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Q What can teach us about the ? This man, known only as Tollund Man, died about 2200 ago in what is now Denmark. Details such as his skin and were preserved by the acid of the bog in which he was found. A bog is a type of wetland that accumulates peat, the deposits of dead material. Older remains from bogs can add information to the fossil record, which tends to consist mostly of hard shells, teeth, and bones.

RE a d IN G T o o l b o x This reading tool can help you learn the material in the following pages.

USING Your Turn Describing Time Certain words and phrases can help Read the sentences below, and write the specific time you get an idea of a past ’s time frame (when it markers. happened) and (for how long it happened). 1. Jennifer celebrated her 16th birthday on Saturday two These phrases are called specific time markers. Specific ago. time markers include phrases such as 1 , , 2. became extinct about 65 million years ago, the 20th century, and 30 years later. at the end of the Period.

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12.1 The Fossil Record

Key Concept VOCABULARY Fossils are a record of life that existed in the past. MAIN IDEAS Fossils can form in several ways. half-life Radiometric dating provides a close estimate of a fossil’s .

Connect to Your Do you ever consider how much the world and its inhabitants have changed in the past 15, 50, or 100 years? Have you studied ancient civilizations that existed thou- sands of years ago? These time frames are tiny blips on the scale of time revealed by the fossil record. Some of the world’s oldest fossils, found at the Burgess site in , offer a glimpse of what life was like 500 million years before Tollund Man lived. These specimens are keys to understanding the history of life on .

MAIN IDEA Fossils can form in several ways. Fossils are far more diverse than the giant we see in museums. The following processes are some of the ways fossils form. Figure 1.1 shows examples of fossils produced in these different ways. • occurs when carried by are deposited around a hard structure. They may also replace the hard structure itself. • Natural casts form when flowing water removes all of the original bone or , leaving just an impression in sediment. Minerals fill in the mold, recreating the original shape of the . • Trace fossils record the activity of an organism. They include nests, burrows, imprints of leaves, and footprints. • -preserved fossils are organisms that become trapped in resin FIGURE 1.1 The fossil record that hardens into amber after the tree gets buried underground. includes fossils that formed in • Preserved remains form when an entire organism becomes encased in many different ways. material such as ice or volcanic ash or immersed in bogs. (l) ©Louis Bergman/Corbis; Psihoyos/Corbis; (cl) ©Jim Jurica/ShutterStock; (c) V. ©Lester (cr) ©Colin Keates/Dorling Kindersley/Getty Images; (r) ©Corbis

Permineralized Natural cast of a , Trace fossils of footprints Amber‑preserved spider Ice‑preserved of a Veloci- a marine from a Dimetrodon dinosaur 5000‑‑old remains raptor dinosaur of a man found in the Italian Alps

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FIGURE 1.2 the Process of Permineralization The process of permineralization requires rapid burial in an area with water and continuous sedimentation.

An organism dies in a location, such as a Over time, pressure from additional Earthquakes or may expose the riverbed, where sediments can rapidly sediment compresses the body, and min- fossil millions of years after formation, or cover its body. erals slowly replace all hard structures, it may be uncovered by paleontologists, such as bone. hikers, or road-building crews.

Infer What conditions could occur that would prevent an organism from being preserved through permineralization?

Most fossils form in sedimentary , which is made by many layers of sediment or small rock . The best environments for any type of fossilization include wetlands, bogs, and areas where sediment is continuously deposited, such as river mouths, lakebeds, and floodplains.

The most common fossils result from permineralization. Several circum- R E ADI N G TO OLB ox stances are critical for this process, as shown in FIGURE 1.2. The organism must be buried or encased in some type of material—such as sand, sediment, mud, TAKING NOTES Make a cause-and-effect chain or tar—very soon after , while the organism’s features are still intact. of the conditions required for After burial, groundwater trickles into tiny pores and in , bones, fossilization. Fill in important and shells. During this process, the excess minerals in the water are deposited details. on the remaining cells and tissues. Many layers of deposits are left An organism dies and is behind, creating a fossilized record by replacing organic tissues with hard encased in mud. Cause minerals. The resulting fossil has the same shape as the original structure and

may contain some original tissue. Effect With such specific conditions needed for fossilization, it is easy to see why only a tiny percentage of living things that ever existed became fossils. Cause Most remains decompose or are destroyed before they can be preserved. Even Effect successful fossilization is no guarantee that an organism’s remains will be added to the fossil record. Natural events such as earthquakes and the recycling of Cause rock into magma can destroy fossils that took thousands of years to form. Summarize Why are so few complete fossils discovered?

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MAIN IDEA Radiometric dating provides a close estimate of a fossil’s age.

CONNECT TO Recall that in the 1700s had realized that rock layers at the bottom of an undisturbed sequence of rocks were deposited before those at the top, Chemistry of Life and therefore are older. The same logic holds true for the fossils found in rock Recall from the chapter Chemistry of Life that all layers. Relative dating estimates the time during which an organism lived by of a given element have the comparing the placement of fossils of that organism with the placement of same of protons. fossils in other layers of rock. Relative dating allows scientists to infer the are named for the total number of protons and in which groups of existed, although it does not provide the in their nuclei. actual ages of fossils.

neutrons protons To estimate a fossil’s actual, or absolute, age, scientists use radiometric dating—a technique that uses the natural decay rate of unstable isotopes found in materials in order to calculate the age of that material. Isotopes are atoms of an element that have the same number of protons but a different number of neutrons. Most elements have several isotopes. For -12 CARBON-14 example, the element carbon (C) has three naturally occurring isotopes. All NUCLEUS NUCLEUS 6 protons 6 protons carbon isotopes have six protons. Isotopes are named, however, by their 12 6 neutrons 8 neutrons number of protons plus their number of neutrons. Thus, carbon-12 ( C) has six neutrons, carbon-13 (13C) has seven neutrons, and carbon-14 (14C) has eight neutrons. More than 98 percent of the carbon in a living organism is 12C. Some isotopes have unstable nuclei. As a result, their nuclei undergo radioactive decay—they break down—over time. This releases radiation in the form of particles and . As an isotope decays, it can transform into a different element. The decay rate of many radioactive isotopes has been measured and is expressed as the isotope’s half-life, as shown in FIGURE 1.3. A half-life is the amount of time it takes for half of the isotope in a sample to decay into a different element, or its product isotope. An element’s half-life is not affected by environmental conditions such as temperature or pressure. Both 12C and 13C are stable, but 14C decays into nitrogen-14 (14N), with a half‑life of roughly 5700 years. Radiocarbon Dating The isotope 14C is used commonly for radiometric dating of recent remains, such as those of Tollund Man shown at the beginning of this chapter. - isms absorb carbon through eating and breathing, so 14C is constantly being resupplied. When an organism dies, its intake of carbon stops, but the decay of 14C continues. The fossil’s age can be estimated by FIGURE 1.3 DECAY OF ISOTOPES comparing the ratio of a stable isotope, such as 12C, to 14 Isotope (parent) Product (daughter) Half-life (years) C. The longer the organism has been dead, the larger the difference between the amounts of 12C and 14C -87 strontium-87 48.8 billion there will be. The half-life of carbon-14 is roughly 5700 14 -238 -206 4.5 billion years, which means that after 5700 years, half of the C in a fossil will have decayed into 14N, its decay product. chlorine-36 -36 300,000 The other half remains as 14C. After 11,400 years, or carbon-14 nitrogen-14 5730 two half-, 75 percent of the 14C will have decayed.

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FIGURE 1.4 Radiometric Dating Using Carbon-14 Radiometric dating uses the natural decay rate of unstable isotopes to calculate the age of a fossil. 100 Percent of carbon-14 Percent of nitrogen-14 1 half-life C remaining C remaining 14 50 2 half-lives 3 half-lives 4 half-lives

Percent of Percent 25 12.5 6.25 0 5730 11,460 17,190 22,920 Time passed in years

14 14 One-quarter of the original C remains. Radioactive decay of C is shown Web in FIGURE 1.4. Carbon-14 dating can be used to date objects only up to about 45,000 years old. If the objects are older than that, the fraction of 14C will be HMDScience.com too small to measure accurately. Older objects can be dated by using an GO ONLINE isotope that has a longer half-life, such as uranium. Geologic Dating Determining Earth’s Age Scientists have used radiometric dating to determine the . Because Earth constantly undergoes erosion and rock recycling, rocks on Earth do not remain in their original state. Unlike Earth’s rocks, —which are mostly pieces of rock and that have fallen to Earth’s surface from — do not get recycled or undergo erosion. Meteorites are thought to have formed at about the same time as Earth. Therefore, meteorites provide an unspoiled sample for radiometric dating. Uranium-to-lead isotope ratios in many samples consistently estimate Earth’s age at about 4.5 billion years. Summarize Why are meteorites helpful for determining the age of Earth?

Self-check Online HMDScience.com 12.1 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. What types of evidence of ancient 3. Apply Considering that millions of Earth life can be preserved as fossils? species have lived on Earth, why are 5. When mountains form, 2. In radiometric dating, why is a there relatively few fossils? the order of rock layers can uranium isotope often used instead 4. Contrast Explain the difference be disturbed. How could of 14C to determine the age of Earth? between relative dating and radiometric dating be used absolute dating. to sort out the relative ages of such rock layers?

