its host to live as long as possible, protozoa and various species within the animal since it depends on the host for support and sus- kingdom, mostly worms and arthropods. Includ- tenance. ed among these organisms are countless varieties A second type of symbiosis, commensalism, of tapeworm and roundworm as well as a parade likewise involves a benefit to only one of the two of insects that have plagued humankind since the organisms, but in commensalism the beneficiary, dawn of time: cockroaches, lice, bedbugs, flies, or commensal, manages to receive its benefits fleas, ticks, mites, and mosquitoes. without causing any detriment to the host. The other variety of symbiosis is mutualism, an HOW IT WORKS arrangement from which both partners receive benefits. (Commensalism and mutualism, along Symbiosis with the overall concept of symbiosis, are dis- cussed at much greater length in Symbiosis.) Parasitism belongs within the context of symbio- In addition to the distinctions among para- sis, a term for a biological relationship in which sitism, commensalism, and mutualism, symbiot- two species live in close proximity to each other ic relationships are distinguished according to and interact regularly in such a way as to benefit the participants’ ability to live without each one or both of the organisms. In addition to the other. In a facultative relationship, the partners symbiosis of two species (that is, at least two can live apart successfully, whereas in an obligate individuals representing two different species), it one, the interacting species are incapable of liv- is also possible for a symbiotic relationship to ing separately. Needless to say, for most parasites exist between two organisms of the same species. the relationship is obligate, whereas from the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 273 set_vol3_10 8/19/02 11:18 AM Page 274
PROTOZOA, WORMS, AND Parasites host’s viewpoint, the arrangement is more than ARTHROPODS. and Para- facultative: not only could the host live without After making all the exclu- sitology the parasite, but it actually would be better off sions indicated in the foregoing paragraph, the without the tiny hanger-on. species studied within the realm of parasitology are only those creatures within certain phyla from Defining the Limits of two kingdoms: animals and protists. The latter Parasitology category, which includes algae (other than blue- green algae), slime molds, and protozoa, is made The realm of parasitism and parasitic creatures is up primarily of unicellular, eukaryotic organisms. far larger and more varied than that of parasitol- (A eukaryote is a cell that has a nucleus, as well as ogy. In other words, the range of organisms that organelles, or sections of the cell that perform exhibit parasitic behavior is much greater than specific functions, enclosed in membranes.) the variety of species that are considered within the realm of parasitological study. Among the Among protists, of principal concern is pro- parasitic species excluded from ordinary para- tozoa, which, unlike other protists, are capable of sitology are two named in the title of a book in moving on their own. All the species we discuss the bibliography at the end of this essay: Despica- in the remainder of this essay fall into one of ble Species: On Cowbirds, Kudzu, Hornworms, and three general categories: protozoa, worms, and Other Scourges, by Janet Lembke. arthropods, the latter two being groups of crea- tures within the animal kingdom. The cowbird is a species that exploits the instinctive tendency of other birds to care for The Taxonomy of Worms and their young. Rather than raise its offspring, it Arthropods leaves its eggs with other birds, which mistake the eggs for their own and provide them with food Taxonomy, or the area of the biological sciences and care. Meanwhile, the adult cowbird goes on devoted to the classification of species, is an about its business, freed from the responsibility exceedingly complex area of study. Within that of raising its own progeny. (See Instinct and realm, there are numerous matters that either are Learning for more on this subject.) or have been areas of dispute, among them the classification of protozoa as protists rather than Kudzu is a plant that grows at an amazing animals. Owing to differences between taxonom- rate, covering hillsides and virtually every surface to which it can attach itself. During the 1930s ic systems and the complications involved in agricultural officials in the American South explaining the characteristics that unify members advocated the planting of kudzu, imported from of particular phyla (the next-largest major taxo- China, as a means of controlling erosion on hill- nomic ranking after kingdom), we will dispense sides. The plant did help control erosion, but it with any effort to delineate classifications of virtually took over parts of Georgia, Alabama, species rigorously. Mississippi, and neighboring states. Only In other words, we will take a much simpler through successful, or at least partially successful, approach, a fact signaled by the use of the very eradication campaigns did county and state gov- general word worm. Whereas arthropods are a ernments, as well as private landowners, manage genuine phylum whose characteristics we exam- to stem the tide of kudzu’s onslaught. ine shortly, worms is simply a broad term that Cowbirds are clearly parasitic in their behav- encompasses numerous phyla: Platyhelminthes ior, and kudzu is unquestionably a pest in plant (flatworms), Nemertea (or Rhynchocoela, ribbon form (i.e., a weed), but this does not truly make worms), Acanthocephala (spiny-headed worms), them parasites in the sense that parasitologists use Aschelminthes, Priapulida (priapulids), and the term. Nor, as we noted earlier, does the capac- Annelida (annelid worms). ity to cause or carry disease necessarily define a The classification of the many species of parasite, though many parasites (known collec- worm serves to illustrate just how complex a sub- tively as pathogens) do carry diseases. And even ject taxonomy can be. The group Aschelminthes, though many species of viruses, bacteria, and for example, is divided into five classes—Rotifera, fungi exhibit parasitic behavior and can be trans- Gastrotricha, Kinorhyncha, Nematoda, and mitted by parasites, scientists usually study them Nematomorpha—that sometimes are treated as separately in the context of infectious diseases. separate phyla. Yet another classification system
274 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 275
groups the worms into phyla completely different worms are endoparasites, or parasites that live Parasites from the ones named here. Therefore, rather than inside a host’s body. As we shall see, insects are and Para- sitology hopelessly confusing the issue, in the present con- often vectors, or organisms that transmit text we will call them all simply worms and not pathogens, meaning that they serve as a delivery attempt to map the complex relationships vehicle for the disease-causing protozoa and between the species that fall under that heading. worms. Because endoparasites interact with the ARTHROPODS. In contrast to worms, bodies of their hosts in much more complex arthropods, or members of the phylum Arthro- ways, we devote more space to those species. poda, are much easier to identify. Arthropoda is In general, the life cycle of most worms the largest phylum in the animal kingdom, begins in the body of a definitive host, a host that accounting for some 84% of all known animal provides a setting for the sexual reproduction of species. Nearly 900,000 arthropod species have parasites. (For most of the parasites we consider, been identified, the vast majority of them being humans are the definitive host.) The parasite’s insects, but some zoologists maintain that this eggs pass from the body of the definitive host, number represents but a tiny fraction of all usually through the feces, and hatch in an inor- species within this enormous phylum. In fact, ganic medium, typically water. From there, the there may be as many as ten million species of parasites enter the body of either a vector or an insect alone. intermediate host. The latter is defined by the fact Arthropods are identified by a nonliving that sexual reproduction of parasites does not exoskeleton (an external skeleton), by segmented take place in its body; for the parasites to repro- bodies, and by jointed appendages in pairs. The duce sexually, they must enter the body of a defin- four subphyla of phylum Arthropoda are Trilobi- itive host, where the cycle begins again. There ta, Crustacea, Chelicerata, and Uniramia. The may be more than one intermediate host, and first of these subphyla became extinct during the they are identified, respectively, as first intermedi- “Great Dying” that marked the end of the Permi- ate host, second intermediate host, and so on. an period some 245 million years ago. (See Pale- For protozoa the life cycle is much as we ontology for more on this subject.) The second have described; however, the concept of a defini- includes numerous organisms that humans eat, tive host, as we have defined it, does not usually such as crabs and shrimp, as well as a few para- enter in, since protozoa reproduce asexually. sitic species. The most familiar parasites, howev- Nonetheless, they typically begin life in the intes- er, belong to the other two subphyla. Within Che- tinal tracts or other organs of a host, pass licerata, by far the largest class is Arachnida, through the feces into the water supply, and enter whose name might be familiar from that of the the body either of a vector or a new host. 1990 horror film about spiders, Arachnophobia. In addition to spiders, Arachnida includes scor- pions, ticks, and mites, among which are many REAL-LIFE parasitic creatures. APPLICATIONS Finally, there is subphylum Uniramia, which includes several classes, such as Chilopoda and Protozoa Diplopoda (centipedes and millipedes, respective- They may be single-cell organisms, compared ly); the largest class is unquestionably Insecta. In with the many millions of cells that make up the the latter group are some of the most obnoxious vastly more complex human body, but protozoa creatures known, including the genera Anopheles have been the cause of more human suffering (mosquitoes), Glossina (tsetse fly), and Climex and death than almost any category of disease- (bedbugs) as well as the species Periplaneta ameri- carrying organism. Intestinal protozoa are com- cana (American cockroach), Phthirus pubis (pubic mon throughout the world, particularly in areas or crab louse), and Tunga penetrans (sand flea). where food and water sources are subject to con- Parasite Homes and Life tamination from animal and human waste. Like Cycles soldiers sneaking into a city disguised as civilians, protozoa typically enter the body while still in an Most insects are ectoparasites, or parasites that inactive state, thus getting around any defenses live outside a host’s body, whereas protozoa and the immune system might put up against them.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 275 set_vol3_10 8/19/02 11:18 AM Page 276
Parasites Protozoa in this inactive state, encased in a pro- the ease with which the pathogen can contami- and Para- tective outer membrane, are known as cysts. As nate water supplies makes it a condition known sitology cysts, they enter the gastrointestinal tract of the the world over. Even in the United States and host before developing into a mature form that other industrialized nations, G. lamblia may find feeds and reproduces. its way into water supplies when waste-disposal Other types of parasitic protozoa infect the systems are placed too close to wells. Campers blood or tissues of their hosts. These protozoa who unwisely drink water from mountain typically enter the host through a vector. Often streams also may contract giardiasis, which may the vector is an invertebrate (an animal without come from beavers—hence the nickname an internal skeleton), such as an insect, that feeds “beaver fever,” which is sometimes given to giar- on the host and passes the protozoan on through diasis contracted in the wild. the bite wound. The effects that protozoan para- Far more notorious and destructive is Enta- sites can have on human bodies range from no moeba histolytica, carrier of amoebic dysentery. symptoms to extreme ones. Some victims of Tri- The process of infection is much the same as with chomonas vaginalis, known colloquially as trick, G. lamblia, but once E. histolytica enters the are asymptomatic, meaning that they show no host’s body, it can wreak considerably more dam- obvious symptoms of their condition. It is an age. In most cases, the parasite causes only diar- STD, or sexually transmitted disease, for which rhea and relatively minor gastrointestinal prob- men are typically carriers, passing it on to female lems, but it can bring about a rupture of the gas- sexual partners. Males do exhibit some symp- trointestinal tract (the stomach and intestines). toms, but usually it is the female who experiences Trophozoites may even pass into the circulatory the worst symptoms of the infection, including a system or other organs, such as the liver, and burning sensation when urinating, genital itch- amoebic dysentery can be fatal. ing, and a white discharge. When such pain is not BLOOD INFECTIONS. Among the present, the disease can be detected only by find- most significant protozoan blood infections is ing the trophozoites (protozoa in a feeding stage, African trypanosomiasis, or “sleeping sickness.” as opposed to a reproductive or resting stage) in The parasite enters humans through the bite of secretions within the victim’s genital tract. the tsetse fly, a bloodsucking pest that serves as INTESTINAL PROTOZOA. A clas- vector for the Trypanosoma genus of parasites. sic intestinal protozoan is Giardia lamblia. Like The result is fever, inflammation of the lymph all unicellular organisms, it is extremely small, nodes (masses of tissue at certain places in the with a length of about 15 microns, or microme- body that filter blood), and various negative ters, equal to about 0.00059 in. It has a teardrop effects on the brain and spinal cord that bring shape with two nuclei on either side of a rod about extreme lethargy or tiredness—hence the called an axostyle, and under a scanning electron name. Sleeping sickness, prevalent throughout microscope the nuclei look like eyes separated by central Africa, is frequently fatal. a nose. For this reason, people who have Still worse is the disease carried by the Plas- glimpsed G. lamblia under such magnification modium genera: malaria, which has been have reported the decidedly unsettling sensation described at the leading health problem in the of a microorganism “staring back at them.” world today. It is estimated that more than two These protozoa carry a condition known as billion people live in regions where malaria is giardiasis. As cysts, they pass through the feces of endemic (native), that the number of persons one host and enter the body of the next through infected may be as high at 750 million, and that contaminated food or water. The resulting symp- as many as three million people die of the disease toms can range from nothing to severe diarrhea. each year. Four species of Plasmodium, borne by Not surprisingly, giardiasis is fairly common in mosquitoes, are capable of infecting humans and developing countries, where open sewers com- causing malaria. P. falciparum is the most dan- monly run through the city streets. Also, there are gerous of the four strains; it can kill a healthy places in the third world where farmers use adult in 48 hours. human feces as a fertilizer, another significant Inside the human host, the parasite finds a source of infection. Yet giardiasis is not simply an home in the red blood cells, where it reproduces affliction of people in developing nations; rather, until the cell bursts, spreading more parasites
276 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 277
throughout the blood. These new parasites infect Parasites and destroy more red blood cells. As a new gen- and Para- sitology eration of parasites bursts from the blood cells, the body tries to defend itself by causing a fever, which does destroy many (but not all) of the pathogens. Scientists have developed several chemical antidotes to malaria; unfortunately, strains of pathogen resistant to these antidotes are on the rise in various parts of the world. Worms
The protozoa we have discussed up to this point, as well as the conditions that they carry, are far from pleasant. But the many species we examine under the general heading of “worms” are vastly more disgusting, if that is to be believed. It should be stated that much of what follows is not for those of weak stomach and probably should not be read while eating. In contrast to protozoa, worms are relatively
complex, large creatures with fairly long life FLUKES AND OTHER WORMS CAN BE AS DESTRUCTIVE AS spans. For example, the genera Schistosoma, or MANY OF THEIR PROTOZOAN COUNTERPARTS; ORIENTAL blood flukes, average 0.4 in. (10 mm) in length, LIVER FLUKES, FOR EXAMPLE, PASS THROUGH THE BOD- IES OF SNAILS TO FISH AND THEN TO HUMANS, WHO while Clonorchis sinensis (Chinese, or oriental, ARE INFECTED BY EATING RAW OR UNDERCOOKED FISH. liver fluke) may attain lengths of about 1 in. (25 THEY BLOCK THE BILE DUCTS IN THE LIVER, A CONDI- mm). Additionally, they can live for very long TION THAT CAN BE FATAL. (Photo Researchers. Reproduced by per- spans of time: a blood fluke (fluke is simply a mission.) more common way of designating a nematode, a type of worm) may last as long as 30 years. fish. They block the bile ducts in the liver, a con- During that time, a female blood fluke can dition that can be fatal. produce several hundred eggs a day. Blood flukes TAPEWORMS AND THE DAN- are one of the few worm species in which the GERS OF UNDERCOOKED MEAT. sexes are separate; most worms are hermaphro- There are numerous species of tapeworm, which dites, or both male and female. This brings up may be as long as 50 ft. (15 m). Their bodies are another distinction between worms and proto- made up of segments called proglottids, which zoa: the fact of sexual reproduction. From this contain male and female sexual organs and follows yet another distinction, the involvement which house their eggs. Proglottids break off, (typically) of one or more intermediate hosts as well as a definitive host, in whose body sexual usually into the feces of the host, thus enabling reproduction takes place. the spread of the tapeworm. One variety, Dipy- lidium caninum (cucumber tapeworm), uses Flukes and other worms can be as destruc- dogs or cats as a definitive host, entering the tive as many of their protozoan counterparts. body when the animal ingests a flea or louse, Blood flukes, for instance, kill some one million people a year by inflaming tissues and causing which serves as an intermediate host. Several organs, including the liver and small intestine, to varieties of worm can enter the body through cease functioning. Oriental liver flukes, so named improperly cooked pork. This is a testament to because they are common throughout eastern the wisdom of the injunctions against eating Asia, pass through the bodies of snails (first pork in the Old Testament and Koran, which intermediate host) to fish (second intermediate were written for peoples in a world without host) and ultimately to humans (definitive host), refrigeration or sophisticated medical knowl- who are infected by eating raw or undercooked edge.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 277 set_vol3_10 8/19/02 11:18 AM Page 278
Parasites New World, and was a widespread affliction and Para- throughout the southeastern United States during sitology the early part of the twentieth century. Another common species, Ancylostoma duodenale, is found in southern Europe, northern Africa, northern Asia, and parts of South America. Pinworms, carriers of a condition known as enterobiasis, are also common in the United States, and in much of the world as a whole. They primarily afflict preschool and school-age chil- dren who live in crowded conditions, though adults can often contract enterobiasis as well. Pinworms infect about half as many people as hookworms, but the effects can be extremely painful, especially because they are felt in the perianal skin, or the skin around the anus, and (for adult women) in the genitals. The eggs hatch on the perianal skin, which causes it to itch, but if the victim scratches the area, this can cause addi- tional bacterial infections. The affliction of pin- worms can be especially severe for children, who often experience irritability, insomnia, restless- ness, and other behavioral changes. They are FOUND IN CENTRAL AFRICA AND PARTS OF ASIA, highly contagious and can contaminate nonliving WUCHERERIA BANCROFTI is carried by mosquitoes and causes a condition known as elephanti- surfaces such as bed linens and carpets; therefore, asis. This parasite makes its way to the lymph if one family member is infected, the whole fam- nodes and brings about grotesque swelling. ily usually must receive treatment. Drugs for (© Sheldan Collins/Corbis. Reproduced by permission.) treating pinworm are sold either as prescription or over-the-counter drugs, and treatment usually The Taenia genus of tapeworm includes T. involves two doses over the course of two weeks. solium, the pork tapeworm, which has been Ascaris lumbricoides, or intestinal round- known to crawl out of the anus of an infected worms, may infect as much as one-fourth of the human. Another worm often found in under- world’s population. Affected areas are mostly in cooked pork is Trichinella spiralis, which brings the third world, though in the United States, an about the condition known as trichinosis. The estimated 10,000 cases of roundworm infec- latter disease is rarely fatal, but it can cause tion—frequent in dogs and cats—occur in extreme discomfort, characterized by sore, tender humans. The roundworm pathogen goes muscles. Thanks to enhanced efforts at meat through a strange life cycle inside the human inspection, the incidence of trichinosis in U.S. body, hatching in the small intestine and making pigs has dropped to less than 1%. Nonetheless, it its way into the air passages, only to be swallowed is still quite possible to ingest the parasite from and returned to the small intestine. It can cause a eating undercooked game, particularly bear. fluid buildup in the lungs, resulting in what is WORMS THAT AFFLICT LARGE known as ascaris pneumonia, an often fatal dis- POPULATIONS. Hookworms such as ease. As with T. solium, the intestinal roundworm Necator americanus afflict more than a billion has been known to escape the body, though in an people as well as untold numbers of dogs and cats. even more shocking way. Because it is very sensi- In fact, one of the ways that the parasites spread is tive to anesthetics, the parasite may evacuate the through human exposure to dog and cat feces. body of a patient in a surgical recovery room by Some hookworms can produce as many as 25,000 moving from the small intestine to the stomach eggs a day, wreaking havoc on the host, on whose and out the nose or mouth. blood the hookworms depend for sustenance. As OTHER WORMS. Several other its name implies, N. americanus is found in the worms have been associated with various dis-
278 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 279
Parasites and Para- sitology
SCANNING ELECTRON MICROGRAPH OF A TICK FEEDING IN HUMAN SKIN. A TICK IS A PARASITIC ARTHROPOD THAT CAN SERVE AS A VECTOR FOR BACTERIAL INFECTIONS, SUCH AS ROCKY MOUNTAIN SPOTTED FEVER. (Photo Researchers. Reproduced by permission.)
eases in parts of the third world. Among them are and causes a condition known as elephantiasis. Loa loa, or the “eye worm,” found primarily in The parasite makes its way to the lymph nodes, equatorial Africa. These worms live just under particularly those that drain lower parts of the the skin and, as their name indicates, just under body, and brings about grotesque swelling, usual- the surface of the eyeball. Onchocerca volvulus, ly in the legs and genitals. (Visitors to the “Para- also common in Africa as well as Arabia and sites and Parasitological Resources” Web site of Latin America, afflicts an estimated 30 million Ohio State University’s College of Biological Sci- people. When it enters the human eye, it dies, ences, will find links to photographs of elephan- eventually bringing about blindness. Because the tiasis victims. It should be noted that these vector, a black fly, breeds in rivers, the disease is images include grotesquely swollen male geni- commonly called river blindness, and in some talia and may be too graphic for some viewers.) towns it has blinded as much as 40% of the adult Arthropods population. Found in central Africa and parts of Asia, After the preceding discussion of worms, Wuchereria bancrofti is carried by mosquitoes mere insects and other arthropods probably will
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 279 set_vol3_10 8/19/02 11:18 AM Page 280
Parasites and Para- sitology KEY TERMS
ARTHROPOD: A term for members of ENDOPARASITES: Parasites that live the phylum Arthropoda, largest in the ani- inside a host’s body. mal kingdom. Arthropods are identified by EUKARYOTE: A cell that has a nucleus, a nonliving exoskeleton (an external skele- as well as organelles (sections of the cell ton), by segmented bodies, and by jointed that perform specific functions) that are appendages that appear in pairs. Among bound by membranes. the classes within this phylum are Arachni- FACULTATIVE: A term for a symbiotic da (spiders, scorpions, ticks, and mites) relationship in which partners are capable and Insecta. of living apart. ASYMPTOMATIC: Displaying no GASTROINTESTINAL TRACT: The symptoms of a disease or other condition. stomach and intestines. CLASS: The third-largest major, or GENUS: The second-smallest major obligatory, taxonomic classification rank, taxonomic classification rank, smaller than smaller than phylum but larger than order. family and larger than species. COMMENSALISM: A symbiotic rela- HOST: The term for an organism that tionship in which one organism, the com- provides a benefit or benefits to another mensal, benefits without causing any detri- organism in a symbiotic relationship of ment to the other organism, the host. commensalism or parasitism. See also CYST: A protozoan in an inactive state, intermediate host and definitive host. encased in a protective outer membrane. INTERMEDIATE HOST: A creature Protozoa usually enter the bodies of hosts that serves as host to a parasite, which it in the form of cysts. receives either directly (typically through DEFINITIVE HOST: The host in whose the water supply) or from a vector before body a parasite reproduces sexually. A par- passing it on to a definitive host. Parasites asite enters a definitive host either through do not reproduce sexually inside an inter- a vector, an intermediate host, or through mediate host. some other source, such as a water supply. LYMPH NODES: Masses of tissue at ECTOPARASITES: Parasites that live certain places in the body that act as filters outside a host’s body. for blood.
not have the same power to disgust the reader as Certainly many insect parasites, such as they otherwise might have had. This is particu- mosquitoes (genus Anopheles), are notorious larly so because most arthropods are ectopara- vectors. Of the 400 or so mosquito species, about sites, and not as many of them are associated a dozen carry malaria, and many others transmit with diseases. This is not to say that arthropod canine heartworm. Some parasites are known as parasites are anything pleasant or that they could mechanical, as opposed to biological, vectors. be described as upstanding citizens of the biolog- This means that they may happen to carry cysts ical world—only that, compared with worms, of protozoa, or worm eggs, on their exteriors, but they are not quite so revolting. they do not serve as the biological carrier of any
280 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 281
Parasites and Para- KEY TERMS CONTINUED sitology
OBLIGATIVE: A term for a symbiotic PROTOZOAN: A term that refers to relationship in which the partners, if they some 50,000 species of mobile protists. were separated, would be incapable of con- SUBPHYLUM: A classification rank, tinuing to live. smaller than that of phylum but larger than PARASITE: A general term for any that of class, that is sometimes used to organism that depends on another organ- indicate a level between those two major ism for support, which it receives at the ranks. expense of the other organism. SYMBIOSIS: A biological relationship PARASITISM: A symbiotic relation- in which (usually) two species live in close ship in which one organism, the parasite, proximity to each other and interact regu- benefits at the expense of the other organ- larly in such a way as to benefit one or both ism, the host. of the organisms. Symbiosis may exist PARASITOLOGY: A biological disci- between two or more individuals of the pline devoted to the study of parasites but same species as well as between two or primarily those among the animal and more individuals representing two differ- protist kingdoms. Parasitic bacteria, fungi, ent species. The three principal varieties of and viruses usually are studied within the symbiosis are mutualism, commensalism, context of infectious diseases. and parasitism. PATHOGEN: A disease-carrying para- TAXONOMY: The area of the biological site, typically a microorganism. sciences devoted to the classification of PHYLUM: The second-largest major organisms according to apparent common taxonomic classification rank, smaller than characteristics. kingdom but larger than class. TROPHOZOITE: A protozoan in a PROTISTS: A term for members of the feeding stage, as opposed to a reproductive kingdom Protista, which includes algae or resting stage. (other than blue-green algae, which are monerans), slime molds (a group of about VECTOR: An organism, such as an 500 species that resemble fungi), and pro- insect, that transmits a pathogen to the tozoa. body of a host.
disease. Such is the case with the American cock- tor, either mechanical or biological, for any dis- roach (Periplaneta americana). But one thing ease—yet it certainly gives rise to more than its unites all insect parasites: they are pests, and if share of misery. Like many another arthropod any of their species ever became endangered parasite, it causes its hosts to itch. Most familiar (something that is not likely to happen), very few from the old saying “don’t let the bedbugs bite,” bedbugs live not only in beds but also in clothing, people would be upset. furniture, laundry, and numerous other areas ARTHROPODS THAT CAUSE around a house. And though they depend on ITCHING. The bedbug (genus Cimex) is an human blood, it does no good simply to vacate a insect parasite that is not known to serve as a vec- house for a few days in the hope of starving
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 281 set_vol3_10 8/19/02 11:18 AM Page 282
Parasites them; they can survive for up to year on the itching to severe neurological disorders. There and Para- blood supplies they have stored in their bodies. are also several varieties of mange mites, among sitology Then there are body lice (Pediculus them Sarcoptes scabiei, which afflicts humans and humanus) and pubic, or crab, lice (Phthirus animals with scabies, characterized by severe skin pubis). Although people may tend to associate inflammation and itching. the problem of lice with the poor, lice afflict all socioeconomic classes. Body lice, as their name WHERE TO LEARN MORE indicates, spread all over the body and particu- Berenbaum, May. Bugs in the System: Insects and Their larly the head, while crab lice nest either in the Impact on Human Affairs. Reading, MA: Addison- pubic area or anywhere else they can find short, Wesley, 1995. thick hair: armpits, eyebrows, eyelashes, beards, “Biology 160, Animal Behavior: Symbiosis and Social Par- sideburns, and mustaches. Other than inducing asitism.” Department of Biology, University of Cali- itching, most lice do not pose a serious threat— fornia at Riverside (Web site).
282 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 283
Infection INFECTION
CONCEPT Infections fall into two general categories: exoge- Humans may hold dominance over most other nous, or those that originate outside the body, life-forms on Earth, but a few varieties of organ- and endogenous, which occur when the body’s ism have long held mastery over us. Ironically, resistance is lowered. Examples of exogenous these life-forms, including bacteria and viruses, infection include catching a cold by drinking are so small that they cannot be seen, and this, in after someone else from the same glass; coming fact, has contributed to their disproportionate down with salmonella after ingesting under- influence in human history. For thousands of cooked eggs, meat, or poultry; getting rabies years, people attributed infection to spiritual from a dog bite; or contracting syphilis, AIDS causes or, at the very least, to imbalances of (acquired immunodeficiency syndrome), or “humors,” or fluids, in the human body. Today some other sexually transmitted disease from an germ theory and antisepsis—the ideas that infected partner. microbes cause infection and that a clean body Any number of factors—lack of sleep, pro- and environment can prevent infections—are longed exposure to extreme cold or moisture, ingrained so deeply that we almost take them for and so on—can lower the body’s resistance, granted. Yet these concepts are very recent in ori- opening the way for an endogenous infection. gin, and for a much longer span of human histo- Malnutrition, illness, and trauma also can be fac- ry people quite literally wallowed in filth—with tors in endogenous infection. Substance abuse, predictable consequences. whether it be the use of tobacco in its many forms, excessive drinking, or drug use, lowers the HOW IT WORKS body’s resistance. Furthermore, all of these behaviors tend to be coupled with poor eating What Is Infection? habits, which invite infection by denying the body the nutrients it needs. The term infection refers to a state in which par- Some Terms asitic organisms attach themselves to the body, or to the inside of the body, of another organism, A whole array of terminology attends the study causing contamination and disease in the host of infection and infectious diseases, a subject that organism. Parasite refers generally to any organ- is touched upon in the present context but ism that lives at the expense of another organism, explored at length in its own essay as well. on which it depends for support. Numerous par- Among these terms are the names for the differ- asites and the diseases they cause are discussed in ent branches of study relating to infection, its the essay Parasites and Parasitology; in the pres- agents, and the resulting diseases. Although germ ent context, we are concerned primarily with theory is a term (defined later) that is used wide- infections that relate to bacteria and viruses. ly in the context of infection, germ itself—a com- Almost all infections contracted by humans mon word in everyday life—is not used as much are passed along by other humans or animals. as microorganism or pathogen. The latter word
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 283 set_vol3_10 8/19/02 11:18 AM Page 284
Infection refers to disease-carrying parasites, which are • Inoculation: The prevention of a disease by usually microorganisms. Two of the principal the introduction to the body, in small quan- types of pathogen, bacteria and viruses, are dis- tities, of the virus or other microorganism cussed later in this essay. that causes the disease Words relating to the effects of infectious • Vaccine: A preparation containing microor- agents include epidemic, an adjective meaning ganisms, usually either weakened or dead, “affecting or potentially affecting a large propor- which are administered as a means of tion of a population”; as a noun, the word refers increasing immunity to the disease caused to an epidemic disease. Pandemic also doubles as by those microorganisms an adjective, meaning “affecting an extremely Some of these words appear in this essay and high proportion of a population over a wide geo- others in related essays on infectious diseases and graphic area,” and a noun, referring to a disease immunity. of pandemic proportions. Areas of study relating to pathogens, their effects, and the prevention of Bacteria those effects include the following. Five major groups of microorganisms are • Bacteriology: An area of the biological sci- responsible for the majority of infections. They ences concerned with bacteria, including include protozoa and helminths, or worms— their importance in medicine, industry, and both of which are considered in Parasites and agriculture Parasitology—as well as bacteria and viruses. • Epidemiology: An area of the medical sci- Bacteria and viruses often are discussed, along ences devoted to the study of disease, with fungi (the fifth major group), in the context including its incidence, distribution, and of infection and infectious diseases. In the pres- control within a population ent context, however, we limit our inquiry to • Etiology: A branch of medical study con- viruses and bacteria. cerned with the causes and origins of dis- Bacteria are very small organisms, typically ease. Also, a general term referring to all the consisting of one cell. They are prokaryotes, a causes of a particular disease or condition term referring to a type of cell that has no nucle- • Immunology: The study of the immune sys- us. In eukaryotic cells, such as those of plants and tem, immunity, and immune responses animals, the nucleus controls the cell’s functions • Pathology: The study of the essential nature and contains its genes. Genes carry deoxyribonu- of diseases cleic acid (DNA), which determines the charac- • Virology: The study of viruses teristics that are passed on from one generation In addition, there are several terms relating to the next. The genetic material of bacteria is to the prevention of infection. contained instead within a single, circular chain • Antibiotic: A substance produced by, or of DNA. derived from, a microorganism, which in Members of kingdom Monera, which also diluted form is capable of killing or at least includes blue-green algae (see Taxonomy), bacte- inhibiting the action of another microor- ria generally are classified into three groups ganism. Antibiotics typically are not effec- based on their shape: spherical (coccus), rodlike tive against viruses. (bacillus), or spiral- or corkscrew-shaped (spiro- • Antisepsis: The practice of inhibiting the chete). Some bacteria also have a shape like that growth and multiplication of microorgan- of a comma and are known as vibrio. Spirochetes, isms which are linked to such diseases as syphilis, • Germ theory: A theory in medicine, widely sometimes are considered a separate type of crea- accepted today, that infections, contagious ture; hence, Monera occasionally is defined as diseases, and other conditions are caused by consisting of blue-green algae, bacteria, and the actions of microorganisms spirochetes. • Immunity: A condition of being able to The cytoplasm (material in the cell interior) resist a particular disease, particularly of all bacteria is enclosed within a cell membrane through means that prevent the growth and that itself is surrounded by a rigid cell wall. Bac- development of pathogens or counteract teria produce a thick, jellylike material on the their effects surface of the cell wall, and when that material
284 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 285
forms a distinct outer layer, it is known as a cap- asites, including bacteria and those species dis- Infection sule. Many rod, spiral, and comma-shaped bacte- cussed in Parasites and Parasitology, depend on ria have whiplike limbs, known as flagella, other organisms to serve as hosts, they can live attached to the outside of their cells. They use when they are between hosts. They are rather like these flagella for movement by waving them back a person between jobs: without other means of and forth. Other bacteria move simply by wig- support, the person eventually will go broke or gling the whole cell back and forth, whereas still starve, but typically such a person can hang on others are unable to move at all. for a few months until he or she finds a new job. Bacteria most commonly reproduce by fis- A virus without a host, on the other hand, is sim- sion, the process by which a single cell divides to ply not alive—not dead, like a formerly living produce two new cells. The process of fission thing, but more like a machine that has been may take anywhere from 15 minutes to 16 hours, switched off. depending on the type of bacterium. Several fac- Once a virus enters the body of a host, it tors influence the rate at which bacterial growth switches on, and the result is truly terrifying. In occurs, the most important being moisture, tem- order to produce new copies of itself, a virus perature, and pH, or the relative acidity or alka- must use the host cell’s reproductive “machin- linity of the substance in which they are placed. ery”—that is, the DNA. The newly made viruses Bacterial preferences in all of these areas then leave the host cell, sometimes killing it in vary: for example, there are bacteria that live in the process, and proceed to infect other cells hydrothermal vents, or cracks in the ocean floor, within the organism. As for the organisms that where the temperature is about 660°F (350°C), viruses target, their potential victims include the and some species survive at a pH more severe whole world of living things: plants, animals, and than that of battery acid. Most bacteria, however, bacteria. Viruses that affect bacteria are called favor temperatures close to that of the human bacteriophages, or simply phages. Phages are of body—98.6°F (37°C)—and pH levels only slight- special importance, because they have been stud- ly more or less acidic than water. Since they are ied much more thoroughly than most viruses; in composed primarily of water, they thrive in a fact, much of what virologists now know about moist environment. viruses is based on the study of phages. Viruses REAL-LIFE One of the interesting things about bacteria is APPLICATIONS their simplicity, coupled with the extraordinary complexity of their interactions with other Bacteria and Humans organisms. As simple as bacteria are, however, viruses are vastly more simple. Furthermore, the Not all bacteria are harmful; in fact, some even diseases they can cause in other organisms are at are involved in the production of foods con- least as complex as those of bacteria, and usually sumed by humans. For example, bacteria that much more difficult to defeat. Whereas there are cause milk to become sour are used in making “good” bacteria, as we shall see, scientists have yet cottage cheese, buttermilk, and yogurt. Vinegar to discover a virus whose impact on the world of and sauerkraut also are produced by the action of living things is beneficial. There is something bacteria on ethyl alcohol and cabbage, respective- downright creepy about viruses, which are not ly. Other bacteria, most notably Escherichia coli exactly classifiable as living things; in fact, a virus (E. coli) in the human intestines, make it possible is really nothing more than a core of either DNA for animals to digest foods and even form vita- or RNA (ribonucleic acid), surrounded by a shell mins in the course of their work. (See Digestion of protein. for more on these subjects.) Others function as Two facts separate viruses from the world of decomposers (see Food Webs), aiding in the the truly living. First, unlike all living things chemical breakdown of organic materials, while (even bacteria), viruses are not composed of even still others help keep the world a cleaner place by a single cell, and, second, a virus has no life if it consuming waste materials, such as feces. cannot infect a host cell. When we say “no life” in Despite its helpful role in the body, certain this context, we truly mean no life. Although par- strains of E. coli are dangerous pathogens that
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 285 set_vol3_10 8/19/02 11:18 AM Page 286
Infection can cause diarrhea, bloody stools, and severe blood as the body tries to shunt blood to vital abdominal cramping and pain. The affliction is organs. If the syndrome is severe enough, gan- rarely fatal, though in late 1992 and 1993 four grene may develop in the fingers and toes. people died during the course of an E. coli out- In 1980, several women in the United States break in Washington, Idaho, California, and died from TSS, and several others were diagnosed Nevada. More often the outcome is severe illness with the condition. As researchers discovered, all that may bring on other conditions; for example, of them had been menstruating and using high- two teenagers among a group of 11 who became absorbency tampons. It appears that such tam- sick while attending a Texas cheerleading camp pons provide an environment in which TSS- had to receive emergency appendectomies. The causing bacteria can grow, and this led to recom- pathogen is usually transmitted through under- mendations that women use lower-absorbency cooked foods, and sometimes through other tampons if possible, and change them every two means; for example, a small outbreak in the to four hours. Since these guidelines were insti- Atlanta area in the late 1990s occurred in a recre- tuted, the incidence of toxic shock has dropped ational water park. significantly, to between 1 and 17 cases per BACTERIAL INFECTIONS. Many 100,000 menstruating women. bacteria attack the skin, eyes, ears, and various Many bacteria produce toxins, poisonous systems in the body, including the nervous, car- substances that have effects in specific areas of diovascular, respiratory, digestive, and genitouri- the body. An example is Clostridium tetani, nary (i.e., reproductive and urinary) systems. responsible for the disease known as tetanus, in The skin is the body’s first line of defense against which one’s muscles become paralyzed. A related infection by bacteria and other microorganisms, bacterium, C. botulinum, releases a toxin that although it supports enormous numbers of bac- causes the most severe form of food poisoning, teria itself. Bacteria play a major role in a skin botulism. Salmonella poisoning comes from condition that is the bane of many a young man’s another genus, Salmonella, which includes S. (and, less frequently, a young woman’s) existence: typhi, the cause of typhoid fever. acne. Pimples or “zits,” known scientifically as Acne vulgaris, constitute one of about 50 varieties Viral Infections of acne, or skin inflammation, which are caused by a combination of heredity, hormones, and With viruses, as we have noted, there is no need bacteria—particularly a species known as Propi- even to discuss “good” kinds, because there is no onibacterium acnes. When a hair follicle becomes such thing—all viruses are harmful, and most are plugged by sebum, a fatty substance secreted by killers. The particular strains of virus that attack the sebaceous, or oil, glands, this forms what we animals have introduced the world to a variety of know as a blackhead; a pimple, on the other ailments, ranging from the common cold to hand, results when a bacterial infection, brought AIDS and some types of cancer. Other diseases about by P. acnes, inflames the blackhead and related to viral infections are hepatitis, chicken turns it red. For this reason, antibiotics may pox, smallpox, polio, measles, and rabies. sometimes cure acne or at least alleviate the One reason why physicians and scientists worst symptoms. have never found a cure for the common cold is Acne may seem like a life-and-death issue to that it can be caused by any one of about 200 a teenager, but it goes away eventually. On the viruses, including rhinoviruses, adenoviruses, other hand, toxic shock syndrome (TSS), caused influenza viruses, parainfluenza viruses, syncytial by other bacteria at the surface of the skin— viruses, echoviruses, and coxsackie viruses. Each species of Staphylococcus and Streptococcus—can has its own characteristics, its favored method of be extremely dangerous. The early stages of TSS transmission, and its own developmental period. are characterized by flulike symptoms, such as These viruses can be transmitted from one per- sudden fever, fatigue, diarrhea, and dizziness, but son to another by sneezing on the person, shak- in a matter of a few hours or days the blood pres- ing hands, or handling an object previously sure drops dangerously, and a sunburn-like rash touched by the infected person. Surprisingly, forms on the body. Circulatory problems arise as some more direct forms of contact with an a result of low blood pressure, and some extrem- infected person, as in kissing, seldom spread ities, such as the fingers and toes, are deprived of viruses.
