Fossils Everywhere

Neil H. Shubin

Abstract: History is omnipresent in the natural world, from inside rocks on the continents to the genes, cells, and organs of each creature on the planet. Linking the historical records of rocks, fossils, and genes has been a boon to understanding the major events in evolution. We use these seemingly different lines of evidence as tools for discovery: analyses of genes can predict likely places to ½nd fossils, and new fossils can provide the means to interpret insights from genetics. Viewed in this way, every living thing on Earth is the extreme tip of a deeply branched tree of life that extends three billion years into the past. Genes and fossils reveal how deeply connected our species is to the rest of the living world and the planet itself.

More than a century of discovery has led us to the realization that the descendants of ½sh now walk on land, those of dinosaurs fly in the air, and the evolutionary offspring of arboreal primates fly in space and have left footprints on the moon. One hundred years ago these evolutionary transitions would have seemed utterly impossible, or worse, unthinkable. For example, most ½sh reproduce, feed, and breed in water; for their relatives to invade land, almost every system of their bodies NEIL H. SHUBIN, a Fellow of the would apparently need to change. If the same con- American Academy since 2009, is ceptual challenges hold for every major step in the the Robert R. Bensley Distinguished history of life, how could we ever come to terms Service Professor of Organismal with ancient events, let alone understand their rele- Biology and Anatomy and Associ- vance to our lives today? We must look to the genes, ate Dean of Biological Sciences at cells, and organs of every creature alive today to the University of Chicago. He has performed expeditionary research understand the more than 3.5 billion years of the programs in Canada, Africa, the history of life. Each new piece of evidence that continental United States, Asia, emerges helps reveal how the past has shaped us and Greenland that have led to new and our world. insights into the origin of major We live in an age of invention; new technology groups of vertebrates. He is the changes what we can do, how we live, and what author of Your Inner Fish: A Journey kinds of questions we can ask about our world. The Into the 3.5-Billion-Year History of the Human Body (2008), and his work doubling time of computer chip speeds is surpassed has appeared in Nature and the by the rate at which we can sequence whole genomes Journal of Vertebrate Paleontology, at ever-decreasing cost. The genome of any species among other publications. can now be identi½ed and compared among crea-

Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils tures as different as yeast and humans.1 in an unfortunate marriage. Friends bailed Everywhere Genes can even be swapped between him out of the union, ultimately helping species, moving basic bits of dna between him travel to France to study medicine. flies, worms, and mice. The exponential Returning to Zurich, Gessner had many rate of technological change in biology loves, not least of which were the moun- once prompted a colleague of mine to tains of Switzerland. His passions were the admit that he could have collected all the beauty of nature and the physical exer- data for his Ph.D. thesis–written in the cise of climbing. Lured by the majesty of early 1990s–in a single week. My col- the snow line, he declared a goal of reach- league made that comment about ½ve ing the summit of a different challenging years ago; he could now execute the dis- Swiss mountain each year. sertation in an afternoon. Gessner developed an ardor for describ- In the face of this technological revolu- ing nature–½rst the plants, then the ani- tion, fossil bones seem almost quaint. mals. His four-volume opus Historiae Ani- Most of us encounter these relics in muse- malium, published between 1551 and 1558, ums, where the dinosaurs, ground sloths, was remarkable for its richly detailed de- and mammoths stand motionless in neo- scriptions and illustrations of the world’s classical buildings of marble and granite. living things. In this tome, Gessner did Both the subject and the ½eld appear something that relatively few had done frozen in time; paleontologists digging in before him: he speci½cally compared en- rocks to ½nd remnants of long-lost life is tities inside rocks to bones and shells of a far cry from a roomful of humming com- contemporary organisms. He illustrated puters and gene sequencers. But these are crabs, clamshells, and urchins, revealing special times; profound insights into the that a number of rocks contained similar great transformations of life have come entities. from linking new genetic, developmental, How could rocks look like living crea- and computational tools with approaches tures? In the 1500s, answers to this ques- that date from the days of Leonardo da tion took several forms.3 One common Vinci. explanation was that the rocks held mon- Fossils are a kind of window into our strosities that were destroyed in the great perceptions of life, the planet, and our his- flood during Noah’s time. Another theory, torical connection to them.2 Most of us common in Gessner’s day, was that life- take their meaning entirely for granted, like objects were produced by the same so much so that it is hard to envision how forces that made the rocks: some stones strange these objects seemed when ½rst contained things that only coincidentally encountered by philosophers centuries looked like wood, bones, and teeth. These ago. Our own conception of them–as evi- objects were not associated with creatures dence of creatures that inhabited long- alive or dead because they were consid- lost worlds–arose in parallel with an ered natural outgrowths of the rock itself. entirely novel way of thinking about the The other explanation was that fossils re- natural world. flected a kind of Loch Ness phenomenon: perhaps they were mysterious animals In 1541, Conrad Gessner, then twenty- that could be found alive in remote or ½ve years old, landed in Zurich as a lectur- unexplored regions of the planet. er in physics. His path to his new post was All these conceptions changed in 1666, anything but easy: having lost his father when ½shermen working on the coast of in battle at a young age, he found himself Italy caught a giant shark. This monster