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When WAS THAT?

VIRTUAL Lab Web BIOLOGY Comparing Hominoid Geologic Dating Endosymbiosis Skulls Examine hominoid How do scientists decide which type endosymbiosis in action to skulls to determine how of geologic dating to use for a sample? discover how mitochondria modern are Decide which method of geologic and likely similar to chimps and dating would be best to date two evolved. . different types of fossil samples. ©James L. Amos/Corbis ©James L.

online biology 352 HMDScience.com CorrectionKey=B DO NOT EDIT--Changes mustbemadethrough “File info”

©Ron Sturm period oftime. and representative ofaspecific sediment, widely distributed, They are abundantinmarine wide, are good index fossils. usual FIGURE 2.1 period scale time geologic index fossil VOCABULARY 12.2 ly less than2millimeters

Fusulinids, tinyfossils The Geologic TimeScale based onmajorpastevents. age ofrock layers. Index fossils tool the are to another determine units based ontheoccurrence ofmajorgeologic changes. years.about 4.5billion Scientistshave dividedtheEarth’s progress into manageable school graduation, each new year isastep inyour life development. Earth’s life spans Life ismarked by increments ofprogress. From your first year ofschool through high Connect to Your World Ke Apply

y the samethe period. time presence of organisms both inone layer shows that lived they during are for useful dating fossils of other organisms instrata, the because must 251million and between be 359million years old. Fusulinid fossils 251 million years ago. The presence of fusulinids indicates that arock layer common, very time but disappeared after amass event about the agethe of fossils or strata the are inwhich they found. to results. confirm Index fossils provide an additional tool for determining determine age the of arock layer almost always two use or more methods scientists determine age the of rock layers. Scientists to are who trying You have that learned relative both dating and radiometric dating can help shown in marineThe extinct known as fusulinids (fyoo found widely around world, the and existed only for arelatively brief time. strata correlated. can be The index best fossils are common, to easy identify, was exposed. as markers, Smith layer could identifyaparticular of rock wherever it layers contained fossils inother those unlike layers. Using key fossils these latethe 1700s,English William Smith discovered that rock certain geographic areas. of organisms that existed only spans during of specific over time large MAI MAI C

o The s Usin The geologic timescale organizes Earth’s history. Index fossils are another tool to determine theage ofrock layers.

N Could a rock layer with fusulinid fossils be 100 million years old? million 100 be fossils fusulinid layer arock with Could n N ID c ID g index fossils for age estimates of rock layers is not In idea. anew ep horter life the span of more the a species, precisely different the E E FIGURE 2.1, t A AS

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FIGURE 2.2 Unit 4:Evolution magnification 50 fossils. (colored SEM; and theoldest animal the originofeukaryotes, known rocks andfossils, includes theoldest of Earth’s history. It up thevast majority This timespanmakes p r e cambria 2000 1000 250 550 Millions of years ago (mya) 100

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m e X X X X X X = Majorextinction CR Me and diversified. evolved. 65–1.8 mya T modern forms oflife. 1.8 mya– QUAT c short periodoftime knownshort astheCambrian Explosion. 542–488 mya CAMBRIA drop anda mass extinction ofmarinelife to occur. earliest . Massive glaciers formed, causing levels to 488–444 mya ORDOVICIA to form. Jawless andfreshwater fishesevolved. 444–416 mya SILURIA appeared. Firstferns, , andforests arose. 416–359 mya D ians, winged , early , andsmall reptiles. swamps. Fishcontinued to diversify. Life forms includedamphib- 359–299 mya CARBO was formed asmajorlandmasses joinedtogether. 299–251 mya PE Pal flying reptiles () arose. evolved,saurs asdidplants suchasferns andcycads. and 251–200 mya pe common today. were offishandsquid. full Firstbirds arose. 200–145 mya plants arose. Birds survived to radiate intheTertiary period.Flowering 145–65 mya E en E RMIA RTIARY VO E so e TAC o o N E z z z le R N oic oic IA E oic N N N IF OUS

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MAIN IDEA The organizes Earth’s history. The geologic time scale, shown in Figure 2.2, is a representation of the history Biology VIDEO of Earth. It organizes Earth’s development by major changes or events that CLIP have occurred, using evidence from the fossil and geologic records. Scientists HMDScience.com worked out the entire geologic time scale during the 1800s and early 1900s. GO ONLINE Although the scale is still being changed a little bit here and there, the main Geologic Time divisions of geologic time have stayed the same for over a hundred years. The time scale is divided into a series of units based on the order in which different groups of rocks and fossils were formed. The geologic time scale consists of three basic units of time. • last tens to hundreds of millions of years and consist of two or more periods. • Periods are the most commonly used units of time on the geologic time scale, lasting tens of millions of years. Each period is associated with a particular type of rock system. • Epochs (EHP-uhks) are the smallest units of geologic time and last several million years. The names of the eras came from early ideas about life forms preserved CONNECT TO as fossils. means “ancient life,” means “middle life,” and means “recent life.” Within the eras, the boundaries between many Recall from the chapter The of the geologic periods are defined by mass extinction events. These events Evolution of Populations that help to define when one period ends and another begins. The largest adaptive adaptive radiation refers to the change of a single species into tend to follow large mass . Recall that adaptive radiation several forms that are each happens when a group of organisms diversifies into several species. Those adapted to a specific species adapt to different ecological niches because mass extinctions make environmental niche. many niches available. Over generations, the adaptive traits favored within these newly opened niches may become common for that population of organisms, and may occur. Summarize Why do adaptive radiations often occur after mass extinctions?

Self-check Online HMDScience.com GO ONLINE 12.2 Formative Assessment CONNECT TO Reviewing Main Ideas Critical thinking Scientific Process 1. How are index fossils used to date 3. Infer The most common index fossils 5. French physicist Henri rock layers? are shells of invertebrates. Give two Becquerel discovered 2. What is the usefulness of reasons why this is so. radioactivity in 1896, after categorizing Earth’s history 4. Analyze Scientists have inferred geologists had developed into the geologic time scale? that there have been at least five the geologic time scale. How mass extinctions in Earth’s history. did Becquerel’s discovery How would fossil evidence support help later geologists as they this inference? refined the time scale? 3f

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about 4.6 billion yearsabout 4.6billion ago. rotating diskofgas anddust formed from a propo figure nebula VOCABULARY replicating life as theDNAmolecule for self- molecules having information such organization into long complex simple organic molecules andtheir evidence r Unit 4:Evolution ses that theSunand 9 3.1 D egarding formation of 12.3

analyze andevaluate the One hypothesis 9d Origin ofLife E as withanybranch ofscience, remain. questionsstill about theway formed Earth andlife began have beenproposed andresearched. But we go, thetougher itisto piece together what life was like at that time.Hypotheses By studyingthegeologic timescale, itisclear that thefarther back inEarth’s history Connect to Your World Ke arth was very different billionsofyears was different very ago.arth

y as and that inpools larger collected of water. bodies . The began to form. condensed and fell tion and produced for energy reactions and on early inthe Earth impacts less became frequent. That to down. cool allowed radia­ Solar Earth about 2billion years ago, after first forms the of life to had evolve. begun . Most scientists agree that free was not abundant until sphere containing compounds such as , water vapor, methane, and nitrogen were released from interior. the combined They to form an atmo­ materialsthese separated into Earth’s layers. , , and intense heat kept materials the inamolten making up Earth state. time, Over active decay of elements released heat trapped within deep as Earth well. This , the struck releasing enormous amounts of heat. Meanwhile, radio­ the nowa time the called eon. Many , meteorites, and system. repeated collisions of space this debris built up into planets the of our solar mained nebula’s inthe disk circled formed newly the millions . Over of years, material nebula inthe together pulled of because . Materials that re­ 4.6 billion years ago, sun the formed from anebula. time, most Over of the and observations made with Hubble the Space Telescope. It suggests that about solarthe system was formed by acondensing differentvery from of those today. years old, and conditions (2)the of early the planet and its atmosphere were Earth’s origins, most scientists agree on two key points: is billions (1)Earth of andorigin of its Earth living things. Despite differences over details the of For centuries, many of history’s greatest minds have wondered about the space, as shown in MAI MAI C

o Towa Tod Earth was most likelyEarth violent hot and for very its first 700million years, Several sets ofhypotheses propose how life began onEarth. wasEarth very different ofyears billions ago. N n N ID c ID ay, most the widely accepted hypothesis of Earth’s origins suggests that ept rd end the of Hadean the 4and eon, between 3.8billion years ago, E E A AS t he originoflife on FIGURE 3.1.

This hypothesis is supported by models E arth remainsarth apuzzle. nebula, acloud of gas and dust in

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Once liquid water was present, organic compounds could be formed from READING TOOLBox inorganic materials. All living is organic, as are the building blocks of life, such as sugars and amino acids. However, you’ll see below that the leap VOCABULARY Organic compounds are that resulted in life on Earth required conditions other than just the presence carbon based and contain of water. carbon–carbon bonds. Summarize Describe the of Earth’s origin.