286 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 287
A group of viruses called the orthomyx- Infection oviruses transmit influenza, an illness usually characterized by fever, muscle aches, fatigue, and upper respiratory obstruction and inflammation. The most common complication of influenza is pneumonia, a disease of the lungs that may be viral or bacterial. The viral form of pneumonia that goes hand in hand with influenza can be very severe, with a high mortality (death) rate; by contrast, bacterial pneumonia, which typically appears five to ten days after the onset of flu, can be treated with antibiotics. THE EVER ELUSIVE VIRUS. Viruses are tricky. Because their generations are very short and their structures extremely simple, they are constantly mutating (altering their DNA and hence their heritable traits) and thus becom- ing less susceptible to vaccines. This is the reason why flu vaccine has to be prepared anew each year to target the current strains, and even then the vaccine is far less than universally effective. On the other hand, vaccination has a high rate of TWO CHILDREN ARE CONFINED TO IRON LUNGS AS THE ; SIXOFTHE success for strains of virus that undergo little RESULT OF INFECTION WITH POLIOVIRUS CHILDREN IN THIS ONE FAMILY WERE STRICKEN WITH THE mutation—for example, the smallpox virus. VIRUS. AN EPIDEMIC DISEASE CAN AFFECT A LARGE One particularly elusive type of virus is PROPORTION OF A POPULATION, AS HAPPENED IN THE CASE OF POLIO IN THE MIDDLE OF THE TWENTIETH CEN- known as a retrovirus, which reverses the normal TURY. (© Bettmann/Corbis. Reproduced by permission.) process by which living organisms produce pro- teins. Ordinarily, DNA in the cell’s nucleus car- ries directions for the production of new protein. trast, believed that disease was caused by evil Coded messages in the DNA molecules are spirits, cast upon individuals or populations by copied into RNA molecules, which direct the an angry God as punishment for disobedience. manufacture of new protein. In retroviruses, that Personal hygiene and public health were process is reversed, with viral RNA used to make completely foreign concepts: not only did people new viral DNA, which then is incorporated into bathe infrequently, but they also thought nothing host cell DNA, where it is used to direct the man- of throwing trash—including rotting food and ufacture of new viral protein. Among the diseases even human excrement—into the city streets. caused by retroviruses is AIDS, discussed in This image of trash in the streets may call to Infectious Diseases and The Immune System. mind a city of medieval Western Europe, a place Fighting the Invisible War and time widely known for its filth, squalor, and ignorance. Yet such an image also describes Every day of our lives, we are at war with Athens during the fifth century B.C., when microorganisms, both individually and as a human imagination, wisdom, and appreciation species. It is a war that has lasted for several mil- for beauty reached perhaps their highest points lion years, with billions of lives in the balance, yet in all of history. In the Athens of Socrates, it is an invisible war. Up until a few centuries ago, Herodotus, Hippocrates, and Sophocles, the in fact, we had no idea what we were fighting. streets were piled with trash and crawling with Before the advent of germ theory, the most scien- vermin. In fact, this lack of concern for cleanli- tific theories of disease blamed them either on an ness contributed directly to the end of the Greek imbalance of “humors” (blood, phlegm, yellow golden age, sometimes known as the Age of Per- bile, and green bile), or on inhaling bad air. These icles, after Athens’s great leader (495–425 B.C.)— were the most advanced ideas, the ones held by who died in a great plague that swept the germ- men of learning; most of the populace, by con- ridden city.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 287 set_vol3_10 8/19/02 11:18 AM Page 288
Infection patient. Semmelweis’s idea resulted in a drastic reduction of puerperal fever cases, but his col- leagues denounced his outlandish notion as a useless and foolish waste of time. Six years later, in 1854, modern epidemiology was born when the English physician John Snow (1813–1858) determined that the source of a cholera epidem- ic in London could be traced to the contaminat- ed water of the Broad Street pump. After he ordered the pump closed, the epidemic ebbed— and still many physicians refused to believe that invisible organisms could spread disease. GERM THEORY. A major turning point came just three years later, in 1857, when the great French chemist and microbiologist Louis Pasteur (1822–1895) discovered that heat- ing beer and wine to a certain temperature killed bacteria that caused these liquids to spoil or turn into vinegar. Thus was born the process of pas- teurization, still used today to purify such foods as milk, because, as Pasteur observed, “There are ANTON VAN LEEUWENHOEK’S PROTOTYPE MICROSCOPE. similarities between the diseases of animals or THE INVENTION OF THE MICROSCOPE MADE IT POSSIBLE TO SEE BACTERIA AND OTHER MICROORGANISMS, WHICH man and the diseases of beer and wine.” Pasteur VAN LEEUWENHOEK, THE FIRST HUMAN BEING TO also dealt the final blow to spontaneous genera- OBSERVE THEM, DUBBED ANIMALCULES, or “tiny ani- tion, a centuries-old belief that living organisms mals.” (© Bettmann/Corbis. Reproduced by permission.) could originate from nonliving matter. As he showed in 1861, microorganisms present in the BACTERIOLOGY AND ANTI- air can contaminate solutions that seem sterile. SEPSIS. The first inkling of any etiology Then, in 1876, the German physician Robert other than that of imbalanced humors and Koch (1843–1910) proved what Kircher had pos- demons was the work of the Italian physician tulated two centuries earlier: that bacteria can Girolamo Fracastoro (ca. 1483–1553), who put cause diseases. Koch showed that the bacterium forth the theory that disease is caused by particles Bacillus anthracis was the source of anthrax in cat- so small they are almost imperceptible. The tle and sheep and generalized the methodology he invention of the microscope in 1590 made it pos- had used in that situation to form a specific set of sible to glimpse those particles, which Holland’s guidelines for determining the cause of infectious Anton van Leeuwenhoek (1632–1723)—the first diseases. Known as Koch’s postulates, these guide- human being to observe bacteria and other lines define a truly infectious agent as one that can microorganisms—dubbed animalcules, or “tiny be isolated from an infected animal, cultured in a animals.” The German scholar Athanasius laboratory setting, introduced into a healthy ani- Kircher (1601–1680) also observed “tiny worms” mal to produce the same infection as in the first in the blood and pus of plague victims and theo- animal, and isolated again from the second ani- rized that they were the source of the infection. mal. These ideas formed the basis of research into This was the first theory that dealt with microbial bacterial diseases and are still dominant in the sci- agents as infectious organisms. ences devoted to the study of disease. In 1848 Ignaz P. Semmelweis (1818–1865), a Koch’s postulates helped usher in what has Hungarian physician working in German hospi- been called the golden era of medical bacteriolo- tals, came up with a novel idea: after examining gy. Between 1879 and 1889 German microbiolo- the bodies of women who had died of puerperal gists isolated the organisms that cause cholera, (childbed) fever, he suggested that doctors typhoid fever, diphtheria, pneumonia, tetanus, should wash their hands in a solution of chlori- meningitis, and gonorrhea as well the Staphylo- nated lime water before touching a pregnant coccus and Streptococcus organisms. Even as
288 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 289
Infection KEY TERMS
ANTIBIOTIC: A substance produced by to offspring, stored in DNA molecules or derived from a microorganism, which in called chromosomes. diluted form is capable of killing or at least GERM THEORY: A theory in medi- inhibiting the action of another microor- cine, widely accepted today, that infections, ganism. Antibiotics are not usually effec- contagious diseases, and other conditions tive against viruses. are caused by the actions of microorgan- ANTISEPSIS: The practice of inhibit- isms. ing the growth and multiplication of IMMUNITY: The condition of being microorganisms, generally by ensuring the able to resist a particular disease, particu- cleanliness of the environment. larly through means that prevent the BACTERIOLOGY: An area of the bio- growth and development of pathogens or logical sciences concerned with bacteria, counteract their effects. including their importance in medicine, IMMUNOLOGY: The study of the industry, and agriculture. immune system, immunity, and immune DNA: Deoxyribonucleic acid, a mole- responses. cule in all cells, and many viruses, contain- INFECTION: A state or condition in ing genetic codes for inheritance. which parasitic organisms attach them- ENDOGENOUS: A term for an infec- selves to the body or to the inside of the tion that occurs when the body’s resistance body of another organism, producing con- is lowered. Compare with exogenous. tamination and disease in the host. EPIDEMIC: Affecting or potentially INOCULATION: The prevention of a affecting a large proportion of a popula- disease by the introduction to the body, in tion (adj.) or an epidemic disease (n.) small quantities, of the virus or other EPIDEMIOLOGY: An area of the med- microorganism that causes the disease. ical sciences devoted to the study of dis- MUTATION: Alteration in the physical ease, including its incidence, distribution, structure of an organism’s DNA, resulting and control within a population. in a genetic change that can be inherited. ETIOLOGY: A branch of medical study PANDEMIC: Affecting an extremely concerned with the causes and origins of high proportion of a population over a disease; also, a general term referring to all wide geographic area (adj.) or a disease of the causes of a particular disease or condi- pandemic proportions (n.) tion. PARASITE: A general term for any EXOGENOUS: A term for an infection organism that depends on another organ- that originates outside the body. Compare ism for support, which it receives at the with endogenous. expense of the other organism. GENE: A unit of information about a PARASITOLOGY: A biological disci- particular heritable (capable of being pline devoted to the study of parasites, pri- inherited) trait that is passed from parent marily those among the animal and protist
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 289 set_vol3_10 8/19/02 11:18 AM Page 290
Infection KEY TERMS CONTINUED
kingdoms. Parasitic bacteria, fungi, and RNA: Ribonucleic acid, the molecule viruses usually are studied within the con- translated from DNA in the cell nucleus, text of infectious diseases. the control center of the cell, that directs protein synthesis in the cytoplasm, or the PATHOGEN: A disease-carrying para- space between cells. site, typically a microorganism. VACCINE: A preparation containing PATHOLOGY: The study of the essen- microorganisms, usually either weakened tial nature of diseases. or dead, which is administered as a means PUBLIC HEALTH: A set of policies of increasing immunity to the disease and methods for protecting and improving caused by those microorganisms. the health of a community through efforts VECTOR: An organism, such as an that include disease prevention, health insect, that transmits a pathogen to the education, and sanitation. body of a host.
Koch’s work was influencing the development of notatum, Fleming made a juice with it that he the germ theory, the influence of the English called penicillin. He administered it to laborato- physician Joseph Lister (1827–1912) was being ry mice and discovered that it killed bacteria in felt in operating rooms. Building on the work of the mice without harming healthy body cells. both Semmelweis and Pasteur, Lister—for whom It would be more than a decade before the the well-known antiseptic mouthwash Listerine development of a form of penicillin that could be was named—began soaking surgical dressings in synthesized easily. This drug arrived on the scene carbolic acid, or phenol, to prevent postoperative in 1941—just in time for the years of heaviest infection. fighting in World War II—and after the war ANTIBIOTICS. Whereas antisepsis was pharmaceutical companies began to manufac- the great battleground of the invisible war during ture numerous varieties of antibiotic. By the last the nineteenth century, in the twentieth century decade of the twentieth century, however, a new the most important struggle concerned the problem emerged: bacteria were becoming resist- development of antibiotics. The first effective ant to antibiotics. This has been the case with medications to fight bacterial infection in medications used to treat conditions ranging humans were sulfa drugs, developed in the 1930s. from children’s ear infections to tuberculosis. They work by blocking the growth and multipli- An example is amoxicillin, a penicillin deriv- cation of bacteria and were initially effective ative developed in the late twentieth century. against a broad range of bacteria, but many Many pediatricians found it a better treatment strains of bacteria have evolved resistance to than penicillin for ear infections, because it did them. Today, sulfa drugs are used most common- not tend to cause allergic reactions sometimes ly in the treatment of urinary tract infections and associated with the other antibiotic. However, by for preventing infection of burn wounds. the late 1990s evidence surfaced indicating that The importance of sulfa drugs was eclipsed certain types of bacteria had developed a protein by that of penicillin, first discovered in 1928 by that rendered amoxicillin ineffective against ear the British bacteriologist Alexander Fleming infections. Critics of amoxicillin (or of antibiotic (1881–1955). Working in his laboratory, Fleming treatments in general) maintained that wide- noticed that a mold that had fallen accidentally spread prescription of the antibiotic actually into a bacterial culture killed the bacteria. Having helped create that situation, because the bacteria identified the mold as the fungus Penicillium developed the protein mutation defensively.
290 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_10 8/19/02 11:18 AM Page 291
Because of these and similar concerns associated Centers for Disease Control and Prevention (Web site). Infection with antibiotics, doctors have begun taking
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 291 set_vol3_11 8/19/02 11:21 AM Page 293
SCIENCE OF EVERYDAY THINGS real-life biology BRAIN AND BODY
CHEMORECEPTION BIOLOGICAL RHYTHMS
293 set_vol3_11 8/19/02 11:21 AM Page 295
Chemoreception CHEMORECEPTION
CONCEPT matter or energy of the environment, into Chemoreception is a physiological process processes within the body and brain. For exam- whereby organisms respond to chemical stimuli. ple, if you eat a cookie, your sense of taste trans- Humans and most higher animals have two prin- lates the chemical data from that cookie into the cipal classes of chemoreceptors: taste (gustatory sensation of sweetness, a sensation your brain receptors), and smell (olfactory receptors). most likely perceives as a pleasing one. Though our sense of smell assists us in distin- In everyday language, people are accus- guishing among tastes, the gustatory and olfacto- tomed to speaking of five senses possessed by ry receptors are different in many respects—not humans and, to a greater or lesser degree, by only in their locations but also in terms of their other animals: sight, touch, smell, hearing, and chemical and neurological makeup. Capabilities taste. The reality is rather more complex. For one of taste and smell vary widely among people, as a thing, there are not really just five senses; rather, function of genetics, age, and even personal there are at least five others—and there may be habits. Likewise, culture influences attitudes more, depending on just how one defines and toward taste and smell. As for the animal king- classifies the senses. These other senses include dom, certain creatures are gifted with exceeding- the kinesthetic sense, or the discernment of ly acute senses, particularly where smell is con- motion; the sensation of temperature, or distin- cerned, but for some invertebrates, such as guishing relative heat and cold; the awareness of worms, there is really little distinction between pressure; the sense of equilibrium or balance; taste and smell. Among the most interesting and the perception of pain. aspects of chemoreception in animals is the use of smell for communication, particularly All of these senses involve a response to through the release of special chemicals called stimuli, which may be defined as any phenome- pheromones. As to whether pheromones, which non (that is, an observable fact or event, such as function as sex attractants, play a role in human an environmental change) that directly influ- interaction, many scientists remain skeptical. ences the activity or growth of a living organism. Each of these senses has a biological component as well as a physical or a chemical one. In the next HOW IT WORKS few paragraphs, we briefly discuss what this means, first by considering the nervous system in The Senses general terms and then by looking at the physical The term sense, which also may be called sensory and chemical receptors that transmit data reception and sensory perception, refers to the through that system. means by which an organism (usually an animal) THE NERVOUS SYSTEM. All sen- receives signals regarding physical or chemical sation is biological in the sense that an organism changes or both in its environment. Sensory not only must be living to experience it but also reception and perception entail the translation of must have a functioning nervous system. The lat- these signals, which represent changes in the ter is a network, found in the bodies of all verte-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 295 set_vol3_11 8/19/02 11:21 AM Page 296
Chemorecep- tion
CHEMORECEPTION IS THE MEANS BY WHICH AN ORGANISM RECEIVES SIGNALS REGARDING CHEMICAL CHANGES IN ITS ENVIRONMENT, TRANSLATING THEM INTO PROCESSES WITHIN THE BODY AND BRAIN. NERVE CELLS, SUCH AS THE ONES SHOWN HERE, MAKE UP A NETWORK OF PRIMARY RECEPTORS THAT RECEIVE AND INTERPRET STIMULI AND TRANSMIT MESSAGES BASED ON THESE SENSORY DATA TO THE BRAIN. (Photograph by Secchi-Lecague/Roussel-UCLAF/CNRI/Sci- ence Photo Library. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)
brates (animals with internal skeletons), whose PHYSICAL AND CHEMICAL purpose is to receive and interpret stimuli and to RECEPTORS. As we have noted, the transmit impulses. Parts of the nervous system changes in the outside environment that the include the brain, spinal cord, nerves, and other brain interprets as sensation are either physical components. or chemical in origin. By physical, we mean the types of data and phenomena that are studied in To be experienced through the senses, all the science of physics: matter, energy, and the data must be transmitted to the brain through interactions between them. Likewise, chemical, the nervous system. This happens through the refers to objects of study in the scientific realm of conversion and transmission of physical or chemistry: elements, compounds, and mixtures. chemical information, which takes place in sen- In the human body there are sensory nerves sory nerves known as receptors. A receptor is a devoted to the interpretation of physical or structure in the nervous system that receives spe- chemical data. Among the receptors of physical cific stimuli and is affected in such a way that it data are photoreceptors, which respond to light sends particular messages to the brain. The brain and therefore play a part in the sense of sight; interprets these messages as sensations corre- thermoreceptors, which respond to temperature sponding to the stimuli. Primary receptors are and are concerned with the sense of heat and those that directly convert stimuli to electronic cold; and nociceptors, or pain receptors. In addi- signals, which they send to the brain. Neurons, or tion, there are mechanoreceptors, which respond nerve cells, serve as primary receptors. In addi- to the mechanical properties of matter and are tion, there are secondary receptors, which simply involved in the senses of touch, hearing, and transmit signals between neurons. Secondary equilibrium. receptors induce a response in an adjoining neu- As for chemical data, they are interpreted ron, thus sending the signal down the line toward through chemoreceptors, which govern the sens- the brain. es of smell and touch. For this reason, chemore-
296 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:21 AM Page 297
ceptors sometimes are referred to as the chemical cally bipolar, with all the negative charges on the Chemorecep- senses. Our focus in the present context, of end or side where the oxygen atom is located and tion course, is on chemoreception, the term for the all the positive charges on the end or side of the physical process whereby organisms—not just hydrogen molecules. In petroleum and most humans, but virtually all animals—respond to other oily substances, however, the molecules chemical stimuli. typically include some combination of carbon and hydrogen, which have similar electronegativ- Distinguishing Gustation and ity values. For that reason, the electric charges are Olfaction distributed more or less evenly throughout the molecule. As a result, oily substances form very We tend to associate taste and smell, and indeed loose intermolecular bonds, and they tend not to there is some relation between them, but there bond with water-based substances. are also numerous distinctions. Not everyone with an acute sense of smell, for instance, has as As for taste buds and olfactory membranes, finely honed a sense of taste. Conversely, people it is fitting that the taste buds would be more with poor senses of smell do not necessarily suf- receptive to water-based foods and liquids, since fer a corresponding impairment in their taste most foods (including jalapeño peppers) contain buds. The neurological structures that have to do at least some water. Obviously, if we can taste with taste and smell, at least in higher animals, spicy-hot substances, then there must be are not the same. Whereas gustatory sensation some receptivity to oil-based foods. Part of this travels via secondary receptors called epithelial receptivity involves the olfactory membranes, cells, which pass messages to adjacent neurons, which, as we have noted, are more receptive to olfactory signals enter the nervous system oily materials. through primary receptors. Animals and Chemoreception The principal sensory organs of gustation and olfaction in a human being are, respectively, Just as taste and smell are sharply distinguished the taste buds on the tongue and the olfactory in humans (despite the fact that smell aids us in receptors located within the olfactory epithelium the process of tasting), the same is true of most in the nasal cavity. They are designed to respond vertebrates. In the case of invertebrates, such as to two entirely different types of chemical mate- worms, however, there is much less differentia- rials. Taste buds are best at receiving sensory data tion between gustatory and olfactory receptors. from water-soluble chemicals, or those that dis- These animals, in fact, may have only one chem- solve in water, while the olfactory receptors are ical sense, with only slight differences between most attuned to vapors that are water-insoluble. what scientists call distance chemoreception, WATER AND OIL. Almost everyone more or less the same as smell in vertebrates, and who has ever eaten a hot, spicy dish has tried to contact chemoreception, which corresponds to “put out the fire” in their mouths by drinking vertebrates’ sense of taste. water, only to discover to their dismay that water Such analogies can be made because dis- only seemed to make the problem worse. On the tance chemoreceptors appear to respond to non- other hand, milk is usually quite effective. The water-soluble substances in the same way that the reason is that most spicy-hot substances tend to olfactory receptors in vertebrates do, while con- be oily, and therefore they do not readily form tact chemoreceptors are more responsive to intermolecular bonds with water. On the other water-soluble chemicals. As with many other ani- hand, milk, though it is largely composed of mals, these senses are linked to numerous behav- water, also contains oily fat particles. Thanks to iors—not just feeding—in invertebrates. Dis- the chemistry of water- and oil-based substances, tance chemoreception enables invertebrates to it is true (as the old saying goes) that “oil and sense the presence of chemicals that pose a dan- water don’t mix.” ger, signaling the need to move away, while con- In water, hydrogen and oxygen have signifi- tact chemoreception assists the invertebrate in cantly different levels of electronegativity, or the determining when to mate and when to lay eggs. relative ability of an atom to attract valence elec- THE MANY FUNCTIONS OF trons, which are used in chemical bonding. CHEMORECEPTION. Terrestrial, or Therefore, a water molecule tends to be electri- land-based, animals whose skins secrete mucus
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 297 set_vol3_11 8/19/02 11:21 AM Page 298
Chemorecep- tion
TERRESTRIAL ANIMALS WHOSE SKINS SECRETE MUCUS, LIKE THE SNAIL, HAVE WHAT SCIENTISTS CALL THE COMMON CHEMICAL SENSE, which makes them sensitive to the presence of foreign chemicals on the sur- face of their bodies. (JLM Visuals. Reproduced by permission.)
(e.g., snails and slugs) as well as aquatic animals Mechanisms of Taste and have what scientists call the common chemical Smell sense, which makes them sensitive to the presence Scientists in the nineteenth century believed that of foreign chemicals anywhere on the surface of human tongues have receptors for four basic their bodies. Even humans and other animals tastes: sweet, sour, salty, and bitter. More current whose skin does not secrete mucus across its research, however, shows that the taste receptors entire surface have an evolutionary remnant of on the tongue are more complicated than was this sense. Thus, the mucous membranes in the previously thought. Still, it does seem to be the eyes, mouth, nose, and genitals respond to chem- case that specific kinds of taste buds are clustered ical irritants. in certain areas. Taste buds, so named because This is yet another example of the fact that, they look like plant buds when viewed under a while chemoreception in animals is associated microscope, cover the tongue and, to a lesser most readily with tasting and smelling, it is extent, can be found on the cheek, throat, and linked to myriad other functions as well. Some roof of the mouth. As we shall see, however, some insects may use chemoreception to detect the people have a greater concentration of taste buds presence of moisture, and many animals apply it than do others. In the mouth, saliva breaks down for a variety of purposes. Such purposes include the chemical components of substances, which selection and courtship of a mate as well as the travel through the pores in the papillae (small identification of friends or foes. A dogs’ sense of protuberances on the surface of the tongue) to smell tells it everything it needs to know about a reach the taste buds themselves. new animal or person it encounters; similarly, a When specific proteins in food bind to cat may identify another cat by sniffing its rec- receptors on the taste buds, these receptors send tum. Smell also can be used to mark territory, messages to the cerebral cortex, a surface layer on which is why dogs and cats mark their “turf” by the brain that coordinates sensory information. urinating. Both chemical senses, particularly And though taste buds in certain regions of the smell, are important mediums of communica- tongue have an affinity for particular flavors, as tion in the animal kingdom. we discuss later, the intricacies of the neural and
298 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:21 AM Page 299
chemical networks tend to suggest that nothing is ry and emotion, are coordinated with the sense Chemorecep- clear-cut about this highly complex biological of smell. Hence, as many people have observed, tion process. the sense of smell is linked strongly with long- OLFACTION: A DIRECT SENSE. term memory in a way that such senses as sight Before modern times, several of the leading the- and touch are not. ories of vision maintained that the eye actually interacts with the objects it sees. Now we know REAL-LIFE that our eyes simply receive light reflected off those objects. By contrast, smelling is a direct APPLICATIONS experience, because we inhale microscopic por- Taste Buds tions of substances that have evaporated and make their way into the nasal cavity, where they Humans have only about 10,000 taste buds, chemically interact with sense receptors. Cells in whereas rabbits have 17,000 and cows some the nose detect odors through receptor proteins 25,000. This seems more than a little ironic, since on the cell surface, which bind to odor-carrying humans enjoy by far the most varied diet. Both of molecules. A specific odorant docks with an the other animals are herbivores, meaning that olfactory receptor protein in much the same way they do not eat meat, nor are they accustomed to that a key fits into a lock. This, in turn, excites the sweets and the many other varieties of taste in the nerve cell, causing it to send a signal to the brain. diet of the average well-fed American. If any- Although there are many tens of thousands thing, cows, with about 50% more taste buds of odor-carrying molecule types in the world, than rabbits, eat a diet even more plain than that meaning that there are as many different smells, of their furry, fleet-footed fellow mammals. there are only hundreds (or at most about 1,000) Though our tasting equipment (that is, the different types of olfactory receptors in even the chemoreceptors for taste in our tongues) may be most sensitive animal species. This finding has led scientists to speculate that not every receptor much less sophisticated than that of cows or rab- recognizes a unique odorant molecule; rather, bits, the number of tastes our palate can recog- similar odorants can bind to the same receptor. nize is as varied as a spectrum of color swatches Another way to put this, in light of the lock and at a paint store. Despite such variation, there are key analogy, is that a few loose-fitting odorant only a few basic tastes, most notably, the ones “keys” of roughly similar structure can enter the that once were thought to constitute primary same receptor “lock.” tastes analogous to the primary colors: sweet, sour, salty, and bitter. Just as we sense smells directly, the olfactory sense is also direct in the way that signals are THE FOUR “BASIC TASTES.” transmitted to the brain. Vision, by contrast, puts When you lick an ice cream cone, you may notice into play several steps: a receptor cell detects light that you are experiencing the sweetness of it pri- and passes the signal to a nerve cell, which passes marily at the tip of your tongue. This is not sim- it on to another nerve cell in the central nervous ply because you are licking it with the tip but also system, which then relays it to the visual center of because there is a heavier concentration of sweet- the brain. In olfaction, on the other hand, the ness receptivity in that area. On the other hand, olfactory nerve cells perform all these functions. if you eat a sour gumball, you experience the In most animals these cells take scent mes- taste most notably on the sides of your tongue, sages directly to the nerve cells of the olfactory where receptivity to sour tastes is strongest. bulb in the brain. With insects and other inverte- Reception of salty tastes takes place near the front brates whose brains are relatively simple, func- of the tongue, just behind the tip. As for bitter tioning primarily as clearinghouses for sensa- tastes, the focal point of receptivity appears to be tions, the olfactory nerves send signals to the near the back of the tongue. The latter may be a olfactory ganglia, a mass of nerve tissue that con- highly useful adaptive mechanism we have devel- nects nerve cells external to the brain and spinal oped along the way, since many poisons are bit- cord. In higher animals, such as humans, olfacto- ter, and the gagging reflex takes place near the ry signals go to the olfactory cortex, a structure in back of the mouth. Not all bitter tastes are revolt- the brain where higher functions, such as memo- ing, however: olives, which many people love, are
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 299 set_vol3_11 8/19/02 11:21 AM Page 300
Chemorecep- bitter as well, as is coffee when it has no cream or eggs cooked over an open campfire are likely (all tion sugar to alter its flavor. other things being equal) to be more appealing to As we noted earlier, the organization of taste most people than eggs cooked in a skillet on an receptors on the tongue is not quite as simple as electric stove. But if the campfire is fueled with once was believed. For example, though our burning dung instead of wood, most Americans receptivity to sweetness seems to take place pri- would choose the stove. In sparsely forested parts marily at the tip, to a lesser extent we taste sweet- of the third world, however, animal dung is a ness and other flavors all over the tongue. Fur- principal form of fuel, and without it people thermore, such flavors as sweet, sour, bitter, and might have to eat meals raw. salty are not the sum total of “taste” as we experi- TEXTURE AND TEMPERATURE. ence it; not only smell but also texture affects the Texture and temperature may have less impact taste of substances. Genetic and cultural factors on taste than does smell, but these are still signif- also influence a person’s unique tastes and may icant factors. Take the example of three plates of explain why one person loves sweets while anoth- French fries. One is limp and soggy in consisten- er cannot get enough of tangy tastes. cy, and another is so crispy that the French fry crunches like potato chips. The third, however, is Taste and Other Senses just a bit crispy on the outside and just a bit soft on the inside. Most people, though certainly not Taste does not work alone; on the contrary, our all, would judge the third plate of French fry the sense of the smell, texture, and temperatures of most delectable—purely on the basis of texture. foods affects our overall perception of its flavor and in some cases its desirability. When food is in By the same token, many people are less than the mouth, it produces a scent, which enters the enthusiastic about boiled okra, owing to its slimy nose through the nasopharynx, an opening that consistency, whereas fried okra is more appealing links the mouth and the nose. Because we experi- to most Americans, since it lacks that gooey tex- ence smell more directly and our noses are more ture. Likewise, temperature plays more of a role sensitive to olfactory sensations than our taste than one might think. Many people, for example, buds are to gustatory ones, people often experi- find cold coffee unappealing, though others have ence the flavor of food first by its smell. This a fondness for iced coffee, at least if it has milk greatly affects our perception of what we eat. and sweetener. Similarly, many Americans enjoy the combination of cold ice cream and hot pie or For example, while many people like blue cobbler, partly because the contrast of tempera- cheese, many others despise it, and this probably tures adds to the overall flavor. has more to do with its smell than with its taste. While its taste is rather tangy, it is not quite as Different People, Different “radical” as the aroma of this cheese, which has Sensations been compared to everything from old socks to vomit. Other cheeses, such as gorgonzola and, Not all people “taste” the same—that is, not all particularly, Limburger, are even more pungent people have the same sense of taste or the same and therefore have more than their share of level of acuity for distinguishing different flavors. detractors—but again, the smell is more extreme One person may have 10–1,100 taste buds per than the taste. square inch (6.45 cm2) on the tongue, indicating On a more pleasant note, anyone who has a huge range of sensitivities with regard to gusta- ever enjoyed a good steak or a hamburger cooked tory data. Research also has shown that women, on an open grill will attest to the fact that a great on average, have more taste buds than men, deal of that enjoyment comes from the aromas of proving what many a woman has long asserted— cooking. Usually it is the smell of a delectable that women have “better taste” than men. food item, which we detect long before we taste Though it does appear that the average it, that causes our salivary glands to begin oper- female has a more acute sense of gustation than ating, preparing us for the process of consuming the typical male, taste buds are not the only bio- and digesting the dish. Additionally, different logical factor involved in recognizing flavors. For types of cooking have particular smells and tastes example, the amount of saliva a person naturally associated with them, which people may find generates and the amount of salt that appears in more or less appealing. For example, scrambled one’s saliva play a major role in determining an
300 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:21 AM Page 301
individual’s response to salty foods. A person mature, so do our taste buds, and their numbers Chemorecep- whose mouth generates less saliva is more sensi- increase, leading to greater sophistication of taste. tion tive to the salt in foods, whereas a person prone to Children tend to like very basic tastes, particularly generating a greater quantity of saliva is less like- sweet and sour, and respond much less favorably ly to taste the salt that has been added to a dish. to the subtlety in more complex dishes. This fact, That person is therefore more likely to add salt. combined with an increased awareness of health Ability to smell also varies from person to issues as one ages, explains why an adult might rel- person, though it appears that a less acute sense ish a broccoli casserole but find cotton candy so of smell may be a sign not merely of fewer olfac- sweet as to be repugnant, whereas a child’s reac- tory receptors but also of an actual olfactory dis- tion would probably be just the opposite. orders. Just as some people may be color-blind, it Just as taste buds mature along with the peo- appears that others are “smell-blind.”And just as ple who own them, they also age. Every 3-10 being color-blind can have very serious conse- days, on average, our taste buds regenerate them- quences, for instance by causing a color-blind selves, replacing old ones that have been worn driver to miss a red light, much the same is true out by foods that are too hot, too cold, or other- with an olfactory disorder. To a greater extent wise too taxing to the chemoreceptors in our than one might immediately guess, smell serves a tongues. But as people grow older, their taste protective function. For example, without a sense buds replace themselves less frequently, and of smell, one cannot tell if food is spoiled, unless, therefore their sense of taste becomes less finely of course, it has reached such a state of putrefac- tuned. An older person may require much more tion that it shows visible signs. For a person with sweetness or spice to taste a particular food. an ordinary sense of smell, however, rotten food sends a signal through the olfactory receptors, Chemoreception Impairment which may cause a gag reflex when smelling food and Disorders that has spoiled. Sense of smell also deteriorates with age; as we AGE AND SENSE OF TASTE. It noted earlier, this can pose dangers, because a is an experience familiar to many people, and it person depends on the sense of smell for protec- goes something like this. Let us say that in his tion more than one might imagine. For example, boyhood, a man enjoyed a particular brand of in addition to the inability to detect spoiled food, candy, of which he could never get enough. Left an elderly person would be far less likely to smell to his own devices, he probably would have eaten smoke if a building were on fire. Older people so much that he would have become sick. The also are less likely to be cognizant of olfactory fact that this never happened had more to do data that send messages concerning unpleasant with his parents—and the fact that his allowance smells of a less critical nature—body odor, for money had to go for other things as well—than it example. did with his own natural sense of restraint. So he Many of the problems of gustation and dreamed of the day when he became a grown-up, olfaction suffered by the elderly are reflected, at a when he could eat whatever he wanted. much younger age, in the bodies of smokers. In Eventually, he forgets this dream amid the addition to its many other negative effects, smok- many distractions of adolescence, but then one ing deadens taste buds and desensitizes the olfac- day many years later, as a grown man, he happens tory receptors. It is not uncommon to see a heavy to see this particular item of candy in a store, and smoker salting pizza or some other food that for all his childhood memories come back to him. people with ordinarily functioning taste buds He buys several pieces, thrilled that he can enjoy would not seem to require any salt. As for olfac- in complete freedom what was once a rare treat. tory sensation, a smoker becomes accustomed to He can barely wait to get into his car, open the the reek of stale smoke and ashes. first piece, put it in his mouth—and then he On a more temporary basis, many people recoils in disgust, thinking, How did I ever enjoy find their senses of taste and smell impaired by that? Disappointed, he throws away the rest of illness. A person with a cold or flu, even at its the candy. final stages, usually has enough congestion that The candy, of course, has not changed, but the senses of both smell and taste are limited, if the man—and his taste buds—have. As we not almost nonexistent. In this instance, the lack
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 301 set_vol3_11 8/19/02 11:21 AM Page 302
Chemorecep- tion
TASTE AND SMELL ARE NOT PURELY BIOLOGICAL, BUT ALSO REFLECT CULTURAL FACTORS. AS LATE AS THE 1970S, MOST AMERICANS WOULD HAVE SAID THAT THE IDEA OF EATING RAW FISH WAS REVOLTING. TODAY, AMERICA’S CITIES BRISTLE WITH JAPANESE RESTAURANTS THAT SERVE SUSHI, SHOWING THAT CULTURAL TASTES CAN CHANGE. (© Japack Company/Corbis. Reproduced by permission.)
of ability to taste serves to illustrate the strong cause more permanent damage to the intricate link between gustation and olfaction: the taste neural networks that process tastes and smells. buds themselves are working fine, but the lack of Drugs such as lithium, used to treat bipolar smell, resulting from congestion, hinders the disorder (what used to be called manic depres- brain’s ability to process flavor. sion) also may cause taste and smell disorders. TASTE AND SMELL DISOR- This occurs because certain drugs (lithium is just DERS. In addition to people whose olfactory one example among many) inhibit the action of and gustatory senses are impaired by age, illness, certain enzymes, affect the body’s metabolism, or smoking, between two million and four mil- and interfere with the neural networks and lion Americans suffer from some sort of taste or receptors involved in tasting and smelling. Expo- smell disorder. The inability to taste or smell not sure to such environmental toxins as lead, mer- only robs an individual of certain sensory pleas- cury, insecticides, and solvents (e.g., paint thin- ures, it also can be dangerous to one’s mental ner) also can damage taste buds and sensory cells health. Some psychiatrists believe that the lack of in the nose or brain. taste and smell can have a profoundly negative effect on a person’s quality of life, leading to Culture and Chemoreception depression or other psychological problems. One of the favorite delicacies in the Philippines is Whereas impairments of smell and taste known as dinuguan, or pork cooked in pork brought on by cold, flu, various viral and bacter- blood. Chances are that a visitor from England or ial infections, and even allergies are usually tem- northern Europe, when told the constituents of porary, some other illness-related taste and smell the dish, would feel right at home; an American, disorders are more long term. Such is the case however, most likely would try to think of an with neurological disorders due to brain injury excuse to pass up this Filipino delight. To most or diseases such as Parkinson’s or Alzheimer’s Americans, it would seem that eating dinuguan, (conditions marked by tremors and mental dete- or the many varieties of blood pudding or blood rioration, respectively). These conditions can sausage common in England and Scandinavia, is
302 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:21 AM Page 303
simply “gross”—that it is objectively and unques- ble of detecting odors so faint that their propor- Chemorecep- tionably disgusting. But in the 1960s or even the tion of the surrounding air is in the range of only tion 1970s, most Americans, even in large cities, a few parts per trillion. Many researchers are would have said that the idea of eating raw fish beginning to wonder whether smell does not play was revolting. Today, however, America’s cities a greater role in human behavior and biology and suburbs bristle with Japanese restaurants than previously was believed. For example, that serve sushi, an indication of the fact that cul- research has shown that only a few days after the tural tastes can change. birth of her baby, a human mother can smell the As is discussed in Parasites and Parasitology, difference between a vest worn by her baby and it should be noted that improperly cooked pork one worn by another. and fish (especially raw fish) are quite likely to Nevertheless, the fact remains that the olfac- serve as hosts for disease-carrying worms, so one tory abilities of many animals are far beyond should exercise care before sitting down to a plate those of humans. This is a fact that hardly needs of dinuguan or sushi. But then again, many scientific verification, since most of us have Americans like their steaks on the rare side, and observed dogs’ reliance on their strong sense of undercooked beef certainly has its share of smell. This explains why police use dogs to detect pathogens (disease-carrying parasites) as well. It illegal drugs and explosives and to track runaway seems that it is not health concerns that explain prisoners or the bodies of murder victims. Dogs our cultural double standards about certain food are not the only animals gifted with acute senses items. of smell, which aid them in finding their way to Of course, Americans are not the only peo- specific targets. Salmon, for example, manage to ple with quirky standards regarding tastes and find their way back to the streams where they smells; in fact, every culture has its idiosyn- were hatched, guided by their sense of smell. (For crasies. In China it is not considered at all offen- more about animals’ navigational abilities, see sive for one’s breath to smell heavily of onions; Migration and Navigation.) on the other hand, the smell of dairy products, Most vertebrates other than humans have virtually nonexistent in Chinese cuisine, is con- many more olfactory nerve cells in a proportion- sidered highly offensive. What does all of this ately larger olfactory epithelium, and this proba- prove? Only that taste and smell are not purely bly gives them much more sensitivity to odors. In biological but also reflect cultural factors. addition, most land vertebrates have a specialized During the nineteenth century, many Euro- scent organ in the roof of their mouths called the pean scientists embraced a racist theory concern- vomeronasal organ, which gives them far more ing the olfactory capabilities of different peoples sensitivity to odors than humans have. around the world. According to this highly unsci- PHEROMONES. Some animals are entific “theory,”non-Europeans were more prim- known particularly for the odors they excrete, itive than Europeans and therefore closer to ani- especially when it is an animal such as a skunk or mals, which meant that they had a stronger sense stinkbug that puts off a repellent odor as a of smell. Completing the loop of this circular defense mechanism. But animals also send out logic, subscribers to this nonsense maintained much more subtle smells known as pheromones. that because non-Europeans had a stronger sense Chemical substances produced and secreted by of smell, it proved they were more primitive than animals, pheromones serve as stimuli for behav- Europeans! ioral responses on the part of other animals of the same species. Pheromones are common Humans, Animals, and Smell among insects as well as many vertebrates, but By the early twentieth century, physiologists had they are nonexistent among bird species. begun to explore much more scientific ideas con- Among so-called “social insects” such as bees cerning olfactory and gustatory abilities in and ants, pheromones play a particularly strong humans—abilities that, needless to say, are not a role. The queen honeybee gives off what is called function of race or ethnicity. Only one assump- the queen substance, a pheromone that acts to tion of the old-fashioned European scientists was prevent the development of ovaries among work- correct: that animals have a stronger sense of ers, which are biologically unproductive females. smell than do humans. The human nose is capa- Pheromones are vital for communication among
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 303 set_vol3_11 8/19/02 11:22 AM Page 304
Chemorecep- social insects, which have little or no sense of tion KEY TERMS sight. Ants, bees, and wasps send out smells to alarm others of danger, and ants may create a path of pheromones to guide others to a food CHEMORECEPTION: A physiological source. process whereby organisms respond to The function for which pheromones are chemical stimuli. most widely known, however, is as a sex attrac- GUSTATORY: Of, or involving, the tant. Male musk deer are noted for their excre- tion of musk, which, as the result of to its pleas- sense of taste (gustation). ant and powerful smell, is often an ingredient in INVERTEBRATE: An animal without perfume manufacture. Though musk is a sex an internal skeleton. attractant, it is not a pheromone, which is a much less obvious scent and which, as we have NERVOUS SYSTEM: A network, noted, likely has an effect only on animals of the found in the bodies of all vertebrates, same species. whose purpose is to receive and interpret Experiments have shown that male mice stimuli and to transmit impulses. Parts of who lack a gene for a pheromone receptor are the nervous system include the brain, likely to attempt to mate with males, simply spinal cord, and nerves. because they cannot tell the difference. With humans, of course, it is easy to tell the difference OLFACTORY: Of, or involving, the between males and females on sight, but do sense of smell (olfaction). humans also respond to pheromones? The fact that these chemicals theoretically could induce PHEROMONES: Chemical substances mating behavior led cologne and perfume mak- produced and secreted by animals, which ers long ago to embrace the idea of human serve as stimuli for one or more behavioral pheromones, whose presence in a manufactured responses on the part of other animals of scent obviously would be a boon to many a sin- the same species. gle man or woman. Despite the enthusiastic claims of perfume manufacturers, however, RECEPTOR: A structure in the nerv- many scientists have yet to be convinced that ous system that receives specific stimuli pheromones play a significant role for humans. and is affected in such a way that it sends Regarding the fact that the human anatomy particular messages to the brain. These includes the vestige of a vomeronasal organ, the messages are interpreted by the brain as olfactory researcher Charles Wysocki told Lee Bowman in an article published on the National sensations corresponding to the stimuli. Library of Medicine Web site,“It’s like the appen- SENSE: The means by which an dix—it’s there, but it doesn’t seem to do any- organism (usually an animal) receives sig- thing.” As for scent makers’ promises that nals regarding physical or chemical pheromones will help users attract partners, Wysocki said, “Sure the claims are out there. ... changes or both in its environment. Some- ‘All you have to do is put this on and you’ll score.’ times called sensory reception or sensory But there’s nothing in the published biomedical perception. literature [to indicate] that we have any kind of pheromone that draws a partner.” Research does STIMULUS: Any phenomenon (i.e., an suggest that people give off chemical messages observable fact or event, such as an envi- that correspond to certain moods, but it is a long ronmental change) that directly influences way from this to the idea of a spray-on aphro- the activity or growth of a living organism. disiac. VERTEBRATE: An animal with an WHERE TO LEARN MORE internal skeleton. Ackerman, Diane. A Natural History of the Senses. New York: Vintage Books, 1991.
