15 Evidence for Evolution DRIVING QUESTIONS 1. How does the fossil record reveal information about evolutionary changes? 2. What features make Tiktaalik a transitional fossil, and what role do these types of fossil play in the fossil record? 3. What can anatomy and DNA reveal about evolution? Copyright ©2018 W.H. Freeman Publishers. Distributed by W.H. Freeman Publishers. Not for redistribution. 21_SHU_31905_ch15_346_367.indd 346 13/07/17 11:28 AM A Fish with Fingers? A transitional fossil fills a gap in our knowledge of evolution OR 5 YEARS, BIOLOGISTS NEIL SHUBIN AND TED DAESCHLER spent their summers trekking through one of the most desolate regions on Earth. FThey were fossil hunting on remote Ellesmere Island, in the Canadian Arctic, about 600 miles from the north pole. Even in summer, Ellesmere is a forbidding place: a windswept, frozen desert where sparse vegetation grows no more than a few inches tall, where sleet and snow fall in the middle of July, and where the sun never sets. Only a handful of wild animals survive here, but those that do make for dangerous working conditions: hungry polar bears and charging herds of muskoxen are known hazards of working in the Arctic, says Daeschler, who carried a shotgun for protection. Photo used with permission from Tangled Bank Studios, LLC. Copyright 2014. All rights reserved. Tangled Photo used with permission from 347 Copyright ©2018 W.H. Freeman Publishers. Distributed by W.H. Freeman Publishers. Not for redistribution. 21_SHU_31905_ch15_346_367.indd 347 13/07/17 11:29 AM 348 15 UNIT 3 • HOW DOES LIFE CHANGE OVER TIME? EVOLUTION AND DIVERSITY Ted Daeschler/Academy of Natural Sciences/VIREO Ted Fossil hunting in the vast wilderness of Ellesmere Island in the Canadian Arctic. When not looking over their shoulders, of four-legged, land-dwelling vertebrates— the researchers drilled, chiseled, and ham- including humans. mered their way through rocks looking for Tiktaalik “splits the difference between fossils. Not just any rocks and fossils, but ones something we think of as a fish and something dating from 375 million years ago, when ani- we think of as a limbed animal,” says Daeschler, mals were taking their first tentative steps on a curator of vertebrate zoology at the Acad- land. For three summers, they scoured the emy of Natural Sciences in Philadelphia. “In site of what was once an active streambed that sense, it is a wonderful transitional fossil but found little of interest. Then, in 2004, between two major groups of vertebrates.” the team made a tantalizing discovery: the Today, of course, four-legged animals snout of a curious-looking creature protrud- roam far and wide over land. But 400 mil- ing from a slab of pink rock. Further excava- lion years ago it was a different story. Life was tion revealed the well-preserved remains of mostly aquatic then, restricted to oceans and several flat-headed animals between 4 and 9 fresh water streams. How life made the jump feet long. In some ways, the animals resem- from water to land is a question that has long bled giant fish—they had fins and scales. But intrigued evolutionary biologists. In fact, sci- they also had traits that resembled those of entists have been searching for evidence of land-dwelling amphibians—notably, a neck, this milestone ever since Charles Darwin first wrists, and fingerlike bones. The research- proposed that all life on the planet is related ers named the new species Tiktaalik roseae; by a tree of common descent. According to tiktaalik (pronounced tic-TAH-lick) is a native Shubin, a professor of biology at the Univer- VERTEBRATE word meaning “large freshwater fish.” This sity of Chicago and the Field Museum of Nat- An animal with a bony or ancient hybrid animal no longer exists, but ural History, Tiktaalik is the most compelling cartilaginous backbone. it represents a critical phase in the evolution example yet of an animal that lived at the cusp Copyright ©2018 W.H. Freeman Publishers. Distributed by W.H. Freeman Publishers. Not for redistribution. 21_SHU_31905_ch15_346_367.indd 348 13/07/17 11:29 AM 349 of this important transition. Not only does it Because not all organisms are preserved, DESCENT WITH 15 fill a gap in our knowledge, the discovery also the fossil record is not a complete record MODIFICATION EVIDENCE FOR EVOLUTION provides persuasive evidence in support of of past life. Nevertheless, the existing fossil Darwin’s term for evolution, Darwin’s theory. record is remarkably rich and offers a revealing combining the ideas that all living things are related and window into the past. Paleontologists, scien- that organisms have changed tists who study ancient life, have uncovered over time. Reading the Fossil hundreds of thousands of fossils throughout FOSSILS Record the world, from many evolutionary time peri- The preserved remains or ods. When fossils