MAIN IDEA 9d Several sets of hypotheses propose how life began on Earth. Since the 1950s, scientists have proposed several hypotheses to explain how life began on Earth. These hypotheses have taken into consideration early organic molecules, the formation of organic from these organic building blocks, the evolution of structures, and early genetic material. Organic Molecule Hypotheses There are two general hypotheses about the way life-supporting molecules appeared on . Miller-Urey experiment In 1953 and designed an experiment to test a hypothesis first proposed by Alexander Oparin in the 1920s. Earlier scientists had proposed that an input of energy from lightning led to the formation of organic molecules from inorganic molecules present in the atmosphere of early FIGURE 3.2 Miller‑Urey Experiment Earth. Miller and Urey built a system to model A laboratory model was used to represent the conditions conditions they thought existed on early Earth, as of early Earth. This experiment demonstrated that shown in Figure 3.2. They demonstrated that organic molecules can be made from inorganic molecules. organic compounds could be made by heating and passing an electrical current, to simulate A boiling chamber was An electric spark in used to heat “” a mixture of lightning, through a mixture of gases. These water to simulated lightning. gases—methane (CH4), ammonia (NH3), hydro­ produce water gen (H ), and water vapor (H O)—were thought vapor. The 2 2 vapor traveled to be present in the early atmosphere. The Miller- through a tube to electrodes Urey experiment produced a variety of organic the “atmosphere.” “atmosphere” compounds, such as amino acids. After Miller’s death in 2007, scientists found water sealed vials from his early experiments that when tested, proved that more than 20 different amino acids had been formed. Since Miller’s experiments occurred, it has been suggested by some scientists that due to volcanic eruptions more than 4 billion “ocean” years ago, different compounds were present in The experiment produced the early atmosphere. Experiments using more simple organic molecules recent estimates of conditions on early Earth have heat source such as amino acids. also produced organic molecules, including amino acids amino acids and .

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Meteorite hypothesis Analysis of a meteorite that fell near Murchison, , in 1969 revealed that organic molecules can be found in space. More than 90 amino acids have been identified from this meteorite. Nineteen of these amino acids are found on Earth, and many others have been made in experi­ ments similar to the Miller-Urey study. This evidence suggests that amino acids could have been present when Earth formed, or that these organic molecules may have arrived on Earth through meteorite or impacts. Organic Hypotheses Once amino acids and other organic building blocks existed on early Earth, certain requirements would have to have been met in order for more complex organic polymers to form. Planetary conditions were still extreme, ranging from ice sheets to areas having high temperatures and extensive volcanic activ­ ity. Since there was no layer at that time, high levels of radiation would have permeated the atmosphere. All of these factors would have contributed to breaking apart the chemical bonds of organic molecules unless they were protected in some way. The survival of organic molecules would have depended on them either dissolving in water or adsorbing to some type of mineral. Frozen seawater hypothesis If you fill a container to the top and place it into your freezer, the ice that forms will expand beyond the rim of the container. This occurs as water molecules form into rigid . Because of this, a solution that is water-based will push any dissolved materials into the spaces between the crystals as it freezes. Recent research has shown that when solutions containing nucleotides are frozen, the nucleotides are pushed into the spaces between the water crystals. As Figure 3.3 illustrates, when ­nucleotides are ­concentrated into a tiny area, they can bind together and form long, complex molecules that can carry information. On the basis of these experiments, Scripps Oceanographic scientist Jeffrey Bada proposed in 2004 that biological polymers may have formed in sea ice on the early planet. adsorption hypothesis In the late 1950s and early 1960s, scientist Sidney Fox found that when dilute solutions of amino acids and cyanide were dripped onto hot, dry sand, rock, or clay, polymers that he called “proteinoids” were formed spontaneously. The metallic ions on the particles of clay acted as catalysts, binding to the monomers and concentrat­ ing them closely enough for the molecules to join together, forming more-complex organic polymers similar to .

FIGURE 3.3 Frozen Seawater Hypothesis Nucleotides trapped within the spaces between ice crystals and kept in close proximity to one another may have combined to form more-complex polymers.

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(bl) ©Emory Kristof/National Geographic Image Collection FIGURE 3.4 ments and spread out into other environments. membranesThese would have let early microbes leave rocky their compart­ rightthe ingredients combined, first organic the membranes cell could form. compartments,the Russell proposed, asmembranes. first cell the acted Once concentratedwalls basic organic the space. inasmall of molecules The walls combinedecules compartments inthe of chimneys. The these compartment . Russell proposed that, around 4billion years ago, biological mol­ Iron bubbles sulfide quickly formed, making achimney structure within laboratory by into injecting sulfide warm sodium acool, iron compartments, such shown as those in waterreacts withocean cooler the to form chimneylike structures with many Michael Russell noted that hot iron rising from sulfide below floor ocean the Iron-sulfide bubbles hypothesis addresses way the membranes cell may have formed. on way the organic could molecules have brought been together, and others There are hypotheses several about formation the of first cells. the One focuses E thatmolecules more carried information. advantageous functions may have leading to selected, more been withmolecules variations that improved replication or other added efficiency inherited bybeen generation each new of In molecules. successive molecule, as well as any variation introduced during replication, would have capable of replication formed, sequence the information contained inthe either or monomers—on Once amolecule early Earth. life rely on Earth on formation the of self Implications ofpolymer formation arly Cell Formation Hypotheses

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Man e 1990s, William Martin and F igure y hypotheses describing origin of the - rep pothesis licating polymers—made from 3.4 early life to form. created conditions necessary for rock. These structures may have water to make compartments of that mixes withocean Hydrothermal vents produce . Russell inthe this modeled - ri ch solution. - co ­ gen mplex erations, the cell. environments insideandoutside boundary between the maintains a two layers oflipids,orfats. The cell membranes are composed of Structure andFunction that most Recall from thechapter Cell Ce CO lls Chapter TheHistory ofLife 12: NNE C T

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figure (colored SEM;magnification 1500 the membrane ofalivingcell. membrane tha Unit 4:Evolution 3.5

Liposomes have a t issimilar to  ) can catalyze specific chemicalcan reactions. catalyze specific As that RNA can catalyze reactions. Colorado and Sidney Altman from Yale University independently discovered living things on In early 1980s,Thomas the Earth. from University Cech the of RNA, rather than DNA, was genetic the material that stored information in A hypothesis that has gained much support inrecent years proposes that R needs enzymesneeds to replicate itself. into pieces, and from pieces these make even more RNA. Unlike RNA, DNA catalyze own their replication and synthesis. RNA can copy itself, chop itself

proteinoids had awater ping into them water, spontaneously they formed into microspheres. The that proteinoids, the when produced inhot conditions, were by cooled drop­ claythe adsorption hypotheses astep further. The two scientists discovered foundations upon which current research is based. researchthe of scientists these into how cells originally formed provided the replication. orcoding true In other words—none of were them alive. However, slowly grow larger and eventually bud, forming spheres. new Under laboratory certain conditions, proteinoid these microspheres would group outwards, forming abilayer similar to structure the of membranes. cell group. that The parts were water N Lip A as A as osomes, and coacervates, microspheres were not capable of genetic E arly Genetic Material arly Genetic enc origin of life. Lipid spontaneously molecules form membrane that evolution the of lipid membranes step for was acrucial the Lipid membrane hypothesis Fox, working with Kaoru Harada, took his previous work with Proteinoid microsphere hypothesis and feed excretebacteria then wastes. release some compounds inaway that was similar to how surrounded when coacervates, These by water, could absorb and down, tiny capsules that “coacervates” he called were produced. arabic into awater Alexander found Oparin that by adding asubstance gum called cell separated organic these from molecules environment. the These nucleotides. The would as membranes act that organic such molecules, as amino acids, fatty acids, sugars, and brane. liposomes These could form then around avariety of liposomes were formed with adouble, or bilayer, lipid mem­ biochemist Harold Morowitz tested that idea the at some point proposed that cell Coacervate hypothesis cells. - losed spheres,losed liposomes, shown called in lik e structures may have later givento first true the rise - so luble chemical group attached to awater - - - lik ba so e structures formed from liposomes, luble turned inwards and insoluble the sed solutionsed of gelatin, cooling then it

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Figure 3.6 and RNA, in the form of a ribozyme, is able to replicate DNA requires many enzymes to replicate. Helicase enzymes separate the DNA itself without the help of additional enzymes. strands and polymerase enzymes add nucleotides to the DNA strands. DNA RNA RNA RNA helicase enzyme polymerase enzyme duplication DNA

DNA

Along with the discovery of ribozymes, several other types of evidence support the RNA hypothesis. Short chains of RNA will form from inorganic materials in a test tube. If is added as a catalyst, longer chains will grow. Also, RNA will fold into different shapes depending upon its sequence of nucleotides. Thus, it can perform more functions than DNA. But RNA does not catalyze chemical reactions as well as proteins do, nor does it store genetic information as well as DNA does. Over time, RNA may have become less important for these functions. Perhaps the earliest replicating RNA molecule gained simple membranes over many generations through . Membranes might protect chemical reactions and make them work more efficiently. RNA molecules that made copies of themselves in a double-stranded form, similar to DNA, might eventually have been selected because fewer would occur. Because DNA is more stable than RNA, it could reliably store more sequence information for a longer period of time. This may have led to DNA replacing RNA as the primary genetic material. Currently, there are several hypotheses about how RNA could have led to life as we know it today. Laboratory experiments in which RNA molecules survive and self-replicate support the idea of early cells being based on RNA. This model of the origins of life on Earth is sometimes called the RNA world. Synthesize Could cell structures or RNA have been present before organic Self-check Online molecules existed on Earth? Explain. HMDScience.com GO ONLINE