304 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 305
Bowman, Lee. “A Nose for Romance?” U.S. National Finger, Thomas E., Wayne L. Silver, and Diego Restrepo. Chemorecep- Library of Medicine/National Institutes of Health The Neurobiology of Taste and Smell. New York: tion (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 305 set_vol3_11 8/19/02 11:22 AM Page 306
BIOLOGICALBiological Rhythms RHYTHMS
CONCEPT Nor do all cycles involve sleep and wakeful- People frequently talk about body clocks, a term ness: menstruation, for instance, is a monthly cycle that refers to the patterns of energy and exhaus- related to the sloughing off of the lining of the tion, functioning and resting, and wakefulness uterus, a reproductive organ found in most female and sleep that characterize everyday life. In fact, mammals. Another biological rhythm is the beat- the concept of the body clock, or circadian ing of the heart, which, of course, takes place at rhythm, is part of a larger picture of biological very short intervals. Nonetheless, the circadian cycles, such as menstruation in mammalian rhythm is the most universal of biological cycles, females. Such cycles, which assume a variety of and it is the focus of our attention in this essay. forms in a wide range of organisms, are known as BIOLOGICAL CLOCKS. In dis- biological rhythms. These rhythms may be cussing the operation of biological rhythms, the defined as processes that occur periodically in an term biological clock often is used. A biological organism in conjunction with and often in clock is any sort of mechanism internal to an response to periodic changes in environmental organism that governs its biological rhythms. conditions—for example, a change in the One such mechanism, which we examine in the amount of available light. Not all aspects of the next section, is the pineal gland. Internal clocks body clock are part of day-to-day experience, and operate independently of the environment but this is fortunate, since these interruptions in the also are affected by changes in environmental healthy flow of biological rhythms can threaten conditions. the well-being of the human organism. Among these challenges to the ordered working of bodi- Examples of such alterations of conditions ly “clocks” are jet lag, seasonal affective disorder include a decrease (or increase) in the hours of (SAD), and other disorders linked to a range of available light due to a change of seasons or a causes, including drug use. change in time alteration due to rapid travel from west to east or north to south. In the latter instance, a condition known as jet lag—increasingly familiar HOW IT WORKS to humans since the advent of regular air travel in the mid–twentieth century—may ensue. Understanding Biological Rhythms The Pineal Gland
Among the many varieties of biological rhythm, Governing human biological cycles—the “com- the most well known are those relating to sleep puter” that operates our biological clocks—is the and wakefulness, which are part of the circadian pineal gland, a cone-shaped structure about the rhythm that we discuss later in this essay. Circa- size of a pea located deep inside the brain. At one dian, or daily, cycles are only one type of biolog- time, the great French philosopher and mathe- ical rhythm. Some rhythms take place on a cycle matician René Descartes (1596–1650) held that shorter than the length of a day, while others are the pineal gland was actually the seat of the soul. based on a monthly or even an annual pattern. Though it might seem absurd now that a respect-
306 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 307
ed thinker would seriously attempt to locate the the body clock. Complementary to melatonin is Biological soul in space, as though it were a physical object, serotonin, which is as important to waking func- Rhythms Descartes’s claim resulted from hours of tions as melatonin is to sleepiness. Like mela- painstaking dissection conducted on animals. tonin, serotonin serves several functions, includ- In searching for the human soul, Descartes ing the regulation of attention. sought that ineffable quality described some fif- Serotonin is among the substances responsi- teen centuries earlier by the Roman emperor and ble for the ability of a human with a healthily philosopher Marcus Aurelius (121–180), who functioning brain to filter out background noise wrote, “This being of mine, whatever it really is, and sensory data. Thanks in part to serotonin, consists of a little flesh, a little breath, and the you are able to read this book without having part which governs.” As it turns out, the pineal your attention diverted by other sensory data gland is, in a sense, “the part which governs”: it around you: the voice of someone talking nearby, may not be the home of the soul (which, in any the sunlight or a bird singing outside, the hum of case, is not a question for science), but it does a light or a fan in the room. govern human circadian rhythms and thus has a By contrast, a person under the influence of powerful effect on the manner in which we expe- the drug LSD (lysergic acid diethylamide) is not rience the world. able to make those automatic filtering adjust- MELATONIN. The pineal gland secretes ments facilitated by serotonin. Instead, he or she two hormones (molecules that send signals to the is at the mercy of seemingly random intrusions body), melatonin and serotonin. During the late of outside stimuli, such as the color of paint on a 1990s, melatonin became a popular over-the- wall or the sound of music playing in the back- counter treatment for persons afflicted with sleep ground. The secret of LSD’s powerful hallucina- disorders, because it is believed that the hormone tory effect can be attributed in part to the fact is associated with healthful sleep. Scientists do that it apparently mimics the chemistry of sero- not fully understand the role that melatonin tonin in the brain, “tricking” the brain into plays in the body, although it appears that it reg- accepting the LSD itself as serotonin. ulates a number of diurnal, or daily, events. With regard to body clocks and biological In addition, melatonin seems to serve the rhythms, serotonin plays an even more vital gov- function of controlling fat production, which is erning role than does melatonin, since melatonin, one reason why good sleep is associated not only in fact, is created by the chemical conversion of with a healthy lifestyle but also with a healthy serotonin. On regular daily cycles the body con- physique. Many health specialists maintain that verts serotonin to melatonin, thus influencing the for adults there is a close link between a “spare organism to undergo a period of sleep. Then, as tire” (that is, fat accumulation around the waist) the sleeping period approaches its end, the body and stress, lack of sleep, and low melatonin levels. converts melatonin back into serotonin. Among the many roles melatonin plays in the body is its job of regulating glucose levels in REAL-LIFE the blood, which, in turn, serve to govern the APPLICATIONS production of growth hormone, or soma- totropin. Growth hormone is associated with the Circadian Rhythms development of lean body mass, as opposed to fat, which is why athletes involved in the The term circadian derives from the Latin circa Olympics and other major sporting competi- (“about”) and dies (“day”), and, indeed, it takes tions sometimes have illegally “doped” with it as “about” a day for the body to undergo its entire a means of increasing strength. It is not surpris- cycle of serotonin-melatonin conversions. In ing, then, to learn that children—who clearly fact, the cycle takes almost exactly 25 hours. Why need and use more growth hormone and who 25 hours and not 24? This is a fascinating and also need more hours of sleep than adults—also perplexing question. have higher melatonin levels. It would be reasonable to assume that natu- SEROTONIN. Melatonin is not the ral selection favors those organisms whose body only important hormone that is both secreted by clocks correspond to the regular cycles of Earth’s the pineal gland and critical to the regulation of rotation on its axis, which governs the length of a
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 307 set_vol3_11 8/19/02 11:22 AM Page 308
Biological day—or, more specifically, a solar day. Yet the This fact, by the way, points up the great wis- Rhythms length of the human daily cycle has been con- dom of a practice common in Spanish-speaking firmed in countless experiments, for instance, countries and some other parts of the world: with subjects in an environment such as a cave, siestas. The siesta devotes one of the least pro- where levels of illumination are kept constant for ductive parts of the day, the post-lunch lag, to weeks on end. In each such case, the subject’s rest, so that a person is equipped with energy for body clock adopts a 25-hour cycle. the rest of the afternoon and early evening—at POSSIBLE EXPLANATIONS precisely the time when energy is at a high. To FOR THE 25-HOUR CYCLE. One compensate for the time “lost” on napping, many might suggest that the length of the cycle has such societies maintain a later schedule, with something to do with the fact that Earth’s rate of offices closing in the early evening rather than rotation has changed, as indeed it has. But the late afternoon and with evening meals served at speed of the planet’s rotation has slowed, about 9:00 P.M. because—like everything else in the universe—it Note that even though our body clocks run is gradually losing energy. (This is a result of the on a 25-hour day, they readily adjust to the 24- second law of thermodynamics.) hour world in which we live. As long as a person About 650 million years ago, long before is exposed to regular cycles of day and night, the humans or even dinosaurs appeared on the pineal gland automatically adapts to the length of scene, Earth revolved on its axis about 400 times a 24-hour solar day. If a person has been living in in the interval required to revolve around the a sunless cave, with no exposure to daylight for a Sun. This means that there were 400 days in a length of time, it would take about three weeks year. By the time Homo sapiens emerged as a for the pineal gland to reset itself, but thereafter species about two million years ago, days were it would track with Earth time consistently. considerably longer, though still shorter than The adjustment of the body clock is not sim- they are now. This only means that the 25-hour ply a matter of sending signals for sleep and human body clock would have been even less wakefulness. In fact, the pineal gland is at the compatible with the length of a day in the distant center of a complex information network that past of our species. controls sleep cycles, body temperature, and One possible explanation of the 25-hour body stress-fighting hormones. Hence the link that we clock is the length of the lunar day, or the amount noted earlier between body temperature and cir- of time it takes for the Moon to reappear in a given cadian rhythms: just as the body reaches its low- spot over the sky of Earth. In contrast to the 24- est temperature in the circadian trough, it also hour solar day, the lunar day lasts for 24 hours and enters a period of extremely deep sleep. 50 minutes—very close in length to the natural REGULATING THE BODY human cycle. Still, the exact relationships between CLOCK. Tied in with these sleep patterns are the Moon’s cycles and those of the human body many other bodily functions. For example, body- have not been established fully: the idea that lunar builders and others who work out with weights cycles have an effect on menstruation, for instance, experience their greatest benefits not when lifting appears to be more rumor than fact. (which, in fact, tears muscles down rather than PEAKS AND TROUGHS. On the building them up) but when resting—and par- other hand, circadian rhythms do mirror the pat- ticularly when sleeping—after having worked terns of the Moon’s gravitational pull on Earth, out earlier in the day. Likewise, deep sleep is asso- which results in a high and low tide each day. ciated with growth, as we have noted. Further- Likewise, the human circadian rhythm has its more, it appears that dreaming may be essential highs and lows, or peaks and troughs. In the cir- to the well-being of the psyche, providing an cadian trough, which occurs about 4:00 A.M., opportunity for the brain to “clean out” the sig- body temperature is at its lowest, whereas at the nals and data it has been receiving for the pre- peak, around 4:00 P.M., it reaches a high. A person ceding 16 hours of wakefulness. may experience a lag in energy after lunchtime, Given these and other important functions but usually by about 4:00 in the afternoon, ener- associated with deep sleep, it follows that the gy picks up—a result of the fact that the body has maintenance of the body clock is of great impor- entered a peak time in its cycle. tance to the health of the human organism. For-
308 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 309
Biological Rhythms
VIEW OF THE MIDNIGHT SUN IN LYNGENFJORD, NORWAY. THE “BODY CLOCK” CAN BE DISRUPTED BY CHANGES IN THE AMOUNT OF AVAILABLE LIGHT, SUCH AS OCCUR IN REGIONS OF THE EXTREME NORTH THAT UNDERGO PERIODS OF ALMOST CONSTANT DAYLIGHT FROM MID-MAY TO LATE JULY. (© Bettmann/Corbis. Reproduced by permission.)
tunately, animals’ brains are programmed to person may drink coffee to stay awake at night, make adjustments of the body clock so as to but he or she also may experience a sleep disor- accommodate the daily cycles of light and dark. der as a result of some other situation, which may We have discussed the means by which the or may not be the result of purposeful action. human brain achieves this accommodation, but Examples of sleep disorders that are the by-prod- it is not the only animal brain thus equipped. uct of other activities include jet lag as well as the “Bird brains” (quite literally) are similarly able to malfunctioning of the body clock that often make an adjustment: whereas humans have a stems from recreational drug use. natural 25-hour clock, birds run on a 23-hour The causes for interference with a person’s circadian cycle, but their pineal glands likewise body clock may be outside that person’s control assist them in adapting to the 24-hour solar day. to one degree or another. Working at night, for The brains of birds, humans, and other ani- instance, is a condition that almost never suits a mals respond to environmental features known human being, no matter how much a person may collectively as zeitgebers (German for “time insist that he or she is a “night person.” Never- givers”), which aid in the adjustment to the solar theless, a person may be required by circum- schedule. The most obvious example is the stances, such as schedule, economic necessity, or change from day to night, but there are other job availability, to take a night job. Another zeitgebers of which we are less aware in our ordi- example of interference with the body clock nary experience. For example, Earth’s magnetic would be narcolepsy (a condition characterized field goes through its own 24-hour cycle, which by brief attacks of deep sleep) or some other con- subtly influences our biological rhythms. dition that is either congenital (something with which a person is born) or symptomatic (a Interfering with the Body symptom of some other condition rather than a Clock condition in and of itself). In modern life humans often interfere with their WHITE NIGHTS. At least one example own body clocks, either deliberately and directly of human experience involving interference with or indirectly and by accident. On the one hand, a the body clock relates to conditions completely
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 309 set_vol3_11 8/19/02 11:22 AM Page 310
Biological outside people’s control. This is the situation of nia, which is a result of lost sleep due to the fact Rhythms the “white nights” or “midnight sun,” whereby that the sufferer actually is waking up numerous regions in the extreme north—Russia, Alaska, times throughout the night. and Scandinavia—undergo periods of almost At the other extreme from apnea, in terms of constant daylight from mid-May to late July. prevalence among the population, is Kleine- (These are matched by a much less pleasant phe- Levin syndrome, which typically affects males in nomenon: near constant darkness from mid- their late teens or twenties. The syndrome may November to late January.) bring about dramatic symptoms that range from During those times people often line their excessive sleepiness, overeating, and irritability to windows with dark material to make it easier to abnormal behavior, hallucinations, and even loss go to sleep in a world where the Sun is nearly as of sexual inhibitions. Added to this strange mix is bright at 3:00 A.M. as it is at 3:00 P.M. The situa- the fact that Kleine-Levin syndrome typically tion is even more pronounced in Antarctica, disappears after the person reaches the age of 40. where researchers and adventurers may find JET LAG. There are numerous classes of themselves much closer to the South Pole than sleep disorders, among them circadian rhythm people in Saint Petersburg, Anchorage, or Oslo disorders—those related to jet lag or work sched- are to the North Pole. ules. As we have seen, the pineal gland can adjust In Antarctica the human population is much easily from a natural 25-hour cycle to a 24-hour higher in the summer, a period that coincides one, but it can do so only gradually, and it cannot with the depth of winter in the Northern Hemi- readily adapt to sudden changes of schedule, sphere, and scientists or mountaineers trekking such as those brought about by air travel. through remote regions may be forced to sleep in tents that keep out the cold but let in the light. Jet lag is a physiological and psychological Usually, however, the rugged conditions of life condition in humans that typically includes near the South Pole involve such exertions that fatigue and irritability; it usually follows a long by nighttime people are ready to sleep, light or no flight through several time zones and probably light. results from disruption of circadian rhythms. SOME SLEEP DISORDERS. The name is fitting, since jet lag is associated almost exclusively with jets: traveling great dis- Few people ever get to experience the white tances by ship, even at the speeds of modern nights, but almost everyone has suffered through craft, allows the body at least some time to adjust. a temporary bout of insomnia—a condition known specifically as transient insomnia. An Older modes of travel were too slow to unfortunate few suffer from chronic insomnia or involve jet lag; for this reason, the phenomenon some other sleep disorder. Insomnia, the inabili- is a relatively recent one. The only people who ty to go to sleep or to stay asleep, is one of the two manage to experience jet lag without riding in a most common sleep disorders, the other being jet are those traveling in even faster craft—that is, hypersomnia, or excessive daytime sleepiness. astronauts. An astronaut orbiting Earth in a Transient forms of insomnia are usually space shuttle experiences rapid shifts from day to treatable with short-term prescription drugs, but night; if manned vessels ever go out into deep more serious conditions qualify as actual disor- space, scientists will face a new problem: assisting ders and may require long-term treatment. These the adjustment of circadian cycles to that sunless disorders may have as their cause drug use (either realm. prescription or illegal) as well as medical or psy- On a much more ordinary level, there is the chological problems. Among the most common jet lag of people who travel from the United of these more specialized disorders is apnea, the States East Coast to Europe or between the East regular cessation of breathing whose most Coast and West Coast of the United States. The noticeable symptom is snoring. worst kinds of jet lag occur when a person flies Apnea, which affects a large portion of the from west to east across six or more time zones: United States population, is a potentially very anyone who flies to Europe from the East Coast serious condition that can bring about suffoca- is likely to spend much of the first day after tion or even death. More often its effects are less arrival sleeping rather than sightseeing. There- dramatic, however, and manifest in hypersom- after, it may take up to ten days (usually as long
310 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 311
as or longer than most European vacations) for of working at night so unattractive, even though Biological the body to adjust fully. it is clear that in our modern society some night- Rhythms By contrast, someone who has flown from shift positions are essential. the East Coast to the West Coast feels unexpect- People who have offices in their homes may ed energy. The reason is that when it is 6:00 A.M. find it beneficial to work at late hours, when the in the Pacific time zone, it is 9:00 A.M. in the east- phone is not ringing and the world is quiet, but ern time zone, to which a person’s body clock (in the “extra time” gained by working at night ulti- this particular scenario) is still adapted. There- mately is counterbalanced by the body’s reaction fore, at 6:00 in the morning, the newly arrived to changes in its biological rhythms. Such is also traveler will feel as good as he or she would nor- the case with night-shift workers, who never real- mally feel at 9:00 A.M. back east. Conversely, at ly adjust to their schedules even after years on the 9:00 P.M. in the west, it is midnight in the east. job. This means that the traveler is likely to feel tired There is such a thing as a “night person,” or long before his or her ordinary bedtime. someone with a chronic condition known as There are steps one can take to avoid, or at delayed sleep phase syndrome. A person with this least minimize the effects of, jet lag. One is to syndrome is apt to feel most alert in the late ensure a regular sleep schedule prior to traveling, evening and night, with a corresponding lag of so as to minimize the effects of sleep deprivation, energy in the late mornings and afternoons. Even if the latter does occur. It is even better if one can, so, given the role of sunlight in governing the in the days prior to leaving, adopt a schedule body clock, the condition does not really lend adjusted to the new time zone. For example, if itself to regular night work but rather merely one were traveling from the East Coast to Cali- causes a person to experience problems adapting fornia, one would start going to bed three hours to the schedule maintained by most of society. earlier, and rising three hours earlier as well. One possible means of dealing with this problem Changing eating habits in the days prior to is to go to bed three hours later than would be departure may also help. Some experts on the normal for an ordinary 9-to-5 schedule, and subject recommend a four-day period in which wake up three hours later as well; unfortunately, one alternates heavy eating (days one and three) that is not practical for most people. Another and very light eating (days two and four.) It is treatment applied with success is exposure of a believed that high-protein breakfasts stimulate person to artificial, high-intensity, full-spectrum the active, waking cycle, while high-carbohydrate light, which augments the effect of sunlight, evening meals stimulate the resting cycle; con- between the hours of 7:00 and 9:00 A.M. versely, depriving the liver of carbohydrates may COLONIZING THE NIGHT? In prepare the body clock to reset itself. this vein it is interesting to note that some of the ON THE NIGHT SHIFT. At least optimistic predictions made in 1987 by Murray the body does adjust to jet lag; on the other hand, Melbin in his fascinating book Night as Frontier: it may never become accustomed to working a Colonizing the World After Dark have not come to night shift. If you stay up all night studying for a pass. Melbin, who explains circadian rhythms test, you will find that around 4:00 A.M. you hit a and the body clock in a highly readable and “lull” when you feel sleepy—and because of the understandable fashion, makes a brilliant analy- lowered temperature at the circadian trough, you sis of the means by which industrialized societies also feel cold. You might assume that this situa- have extended their daily schedules into the tion would improve if you worked regularly at nighttime hours. Thus, to use his analogy, such night, but the evidence suggests that it does not. societies have “colonized” the night. As long as a person lives in a sunlit world of Until the invention in 1879 of the first suc- 24-hour solar days, the body clock remains cessful incandescent lamp by the American adapted to that schedule, and this will be true inventor Thomas Edison (1847–1931), activity at whether the person is at home and in bed or at night was limited. Torches, crude lamps, and can- work behind a desk or counter during the hours dles in ancient times; metal lamps in the Middle of night. In other words, the person always will Ages; and the various oil-burning lamps that hit the circadian trough about 4:00 A.M. This is applied the glass lantern chimney devised in one of the reasons why most people find the idea 1490 by the Italian scientist and artist Leonardo
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 311 set_vol3_11 8/19/02 11:22 AM Page 312
Biological Before the events of September 11, 2001, Rhythms when terrorists crashed hijacked planes into the World Trade Center in New York City and the Pentagon in Washington, D.C., the burden on America’s airports had become almost unbear- able. The concourses of Hartsfield International in Atlanta, Georgia, the world’s busiest airport, were a nonstop melee of people, luggage, and noise, as travelers fought to change flights or pick up their bags. One obvious solution to the prob- lem would have been to adopt a round-the-clock airport schedule, with flights regularly leaving at 3:00 or 4:00 in the morning. No airport rushed to enact such a measure, however, and after September 11 heightened security concerns made it unlikely that any facil- ity would adopt a 24-hour schedule, with the additional security threats it entailed. For a time at least, the volume of air traffic decreased dra- matically, but even as it climbed back up in the months after the terrorist attacks, airports con- tinued to operate on their ordinary schedules. The reason appears to be the difficulty of per- UNTIL THOMAS EDISON INVENTED THE FIRST SUCCESS- suading people to adjust to a late-night sched- FUL INCANDESCENT LAMP IN 1879, ACTIVITY AT NIGHT WAS LIMITED. WITH ELECTRIC LIGHTING, INDUSTRIALIZED ule—that is, finding enough people willing to fly SOCIETIES HAVE BEEN ABLE TO “COLONIZE” THE NIGHT, in the middle of the night and enough baggage EXTENDING DAYTIME ACTIVITIES INTO THE NIGHTTIME handlers and ticket agents willing to service HOURS. (© Bettmann/Corbis. Reproduced by permission.) them. There are, it seems, limits to the extent to which nighttime can be colonized. da Vinci (1452–1519) all made it possible for a Other Examples of person to read at night and to perform other lim- Biological Rhythms ited functions. After their introduction in the nineteenth century, street lamps in London, the Although the circadian rhythms of sleep and first of their kind, made the streets safe for walk- wakefulness are particularly important examples ing at late hours, but travel, large gatherings, and of biological cycles, they are far from the only outdoor work after dark remained difficult ones. Not all rhythms, in fact, are circadian. Some before the advent of electric light. are ultradian, meaning that they occur more than once a day. Examples include the cycles of taking Since 1879 the Western world has indeed in fluid and forming urine as well as cell-division “colonized” the night with all-night eateries, cycles and cycles related to hormones and the roads that are never free of traffic, and round- endocrine glands that release them. For instance, the-clock entertainment on radio, TV, and now the pituitary gland in the brain of a normal male the Internet. There are even hardware stores open mammal secretes hormones about every one to all night in some major cities. Certainly today two hours during the day. there are more gas stations, restaurants, televi- The overall cycle of sleeping and waking is sion programs, and customer-service telephone circadian, but there is an ultradian cycle within lines that operate 24 hours than there were in sleep as the brain moves from drowsiness to REM 1987, when Melbin wrote his book, but it is (rapid eye movement, or dream, sleep) to dozing, unlikely that Americans will ever fully “colonize” then to light and deep sleep, and finally to slow- the night in the thoroughgoing fashion that their wave sleep. Over the course of the night, this ancestors colonized the New World. An example cycle, which lasts about 90 minutes, repeats itself of the limits to night colonization is in air travel. several times. Among the functions affected by
312 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 313
Biological Rhythms
A ROCKY MOUNTAIN GOAT SHEDS ITS THICK WINTER FUR. THE SHEDDING OF FUR, SKIN, OR ANTLERS IS ONE EXAM- PLE OF A CIRCANNUAL CYCLE, WHICH TAKES A YEAR TO COMPLETE. (© W. Wayne Lockwood, M.D./Corbis. Reproduced by permis- sion.)
this cycle are heart rate and breathing, which The reasons for this alignment of menstrual slow down in deep sleep. Additionally, heartbeat cycles are not completely understood. Nor is the and respiration are themselves ultradian cycles of cause of the 28-day cycle evident. If it were the very short duration. result of the Moon’s cycles, all women on Earth MENSTRUATION AND OTHER would have menstrual cycles that last 29.5 days, which is how long it takes the Moon to travel INFRADIAN CYCLES. In contrast to the around Earth. Furthermore, if there were a clear ultra-quick ultradian cycles of the beating heart connection between the Moon and menstrua- and the lungs’ intake and outflow of oxygen, tion, the periods of all menstruating females on there are much longer infradian, or monthly, Earth would be aligned with the Moon’s phases. cycles. By far the most common is menstruation, Neither of these, of course, is the case. which begins when a female mammal reaches a CIRCANNUAL CYCLES. Longer state of physical maturity and continues on a still than infradian cycles, circannual cycles, as monthly basis until she is no longer able to con- their name suggests, take a year to complete. ceive offspring. Among them is the cycle of dormancy and activ- When she becomes pregnant, the menstrual ity marked by the hibernation of certain species cycle shuts down and, in some cases, does not in the winter. There are also certain times of the resume until several months after delivery of the year when animals shed things—fur, skin, offspring. Assuming she is in good health, the antlers, or simply pounds. Likewise, at some human female will experience fairly regular men- points in the year animals gain weight. strual periods at intervals of 28 days. Among People are affected strongly by the seasonal human females, it has long been known that the changes associated with the circannual cycle. menstrual cycles of women who live or work in There is almost no person who lives in a temper- close proximity to one another tend to come into ate zone (that is, one with four seasons) who is alignment. For example, college girls on the same not capable of calling strong emotions to mind floor in a dormitory are likely to share menstru- when imagining the sensations associated with al cycles. winter, spring, summer, or fall. Some sensations,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 313 set_vol3_11 8/19/02 11:22 AM Page 314
Biological Rhythms KEY TERMS
BIOLOGICAL CLOCK: A mechanism JET LAG: A physiological and psycho- within an organism (for example, the logical condition in humans that typically pineal gland in the human brain) that gov- includes fatigue and irritability; it usually erns biological rhythms. follows from a long flight through several BIOLOGICAL RHYTHMS: Processes time zones and probably results from dis- that occur periodically in an organism in ruption of circadian rhythms. conjunction with and often in response to MENOPAUSE: The point at which periodic changes in environmental condi- menstrual cycles cease, a time that typical- tions. ly corresponds to the cessation of the CHRONOBIOLOGY: A subdiscipline female’s reproductive abilities. of biology devoted to the study of biologi- MENSTRUATION: Sloughing off of cal rhythms. the lining of the uterus, which occurs CIRCADIAN RHYTHM: A biological monthly in nonpregnant females who have cycle that takes place over the course of not reached menopause (the point at approximately a day. In humans circadian which menstrual cycles cease) and which rhythms run on a cycle of approximately manifests as a discharge of blood. 25 hours and govern states of sleep and PINEAL GLAND: wakefulness as well as core body tempera- A small, usually ture and other biological functions. cone-shaped portion of the brain, often located between the two lobes, that plays a HORMONE: Molecules produced by principal role in governing the release of living cells, which send signals to spots remote from their point of origin and certain hormones, including those associ- which induce specific effects on the activi- ated with human circadian rhythms. ties of other cells. ULTRADIAN RHYTHM: A biological INFRADIAN RHYTHM: A biological cycle that takes place over the course of less cycle that takes place over the course of a than a day. Compare with circadian month. rhythm.
however, are better than others, and though there ly, to the altered circadian rhythm) in winter- can be negative associations with spring or sum- time. mer, by far the season most likely to induce ill As we have noted, the body responds to the effects in humans is winter. onset of night and sleep by the release of mela- tonin, but when darkness lasts longer than nor- The thirteen weeks between the winter sol- mal, melatonin secretions become much more stice in late December and the vernal equinox in pronounced than they would be under ordinary late March have such a powerful impact on the conditions. The result of this hormone imbal- human psyche that scientists have identified a ance can be depression, which may be com- mental condition associated with it. It is SAD, or pounded by other conditions associated with seasonal affective disorder, which seems to be winter. Among these conditions is “cabin fever,” related to the shortened days (and thus, ultimate- or restlessness brought about by lengthy confine-
314 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_11 8/19/02 11:22 AM Page 315
ment indoors. An effective treatment for SAD is in Scotland, who in June 2000 announced find- Biological exposure to intense bright light. ings linking the drug, ecstasy, to long-term dam- Rhythms age to the body clock. As with LSD and many Studying Biological Rhythms another drug, ecstasy plays havoc with serotonin and may exert such a negative impact on the Treatment of SAD is just one example of the pathways of serotonin release in the pineal gland issues confronted by scientists working in the that it permanently alters the brain’s ability to realm of chronobiology, a subdiscipline devoted manufacture that vital hormone. Thus the drug, to the study of biological rhythms. Naturally, a which induces a sense of euphoria in users, can particularly significant area of chronobiological induce serious sleep and mood disorders as well study is devoted to sleep research. The latter is a as severe depression. relatively new field of medicine stimulated by the discovery of REM sleep in 1953. In addition to studying such disorders as sleep apnea, sleep WHERE TO LEARN MORE researchers are concerned with such issues as the Biological Rhythms (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 315 set_vol3_12 8/19/02 11:25 AM Page 317
SCIENCE OF EVERYDAY THINGS real-life biology LEARNING AND BEHAVIOR
BEHAVIOR INSTINCT AND LEARNING MIGRATION AND NAVIGATION
317 set_vol3_12 8/19/02 11:25 AM Page 319
Behavior BEHAVIOR
CONCEPT courtship and mating and responds automatical- In biology the term behavior refers to the means ly to changes in its body. by which living things respond to their environ- All creatures, even humans, are capable of ments. At first glance, this might seem to encom- automatic responses to stimuli. When a person pass only animal behavior, but, in fact, plants dis- inhales dust, pepper, or something to which he or play observable behavior patterns as well. One of she is allergic, a sneeze follows. The person may the principal manifestations of plant behavior is suppress the sneeze (which is not a good practice, tropism, a response to a stimulus that acts in a since it puts a strain on blood vessels in the particular direction, thus encouraging growth head), but this does not stop the body from either toward or away from that stimulus. Behav- responding automatically to the irritating stimu- ior in plants is primarily a matter of response to lus by initiating a sneeze. Similarly, plants stimuli, which may be any one of a variety of respond automatically to light and other stimuli influences that derive either from inside or out- in a range of behaviors known collectively as tro- side the organism. Response to stimuli is auto- pisms, which we explore later in this essay. matic, and even humans are capable of making INNATE AND LEARNED BEHAV- these types of programmed responses. In most IOR. Not all responses to stimuli are automat- cases, behaviors in organisms are designed to ic, however. Certainly not all behavior on the part ensure their survival. Such is the case, for of higher animals is automatic, though, as we instance, with the complex of behaviors known have noted, even humans are capable of some as territoriality, whereby animals defend what automatic responses. In general, behavior can be they perceive to be their own. categorized as either innate (inborn) or learned, but the distinction is frequently unclear. In many HOW IT WORKS cases it is safe to say that behavior present at birth is innate, but this does not mean that behavior Stimulus and Response that manifests later in life is learned. (Later in this essay we look at an example of this behavior as it A stimulus is any phenomenon that directly relates to chickens and pecking.) influences the activity or growth of a living Behavior is considered innate when it is organism. Phenomenon, meaning any observable present and complete without any experience fact or event, is a broad term and appropriately whereby it was learned. At the age of about four so, since stimuli can be of so many varieties. weeks, human babies, even blind ones, smile Chemicals, heat, light, pressure, and gravity all spontaneously at a pleasing stimulus. Like all can serve as stimuli, as indeed can any environ- innate behavior, babies’ smiling is stereotyped, or mental change. Nor are environmental changes always the same, and therefore quite predictable. limited to the organism’s external environment. Plants, protista (single-cell organisms), and ani- In some cases its internal environment can act as mals that lack a well-developed nervous system a stimulus, as when an animal reaches the age of rely on innate behavior. Higher animals, on the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 319 set_vol3_12 8/19/02 11:25 AM Page 320
Behavior other hand, use both innate and learned behav- ular stimulus even when that stimulus is ior. A fish is born knowing how to swim, where- removed, so long as the stimulus has been associ- as a human or a giraffe must learn how to walk. ated with a secondary one. Ethology Pavlov began his now famous set of experi- ments by placing powdered meat in a dog’s Ethology is the study of animal behavior, includ- mouth and observing that saliva flowed into the ing its mechanisms and evolution. The science mouth as a reflex reaction to the introduction of dates back to the British naturalist Charles Dar- the meat. He then began ringing a bell before he win (1809–1882), who applied it in his research gave the dog its food. After doing this several concerning evolution by means of natural selec- times, he discovered that the dog salivated mere- tion (see Evolution). Darwin presented many ly at the sound of the bell. Many experiments of examples to illustrate the fact that, in addition to this type demonstrated that an innate behavior other characteristics of an organism, such as its can be modified, and thus was born the scientif- morphologic features or shape, behavior is an ic concept of conditioning, or learning by associ- adaptation to environmental demands and can ation with particular stimuli. increase the chances of species survival. The variety of conditioning applied by The true foundations of ethology, however, Pavlov, known as classical conditioning, calls for lie in the work of two men during the period pairing a stimulus that elicits a specific response between 1930 and 1950: the Austrian zoologist with one that does not, until the second stimulus Konrad Lorenz (1903–1989) and the Dutch elicits a response like the first. Classical condi- ethologist Nikolaas Tinbergen (1907–1988). tioning is contrasted with operant conditioning, Together with the Austrian zoologist Karl von which involves administering or withholding Frisch (1886–1982), most noted for his study of reinforcements (that is, rewards) based on the bee communication and sensory perception, the performance of a targeted response. two men shared the 1973 Nobel Prize in physiol- OPERANT CONDITIONING. ogy or medicine. During operant conditioning, a random behav- Lorenz and Tinbergen, who together are ior is rewarded and subsequently retained by an credited as founders of scientific ethology, con- animal. According to operant conditioning theo- tributed individually to the discipline and, dur- ry, if we want to train a dog to sit on command, ing the mid–twentieth century, worked together all we have to do is wait until the dog sits and on a theory that animals develop formalized, then say, “Sit,” and give the dog a biscuit. After a rigid sequences of action in response to specific few repetitions, the dog will sit on command stimuli. According to Lorenz and Tinbergen, ani- because the reward apparently reinforces the mals show fixed-action patterns (FAPs) of behav- behavior and fosters its repetition. ior which are strong responses to particular stim- Human parents apply operant conditioning uli. Later in this essay, we look at examples of when they admonish their offspring with such FAPs in action. In addition, Lorenz put forward phrases as “You can’t watch TV until you’ve the highly influential theory of imprinting, dis- cleaned your room.” Likewise, young chim- cussed briefly in this essay and in more detail panzees learn through a form of operant condi- elsewhere (see Instinct and Learning). tioning. By observing their parents, young Behaviorism and Conditioning chimps learn how to strip a twig and then use it to pick up termites (a tasty treat to a chim- The development of ethology by Lorenz and Tin- panzee) from rotten logs. Their behavior thus is bergen occurred against the backdrop of the rise rewarded, an example of the way that operant of the behaviorist school in the realms of philos- conditioning enables animals to add new, nonin- ophy, psychology, and the biological sciences. herited forms of behavior to their range of skills. This school of thought had its roots in the late Though the theory of operant conditioning nineteenth century, with the writings of a num- goes back to the work of the American psycholo- ber of philosophers and psychologists as well as gist Edward L. Thorndike (1874–1949), by far its practical scientists, such as the Russian physiolo- most famous proponent was another American gist Ivan Pavlov (1849–1936). Pavlov showed that psychologist, B. F. Skinner (1904–1990). In an animal can be trained to respond to a partic- applying operant conditioning to human beings,
320 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:25 AM Page 321
Skinner and his followers took the theory to Behavior extremes, maintaining that humans have no ideas of their own, only conditioned responses to stimuli. Love, courage, faith, and all the other emotions and attitudes that people hold in high esteem are, according to this school of thought, simply a matter of learned responses, rather like a parrot making human-like sounds to earn treats. This extreme form of behaviorism is no longer held in high regard within the scientific or medical communities.
REAL-LIFE APPLICATIONS Behavior in Plants
As noted earlier, the term behavior would seem at first glance to apply only to animals and not to plants. Certainly the majority of attention in behavioral studies, outside the realm of humans, is devoted to ethology, but plants are not without their observable behavioral characteristics. These ONE EXAMPLE OF INNATE ANIMAL BEHAVIOR IS THE features primarily manifest in the form of REFLEX, A SIMPLE, INBORN, AUTOMATIC RESPONSE TO tropism, a response to a stimulus that acts in a A STIMULUS BY A PART OF AN ORGANISM’S BODY. SUCH particular direction, thus encouraging growth A MECHANISM IS AT WORK, FOR INSTANCE, WHEN JEL- either toward or away from that stimulus. Tro- LYFISH WITHDRAW THEIR TENTACLES. (© Henry Horen- pism primarily affects members of the plant stein/Corbis. Reproduced by permission.) kingdom, though it has been observed in algae and fungi as well. toward light or water, for instance. On the other Though the word tropism itself may be hand, some kinds of stimuli tend to evoke diat- unfamiliar to most people, the phenomenon ropism, or growth away from the stimulus. Such itself is not. There are plenty of opportunities in is bound to be the case, for instance, with trau- daily life to observe the response of plants to matropism and electrotropism. energy, substances, or forms of stimulation. For Tropism, along with movement due to example, perhaps you have noticed the way that changes in water content, is one of the two prin- trees or flowers grow toward sunlight, even bend- cipal forms of innate behavior on the part of ing in their growth if it is necessary to reach the plants. In general, stems and leaves experience energy source. Similarly, plants in a parched positive phototropism, as they grow in the direc- region are likely to develop roots directed lateral- tion of a light source, the Sun. At the same time, ly toward a water source. roots exhibit positive gravitropism, or growth Among the various forms of tropism are toward the gravitational force of Earth, as well as phototropism (response to light), geotropism positive hydrotropism, since they grow toward (response to gravity), chemotropism (response water sources below ground. On the other hand, a plant may move in a specific way regardless of to particular chemical substances), hydrotropism the direction of the stimulus. Such movements (response to water), thigmotropism (response to are temporary, reversible, and result from mechanical stimulation), traumatropism changes in the water pressure inside the plant. (response to wounds), and galvanotropism or electrotropism (response to electric current). Animal Behavior Most of these types involve growth toward a stimulus, a phenomenon known as positive An excellent example of an innate animal behav- growth, or orthotropism. Plants tend to grow ior, and one in which humans also take part, is
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 321 set_vol3_12 8/19/02 11:25 AM Page 322
Behavior it is a form of innate behavior that has undergone improvement as the organism matures. For example, chickens become more adept at pecking as they get older, but this does not mean that pecking is a learned behavior; on the contrary, it is innate. The improvement in pecking aim is not the result of learning and correction of errors but rather is due to a natural maturing of muscles and eyes and the coordination between them. FAPS. In studying fixed-action patterns of behavior, or FAPs, Lorenz and Tinbergen observed numerous interesting phenomena. Male stickleback fish, for example, recognize potential competition—other breeding stickle- back males—by the red stripe on their underside and thus engage in the FAP of attacking anything red on sight. Tinbergen discovered that jealous stickleback males were so attuned to the red stripe that they tried to attack passing British mail trucks, which were red, when they could see them through the glass of their tanks. Tinbergen IN CONTRAST TO SIMPLE FIXED-ACTION PATTERNS OF termed the red stripe a behavioral releaser, or a BEHAVIOR, OR FAPS, ARE COMPLEX PROGRAMMED BEHAVIOR PATTERNS, WHICH COMPRISE SEVERAL STEPS simple stimulus that brings about a FAP. AND ARE MUCH MORE COMPLICATED. ONE TYPE OF Once a FAP is initiated, it continues to com- COMPLEX BEHAVIOR IS THE BUILDING OF DAMS BY pletion even if circumstances change. If an egg BEAVERS. (© Harry Engels/Nas. Photo Researchers. Reproduced by permission.) rolls out of a goose’s nest, the goose stretches her neck until the underside of her bill touches the egg. Then she rolls the egg back to the nest. If the reflex. A reflex is a simple, inborn, automatic someone takes the egg away while she is reaching response to a stimulus by a part of an organism’s for it, the goose goes through the motions any- body. The simplest model of reflex action way, even without an egg. Not all animal behav- involves a receptor and sensory neuron and an ior is quite so predictable, however. In contrast to effector organ. Such a mechanism is at work, for FAPs are complex programmed behavior pat- instance, when certain varieties of coelenterate (a terns, which comprise several steps and are much phylum that includes jellyfish) withdraw their more complicated. Birds making nests or beavers tentacles. building dams are examples of complex pro- More complex reflexes require processing grammed behavior. interneurons between the sensory and motor IMPRINTING. As we noted earlier, neurons as well as specialized receptors. These Lorenz initiated the study of a learning pattern neurons send signals across the body, or to vari- that came to be known as imprinting. Witnessed ous parts of the body, as, for example, when food frequently in birds, imprinting is the learning of in the mouth stimulates the salivary glands to a behavior at a critical period early in life, such produce saliva or when a hand is pulled away that the behavior becomes permanent. The very rapidly from a hot object. young bird or other organism is like wet con- Reflexes help animals respond quickly to a crete, into which any pattern can be etched; once stimulus, thus protecting them from harm. By the concrete has dried, the pattern is set. contrast, learned behavior results from experi- Newly hatched geese are able to walk. This is ence and enables animals to adjust to new situa- something they learn the moment they are tions. If an animal exhibits a behavior at birth, it hatched, and they do so by following their par- is a near certainty that it is innate and not ents. But how, Lorenz wondered, do young geese learned. Sometimes later in life, however, a distinguish their parents from all other objects in behavior may appear to be learned when, in fact, their environments? He discovered that if he
322 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 323
Behavior
ROCKY MOUNTAIN BIGHORN RAMS MEET HEAD TO HEAD AT THE BOUNDARIES OF THEIR RESPECTIVE TERRITORIES. USING THEIR HORNS, THESE RAMS WILL STRONGLY DEFEND THEIR TERRITORIES AGAINST INVADERS. (© W. Perry Con- way/Corbis. Reproduced by permission.)
removed the parents from view the first day after which informs the other bees that food is near the goslings hatched and if he walked in front of (about 250 ft., or 75 m, from the hive), and the the young geese at that point, they would follow other is a wagging dance, which conveys the fact him. This tactic did not work if he waited until that food is farther away. the third day after hatching, however. There are numerous other forms of commu- Lorenz concluded that during a critical peri- nication using one or more sense organs. Birds od following birth, the goslings follow their par- hear each other sing, a dog sees and hears the spit ents’ movement and learn enough about their and hiss of a cornered cat, and ants lay down parents to recognize them. But since he also had scent signals, or pheromones, to mark a trail that determined that young geese follow any moving leads to food. This is only one level of interactive object, he reasoned that they first identify their behavior, however. Quite a different variety of parents by their movement, which acts as a interaction is courtship, discussed in Reproduc- releaser for parental imprinting. (Imprinting is tion. Other forms of interactive behavior include discussed further in Instinct and Learning.) the establishment of an animal’s territory, a sub- ject we discuss at the conclusion of this essay. Interactive Behavior LIFE IN COMMUNITIES. Interac- Much of an animal’s behavior (this is true of the tive behavior comes into play when animals live in human animal as well) takes place in interaction close proximity to one another. Certainly there are with others. This interaction may include rudi- benefits to group life for those species that practice mentary forms of communication, such as bee it: the group helps protect individuals from pred- dances, studied by Lorenz and Tinbergen’s col- ators and, through cooperation and division of league Frisch. As he showed in perhaps the most labor, ensures that all are fed and sheltered. In important research of his career, bees communi- order to be workable, however, a society must have cate information about food supplies, including a hierarchy. Thus, in a situation quite removed their direction and the distance to them, by from the human ideals of freedom and democra- means of two different varieties of “dance,” or cy, insect and animal societies are ones in which rhythmic movement. One is a circling dance, every creature knows its place and sticks to it.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 323 set_vol3_12 8/19/02 11:26 AM Page 324
EXAMPLES OF TERRITORIES. Behavior Bees, ants, and termites live in complex com- munities in which some individuals are responsi- A territory established only for a single night, for ble for finding food, others defend the colony, the sole purpose of providing the animal or ani- and still others watch over the offspring. In such mals with a place to rest, is known as a roost. a highly organized society, a dominance hierar- Even within the roost, there may be a battle for chy or ranking system helps preserve peace and territory, since not all spots are created equal. discipline. Chickens, for example, have a pecking Because roosting spots near the interior are the order from the most dominant to the most sub- safest, they are the most highly prized. missive. Each individual knows its place in the Another type of specialized territory is the order and does not challenge individuals of high- lek, used by various bird and mammal species er rank. This, again, is quite unlike humans, who during the breeding season. Leks are the “singles at least occasionally step out of line and challenge bars” of the animal world: here animals engage in bullies; by contrast, that never happens with behavior known as lekking, in which they display chickens (fittingly enough). their breeding ability in the hope of attracting a mate. Not surprisingly, leks are among the most Territoriality strongly defended of all territories, since holding Almost everyone has seen a dog “mark its territo- a good lek increases the chances of attracting a ry” by urinating on a patch of ground or has mate. Like the singles-only communities that watched a cat arch its back in fury at an intruder they mimic, leks are no place for families: gener- to what it perceives as its territory. In so doing, ally of little use for feeding or bringing up young, these household pets are participating in a form the lek usually is abandoned by the animal once of behavior that cuts across the entire animal it attracts a mate or mates. kingdom: territoriality, or the behavior by which THREATENING DISPLAYS. An an animal lays claim to and defends an area animal has to be prepared to defend its territory against others of its species and occasionally by fighting off invaders, but naturally it is prefer- against members of other species as well. able to avoid actual fighting if a mere display of The physical size of the territory defended is strength will suffice. Fighting, after all, uses up extremely varied. It might be only slightly larger energy and can result in injury or even death. than the animal itself or it might be the size of a Instead, animals rely on various threats, through small United States county. The population of vocalizations, smells, or visual displays. the territory might consist of the animal itself, The songs of birds, the drumming of wood- the animal and its mate, an entire family, or an peckers, and the loud calls of monkeys may seem entire herd or swarm. Time is another variable: innocuous to humans, but they are all warnings some animals maintain a particular territory that carry for long distances, advertising to year-round, using it as an ongoing source of food potential intruders that someone else’s territory and shelter. Others establish a territory only at is being approached. As noted earlier, many ani- certain times of the year, when they need to do so mals, such as dogs, rely on smells to mark their for the purposes of attracting a mate, breeding, territories, spraying urine, leaving droppings, or or raising a family. rubbing scent glands around the territories’ bor- Territorial behavior offers several advantages ders. Thus, an approaching animal will be to the territorial animal. An animal that has a warned off the territory without ever encounter- “home ground” can react quickly to dangerous ing the territory’s defender. Or, if the invader is situations without having to seek hiding places or unfortunate enough to have trespassed on a defensible ground. By placing potential competi- skunk’s territory, it may get a big blast of scent tors at spaced intervals, territoriality also pre- when it is too late to retreat. vents the depletion of an area’s natural resources Suppose an animal ignores these warnings, and may even slow down the spread of disease. or suppose, for one reason or another, that two Furthermore, territorial behavior exposes weaker animals meet nose to nose at the boundaries of animals (which are unable to defend their terri- their respective territories. Usually there follows a tory) to attacks by predators and thus assists the threatening visual display, often involving exag- process of natural selection in building a geration of the animals’ sizes by the fluffing up of stronger, healthier population. feathers or fur. The animals may show off their
324 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 325
Behavior KEY TERMS
CONDITIONING: Learning by associa- ence to learn them. For example, fish have tion with particular stimuli. There are two an innate ability to swim, whereas humans varieties of conditioning: classical condi- must learn how to walk. tioning, which involves pairing a stimulus NATURAL SELECTION: The process that elicits a specific response with one that whereby some organisms thrive and others does not until the second stimulus elicits a perish, depending on their degree of adap- response like the first, and operant condi- tation to a particular environment. tioning, which involves administering or REFLEX: An inborn, automatic withholding reinforcements (i.e., rewards) response to a stimulus by a part of an based on the performance of a targeted organism’s body. response. STIMULUS: Any phenomenon (for ETHOLOGY: The study of animal example, an environmental change) that behavior, including its mechanisms and directly influences the activity or growth of evolution. a living organism. FAPs: Fixed-action patterns of behav- TERRITORIALITY: The behavior by ior, or strong responses on the part of an which an animal lays claim to and defends animal to particular stimuli. an area against others of its species and IMPRINTING: The learning of a behav- occasionally against members of other ior at a critical period early in life, such that species as well. the behavior becomes permanent. TROPISM: A response to a stimulus INNATE: A term to describe behaviors that acts in a particular direction, thus that are present and complete within the encouraging growth either toward or away individual and which require no experi- from that stimulus.
weapons, whether claws or fangs or other does break out, it is an aberration. This typically devices. Or the two creatures may go through all happens only in overcrowded conditions, when the motions of fighting without ever actually resources are scarce—again, not unlike the situa- touching, a behavior known as ritual fighting. tion with humans. FIGHTING. The degree to which a crea- Late in his career, Lorenz devoted himself to ture engages in these displays of bravado helps studying human fighting behavior. In Das soge- define its territory. If the creature perceives that it nannte Böse (On Aggression, 1963), he main- is at the center of its own territory and is being tained that fighting and warlike behavior are attacked on home ground, it will go into as innate to human beings but that they can be threatening a mode as it can muster. If, on the unlearned through a process whereby humans’ other hand, the animal is at the edge of its terri- basic needs are met in less violent ways. Just as torial boundaries, it will be much more half- fighting in animal communities has its benefits, hearted in its efforts at intimidation. As with Lorenz maintained, inasmuch as it helps keep humans, few animals want to fight when there is competitors separated and enables the larger nothing really at stake. Also like humans, animals group to hold on to territory, so fighting among many times may seem to be spoiling for a fight humans might be directed toward more useful without actually fighting, such that when a fight means. As discussed in Biological Communities,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 325 set_vol3_12 8/19/02 11:26 AM Page 326
Behavior it is possible that sports and business competi- “Growth Movements, Turgor Movements, and Circadian tion in the human community provides a more Rhythmics.” Department of Biology, University of Hamburg (Germany) (Web site).