are arranged in order of impressions of once-living The theory of evolution—what Darwin called age, they provide a tangible history of life on organisms. descent with modification—draws two Earth. The fossil record also allows biologists FOSSIL RECORD main conclusions about life on Earth: that all to test certain tenets of Darwin’s theory. An assemblage of fossils living things are related, and that the differ- arranged in order of age, For example, if all organisms have de- ent species we see today have emerged over providing evidence of scended from a single common ancestor that time as a result of natural selection operat- changes in species over time. lived billions of years ago, as the theory of ing over millions of years. Many lines of evi- PALEONTOLOGIST evolution concludes they did, then we would A scientist who studies dence support this theory (remember that in expect the fossil record to show an ordered ancient life by examining the science a “theory” is an idea supported by a fossil record. succession of evolutionary stages as organisms tremendous amount of evidence and which evolved and diversified. And, indeed, that is has never been disproved; see Chapter 1). exactly what we see: prokaryotes appear be- One of the most compelling lines of evidence fore eukaryotes, for evolution comes from fossils, the pre- single-celled organisms before served remains or impressions of once-living multicellular ones, water-dwelling organisms organisms. Fossils are like snapshots of past before land- dwelling ones, fish before amphibi- life, capturing what life was like at particular ans, reptiles before birds, and so on. moments in time. Moreover, we would expect to see changes Fossils are formed in a number of ways: over time within a family of organisms, and an animal or plant may be frozen in ice, we do. One exceptionally well studied exam- trapped in amber (hardened tree sap), or ple is horses. Comparisons of modern-day buried in a thick layer of mud. The entombed horse bones with fossils of horse ancestors organism is thereby protected from being eaten by scavengers or rapidly decomposed Ted Daeschler/Academy of Natural Sciences/VIREO by bacteria. Over time, if conditions are right— for example, if the mud encasing the speci- men remains undisturbed long enough for hardening to occur—the organism’s shape is preserved. Not all organisms are equally likely to form fossils, however: animals with bones or shells are more likely to be preserved than animals without such hard parts (think earthworms or jel- lyfish) that decay quickly. And conditions per- mitting fossilization are rare: the organ- ism has to be in just the right place at just the right time A Tiktaalik roseae fossil. (INFOGRAPHIC 15.1). Copyright ©2018 W.H. Freeman Publishers. Distributed by W.H. Freeman Publishers. Not for redistribution. 21_SHU_31905_ch15_346_367.indd 349 13/07/17 11:29 AM 350 15 INFOGRAPHIC 15.1 Fossils Form Only in Certain Circumstances UNIT 3 • Not every organism that dies forms a fossil. Organisms are more likely to fossilize if they have bony skeletons or hard shells. In addition, the organism must be preserved quickly and kept undisturbed while mineralization or mud hardening occurs. Therefore, the fossil record is not a complete record of past life, but it has supplied an impressive body of evidence for evolution. HOW DOES LIFE CHANGE OVER TIME? EVOLUTION AND DIVERSITY Organism dies. choicegraphx/iStockphoto Is the dead organism No Fossil Forms No Organism is eaten preserved quickly? or broken down by aerobic microbes. Yes Organism is preserved by rapid Organism is preserved by rapid burial freezing, desiccation, or burial in in sediment layers. amber (hardened tree sap). In these cases, Quick burial in sediment organisms are (for example, by a preserved largely mudslide or volcanic in their original eruption) protects the state. body from rapid decay. Colin Keates/Getty Images Spider in amber Are the conditions right for fossilization of this No Fossil Forms buried organism? No Organism is slowly broken down by Yes bacteria, and water contributes to erosion. Organism is fossilized by mineralization. Organism is fossilized by imprint or mold. Hard parts of the body The soft mud in which a such as teeth and bones dead organism is buried do not decay rapidly. hardens into rock around Over time minerals in the organism. Over time the water are deposited in the organism decays, leaving a spaces within bones or space in the surrounding Arpad Benedek/Getty Images replace the bone as it rock. The space has the breaks down. The result Ralf Hettler/iStockphoto same shape as the exterior is a mineralized fossil. of the organism. ? Why are flies in amber more common than fossilized flies? reveal how, in the course of evolution, horses There are several branches and lineages of have lost most of their toes.