12.3 Formative Assessment CONNECT TO Synthesis Reviewing Main Ideas Critical thinking 5. RNA is hypothesized to be 1. Describe the environmental 3. Analyze What factors do the two the earliest form of genetic conditions that are thought to have organic polymer hypotheses have material because it can store existed during the Hadean eon. in common regarding how more- information, catalyze its own complex organic polymers originated? 2. What evidence do the two organic 9d replication, and catalyze molecule hypotheses provide other reactions. Which two regarding the formation of simple 4. Compare and Contrast Choose two of these functions can DNA organic molecules? 9d of the early cell formation hypoth- not do? Which two can eses and discuss their differences. proteins not do? 9A, 9D

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early Earth. among thefirst organisms on are considered to have been by cyanobacteria. Cyanobacteria these f f endosymbiosis cyanobacteria VOCABULARY organisms andecosystems and disrupting thehealth ofboth inboth maintaining cell; 11Csummarizetherole of concerning thecomplexity ofthe evaluate scientific explanations the effects ofo recombination; 7 flow, , and mechanisms, includinggenetic drift, igure 4.1 Unit 4:Evolution ound inAustralia, are made 7 F 12.4 analyze andevaluate

Stromatolites, like ther evolutionary G anal 7f, 7g, 11C yze and Early Single-Celled Organisms composition ofEarth. composition haveMicrobes changed physical the chemical and more complicated organisms, suchashumans,truly began. together. Once thefirst cells arose from these molecules, thesteps toward even years ago, organic molecules were everywhere. However, they didn’t yet fully work together theparts putting to get aworking result can bevery of Billions difficult. If you have ever assembled acomplicated modelorworked onacar, you know that Connect to Your World K ye Apply ey

oxygen to live. and allowed ocean evolution the the of aerobic , which need released oxygen as abyproduct. Higher oxygen levels atmosphere inthe prokaryotes probably got from energy their organic molecules. would have anaerobic, been or living without oxygen. Many of early these photosyn organisms changed atmosphere the by giving off oxygen as abyproduct of Single-celled organisms changed Earth’s by surface depositing minerals. These ars ago. MAI MAI C

o The evolution ofsexual led to increased diversity. prokary Several theorieshave beenproposed for how eukaryotic cells evolved from Microbes have changed thephysical andchemical composition ofEarth.

N How are evidence of Earth’s early life? early Earth’s of evidence stromatolites How are n N ID ce IDEAS Communities of photosynthesizing instromatolites cyanobacteria Fossils of stromatolites as old as 3.5billion years have found. been fossils. fossils These are of agroup of marine around 3.5billion years ago, since that is age the of oldest the known membrane-bound . was asingle prokaryotic that prokaryotic cell. Recall cells have no out . early all Like life forms, each cyanobacterium fossils, but some are living communities, as shown in of layers of and cyanobacteria sediment. There are many (stroh-MAT-l-yts ( pt ­the EA sy otic cells.

-ah-noh-bak-TEER-ee-uh), which are that can carry

sis. Before photosynthesis evolved, however, first prokaryotes the Scie Som Single-celled organisms existed 3.8billion ntists have found that evidence photosynthetic life evolved e cyanobacteria livee cyanobacteria incolonies and form stromatolites 11C

). Stromatolites are domed, rocky structures made cyanobacteria Figure 4.1.

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MAIN IDEA 7g Several theories have been proposed for how eukaryotic cells evolved from prokaryotic cells. The fossil record shows that eukaryotic organisms had evolved by 1.5 billion years ago. A is more complex than a , having a nucleus and other membrane-bound organelles. While the first were made of only one cell, later eukaryotic organisms became multicellular. Bacterial, plant, and animal cells share certain complex traits, including specific enzymes, metabolic pathways, , cell membranes, and the genetic code carried in DNA. While these traits originated in prokaryotes, other READING TOOLBox eukaryotic traits such as organelles arose through the processes of evolution. theory One hypothesis of eukaryote evolution did not get VOCABULARY Endosymbiosis much attention until the 1970s. found evidence to can be broken down into endo‑, meaning support the endosymbiont theory. Endosymbiosis (ehn-doh-sihm-bee-OH- “within,” sym-, meaning sihs) is a relationship in which one organism lives within the body of an- “together,” and biosis, meaning other—with both organisms benefitting. “way of life.” The endosymbiont theory suggests that mitochondria and chloroplasts were once simple prokaryotic cells that were engulfed by larger prokaryotes around 1.5 billion years ago. Instead of being digested, some of the smaller prokaryotes may have survived inside the larger ones as illustrated in Figure 4.2. If it took in a prokaryote that acted as a , the larger cell got energy in the form of ATP. If it took in a prokaryote that acted as a , the larger cell could use photosynthesis to make sugars. In exchange, the mitochondria and the chloroplasts found a stable environment and nutrients. Margulis based her theory on several factors. Unlike other organelles, mitochondria and chloroplasts have their own DNA and ribosomes. They can copy themselves within the cell in which they are found. Mitochondria and chloroplasts are also about the same size as prokaryotes, their DNA forms a circle, and their gene structures are similar to those of prokaryotes. Biology 7g Analyze What evidence supports the theory of endosymbiosis? HMDScience.com FIGURE 4.2 Endosymbiosis GO ONLINE Endosymbiosis The theory of endosymbiosis proposes that the mitochondria found in eukaryotic cells descended from ancestors of infection-causing bacteria. Likewise, chloroplasts are considered descendants of cyanobacteria. host cell Infection-causing bacteria entering a host cell bacterium Over generations, Bacterium enters cell. bacteria evolve as early nuclear mitochondria. envelope cell

colored SEM; magnification 13,000 ©David H. Walker, M.D. and Vsevolod L. Popov, Ph.D. Popov, Vsevolod L. and M.D. Walker, ©David H.

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Autogenous theory In 1976 botanist F.J.R. Taylor proposed an alternative to the endosymbiont theory. Taylor’s theory, known as autogeny, is based upon the extensive folding of the internal membranes found within chloroplasts and mitochondria, as seen in Figure 4.3. You may recall that one of the major differences between prokaryotes and eukaryotes is that eukaryotes have membrane-bound organelles and prokary- otes do not. This provides eukaryotes with a distinct adaptive advantage. Since prokaryotic metabolic reactions all occur in the cytoplasm, it is possible for them to interfere with one another. Eukaryotes, on the other hand, have the specialized chemical reactions involved in separated by the membranes surrounding each . The autogenous theory proposes that eukaryotic organelles evolved from infoldings of the plasma membrane, creating pockets that eventually pinched off. When they pinched off into separate structures, small sections of nucleic acids and ribosomes were trapped inside. These new structures became specialized in performing different metabolic processes. Having these reac- tions isolated from one another is much more efficient, allowing eukaryotic cells to evolve even greater complexity. According to the autogenous theory, those organelles that performed photosynthesis eventually developed into chloroplasts. The theory also proposes that those new structures that special- ized in providing energy to the other organelles through cellular evolved into mitochondria. Analyze Why do membrane-bound organelles give eukaryotes an adaptive advantage over prokaryotes? 7g

FIGURE 4.3 Autogenous Theory According to the autogenous theory, eukaryotes arose directly from a single prokaryotic ancestor through isolation of metabolic functions by infoldings of the plasma membrane. Such infoldings and pockets can be seen in chloroplasts and in mitochondria.

chloroplast mitochondria

Infolding of Infolding of the plasma the plasma membrane membrane

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FIGURE 4.4 Horizontal gene transfer occurs commonly among bacteria. The donor cell forms a cytoplasmic bridge called a pilus, connecting to the recipient cell. A strand of DNA separates from a plasmid within the donor and moves into the recipient. Each cell then replicates a new, complementary strand of DNA and forms a new plasmid. In time, the new DNA may be incorporated into the genetic material of the recipient cell.

donor donor donor donor Pilus forms

plasmid

recipient recipient recipient recipient A single strand of Both cells make a Plasmid DNA plasmid DNA separates complementary strand is integrated into and is transferred. of DNA to produce a the recipient’s DNA. new plasmid.

Horizontal gene transfer theory In 2002 biologist proposed a third theory of how the complexity of eukaryotes evolved. Woese’s earlier research into the genetic differences among prokaryotes led to the establish- ment of the currently accepted classification of all living things into three domains: eukaryotes and two groups of prokaryotes, bacteria and . Woese pointed out that the endosymbiont theory posits that eukaryotes arose when fully evolved prokaryotic cells engulfed and incorporated other prokaryotes.. Based on his genetic research, Woese proposed instead that the genetic makeup of early cells was highly fluid and that eukaryotes, bacteria, and archaea evolved more or less in parallel. Woese further hypothesized that extensive exchange of genetic information took place among these evolving cell types through a process of horizontal gene transfer (HGT). This type of gene transfer continues to take place today when modern prokaryotes repro- duce by a process called conjugation, as illustrated in Figure 4.4. According to Woese’s theory, horizontal gene transfer enabled the appear- ance of more complex cell structures. Distinct cell types emerged only when each of the cell organizations we know today reached a degree of complexity and interconnectedness that made it impossible for HGT to further change the organization in a fundamental way. Woese called this critical point the “Dar- winian threshold,” because this was the point in the evolution of life when natural selection on distinctly separated gene pools could begin to act. Evaluate Some scientists propose that a combination of theories may best describe the evolution of eukaryotes. What information would you use to support this view?