326 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 327
INSTINCT AND
Instinct and Learning LEARNING
CONCEPT stimuli. These sequences they called fixed-action Among the most fascinating areas in the biologi- patterns of behavior, or FAPs. cal sciences is ethology, or the study of animal In studying male stickleback fish, Tinbergen behavior—in particular, the areas of ethology discovered an excellent example of a FAP. Males that deal with instinct and learning. Instinct is a of that species recognize potential competitors, stereotyped, or largely unvarying, behavior that in the form of other stickleback males capable of is typical of a particular species. An instinctive breeding, by the red stripe on their underside. behavior does not have to be learned; rather, it is Tinbergen termed the red stripe a behavioral fully functional the first time it is performed. On releaser, or a simple stimulus that brings about a the other hand, learning, in an ethological con- FAP. (A stimulus is any observable fact or event text, is the alteration of behavior as the result of that directly influences the activity or growth of a experience. Clearly, the distinction between living organism.) instinct and learning revolves around the ques- tion of whether an animal, in responding to a This particular FAP compels the male stick- specific situation, is acting on the basis of experi- leback to attack anything red, even when it is not ence or instead is guided by instincts “hardwired” a competitor. Thus, as Tinbergen observed, jeal- within its brain. The difference would seem to be ous stickleback males actually would try to attack a simple one, but nothing is simple in the study red British mail trucks when they could see them of instinct and learning. Plenty of gray area exists through the glass of their tanks. Clearly, then, a between pure instinct and genuine learning, and FAP such as the response to the red stripe is not within that gray area is a fascinating concept something that an animal thinks through; rather, known as imprinting, or the learning of a behav- it is automatic, almost as though the animal were ior at a critical period early in life, such that the being acted upon, instead of acting in its own behavior becomes permanent. right. There have been stories and scientific studies HOW IT WORKS of animals seemingly acting above and beyond the call of duty by raising the offspring of other Instinct individuals—even those of another species. Tin- bergen, for example, observed and photographed The founding fathers of ethology (the study of a cardinal feeding baby minnows at water’s edge. animal behavior, including its mechanisms and Such behavior seems altruistic, touching, and evolution) were the German zoologist Konrad even inspiring, yet for all the significance we Lorenz (1903–1989) and the Dutch ethologist might be inclined to place on it from our human Nikolaas Tinbergen (1907–1988). In addition to perspective, it is nothing but a FAP. Most likely their separate contributions to the field, which the bird had lost its own offspring, and was sim- we discuss later, the two scientists together devel- ply acting on a parental instinct, which caused it oped a theory that animals employ formalized, to respond to the sight of open, upturned, hun- rigid sequences of action in response to specific gry mouths. It so happened that the open
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 327 set_vol3_12 8/19/02 11:26 AM Page 328
Instinct and mouths were not those of its offspring at all, but In such situations improvements in the crea- Learning of fish, yet this made no difference in the behav- tures’ ability to perform an innate behavior may ior of the cardinal, acting as it was on an inborn, seem to indicate that the animal is learning, stereotypic pattern of response. when, in fact, another process is at work. For SURVIVAL-FRIENDLY, CON- example, chickens exhibit the innate tendency TROLLED BEHAVIOR. This is one char- toward pecking as a way of establishing and acteristic of instinct, a concept virtually synony- maintaining a dominance hierarchy. This is the mous with that of the FAP: the creature acting on “pecking order,” to which people often refer in instinct is not thinking about what it does but is everyday speech as a metaphor for various hier- behaving almost as though it were controlled by archies in human experience, such as those at a some outside force. The use of the “outside force” workplace. Though pecking is innate, chickens’ concept perhaps makes instinct sound like some- ability to perform it actually improves as they thing mystical when it is not. On the contrary, grow older. instinctive behavior is simply a survival mecha- Older chickens display a better aim when nism in the brains of each member of a particu- pecking than do younger ones, but this does not lar species, developed through countless genera- mean that they have learned from experience. tions of natural selection. Indeed, one clue that pecking is innate in older If a particular behavior helps foster the sur- chickens is the fact that they uniformly improve vival of a species, natural selection favors it. In in their ability to peck. On the other hand, if they other words, those individuals of a species who were simply learning, with practice, how to peck possess the tendency toward a particular sur- more accurately, one could expect that some vival-friendly behavior are the ones that survive chickens would exhibit more dramatic improve- and pass on their genes to others, while those that ment than others, in the same way that some do not possess this tendency do not. The sur- humans play basketball (or sing or write poetry) vival-friendly instinct may be geared toward the better than others. In fact, what has happened is survival of the individual, its offspring, or others. that the ability to perform an innate behavior simply has improved as a result of growth: as the Closely tied to instinct is the innate animal chickens’ eyes and muscles mature, their aim behavior known as a reflex: a simple, inborn, improves, but this has nothing to do with experi- automatic response to a stimulus by a part of an organism’s body. Reflexes help animals (includ- ence per se. ing humans) respond quickly to a stimulus, thus Imprinting protecting them from harm. Again, the animal does not think about what it is doing. If you This is one example of the ways in which instinc- touch a hot stove or receive an electric shock, you tive and learned behavior can become confused, do not decide to pull your hand back: you whip though in the pecking example there is really no your hand away from the painful stimulus faster gray area; rather, what is actually an innate than you can blink. As with instinct, a reflex is a behavior merely seems to be a learned one. Yet survival-friendly behavior over which the animal there truly is a great deal of gray area between (in this case, you) has little control. instinct and learning. Many behaviors that at first glance might appear purely instinctive can be Innate Versus Learned shown to have an experiential component—that Behavior is, an aspect of the behavior has been modified Instinct is innate, meaning that instinctive through experience or learning. behaviors and responses are present and com- A fascinating example of how instinct and plete within the individual at birth. In other learning can be blurred or combined is imprint- words, the individual does not have to undergo ing, or the learning of a behavior at a critical any experience to acquire such behaviors. For period early in life, such that the behavior example, fish have an innate ability to swim, becomes permanent. Lorenz, who first developed whereas most mammals must learn how to walk. the theory of imprinting, noted that newly It is fairly easy to identify innate behavior when hatched geese learn to walk by following their an animal exhibits it at birth, but in some cases parents, but he wondered how they distinguished innate behavior manifests only later in life. their parents from all other objects in their envi-
328 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 329
Instinct and Learning
INSTINCT AND LEARNING CAN BE BLURRED IN THE CASE OF IMPRINTING, OR THE LEARNING OF A BEHAVIOR AT A CRIT- ICAL PERIOD EARLY IN LIFE, SUCH THAT THE BEHAVIOR BECOMES PERMANENT. NEWLY HATCHED GEESE LEARN TO WALK BY FOLLOWING THEIR PARENTS. IF THE GOSLINGS ARE REMOVED FROM THEIR PARENTS, THEY WILL IMPRINT INSTEAD ON ANOTHER MOVING OBJECT, EVEN A HUMAN BEING. (© Galen Rowell/Corbis. Reproduced by permission.)
ronments. He discovered that if he removed the between the offspring and its parents, causing the parents from view the first day after the goslings offspring to imprint on it. hatched and if he walked in front of the young Once imprinting is complete, it brings about geese at that point, they would follow him. numerous consequences that are more or less From these experiments, Lorenz concluded automatic or inevitable. Yet for all their automat- two things. First of all, during a critical period ic, inevitable qualities, these consequences are following birth, goslings follow their parents’ not the result of innate behaviors or instincts but movements and learn enough about their par- of learned behaviors. Thus, learning is destiny, at ents to recognize them. This critical period is least to an extent. The individual’s mind, at an short: if he walked in front of three-day old geese, early stage, is like wet concrete into which virtu- they were already too old to imprint on him. Sec- ally any impression can be made. Once the indi- ond, Lorenz determined that the parents’ move- vidual has imprinted on something or someone, ment must be a behavioral releaser for imprint- however, the concrete begins to set, and it hard- ing; thus, if the tiny goslings happened to fix on ens around the impressions made in it. another moving object, they would imprint on that one. REAL-LIFE We can see in this example that imprinting APPLICATIONS has both innnate, or instinctive, and learned components. The tendency to imprint is innate, Instinct at Work but if the act of imprinting itself were likewise innate, then it would be the same for all individ- When a kangaroo rat hears a rattling sound, even uals within a species, and this is not the case. The if that sound comes from a drum or a fan or vast majority of goslings will imprint on adult some otherwise harmless contraption, it per- geese, of course, but in an experimental setting forms a lightning-quick escape jump maneuver. (or through some freak accident in nature), it is Why? Because its brain interprets the rattling possible that some other object will come sound as that of a rattlesnake ready to strike, and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 329 set_vol3_12 8/19/02 11:26 AM Page 330
Instinct and it acts automatically. In other words, the kanga- though there is no base plate to hold them. The Learning roo rat does not reason out a course of action; it eggs, therefore, will fall out of the bottom of the simply moves. Nor is it acting on experience in incomplete web, but the spider will continue any way: even if it has never encountered a rat- working as before, building the lid for the top. tlesnake, it will respond in exactly the same way. Fortunately for the spider, she has several Virtually all instinctive behavior, as we noted cocoons. If she is returned to her original com- earlier, is geared toward preserving the life of the pleted base plate as she prepares to spin the next individual or of its offspring, and thus it serves to cocoon, she will not use the base plate she already preserve the life of the species as well. Such is the has spun; rather, like a robot, she will start from case with the rat acting to protect itself from the the very beginning, spinning a new base plate as snake, and so, too, with a mother when her young though the original were not there. are threatened. People do not normally think of Many of these automatic behaviors are trig- ducks as intimidating creatures, and in most cases gered by a releaser. For example, a bright red spot they are not, but try stepping anywhere near a on the bill of an adult gull serves as a releaser to mother’s ducklings, and watch her response. She its offspring, which peck on the parent’s bill to will start to hiss, spit, and waddle forward menac- obtain food. For a female rat in heat, rubbing of ingly in such a way as to terrify any would-be her hindquarters acts as a releaser for an instinc- intruder. If the outsider is still foolhardy enough tive behavior pattern known as lordosis. In lordo- to press forward, the duck or goose will readily sis the female flexes her front legs, lowers her become a squawking, flapping, biting, pecking torso, raises her rump, and moves the tail to one army of one. This behavior does not vary from side. This posture, in turn, acts as a releaser for a mother to mother but is the same in all instances male rat, who initiates copulation—yet another of mother ducks who perceive a threat to their example of instinctive behavior as life-preserv- ducklings—a hallmark of an instinctive action. ing. In this case, however, what is being preserved AUTOMATIC BEHAVIORS AND is not the life of an individual but of the entire RELEASERS. There is an automatic, species, since intercourse yields offspring. almost robotlike character to many an example Challenging Situations of of instinctive behavior. If an egg rolls out of a Instinct and Imprinting goose’s nest, the goose stretches her neck until the underside of her bill touches the egg—an Instincts are ingrained so deeply that some ani- action that, like all instinctive ones, clearly is mals possess what appears to be an instinct for geared toward the survival of her offspring. Sup- exploiting, or taking advantage of, the instinctive pose someone takes the egg away while the goose behaviors of other animals. We typically think of is reaching for it: she continues to go through the parasites as microbes, or at least as no larger than motion of stretching to retrieve the egg. This may insects, but there are species of bird regarded as not seem very “smart,”but, of course, instinct has parasites, inasmuch as they take advantage of nothing to do with intelligence. other species. When people speak metaphorical- Likewise, a spider preparing to lay its eggs ly of another person as a “parasite,” what they spins a silk cocoon in a particular way, always the really mean is that the person in question same and without any regard for outside factors. exploits the good behavior of others. A human She begins by building a base plate, then con- “parasite” never has to pay for dinner at a restau- structs the walls of her cocoon before laying her rant, for instance, because he or she can count on eggs within it and sealing it with a lid. So rigid are good, decent people always to pick up the tab. these behavioral patterns that they cannot be Likewise, parasitic bird species, such as the North altered, even if the spider needs to do so. (It is American cowbird or the European cuckoo, can hard to imagine a spider “wanting” to do some- rely on other species’ instinctive tendency to do thing, a term that implies decision-making abili- what all animals (and humans) should do: take ties and a degree of self-awareness common only care of their own offspring. among higher mammals, particularly humans.) Avian parasites lay their eggs in the nest of If the spider is moved physically after she has an unwitting host and then leave, “knowing” built the base plate, she nonetheless will set about instinctively (which is not really the same thing spinning walls and depositing her eggs, even as knowing in the way that humans think of it)
330 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 331
that their victims will take care of the eggs. Cow- happens to the animal in the first minutes of its Instinct and birds and cuckoos have developed the practice of life, imprinting will affect its entire destiny both Learning laying their eggs in the nests of birds that are as an individual and as a member of its species. smaller than they, meaning that their hatchlings Suppose an animal has imprinted on a creature will be larger than the victims’ offspring. This of another species: this will haunt it for the rest of only adds to the detriment caused by the inter- its life, for instance, determining its choice of a lopers: since the parasites’ children are larger mate and its courtship behavior. than the hosts’, this triggers a more powerful Many species will avoid social contact with ani- release of the host parents’ instinctive feeding mals that are not similar to the one to which they behavior. In other words, the parasites’ offspring have imprinted. In other words, a duckling that get more food than the hosts’.As for the parasitic has imprinted properly on its duck parents will adults, they are long gone, having deposited their want to spend time with other ducks. From the young in the care of the hosts. larger perspective of nature and the continuing ERRORS IN IMPRINTING. The propagation of life-forms, this is a good thing, situation of the parasitic bird species is one because it helps prevent attempts to breed example of the “dark side” to instinct and learn- between different species. If, however, through ing mechanisms in animals. Other examples some accident (or as the result of exposure to include the many possible errors that can occur artificial conditions, such as those in an experi- in imprinting. Such errors arise when an animal ment), an animal has imprinted on an individual either fails to receive an imprint from an appro- of a different species, that animal will attempt to priate parent figure or receives an imprint from a court a member of that other species later in creature of another species. life—usually with disastrous results. TARZAN THE APE MAN. In their efforts to establish and maintain In dis- their territories and attract females, male birds of cussing the critical nature of imprinting, we a given species learn a particular song. This takes could use all sorts of animal examples, some of place at a critical period when, as a nestling, the which we have discussed. More compelling, how- bird hears the song of its father and from this ever, is the example of a man, albeit a fictional exposure eventually develops its own mature one, who lived among the apes and attempted to become one of them. That man, of course, is song. The process is a lengthy one: the immature Tarzan, creation of the American novelist Edgar bird does not begin singing until the following Rice Burroughs (1875–1950). First appearing in spring, when it starts trying to match its own, Tarzan of the Apes (1914) and immortalized in juvenile song with the one it heard from its father countless novels, movies, comic books, and car- during the critical period. toons, Tarzan is the epitome of the noble savage, If during that early critical period, the or the man at one with nature. nestling is prevented from hearing an adult song The son of an English lord, the boy is left of its own species, it never will develop a species- alone as an infant in Africa after his parents die typical song, and thus its very life and its ability and is raised by an ape who has lost her own to propagate (which, in turn, affects the well- child. The apes name him Tarzan, which, in their being of the entire species) are threatened. The language, means “white skin,”and he grows up as bird may hear the songs of other species, but this one of them. Later, he discovers his parents’ cabin does it little good. It appears that there is a and the books left there, teaches himself to read, strongly instinctive aspect to what the bird can learns English, and begins to uncover the truth of learn during the critical period and also that his background. As a grown man, he meets and birds are highly selective toward songs produced falls in love with Jane Porter, a beautiful young by other members of their species. Therefore, it American who, like his parents when he was learns either the right song or no song at all. infant, was marooned nearby after a shipboard Why Imprinting Is Crucial mutiny. Jane helps expose him to civilization, and Tarzan eventually travels to France, the United Imprinting is crucial for an animal’s develop- States, and other far-off lands. ment and not only because the act of imprinting One can find in this astonishing tale helps the animal learn one or another important antecedents in other rough-and-ready characters early function, such as walking. Far beyond what from American literature, most notably Buck, the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 331 set_vol3_12 8/19/02 11:26 AM Page 332
Instinct and Learning
THERE IS AN AUTOMATIC CHARACTER TO MANY EXAMPLES OF INSTINCTIVE BEHAVIOR. A SPIDER SPINS A COCOON IN A PARTICULAR WAY, BEGINNING WITH A BASE PLATE AND THEN CONSTRUCTING WALLS BEFORE LAYING EGGS AND SEAL- ING THE COCOON WITH A LID. IF THE SPIDER IS MOVED AFTER SHE HAS BUILT THE BASE PLATE, SHE WILL CONTIN- UE TO SPIN WALLS AND DEPOSIT HER EGGS, LEAVING THEM TO FALL OUT THE BOTTOM OF THE INCOMPLETE WEB. (© Science Pictures Limited/Corbis. Reproduced by permission.)
dog turned wolf in Jack London’s (1876–1916) Tarzan falling in love with a female ape, thinking novel The Call of the Wild (1903). Buck is a par- that he is of the same species. The ape, for her ticularly interesting example from the standpoint part, recognizes the difference between them and of biological study, because London, an advocate rejects the human in favor of a male ape. of Darwin’s theory of evolution by natural selec- A real-life Tarzan would be human only in a tion, was attempting to show the common evolu- biological sense; otherwise, he would lack virtu- tionary thread linking dog and wolf and, by ally all human characteristics. Not only would he inference, ape and human. be foul smelling (from our perspective) and cov- While Burroughs is not so didactic as Lon- ered with hair, but he also would be entirely don, he had a purpose as well, an aim opposite to ignorant of human speech or even human that of London: to show that a human, thrust by thought processes, which have to be learned. circumstances beyond his control into a world of Most preposterous of all is the idea that he would animals, would assert his humanness. There may find books and recognize them as a mode of even have been a class element to the Tarzan sce- communication, let alone teach himself to read nario, inasmuch as the young Tarzan is the off- them. Suppose that he had been raised as a spring of nobility and ultimately proves his noble human but that he had never seen a book or even lineage by rising above his circumstances. (This, so much as a written word. Even if he were the too, is a theme quite far removed from the heart most brilliant human being who had ever lived, of London, who was a socialist.) he would treat the books the way an ape would— as mere objects. All literary analysis aside, is there any truth, from the standpoint of what we know about Comparing Humans and imprinting, instinct, and learning, to Burroughs’s Animals portrayal of Tarzan? The answer is a resounding “no.”Assuming that a boy could be raised by apes, The case of Tarzan provides an appropriate place he would become socialized as an ape. Appropri- to close this discussion, with a few words on ately enough, in one story Burroughs shows humans and their place in the context of the larg-
332 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 333
er concepts of instinct, imprinting, and learning. Instinct and Do humans possess instincts? It appears that we KEY TERMS Learning do, inasmuch as we are prone to certain auto- matic, innate responses. For example, human babies are capable of smiling at the age of four BEHAVIORAL RELEASER: A simple weeks, even though they do not know what a stimulus that brings about a fixed-action smile “means.” On the other hand, humans are pattern of behavior, or FAP. not nearly as inclined to instinctive behavior as ETHOLOGY: animals; in general, with higher mammals and The study of animal especially humans, instinctive characteristics behavior, including its mechanisms and diminish in favor of the capacity to learn. evolution. Our methods of learning are quite different FAPs: Fixed-action patterns of behav- from those of lower animals, who “learn” in ways ior, or strong responses on the part of an that do not involve conscious thought. For animal to particular stimuli. FAP is virtual- instance, a snail will pull its head back into its shell when touched, but when it is touched ly synonymous with instinct. repeatedly with no subsequent harm, the with- IMPRINTING: The learning of a behav- drawal response ceases. The snail has experienced ior at a critical period early in life, such that habituation, a type of behavior in which an ani- the behavior becomes permanent. mal develops a tendency to ignore a stimulus that is repeated over and over. Apparently, the snail’s INNATE: A term to describe behaviors nervous system has “learned” that the stimulus is that are present and complete within the not threatening and so stops the reflex. individual and which require no experi- This is a far cry from learning as humans ence to learn them. For example, fish have experience it, particularly as we move past the an innate ability to swim, whereas humans first few weeks and months of life. Even from the earliest moments of a human’s existence—that is, must learn how to walk. even in the womb—a human is self-aware in a INSTINCT: A stereotyped, or largely way that few, if any, animals other than mammals unvarying, behavior that is typical of a par- are. Furthermore, the self-awareness of a human ticular species. An instinctive behavior does from about the age of two years old is far beyond any concept of self possessed by even the highest not have to be learned; rather, it is fully forms of mammal. From what we can discern, a functional the first time it is performed. house cat or even an ape does not experience any LEARNING: The alteration of behavior thought along the lines of Who am I? or What is as the result of experience. my place in the order of things? (Of course, almost everyone who has had a pet has at one time or NATURAL SELECTION: The process another believed that a cat or dog was embroiled whereby some organisms thrive and others in such types of philosophical inquiry, but this perish, depending on their degree of adap- may be merely anthropomorphism—ascribing human qualities to animals.) By contrast, a two- tation to a particular environment. year-old human already is forming complex REFLEX: An inborn, automatic judgments about his or her role with regard to response to a stimulus by a part of an Mommy, Daddy, siblings, other relatives, and organism’s body. household pets. IMPRINTING IN HUMANS. RELEASER: See Behavioral releaser. Whereas humans have fewer instinctive respons- STIMULUS: Any phenomenon (that is, es than most animals and are far more capable of an observable fact or event, such as an envi- learning than any other creature, in the area of ronmental change) that directly influences imprinting we are not so different from cows or even birds. We have discussed the importance of the activity or growth of a living organism. imprinting in birds, but it should be noted that
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 333 set_vol3_12 8/19/02 11:26 AM Page 334
Instinct and imprinting is at least as critical among hoofed (1994), in which Jodie Foster plays a young Learning mammals, such as cattle or sheep, since they tend woman who has grown up with little exposure to to congregate in large herds wherein a young ani- other humans. mal could be separated easily from its mother. Likewise, with humans imprinting of some sort WHERE TO LEARN MORE is critical. Animal Behaviour. University of Plymouth Department Humans do not imprint as rigidly as geese of Psychology (Web site).
334 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 335
MIGRATION AND
Migration and Navigation NAVIGATION
CONCEPT put it another way, the physical “cost” of migrat- Among the most intriguing aspects of animal ing must be less than the cost of staying home. behavior and perception is the tendency to Human beings would perform such calcula- migrate long distances, coupled with the naviga- tions rationally, of course, by thinking through tional ability that makes this possible. Most such the options and weighing them. Animals, on the migration is seasonal, a primary example being other hand, rely on instinct—a word that, like birds’ proverbial flight south for the winter. many terms in science, has a somewhat different Sometimes, however, animals widely separated meaning within the scientific community than it from their home environments nonetheless does for the world at large. People tend to think manage to find their way home. This fact has of instinct as a matter of “just knowing” some- long fascinated humans, as reflected in a number thing, as in “I just know he/she is a good/bad per- of true and fictional stories on the subject that son,” but, in fact, instinct seems to have nothing have circulated over the years. For example, The to do with “knowing” at all. Incredible Journey, a 1963 Disney film remade in On the contrary, instinct can be defined as a 1993, is a fictional tale, but there are numerous stereotyped (that is, largely unvarying) behavior true stories of dogs and cats making their way that is typical of a particular species. Instinctive home to their masters across thousands of miles. behavior does not have to be learned; rather, it is How do animals do this? Scientists do not fully fully functional the first time it is performed. understand the answers, but theories regarding Though animals do exhibit some problem-solv- animal navigation abound. In any case, there is ing ability, when a bird flies south for the winter, no question that animals possess navigational it has not thought that process through in any abilities unavailable to humans, for example, way. Instead, it is on “autopilot.” This may seem echolocation, used by bats, whales, and dolphins almost magical, but it probably just reflects the for local navigation, requires an ability to hear processes of natural selection (see Evolution): for sounds far beyond the range of the human ear. a particular bird species, those individuals “hard- wired” with a tendency to fly south were those HOW IT WORKS that survived harsh winters, and therefore this tendency became favored in the gene pool. Reasons for Migration Just as circumstances in the creature’s home environment present a compelling need for Why do animals migrate? Seasonal temperature migration, so there are other circumstances in changes, of course, are a factor, as in the well- the wintering environment that force the animal known instance of birds flying south for the win- to leave as spring approaches. The wintering ter. But to justify enduring the dangers and hard- environment, after all, has its own native species, ships of long-distance migration, there must be which most likely remain in the area even as the an underlying cost-benefit equation whereby the influx of visitors from up north arrives. Compe- benefits of migration outweigh the costs. Or, to tition for food and shelter thus can become
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 335 set_vol3_12 8/19/02 11:26 AM Page 336
Migration rather intense. Over time, this increased compe- movement probably is related to seasonal and tition creates a situation in which it is advanta- changes in the location, availability, and choice of Navigation geous for the migrating creature to return home. prey. Nor does all migration involve movements across Earth’s surface; there is also elevational Types of Migration migration, which entails a change of altitude or The idea of birds flying south en masse for an depth beneath the sea. entire winter represents only one of four different Animals that live on mountains, for types of migration: complete, as opposed to par- instance, may take part in elevational migration, tial, differential, or irruptive migration. Complete moving to lower elevations in winter just as other migration involves the movement of all individu- species move to lower latitudes. For zooplankton, als within a population away from their breeding tiny animals that float on the waters of the open grounds at the conclusion of the breeding season. ocean, migration is a matter of changing depths Usually this entails migration to a wintering site in the water. During the summertime, when pop- that may be thousands of miles away. ulations of zooplankton are large, these organ- Some species practice partial migration, isms live on the surface and feed on the plant life whereby some individuals remain at the breeding there. During the cold months, however, zoo- ground year-round, while others migrate. Others plankton migrate to depths of about 3,300 ft. (1 employ differential migration, in which all mem- km) and do not feed at all. bers of the population migrate, but for periods of time and over distances that vary as a function of age or sex. For example, herring gulls migrate for REAL-LIFE increasingly shorter distances the older they get, APPLICATIONS and male American kestrels remain at the breed- ing grounds longer than females. Even when the The Process of Migration male birds do set out on their journeys, they do When animals migrate, they move along more or not travel as far as their female counterparts. less the same corridors or paths each year. For Finally, there is irruptive migration, whereby cer- North American birds migrating south for the tain species do not migrate at all during some winter, one of the most commonly used “fly- years but may do so during other years. The like- ways” is across the Gulf of Mexico, a journey of lihood of migration seems to be tied to climate and resource availability: the colder the winter 500-680 mi. (800–1,000 km). To make it across and the more scarce the food, the more likely the open waters of the gulf, birds have to store up migration will occur in species prone to irruptive fat, on which they can live for some time while migratory behavior. out of sight of food sources. Migrating birds are not like the proverbial Directions of Migration parent (usually a father) who will not let the chil- dren stop to go to the bathroom on a long road Though southward migration is the most widely known form of migratory behavior, not all trip. As they make their way south, birds stop reg- migration is from the north to the south. Actual- ularly to rest and eat, sometimes for days at a ly, this type of movement is more properly called time. These stops are particularly frequent and latitudinal migration, since it also takes place in long just before crossing a large expanse of water. the Southern Hemisphere, where, of course, it is Only when the bird has stored sufficient quanti- from south to north. (Also, winter in those lati- ties of body fat does it resume the journey. tudes occurs at the same time as summer in the DAY AND NIGHT TRAVEL. In Northern Hemisphere.) There is far less habit- North America it is common to see flocks of able land below the equator than above it, how- birds apparently flying south during the daytime ever, so latitudinal migration in the Southern in the autumn months. Yet most migrating bird Hemisphere is not nearly as significant as it is at species travel at night. The fewer species that northerly latitudes. travel by day tend to follow paths that are slower There are, in fact, species of bird, such as the and less direct than those of nocturnal migrants. prairie falcon (Falco mexicanus), that travel lon- The reason for this difference has to do with the gitudinally, or from east to west. This type of differences in feeding opportunities.
336 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 337
Migration and Navigation
BIRDS ARE NOT THE ONLY CREATURES CAPABLE OF GREAT NAVIGATIONAL FEATS. SALMON COME BACK FROM THE OCEANS AND FIGHT THEIR WAY UPSTREAM TO SPAWN IN THE VERY SAME SPOT AT WHICH THEY WERE HATCHED. (© Ralph A. Clevenger/Corbis. Reproduced by permission.)
Nocturnal migrants have the entire day in have to forage at the same time that they are trav- which to rest, forage for food, and eat, thus build- eling. For this reason, they tend to stick close to the coastlines, which offer abundant supplies of ing up reserves of energy for the nonstop flight insect life. This slows them down but offers a that night. Daytime migrants, on the other hand, dependable food supply.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 337 set_vol3_12 8/19/02 11:26 AM Page 338
Feats of Navigation Migration patterns. Auditory cues and the sensitivity of and migratory species to these cues often have been Navigation In making their journeys, some creatures display navigational skills that would put such great advanced as a key to migration behavior. Finally, mariners as Ferdinand Magellan and James Cook one theory, which we discuss shortly, holds that to shame. For example, the arctic tern (Sterna sensitivity to Earth’s magnetic field provides paradisaea) is a complete migrant in every sense long-distance travelers with the navigational aid of the word. Not only does the tern engage in they need. complete migration, as defined earlier, but it also Several experiments have been performed literally crosses the globe from pole to pole. In on a variety of creature well known for its navi- traveling from the Arctic to the Antarctic and gational abilities: the homing pigeon, which back again in a single year, the arctic term com- often can return across many hundreds of miles pletes a round-trip migration of more than to its home. While they are undergoing training 21,750 mi. (35,000 km). This distance, nearly the by humans, these pigeons are released from a circumference of the planet, is the longest regular series of sites, each just a little farther from the migratory path of any animal. birds’ home area. This training seems to make A specimen of another bird species, the them accustomed to traveling long distances and Manx shearwater, was taken via airplane to to finding their way back. Thus, once trained, a Boston from its home on an island off the coast pigeon released 100 mi. (63 km) or more from its of Wales. Released experimentally in Boston, the home will begin flying in the correct direction bird took only 13 days to return to its point of within a few minutes. origin, a flight of some 3,050 mi. (4,880 km). In BIOLOGICAL CLOCKS AND another experiment, an albatross from Midway NAVIGATION. Several theories regarding Island, deep in the Pacific, was released in the pigeons’ homing skills cite internal or biological Philippines and made its way to its home area, a clocks (see Biological Rhythms). One such theo- distance of 4,120 mi. (6,592 km), in only 32 days. ry, which is no longer accepted widely, held that There are also many examples of swallows find- the pigeon’s perception of the Sun’s position in ing their way across great distances. For instance, the sky, combined with its biological clock, swallows that winter in southern Africa still man- helped it navigate. Experiments have not proved age to get back to their homes in northern Europe each spring. Then there are the swallows this to be the case, however. of San Juan Capistrano, southwest of Los Ange- In one such trial pigeons were kept in a lab- les, which leave every year on October 23 and oratory from which they could see the Sun for return like clockwork on March 19. only very limited periods of time each day. After Birds are not the only creatures capable of an extended period, the pigeons, with their eyes such great navigational feats. Monarch butterflies covered, were taken away about 40 mi. (64 km) to (Danas plexippus), which are born in Canada or the south and released, so that the moment of the northern United States, winter each year in release was their first unobstructed view of the southern California, just as they have done for Sun in weeks. Assuming the theory was correct, countless years. Then, when winter is over, they the pigeons would have been disoriented, but, in make their way back to their home regions. Like- fact, they quickly took stock of their position and wise, salmon come back from the oceans and began flying north. fight their way upstream to spawn in the very “MAGNETIC MAPS.” One intrigu- same spot at which they were hatched. ing theory of animal navigation holds that crea- How Do They Do It? tures carry in their brains “magnetic maps,” or strong sensitivities to Earth’s magnetic field, that Based on these and other examples, one is left assist them in finding their way across distances. wondering, how do they do it? Numerous obser- As reported in National Geographic Today on-line vations and theories have been put forward to (October 12, 2001), research on loggerhead tur- answer this question. Salmon, for instance, seem tles has shown that hatchlings are sensitive to the to distinguish their home streams on the basis of strength and direction of Earth’s magnetic field smell, whereas some birds appear to use visual and apparently use this in their migratory navi- signals, primarily the position of the Sun or star gation.
338 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 339
Migration and Navigation
A FASCINATING EXAMPLE OF ANIMAL NAVIGATION IS ECHOLOCATION, WHICH IS NOT NECESSARILY TIED TO MIGRATION. RATHER, IT PROVIDES A MEANS OF LOCAL NAVIGATION—AND ESCAPE FROM PREDATORS—FOR CREATURES THAT LACK THE ABILITY TO SEE IN THEIR ENVIRONMENTS, SUCH AS THIS WHALE IN THE CLOUDY REALM OF THE OCEAN. (© Terry Whittaker. Photo Researchers. Reproduced by permission.)
By rigging up harnesses attached to elec- these magnetic stimuli by turning in the appro- tronic tracking units and outfitting the turtles priate direction—for instance, south when they with these devices, researchers were able to fol- perceived that they were in the magnetic field low the course of their migration. Because these equivalent to that of Portugal and west in the are, after all, turtles (though seaborne rather than field resembling that of Cape Verde. terrestrial or land-based), migration is no speedy The research team, which published its affair. To complete the trip, an 8,000-mi. (12,900- results in the distinguished journal Science, con- km) circuit from south Florida around the Sara- cluded that the turtles were hardwired with mag- gasso Sea in the north Atlantic and back again, netic sensitivities. The team leader Kenneth takes 5–10 years. Lohmann, a biologist at the University of North In the experiment, baby loggerheads were Carolina in Chapel Hill, told National Geograph- tagged with tracking systems just after they came ic Today, “These turtles have never been exposed out of their underground nests on the eastern to water, yet they were able to process magnetic coast of Florida. Still babies, they would normal- information and change their swimming direc- ly begin a journey across the Atlantic, past the tion accordingly. It seems they inherited some Canary and Cape Verde islands on the west coast sort of magnetic map.” and Africa and then back to Florida. But instead Echolocation of going on this journey, the turtles in the exper- iment were placed in a large circular water tank One final, fascinating example of animal naviga- surrounded by an electric coil capable of gener- tion is echolocation, which differs from most of ating specific magnetic fields. the navigational behaviors we have discussed so By turns, the research team exposed the ani- far in that this one is not necessarily tied to mals to fields simulating those in three key spots migration. Rather, echolocation provides a along the route: northern Florida, the area off the means of local navigation for creatures that lack coast of Portugal, and the region near the Cape the ability to see in their environments: bats fly- Verde Islands. In each case, turtles responded to ing through caves and dolphins, porpoises, and
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 339 set_vol3_12 8/19/02 11:26 AM Page 340
Migration with that of bats and oceangoing mammals, and Navigation KEY TERMS which can hear tones up to 150,000 Hz. Echolocation represents an evolutionary tri- BIOLOGICAL CLOCK: A mechanism umph in the form of adaptation to environ- ments—the dark, nocturnal world of the bat and within an organism (for example, the the cloudy realm of sea mammals. In the distant pineal gland in the human brain) that gov- past, bats that hunted for insects during the day erns biological rhythms. would have been at a disadvantage compared ECHOLOCATION: The use of sound with birds, which are nimble of movement and extremely sharp-eyed in spotting their insect waves, which are reflected back to the emit- prey. Whales, porpoises, and dolphins were in an ter, as a means of navigating. even worse situation with regard to sharks. INSTINCT: A stereotyped, or largely Sharks, known for a finely tuned sense of smell, unvarying, behavior that is typical of a par- were not only competitors for prey but, as terti- ticular species. An instinctive behavior ary consumers (see Food Webs), they were and are also potential predators of sea mammals. does not have to be learned; rather, it is IN BATS. Only certain kinds of bats use fully functional the first time it is per- echolocation. These are the carnivorous varieties formed. that live on frogs, fish, and insects, as opposed to MIDDLE EAR: A small cavity that the herbivorous eaters of fruit and nectar. These transmits sound waves, via a network of carnivorous bats fly through the darkness, emit- ting extremely high frequency sounds and receiv- tiny bones, from the eardrum, which lies ing the echoes from these sounds. Contained in between it and the outer ear. Bats use the the echo is a whole database of information middle ear to separate transmission and regarding the object off which it has reflected: its reception signals in echolocation. distance, direction, size, surface texture, and even material composition. MIGRATION: A pattern of movement, usually regular and seasonal, whereby ani- Interestingly, the volume of these sounds is so great—as high as 100 decibels (dB)—that if mals travel (typically guided by instinct) to people were able to hear them, the noises would specific locations. be almost literally ear-splitting. (The upper range NATURAL SELECTION: The process of safety is 120 dB.) As it is, a person would hear whereby some organisms thrive and others only clicks or chirps. Scientists have long won- dered how the bats can both emit these sounds, perish, depending on their degree of adap- which would be deafening to the bat, and hear tation to a particular environment. them at the same time. Experimental evidence indicates that when the bat emits a sound, the middle ear (that is, the middle portion of the ear’s interior) adjusts in such a way as to momen- toothed whales swimming beneath the ocean’s tarily deafen the bat. A split second later, the bat’s surface. inner ear readjusts so as to permit it to hear the The frequency of a sound is related not to echo from the previous sound. volume but to pitch: the higher the note, the IN OCEANGOING MAMMALS. It higher the frequency. The frequency is measured is true, as the tagline for the 1979 film Alien in Hertz, or cycles per second. Human beings are threatened, that “In space no one can hear you capable of hearing sounds between 20 Hz and scream.” Another way of putting this is that 20,000 Hz, whereas cats can hear sounds up to sound requires a material medium through 32,000 Hz and dogs up to 46,000 Hz. This is why which to travel, and the more dense the medium, these creatures can hear dog whistles and other the more efficiently it moves. Thus, sound moves sounds inaudible to humans. Even the hearing more effectively through water than through air, ability of these household pets pales compared and for this reason echolocation is even more
340 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_12 8/19/02 11:26 AM Page 341
suited to the deep-sea environment than it is to it sends out noises. Some species have bony insu- Migration the world above ground. lating structures, which separate the portion of and Navigation As with bats, undersea mammals send out the head where sounds are received from that sounds and then listen for the echoes. Owing to part where sounds are generated. Likewise, struc- their heightened sense of hearing, these creatures tures in the middle ear assist the whale in distin- obtain far more information from sounds than a guishing whether sounds come from the left or human would. Sound, in fact, does for them what the right, thus facilitating the whale’s navigation. sight would do for a human, providing a detailed, three-dimensional image of their surroundings, WHERE TO LEARN MORE but the images it offers are even more precise Caras, Roger A. The Endless Migrations. New York: Dut- because sound waves are less subject to interfer- ton, 1985. ence and diffraction than light waves. (For exam- Journey North: A Global Study of Wildlife Migration (Web ple, sound waves can simply go around a build- site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 341 set_vol3_13 8/19/02 11:31 AM Page 343
SCIENCE OF EVERYDAY THINGS real-life biology THE BIOSPHERE AND ECOSYSTEMS
THE BIOSPHERE ECOSYSTEMS AND ECOLOGY BIOMES
343 set_vol3_13 8/19/02 11:31 AM Page 345
The Biosphere THE BIOSPHERE
CONCEPT that can be set apart mentally from the rest of the The biosphere is simply “life on Earth”—the sum universe for the purposes of study, observation, total, that is, of all living things on Earth. Yet the and measurement. Thus, virtually by definition, a whole is more than the sum of the parts: not only system is something in which the various parts fit is the biosphere an integrated system whose many together harmoniously, as though they were components fit together in complex ways, but it designed—or adapted over millions of years—to also works, in turn, in concert with the other do so. major earth systems. The latter include the Anything outside the system is known as the geosphere, hydrosphere, and atmosphere, environment, and numerous qualifying terms through which circulate the chemical elements identify the level of interaction between the sys- and compounds essential to life. Among these ele- tem and its environment. An isolated system is ments is carbon, a part of all living things, which one so completely sealed off from its environ- also cycles through the nonliving realms of soil, ment that neither matter nor energy passes water, and air—just one of many vital biogeo- through its boundaries. This is a merely theoret- chemical cycles. As for the compounds on which ical construct, however, because, in practice, life depends, none is more important than water, some matter always flows between system and which, though it is the focal point of the hydros- environment. For example, regardless of how phere, passes through the various earth systems as tightly a vault or other interior chamber is sealed, well. Organisms participate in the hydrologic there is always room for matter at the microscop- cycle by providing moisture to the air through the ic or atomic level to pass through the barrier; process of transpiration, and they likewise benefit moreover, energy, which in many forms does not from the downward movement of moisture in the require any material medium for its transmis- form of precipitation. These and many other sion, will pass through as well. interactions make it easy to see why scientists Earth itself is an approximation of a closed speak of Earth as a system—and why some go system, or a system in which, despite the sound of even further and call it a living thing. its name, exchanges of energy (but not matter) with the environment are possible. Earth absorbs HOW IT WORKS electromagnetic energy from the Sun and returns that energy to space in a different form, but very Earth Systems little matter enters or departs Earth’s system. Earth is not a perfectly closed system, however, Chances are that the mention of the word system since meteorites can enter the atmosphere and calls to mind something mechanical or electrical, hydrogen can escape. Without the intrusion of produced by humans: for example, a heating and meteorites, in fact, it is unlikely that life could cooling system. This application of the word is exist on the planet, because these projectiles from close to the scientific meaning, but in the sciences space first brought water (and possibly even the system identifies a wider range of examples. In carbon-based rudiments of life) to the planet scientific terms a system is any set of interactions Earth more than four billion years ago.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 345 set_vol3_13 8/19/02 11:31 AM Page 346
The Nevertheless, Earth more closely resembles a the noble gases, is simply “hanging in the air” Biosphere closed system than it does an open system, or one (literally), left over from the time when volcanoes that allows the full and free exchange of both hurled it into the atmosphere several billion years
matter and energy with its environment. The ago. By contrast, oxygen (both in O2 and O3, or human circulatory system is an example of an ozone, molecules) and the other elements in air open system, as are the various “spheres” of Earth are vital to life. Furthermore, oxygen is one of (geosphere, hydrosphere, biosphere, and atmos- two elements, along with hydrogen, that goes phere) that we discuss later in this essay. The dis- into the formation of water. tribution of matter and energy within these earth OVERLAP BETWEEN SUBSYS- systems may vary over time, but the total amount TEMS. The present atmosphere would not of energy and matter within the larger earth sys- exist without the biosphere. In order to put oxy- tem is constant. gen into the air, there had to be plants, which take THE FOUR “SPHERES.” On the in carbon dioxide and release oxygen in the other hand, the four subsystems, or “spheres,” process of photosynthesis. This resulted from an within the larger Earth system are very much exceedingly complex series of evolutionary open systems. Of these four subsystems, the developments from anaerobic, or non-oxygen- name of only one, the atmosphere, is familiar breathing, single-cell life-forms to the appear- from everyday life, whereas those of the other ance of algae. As plant life evolved, eventually it three sectors (geosphere, hydrosphere, and bios- put more and more oxygen into the atmosphere, phere) may sound at first like scientific jargon. until the air became breathable for animal life. Yet each has a distinct identity and meaning, and Thus, the atmosphere and biosphere have sus- each represents a part of Earth that is at once tained one another. clearly defined and virtually inseparable from the Such overlap is typical and indeed inevitable rest of the planet. where the open earth subsystems are concerned, The geosphere is the upper part of the plan- and examples of this overlap are everywhere. For et’s continental crust, the portion of the solid instance, plants (biosphere) grow in the ground earth on which human beings live and which (geosphere), but to survive they absorb water provides them with most of their food and natu- (hydrosphere) and carbon dioxide (atmosphere). ral resources. It is also the oldest, followed by the Nor are plants merely absorbing: they also give hydrosphere, which had its beginnings with sev- back oxygen to the atmosphere, and by providing eral hundred millions years’ worth of rains that nutrition to animals, they contribute to the bios- took place about four billion years ago. Today the phere. At the same time, the many components of hydrosphere includes all water on Earth, except the picture just described are involved in complex for water vapor in the atmosphere. The latter, biogeochemical cycles, which we look at later. incidentally, was probably the last of the four sub- THE BIOSPHERE IN CONTEXT. systems to take shape: though Earth in its early The biosphere is, of course, integral to the func- stages had a blanket of gases around it, there was tioning of earth systems. First of all, the present no oxygen. (See Paleontology for more about the atmosphere, as we have noted, is the product of early atmosphere, oxygen, and early life.) respiration on the part of plants, which receive THE ATMOSPHERE. Today the carbon dioxide and produce oxygen. In addition, atmosphere is 78% nitrogen, 21% oxygen, and transpiration, a form of evaporation from living 0.93% argon. The remaining 0.07% is made up organisms (primarily plants), is a mechanism of of water vapor, carbon dioxide, ozone (a form of fundamental importance for moving moisture oxygen in which three oxygen atoms bond chem- from the hydrosphere through the biosphere to ically), and noble gases. The noble gases, includ- the atmosphere. ing argon and neon, are noted for their lack of We examine transpiration later, within the reactivity, meaning that they are extremely resist- larger context of evapotranspiration, along with ant to chemical bonding with other elements. another area in which the biosphere interacts Nitrogen also tends to be unreactive, and the closely with one or more of the other earth sys- reason for its abundance in the atmosphere lies tems: soil. Though soil is part of the geosphere, its in the fact that it never attempted to bond with production and maintenance is an achievement other elements. Therefore, nitrogen, along with of all spheres. The role of the biosphere in this
346 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 347
instance is particularly important: the amount of of leaves. Scientists usually speak of the two as a The decayed organic material (i.e., dead plants and single phenomenon, evapotranspiration. The Biosphere animals) is critical to the quality of the soil for atmosphere is just one of several “compart- sustaining further life in the form of plants and ments” in which water is stored within the larger other organisms that live underground. environment. In fact, the atmosphere is the only As important as the biosphere is, it may be major reservoir of water on Earth that is not con- surprising to learn just what a small portion of sidered part of the hydrosphere. the overall earth system it occupies. As living ACCOUNTING FOR EARTH’S beings, we tend to have a bias in favor of the liv- WATER SUPPLY. The water that most of ing world, but the overwhelming majority of the us see or experience is only a very small portion planet’s mass and space is devoted to nonliving of the total. Actually, that statement should be matter. The geosphere alone accounts for almost qualified: the oceans, parts of which most people 82% of the combined mass of the four subsys- have seen, make up about 5.2% of Earth’s total tems. (This is not the combined mass of Earth, water supply. This may not sound like a large which would be much larger; remember that the portion, but, in fact, the oceans are the second- geosphere is only the extreme upper layer of largest water compartment on Earth. If the Earth, and does not include the vast depths of the oceans are such a small portion yet rank second lower mantle and core.) in abundance, two things are true: there must be Of the remaining mass that makes up the a lot of water on Earth, and most of it must be in four earth systems, the hydrosphere is a little one place. more than 18%, the atmosphere less than 1%, In fact, the vast majority of water on Earth is and the biosphere a tiny 0.00008%. Note just stored in aquifers, or underground rock forma- how much greater the amount of mass is in the tions, that hold 94.7% of the planet’s water. Thus, air, which we tend to think of as being weightless deep groundwater and oceans account for 99.9% (though, of course, it is not), than in the bios- of the total. Glaciers and other forms of perma- phere. Even within the biosphere’s almost infini- nent and semipermanent ice take third place, tesimal fraction of total mass, the animal king- with 0.065%. Another 0.03% appears in the form dom accounts for less than 2%, the remainder of shallow groundwater, the source of most local being devoted to other kingdoms: plants, fungi, water supplies. Next are the inland surface monera (including bacteria), and protista, such waters, including such vast deposits as the Great as algae. (See Taxonomy and Species for more Lakes and the Caspian Sea as well as the Missis- about the kingdoms of living things.) It need sippi-Missouri, Amazon, and Nile river systems hardly be added that humans, in turn, are a very, and many more, which collectively make up just very small portion of the animal world. 0.003% of Earth’s water. Water and the Hydrologic ATMOSPHERIC MOISTURE AND Cycle WEATHER. That leaves only 0.002%, which is the proportion taken up by moisture in the atmos- As any backyard horticulturist knows, plants phere: clouds, mist, and fog, as well as rain, sleet, need good soil and water. In the course of circu- snow, and hail. While it may seem astounding that lating throughout Earth, water makes its way atmospheric moisture is such a small portion of through organisms in the biosphere as well as the total, this fact says more about the vast reservoirs housed within the geosphere. It also amounts of water on Earth than it does about the circulates continuously between the hydrosphere small amount in the atmosphere. That “small” and the atmosphere. This movement, known as amount, after all, weighs 1.433 ϫ 1013 tons (1.3 ϫ the hydrologic cycle, is driven by the twin 1013 tonnes), or 28,659,540,000,000,000 pounds processes of evaporation and transpiration. (12,999,967,344,000,002 kg). The first of these processes, of course, is the This moisture in the atmosphere is the means whereby liquid water is converted into a source of all weather, which clearly has an effect gaseous state and transported to the atmosphere, on Earth’s life-forms. (Weather is the condition while the second one—a less familiar term—is of the atmosphere at a given time and in a given the process by which plants lose water through place, whereas climate is the pattern of weather their stomata, small openings on the undersides in a particular area over an extended period of
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 347 set_vol3_13 8/19/02 11:31 AM Page 348
THE CARBON AND NITROGEN The time.) On the one hand, rain is necessary to pro- Biosphere vide water to plants, and desert conditions can CYCLES. Carbon, for instance, is present in sustain only very specific life-forms; on the other all living things and is integral to the scientific hand, storms, icy precipitation, and flooding can definition of the word organic. The latter term be deadly. does not, as is popularly believed, refer only to living and formerly living things, their parts, and Biogeochemical Cycles their products, such as sweat or urine. Organic refers to the presence of compounds containing Water is not the only substance that circulates carbon and hydrogen. The realm of organic sub- through the various earth systems. So, too, do six stances encompasses not only the world of the other substances or, rather, chemical elements. These elements are composed of a single type of living, the formerly living, and their parts and atom, meaning that they cannot be broken down products, but also such substances as plastics that chemically to make a simpler substance, as is the have never been living. case with such compounds as water. The six ele- The carbon cycle itself involves movement ments that cycle throughout Earth’s systems are between the worlds of the living and nonliving, hydrogen, oxygen, carbon, nitrogen, phosphorus, the organic and inorganic. This highly complex and sulfur. The two following lists provide rank- biogeochemical cycle circulates carbon from soil ings for their abundance. The first shows their and carbonate rocks (which are inorganic ranking and share in the entire known mass of because they do not contain carbon-hydrogen the planet, including the crust, living matter, the compounds) to plants and hence to animals, oceans, and atmosphere. The second list shows which put carbon dioxide into the air. Likewise, their relative abundance and ranking in the the nitrogen cycle moves that element among all human body. these reservoirs. Abundance of Selected Elements on Earth Because nitrogen is highly unreactive, the (Ranking and Percentage): participation of microorganisms in the nitrogen • 1. Oxygen (49.2%) cycle is critical to moving that element between • 9. Hydrogen (0.87%) the various earth systems. These organisms “fix” • 12. Phosphorus (0.11%) nitrogen, meaning that by processing the element • 14. Carbon (0.08%) through their bodies, they bring about a chemi- • 15. Sulfur (0.06%) cal reaction that makes nitrogen usable to plant • 16. Nitrogen (0.03%) life. Additionally, detritivores and decomposers Abundance of Selected Elements in the in the soil are responsible for transforming Human Body (Ranking and Percentage): nitrites and nitrates (compounds of nitrogen and • 1. Oxygen (65%) oxygen) from the bodies of dead animals into elemental nitrogen that can be returned to the • 2. Carbon (18%) atmosphere. • 3. Hydrogen (10%) • 4. Nitrogen (3%) • 6. Phosphorus (1%) REAL-LIFE • 9. Sulfur (0.26%) APPLICATIONS Note that the ranking of all these elements (with the exception of oxygen) is relatively low in Soil and the Life in It the total known elemental mass of Earth, where- as their relative abundance is much, much high- The soil is a sort of anchor to the biosphere. It er within the human body. This is significant, teems with life like few other areas within the given the fact that these elements are all essential earth system, and, indeed, there are more crea- to the lives of organisms. All six of these elements tures—plant, animal, monera, protista, and take part in biogeochemical cycles, a term used to fungi—living in the soil than above it. Minerals refer to the changes that a particular element from weathered rock in the soil provide plants undergoes as it passes back and forth through the with the nutrients they need to grow, setting in various earth systems and particularly between motion the first of several steps whereby organ- living and nonliving matter. isms take root in and contribute to the soil.
348 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 349
The Biosphere
NUMEROUS ANT SPECIES PERFORM A POSITIVE FUNCTION FOR THE ENVIRONMENT. LIKE EARTHWORMS, THEY AERATE SOIL AND HELP BRING OXYGEN AND ORGANIC MATERIAL FROM THE SURFACE WHILE CIRCULATING SOILS FROM BELOW. (© Ralph A. Clevenger/Corbis. Reproduced by permission.)
Plants provide food to animals, which, when that result in the breakdown of their compo- they die, likewise become one with the soil. So, nents. As a result, simple and usable nutrients are too, do plants themselves. Organisms from both made available to the soil. the plant and animal kingdoms leave behind Earthworms, which are visible and relatively material to feed such decomposers as bacteria large, are not the only worms at work in the soil. and fungi, which, along with detritivores, are There are also the colorless creatures of the phy- critical to the functioning of food webs (see lum Nematoda, or roundworms. Included in this entry). Detritivores, of which earthworms are a phylum are hookworms and pinworms, which great example, are much more complex organ- can be extremely detrimental to the body when isms than the typically single-cell decomposers. they live inside it as parasites. (See Parasites and We cannot see bacteria, but almost anyone Parasitology.) This is one good reason (among who has ever dug in the dirt has discovered many) not to eat dirt. But nematodes in the soil, another type of organism: the Annelida phylum most of them only slightly larger than microor- of the animal kingdom, which includes all seg- ganisms, perform the vital function of processing mented worms, among them, the earthworm. organic material by feeding on dead plants. Even (Incidentally, the leech family also falls within in a soil situation, however, some nematodes are phylum Annelida.) These slimy creatures at first parasites that live off the roots of such crops as might seem disgusting, but without the appro- corn or cotton. priately named earthworm, our world could not In addition to earthworms, ants and other exist as it does. creatures are also significant inhabitants of the Detritivores consume the remains of plant soil. Like earthworms, ants aerate soil and help and animal life, which usually contain enzymes bring oxygen and organic material from the sur- and proteins far too complex to benefit the soil in face while circulating soils from below. Among their original state. By feeding on organic the larger creatures that call the soil home are remains, detritivores cycle these complex chemi- moles, which live off earthworms, grubs (insect cals through their internal systems, thus causing larvae), and the roots of plants. By burrowing the substances to undergo chemical reactions under the ground, they help to loosen the soil,
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 349 set_vol3_13 8/19/02 11:31 AM Page 350
The making it more porous and thus receptive to Given the poor quality of equatorial soil, one Biosphere both moisture and air. Other large burrowing might wonder how it is that some of the most creatures include mice, ground squirrels, and, in lush rain forests are located in the equatorial some areas, even prairie dogs. regions of Africa and South America. The answer is that a rain forest has such a wide assortment of Soils and the Environments life-forms that there is never a shortage of They Help Create decayed organic material to “feed” the soil. Due to the sheer breadth and scope of the rain forest’s Five different factors determine the quality of ecological diversity, there are bound to always be soil: parent material (the decayed organisms and plants and animals dying, replenishing what weathered rock that make it up), climate, the would otherwise be poor soil. The rapid rate of presence of living organisms, topography (the decay common in warm, moist regions (which shape of the land, including prominent natural are extremely hospitable to bacteria and other features), and the passage of time. These factors microbes) further supports the process of renew- influence the ability of the soil to sustain life. ing minerals in the ground. For example, in a desert, a place that obvi- The fact that decayed organic material is ously has a smaller abundance and complexity of critical to the life of soil helps to explain why life-forms (see Biological Communities), the soil many environmentalists have long been con- itself is lacking in this life-sustaining quality. cerned about the destruction of tropical rain Desert soil, in fact, is usually referred to as imma- forests. For example, in Brazil, vast portions of ture soil. Healthy soil normally has a deep A hori- the Amazon rain forest have been clear-cut, sub- zon, the area in which decayed organic material jected to an extremely destructive form of slash- appears and atop which humus sits. In general, and-burn agriculture that is motivated by mod- the deeper the A horizon, the better the soil. ern economic concerns—the desire of the coun- Immature soil, on the other hand, has a very thin try’s leaders to create jobs and exports—but A horizon and no B horizon, which is the subsoil which resembles practices applied (albeit on a that typically separates the A horizon from a much smaller scale) by premodern peoples in layer of weathered material that rests even lower, Central and South America. at the C horizon just above bedrock. By removing the heavy jungle canopy of tall In deserts, by definition, the water supply is trees, clear-cutting exposes the ground to the very limited, and only those species that require heat of the Sun and the pounding of monsoon very little water—for example, the varieties of rains. Sun and rain, thanks to the removal of this cactus that grow in the American Southwest— protection, fall directly on the ground, parching are able to survive. But lack of water is not the it in the first instance and eroding it in the sec- only problem. Desert subsoils often contain ond. Furthermore, when trees and other vegeta- heavy deposits of salts, and when rain or irriga- tion are removed, the animal life that these plants tion adds water to the topsoil, these salts rise. supported disappears as well, and this has a Thus, watering desert topsoil actually can make it direct impact on the soil by removing organisms a worse environment for growth. whose waste products and bodies eventually TROPICAL RAIN FORESTS. The would have decayed and enriched it. soil in rain forests has just the opposite problem WHAT MAKES GOOD SOIL? The of desert soil: instead of being immature, it has soil of the Brazilian rain forest may be old and gone beyond maturity and reached old age, a weak, but thankfully there are places in the world point at which plant growth and water percola- where the soil tells an entirely different story. tion (the downward movement of water through Instead of being nutrient-poor, this soil is nutri- the soil) have removed most of its nutrients. In ent-rich, and instead of being red, such soil is a environments located near the equator, whether deep, rich black. Sometimes regions of good and these regions be desert or rain forest, soils tend to bad soil exist in close proximity, as in ancient be “old.” This helps to explain the fact that equa- Egypt, where the Nile made possible a narrow torial regions are usually low in agricultural pro- strip of extraordinarily productive land, running ductivity, despite the fact that they enjoy an oth- the length of the country, flanked by deserts. The erwise favorable climate. latter the Egyptians called “the red land” because
350 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 351
of its nutrient-poor soil, whereas their own fer- The tile region was “the black land.” Biosphere Just as Egypt, after its annexation by the Romans in 30 B.C., became known as the bread- basket of the Roman Empire, there are other “black lands” that serve as the breadbaskets of today’s world. Unlike Egypt, however, most are located in temperate rather than equatorial regions. (See Biomes for more about temperate regions.) Examples include the midwestern Unit- ed States, western Canada, and southern Russia, regions characterized by vast plains of fertile black soil. Below this rich topsoil is a thick sub- soil that helps hold in moisture and nutrients. Rivers such as the Nile, the Mississippi-Mis- souri, or the Volga in Russia make possible the richest variety of soil on Earth, alluvial soil, a youngish sediment of sand, silt, and clay. A river pulls soil along with it as it flows, and with this comes nutrients from the regions through which the river has passed. The river then deposits these nutrients in the alluvial soil at the delta, the area where it enters a larger body, usually of salt water. DESERT TORTOISE AND BEAVERTAIL CACTUS IN THE (A delta is so named because, as it widens in the MOJAVE DESERT. ONLY THOSE PLANT AND ANIMAL region near the sea, the shape of the river is like SPECIES THAT CAN ENDURE A LIMITED WATER SUPPLY that of the Greek letter delta, or ∆.) AND IMMATURE SOIL WITH HEAVY DEPOSITS OF SALT IN THE LOWER LAYERS CAN SURVIVE A DESERT ENVIRON- Because they are depositories for accumulat- MENT. (© D. Suzio. Reproduced by permission.) ed alluvial soil, delta regions such as Mississippi and Louisiana in the United States (where the mighty Mississippi-Missouri, the largest river hydrogen compound. The latter may be ammo- + system in North America, empties into the Gulf nia (NH3) or ammonium (NH4 ). Note that of Mexico), are exceedingly fertile. The same is ammonia is much more than the product with true in the Volga and Nile deltas, the deltas of the which it is most readily associated: a household Danube and other major European rivers, and cleaner. In fact, ammonia is an extremely abun- those of lesser-known rivers in Canada or Aus- dant substance, occurring naturally, for instance, tralia. in the atmospheres of Venus and other planets in FERTILIZER. It is also possible to arti- our solar system. The importance of ammonia is ficially improve soil that is not in a river delta, or reflected by the fact that it and water are the only that has not otherwise been blessed by nature. two substances that chemists regularly refer to by The most significant way to achieve this is by their common names, as opposed to a scientific using fertilizer, which augments the nutrients in name such as carbon dioxide. the soil itself. As noted earlier, nitrogen is highly As for ammonium, its extra hydrogen atom nonreactive, meaning that it tends not to bond makes it a substance that dissolves in water and is chemically with other substances. However, attracted to negatively charged surfaces of clays because it is a necessary component of biogeo- and organic matter in soil. Therefore, it tends to chemical cycles, it is critical that nitrogen be become stuck in one place rather than to move introduced to the soil in such a way that it around, as nitrate does. Plants in acidic soils typ- becomes useful, and typically this is done by com- ically receive their nitrogen from ammonium, bining it with a highly reactive element: oxygen. but in nonacidic soils, nitrate is typically the Fertilizers may contain nitrogen in the form more useful form of fertilizer. The two fertilizers of a nitrate, which is a compound of nitrogen are also combined to form ammonium nitrate, and oxygen, or they may include a nitrogen– which is powerful both as a fertilizer and as an
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 351 set_vol3_13 8/19/02 11:31 AM Page 352
The explosive. (Ammonium nitrate was used both in Years after the dust bowl, the American West Biosphere the first World Trade Center bombing, in 1993, could have again become the site of another dis- and in the even more devastating Oklahoma City aster, had not farmers and agricultural officials bombing two years later.) learned from the mistakes of an earlier genera- tion. During the 1970s, American farms enjoyed Erosion and Soil such a great surplus that farmers increasingly Conservation began to sell their crops to the Soviet Union, and farmers were encouraged to cultivate even mar- The mismanagement of agricultural lands, ginal croplands to increase profits. This alarmed and/or the influence of natural forces, can pro- environmental activists, who called attention to duce devastating results, as illustrated by events the flow of nutrients from croplands into water during the years 1934 and 1935 in a region resources. As a result of public concerns over including Texas, Oklahoma, Kansas, and eastern these and related issues, Congress in 1977 passed Colorado. In just a few months, once-productive farmland turned into worthless fields of stubble the Soil and Water Resource Conservation Act, and dust, good for virtually nothing. By the time mandating measures to conserve or protect soil, it was over, the region had acquired a bitter nick- water, and other resources on private farmlands name: the “dust bowl.” and other properties. Ironically, in the years leading up to the early Leaching and Its Effect on 1930s the future dust bowl farmlands had Soil seemed remarkably productive. Farmers happily reaped abundant yields, year after year, not Like erosion, leaching moves substances through knowing that they were actually preparing the soil, only in this case it is a downward movement. way for soil erosion on a grand scale. Farmers in Leached water can carry all sorts of dissolved the 1930s had long known about the principle of substances, ranging from nutrients to contami- crop rotation as a means of giving the soil a rest nants. The introduction of manufactured con- and restoring its nutrients. But to be successful, taminants to the soil, and hence the water table, crop rotation must include fallow years (i.e., no is of course a serious threat to the environment. crops are planted), and must make use of crops On the other hand, where human waste and that replenish the soil of nutrients. other, more natural forms of toxin are con- cerned, nature itself is able to achieve a certain Cotton and wheat are examples of crops that deplete nutrient content in the soil, and in fact amount of cleanup on its own. wheat was the crop of choice in the future dust In a septic tank system, used by people who bowl. In some places, farmers alternated between are not connected to a municipal sewage system, wheat cultivation and livestock grazing on the anaerobic bacteria process wastes, removing a same plot of land. The hooves of the cattle fur- great deal of their toxic content in the tank itself. ther damaged the soil, already weakened by rais- (These bacteria usually are not introduced artifi- ing wheat. The land was ready to become the site cially to the tank; they simply congregate in what of a full-fledged natural disaster, and in the is a natural environment for them.) The waste- depths of the Great Depression, that disaster water leaves the tank and passes through a drain came in the form of high winds. These winds field, in which the water leaches through layers of scattered vast quantities of soil from the Great gravel and other filters that help remove more of Plains of the Midwest to the Atlantic seaboard, its harmful content. In the drain field, the waste and acreage that once had rippled with wheat is subjected to aerobic decay by other forms of turned into desert-like wastelands. bacteria before it either filters through the drain- The farmlands of the plains states have long pipes into the ground or is evaporated. since recovered from the dust bowl, and farming In addition to purifying water, leaching also practices have changed considerably. Instead of passes nutrients to the depths of the A horizon alternating one year of wheat with a year of graz- and into the B horizon—something that is not ing livestock, farmers in the dust bowl region always beneficial. In some ecosystems, leaching apply a three-year cycle of wheat, sorghum, and removes large amounts of dissolved nitrogen fallow land. They also have planted trees to serve from the soil, and it becomes necessary to fertil- as barriers against wind. ize the soil with nitrate. However, soil often has
352 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 353
difficulty binding to nitrate, which tends to leach stances that scientists today do not understand The easily, and this leads to an overabundance of fully, desertification—the slow transformation of Biosphere nitrogen in the lower levels of the soil and ordinary lands to desert—had begun to set in. As groundwater. This is a condition known as nitro- the Sahara became drier and drier, the herds dis- gen saturation, which can influence the eutroph- appeared. ication of waters (a topic discussed later) and Eventually, the Egyptians began bringing in cause the decline and death of trees with roots in domesticated horses to cross the desert: hence an affected area of ground. the name of the Horse period (ca. 1500–ca. 600 How Deserts Are Formed B.C.), when the Sahara probably resembled the dry grasslands of western Texas or the sub-Saha- Let us now consider what one might call an ran savanna in Africa today. By about 600 B.C., extreme ecosystem: a desert. Of the world’s however, the climate had become so severe, the deserts, by far the most impressive is the Sahara, water supply so limited, and the ecosystem so which today spreads across some 3.5 million sq. depleted of supporting life-forms that not even mi. (9.06 million sq km), an area that is larger horses could survive in the forbidding climate. than the continental United States. Only about There was only one mammal that could: the 780 acres (316 hectares) of it, a little more than 1 hardy, seemingly inexhaustible creature that gave sq. mi. (2.6 sq km), is fertile. The rest is mostly its name to the Camel era, which continues to the stone and dry earth with scattered shrubs—and present day. here and there the rolling sand dunes typically CONTROLLING DESERTIFICA- used to depict the Sahara in movies. TION. What happened to the Sahara? The Just 8,000 years ago—the blink of an eye in answer is a complex one, as is the subject of terms of Earth’s timescale—it was a region of desertification. Desertification does not always flowing rivers and lush valleys. For thousands of result in what people normally think of as a years it served as a home to many cultures, some desert; rather, it is a process that contributes of them quite advanced, to judge from their art- toward making a region more dry and arid, and work. Though they left behind an extraordinary because it is usually gradual, it can be reversed in record in the form of their rock-art paintings and some cases. Nor is it necessary for a society to carvings, which show an understanding of realis- tic representation that would not be matched undertake large-scale mechanized agricultural until the time of the Greeks, the identity of the projects, such as those of the American dust bowl early Saharan peoples themselves remains largely of the 1930s, to do long-term damage that can a mystery. result in desertification. The Pueblan culture of EVOLUTION OF THE ANCIENT what is now the southwestern United States depleted an already dry and vulnerable region SAHARAN ECOSYSTEM. The phases after about A.D. 800 by removing its meager in ancient Saharan cultures are identified by the stands of mesquite trees. names of the domesticated animals that dominat- ed at given times, and collectively these names tell And though human causes, either in the the story of the Saharan ecosystem’s transforma- form of mismanagement or deliberate damage, tion from forest to grassland to desert. First was certainly have contributed to desertification, the Hunter period, from about 6000 to about 4000 sometimes nature itself is the driving force. B.C., when a Paleolithic, or Old Stone Age, people Long-term changes in rainfall or general climate, survived by hunting the many wild animals then as well as water erosion and wind erosion such as available in the region. Next came the Herder peri- caused the dust bowl, can turn a region into a od, from about 4000 to 1500 B.C. As their name permanent desert. An ecosystem may survive suggests, these people maintained herds of ani- short-term drought, but if soil is forced to go too mals and also practiced basic agriculture. long without proper moisture, it sets in motion a A high point of ancient Saharan civilization chain reaction in which plant life dwindles and, came with the Herder period, which, not surpris- with it, animal life. Thus, the soil is denied the ingly, also marked the high point of the local fresh organic material necessary to its continued ecosystem in terms of its ability to sustain varied sustenance, and a slow, steady process of decline life-forms. Yet through a complex set of circum- begins.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 353 set_vol3_13 8/19/02 11:31 AM Page 354
The Phosphorus Cycle The less, scientists believe that phosphorus is steadily Biosphere Having examined what can go wrong (and right) being transferred to the ocean, from whence it is in soil, a key component of the biosphere, we will not likely to return. It is for this reason that phos- devote the remainder of this essay to two other phorus-based fertilizers are important, because aspects of the biosphere mentioned earlier: bio- they feed the soil with nutrients that would oth- geochemical cycles and the hydrologic cycle. In erwise be steadily lost. However, phosphorus still the context of biogeochemical cycles, we will ends up making its way through the waters, and look at the phosphorus cycle, along with this creates a serious problem in the form of eutrophication—another instance of “what can eutrophication. go wrong.” EUTROPHICATION. Eutrophication Because of its high reactivity with oxygen, (from a Greek term meaning “well nourished”) is phosphorus is used in the production of safety a state of heightened biological productivity in a matches, smoke bombs, and other incendiary body of water. One of the leading causes of devices. It is also important in various industrial eutrophication is a high rate of nutrient input, in applications and in fertilizers. In fact, ancient the form of phosphate or nitrate, a nitrogen-oxy- humans used phosphorus without knowing it gen compound. As a result of soil erosion, fertil- when they fertilized their crops with animal izers make their way into bodies of water, as does bones. In the early 1800s chemists recognized detergent runoff in wastewater. Excessive phos- that the critical component in the bones was the phates and nitrates stimulate growth in algae and phosphorus, which plants use in photosynthe- other green plants, and when these plants die, sis—the biological conversion of energy from the they drift to the bottom of the lake or other body Sun into chemical energy. With this discovery of water. There, decomposers consume the came the realization that phosphorus would remains of the plants, and in the process of doing make an even more effective fertilizer when treat- so, they also use oxygen that otherwise would be ed with sulfuric acid, which makes it soluble, or available to fish, mollusks, and other forms of capable of being dissolved, in water. This com- life. As a result, those species die off, to be pound, known as superphosphate, can be pro- replaced by others that are more tolerant of low- duced from phosphate, a type of phosphorus- ered oxygen levels—for example, worms. Need- based mineral. less to say, the outcome of eutrophication is dev- astating to the lake’s ecosystem. Microorganisms in the biosphere absorb insoluble phosphorus compounds and, through Lake Erie—one of the Great Lakes on the the action of acids within the microorganisms, border of the United States and Canada— turn them into soluble phosphates. These soluble became an extreme example of eutrophication in phosphates then are absorbed by algae and other the 1960s. As a result of high phosphate concen- green plants, which are eaten by animals. When trations, Erie’s waters were choked with plant they die, the animals, in turn, release the phos- and algae growth. Fish were unable to live in the phates back into the soil. As with all elements, the water, the beaches reeked with the smell of decay- total amount of phosphorus on Earth stays con- ing algae, and Erie became widely known as a stant, but the distribution of it does not. Some of “dead” body of water. This situation led to the the phosphorus passes from the geosphere into passage of new environmental standards and the biosphere, but the vast majority of it winds pollution controls by both the United States and up in the ocean. It may find its way into sedi- Canada, whose governments acted to reduce the ments in shallow waters, in which case it contin- phosphate content in fertilizers and detergents ues to circulate, or it may be taken to the deep drastically. Within a few decades, thanks to the parts of the seas, in which case it is likely to be new measures, the lake once again teemed with deposited for the long term. life. Thus, Lake Erie became an environmental success story. Because fish absorb particles of phosphorus, some of it returns to dry land through the catch- Evapotranspiration ing and consumption of seafood. Also, guano or dung from birds that live in an ocean environ- Many of the phenomena and processes we have ment (e.g., seagulls) returns portions of phos- described tie together the biosphere with other phorus to the terrestrial environment. Neverthe- “spheres” of Earth. Such is the case with evapo-
354 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 355
The Biosphere
CROSS SECTION OF A LILAC LEAF. A KEY ELEMENT IN CIRCULATING LIFE-SUSTAINING MATERIALS AMONG THE VARI- OUS EARTH SYSTEMS IS TRANSPIRATION, THE EVAPORATION OF MOISTURE FROM PLANTS. PLANTS LOSE THEIR WATER THROUGH MEMBRANES OF A TISSUE KNOWN AS SPONGY MESOPHYLL (SHOWN HERE), FOUND IN THE TINY CAVITIES THAT LIE BENEATH THE MICROSCOPIC LEAF PORES CALLED STOMATA. (© Lester V. Bergman/Corbis. Reproduced by permission.)
transpiration, the sum total of evaporation and they keep them open—just as a human’s pores transpiration. The second of these terms is less must remain open. Otherwise, the person would well-known than the first, but the words in fact not take in enough oxygen, and would perish. refer to the same process. The only difference is The fact that the stomata are exposed in order that evaporation involves the upward movement to receive carbon dioxide for the plant’s photosyn- of water from nonliving sources, while transpira- thesis also means that the stomata are open to tion is the evaporation of moisture from living allow the loss of moisture to the atmosphere. It sources. can be said, then, that transpiration in plants— Transpiration is at least as important as vital as it is to the functioning of our atmos- evaporation when it comes to putting moisture phere—is actually an unavoidable consequence of into the atmosphere. It actually puts more water photosynthesis, an unrelated process. (See Carbo- into the air than evaporation does: any large area hydrates for more about photosynthesis.) of vegetation tends to transpire much larger ANIMAL TRANSPIRATION. Tran- quantities of moisture than an equivalent nonfo- spiration in animals (including humans) takes liated region, such as the surface of a lake or place for much the same reason as it does with moist soil. Though animals can play a part in plants: as a by-product of breathing. Animals transpiration, plant transpiration has much have to keep their moist respiratory surfaces, greater environmental significance. such as the lungs, open to the atmosphere. We Water in plants is lost through moist mem- may not think of our own breathing as transfer- branes of a tissue known as spongy mesophyll, ring moisture to the air, but the presence of found in the tiny cavities that lie beneath the moisture in our lungs can be proved simply by microscopic leaf pores called stomata. Stomata breathing on a piece of glass and observing the remain open most of the time, but when they misty cloud that lingers there. need to be closed, guard cells around their bor- Transpiration can cause animals to become ders push them shut. Because plants depend on dehydrated, but it also can be important in cool- stomata to “breathe” by pulling in carbon dioxide, ing down their bodies. When human bodies
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 355 set_vol3_13 8/19/02 11:31 AM Page 356
The Biosphere KEY TERMS
A HORIZON: Topsoil, the uppermost bedrock. The C horizon is made of of the three major soil horizons. This layer regolith, or weathered rock. and the humus that lies above it house all CANOPY: The upper portion or layer the organic content in soil. of the trees in a forest. A forest with a AEROBIC: Oxygen-breathing. closed canopy is one so dense with vegeta- tion that the sky is not visible from the ANAEROBIC: Non-oxygen-breathing. ground. ATMOSPHERE: Earth’s atmosphere is CLIMATE: The pattern of weather con- a blanket of gases that includes nitrogen ditions in a particular region over an (78%), oxygen (21%), argon (0.93%), and extended period. Compare with weather. a combination of water vapor, carbon CLOSED SYSTEM: dioxide, ozone, and noble gases such as A system that per- neon (0.07%). Most of these gases are con- mits the exchange of energy with its exter- nal environment but does not allow matter tained in the troposphere, the lowest layer, to pass between the environment and the which extends to about 10 mi. (16 km) system. Compare with isolated system on above the planet’s surface. the one hand and open system on the other. B HORIZON: Subsoil, beneath topsoil COMPOUND: A substance made up of and above the C horizon. Though the B atoms, chemically bonded to one another, horizon contains no organic material, its of more than one chemical element. presence is critical if the soil is to be suit- DECOMPOSERS: able for sustaining a varied ecosystem. Organisms that obtain their energy from the chemical BIOGEOCHEMICAL CYCLES: The breakdown of dead organisms as well as changes that particular elements undergo from animal and plant waste products. The as they pass back and forth through the principal forms of decomposer are bacteria various earth systems (e.g., the biosphere) and fungi. and particularly between living and non- DECOMPOSITION REACTION: A living matter. The elements involved in chemical reaction in which a compound is biogeochemical cycles are hydrogen, oxy- broken down into simpler compounds, or gen, carbon, nitrogen, phosphorus, and into its constituent elements. In the bios- sulfur. phere, this often is achieved through the BIOSPHERE: A combination of all liv- help of detritivores and decomposers. ing things on Earth—plants, animals, DETRITIVORES: Organisms that feed birds, marine life, insects, viruses, single- on waste matter, breaking organic material cell organisms, and so on—as well as all down into inorganic substances that then formerly living things that have not yet can become available to the biosphere in decomposed. the form of nutrients for plants. Their C HORIZON: The bottommost of the function is similar to that of decomposers; soil horizons, between subsoil and however, unlike decomposers—which tend
356 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 357
The Biosphere KEY TERMS CONTINUED
to be bacteria or fungi—detritivores are it along to other organisms. Earth scientists relatively complex organisms, such as typically prefer this name to food chain, an earthworms or maggots. everyday term for a similar phenomenon. ECOLOGY: The study of the relation- A food chain is a series of singular organ- ships between organisms and their envi- isms in which each plant or animal ronments. depends on the organism that precedes it. Food chains rarely exist in nature. ECOSYSTEM: A community of inter- GEOSPHERE: dependent organisms along with the inor- The upper part of ganic components of their environment. Earth’s continental crust, or that portion of the solid earth on which human beings live ELEMENT: A substance made up of and which provides them with most of only one kind of atom. Unlike compounds, their food and natural resources. elements cannot be broken down chemi- HUMUS: Unincorporated, often par- cally into other substances. tially decomposed plant residue that lies at EUTROPHICATION: A state of height- the top of soil and eventually will decay ened biological productivity in a body of fully to become part of it. water, which is typically detrimental to the HYDROCARBON: Any organic chemi- ecosystem in which it takes place. Eutroph- cal compound whose molecules are made ication can be caused by an excess of nitro- up of nothing but carbon and hydrogen gen or phosphorus, in the form of nitrates atoms. and phosphates, respectively. HYDROLOGIC CYCLE: The continu- EVAPORATION: The process whereby ous circulation of water throughout Earth liquid water is converted into a gaseous and between various earth systems. state and transported to the atmosphere. When discussing the atmosphere and pre- HYDROSPHERE: The entirety of cipitation, usually evaporation is distin- Earth’s water, excluding water vapor in the guished from transpiration. In this context, atmosphere but including all oceans, lakes, evaporation refers solely to the transfer of streams, groundwater, snow, and ice. water from nonliving sources, such as the ISOLATED SYSTEM: A system that is soil or the surface of a lake. separated so fully from the rest of the uni- EVAPOTRANSPIRATION: The loss of verse that it exchanges neither matter nor water to the atmosphere via the combined energy with its environment. This is an (and related) processes of evaporation and imaginary construct, since full isolation is transpiration. impossible. LEACHING: FOOD WEB: A term describing the The removal of soil mate- interaction of plants, herbivores, carni- rials that are in solution, or dissolved in vores, omnivores, decomposers, and detri- water. tivores in an ecosystem. Each of these MINERAL: A naturally occurring, typi- organisms consumes nutrients and passes cally inorganic substance with a specific
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 357 set_vol3_13 8/19/02 11:31 AM Page 358
The Biosphere KEY TERMS CONTINUED
chemical composition and a crystalline PATHOGEN: A disease-carrying para- structure. A crystalline structure is one in site, usually a microorganism. which the constituent parts have a simple PHOTOSYNTHESIS: The biological and definite geometric arrangement that is conversion of light energy (that is, electro- repeated in all directions. magnetic energy) from the Sun to chemical OPEN SYSTEM: A system that allows energy in plants. complete, or near complete, exchange of SOLUBLE: Capable of being dissolved. matter and energy with its environment. SYSTEM: Any set of interactions that ORGANIC: At one time, chemists used can be set apart mentally from the rest of the term organic only in reference to living the universe for the purposes of study, things. Now the word is applied to most observation, and measurement. compounds containing carbon, with the WEATHER: The condition of the exception of carbonates (which are miner- atmosphere at a given time and place. als) and oxides, such as carbon dioxide. Compare with climate.
become overheated, they produce perspiration, the case in a temperate region, which, by defini- which cools the surface of the skin somewhat. If tion, is one that has the four seasons to which the air around us is too humid, however, then it most people in the United States (outside already is largely saturated with water, and the Hawaii, Alaska, and extreme southern Florida perspiration has no place to evaporate. There- and Texas) are accustomed. In a tropical region, fore, instead of continuing to cool our bodies, the by contrast, there is a “dry season,”in which tran- perspiration simply forms a sticky film on the spiration takes place, and a “rainy season,” in skin. But assuming the air is capable of absorbing which moisture from the atmosphere inundates more moisture, the sweat will evaporate, cooling the solid earth. This rainy season may be so our bodies considerably. intense that it produces floods. EFFECTS OF HEAT AND COLD. When summer is at its height, air tem- WHERE TO LEARN MORE peratures are warm and the trees are fully foliat- ed (i.e., covered in leaves), and a high rate of Bial, Raymond. A Handful of Dirt. New York: Walker, 2000. transpiration occurs. So much water is pumped into the atmosphere through foliage that the rate Biogeochemical Cycles (Web site).