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MAIN IDEA 7F CONNECT TO The evolution of led to Recall from the chapter increased diversity. and Mendel that recombination is an important source of genetic The first prokaryotes and eukaryotes could only reproduce asexually. Some variation. Sexual reproduction time later, eukaryotic cells began to reproduce sexually. Of the groups of results in many phenotypes organisms that reproduce asexually today, only a few—such as bacteria— through the processes of producing and appear to have ancient asexual origins. crossing over in meiosis. Recall that in , a single parent produces offspring that are genetically identical to itself. Asexual reproduction lets organisms have many offspring quickly. Sexual reproduction, on the other hand, needs two parents. Both parents give to their offspring. This means that individuals must use time and energy to produce gametes, find a mate, and pass on genetic information. Recall that each parent passes on only half of its genes to offspring. The evolution of sexual reproduction is still an active area of research. The disadvantages of sexual reproduction—needing a partner and passing on only half of a set of genes—seem clear. One advantage to sexual reproduction, however, is . Sexual reproduction allows new combinations of genes to come together. This process may mask harmful mutations, and in some cases it may also bring beneficial mutations together. Sexual reproduction may also have resulted in an increase in the rate of evolution by natural selection. Sexual reproduction creates more genetic variation, which lets a population adapt quickly to new conditions. Over a long time, early eukaryotes may have gained variations that made living closely together, and eventually cooperating, beneficial. Thus, sexual repro- duction may have been the first step in the evolution of multicellular life. Infer How can mutations be beneficial to organisms?

Self-check Online HMDScience.com 12.4 Formative Assessment GO ONLINE CONNECT TO Reviewing Main Ideas Critical thinking Scientific Process 1. How did early cyanobacteria affect 4. Apply How does sexual reproduc- 6. According to the theory of the physical and chemical conditions tion increase the chances that some endosymbiosis, mitochondria on Earth? 11C individuals will survive in changed were once independent 7F 2. How does the theory of environmental conditions? organisms. Is it possible that endosymbiosis differ from the 5. Infer For photosynthetic organisms mitochondria might now be autogenous and horizontal gene to become more common than those able to exist independently if transfer theories in explaining the that get energy from eating organic removed from a cell? evolution of eukaryotes? 7g molecules, what environmental Describe how you could 3. How does sexual reproduction conditions must have changed? investigate this increase diversity among living things? question. 7g 7F

366 Unit 4: Evolution D A T A ANALYSIS 367

5 . Coli n E i 4 e im 3 on t i Generation t 85 125 a 240 2 Chapter 12:Chapter of Life The History Time (min) Generation 5 Generation ener 1 G bacteria dividing (colored dividing (colored bacteria E. coli 32,000+) SEM; magnification HMDScience.com 0 12 ph 1. 72 36 24 84 48 96 60 a

68 192 100 GO ONLINE charts and animated Create Smart Grapher. graphs using (min) time Generation Gr Time (min) Generation 4 Generation 2g ia 51 75 144 ter Time (min) ac Generation 3 Generation on b mm 34 50 96 o c Time (min) Generation 2 Generation erval. erval. es for es im as an example, explain how changing the axes of a graph can can of a graph the axes changing how explain as an example, or E. coli 17 25 48 arest convenient number, such as 2, 5, or number, convenient arest on t i Time (min) 10 bacteria 20 bacteria bacteria 40 80 bacteria 160 bacteria t Generation 1 Generation -axis for a graph that compares the generation the generation compares that a graph the y-axis and x-axis for for the intervals ate a Calcul Gener

le on the axis at zero (or at one interval lower than the lower one interval at (or zero on the axis at le Using your graph f graph Using your

, the scale ranges from 0 to 96. 0 to from ranges , the scale E. coli For the highest value. above le Calculate Axes Intervals Axes Calculate E. coli S. lactis Bacteria

ble 1. able and divide the difference by the number of data points. of data the number by and divide the difference able B. megaterium Ta Graph Data Graph of all three of the bacteria species listed below. Draw the axes, plot the data, and label each of each and label the data, plot the axes, Draw below. species listed of the bacteria three times of all axes. your and label graph your title to lines. Be sure plotted the three Analyze influence how data are interpreted. are data how influence

1. 2. , the interval was rounded down to 12. to down rounded was , the interval E. coli 10. For int number as the the rounded Use Begin the sca End the sca Calculate the difference between the smallest and largest values of values and largest the smallest between the difference Calculate the vari the ne to Round the result than the interval). much larger are be graphed to if the values value lowest data, 85 – 17 = 68. Divided by 5, this equals 13.6. 5, this equals by 85 – 17 = 68. Divided data, the E. coli For ractice

• • • • • P

Model of short the benefit generation have asexually reproduce Some species that changing environmental to quickly more adapt to be able may times. They resistant become quickly can instance, for populations, Bacteria conditions. will pass treatment antibiotic survive that bacteria Individual antibiotics. to reproduce. when they their offspring to resistance for the gene The following generation. with each doubles of bacteria The population of the of the line graph axis intervals determine to used the steps are 5 generations: over coli of Escherichia growth Determining the correct scales of axes on graphs is important so that all data points can can points data all that so important is graphs on axes of scales correct the Determining the If results. the of perception reader’s the influence also can scale The plotted. be rate fast a steep—indicating seem will graph the of slope the apart, far too are intervals with flatter, be will graph the small, too are intervals the If data. the in change large a or nonexistent. or small seems that change Calculating Axes Intervals ©Photo Researchers, Inc. Researchers, ©Photo

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12.5 Radiation of Multicellular Life

7e Key Concept Multicellular life evolved in distinct phases. MAIN IDEAS VOCABULARY Life moved onto land during the Paleozoic era. Paleozoic Reptiles radiated during the Mesozoic era. explosion Mammals radiated during the Cenozoic era. Mesozoic Cenozoic Connect to Your World Do you know any people who can get everyone’s attention and change the social 7E analyze and evaluate the relationship of natural selection to atmosphere in a room with their personalities? When photosynthetic organisms and to the development appeared on Earth, the increase in oxygen changed the atmosphere dramatically. of diversity in and among species New ecological opportunities arose, and multicellular organisms began to evolve.

MAIN IDEA 7E Life moved onto land during the Paleozoic era. The trend toward multicellular organisms was one of the most important transitions in the history of life. One hypothesis suggests that it was an advantage for early one-celled organisms to increase in size by becoming figure 5.1 This illustration depicts a scene from the Carbon- multicellular. Cells that cooperated could compete more effectively for energy, iferous period of the Paleozoic by processes such as cooperative feeding. At some point, increased depen- era. Note the diversity of animals dence on neighboring cells would have led the cells to function as a . and plants represented. Multicellular organisms first appeared during the Paleozoic (pay-lee- uh-ZOH-ihk) era, which began 542 million years ago. Members of every major animal group evolved within only a few million years. The era ended figure 12.14 This illustration 251 million years ago with a mass extinction. More than 90 percent of marine depicts a scene from the Carbon- iferous period of the Paleozoic species and 70 percent of land species of that time became extinct. In between era. Note the diversity of animals these remarkable events, multicellular animals radiated, the first vertebrates and plants represented. evolved, and early plants moved onto land. The earliest part of the Paleozoic era is often called the . During the Cambrian explosion, a huge diversity of animal species evolved. At the start of the Paleozoic era, all life was found in the ocean. Among the earliest vertebrates was a group of jawless . The only species of jawless fish still remain- ing are the Agnathan fish, which you will read about in the chapter Diversity. Marine invertebrates, such as the , were especially abundant. This highly diverse group of had thousands of species, though almost half of these species died in the mass at the end of the Cambrian period. Many other animals from this time period are also extinct. The best known of these are found at the site in British Co- lumbia, where many fossils were well preserved.