358 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:31 AM Page 359
Richardson, Joy. The Water Cycle. Illus. Linda Costello. tem Science. 2d ed. New York: John Wiley and Sons, The New York: Franklin Watts, 1992. 1999. Biosphere Skinner, Brian J., Stephen C. Porter, and Daniel B. Smith, David. The Water Cycle. Illus. John Yates. New Botkin. The Blue Planet: An Introduction to Earth Sys- York: Thomson Learning, 1993.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 359 set_vol3_13 8/19/02 11:32 AM Page 360
ECOSYSTEMS AND
ECOLOGYEcosystems and Ecology
CONCEPT The biosphere consists of all living things— Composed of living organisms and the remains microorganisms, plants, insects, birds, marine of living things as well as the nonliving materials life, and all other forms of animals—as well as in their surroundings, an ecosystem is a complete formerly living things that have not yet decom- community. Its components include plants, ani- posed. (Decomposed remains of organic materi- mals, and microorganisms, both living and dead; als become part of the geosphere, specifically the soil, rocks, and minerals; water sources above and soil.) Organisms in the biosphere, whether living below ground; and the local atmosphere. An or formerly living, are united by the interrelation ecosystem can consist of an entire rain forest, of energy transfer that takes place through the geographically larger than many nations, or it food web. can be as small as the body of an animal, which is A food web is similar to the more well likely to comprise far more microorganisms than known expression food chain. In scientific terms, there are people on Earth. Among the most sig- however, a food chain is defined as a series of sin- nificant areas of interest in the realm of ecosys- gular organisms in which each plant or animal tems is ecology, or the study of the relationships depends on the organism that precedes or follows between organisms and their environments. it. This rarely exists in nature; instead, the feeding Forests, a broad category of ecosystem, provide a relationships between organisms in the real world living laboratory in which to investigate the ways are much more complex and are best described as in which organisms interact with their environ- a web rather than a chain. (For more on the bios- ments. They also aptly illustrate the stresses phere and earth systems, see The Biosphere.) placed on ecosystems by human activities. Energy Flow and Nutrients
HOW IT WORKS Energy is the ability of objects or systems to accomplish work. (The latter is defined as the The Biosphere and Food exertion of force over a given distance: for exam- Webs ple, a plant growing from the ground, an insect or In the sciences, a system is any set of interactions bird flying, or a human or pack animal moving an that can be set apart mentally from the rest of the object.) Food webs are built around energy trans- universe for the purposes of study, observation, fer, or the flow of energy between organisms, and measurement. Modern earth scientists regard which begins with plant life. Hence the impor- the planet as a massive ecosystem, the stage on tance of plants to ecosystems, as we illustrate later which four extraordinarily complex earth systems in discussing various types of forest, which are interact. These systems are the atmosphere, the defined by their dominant varieties of tree. hydrosphere (all the planet’s waters, except for Plants absorb energy in two ways. From the moisture in the atmosphere), the geosphere (the Sun they receive electromagnetic energy in the soil and the extreme upper portion of the conti- form of visible light and invisible infrared waves, nental crust), and the biosphere. which they convert to chemical energy by means
360 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 361
of photosynthesis. In addition, plants take in Nonetheless, detritivores are relatively large, Ecosystems nutrients from the soil, which contains energy in complex organisms compared with another vari- and Ecology the forms of various chemical compounds. These ety of species that occupies a level or position in compounds may be organic, which typically the food web that comes “after” carnivores and means that they came from living things, though, omnivores: decomposers, including bacteria and strictly speaking, organic refers to characteristic fungi. carbon-based chemical structures. Plants also This illustrates the reason why ecological sci- receive inorganic compounds from minerals in ences treat the expression food chain with disfa- the soil. (See Paleontology for an explanation of vor. There is no such thing as “the top of the food the scientific distinction between organic and chain,” rather, there are simply stages, like a cir- inorganic.) cular assembly line, with detritivores occupying a Contained in these minerals are six chemical position between meat-eating animals and elements essential to the sustenance of life on plants. Earthworms, in particular, help convert planet Earth: hydrogen, oxygen, carbon, nitro- animal bodies into soil nutrients useful to the gen, phosphorus, and sulfur. These are the ele- growth of plant life, yet even these and other ments involved in biogeochemical cycles, detritivores must themselves eventually be con- through which they are circulated continually verted to soil as well. between the living and nonliving worlds—that is, This “final” stage of conversion—that is, the between organisms on the one hand and the last stop after the animal portion of the food web inorganic realms of rocks, minerals, water, and and before the cycle comes back around to ordi- air on the other. (See The Biosphere for more nary plants—is occupied by decomposers, such about biogeochemical cycles.) as bacteria and fungi. These decomposers obtain FROM PLANTS TO MEAT their energy from the chemical breakdown of EATERS. As plants take up nutrients from dead organisms as well as from animal and plant the soil, they convert them into other forms. waste products. Like detritivores, they aid in Eventually, the plants themselves become food decomposition, a chemical reaction in which a either for herbivores (plant-eating organisms) or compound is broken down into simpler com- for omnivores (organisms that eat both plants pounds, or into its constituent elements. and other animals), thus passing along these Often an element such as nitrogen appears usable, energy-containing chemical compounds in forms that are not readily usable to organisms, to other participants in the food web. It is likely and therefore such elements (which may appear that an herbivore will be eaten in turn either by individually or in compounds) need to be an omnivore (for example, humans as well as a processed chemically through the body of a number of bird species and many others) or by a decomposer or detritivore. This process brings carnivore, an organism that eats only meat. about a chemical reaction in which the substance Carnivores and omnivores are not usually (whether an element or compound) is trans- prey for other carnivores or omnivores, but this formed into a more usable version. By processing does happen in the case of what are known as ter- these chemical compounds, decomposers and tiary consumers (see Food Webs). There is also detritivores provide nutrients necessary to plant the matter of cannibalism, discussed in Biologi- growth. cal Communities. For the most part, however, the only creatures that eat carnivores and omnivores REAL-LIFE do so after these organisms have died or been killed. APPLICATIONS DETRITIVORES AND DECOM- Forests and Ecology POSERS. An animal that obtains its energy in this way, from consuming the carcasses of car- One easily understandable example of ecosys- nivores and omnivores (as well as herbivores and tems and ecology in action is the forest. Virtually perhaps even plants), is known as a detritivore. everyone has visited a forest at one time or Large and notable examples include vultures and another, and those who are enthusiasts for the hyenas, though most detritivores—earthworms great outdoors may spend a great deal of time in or maggots, for instance—are much smaller. one. In the past, of course, people interacted with
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 361 set_vol3_13 8/19/02 11:32 AM Page 362
Ecosystems and Ecology
A VARIETY OF ANGIOSPERM, MANGROVE TREES ARE FOUND IN LOW-LY I N G , MUDDY REGIONS NEAR SALTWATER, WHERE THE CLIMATE IS HUMID. A MANGROVE FOREST IS POOR IN SPECIES: ONLY ORGANISMS THAT CAN TOLERATE FLOOD- ING AND HIGH SALT LEVELS ARE CAPABLE OF SURVIVING. (© Wolfgang Kaehler/Corbis. Reproduced by permission.)
forests not so much out of choice, and certainly species of vegetation do not all shed at the same not with recreation as the foremost aim in mind, time, the rain forest canopy—the upper layer of but simply because they depended on the forest trees in the forest—remains rich in foliage year- for survival. Not only did the forest provide round. Hence, the tropical rain forest is an exam- hunters and food-gatherers with an abundance ple of an evergreen forest. Climate can determine of wildlife and fruit, but trees provided material the type of life forms capable of surviving in the for building dwellings. It is no wonder, then, that forest ecosystem. This can be illustrated by refer- many early human settlements tended to be in, or ring to a forest almost perfectly opposite in char- at the edges of, forests. acter to a rain forest: the taiga, or boreal forest, A forest is simply an ecosystem dominated that spans much of northern Eurasia. by trees. There are many varieties of forest, how- The taiga is a deciduous forest, meaning that ever, because so many factors go into determin- its trees shed their leaves seasonally; indeed, ing the character of a forest ecosystem. The fact because of the very cold climate in taiga regions, that the forest is an ecosystem means that its where the temperature during winter is usually qualities are defined by far more than just the well below the freezing point, trees spend a great varieties of trees, which are simply the most visi- portion of the year bare. Rainfall is much, much ble among many biological forms in the forest. lower than in a rain forest, of course: only about Numerous abiotic, or nonbiological, factors also 10-20 in. (250–500 mm) per year, as compared affect the characteristics of a forest as well. For with more than 70 in. (1,800 mm) for a typical instance, there is weather, defined as the condi- rain forest. The dry, inhospitable climate of the tion of the atmosphere at a given time and place, taiga makes it a forbidding place for reptiles and and climate, the overall patterns of weather for amphibians, though the taiga is home to many extended periods. endothermic (warm-blooded) creatures such as These play a clear role, for instance, in defin- mammals or birds. ing the tropical rain forest, a place where con- LATITUDE, ALTITUDE, AND stant rainfall ensures that there are always plenty FORESTS. Elevation or relief—that is, of plants in flower. Because the trees and other height above sea level—also determines the char-
362 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 363
acter of a forest, as do latitude (distance north or modern times, a growing awareness of ecology, Ecosystems south of the equator) and topography, or the and of the distance that technology has placed and Ecology overall physical configuration of Earth’s surface between modern society and the forests, led to in a given area. Rain forests can exist anywhere, the movement for the establishment of national but by definition a tropical rain forest, such as parks in general, and of national forests in par- those along the Amazon River in South America ticular. or the Congo River in Africa, must lie between The first of these preserves—places where the Tropic of Cancer in the north and the Tropic commercial development is forbidden and com- of Capricorn in the south. mercial activity is limited—was Yellowstone Naturally, in the tropical rain forest, temper- National Park in Wyoming, established by the atures are high—typically, about 86°F (30°C) administration of President Ulysses S. Grant in during the day, cooling down to about 68°F 1872. Though Yellowstone contains enormous (20°C) at night. By contrast, there are much cool- areas of forest land, the first national forest er rain forests in the temperate zones. An exam- reserve (as national forests were called at the ple is the Cherokee National Forest on the border time) was Sequoia National Park, established in between North Carolina and Tennessee, which, 1891. Home to some of the largest, most awe- though located in the southeastern United States, inspiring trees in the world, Sequoia is part of a is chilly even in the summer months. group of national forests and parks to the north- Just as latitude affects a forest, so does alti- east of Bakersfield, California. tude. Rain forests at relatively high elevations, The United States Forest Service was actu- such as the highlands of New Guinea, are known ally founded earlier (1905) than the National as montane forests. These forests, though they Park Service (1916), a fact that illustrates the may be located at the same latitude as tropical importance of pristine forests to maintaining a rain forests—most montane forests are in eastern proper balance between humans and their envi- Brazil, southeastern Africa, northern Australia, ronment. Since the establishment of the forest and parts of southeast Asia—are much cooler. service under the aegis of the U.S. Department Lush by comparison to most non-rain forests, of Agriculture, the lands controlled by the forest their vegetation is nonetheless much less dense service have grown to encompass about 191 than in a typical tropical rain forest. million acres (77.3 million hectares), an area In addition to its role in defining the overall larger than Texas. During the same period, the character of the forest, differences in relative alti- U.S. example of national parks and forests has tude or elevation resulting from the great height inspired nations around the world to create of trees in the rain forest also influence the for- their own preserves. mation of differing biological communities. For Coupled with the rise of national parks and example, monkeys, flying squirrels, and other forests at the turn of the nineteenth century was animals capable of swinging, gliding, or other- a growing interest in conservation and manage- wise moving from tree to tree inhabit the canopy, ment of environmental resources. This interest which is rich in well-watered leaves and other manifested across a broad spectrum, from envi- food sources. These top-dwellers seldom even ronmentalists who urged that the forests be left need to come down to the ground for anything. in their original state to industrial foresters who The rain forest floor, by contrast, is mostly bare, view the forest as a resource that can be utilized. since the trees above shade it. On this level live Both sides have their merits, and both have their creatures such as chimpanzees and gorillas, who complaints about the other. The close historical feed off of low-lying plant forms. Other biologi- ties between conservationism and the science of cal communities exist above or below the forest forestry (the management of forest ecosystems floor. (For more on these subjects, including the for purposes such as harvesting timber), both of various types of forests, see Biomes. See The which had their origins during the nineteenth Biosphere for a discussion of soil quality in the century, suggest that there is no inherent reason rain forest.) that the two sides should be in conflict. If any- HUMANS AND FORESTS. Earlier thing, responsible forestry goes hand-in-hand we noted the fact that humans’ early history kept with an attitude of conserving as many resources them, like other primates, close to the forest. In as is feasible.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 363 set_vol3_13 8/19/02 11:32 AM Page 364
Ecosystems Flowering plants evolved only about 130 and Ecology million years ago, by which time gymnosperms (of which modern pines are an example) had long since evolved and proliferated. Yet in a rela- tively short period of time, angiosperms have become the dominant plants in the world today. About 80% of all living plant species are flower- ing plants, and based on the record of angiosperms and gymnosperms heretofore, it is likely that the world of 100 million years from now will be one in which the forests are typified by angiosperms. Gymnosperms, meanwhile, may well become a dying, if not a dead, breed. POLLINATION BY GYMNOSPERMS. Gymnosperms reproduce sexually as well, but they do so by a much less efficient method than that of the angiosperm. Whereas the angiosperm keeps its seeds safely tucked away inside the ovary and coexists with its ecosystem most favorably by putting the insect and animal life to work, gym- nosperm reproduction is an altogether less effec- BY EVOLVING BRIGHT COLORS, SCENTS, AND NECTAR, THE FLOWERS OF ANGIOSPERMS ATTRACT ANIMALS, tive—and, indeed, less pleasant—process. , MOVING WHICH TRAVEL FROM ONE FLOWER TO ANOTHER For starters, gymnosperms produce their POLLEN AS THEY GO. WHEREAS INSECTS AND ANIMALS POSE A THREAT TO GYMNOSPERMS, ANGIOSPERMS PUT seeds on the surface of leaflike structures, and BEES, BUTTERFLIES, HUMMINGBIRDS, AND OTHER this makes the seeds vulnerable to physical dam- FLOWER-SEEKING CREATURES TO WORK ASSISTING THEIR age and drying as the wind whips the trees’ REPRODUCTIVE PROCESS. (© Gallo Images/Corbis. Reproduced branches back and forth. Furthermore, insects by permission.) and other animals view gymnosperm seeds as a source of nutrition, as indeed they are. And in Comparing Angiosperms and contrast to the angiosperm, which attracts bees Gymnosperms and other creatures to it, gymnosperms package Several times we have referred to angiosperms, a the male reproductive component in tiny pollen name that encompasses not just certain types of grains, which it releases into the wind. tree but also all plants that produce flowers dur- Eventually, the grains make their way toward ing sexual reproduction. The name, which comes the female component of another individual from Latin roots meaning “vessel seed,”is a refer- within the same species, but the fact that they do ence to the fact that the plant keeps its seeds in a is an example of the wonder inherent in life itself vessel whose name, the ovary, emphasizes the and not of the efficiency of gymnosperm repro- sexual quality of the reproductive process it duction. Gymnosperms shower their ecosystems undergoes. with pollen, a fact familiar to anyone who lives in Angiosperms are a beautiful example of how a place with a high gymnosperm population— a particular group of organisms can adapt to spe- and hence a high pollen count in the spring. In cific ecosystems and do so in a highly efficient gymnosperm-heavy environments, yellowish manner, such that the evolutionary future her- dust forms on everything, and where humans alds only greater dominance for these species. interact with the natural world, this can create a This is all the more interesting in light of the con- great deal of discomfort in the form of hay fever trast between the success of the angiosperm and and allergies. Meanwhile, cars, windowsills, mail- the rather less impressive results achieved by boxes, and virtually every other available surface another broad category of sexually reproducing takes on a yellow film that usually is not relieved plant, one that formerly dominated Earth’s for- until a good rain falls or, more likely, pollination est: the gymnosperm. ends for the year.
364 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 365
Deforestation Though pollen is unpleasant to humans, it Ecosystems and Ecology should be noted that like all natural mechanisms, Returning to the subject of forests in general, if a it benefits the overall ecosystem. Packed with forest experiences significant disturbance, it may energy, pollen grains contain large quantities of undergo deforestation. Despite the finality in the nitrogen, making them a major boost to the sound of the word, deforestation does not neces- nutrient content in the soil. But it costs the gym- sarily imply complete destruction of the forest. In nosperm a great deal, in terms of chemical and fact, deforestation can describe any interruption biological energy and material, to produce pollen in the ordinary progression of a forest’s life, grains, and the benefits are uncertain. including clear-cut harvesting—even if the forest POLLINATION BY ANGIOSPERMS. fully recovers. If the gymnosperm and angiosperm varieties of Deforestation can occur naturally, as a result pollination were compared to marketing cam- of changes in the soil and climate, but the most paigns, gymnosperm reproduction would significant cases of deforestation over the past involve the client (i.e., the gymnosperms them- few thousand years have been the consequence of selves) investing maximum capital for minimal human activities. Usually deforestation is driven returns. In a very real sense, gymnosperm polli- by the need to clear land to harvest trees for fuel nation is like the marketing of a company or, in some cases, to obtain building materials in that bombards a neighborhood with leaflets, the form of lumber. Though deforestation has such that advertising rapidly becomes another been a problem the world over, since the 1970s it form of trash simply to be swept up and thrown has become an issue primarily in developing away. countries. By contrast, the “marketing” of angiosperms In developed nations such as the United is like that of a company that uses carefully tar- States, environmental activism has raised public geted, researched advertising, utilizing as many awareness concerning deforestation and led to free means as possible for getting out word about curtailment of large-scale cutting in forests that itself. Just as a smart marketer sets in place the are deemed important environmental habitats. conditions to get consumers talking about a By contrast, developing nations, such as Brazil, product—thus using advertising that is both free are cutting down their forests at an alarming rate. and extremely effective—the angiosperm enlists Generally, economics is the dominant factor, with the need for new agricultural land or the the aid of mobile organisms in its environment. desire to obtain wood and other materials typi- In addition, the angiosperm puts a great deal cally driving the deforestation process. of its energy into producing reproductive struc- CONSEQUENCES OF DEFOR- tures, an effort that pays off bountifully. By ESTATION. The deforestation of valuable evolving bright colors, scents, and nectar, the reserves such as the Amazon rain forest is an flowers of angiosperms attract animals, which environmental disaster in the making. As dis- travel from one flower to another, unintentional- cussed in the essay The Biosphere, the soil in rain ly moving pollen as they go. Thus, whereas forests as a rule is “old,”and leached of nutrients. insects and animals pose a threat to gym- Without the constant reintroduction of organic nosperms, angiosperms actually put bees, butter- material from the plants and animals of the rain flies, hummingbirds, and other flower-seeking forests, it would be too poor to grow anything. creatures to work assisting their reproductive Therefore, when nations cut down their own rain process. forest lands, they are in effect killing the golden Because of this remarkably efficient system, goose to get at the egg: once the rain forest is animal-pollinated species of flowering plants do gone, the land itself is worthless. not need to produce as much pollen as gym- Deforestation has several other extremely nosperms. They can put their resources into serious consequences. From a biological stand- other important functions instead, such as point, it greatly reduces biodiversity, or the range growth and greater seed production. In this way, of species in the biota. In the case of tropical rain the angiosperm solves its own problem of repro- forests as well as old-growth forests (see Biologi- duction—and, as a side benefit, adds enormous- cal Communities), certain species cannot survive ly to the world’s beauty. once the environmental structure has been rup-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 365 set_vol3_13 8/19/02 11:32 AM Page 366
Ecosystems and Ecology
A LONE TREE TRUNK STANDS IN AN AREA OF DEFORESTED GRASSLAND IN MARANHÃO, BRAZIL. THE DEFORESTATION OF VALUABLE RESERVES SUCH AS THE AMAZON RAIN FOREST IS AN ENVIRONMENTAL DISASTER IN THE MAKING, DEPLETING AND STARVING THE SOIL, REDUCING BIODIVERSITY, AND BRINGING ABOUT DANGEROUS CHANGES IN ATMOSPHERIC CARBON CONTENT. (© Barnabas Bosshart/Corbis. Reproduced by permission.)
tured. From an environmental perspective, it The Greenhouse Effect leads to dangerous changes in the carbon content The most potentially serious aspect of cutting of the atmosphere, discussed later in this essay. In down forests may well be the greenhouse effect, the case of old-growth forests or rain forests, which some scientists and activists believe is deforestation removes an irreplaceable environ- causing an overall warming of the planet. Today, mental asset that contributes to the planet’s bio- thanks to the popularity of environmental caus- diversity—and to its oxygen supply. es among entertainment figures and on college campuses, terms such as “the greenhouse effect” Even from a human standpoint, deforesta- and “global warming” are commonplace. Howev- tion takes an enormous toll. Economically, it er, these phrases are used so frequently, and depletes valuable forest resources. Furthermore, sometimes so confusingly or misleadingly, that it deforestation in many developing countries is worthwhile to address their meaning briefly; often is accompanied by the displacement of then, we can conclude our discussion by looking indigenous peoples, while still other political at what impact the steady reduction in forest and social horrors may lurk in the shadows. For lands has had on the increasing release of green- example, Brazil’s forests are home to charcoal house gases. factories that amount to virtual slave-labor The greenhouse effect itself is not a conse- camps. Aboriginal peoples (i.e., “Indians”) are quence of any action on the part of human lured from cities with promises of high income beings; rather, it is a part of life on Earth. In fact, without it, there could be no life on Earth. and benefits, only to arrive and find that the sit- Though the planet receives an incredible amount uation is quite different from what was adver- of energy from the Sun, much of it is lost by tised. Having paid the potential employer for being absorbed or reflected in the atmosphere or transportation to the work site, however, they on the surface. So-called greenhouses gases such are unable to afford a return ticket and must as carbon dioxide, however, help to trap this labor to repay the cost. energy, keeping much more of the Sun’s warmth
366 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 367
Ecosystems KEY TERMS and Ecology
ANGIOSPERM: A type of plant that principal forms of decomposer are bacteria produces flowers during sexual reproduc- and fungi. tion. DECOMPOSITION REACTION: A ATMOSPHERE: Earth’s atmosphere is chemical reaction in which a compound is a blanket of gases that includes nitrogen broken down into simpler compounds or (78%), oxygen (21%), argon (0.93%), and into its constituent elements. In the earth a combination of water vapor, carbon system, this often is achieved through the dioxide, ozone, and noble gases such as help of detritivores and decomposers. neon (0.07%). Most of these gases are con- DEFORESTATION: A term for any tained in the troposphere, the lowest layer, interruption in the ordinary progression of which extends to about 10 mi. (16 km) a forest’s life. above the planet’s surface. DETRITIVORES: Organisms that feed BIODIVERSITY: The degree of variety on waste matter, breaking down organic among the species represented in a partic- material into inorganic substances that ular ecosystem. then can become available to the biosphere BIOLOGICAL COMMUNITY: The liv- in the form of nutrients for plants. Their ing components of an ecosystem. function is similar to that of decomposers; CANOPY: The upper portion or layer however, unlike decomposers—which tend of the trees in a forest. A forest with a to be bacteria or fungi—detritivores are closed canopy is one so dense with vegeta- relatively complex organisms, such as tion that the sky is not visible from the earthworms or maggots. ground. ECOLOGY: The study of the relation- CARNIVORE: A meat-eating organ- ships between organisms and their envi- ism, or an organism that eats only meat (as ronments. distinguished from an omnivore). ECOSYSTEM: A community of inter- COMPOUND: A substance made up of dependent organisms along with the inor- atoms, chemically bonded to one another, ganic components of their environment. of more than one chemical element. ELEMENT: A substance made up of CONIFER: A type of tree that produces only one kind of atom. Unlike compounds, cones bearing seeds. elements cannot be broken chemically into DECIDUOUS: A term for a tree or other substances. other form of vegetation that sheds its ENERGY: The ability of an object (or leaves seasonally. in some cases a nonobject, such as a mag- DECOMPOSERS: Organisms that netic force field) to accomplish work. obtain their energy from the chemical ENERGY BUDGET: The total amount breakdown of dead organisms as well as of energy available to a system or, more from animal and plant waste products. The specifically, the difference between the
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 367 set_vol3_13 8/19/02 11:32 AM Page 368
Ecosystems and Ecology KEY TERMS CONTINUED
energy flowing into the system and the vapor, in the atmosphere. These gases are energy lost by it. heated and ultimately re-radiate energy at ENERGY TRANSFER: The flow of an even longer wavelength to space. (Wave- energy between organisms in a food web. length and energy levels are related inverse- ly; hence, the longer the wavelength, the FOOD WEB: A term describing the less the energy.) interaction of plants, herbivores, carni- vores, omnivores, decomposers, and detri- GYMNOSPERM: A type of plant that tivores in an ecosystem. Each of them con- reproduces sexually through the use of sumes nutrients and passes it along to seeds that are exposed, not hidden in an other organisms. Earth scientists typically ovary as with an angiosperm. prefer this name to food chain, an everyday HERBIVORE: A plant-eating organ- term for a similar phenomenon. A food ism. chain is a series of singular organisms in OMNIVORE: An organism that eats which each plant or animal depends on the both plants and other animals. organism that precedes it. Food chains ORGANIC: rarely exist in nature. At one time chemists used the term organic only in reference to living FOREST: In general terms, a forest is things. Now the word is applied to most simply any ecosystem dominated by tree- compounds containing carbon, with the size woody plants. A number of other char- exception of carbonates (which are miner- acteristics and parameters (for example, als) and oxides, such as carbon dioxide. weather, altitude, and dominant species) further define types of forests, such as PHOTOSYNTHESIS: The biological tropical rain forests. conversion of light energy (that is, electro- magnetic energy) from the Sun to chemical GREENHOUSE EFFECT: Warming energy in plants. of the lower atmosphere and surface of Earth. This occurs because of the absorp- SYSTEM: Any set of interactions that tion of long-wavelength radiation from the can be set apart mentally from the rest of planet’s surface by certain radiatively active the universe for the purposes of study, gases, such as carbon dioxide and water observation, and measurement.
within Earth’s atmosphere, much as a green- 932°F (500°C). To many environmentalists, there house helps trap heat. Without the greenhouse is a grave danger that Earth could be slowly going effect, Earth would be so cold that the oceans the way of Venus, building up greenhouse gases would freeze. such that the temperature is slowly increasing. This is the phenomenon of global warming, Obviously, then, the greenhouse effect is a which threatens to melt the polar ice caps and good thing—but only if greenhouse gases are submerge much of Earth’s land surface. At least, kept at certain levels. Earth, after all, is not the that is the opinion of environmentalists and oth- only planet in the solar system that experiences a ers who subscribe to the idea that Earth is steadi- greenhouse effect; there is also Venus, a hellish ly warming as a result of human pollution and place where surface temperatures are as high as industrial activity.
368 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 369
WHERE TO LEARN MORE There is a considerable body of scientific Ecosystems knowledge that challenges the environmentalist Ashworth, William, and Charles E. Little. Encyclopedia of and Ecology position on global warming and the greenhouse Environmental Studies. New York: Facts on File, 2001. effect, but it is not our purpose here to judge the Diamond, Jared M. Guns, Germs, and Steel: The Fates of various positions. Rather, our concern is the link Human Societies. New York: W. W. Norton, 1997. between forests and the increase of greenhouses The Ecological Society of America: Issues in Ecology (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 369 set_vol3_13 8/19/02 11:32 AM Page 370
BIOMESBiomes
CONCEPT types is divided further into marine and freshwa- On a political map of the world, Earth is divided ter biomes. into countries, of which there are almost 200. But Within a biome or ecosystem, the sum of all nature, of course, knows no national boundaries, living organisms is referred to as the biological and therefore the natural divisions of the planet community. Sometimes the term biota, which are quite different from those agreed upon by refers to all flora and fauna (plants and animals) humans. While continents are a useful concept to in a region, is used instead. Thus, biological com- geographers and earth scientists, in the worlds of munity is a larger concept, since it includes biology, ecology, and biogeography, the concept microorganisms, which are vital to the function- of a biome makes much more sense. There are ing of the food web. The food web, which may be more than a dozen basic terrestrial and aquatic thought of as an interconnected network of food biomes or ecosystems, including boreal conifer- chains, is the means by which energy is trans- ous forests, deserts, tundra, and underwater envi- ferred through a biological community. Without ronments. Each is a distinct “world” unto itself, microorganisms known as decomposers, a key with characteristic forms of plant life as well as link in the food web would be missing. (See Food animal species that congregate around the plants Webs for more on this subject.) for food or shelter or both. Combined with these SUCCESSION AND CLIMAX. features of the biological community are aspects of the inorganic realm that likewise define a Over the course of time, ecosystems experience a biome, for instance, climate and the availability process known as succession, the progressive of water. replacement of one biological community by another. This is rather like the series of changes one might witness if one were to record the activ- HOW IT WORKS ity on a major city block over the space of a few decades, as stores come in and shut down and Ecosystems, Biomes, and Bio- buildings are erected and demolished. In the case logical Communities of biological succession, a process akin to natural selection (see Evolution) is occurring: the ecosys- An ecosystem is a community of interdependent tem becomes home, in turn, to a number of dif- organisms along with the inorganic components ferent biological communities until (in the of their environment, including water, soil, and air. Earth is the largest ecosystem, divided into absence of outside interference) the one that is biomes, large areas with similar climate and veg- most suited or adapted to local conditions final- etation. A biome is a large ecosystem, extending ly takes root. (That is, until it is replaced, and the over a wide geographic region, characterized by process of succession continues.) certain dominant life-forms—most notably, trees This most suited or adapted biological com- or the lack thereof. There are two basic varieties munity is described as a climax community, one of biome: terrestrial, or land-based (of which that has reached a stable point as a result of there are six), and aquatic. The second of these ongoing succession. In such a situation, the com-
370 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 371
munity is at equilibrium with environmental cipitation, weather, and climate. Although it does Biomes conditions, and conditions are stable, such that contain organic compounds from the decayed the biota experiences little change thereafter. The vegetable and animal matter that enriches it, soil, most significant forms of climax vegetation are too, is largely inorganic, being formed from the often the defining characteristics of terrestrial weathering of rocks. biomes. (See Succession and Climax for more FLORA. Biomes are differentiated most about this subject.) clearly, however, on the basis of their organic Defining Characteristics of a components. The second term in the phrase Biome boreal coniferous refers to a type of plant that pro- duces cones containing seeds. Thus, the domi- The boreal coniferous forest often is cited by bio- nant plant life in the boreal coniferous forest geographers as a classic example of a biome, for a includes evergreen conifers that can tolerate cold number of reasons. First, like most other terres- weather: pine, fir, and spruce. trial biomes, this one is defined by specific latitu- The varieties that dominate may differ dinal positions: the term boreal means “north- between geographic regions, however. The bore- ern,” and these forests exist between 50 and 60 al coniferous forests of northeastern North degrees north latitude. (Aside from the southern- America, for instance, are dominated by black most tip of South America and a few scattered spruce, while those in the northwest are charac- islands, there is no significant landmass between terized by stands of white spruce. In northeast 50 and 60 degrees south latitude.) Europe, Norway spruce is dominant, while In North America the region between 50 and species of pine and larch occupy the key posi- 60 degrees north latitude is the southerly band of tions in the forests of Siberia. Despite these dif- Canadian provinces (Alberta and Saskatchewan, ferences in dominant species, the conditions are for example). The Eurasian equivalent of this much the same, not only in terms of inorganic region is a band encompassing the British Isles; environment but also with regard to flora and an area of continental Europe that includes fauna. In most boreal coniferous forests, the northern Germany, Poland, and southern Swe- canopy or upper layer is so thick that it allows lit- den; and a vast swath that spans the width of Rus- tle light through. The result is that the understo- sia from Saint Petersburg and Moscow in the west ry, or lower layers of vegetation, is very limited. across the nation’s wide expanse (10 time zones) FAUNA. As for animal life, species in the to the Kamchatka peninsula north of Japan. boreal coniferous forest include bear, moose, The boreal coniferous forest thus illustrates wolf, lynx, deer, weasels, rabbits, beavers, and a key fact about biomes: they can occur in wide- chipmunks. With a few local variations, this ros- ly separated geographic regions as long as the ter of animal life is typical in most such biomes, environmental conditions are the same. In each whether in British Columbia or western Europe of these locales average temperatures are low; or Siberia. summers are short, moist, and of moderate A biome constitutes a complex network of warmth; and winters are long, cold, and dry. interactions among plants, animals, and their Most precipitation is in the form of snow, and surroundings, such that certain animals depend, the A horizon of the soil, home of the organic either directly or indirectly, on certain plants for material in which plants grow, is thin. Moreover, their sustenance. An obvious example is the the soil is acidic and poor in nutrients. (See The beaver’s use of coniferous tree limbs and even Biosphere for more about soil.) trunks for building shelter. Even more funda- Most of the information conveyed in the mental to the functioning of ecosystems is the preceding paragraph refers to the inorganic com- role of plants as food. ponents of the boreal coniferous forest. (Organic Although few animals actually feed off the does not necessarily mean “living,” but it does needles or bark of conifers, they do eat from refer to carbon-based chemical compounds these trees in more indirect ways. The woodpeck- other than carbonates, which are rocks, and car- er, for instance, consumes bugs that live in a tree’s bon oxides, such as carbon dioxide.) As noted bark. Then there are the many insects that live off earlier, inorganic components of a biome include conifer seeds (see Ecosystems and Ecology for a water and air, which in turn are involved in pre- discussion of conifer, or gymnosperm, reproduc-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 371 set_vol3_13 8/19/02 11:32 AM Page 372
Biomes tion), and these bugs, in turn, serve as food for rainfall patterns and other variables are consid- birds, which are the prey of larger carnivores. ered. All climate zones, however, fall into one of Furthermore, though the understory in boreal three basic categories: tropical and subtropical, coniferous forests is not dense, it provides temperate, and polar and subpolar. enough vegetation to meet the needs of deer, rab- The first of these categories is a term com- bits, and other herbivores. prising the region along the equator, extending Classifying Biomes north and south by about 30 degrees in either direction. In North America this would include Earlier it was stated that there are “almost” 200 southern Florida, Texas, and Louisiana. Temper- countries on Earth. It might seem strange that ate zones reach from about 30 to 60 degrees on something like the number of countries could be either side of the equator, thus taking in most of so inexact, when it would seem to be a matter of the United States and southern Canada. Finally, very exact quantities, like the number of states in subpolar and polar regions lie between 60 the United States. But defining sovereign nations degrees and the poles, which are at 90 degrees. is a bit more challenging. Obviously, the United States, Switzerland, and Japan are sovereign REAL-LIFE nations, but many another political entity exists APPLICATIONS in a gray area. If the number of independent nations on Forest Biomes Earth is so open to question, it would stand to reason that the number of basic biomes is as well. The term forest, as used in the realm of ecology, After all, nations typically are delineated by such is one of those rare words that means the same things as borders, seats at the United Nations, thing within a scientific context as it does in the currency, and so forth, whereas the boundaries everyday world. An ecologist or biogeographer between biomes are much less exact. Therefore, it would define forest in more or less the same way would be futile to attempt to say exactly how that a nonscientist would: as any ecosystem dom- many biomes there are on Earth, since the num- inated by tree-size woody plants. Of course, ber varies according to interpretation. numerous other characteristics and parameters, TERRESTRIAL, AQUATIC, AND such as weather, altitude, and dominant species, OTHER CATEGORIES. One of the more further characterize types of forests. useful methods for classifying biomes is that of BOREAL CONIFEROUS FORESTS. the American ecologist Eugene Pleasants Odum Starting with the most northerly of forest biomes, (1913–), introduced in his Fundamentals of Ecol- there is the boreal coniferous forest, which we ogy (1953). The classification scheme that follows have discussed. Called taiga in Russia, boreal is based on that of Odum, who divided biomes coniferous forests often are bordered on the into terrestrial and aquatic. In the present con- north by tundra, discussed later in the context of text, biomes have been grouped into five cate- nonforest ecosystems. An important subset of the gories: forest, nonforest, freshwater, marine, and boreal coniferous grouping is the montane forest, anthropogenic. The last of these categories refers which also is dominated by conifers but which to biomes strongly influenced by humans and most often is found on mountains, at subalpine their activities, though it should be noted that to altitudes where the climate is cool and moist. some degree at least, human activities have influ- In addition to the dominant conifers, boreal enced all of Earth’s biomes. For example, many coniferous forests also have important broad- organisms carry in their fat cells trace amounts of leafed angiosperms (plants that flower during human-manufactured contaminants, such as sexual reproduction), including aspen, birch, DDT. (See Food Webs for more on this subject.) poplar, and willow species. These forests are typ- Biomes are organized here in such a way as ically subject to periodic catastrophes, which to take into account their relative latitudes and result in at least partial destruction of the domi- corresponding climate. (Distinctions of latitude nant trees within stands if not across a given for- and climate are mostly relevant where terrestrial est as a whole. Among these catastrophic events biomes are concerned.) As with biomes, there are are wildfires as well as defoliation by such pests as many possible climate zones, particularly when the spruce budworm.
372 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 373
Biomes
TROPICAL GRASSLAND AND SAVANNA BIOMES, PRIMARILY IN AFRICA, ARE BEST CHARACTERIZED BY THE EXTRAORDI- NARILY ABUNDANT AND DIVERSE ANIMAL LIFE, WHICH INCLUDES SUCH LARGE MAMMALS AS THE CHEETAH AND THE GAZELLE. (© Kevin Schafer/Corbis. Reproduced by permission.)
TEMPERATE DECIDUOUS FORESTS. include ash, basswood, birch, cherry, chestnut, Moving farther away from the poles, the next dogwood, elm, hickory, magnolia, maple, oak, major forest biome is that of the temperate and walnut, among others. (Note that many of deciduous forest. The average American, espe- these species are angiosperms. Likewise, most cially on the East Coast, is likely to be more coniferous trees are gymnosperms, or plants that familiar with the temperate deciduous forest reproduce sexually through exposed seeds as than with any other biome. This type of forest opposed to seeds hidden in a flower.) Among the develops in a climate that is relatively moist, with varieties of animal life are squirrels, rabbits, winters that are fairly cold. The larger grouping skunks, opossums, deer, bobcat, timber wolves, of temperate deciduous forests is divided into foxes, and black bears. smaller categories depending on the relative TEMPERATE RAIN FORESTS. amount of annual rainfall. Temperate rain forests are not necessarily farther The term deciduous refers to a tree that sheds from the poles than temperate deciduous forests, its leaves seasonally, and these forests are domi- but they are subject to milder winters. For exam- nated by such trees, broad-leafed species that ple, the temperate rain forests of Washington
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 373 set_vol3_13 8/19/02 11:32 AM Page 374
Biomes State are north of many a temperate deciduous dinary range of animals and microorganisms. forest on the East Coast, but owing to differences Though biogeographers and ecologists often use in climate patterns, they are subject to milder the boreal coniferous forest as an example when winters than those typical of the deciduous examining biomes, those northerly forests are forests to the east. hardly examples of biological diversity. On the Characterized by abundant precipitation other hand, the tropical rain forest represents (most of it rain rather than snow, due to the such diversity to its greatest extent. milder temperatures), these systems are very Nonforest Biomes moist—as the “rain forest” in their name implies. This, in turn, means that they are seldom subject Making an abrupt shift from the lush world of to catastrophic wildfires, and therefore they often the tropical rain forest, let us look now at the attain the climax stage of old-growth forests. In tundra: a cold, treeless biome in the arctic and the temperate rain forest, coniferous trees are subarctic regions. (The Arctic Circle lies at dominant, and many of these trees are extremely approximately 66.5 degrees north latitude. Lands large and old. Among the tree species typical of north of that line include northern Alaska and this biome are Douglas fir, hemlock, cedar, red- Canada, most of Greenland, extreme northern wood, spruce, and yellow cypress. Scandinavia, and a northern strip of Russia and Tropical forests are discussed at some length Siberia. The subarctic region, less clearly defined, in the essays Ecosystems and Ecology as well as comprises simply those lands that lie directly The Biosphere. Among the two most basic vari- below the Arctic Circle.) Characterized by a short eties of this biome are semi-evergreen tropical growing season, the tundra experiences very little forests and evergreen tropical rain forests. Most precipitation in the form of liquid water. Yet the of the Amazon rain forest in South America, for soil may well be marshy because temperatures instance, is an evergreen tropical forest, while are too low for significant evaporation and surrounding biomes are semi-evergreen. Much because the ground is usually frozen solid, pre- the same is true of biomes in central and south- venting drainage. ern Africa, such as that surrounding the Congo In the most northerly tundras the dominant River, with evergreen forests closest to the river plants are small, hardy species that grow no more and semi-evergreen ones in nearby areas. than 2–4 in. (5–10 cm) tall. In subarctic regions The tropics, in general, are characterized not the dominant shrub species may grow as tall as by the four seasons of the more temperate cli- 3.28 ft. (1 m), and the marshiest subarctic tundra mate zones, but by a dry season and a wet season. may be home to sedge and cotton grass mead- The environment of a semi-evergreen tropical ows. Among the larger forms of animal life on forest is one that is subject to great extremes of the tundra are the caribou and musk ox as well as wet and dry, meaning that water is not available the wolf, one of the larger predatory species. in abundance year-round. This means that most GRASSLANDS AND CHAPAR- trees and shrubs in the biome are seasonally RAL. Temperate grasslands are known as deciduous, shedding their leaves in anticipation prairies in North America and steppes in Eurasia, of the drier season. In an evergreen tropical rain and these grasslands often are divided into small- forest, on the other hand, rainfall is frequent and er subgroups depending on the height of the regular, so there is no seasonal drought. Decidu- dominant grasses. Fire, aided by the dry climate, ous trees may drop their leaves at various times acts as a curb to prevent the tall grass from giving of year, depending on the species, but with a wet way to larger trees and forests. In the United climate and a wide range of trees, there is always States, however, so much prairie has been con- something in bloom. verted to agricultural or other anthropogenic As with the temperate rain forest, the tropi- purposes that it constitutes an endangered cal variety experiences little in the way of wildfire biome. or other catastrophic disturbances, and therefore Much further south are the tropical grass- an old-growth, climax community often devel- land and savanna biomes that appear primarily ops in this biome. For this reason, tropical rain in Africa. Although they do have scattered trees forests usually contain a wide diversity of trees, and shrubs, these biomes are dominated by an enormous richness of species, and an extraor- grasses and other plants. In any case, the plant life
374 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 375
Biomes
ONE TYPE OF FRESHWATER BIOME IS THE SWAMP FOREST, WHICH MAY BE FLOODED EITHER SEASONALLY OR PERMA- NENTLY AND IS NOTABLE FOR ITS ANIMAL LIFE, INCLUDING HERON AND OTHER BIRD SPECIES. (© S. R. Maglione. Photo Researchers. Reproduced by permission.)
is not what best characterizes this biome in the shadows,” areas separated from oceans by high minds of most people. Rather, it is the extraordi- mountains. narily abundant and diverse animal life, which The unavailability of water is the chief defin- includes such large mammals as the rhinoceros, ing feature of the desert, a biome that receives elephant, hippopotamus, buffalo, cheetah, gazelle less than 9.9 in. (25 cm) of precipitation per year. and other antelope, wild dog, and hyena. Then, of Extremely dry deserts support virtually no plant course, there is the lion, “king of beasts,” some- productivity and therefore little, if any, animal times incorrectly portrayed (for instance, in life either. Such is the case, for instance, with the many Tarzan movies) as a jungle creature. extraordinarily forbidding desert known as A biome typical of coastal southern Califor- Rub‘ al Khali (“The Empty Quarter”), which nia, the chaparral is distinguished by what often occupies the lower third of the Arabian peninsu- is described as a Mediterranean climate: dry, la. On the other hand, less dry deserts may sup- rainy in winter, and prone to drought in sum- port relatively diverse plant life, as is the case, for mers. The characteristic plant in a chaparral instance, in Arizona. region has thick, leathery leaves that help it pre- serve moisture during the dry seasons. As with Freshwater Biomes most other nonforest biomes, wildfire is a major Now we make another abrupt shift, in this case controlling factor. from the dry desert to aquatic biomes, beginning DESERT. Deserts, discussed in more with the freshwater variety. Among these are detail within The Biosphere, constitute a biome lentic biomes, which appear in the area of lakes that may be temperate or tropical and which usu- and ponds—any place where water is still. (This ally appears near the center of a continent. Such would include even a vast body of water such as is particularly the case with the Gobi and Takli- Lake Superior, which, though it looks like a sea makan deserts in northwestern and southwestern from the edge, and experiences waves and heavy China, respectively; both deserts are located swells, is nonetheless a freshwater body where almost as far away from ocean as it is possible to water is not flowing. Hence, it is by definition a be on Earth. Deserts also may occur in “rain lentic environment.) The water in these bodies
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 375 set_vol3_13 8/19/02 11:32 AM Page 376
Biomes may take a few days to flush; on the other hand, cypress and silver maple and may be flooded it may take centuries. By sitting for such long either seasonally or permanently. The Okeefeno- periods of time, the water may accumulate large kee is notable for its animal life: not just heron amounts of nutrients, and this is one of the vari- and other bird species but also some of the more ables whereby subgroups of the lentic biome are terrifying reptilian forms, including alligators classified. and water moccasins, for which swamps are In a lotic biome, such as that of a river or notorious. stream, water is flowing. A lotic biome may be as Much less biologically productive than small as a babbling brook that runs for less than either marshes or swamps are bogs, which gener- a mile, or as great as the Mississippi River, which ally have acidic soil that supports only a limited drains much of the continental United States. range of vegetation. Characterized by a cool, wet Where lotic biomes are concerned, the greatest climate (many of them are found in England), variables involve the strength of the flow, includ- bogs often are dominated by sphagnum moss of ing its quantity, velocity, and seasonal variations. one species or another. Later, when the sphag- These characteristics influence other aspects of num moss dies and the remains of several gener- the ecosystem: for example, if water flows calmly ations are compacted together with other plant and slowly, the bottom tends to gather silt. This debris, this becomes the basis for peat, which provides a habitat for certain small species, such provides fuel for some homes in the British Isles as the crustacean, known variously as a crayfish, and Europe. crawfish, or crawdaddy. Finally, there is the fen, another wetland A lotic environment is not fully self-sustain- found throughout the British Isles. Fens resemble ing, and therefore it may not qualify as a true bogs in several ways, including the fact that the biome. Though they support some plant life, local climate is usually cold (unlike the swamp these ecosystems do not have a full complement forest, where the climate is generally hot). The of autotrophs, or life-forms (usually plants) that fen has a better nutrient supply than the bog, depend only on the Sun and the atmosphere, however, and consequently the soil is less acidic, rather than other organisms, for sustenance. meaning that the biome as a whole is more pro- Usually, the lotic ecosystem relies on the input of ductive. organic matter from the nearby terrestrial envi- ronment or from lakes upstream or both. These Marine Biomes other biomes provide the lotic biome with the The largest biome, geographically, is that of the nutrients necessary to feed its fish and other open ocean, sometimes called a pelagic oceanic aquatic species. biome. Yet in terms of primary productivity— WETLANDS. Yet another freshwater the first level in the food web—the ocean might biome is that of the wetlands. Just as rain forests as well be a desert. Tossed by waves and tides and were known as jungles until ecology and environ- heavily affected by the powerful salt content in its mentalism entered the mainstream in the 1970s, chemistry, the open ocean depends for its pri- so the term wetlands replaced a more blunt- mary productivity not on plants but on phyto- sounding word: swamp. That word and several plankton, microscopic organisms that include a other old-fashioned ones are preserved in the range of bacteria and algae. terms for the four major wetland types: marsh, Small crustaceans known as zooplankton eat swamp forest, bog, and fen. Regardless of the the phytoplankton, only to be consumed, in turn, name, this is a biome found in shallow waters, by small fish. Thus it goes up the trophic levels of often in regions known for their pronounced sea- the oceanic food web to the largest predators: sonal variations of water depth. bluefin tuna, sharks, squid, and whales. At the The most biologically productive wetlands, bottom of the ocean are other ecosystems, which marshes typically are dominated by relatively tall depend on the slow rain of dead organic matter, angiosperm varieties, such as the reed, cattail, or biomass, from the surface. Little is known and bulrush, as well as by floating flowers, such as about the deep-ocean biomes, but they appear to the water lily and lotus. Swamp forests, such as be diverse, if low in productivity (i.e., they have a the Okeefenokee on the Georgia-Florida border, relatively wide range of species but a small num- are heavily populated with trees that include bald ber of individuals).
376 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 377
Biomes
CORAL REEFS APPEAR ONLY IN TROPICAL REGIONS. THE PRINCIPAL CHARACTERISTIC OF THIS BIOME IS ITS SUB- STRATE OF CALCIUM CARBONATE, FORMED FROM THE EXOSKELETONS OF DEAD CORAL POLYPS AND OTHER CREA- TURES. ON THIS STRUCTURE IS BUILT A HIGHLY PRODUCTIVE BIOME IN WHICH CORAL, ALGAE, FISH, AND INVERTE- BRATES THRIVE. (© Andrew Martinez. Photo Researchers. Reproduced by permission.)
Upwelling regions are relatively deep, nutri- aries feature characteristics of both marine and ent-rich areas that sustain highly productive bio- freshwater biomes and offer highly productive mes. Among the large variety of species included ecosystems. Many a commercially important in such biomes are fish and shark, marine mam- species of fish, shellfish, and crustacean makes its mals, and birds such as gulls. Upwellings off the early home in an estuary before moving on to west coast of South America and in the Antarctic deeper waters after reaching maturity. Ocean provide some of the human world’s most Seashores constitute a variety of oceanic abundant fisheries. biome or, indeed, several varieties. Environmen- CLOSER TO SHORE. Closer to the tal factors such as the intensity of wave motion major landmasses are the biomes of the conti- determine the characteristics of the seashore nental shelves. There the water is warm com- biome, as do latitude. For example, temperate pared with that of the open ocean, and the nutri- seashore ecosystems can develop kelp “forests.” ent supply is relatively high. This flow of nutri- (See Succession and Climax for more about the ents is fed partly by rivers that empty into the interrelations of species in the kelp forest.) In seas but also by the occasional rising of deeper, other areas, where the bottoms are soft and cov- richer waters to the surface. Not surprisingly, ered with sand or mud, dominant species include then, phytoplankton and animal life are highly mollusks, crustaceans, and marine worms. productive here, and continental shelf regions As with seashores, coral reefs are those rare such as those of the Grand Banks off northeast- oceanic biomes affected by latitude; in fact, this ern North America offer highly abundant and type of biome appears only in tropical regions. commercially important fisheries. The principal characteristic of the coral reef is its Another ocean biome closer to shore is that substrate of calcium carbonate, formed from the of the estuary, an ecosystem that is enclosed by exoskeletons of dead coral polyps and other crea- land on several sides but is still open to the sea. tures. On this structure is built a highly produc- Because they typically experience substantial tive biome in which coral, algae, fish, and inver- inflows of river water from the nearby land, estu- tebrates (animals without a backbone) thrive.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 377 set_vol3_13 8/19/02 11:32 AM Page 378
Biomes KEY TERMS
ABUNDANCE: A measure of the tion that the sky is not visible from the degree to which an ecosystem possesses ground. large numbers of particular species. An CARNIVORE: A meat-eating organ- abundant ecosystem may or may not have ism, or an organism that eats only meat (as a wide array of different species. Compare distinguished from an omnivore). with complexity. CLIMAX: A theoretical notion intended ALPINE: A term that refers to a biogeo- to describe a biological community that graphic zone that includes mountain has reached a stable point as a result of slopes above the timberline. ongoing succession. In such a situation, the ANGIOSPERM: A type of plant that community is at equilibrium with environ- produces flowers during sexual reproduc- mental conditions, and conditions are sta- tion. ble, such that the biota experiences little ANTHROPOGENIC: Influenced by change thereafter. human activity. COMPLEXITY: The range of ecological BIOGEOGRAPHY: The study of the niches within a biological community. The geographic distribution of plants and ani- degree of complexity is the number of dif- mals, both today and over the course of ferent species that theoretically could exist extended periods. in a given biota, as opposed to its diversity, BIOLOGICAL COMMUNITY: The liv- or actual range of existing species. ing components of an ecosystem. CONIFER: A type of tree that produces BIOME: A large ecosystem, character- cones bearing seeds. ized by its dominant life-forms. There are DECIDUOUS: A term for a tree or two basic varieties of biome: terrestrial, or other form of vegetation that sheds its land-based, and aquatic. leaves seasonally. BIOTA: A combination of all flora and DECOMPOSERS: Organisms that fauna (plant and animal life, respectively) obtain their energy from the chemical in a region. breakdown of dead organisms as well as CANOPY: The upper portion or layer from animal and plant waste products. The of the trees in a forest. A forest with a principal forms of decomposer are bacteria closed canopy is one so dense with vegeta- and fungi.
Anthropogenic Biomes their native biomes. New York City would be per- Finally, there are anthropogenic biomes, such as haps the ultimate example of an urban-industri- the urban-industrial techno-ecosystem found in al techno-ecosystem. Though it is far from a nat- many a large metropolitan area. Such an ecosys- ural environment, it teems with life, from the tem may include many species in addition to oaks and elms in Central Park to the rats of the humans, but these—pets, houseplants, and the sewers, and from the pigeons that peck at crumbs like—are not always native to the region, and on the sidewalks to the houseplants on the bal- probably would not flourish unless returned to conies and fire escapes of apartment buildings.
378 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_13 8/19/02 11:32 AM Page 379
Biomes KEY TERMS CONTINUED
DETRITIVORES: Organisms that feed but since the latter rarely exist separately, on waste matter, breaking organic material scientists prefer the concept of a food web down into inorganic substances that then to that of a food chain. can become available to the biosphere in FOREST: In general terms, a forest is the form of nutrients for plants. Their simply any ecosystem dominated by tree- function is similar to that of decomposers; size woody plants. A number of other char- however, unlike decomposers—which tend acteristics and parameters (for example, to be bacteria or fungi—detritivores are weather, altitude, and dominant species) relatively complex organisms, such as further define types of forests, such as earthworms or maggots. tropical rain forests. DIVERSITY: A measure of the number GYMNOSPERM: A type of plant that of different species within a biological reproduces sexually through the use of community. seeds that are exposed, not hidden in an ECOLOGY: The study of the relation- ovary, as with an angiosperm. ships between organisms and their envi- HERBIVORE: A plant-eating organ- ronments. ism. ECOSYSTEM: A community of inter- OMNIVORE: An organism that eats dependent organisms along with the inor- both plants and other animals. ganic components of their environment. ORGANIC: At one time chemists used FAUNA: Animals. the term organic only in reference to living things. Now the word is applied to most FLORA: Plants. compounds containing carbon, with the FOOD WEB: A term describing the exception of carbonates (which are miner- interaction of plants, herbivores, carni- als), and oxides, such as carbon dioxide. vores, omnivores, decomposers, and detri- SUCCESSION: The progressive tivores in an ecosystem. Each of these con- replacement of earlier biological commu- sumes nutrients and passes them along to nities with others over time. other organisms (or, in the case of the TERRESTRIAL: decomposer food web, to the soil and envi- Land-based. ronment). The food web may be thought UNDERSTORY: Layers of vegetation of as a bundle or network of food chains, below the canopy in a forest.
Another anthropogenic biome is the rural area for such a biome, as are the regions around techno-ecosystem, which is not as removed from coal mines and other industrial plants devoted to human civilization as the “rural” in its name the extraction, processing, or manufacture of would imply. This type of biome appears in products from natural resources. This biome regions around transportation and transmission usually supports a mixture of introduced species corridors, including highways, railways, canals, and native species, the latter being those varieties and aqueducts, as well as alongside power and that can survive the disturbances, pollution, and telephone lines. Small towns are a characteristic other stresses associated with the human pres-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 379 set_vol3_13 8/19/02 11:32 AM Page 380
Biomes ence. For example, in the woods along an inter- that do allow native wildlife species to thrive state highway, there are bound to be omnivorous alongside those species introduced for commer- creatures such as raccoons, which thrive on litter cial purposes. thrown out of passing cars. Such creatures must be agile enough to survive the threat of becoming WHERE TO LEARN MORE “road kill,” as well as other hazards associated with the environment. As with the city biome, Biomes of the World (Web site).
380 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 381
SCIENCE OF EVERYDAY THINGS real-life biology BIOLOGICAL COMMUNITIES
SYMBIOSIS BIOLOGICAL COMMUNITIES SUCCESSION AND CLIMAX
381 set_vol3_14 8/19/02 11:34 AM Page 383
Symbiosis SYMBIOSIS
CONCEPT benefits from the symbiotic relationship, and in Symbiosis is a biological relationship in which both instances the creature who does not bene- two species live in close proximity to each other fit—who provides a benefit to the other crea- and interact regularly in such a way as to benefit ture—is called the host. In commensalism the one or both of the organisms. When both part- organism known as the commensal benefits from ners benefit, this variety of symbiosis is known as the host without the host’s suffering any detri- mutualism. The name for a situation in which ment. By contrast, in parasitism the parasite ben- only one of the partners benefits is far more well efits at the expense of the host. known. Such an arrangement is known as para- MUTUALISM: HUMAN AND sitism, and a parasite is an organism that obtains DOG. Mutualism is distinguished from the nourishment or other life support from a host, other two types of symbiosis, because in this vari- usually without killing it. By their very nature, ety both creatures benefit. Thus, there is no host, parasites are never beneficial, and sometimes they and theoretically the partners are equal, though can be downright deadly. In addition to the in practice one usually holds dominance over the extremes of mutualism and parasitism, there is a other. An example of this inequality is the rela- third variety of symbiosis, called commensalism. tionship between humans and dogs. In this rela- As with parasitism, in a relationship characterized tionship, both human and dog clearly benefit: the by commensalism only one of the two organisms dog by receiving food, shelter, and care and the or species derives benefit, but in this case it man- human by receiving protection and loving com- ages to do so without causing harm to the host. panionship—the last two being benefits the dog also receives from the human. Additionally, some dogs perform specific tasks, such as fetching slip- HOW IT WORKS pers, assisting blind or disabled persons, or track- ing prey for hunting or crime-solving purposes. Varieties of Symbiosis For all this exchange of benefits, one of the When two species—that is, at least two individuals two animals, the human, clearly holds the upper representing two different species—live and inter- hand. There might be exceptions in a few unusu- act closely in such a way that either or both species al circumstances, such as dog lovers who are so benefit, it is symbiosis. It is also possible for a sym- obsessive that they would buy food for their dogs biotic relationship to exist between two organisms before feeding themselves. Such exceptions, how- of the same species. Organisms engaging in symbi- ever, are rare indeed, and it can be said that in otic relationships are called symbionts. almost all cases the human is dominant. There are three basic types of symbiosis, dif- Obligate and Facultative ferentiated as to how the benefits (and the detri- Relationships ments, if any) are distributed. These are com- mensalism, parasitism, and mutualism. In the Most forms of mutualism are facultative, mean- first two varieties, only one of the two creatures ing that the partners can live apart successfully.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 383 set_vol3_14 8/19/02 11:34 AM Page 384
Symbiosis Some relationships of mutualism are so close birds to be raised by them. (See Instinct and that the interacting species are unable to live Learning for a discussion of how these species without each other. A symbiotic relationship in exploit other birds’ instinctive tendency to care which the partners, if separated, would be unable for their young.) to continue living is known as an obligate rela- tionship. In commensalism or parasitism, the relationship is usually obligate for the commen- REAL-LIFE sal or the parasite, since by definition they APPLICATIONS depend on the host. At the same time, and also by definition, the host is in a facultative relationship, Mycorrhizae since it does not need the commensal or para- One of the best examples of mutualism is known site—indeed, in the case of the parasite, would be by the unusual name mycorrhiza, which is a much better off without it. It is possible, howev- “fungus root,” or a fungus living in symbiosis er, for an organism to become so adjusted to the with the roots of a vascular plant. (A vascular parasite attached to its body that the sudden plant is any plant species containing a vascular removal of the parasite could cause at least a system, which is a network of vessels for moving short-term shock to the system. fluid through the body of the organism.) The Inquilinism relationship is a form of mutualism because, while the fungus benefits from access to carbohy- A special variety of commensalism is inquilin- drates, proteins, and other organic nutrients ism, in which the commensal species makes use excreted by or contained in the roots of the host of the host’s nest or habitat, without causing any plant, the host plant benefits from an enhanced inconvenience or detriment to the host. Inquilin- supply of inorganic nutrients, especially phos- ism (the beneficiary is known as an inquiline) phorus, that come from the fungus. often occurs in an aquatic environment, though The fungus carries out this function prima- not always. In your own yard, which is your habi- rily by increasing the rate at which organic mat- tat or nest, there may be a bird nesting in a tree. ter in the immediate vicinity of the plant root Supposing you benefit from the bird, through the decomposes and by efficiently absorbing the aesthetic enjoyment of its song or the pretty col- inorganic nutrients that are liberated by this ors of its feathers—in this case the relationship process-nutrients it shares with the plant. (The could be said to be a mutualism. In any case, the term organic refers to the presence of carbon and bird still benefits more, inasmuch as it uses your hydrogen together, which is characteristic not habitat as a place of shelter. only of all living things but of many nonliving The bird example is an extremely nonintru- things as well.) The most important mineral sive case of inquilinism; more often than not, nutrients that the fungus supplies to the plant are however, a creature actually uses the literal nest compounds containing either phosphorus or, to of another species, which would be analogous to a lesser degree, nitrogen. (These elements are a bird nesting in your attic or even the inside of present in biogeochemical cycles—see The Bios- your house. This is where the analogy breaks phere.) So beneficial is the mycorrhizal mutual- down, of course, because such an arrangement ism that about 90% of all vascular plant families, would no longer be one of commensalism, since including mustards and knotweeds (family Bras- you would be suffering a number of deleterious sicaceae and Polygonaceae, respectively), enjoy effects, not the least of which would be bird some such relationship with fungi. droppings on the carpet. SOME EXAMPLES OF MYCOR- Inquilinism sometimes is referred to as a RHIZAE. Many mycorrhizal fungi in the cross between commensalism and parasitism and Basiodiomycete group develop edible mush- might be regarded as existing on a continuum rooms, which are gathered by many people for between the two. Certainly, there are cases of a use in gourmet cooking. Mushroom collectors creature making use of another’s habitat in a par- have to be careful, of course, because some myc- asitic way. Such is the case with the North Amer- orrhizal fungi are deadly poisonous, as is the case ican cowbird and the European cuckoo, both of with the death angel, or destroying angel— which leave their offspring in the nests of other Amanita virosa.
384 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 385
Perhaps the most famous of the edible Symbiosis mushrooms produced by mycorrhizae are the many varieties known by the name truffle. Among these mushrooms is Tuber melanospo- rum, which is commonly mycorrhizal on various species of oak tree. The spore-bearing bodies of the truffle fungi develop underground and are usually brown or black and covered with warts. Truffle hunters require the help of truffle-sniffing pigs or dogs, but their work is definitely worth the trouble: good truffles command a handsome price, and particularly in France the truffle industry is big business. Given the lucrative nature of the undertaking, one might ask why people do not cultivate truffles rather than hunt- ing for them. To create the necessary conditions for cultivation, however, so much effort is required that it is difficult to make a profit, even at the high prices charged for truffles. The soil composition must be just right, and under con- ditions of cultivation this takes about five years. Orchids are an example of a plant in an obli- ONE OF THE BEST EXAMPLES OF MUTUALISM IS THE MYCORRHIZA, a fungus living in symbiosis with gate mutualism: they can thrive only in a mycor- the roots of a vascular plant. Many mycor- rhizal relationship. Tiny and dustlike, orchid rhizal fungi develop edible mushrooms, seeds have virtually no stored energy to support such as the truffle, used in gourmet cook- the seedling when it germinates, or begins to ing. (© Owen Frnaken/Corbis. Reproduced by permission.) grow. Only with the assistance of an appropriate mycorrhizal fungus can these seedlings begin that have a mycorrhizal fungus usually do not. developing. Until horticulturists discovered this Instead, these plants rely heavily on the fungus fact, orchids were extremely difficult to propa- itself to absorb moisture and vital chemical ele- gate and grow in greenhouses; today, they are rel- ments from the ground. This means that it may atively easy to breed and cultivate. be difficult or impossible for plants to survive if THE IMPORTANCE OF MYCOR- they are removed from an environment contain- RHIZAE. Some species of vascular plants do ing mycorrhizal fungus, a fact that indicates an not contain chlorophyll, the chemical necessary obligate relationship. for photosynthesis, or the conversion of light Often, when species of trees and shrubs energy from the Sun into usable chemical energy grown in a greenhouse are transplanted to a non- in a plant. Such a plant is like a person missing a forested outdoor habitat, they exhibit signs of vital organ, and under normal circumstances, it nutritional distress. This happens because the would be impossible for the plant to survive. Yet soils in such habitats do not have populations of the Indian pipe, or Monotropa uniflora, has man- appropriate species of mycorrhizal fungi to colo- aged to thrive despite the fact that it produces no nize the roots of the tree seedlings. If, however, chlorophyll; instead, it depends entirely on myc- seedlings are transplanted into a clear-cut area orrhizal fungus to supply it with the organic that was once a forest dominated by the same or nutrients it needs. This obligate relationship is just closely related species of trees, the plants general- one example of the critical role mycorrhizae per- ly will do well. This happens because the clear- form in the lives of plants throughout the world. cut former forest land typically still has a popula- Mycorrhizae are vital to plant nutrition, tion of suitable mycorrhizal fungi. especially in places where the soil is poor in Plants’ dependence on mycorrhizal fungi nutrients. Whereas many plant roots develop may be so acute that the plants do not do well in root hairs as a means of facilitating the extraction the absence of such fungi, even when growing in of water and nutrients from the soil, plant roots soil that is apparently abundant in nutrients.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 385 set_vol3_14 8/19/02 11:34 AM Page 386
Symbiosis Although most mycorrhizal relationships are not incorrectly called mosses (e.g., reindeer moss). so obligate, it is still of critical important to con- Before the era of microscopy, botanists consid- sider mycorrhizal fungi on a site before a natural ered lichens to be single organisms, but they con- ecosystem is converted into some sort of anthro- stitute an obligate mutualism between a fungus pogenic habitat (that is, an area dominated by and an alga or a blue-green bacterium. The fun- humans—see Biomes). For example, almost all gus benefits from access to photosynthetic prod- the tree species in tropical forests depend on ucts, while the alga or bacterium benefits from mycorrhizae to supply them with nutrients from the relatively moist habitat that fungus provides the soils, which are typically infertile. (See The as well as from enhanced access to inorganic Biosphere for more about the soil in rain forests.) nutrients. If people clear and burn the forest to develop BIG AND SMALL. In contrast to new agricultural lands, they leave the soil bereft these cross-kingdom or interkingdom types of of a key component. Even though some fungi mutualism, there may be intrakingdom (within will survive, they may not necessarily be the the same kingdom) symbiotic relationships appropriate symbionts for the species of grasses between two very different types of animal. and other crops that farmers will attempt to grow Often, mutualism joins forces in such a way that on the cleared land. humans, observing these interactions, see in Interkingdom and Intraking- them object lessons, or stories illustrating the dom Partnerships concept that the meek sometimes provide vital assistance to the mighty. One example of this is Mycorrhizae are just one example of the ways purely fictional, and it is a very old story indeed: that mutualism brings into play interactions Aesop’s fable about the mouse and the lion. between widely separated species—in that par- In this tale a lion catches a mouse and is ticular case, between members of two entirely about to eat the little creature for a snack when different kingdoms, those of plant and fungi. In the mouse pleads for its life; the lion, feeling par- some cases, mutualism may bring together an ticularly charitable that day, decides to spare it. organism of a kingdom whose members are inca- Before leaving, the mouse promises one day to pable of moving on their own (plants, fungi, or return the favor, and the lion chuckles at this algae) with one whose members are mobile (ani- offer, thinking that there is no way that a lowly mals or bacteria). An excellent example is the mouse could ever save a fierce lion. Then one day relationship between angiosperm plants and the lion steps on a thorn and cannot extract it bees, which facilitate pollination for the plants from his paw. He is in serious pain, yet the thorn (see Ecosystems and Ecology.) is too small for him to remove with his teeth, and Another plant-insect mutualism exists he suffers hopelessly—until the mouse arrives between a tropical ant (Pseudomyrmex ferrug- and ably extracts the thorn. inea) and a shrub known as the bull’s horn acacia Many real-life examples of this strong-weak (Acacia cornigera). The latter has evolved hollow or big-small symbiosis exist, one of the more thorns, which the ants use as protected nesting well-known versions being that between the sites. The bull’s horn acacia has the added bene- African black rhinoceros (Diceros bicornis) and fit, from the ant’s perspective, of exuding pro- the oxpecker, or tickbird. The oxpecker, of the teins at the tips of its leaflets, thus providing a genus Buphagus, appears in two species, B. handy source of nutrition. In return, the ants africanus and B. erythrorhynchus. It feeds off protect the acacia both from competition with ticks, flies, and maggots that cling to the rhino’s other plants (by removing any encroaching hide. Thus, this oddly matched pair often can be foliage from the area) and from defoliating seen on the African savannas, the rhino benefit- insects (by killing herbivorous, or plant-eating, ing from the pest-removal services of the oxpeck- insects and attacking larger herbivores, such as er and the oxpecker enjoying the smorgasbord grazing mammals). that the rhino’s hide offers. A much less dramatic, though biologically HUMANS AND OTHER SPECIES. quite significant, example of interkingdom Humans engage in a wide variety of symbiotic mutualism is the lichen. Lichen is the name for relationships with plants, animals, and bacteria. about 15,000 varieties, including some that are Bacteria may be parasitic on humans, but far
386 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 387
Symbiosis
INTRAKINGDOM SYMBIOTIC RELATIONSHIPS CAN EXIST BETWEEN TWO VERY DIFFERENT TYPES OF ANIMAL. THE OXPECK- ER, OR TICKBIRD, FEEDS OFF THE TICKS, FLIES, AND MAGGOTS THAT CLING TO THE HIDE OF THE AFRICAN BLACK RHINOCEROS. THIS ODDLY MATCHED PAIR OFTEN CAN BE SEEN ON THE AFRICAN SAVANNAS. (© Joe McDonald/Corbis. Reproduced by permission.)
from all microorganisms are parasites: without porting tissue. In other words, thanks to selective the functioning of “good” bacteria in our intes- breeding, the corn that grows on farms is tines, we would not be able to process and elimi- enclosed in a husk, and the kernels do not come nate food wastes properly. The relationship of off of the cob readily. Such corn may be desirable humans to animals that provide a source of meat as a crop, but because of these characteristics, it is might be characterized as predation (i.e., the incapable of spreading its own seeds and thereby relationship of predator to prey), which is tech- reproducing on its own. Obviously, agricultural nically a form of symbiosis, though usually it is corn is not on any endangered species list, the not considered in the same context. In any case, reason being that farmers continue to propagate our relationship to the animals we have domesti- the species through breeding and planting. cated, which are raised on farms to provide food, is a mixture of predation and mutualism. For Another example of human-animal mutual- example, cows (Bos taurus) benefit by receiving ism, to which we alluded earlier, is the relation- food, veterinary services, and other forms of care ship between people and their pets, most notably and by protection from other predators, which dogs (Canis familiaris) and house cats (Felis might end the cows’ lives in a much more catus). Fed and kept safe in domestication, these unpleasant way than a rancher will. animals benefit tremendously from their interac- All important agricultural plants exist in tion with humans. Humans, in turn, gain from tight bonds of mutualism with humans, because their pets’ companionship, which might be human farmers have bred species so selectively regarded as a mutual benefit—at least in the case that they require assistance in reproducing. For of dogs. (And even cats, though they pretend not example, over time, agricultural corn, or maize to care much for their humans, have been known (Zea mays), has been selected in such a way as to to indulge in at least a touch of sentimentality.) favor those varieties whose fruiting structure is In addition, humans receive other services from enclosed in a leafy sheath that does not open and pets: dogs protect against burglars, and cats erad- whose seeds do not separate easily from the sup- icate rodents.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 387 set_vol3_14 8/19/02 11:34 AM Page 388
Symbiosis are images of parasitism. Indeed, many parasites KEY TERMS are insects, but insects often interact with other species in relationships of mutualism, such as those examples mentioned earlier (bees and COMMENSALISM: A symbiotic rela- angiosperms, ants and bull’s horn acacia plants). tionship in which one organism, the com- Additionally, there are numerous cases of mutu- mensal, benefits without causing any detri- alism between insect species. One of the most ment to the other organism, the host. intriguing is the arrangements that exists between ants and aphids, insects of the order FACULTATIVE: A term for a symbiotic Homoptera, which also are known as plant lice. relationship in which partners are capable In discussing the ant-aphid mutualism, scien- of living apart. tists often compare the aphids to cattle, with the HOST: The term for an organism that ants acting as protectors and “ranchers.” What aphids have that ants want is something called provides a benefit or benefits for another honeydew, a sweet substance containing surplus organism in a symbiotic relationship of sugar from the aphid’s diet that the aphid excretes commensalism or parasitism. through its anus. In return, ants protect aphid eggs INQUILINISM: A type of symbiosis in during the winter and carry the newly hatched aphids to new host plants. The aphids feed on the which one species, the inquiline, makes use leaves, and the ants receive a supply of honeydew. of a host’s nest or habitat without causing In another mutualism involving a particular any detriment to the host. Inquilinism is ant species, Formica fusca, two organisms appear considered a variety of commensalism. to have evolved together in such a way that each OBLIGATIVE: A term for a symbiotic benefits from the other, a phenomenon known as coadaptation. This particular mutualism involves relationship in which the partners, if they the butterfly Glaucopsyche lygdamus when it is were separated, would be incapable of con- still a caterpillar, meaning that it is in the larval, tinuing to live. or not yet fully developed, stage. Like the aphid, PARASITISM: A symbiotic relation- this creature, too, produces a sweet “honeydew” solution that the ants harvest as food. In return, ship in which one organism, the parasite, the ants defend the caterpillar against parasitic benefits at the expense of the other organ- wasps and flies. ism, the host. WHEN MUTUALISM ALSO CAN SYMBIOSIS: A biological relationship BE PARASITISM. As the old saying goes, in which (usually) two species live in close “One man’s meat is another man’s poison”—in other words, what is beneficial to one person may proximity to one another and interact reg- be harmful to another. So it is with symbiotic ularly in such a way as to benefit one or relationships, and often a creature that plays a both of the organisms. Symbiosis may exist helpful, mutualistic role in one relationship may between two or more individuals of the be a harmful parasite in another interaction. same species as well as between two or Aphids, for instance, are parasitic to many a host plant, which experiences yellowing, stunting, more individuals representing two differ- mottling, browning, and curling of leaves as well ent species. The three principal varieties of as inhibiting of its ability to produce crops. symbiosis are mutualism, commensalism, One particular butterfly group, Heliconiinae and parasitism. (a member of the Nymphalidae, largest of the butterfly families) furnishes another example of the fact that a mutualistic symbiont, in separate Symbiosis Among Insects interaction, can serve as a parasite. Moreover, in this particular case the heliconius butterfly can Where insects and symbiosis are involved, per- be a mutualistic symbiont and parasite for the haps the ideas that most readily come to mind very same plant. Heliconius butterflies scatter the
388 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 389
pollen from the flowers of passionflower vines Among these plants are certain species of orchids, Symbiosis (genus Passiflora), thus benefiting the plant, but ferns, and moss. By “standing on the shoulders of their females also lay eggs on young Passiflora giants,” these plants receive enormous ecological shoots, and the developing larva may eat the benefits: the height of their hosts gives them an entire shoot. As an apparent adaptive response, opportunity to reach a higher level in the canopy several Passiflora species produce new shoots fea- (the upper layer of trees in the forest) than they turing a small structure that closely resembles a would normally attain, and this provides them heliconius egg. A female butterfly that sees this with much greater access to sunlight. At the same “egg” will avoid laying her own egg there, and the time, the hosts are not affected either negatively shoot will be spared. or positively by this relationship. Commensalism Another commensal relationship, known as phoresy, is a type of biological hitchhiking in Years ago a National Geographic article on the which one organism receives access to trans- Indian city of Calcutta included a photograph portation on the body of another animal, without that aptly illustrated the idea of commensalism, the transporting animal being adversely affected though in this case not between animals or plants by this arrangement. The burdock (Arctium but between people. The photograph showed a lappa) is one of several North American plant street vendor in a tiny wooden stall with a win- species that produce fruit that adheres to fur and dow, through which he sold his wares to passers- therefore is dispersed easily by the movement of by. It was a rainy day, and huddled beneath the mammals. The burdock is special from a human window ledge (which also served as a counter- standpoint, however, inasmuch as the anatomical top) was another vendor, protecting himself and adaptation that makes possible its adhesion to fur his own tray of goods from the rain. provided designers with the model for that The photograph provided a stunning exam- extremely useful innovation, Velcro. ple, in microcosm, of the overpopulation prob- As with the illustration of the street vendors lem both in Calcutta and in India as a whole—a in Calcutta, it is always possible that commensal- level of crowding and of poverty far beyond the ism, through a slight alteration, may yield a rela- comprehension of the average American. At the tionship in which the host is affected negatively. same time it also offered a beautiful illustration There are instances in which individual animals of commensalism (though this was certainly not may become loaded heavily with sticky fruit the purpose of including the picture with the from the burdock (or other plants that employ a article). The vendor sitting on the ground acted similar mechanism), thus causing their fur to in the role of commensal to the relatively more mat excessively and perhaps resulting in signifi- fortunate vendor with the booth, who would be cant detriment. This is not common, however, analogous to the host. and usually this biological relationship is truly The relationship was apparently commensal, commensal. Furthermore, phoresy should not be because the vendor on the ground received shel- confused with parasitic relationships in which a ter from the other vendor’s counter without the creature such as a tick attaches itself to the body other vendor’s suffering any detriment. If the of another organism for transport or other pur- vendor in the booth wanted to move elsewhere, poses. (For much more about parasitism, see Par- and the vendor on the ground somehow prevent- asites and Parasitology.) ed him from doing so, then the relationship would be one of parasitism. And, of course, if the WHERE TO LEARN MORE vendor with the booth charged his less-fortunate “Biology 160, Animal Behavior: Symbiosis and Social neighbor rent, then the relationship would not be Parasitism.” Department of Biology, University of truly commensal, because the vendor on the California at Riverside (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 389 set_vol3_14 8/19/02 11:34 AM Page 390
Symbiosis Lembke, Janet. Despicable Species: On Cowbirds, Kudzu, “Parasites and Parasitism.” University of Wales, Aberyst- Hornworms, and Other Scourges. New York: Lyons wyth (Web site).
390 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 391
BIOLOGICAL
Biological Communities COMMUNITIES
CONCEPT Note the importance of the distinctions An ecosystem is a complete community of inter- between living and nonliving and organic and dependent organisms as well as the inorganic inorganic. Although in popular terms, organic components of their environment; by contrast, a means anything that is living as well as anything biological community is just the living members that was once living, along with their parts and of an ecosystem. Within the study of biological products (bones, leaves, wood, sap, blood, urine, communities there are a great number of com- and so on), in fact, the scientific meaning of the plexities involved in analyzing the relationships word is both much broader and more targeted. A between species as well as the characteristics of substance is organic if it contains carbon and specific communities. Yet many of the concepts hydrogen, and thus organic materials also applicable to biological communities as a whole include such items as plastics that have never also apply to human communities in particular, been living. and this makes these ideas easier to understand. For example, the competitive urge that motivates The study of the relationship between living humans to war (and to less destructive forms of things and their environment, pioneered by the strife in the business or sports worlds) may be German zoologist Ernst Haeckel (1834–1919) linked to the larger phenomenon of biological and others, is called ecology. Though the world competition. Indeed, much of the driving force scientific community was initially slow to accept behind the development of human societies, as it ecology as a subject of study, the discipline has turns out, has been biological in nature. gained increasing respect since the mid–twenti- eth century. This change is due to growing HOW IT WORKS acceptance for the idea that all of life is intercon- nected and that the living world is tied to the The Biosphere, Ecosystems, nonliving, or inorganic, world. On the other and Ecology hand, there is also the gathering awareness that certain aspects of industrial civilization may have An ecosystem is a community of interdependent a negative impact on the environment, an aware- organisms along with the inorganic components ness that has spurred further interest in the study of their environment—air, water, and the miner- of ecology. al content of the soil—and a biome is an ecosys- tem, such as a tundra, that extends over a large Introduction to Biological area. All living organisms are part of a larger sys- Communities tem of life-forms, which likewise interact with large systems of inorganic materials in the oper- The term biological community refers to all the ation of a still larger system called Earth. (The living components in an ecosystem. A slightly concepts of ecosystem, biosphere, and biome are different concept is encompassed in the word discussed, respectively, in Ecosystems and Ecolo- biota, which refers to all flora and fauna, or plant gy, The Biosphere, and Biomes.) and animal life, in a particular region.