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contributing to meat the eaters’ extinction. reduced supply food the of meat-eating dinosaurs, The lossextinct. of herbivorous these animals dinosaurs and many other animal became species Without sufficient plants to eat, herbivorous and plants were unable to perform photosynthesis. Sun’sthe light. As aresult, the changed, and debris into atmosphere, the blocking much of is that impact this sent enormous amounts of dust asteroid The most accepted Earth. struck hypothesis cause of debated. which is still Evidence shows that amassive forms. complex more dominated oceans. Sharks the and bony fishes continued to evolve Ic The era, illustrated in United inthe used States formed during the of this period vertebrates,legged such as amphibians, common. became Most of coal the on and organisms were changed buried insediment. they time Over into coal of era. the improved chances their during mass of the extinction at survival end the the Jurassicthe illustrated period, in of animals. This mass extinction allowed radiation the of dinosaurs the in An extinction event near end the of Triassic the destroyed many types first mammalsthe evolved also during that time. this shows On land, earliest the and crocodiles dinosaurs arose. The fossil record Jurassic, and Cretaceous. the Life took off slowly early inthe Triassic. als, w appeared during era. this By era’s the end, mammals—particularly marsupi birds and flowering plants. The oldest direct ancestor of mammals first dinosaurs during roamed Mesozoic era, this the Earth featured also and ended 65million years ago. Age the of Called the Reptiles because MAIN IDEA MAIN ginning of the Paleozoic era to the end of of end to the era Paleozoic the of ginning hthyosaurs (IK-thee-uh-sawrz), agroup of predatory marine reptiles, to land. The number and variety of plant groups greatly increased. Four- The Cr Although life had moved onto land, it abundant was still underwater. The middleof Paleozoic the era was of atime great diversity as life moved The M

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©Michael & Patricia Fogden/Corbis bipedal hominid anthropoid prosimian primate VOCABULARY pros 12.6 more shapesasneeded. primate classification. Add Make aconcept mapof T AKI uha uha such as such as such as im READI ians N G N anthropoids O primates N include T G ES Primate Evolution Toolbox other primates. ancestor sharea common with Humans record offers afascinating glimpse ofourpast. details mustbeinferred through careful study. Thoughfar from complete, thisfossil recently. Manyfossils ofourearly ancestors consist ofpartial skeletons from which In terms ofthegeologic timescale, theevolution ofhumanshasoccurred only very Connect to Your World K ey

anthropoids. subgroups: prosimians the and the ancestor of primates. all Just above tree the splits base, this into two main primate groups forms amany-branched tree. At tree’s the is common the base Similar to other groups of related organisms, relationship the among the Evolution Primate tion that 65million Cretaceous years the closed period ago. The common ancestor of primates all probably arose before mass the extinc- not fused togethernot mature. when fused sizeandsmaller skull plates that are differentiated from anthropoids by shown in , and , the like ones the animals includes , the the night. This group of nocturnal and most are and small active at are oldest the living primate group, to sharing similar physical traits, primates share strong molecular similarities. fingers.their Primates include lemurs, monkeys, , and humans. In addition shouldertheir joint, and many primates have thumbs that can move against relative size. to Primates body have also arms that can rotate inacircle around which allow for excellent three-dimensional vision—and enlarged a category of mammals with hands flexible and feet, forward-looking eyes— MAI MAI C

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estor withother primates. ate inEarth’s history. active at nightand have large eyes and ears. are theol figure 6.1 dest livingprimate group. They are

Prosimians, suchas these tarsiers, Chapter TheHistory ofLife 12: Primates make up

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Unit 4:Evolution which means “.” from theword root anthropo, The anthropoid comes VOCABULARY tools orweapons. and adapt for objects use as toability grasp, manipulate, thumb figure READI s gave hominidsthe 6.3

N O G pposable T OOLB ox as tools or weapons, give hominids an evolutionary advantage over other primates? over other advantage evolutionary an give hominids weapons, or tools as Interpret chapter . You more learn then will about how scientists determine monkeys and agreater ability to manipulate objects. and forage on ground the as well. have They larger brains than do New World feeding. Some Old World monkeys spendintrees, time but also most travel adaptation that allows to them hang by tails their from tree branches while liveAmericas, all intrees. Many have species prehensile, or grasping, tails, an for food. for unlikely that youngsters the would become successful at obtaining adults demonstrate steps the involved. Without observations, it these is more than 40million years ago. physical traits have changed little since appearance their fossil inthe record known as agrooming comb. Tarsiers living have fossils, called as been their ofsecond toe hind their feet and aunique of type horizontal tooth structure Further distinguishing prosimians the is asingle, long grooming claw on the

noids, as shown in subdivided into New the World monkeys, Old World monkeys, and homi- Cla Anthropoids ssification of organisms covered be will inmuch more inthe detail

How d id the ability to manipulate objects, as well as adapt them for use use for them adapt well as as objects, to manipulate ability the id modern humansmodern and immediate their ancestors. humans, but not . The term hominin refers only to originate all they because from same the root Latin word, hominid of to terms anthropoids. the classify used The terms hominoid, confusion, to itshow is necessary difference the some between how to categorize and name an organism. However, to reduce ­(oran Hominids chimps inGombe how learn to “fish” for termites by observing wiping for and mud and sticky Young from bodies. their scientists. The leaves use also for water collecting roots to eat, and open boxes to pry of bananas for left by them and sticks to remove honey from beehives, as well as to digup ­chim but it use for also multiple For purposes. example, wild enables hominids to not only pick up and grasp an object, hominidsthe to manipulate and objects adapt This them. trait thumbs are placedinopposition to four the fingers, allows this for using opposable thumbs to advantage. their their Because (AN-thruh-poydz), the humanlike the (AN-thruh-poydz), primates, are further F igure panzees inTanzaniapanzees adapting have observed grass been gutans, chimpanzees, and ) as well as humans. , meaning “man”. Hom Among primates, hominids are known particularly , and athird term, hominin include , chimpanzees, gorillas, and 6.2. inoids include gibbons and great the apes New World monkeys, which are native to the , all sound very similar sound very , all

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prosimians ©Pete Oxford/Nature Picture Library; old world monkeys baboon ©David A. Northcott/Corbis; new world monkeys tamarin ©Pete Oxford/Nature Picture Library; gibbons ©Sohns, Juergen and Christine/Animals Animals - Earth Scenes; ©Kenneth W. Fink/Ardea London Limited; human ©Getty Images; ©Anup Shah/Nature Picture Library; gorillas ©Wegner, Jorg and Petra/Animals Animals - Earth Scenes FIGURE 6.2 analysis ofchromosomes. between primate groups can beconstructed by scientists usingkaryotype Phylogenetic trees, orcladograms, that evolutionary depict relationships cl humans? to modern closely-related least the is anthropoids Analyze adogram, which group of of group which adogram, ancestor

Based on this this on Based

Evolutionary Rela tionships ofPrimates Anthropoids Hominoids chimpanzees New World orangutans prosimians Old World monkeys monkeys Hominids humans gibbons gorillas Hominins Chapter TheHistory ofLife 12:

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Walking Upright Many hypotheses have been proposed to explain the evolutionary success of the hominins. Enlarged size and the ability to make and use tools were for many years among the most accepted ideas. However, fossil discoveries have revealed that another trait came before tool use and large brains—walking upright on two . Upright posture and two-legged walking required changes in skeletal . Examples of these changes include a more strongly curved, cuplike , the increased alignment of the knees with the body, and a change in the spine from arch-shaped to an S-shaped curve. The skull became more centered on the top of the spine, allowing for greater ease in using forward- facing vision. These changes can be found in intermediate fossils between hominoids that walked only on all fours and early hominins that walked on two legs, as seen in Figure 6.4.

FIGURE 6.4 Walking Upright Changes in the skeletal structure of primates were necessary before was possible.

55 Million 37 Million 3-4 Million 1 Million years 200,000-30,000 Modern Human years ago years ago years ago ago years ago

Smilodectes gracilis Biretia fayumensis Homo Homo sapiens afarensis neanderthalensis

Analyze Give a brief summary of the skeletal changes that occurred through time that allowed for an upright posture and bipedal stance in higher primates.

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walked onwalked hind their feet and front their knuckles. enabled to them travel expending than less while animals farther energy that for The ability survival. to travel upright on two feet would have have had to travel inorder farther toenough find sources of nutrition , would food have become more scarce. Hunter-gatherers would evolved gradually shifted from heavily forested areas to vast grass- food, and using tools. As landscapes the where ancestral humans most importantly, it hands freesthe for , infants carrying and allows higherreach into tree branches foraging, while and perhaps This trait has important adaptive advantages for higherprimates. It have enabled to them over better see obstaclesapproaching to observe threats. offspring. The highervantage point provided by upright their posture would fashion weapons prey to kill or to protect themselves, mates, their and their ed its forelimbs to be free from providing support for its body? its for support providing from free to be forelimbs its ed P covers less thanasecond onour12-hourclock! ancestors—more years than5million ago—to Homosapiens An P 12 noonto about10:30 is to compar O Clock Geologic 2. 2. 3. 1. 1. robl rocedure

ne way to understand therelative length oftimeinEarth’s history Animals that on can walk two legs are called In ad Calcul fit inthisdiagram? Synthesize earliest hominids. eukaryotes, first fishes,first flowering plants, first dinosaurs, first birds, and oldestEarth, rocks, first stromatolites, first aerobic prokaryotes, first also intheapproximate filling timeframes they occurred: formation of Label thefoll they occurred. your along clock, withtheapproximate timeframes inwhich Using thegeologic (Example: thethree o’ years =3600million ago.) appropriate numberofyears, with12o’clock starting years =4800million ago. the sc Draw alarge circle 3,6,and9positions ofaclock face. andmarkthe12, Use of years inEarth’s history would 1minute represent? aly

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MAIN IDEA There are many fossils of extinct hominins. Hominins are classified into several groups. Two important groups are the Homo and the older genus Australopithecus (aw-stray-loh-PIHTH-ih-kuhs). Australopithecus was a long-lived and successful genus. Australopithecus afarensis (af-uh-REHN-sihs), which lived 3 to 4 million years ago in , is one of the better known species of early hominins. Although its brain was much smaller than that of a modern human—about the size of a modern-day chimpanzee’s brain—A. afarensis had very humanlike limbs. The earliest member of the genus Homo was . Nicknamed “handy man” because of the crude stone tools associated with its skeletons, H. habilis lived 2.4–1.5 million years ago in what are now Kenya and Tanzania. This species may have lived alongside the species for about 1 million years. H. habilis is the earliest known hominin to make stone tools. The brain of H. habilis was much larger than that of any of the australopith- ecines, and it more closely resembled the modern human brain in shape. Another hominin species was H. neanderthalensis, commonly called Nean- derthals for the Neander Valley in Germany, where their fossils were first found. This group lived from 200,000 to around 30,000 years ago in and the . Some evidence suggests that H. neanderthalensis coexisted with modern Homo sapiens. Did the two species live side by side? Or did they engage figure 6.5 Computer technol- in a fierce competition for resources, causing the extinction of the ogy allowed scientists to piece by the better-adapted H. sapiens? This puzzle has not yet been solved. together 7-million-year-old skull fragments found in Africa. The Observations from the fossil record, such as the fossil seen in Figure 6.5, three-dimensional reconstruction demonstrate a trend toward increased brain size in the human . Al- suggests that this may be the old- though brain size can only be loosely related to intelligence, the combination est known hominin ancestor of Homo sapiens; it has been named of modern-day humans’ physical and cultural has no doubt tchadensis. contributed to our success as a species. Hypothesize What type of evidence could indicate that H. sapiens and H. neanderthalensis coexisted?

MAIN IDEA T’ ing! maz Modern humans arose nearly 200,000 years ago. A Video Inquiry HMDScience.com Fossil evidence reveals that Homo sapiens evolved nearly 200,000 years ago in

GO ONLINE what is now Ethiopia. However, many of their features were different than Crafty Cavemen those of humans today. After becoming a distinct species, H. sapiens clearly did not stop evolving. The Role of Culture is influenced by culture. Tools are among key markers of culture in human evolution, although they are used by some other animals as well. A comparison of tools from their first appearance some 2.5 million years ago, through their association with later Homo fossil sites, shows a steady trend of increasing sophistication and usefulness. (t), (b) ©AFP Photo/Getty Images (t),

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FIGURE 6.6 Examples of Hominin Skulls Hominin evolution shows changes in brain size.

4–3 Million years ago 2.4–1.5 Million years ago 200,000–30,000 years ago 200,000 years ago–present Australopithecus afarensis Homo habilis Homo neanderthalensis Homo sapiens

Australopithecus afarensis Homo habilis had a brain Homo neanderthalensis’ brain Modern Homo sapiens have a had a brain volume of volume of about 700 cm3. volume may have reached brain volume average of about 430 cm3. 1500 cm3. 1350 cm3. Contrast What characteristics besides brain size differ among the species shown?

VIRTUAL Lab The Evolution of the Human Brain HMDScience.com Human evolution would not have advanced as it did without an enlarging GO ONLINE skull and brain size, as shown in FIGURE 6.6. One recent study has demon- strated that genes controlling the size and complexity of the human brain Comparing Hominoid Skulls evolved faster than analogous genes in nonhuman primates. Researchers compared the DNA sequences for more than 200 genes affecting brain development in humans, Old World monkeys, rats, and mice. They found CONNECT TO that these genes evolved at a much faster rate in the two primates than in the Classification two rodents and that brain-related genes in humans evolved faster than did A genus is a closely related those in the monkeys. The results of the study support the hypothesis that the group of species. You will learn more about categories for rapid evolution of large brain size posed an especially strong selective advan- classification in The Tree of Life. tage among the hominids. Synthesize When might having an increasingly larger brain size no longer be a selective advantage?

Self-check Online HMDScience.com 12.6 Formative Assessment GO ONLINE Reviewing Main Ideas Critical thinking CONNECT TO 1. What characteristics shared by 4. Apply Explain why, according to Anatomy humans and other primates suggest the fossil record, it is not correct to 6. Consider the skull illustrations that they have a common ancestor? say that humans evolved from above. Besides size, how did 2. According to the fossil record, what chimpanzees. skull structure change as other Homo species was present 5. Infer Scientists can often identify hominins evolved? What when modern humans arose? whether a fossil skull was from a features are considered more apelike than humanlike? 3. From the hominin fossils described, bipedal primate. What characteristics what common trends can be found? of a skull might help them make this determination?

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CHAPTER ­hominid that isonyour di evolution. Adddetails aboutcharacteristics ofeach t 12.3 12.2 12.1 Unit 4:Evolution 12 perioD er Key Con t READING READING

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Earth. first genetic material on arose before DNAasthe the hypothesis that RNA help ofproteins, led to reactions withoutthe cules that can catalyze ribozymes, RNAmole- formed. Thediscovery of early cells may have andaboutthewayEarth molecules appeared on are se The originoflife on fossil orrock. with radiometric dating to determine theage ofa on majorpastevents. The geologic timescale divides years isabout4.5billion Earth old. Through radiometric dating, scientists estimate that the age ofafossil ortherock inwhichitisfound. dating, whichuses radioactive isotopes to determine of afossil orrock can bedetermined by radiometric Fossils can form inseveral different ways. Theage Fossils are arecord oflife that existed inthepast. The Geologic TimeSca The Fossil Record

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structure arose. fer provide alternate ideas ofhow more complex cell The theoriesofautogeny andhorizontal gene trans- large prokaryote engulfingasmaller prokaryote. proposes that thefirst eukaryotic cells arose from a anaerobic prokaryotes. Thetheoryofendosymbiosis ago. Thefirst organisms were onEarth most likely Single-celled organisms existed years 3.8billion extinct. extinct. species inthehumanlineage, both modernand relative to theirbodysize.Thehomininsincludeall feet, forward-looking eyes, andenlarged brains mates includeall mammals withflexible handsand share acommon anc Humans appeared late in appear until200,000years ago. ­diversified andf era, mammals, birds, fishes,andflowering plants and mammals inhabited DuringtheCenozoic Earth. the Mesozoic era, dinosaurs, flowering plants, birds, group evolved withinonly afew years. million During the Paleozoic era, members ofevery majoranimal Multicellular life evolved phases. indistinct During Early Single-Celled Primate Evolution Radiation ofMulticellular Life

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CHAPTER 12 TOOLBOXREADING

be “organizes life’s history.” phrasea short to describe geologic timescale could meaning ofeach vocabulary term below. For example, Write precise ashort, phrase that describes the K the two terms ineach ofthefollowing pairs. Describe onesimilarity andonedifference between Compare andContrast R 12.2 12.1 Chapter vo 10. 12. 11. 8. 6. 4. 2. 5. eep ItShort 9. 3. 7. 1. eviewing

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might have formed onearly Earth. two hypotheses describing how complex long, time related to oneanother? into eras, periods,andepochs.How are these unitsof used inradiometric dating. may result. ization andgive anexample ofthetypefossil that perminera Fossils can form inseveral ways, oneofwhichisby primates. What characteristics doall primates share? Humans, apes, following era? Whaton Earth? happenedto these groups inthe In whichera didmammals, dino rel What are some criteria by whichwe can evaluate the advantage?. than asexual reproduction. Whymightthisbean is that itcr One evolutionary advantage ofsexual reproduction ries describing theoriginsofeukaryotic cells. Summarize theevidence suppor physic What are two ways that cyanobacteria have changed the ­molecules tha Compare andc The geologic times How are inde Give anex ative complexity ofacell? HMDScience.com Interact al orchemical composition ofEarth? lization. Describe theprocess ofpermineral- ample oftheway theconcept ofhalf-life is

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Critical Thinking 24. Analyze Some scientists propose that more than one of Analyzing Data Calculate Intervals the theories given in Section 4 may be involved in the Both graphs show the rate of decay of chlorine-36, evolution of the eukaryotic cell. Explain how this might which changes into argon-36. Use the graphs to answer 7G be possible. the next two questions.

25. Apply Why is it likely that autotrophs appeared on Graph 1 of chlorine-36 decay Earth before any aerobes, organisms that depended on oxygen? 100 36Ar 26. Evaluate How does the frozen seawater hypothesis 75 ­suggest that complex molecules that contain ­information, such as DNA, could have formed in spite of conditions on early Earth that would have inhibited their formation? 50

How persuasive do you find the evidence supporting of decay Percent this hypothesis? 9D 25 36Cl 27. Evaluate Thirteen of the 20 amino acids used to make proteins in modern-day cells were made by Miller-Urey’s 0 300,000 600,000 900,000 1,200,000 simulation of early Earth’s conditions. Do the results Years support Miller and Urey’s hypothesis? Why or Why not? 9d Interpreting Visuals Graph 2 of chlorine-36 decay The chart below shows when some human ancestors 100 lived and traits that they had. Use the chart to answer 36Ar the next three questions. 75

H. sapiens Insucient evidence 50 Small brain, large teeth, H. neanderthalensis sometimes bipedal 25 Large brain, small teeth, Percent of decay Percent 36 bipedal H. habilis Cl 0 A. afarensis 1,200,000 2,400,000 3,600,000 4,800,000 Years S. tchadensis

31. Analyze Which graph better shows the concept that 8 7 6 5 4 3 2 1 0 the percentage change of 36Cl and 36Ar slows down Millions of years ago dramatically over time? Explain. 28. Summarize In one or two sentences, summarize the information in the chart. 32. Analyze From which graph can you more accurately determine the half-life in years of 36Cl? Explain. 29. Infer Sahelanthropus tchadensis was pictured in Figure 6.5 of Section 6 as a three-dimensional computer reconstruction. Although skull fragments of this species Making Connections have been found, the chart above shows that there is 33. Write a Detailed Description Choose one of the not enough evidence to describe the traits of periods in geological time and describe it in detail. Be S. tchadensis. Explain why this might be so. Consider the sure to include vivid details about the organisms of the scientific process in your explanation. period. 30. Analyze Some scientists suggest that Homo habilis 34. Connect The time that the Tollund Man on the chapter should be classified as Australopithecus habilis. Based opener lived was determined by radiocarbon dating. upon the information in the chart, explain why this Why can’t 14C be used to date Burgess Shale fossils from might be the case. the Cambrian period?