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 391 set_vol3_14 8/19/02 11:34 AM Page 392
Biological For the biological community to survive and species, as well as between two or more individu- Communities thrive, a balance must be maintained between als representing two different species. The three consumption and production of resources. Nature principal varieties of symbiosis are mutualism, in provides for that balance in numerous ways, but which both participants benefit; commensalism, beginning in the late twentieth century students of in which only one participant benefits, but at no ecology in the industrialized world have become expense to the other participant; and parasitism, more and more concerned with the possible neg- in which one participant benefits at the expense ative impact their own societies exert on Earth’s of the other. These subjects are covered in much biological communities and ecosystems. greater depth within the essays on Symbiosis and It should be noted, however, that nature Parasites and Parasitology. itself sometimes replaces biological communities Evaluating Biological in a process called succession. This process Communities involves the progressive replacement of earlier biological communities with others over time. A few billion years ago, Earth’s oceans and lands Coupled with succession is the idea of climax, a were populated with just a few varieties of single- theoretical notion intended to describe a biolog- cell organisms, but over time increasing differenti- ical community that has reached a stable point as ation of species led to the development of the a result of ongoing succession. (See Succession much more complex ecosystems we know now. and Climax for more about these subjects.) Such differentiation is essential, since the life NICHE. Whereas climax and succession forms in a particular region must adapt to that apply to broad biological communities, a niche biome, whether it be forest or grassland, desert or refers to the role a particular organism or species aquatic environments, mountain setting or jungle. plays within that community. Though the con- Diversity is a measure of the number of dif- cept of niche is abstract, it is unquestionable that ferent species within a biological community, each organism plays a vital role and that the while complexity is the number of niches within totality of the biological community (and, it. Put another way, the complexity of a communi- indeed, the ecosystem) would suffer stress if a ty is the number of species that could exist in it. large enough group of organisms were removed Abundance is the measure of populations within from it. Furthermore, given the apparent interre- individual species; thus, if a biological community latedness of all components in a biological com- has large numbers of individuals, even if it is not munity, every species must have a niche—even diverse in species, it is still said to be abundant. human beings. During its brief summer growing season, the An interesting idea, and one that is some- arctic tundra has vast numbers of insects, migra- what similar to a niche, is that of an indicator tory birds, and mammals, and thus its abundance species. This is a plant or animal that, by its pres- is high, whereas its diversity is low. On the other ence, abundance, or chemical composition, hand, a rain forest might have several hundred or demonstrates a particular aspect of the character even a thousand different tree species, and an or quality of the environment. Indicator species, even larger number of insect species, in only a for instance, can be plants that accumulate large few hectares, but there may be only a few indi- concentrations of metals in their tissues, thus viduals representing each of those species in that indicating a preponderance of metals in the soil. area. Thus, the forest could have extremely high This metal, in turn, could indicate the presence diversity but low abundance of any particular of valuable deposits nearby, or it could serve as a species. Needless to say, the rain forest is likely to sign that the soil is being contaminated. (See have a much greater complexity than the tundra, Food Webs for more about indicator species.) meaning that it is theoretically likely to contain Another concept closely tied to the concept far more species. of niche is that of symbiosis. The latter refers to a Another way to evaluate ecosystems is in biological relationship in which (usually) two terms of productivity. Productivity refers to the species live in close proximity to one another and amount of biomass—potentially burnable ener- interact regularly in such a way as to benefit one gy—produced by green plants as they capture or both of the organisms. Symbiosis may exist sunlight and use its energy to create new organic between two or more individuals of the same compounds that can be consumed by local ani-
392 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 393
mal life. Once again, a forest, and particularly a supply resources is smaller than the potential Biological rain forest, has a very high level of productivity, biological requirement for those resources. Communities whereas a desert or tundra ecosystem does not. Scarcity of resources relative to the need for FOOD WEBS. Food web (in contrast to them is one of the governing facts of human life, the more popular, but less correct term, food reflected in such common expressions as “There is chain) is the designation preferred by scientists no such thing as a free lunch.” In fact, humans to describe the means by which energy is trans- have spent most of their history in a modified ferred through a biological community. Within form of biological competition, war. Further- the food web are various stages, called trophic more, it may well be that our modern predilection levels, that identify the position of various organ- for sports or business competition is simply a isms in relation to the organisms they consume matter of transforming biological competition and the organisms that consume them. into a more refined form. In any case, competition Green plants that depend for their nourish- prevails throughout the world of living things. ment on photosynthesis, or the biological con- INTRASPECIFIC AND INTER- version of electromagnetic energy from the Sun SPECIFIC COMPETITION. For exam- into chemical energy, are primary producers. ple, plants often compete for access to a limited Herbivores, or plant-eating creatures, are pri- supply of nutrients, water, sunlight, and space. mary consumers, whereas the animals that eat Intraspecific competition occurs when individu- herbivores (whether carnivores or omnivores) als of the same species vie for access to essential are secondary consumers. The largest carnivores resources (later we look at intraspecific competi- and herbivores are usually not prey for any other tion between humans), or for mating partners, creatures, but when they die, they, too, will be whereas interspecific competition takes place consumed by detritivores, or scavengers, as well between different species. as decomposers, such as bacteria and fungi. Individuals of the same species have virtual- The second law of thermodynamics, one of ly identical resource requirements: for example, several laws governing energy and the systems in all humans need food, water, air, and some pro- which that energy is applied, holds that in each tection from the natural elements. For this rea- energy transfer some energy is lost. In the case of son, whenever populations of a species are food webs, this means that much of the energy in crowded together, intraspecific competition is each trophic level is unavailable to organisms at intense. This also has been illustrated by experi- the next level. This, in turn, means that each suc- ments involving laboratory mice, which become cessive trophic level generally has far fewer mem- increasingly brutal to one another when con- bers than the prey on which they feed. While fronted with severely diminished resources. there might be thousands of primary producers When intraspecific competition occurs in dense in a particular community, there might be only a populations, the result is a process known as self- few top predators, including humans. (See Food thinning, characterized by the mortality (death) Webs for more on these subjects.) of those individuals less capable of surviving coupled with the survival of individuals that are REAL-LIFE more competitive. If this sounds like the “sur- vival of the fittest,”an idea associated with evolu- APPLICATIONS tion that became the justification for a number of Competition nefarious social movements and activities (see Evolution), it is no accident. The word competition typically brings to mind Ideas related to intraspecific competition images of a basketball court or football field, on influenced the English naturalist Charles Darwin one hand, or a Wall Street trading floor or board- (1809–1882) in developing his theory of evolu- room, on the other. In biological terms, however, tion by means of natural selection. Intraspecific competition is the interaction among organisms competition is an important regulator of popula- of the same or different species vying for a com- tion size and can make the species as a whole mon resource that appears in a limited supply more fit by ensuring that only the hardiest indi- relative to the demand. Put another way, this viduals survive. Likewise, interspecific competi- means that the capability of the environment to tion, or competition between species, plays a
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 393 set_vol3_14 8/19/02 11:34 AM Page 394
Biological Communities
A BIOME IS AN ECOSYSTEM, SUCH AS A TUNDRA, THAT EXTENDS OVER A LARGE AREA. IN THE ARCTIC TUNDRA, COM- PETITION IS LOW, BUT CREATURES LIKE THIS ARCTIC FOX FACE SUCH POWERFUL ENVIRONMENTAL STRESSES FROM THE LOCAL CLIMATE THAT COMPETITION IS NOT THE MOST SIGNIFICANT FACTOR LIMITING POPULATIONS. (Photo Researchers. Reproduced by permission.)
strong role in shaping ecological communities. plants that previously have been suppressed by Furthermore, competition between species is the higher-canopy trees find themselves with also an agent of natural selection. much greater access to such resources as light, COMPETITIVE RELEASE. Envi- moisture, and nutrients. As a result, they thrive. ronmental changes that affect a biological com- Given the distasteful aspects of biological munity may change the competitive relation- competition, such as the destruction of the ships within it, leading to interesting results. “weak” in favor of the “strong” (actually, less During the early 1950s, a fungal pathogen (a dis- adapted and more adapted are much more accu- ease-carrying parasite in the form of a fungus) rate terms in this context), one might wish for a known as chestnut blight, or Endothia parasitica, situation in which no competition exists. Indeed, ended the dominance of the American chestnut there are such situations in nature, but they are in the eastern United States. Up to that time, the far from pleasant. In such biomes as the arctic American chestnut (Castanea dentata) had been tundra, for instance, competition is low, but this the leading species in the canopy, or uppermost is not because all nature lives in happiness and layer, of the deciduous (prone to seasonal shed- harmony; instead, organisms face such powerful ding of leaves) forests in the region. Thanks to environmental stresses from the local climate the chestnut blight, it was as though the winner that competition is not the most significant fac- had been disqualified from a race, meaning that tor limiting populations. all the runners-up changed their standings. THE TUNDRA BIOLOGICAL By being relieved of the stresses associated COMMUNITY. Creatures on the tundra face with competition, other trees were allowed to little stress from competition, but a great deal of become more successful and dominant in their stress in the form of very short growing seasons, habitat. They took advantage of this change to fill thin soil, limited ground cover, low average tem- in the canopy gaps left by the demise of mature peratures and rainfall, high winds, and so on. If chestnut trees. In the same way, if a wildfire, the density of individual plants of the tundra is storm, or other stress disturbs a mature forest, decreased experimentally by thinning, the resid-
394 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 395
ual plants do not thrive as a result, as they might roughly chronological order, these locations were Biological in a less harsh climate. In the case of the tundra, Mesopotamia, Egypt, India, and China. All were Communities it is not competition that constrains their pro- destined to emerge as civilizations, complete with ductivity, and therefore the reduction of poten- written language, cities, and organized govern- tial competition does little to improve conditions ments, between about 3000 and 2000 B.C. for the organisms that survive. In the New World, by contrast, agriculture It is interesting to observe what happens in a appeared much later and in a much smaller way. tundra environment if the intensity of environ- The same was true of Africa and the Pacific mental stress is artificially and experimentally Islands. In seeking to find the reasons why this alleviated by enclosing an area under a green- happened, Diamond noted a number of factors, house and by fertilizing it with nutrients. Such including geography. The agricultural areas of experiments have been performed, and the results the Old World were stretched across a wide area are fascinating: under these more favorable envi- at similar latitudes. This meant that the climates ronmental conditions, competition actually were not significantly different and would sup- increases, resulting in a biological community not port agricultural exchanges, such as the spread of unlike that of a more hospitable biome. wheat and other crops from one region or Biological Communities and ecosystem to another. By contrast, the landmass- Civilizations es of the New World or Africa have a much greater north-to-south distance than they do east In his best-seller Guns, Germs, and Steel: The Fate to west, resulting in great differences of climate. of Human Societies, ethnobotanist Jared M. Dia- DIVERSITY OF SPECIES. Today mond showed that local biological communities such places as the American Midwest support are among the leading determinants of the suc- abundant agriculture, and one might wonder why cess or failure of human civilizations. The book that was not the case in the centuries before Euro- had its beginnings, he wrote, during his many peans arrived. The reason is simple but subtle, and years of work with the native peoples of New it has nothing to do, as many Europeans and their Guinea. One day, a young man put a simple ques- descendants believed, with the cultural “superior- tion to him: why do the societies of the West ity” of Europeans over Native Americans. The fact enjoy an abundance of material wealth and com- is that the native North American biological com- forts, while those of New Guinea have so little? munities were far less diverse than their counter- The question may have been simple, but the parts in the Old World. Peoples of the New World answer was not obvious. As a scientist, Diamond successfully domesticated corn and potatoes, refused to give an answer informed by the poli- because those were available to them. But they tics of the Left or Right, which might have could not domesticate emmer wheat, the variety blamed the problem, respectively, on western used for making bread, when they had no access exploitation or on the failures of the New to that species (it originated in Mesopotamia and Guineans themselves. Instead, he approached it spread throughout the Old World). as a question of environment, and the result was The New World also possessed few animals his thought-provoking analysis, contained in that could be domesticated either for food or for Guns, Germs, and Steel. labor. Every single plant or animal that is a part FAVORABLE AND UNFAVOR- of human life today had to be domesticated— ABLE ECOSYSTEMS. As Diamond adapted in such a way that it becomes useful and showed, the places where agriculture was born advantageous for humans—and the range of were precisely those blessed with favorable cli- species capable of domestication is far from lim- mate, soil, and indigenous plant and animal life. itless. In fact, it is safe to say that all major species Of course, it is no accident that civilization was capable of being domesticated have been, thou- born in the societies where agriculture first sands of years ago. The list of animals that can be developed. Before a civilization can evolve, a domesticated is a short one, much shorter than society must become settled, and for that to hap- the list of animals that can be tamed. A bear, for pen, it must have agriculture. instance, can be captured, or raised from birth in Agriculture came into existence in four places captivity, but it is unlikely that humans would during a period from about 8000 to 6000 B.C.In ever be able to breed bears in such a way that
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 395 set_vol3_14 8/19/02 11:34 AM Page 396
Biological those of the Native Americans. This was also true Communities of the “biological communities” they could not see, and of which people were unaware in 1500: the world of microorganisms, or the “germs” in the title of Diamond’s book. The native peoples of the New World had no natural resistance to smallpox or a host of other diseases, including measles, chicken pox, influen- za, typhoid fever, and the bubonic plague. As with many plants and animals of the Old World, they simply had no exposure to these microor- ganisms. In the Old World, however, close con- tact with farm animals exposed humans to dis- eases, as did close contact with other people in crowded, filthy cities. This exposure, of course, killed off large numbers in such plagues as the Black Death (1347–1351), but those who sur- vived tended to be much stronger and possessed vastly greater immunities. Therefore, the vast majority of Native American deaths that fol- lowed the European invasion were not a result of warfare, enslavement, or massacre of villages (though all of these occurred as well), but of A SMALLPOX VICTIM SHOWS THE CHARACTERISTIC infection. (See Infection and Infectious Diseases . EUROPEANS’ ADVANTAGE OVER LESIONS ON HIS LIMBS for more on these subjects.) THE NATIVE AMERICANS DERIVED FROM THE ECOLOGI- CAL COMPLEXITY OF THEIR BIOLOGICAL COMMUNITIES, CANNIBALISM. Even the practice of EVEN DOWN TO THE WORLD OF MICROORGANISMS. THE cannibalism in such remote locations as the New NATIVE PEOPLES OF THE NEW WORLD HAD NO NATURAL RESISTANCE TO SMALLPOX OR A HOST OF OTHER DIS- Guinea highlands is, according to Diamond, a EASES. (© Corbis. Reproduced by permission.) consequence of a relatively limited biological community. In the past, westerners assumed that only very “primitive” societies engaged in canni- their wild instincts all but disappeared and they balism. However, events have shown that, when became reliable, useful companions. faced with starvation, even people from Euro- The animals that helped make possible the pean and European-influenced civilizations may development of farms, villages, and ultimately consume human flesh in order to survive. empires in the Old World—cows and oxen, hors- For instance, in 1846 members of the Don- es and donkeys, sheep and so on—were absent ner party, making the journey west across North from the New World. (Actually, horses had once America, resorted to eating the bodies of those existed in the Americas, but they had been who had died in the perilous crossing. Much the destroyed through overhunting, as discussed in same happened in the 1970s, when a plane carry- the context of mass extinction within the Paleon- ing Uruguayan athletes crashed in the Andes, and tology essay.) Many Indian tribes domesticated the survivors lived off the flesh of those who had some types of birds and other creatures for food, died. Though their upbringing and cultural but the only animal ever adapted for labor—by norms may have told them that cannibalism was the most developed civilization of the pre- immoral or at the very least disgusting, their Columbian Americas, the Inca—was the llama, bodies told them that if they did not consume the which is too small to carry heavy loads. only available food, they would die. GREATER EXPOSURE TO Whereas these circumstances were unusual MICROORGANISMS. The Europeans’ and temporary, peoples in some parts of the advantage over the Native Americans derived world have been faced with a situation in which ultimately from the ecological complexity of the only sources of protein provided within the their biological communities compared with biological community are ones whose consump-
396 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 397
Biological KEY TERMS Communities
ABUNDANCE: A measure of the CANOPY: The upper portion or layer degree to which an ecosystem possesses of the trees in a forest. A forest with a large numbers of particular species. An closed canopy is one so dense with vegeta- abundant ecosystem may or may not have tion that the sky is not visible from the a wide array of different species. Compare ground. with complexity. CARNIVORE: A meat-eating organ- BIOENERGY: Energy derived from ism, or an organism that eats only meat (as biological sources that are used directly as distinguished from an omnivore). fuel (as opposed to food, which becomes CLIMAX: A theoretical notion intended fuel). Examples of bioenergy include wood to describe a biological community that or manure that can be burned. Usually, has reached a stable point as a result of petrochemicals, such as petroleum or nat- ongoing succession. ural gas, though they are derived from the COMPLEXITY: The range of ecological bodies of dead organisms, are treated sepa- niches within a biological community. The rately from forms of bioenergy. degree of complexity is the number of dif- BIOLOGICAL COMMUNITY: The liv- ferent species that theoretically could exist ing components of an ecosystem. in a given biota, as opposed to its diversity, or actual range of existing species. BIOMAGNIFICATION: The increase in bioaccumulated contamination at higher DECOMPOSERS: Organisms that levels of the food web. Biomagnification obtain their energy from the chemical results from the fact that larger organisms breakdown of dead organisms as well as consume larger quantities of food—and, from animal and plant waste products. The hence, in the case of polluted materials, principal forms of decomposer are bacteria more toxins. and fungi. DECOMPOSITION REACTION: BIOMASS: Materials that are burned A or processed to produce bioenergy. chemical reaction in which a compound is broken down into simpler compounds, or BIOME: A large ecosystem, character- into its constituent elements. In the bios- ized by its dominant life-forms. phere, this often is achieved through the BIOSPHERE: A combination of all liv- help of detritivores and decomposers. ing things on Earth—plants, animals, DETRITIVORES: Organisms that feed birds, marine life, insects, viruses, single- on waste matter, breaking organic material cell organisms, and so on—as well as all down into inorganic substances that then formerly living things that have not yet can become available to the biosphere in decomposed. the form of nutrients for plants. Their BIOTA: A combination of all flora and function is similar to that of decomposers; fauna (plant and animal life, respectively) however, unlike decomposers—which tend in a region. to be bacteria or fungi—detritivores are
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 397 set_vol3_14 8/19/02 11:34 AM Page 398
Biological Communities KEY TERMS CONTINUED
relatively complex organisms, such as NICHE: A term referring to the role earthworms or maggots. that a particular organism plays within its DIVERSITY: A measure of the number biological community. of different species within a biological OMNIVORE: An organism that eats community. both plants and other animals. ECOLOGY: The study of the relation- PATHOGEN: A disease-carrying para- ships between organisms and their envi- site, usually a microorganism. ronments. PHOTOSYNTHESIS: The biological ECOSYSTEM: A community of inter- conversion of light energy (that is, electro- dependent organisms along with the inor- magnetic energy) from the Sun to chemical ganic components of their environment. energy in plants. ENERGY TRANSFER: The flow of PRIMARY PRODUCERS: Green energy between organisms in a food web. plants that depend on photosynthesis for HERBIVORE: A plant-eating organ- their nourishment. ism. PRODUCTIVITY: The amount of bio- INDICATOR SPECIES: A plant or mass produced by green plants in a given animal that, by its presence, abundance, or biome. chemical composition, demonstrates a SUCCESSION: The progressive particular aspect of the character or quali- replacement of earlier biological commu- ty of the environment. nities with others over time. NATURAL SELECTION: The process TROPHIC LEVELS: Various stages whereby some organisms thrive and others within a food web. For instance, plants are perish, depending on their degree of adap- on one trophic level, herbivores on anoth- tation to a particular environment. er, and so on.
tion seems repugnant from the western view- DeLong, J. Bradford. “Review of Diamond,” Guns, Germs, point. Discussing the New Guinea highlands, and Steel (Web site).
398 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 399
NMITA: Neogene Marine Biota of Tropical America (Web Plant Communities of California (Web site). Biological site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 399 set_vol3_14 8/19/02 11:34 AM Page 400
SUCCESSION AND
CLIMAXSuccession and Climax
CONCEPT cussions of biomes, or large ecosystems, and food Eventually almost everyone has the experience of webs, or the means by which energy transfer watching an old neighborhood change. Some- takes place across a biological community. Relat- times we perceive that change for the better, ed to many of these ideas, as well as to succession sometimes for the worse, and the perception can and climax, is the realm of biogeography, or the have more to do with our individual desires or study of the geographic distribution of plants needs than it does with any qualities inherent in and animals, both today and over the course of the change itself. For instance, one person might biological history. regard a new convenience store and gas station as Biogeography, which emerged in the nine- an eyesore, while another might welcome it as a teenth century amid efforts to explore and map handy place to buy coffee, gasoline, or other the planet fully, draws on many fields. Among the items. Likewise, biological “neighborhoods” areas that overlap with this interdisciplinary change, as when a complete or nearly complete realm of study are the biological sciences of community of living things replaces another. botany and zoology, the combined biological and Once again, changes are not necessarily good or earth sciences of oceanography and paleontol- bad in any fundamental sense; rather, one com- ogy, as well as the earth sciences of geology and munity that happens to be better adapted to the climatology. Not only do these disciplines con- changed environment replaces another. Some- tribute ideas to the growing field of biogeogra- times a stress to the ecosystem brings about a phy, but they also make use of ideas developed by change, such that life-forms that once were adapt- biogeographers. Biogeography is concerned with ed to the local environment are no longer. Still, questions regarding local and regional variations there appears to be a point when a community in kinds and numbers of species and individuals. achieves near perfect adaptation to its environ- Among the issues addressed by biogeography are ment, a stage in the levels of succession known as the reasons why particular species exist in partic- climax. This is the situation of old-growth forests, ular areas, the physical and biotic (life-related) a fact that explains much about environmentalist factors that influence the geographic range over opposition to logging in such situations. which a species proliferates, changes in distribu- tion of species over time, and so on. HOW IT WORKS Species interact by three basic means: com- petition for resources, such as space, sunlight, Biogeography water, or food (see Biological Communities for more about competition); predation, or preying, Elsewhere in this book, there is considerable upon one another (see Food Webs); and symbio- material about ecosystems, or communities of sis. An example of the latter form of interaction, interdependent organisms along with the inor- discussed elsewhere (see Symbiosis), occurs ganic components of their environment, as well when an insect pollinates a plant while the plant as about biological communities, or the living provides the insect with nourishment, for components of an ecosystem. There are also dis- instance, in the form of nectar. These interac-
400 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 401
tions can and do affect the geographic distribu- Pioneers modify a site by their presence, for Succession tion of species, and the presence or absence of a instance, by regenerating the soil with organic and Climax particular life-form may serve as a powerful con- material, thus making the area more attractive for trol on the range of another organism. invasion by other species. Eventually, new species Other significant concepts in the realm of move in, edging out the pioneers as they do so. biogeography are dispersal (the spread of a This process may repeat itself several times, until species from one region to another), and barriers the ecosystem reaches the climax stage, which we (environmental factors that act to block disper- examine in greater depth a bit later in this essay. At the climax stage, there are few biological sal). A species may extend its geographic range by “openings” for further change, and change is only gradually colonizing, or taking over, adjacent very slight and slow—at least until another dis- areas, or it may cross a barrier (for instance, a turbance arises and starts the process over again. mountain range, an ocean, or a desert) and colo- nize the lands beyond. Later, we briefly examine THE TOLERANCE MODEL. The the case of a bird that managed to do both. tolerance model is another possible mechanism of succession. According to this concept, all Succession species involved in succession are equally capable of establishing themselves on a recently dis- Succession is the progressive replacement of ear- turbed site, but those capable of attaining a large lier biological communities with others over population size quickly are most likely to become time. It entails a process of ecological change, dominant. Unlike the facilitation model, the tol- whereby new biotic communities replace old erance model does not depict earlier inhabitants ones, culminating in a stable ecological system as preparing the site biologically for new invader known as a climax community. In a climax com- species; rather, this model is more akin to natural munity, climate, soil, and the characteristics of selection, discussed elsewhere (see Evolution). the local biota (the sum of all plants and animals) According to the tolerance model, some are all suited to one another. species will prove themselves more tolerant of At the beginning of the succession process, a biological stresses that occur within the environ- preexisting ecosystem undergoes some sort of ment as succession proceeds. Among these stress- disturbance—for example, a forest fire. This is es is competition, and those species less tolerant followed by recovery, succession, and (assuming of competition may succeed earlier on, when there are no further significant disturbances) cli- there is little competition for resources. Later in max. If the environment has not been modified the succession process, however, such species will previously by biological processes, meaning that be eliminated in favor of others more capable of succession takes place on a bare substrate, such as competing. a sand dune or a dry riverbed, it is known as pri- THE INHIBITION MODEL. Yet mary succession. Primary succession also occurs another model of succession is the inhibition when a previous biological community has been model, which, like the tolerance model, starts obliterated. Secondary succession takes place on with the premise of an open situation at the out- a substrate that has been home to other life- set: in other words, all species have equal oppor- forms and usually in the wake of disturbances tunity to establish populations after a distur- that have not been so sweeping that they pre- bance. In the inhibition model, however, some of vented the local vegetation from regenerating. the early species actually make the site less suit- THE FACILITATION MODEL. able for the development of other species. An Whether the conditions are those of primary or example of this is when plants secrete toxins in secondary succession, the outcome of the preced- the soil, thus inhibiting the establishment and ing disturbance is such that resources are now growth of other species. Nevertheless, in time the widely available, but there is little competition inhibitory species die, thus creating opportunities for them. One way of describing this situation is that can be exploited by later successional species. through what is known as the facilitation model, There is evidence to support all three mod- which identifies “pioneer species” as those life- els—facilitation, tolerance, and inhibition—but forms most capable of establishing a presence on just as each has a great deal of basis in fact, none the site of the disturbance. of the three fully depicts the dynamics of a suc-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 401 set_vol3_14 8/19/02 11:34 AM Page 402
Succession max stage, resources have been allocated almost and Climax completely among the dominant life-forms. Despite its slow rate of change, the climax community is not a perfectly static or unchang- ing one, because microsuccession (succession on the scale of a single tree, or a stand of trees) is always taking place. In fact, frequent enough events of disturbance within small sections of the biological community may prevent climax from even occurring. Once the biota does achieve a state of equilibrium with the environment, how- ever, it is likely that change will slow down con- siderably, bringing an end to the stages of succes- sion. Climax remains a somewhat theoretical notion, and in practice it may be difficult to iden- tify a climax community.
REAL-LIFE APPLICATIONS Colonization and Island Bio- geography
CATTLE EGRET AND WILD HORSE IN SOUTHERN FRANCE. DURING THE NINETEENTH CENTURY, THE OLD WORLD Earlier, in the context of biogeography, there CATTLE EGRET MANAGED TO CROSS THE ATLANTIC AND was a reference to animals “colonizing.”This may FOUND A BREEDING COLONY IN BRAZIL. SINCE THEN, sound like the behavior of humans only, but IT HAS EXPANDED THE RANGE OF ITS HABITATS, SO THAT COLONIES CAN BE FOUND AS FAR NORTH AND EAST AS other animals are also capable of colonizing. Nor ONTARIO AND AS FAR SOUTH AND WEST AS SOUTHERN are humans the only creatures that crossed the CHILE. (© Hellio/Van Ingen. Photo Researchers. Reproduced by per- Atlantic Ocean from the Old World to colonize mission.) parts of the New World. During the nineteenth century, an Old World bird species known as the cessional environment. Put another way, each is cattle egret managed to cross the Atlantic, per- fully right in one particular instance, but none haps driven by a storm, and founded a breeding are correct in all circumstances. Facilitation colony in Brazil. Since then, it has expanded the seems to work best for describing primary suc- range of its habitats, so that cattle egret colonies cession, whereas the more intense, vigorous envi- can be found as far north and east as Ontario and ronment of secondary succession is best pictured as far south and west as southern Chile. by tolerance or inhibition models. All of this THE BIOGEOGRAPHY OF ISO- shows that succession patterns tend to be idio- LATED BIOMES. Colonization is one syncratic, owing to the many variables that deter- example of the phenomena studied within the mine their character. realm of biogeography. Other examples involve Climax islands, and, indeed, island biogeography is a sig- nificant subdiscipline. The central idea of island When a biological community reaches a position biogeography, a discipline developed in 1967 by of stability and is in equilibrium with environ- American biologists R. H. MacArthur mental conditions, it is said to have reached a (1930–1972) and Edward O. Wilson (1929–), is state of climax. Often such communities are that for any landmass a certain number of species described as old growth, and in these situations can coexist in a state of equilibrium. The larger change takes place slowly. Dominant species in a the size of the landmass, the larger the number of climax community are those that are highly toler- species. Thus, reasonably enough, a large island ant of the biological stresses that come with com- should have a great number of species, whereas a petition. And well they might be, since by the cli- small one should support only a few species.
402 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:34 AM Page 403
These principles have helped in the study of Ardennes during the world wars nor logging in Succession other “island” ecosystems that are not necessarily the Amazon valley in the late twentieth century and Climax on islands but rather in or on isolated lakes, managed to destroy those biomes, but it is quite mountain ranges surrounded by deserts, and conceivable that they could have. On the other patches of forest left behind by clear-cut logging. hand, a disturbance can affect an individual life- As a result of such investigations, loggers in the form, as when lightning strikes and kills a mature forests of the Pacific Northwest or the Amazon val- tree in a forest, creating a gap that will be filled ley have been encouraged to leave behind larger through the growth of another tree—an example stands of trees in closer proximity to one another. of microsuccession. This makes possible the survival of species at FORMS OF SUCCESSION. Once higher trophic levels (positions on the food web) succession begins, it can take one of several and of those with very specialized requirements courses. It may lead to the restoration of the as to food or habitat. Examples of the latter ecosystem in a form similar to that which it took species include Amazonian monkeys and the before the disturbance. Or, depending on envi- northern spotted owl, which we discuss later. ronmental circumstances, a very different ecosys- Because studies in island biogeography have tem may develop. For example, suppose that a made land-use planners more aware of the barri- forest fire has wiped out a biological community ers posed by clear-cut foresting, it has become and secondary succession has begun. It is con- common practice to establish “forest corridors.” ceivable that this succession process will restore These long, thin lines of trees connecting sec- the forest to something approaching its former tions of forest ensure that one section will not be state. On the other hand, the wildfire itself may isolated completely from another. well have been a signal of a climate change, in this case, to a drier, warmer environment. In this Succession in Action instance, succession may bring about a commu- nity quite different from that which preceded the In discussing succession earlier, it was noted that disturbance. a disturbance usually sets the succession process in motion. Examples of such disturbances can The “disturbance” itself actually may be the include seismic events (earthquakes, tidal waves, alleviation of a long-term environmental stress or volcanic eruptions) and weather events (hur- that has plagued the community. Suppose that a ricanes or tornadoes). Across larger geologic biological community has suffered from a local timescales, the movement of glaciers or even of source of pollution, for instance, from a factory plates in Earth’s crust (see Paleontology for more dumping toxins into the water supply. Suppose, about plate tectonics and its effect on environ- too, that pressure from state or federal authori- ments) can set succession processes in motion. ties finally forces a cleanup. How does this affect the biotic environment? In all likelihood, species There are also causes directly within the bios- that are sensitive to pollution (i.e., ones that nor- phere, or the realm of all life, that can bring about mally could not survive in polluted conditions) disturbances. Among them are wildfires as well as would invade the area. sudden infestations of insects that act to defoliate, or remove the leaf cover from, a mature forest. Removal of an environmental stress may not Quite a few disturbances can result from activities always be a matter of pollution and cleanup. For on the part of the biosphere’s most complex instance, a herd of cattle may be overgrazing a species: Homo sapiens. Humans can cause ecolog- pasture, thus holding back the growth of plant ical disturbances by plowing up ground, by har- species in the area. Imagine, then, that the cattle vesting trees from forests, by bulldozing land for are moved elsewhere; as a result, new plant construction purposes—even by causing explo- species will proliferate in the area, and, in all like- sions on a military reservation or battlefield. lihood, the biological diversity of that particular ecosystem will increase. Disturbances can take place on a grand scale or a small scale. It is theoretically possible for dis- Primary Succession turbances—even man-made ones—to wipe out forests as large as the Ardennes in northwestern As we noted earlier, primary succession occurs in Europe or the Amazon rain forest in South an environment where there has never been a sig- America. Fortunately, neither shelling in the nificant biological community or in the wake of
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 403 set_vol3_14 8/19/02 11:35 AM Page 404
Succession and Climax
GLACIER BAY, ALASKA, IS AN EXAMPLE OF AN ECOSYSTEM THAT EXPERIENCED PRIMARY SUCCESSION IN THE WAKE OF DEGLACIATION; AS THE GLACIERS MELTED, VARIOUS PLANTS MOVED IN, EACH TAKING ITS TURN AS DOMINANT SPECIES. THE HABITAT NOW HAS REACHED MATURITY, AND ACCESS TO RESOURCES IS ALLOCATED AS FULLY AS IT CAN BE AMONG THE DOMINANT SPECIES. (© Pat O’Hara/Corbis. Reproduced by permission.)
disturbances that have been intense enough to has not possessed a biological community would wipe out all traces of a biological community. An be an abandoned paved parking lot. Eventually, example of primary succession in an area that the asphalt would give way to plant life, and given
404 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:35 AM Page 405
Succession and Climax
ISRAELI FARMERS TRY TO WARD OFF A SWARM OF LOCUSTS. DEFOLIATION BROUGHT ABOUT BY INSECTS IS ONE EXAM- PLE OF THE TYPE OF DISTURBANCE THAT MAY SERVE AS A PRECURSOR TO SECONDARY SUCCESSION. (© Hulton-Deutsch Collection/Corbis. Reproduced by permission.)
enough time, a wide-ranging biological commu- soil. (See The Biosphere for more about nitrogen nity might develop around it. fixing and biogeochemical cycles.) These statements should be qualified in two These were the pioneer species, and over ways, however. When it is said that an area has time they were replaced by larger plants, such as a not maintained a significant biological commu- short version of the willow. Later, taller shrubs, nity, this refers only to the recent or relatively such as the alder (also a nitrogen-fixing species), recent past. In the case of the parking lot area, dominated the area for about half a century. In there probably have been countless biological time, Sitka spruce (Picea sitchensis), western hem- communities in that spot over the ages, each lock (Tsuga heterophylla), and mountain hemlock replaced by the other in a process of primary suc- (T. mertensiana) each had its turn as dominant cession. Also, by significant biological community plant species. With the last group, Glacier Bay we mean a biological community that exists reached climax, meaning that the dominant above ground; even in the instance of the parking species are not those most tolerant of stresses lot, there would be an extensive biological com- associated with competition. The habitat thus has munity underground. (See The Biosphere for reached maturity, and access to resources is allo- more about life in the soil.) cated as fully as it can be among the dominant species. Accompanying these changes have been GLACIER BAY. Glacier Bay, in south- changes in nonliving parts of the ecosystem as ern Alaska, is an example of an ecosystem that well, including the soil and its acidity. experienced primary succession in the wake of deglaciation, or the melting of a glacier. The gla- Secondary Succession ciers there have been melting for at least the past few hundred years, and as this melting began to When a disturbance has not been so intense or occur, plants moved in. The first were mosses and sweeping as to destroy all life within an ecosys- lichens, flowering plants such as the river-beauty tem, regeneration may occur, bringing about sec- (Epilobium latifolium), and the mountain avens ondary succession. But regeneration of existing (Dryas octopetala), noted for their ability to “fix” species is not the only mechanism that makes or transform nitrogen into forms usable by the secondary succession possible; invasions by new
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 405 set_vol3_14 8/19/02 11:35 AM Page 406
Succession plant species typically augment the succession large numbers; if they are to obtain a stake in the and Climax process. While much else changes in the environ- secondary succession, they must do so by a ment of a secondary succession, the quality of the process of re-invasion. Such often happens in the soil itself remains constant, as do other charac- case of coniferous trees. Other species may also teristics, such as climate. invade when they have not previously been a part Because it is rare for a disturbance to be of the habitat, yet they enter now because the powerful enough to obliterate all preexisting life- temporary conditions of resource availability forms, secondary succession is much more com- and limited competition make the prospect for mon than primary succession. Examples of the invasion attractive. A great number of species, type of disturbance that may serve as a precursor from alders and white birch to various species of to secondary succession are windstorms, wild- grasses, fit into this last category. fires, and defoliation brought about by insects— Plants are not the only organisms involved provided, of course, that the destruction caused in secondary succession. In a mature forest of by these phenomena is less than total. The same the type described, the dominant bird forms is true of most disturbances associated with probably include species of warblers, vireos, human activities, such as the abandonment of thrushes, woodpeckers, and flycatchers. When agricultural lands and the harvesting of forests by clear-cutting occurs, however, these birds are cutting down trees for lumber or pulp. likely to be replaced by an entirely new avian In a forest of mixed species in the eastern community—one composed of birds more suit- United States, the dominant trees are a mixture ed to the immature habitat that follows a distur- of angiosperms and coniferous species (respec- bance. As time passes, however, and the forest tively, plants that reproduce by producing flowers regenerates fully, the bird species of the mature and those that reproduce by producing cones forest re-invade and resume dominance, a bearing seeds), and there are plant species capa- process that may well take three to four decades. ble or surviving under the canopy, or “roof,”pro- Old-Growth Forests vided by these trees. Suppose that the forest has been clear-cut. This means that most or all of the Old-growth forests represent a climax ecosys- large trees have been removed, but the entire bio- tem—one that has come to the end of its stages of logical community has not been wiped out, since succession. They are dominated by trees of loggers typically would not bother to cut down advanced age (hence the name old-growth), and smaller plants that are not in their way. the physical structure of these ecosystems is As soon as the clear-cutting is over, regenera- extraordinarily complex. In some places, the for- tion begins. One form that this takes is the forma- est canopy is dense and layered, whereas in others tion of new sprouts from the stumps of the old it has gaps. Tree sizes vary enormously, and the angiosperms. These sprouts are likely to grow rap- forest is littered with the remains of dead trees. idly and then experience a process of self-thinning, An old-growth forest, by definition, takes a in which only the hardiest shoots survive. Within long time to develop. Not only must it have been half a century, a given tree will have only one to free from human disturbance, but it also must three mature stems growing from its stump. have been spared various natural disturbances of At the same time, other species regenerate the kind that we have mentioned, disturbances seemingly from nowhere, though actually they that bring about the conditions for succession. are growing from a “seed bank” buried in the for- Typically, then, most old-growth forests are rain est floor, where trees have dropped countless forests in tropical and temperate environments, seeds over the generations. Species such as the where they are unlikely to suffer such stresses as pin cherry (Prunus pennsylvanica) and red rasp- drought and wildfire. Among North American berry (Rubus strigosus) are particularly adept at old-growth forests are those of the United States regeneration in this form. Therefore, these Pacific Northwest as well as in adjoining regions species are likely to feature prominently in the of southwestern Canada. forest during the first several decades of second- THE SPOTTED OWL. These old- ary succession. growth forests of North America are home to a On the other hand, some tree species simply bird that became well known in the 1980s and do not survive clear-cutting, or at least not in 1990s to environmentalists and their critics: the
406 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:35 AM Page 407
Succession KEY TERMS and Climax
ABUNDANCE: A measure of the ongoing succession. In such a situation, the degree to which an ecosystem possesses community is at equilibrium with environ- large numbers of particular species. An mental conditions, and conditions are sta- abundant ecosystem may or may not have ble, such that the biota experiences little a wide array of different species. Compare change thereafter. with complexity. COMPETITION: Interaction between ANGIOSPERM: A type of plant that organisms of the same or different species produces flowers during sexual reproduc- brought about by their need for a common tion. resource that is available in quantities BIOGEOGRAPHY: The study of the insufficient to meet the biological demand. geographic distribution of plants and ani- COMPLEXITY: The range of ecological mals, both today and over the course of niches within a biological community. The extended periods. degree of complexity is the number of dif- ferent species that could exist, in theory, in BIOLOGICAL COMMUNITY: The liv- a given biota, as opposed to its diversity, or ing components of an ecosystem. the actual range of existing species. BIOME: A large ecosystem, character- CONIFER: A type of tree that produces ized by its dominant life-forms. cones bearing seeds. BIOSPHERE: A combination of all liv- DIVERSITY: A measure of the number ing things on Earth—plants, animals, of different species within a biological birds, marine life, insects, viruses, single- community. cell organisms, and so on—as well as all formerly living things that have not yet ECOSYSTEM: A community of inter- decomposed. dependent organisms along with the inor- ganic components of their environment. BIOTA: A combination of all flora and fauna (plant and animal life, respectively) FOOD WEB: A term describing the in a region. interaction of plants, herbivores, carni- vores, omnivores, decomposers, and detri- BIOTIC: Life-related. tivores in an ecosystem. Each of these CANOPY: The upper portion or layer of organisms consumes nutrients and passes the trees in a forest. A forest with a closed it along to other organisms. Earth scientists canopy is one so dense with vegetation that typically prefer this name to food chain, an the sky is not visible from the ground. everyday term for a similar phenomenon. CLIMATE: The pattern of weather con- A food chain is a series of singular organ- ditions in a particular region over an isms in which each plant or animal extended period. Compare with weather. depends on the organism that precedes it. CLIMAX: A theoretical notion intended Food chains rarely exist in nature. to describe a biological community that FOREST: In general terms, a forest is has reached a stable point as a result of simply any ecosystem dominated by tree-
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 407 set_vol3_14 8/19/02 11:35 AM Page 408
Succession and Climax KEY TERMS CONTINUED
size woody plants. Numerous other char- OLD-GROWTH: An adjective for a cli- acteristics and parameters (for example, max community. weather, altitude, and dominant species) SUCCESSION: The progressive re- further define types of forests, such as placement of earlier biological communi- tropical rain forests. ties with others over time. Succession, MICROSUCCESSION: Succession on which can culminate in a climax commu- a very small scale within a larger ecosystem nity (see climax), is either primary, which or biological community. Microsuccession occurs where there is no preexisting bio- can occur at the level of a stand of trees or even a single tree. logical community (or no such communi- ty has survived), or secondary, in which a NATURAL SELECTION: The process biological community regenerates in the whereby some organisms thrive and others perish, depending on their degree of adap- wake of a disturbance, such as a forest fire. tation to a particular environment. TROPHIC LEVELS: Various stages NICHE: A term referring to the role within a food web. For instance, plants are that a particular organism plays within its on one trophic level, herbivores on anoth- biological community. er, and so on.
northern spotted owl, or Strix occidentalis cauri- economic, and political concerns in such situa- na. A nonmigratory bird, the spotted owl has a tions—continues. breeding pattern such that it requires large tracts CONTINUING CONTROVERSY. of old-growth, moist-to-wet conifer forest as its Another concern raised by the logging of old- habitat. These are the spotted owl’s environmen- growth forests has been the need to preserve dead tal requirements, but given the potential eco- trees, which provide a habitat for woodpeckers nomic value of old-growth forests in the region, and other varieties of species. This concern, too, the situation was bound to generate heated con- has brought about conflict with loggers, who find troversy as the needs of the spotted owl clashed that dead wood gets in the way of their work. with those of local humans. Dead wood, after all, is an expression for some- On the one hand, environmentalists insisted thing or someone that is not performing a useful that the spotted owl’s existence would be threat- function (as in, “We’re removing all the dead ened by logging, and, on the other, representa- wood from the team”), and to loggers this literal tives of the logging industry and the local com- dead wood is nothing more than a nuisance. munity maintained that prevention of logging in Unfortunately, the United States logging the old-growth forests would cost jobs and liveli- industry typically has not pursued a strategy of hoods. The question was not an easy one, pitting attempting to manage old-growth forests as a the interests of the environment against those of renewable resource, which these forests could be, ordinary human beings. By the early 1990s the given enough time. Instead, logging compa- federal government had stepped in on the side of nies—interested in immediate profits and not the environmentalists, having recognized the much else—have tended to treat old-growth spotted owl as a threatened species under the forests as though they were more like coal mines, terms of the U.S. Endangered Species Act of home of a nonrenewable resource. In this “min- 1973. Even so, controversy over the spotted ing” model of tree harvesting, the forest is owl—and over the proper role of environmental, allowed to experience a process of succession
408 VOLUME 3: REAL-LIFE BIOLOGY SCIENCE OF EVERYDAY THINGS set_vol3_14 8/19/02 11:35 AM Page 409
such that a younger, second-growth forest removing only certain trees. Many environmen- Succession emerges. Over time, this might become an old- talists contend, however, that even the new and Climax growth forest, but the need to turn a quick prof- forestry disturbs the essential character of old- it means that the forest likely will be cut down growth forests. before that time comes. The average citizen, who typically has no WHERE TO LEARN MORE vested interest in the side of either the loggers or Browne, E. J. The Secular Ark: Studies in the History of the environmentalists, might well find good and Biogeography. New Haven: Yale University Press, bad on both sides of the issue. Certainly, the 1983. image of radical environmentalists chaining Cox, C. Barry, and Peter D. Moore. Biogeography: An Eco- themselves to trees is as distasteful as the idea of logical and Evolutionary Approach. Malden, MA: loggers removing valuable natural resources. Blackwell Science, 2000. There is also a class dimension to the struggle, The Eastern Old Growth Clearinghouse (Web site).
SCIENCE OF EVERYDAY THINGS VOLUME 3: REAL-LIFE BIOLOGY 409