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Biology End-of-Course Exam Practice

Record your answers on a separate piece of paper. 7G MULTIPLE CHOICE 3 In the evolution of eukaryotes, cells that contained mitochondria-like organelles had an 7D advantage because they— 1 Sexual reproduction and mutation provides A could make use of photosynthesis means for genetic variation in a population. Why B could make use of more available energy is genetic variation an important element of C had more DNA natural selection? D were protected from bacterial invasion A It ensures that all members of a population receive only traits that provide them with a THINK THROUGH THE QUESTION survival advantage in their environment. This question is really just asking about the way B It may provide an individual with traits that do ­mitochondria can help a cell. not provide any survival advantage. C It increases the chances that some individuals in a population will gain genes for traits that 3A, 7G provide a survival advantage in a changing 4 Many scientists believe that the cell parts that are environment, enabling them to pass these now known as mitochondria and chloroplasts genes to generations. were early types of prokaryote cells. The theory of D It increases the likelihood that a population endosymbiosis suggests that early eukaryote cells will go extinct, providing more resources for formed when large prokaryote cells took in other, stronger populations. mitochondria or chloroplasts, enabling them to live inside the larger cell without harm. What 2G, 7G advantage would the larger host prokaryote cell 2 provide to the chloroplast? Percent of Native Fossils in Hawaii A a stable, protected environment Excavated 14C Dating % Bones % Bones B the ability to carry out photosynthesis Section (years before from from Native Non-native C access to present) Species Species D ability to reproduce I 390 100.0 0.0 II 770 98.8 1.2 3A, 7G III 4340 9.2 90.8 5 The theory of endosymbiosis proposes that the IV 7750 0.0 100.0 chloroplasts present in some of today’s eukaryotic cells descended from ancient cyanobacteria. The table above shows the fossil evidence of birds Which piece of evidence supports this theory? in a section of cave wall in Hawaii. What can be A Cyanobacteria are single-celled . determined from the data presented? B Chloroplasts are much larger than today’s A A catastrophic event occurred between 770 prokaryotes. and 4340 years ago. C Chloroplasts are able to copy themselves B Native species out-competed non-native independently of the cell. species. D Chloroplasts help eukaryotic cells process C Most native species died out over 800 years energy more efficiently. ago. D The disappearance of non-native species is a function of time.

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Opinion Poll Issue A bicyclist falls, scrapes Strange Biology his knees, and within a few days is unable to walk. Soccer players with turf burns suddenly find themselves in the hospital with skin infections that require intravenous antibiotics. Why are these young, healthy athletes developing such serious infections? (t) ©Francisco Leong/AFP/Getty Images; (b) ©Hannah Gleghorn/ShutterStock

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Staph Infections These athletes were infected by Staphylococ- MRSA on the Rise cus aureus, or “staph.” Staph is a common 100 bacteria that most people carry on the 90 surface of their skin and in their nose. To 80 cause an infection, staph bacteria must get inside your body. The scrapes athletes 70 commonly get provide an ideal entrance. 60 Serious problems due to staph infections 50 used to be rare. Doctors would prescribe 40 antibiotics, such as penicillin, to kill the 30 staph bacteria. Ordinary staph infections can 20 still be treated this way. The athletes in our

Percent of infections that are resistant are that of infections Percent 10 examples did not have ordinary infections. These athletes’ scrapes were infected by 0 methicillin-resistant Staphylococcus aureus 1987 1991 1995 1999 2003 (MRSA). This bacteria strain is one of many Source: NNIS System and Centers for Control and Prevention that has evolved resistance to antibiotics.

Drug-Resistant Bacteria more than 2 million people every year. Drug- resistant TB kills thousands. Drug-resistant strains of 7C, 7E cholera and bubonic plague also have been reported. Bacteria that can survive antibiotic treatment are How Does Drug Resistance Evolve? called drug-resistant bacteria. Some bacteria have 7C, 7E resistance for one particular When you take antibiotics for a bacterial infection, antibiotic, some have most bacteria may be killed right away, but a few will likely survive. Antibiotics leave behind the more resistance for several, and a This petri dish contains few cannot be treated with Staphylococcus aureus resistant bacteria to survive and reproduce. When any known antibiotic. bacteria. they reproduce, the genes that make them resistant MRSA can resist an entire are passed on to their offspring. Some bacteria of antibiotics. Patients with an MRSA infection reproduce rapidly—E. coli, for example, doubles its must often be treated with what doctors call “the population every 20 minutes. drug of last resort,” vancomycin. Vancomycin is a In addition to their ability to reproduce quickly, drug that must be given intravenously. Not surpris- populations of bacteria evolve rapidly. Bacteria use ingly, doctors began to see cases of vancomycin-resis- plasmids—small loops of DNA—to transfer genetic tant Staphylococcus aureus (VRSA) in 1997. By 2010, material between individual cells. This process is vancomycin-resistant bacteria were being discovered called conjugation. Some plasmids pass on resistance in the droppings of one out of ten seagulls, leading for one particular antibiotic. Others can transfer scientists to postulate that migrating birds may play a resistance for several antibiotics at once. role in spreading drug-resistant “superbugs.” What characteristics do resistant bacteria pass Staph isn’t the only type of bacteria that is making a on to their offspring? Some have cell membranes comeback with drug-resistant strains. In the mid- through which antibiotics cannot easily pass. Others twentieth century, antibiotics nearly wiped out have pumps that remove antibiotics once they enter tuberculosis (TB). But in the 1990s, TB began to the cell. Some can even produce enzymes that attack

©Hank Morgan/Photo Inc. Researchers, approach epidemic numbers again, and now it kills the antibiotic drugs themselves.

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Fighting Back Some scientists are trying to develop ways to treat TECHNOLOGY patients without killing the bacteria that are making them sick. Instead, they target the produced New Drug Delivery System by bacteria. If the bacteria are not harmed by the Researchers at Yeshiva University treatment, no selective pressure is produced. Scien- decided to take on one of the tists hope that by using this approach, bacteria will most difficult bacterial be slower to evolve defense mechanisms against the infections of all, methicillin- antibiotics. Other scientists hope to fight back by resistant staph. They have using bacteria’s ancient rival, bacteriophages, which developed a treatment using are that infect bacteria. nanoparticles that can be delivered directly to a wound on the skin. staph bacteria CAREERS • Tiny nanoparticles carry nitric oxide (NO), which helps the respond to infection. • The nanoparticles are applied topically, to deep, Evolutionary Biologist infected skin abscesses. in Action • The nanoparticles absorb water, swell, and release DR. Richard Lenski NO. NO kills bacteria and dilates blood vessels, to speed healing. TITLE Professor, Microbial , Michigan State University Because the bacteria are “eating” the nanoballs, adaptations that once kept antibiotics out are no EDUCATION Ph.D., , University longer an obstacle. of North Carolina, Chapel Hill Read More >> at HMDScience.com

If you want to observe evolution in action, you must find populations that reproduce quickly. Dr. Richard Lenski, a professor at Michigan State University, has done just that. Dr. Lenski studies populations of E. coli Unanswered Questions 3A, 3D bacteria, which he grows in flasks filled with a sugary Some important research questions involving broth. These bacteria produce about seven generations drug-resistant bacteria include the following: each day. Dr. Lenski has now observed more than 30,000 generations of E. coli. • Can plasmids or bacteriophages be used in The rapid rate of E. coli reproduction allows vaccines to fight bacteria? Dr. Lenski to watch evolution take place. Dr. Lenski • Are bacteria being exposed to antibiotics in can subject each generation of bacteria to the same sewage systems and evolving resistant strains environmental stresses, such as food shortages or there? antibiotics. He then can compare individuals from more • How do antibacterial soaps and household recent generations with their ancestors, which he keeps cleaners contribute to the evolution of drug- in his laboratory freezer. By comparing generations in resistant bacteria? this way, Dr. Lenski can study how the population has evolved. • Can drug-resistant bacteria be transferred from When Dr. Lenski began his research in 1988, watching domestic animals to humans through food? evolution in action was still new. Now, many Read More >> at HMDScience.com evolutionary biologists are following in his footsteps. Read More >> at HMDScience.com (l) ©AP Images/Michigan State University; (r) ©Gary Inc. Arnold, Gaugler/The Medical File/Peter

384 Unit 4: Evolution