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 from the deep drew the attention of the eons of time. Layer after layer reflect Neil H. Grand Duke of Tuscany, who was a great changes in the atmosphere, climate, and Shubin patron of science. He ordered it sent to geography of the planet, and the fossils Niels Stensen, one of the young scientists inside provide a window into the succes- he was supporting at the time. Stensen sion of living things. (known in his publications by the latinized This approach is not just a new way of Steno) studied medicine and had originally thinking; it also is a means of discover- earned the Duke’s favor for his extraordi- ing. Since the days of Steno and his con- nary knowledge of anatomy and his clever temporaries, paleontologists have used a use of experimentation to understand how growing knowledge of the world to iden- bodies function. Steno described the tify places likely to yield fossil discover- bones, muscles, and nerves of the shark ies. While paleontological discovery is head, but his most memorable observa- often accidental–for example, by con- tion came from studying teeth. struction or road crews hitting fossil bones A long-standing puzzle, dating from as they unearth rock–most discoveries before Steno’s time, was the presence of are planned. That is, to examine a question oddly shaped objects, known as “tongue or problem, such as determining links stones,” commonly found in clumps on between ½sh and amphibians, we begin by the ground or still embedded in rocks. narrowing down the mapped regions of These stones had an uncanny resemblance Earth to small sites where fossils might be to shark teeth. The Roman natural histori- found. The approach is straightforward: an Pliny the Elder declared that they either ½nd places where rocks of the right age fell out of the sky or dropped from the and the right type to preserve fossils of moon. Others followed the standard inter- interest are exposed at the surface. Eco- pretation, viewing them as natural out- nomic incentive fuels part of this search: growths of rocks. Steno changed every- geological surveys spurred by the poten- thing. He looked not only at the stones, tial of oil, gas, and mineral development comparing them to teeth, but also at the often prompt states and private industries rocks in which they were found. He noted to map the rocks within their purview. that the stones were recovered from cliffs Geological maps, commonly made at a made up of layer after layer of rock, one on very ½ne scale, are often easy to come by, top of another. To Steno, tongue stones as are aerial photographs that reveal the were actually shark teeth, and not just exposures in any given area. any shark teeth–they were ancient shark Knowing a few relatively simple features teeth, buried under layers of sediment. about the geological landscape greatly Steno developed a theory about what the enhances the odds of ½nding new fossils. stones were and how they were preserved. The ½rst is rock type. Sedimentary rocks The completion of his shark monograph are best: unlike igneous or metamorphic in 1667 was an important moment in the rocks, they have not been superheated or birth of paleontology as a discipline.4 transformed by the tremendous pressures that exist within the Earth. They may Viewing fossils as the remnants of past have been laid down in ancient oceans, life, and layers of rock as reflecting a suc- streams, soils, or even sand dunes. cession of ages, is a relatively new way of Understanding how grains sort inside a understanding the planet and life on it. rock, as well as how different layers of rock The rocks of the world are a library of change relative to one another, can give sorts, whose pages and chapters record clues to the kind of environment in which

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils the rock was deposited. Stream beds, for biology departments splitting into the Everywhere example, can have a lenticular shape, more narrowly focused departments of almost like the cross section of a channel molecular biology, cell biology, organis- bed. Within ancient channels are often mal biology, and so on. But at the same grains that range from rounded cobbles time that the biological disciplines were to ½ne particles. The size of these sedi- becoming more divided, the questions ments and the way they are sorted within and conceptual tools to unite them began the deposit reveal not only if the rock was to emerge with increasing rapidity. deposited in a stream, but also whether One of the greatest boons to paleontol- the channel was big or small and if the ogy originally appeared to be the most sig- water was running fast or slowly. By study- ni½cant threat to its existence as a producing these features, fossil hunters can pre- tive ½eld of inquiry. The approach began dict where they might ½nd fossils in the with a simple notion of Darwin’s: descent ½eld. The likelihood of ½nding a com- with modi½cation.5 If there is a common plete articulated skeleton in the middle history to life on Earth, then descendants of an ancient channel bed is vanishingly should be modi½ed versions of their an- small: moving water may have scattered cestors. Just as each individual is a mod- and broken bones, particularly smaller i½ed descendant of its parents, so, too, and fragile ones. When looking for high- should species be descendants of their quality bones in freshwater settings, we ancestors. This pattern of descent with tend to focus on the margins of streams, modi½cation should yield a pattern in the in the eddies and banks where matter history of life: that is, the features that would settle out at different times of the creatures share with one another should year. Field paleontologists develop a cat- reflect their history. For example, if we alog of rock occurrences like these and, wanted to know how clams, ½sh, mice, when on the rocks, will typically make a and people are all related, we would com- beeline for their favorites. pare their characteristics and discover that Finding new places to look also means these creatures share dna, cells, and other predicting the right age of rock to investi- features, but that ½sh, mice, and people gate. Here, the full suite of biological infor- share characteristics absent in clams– mation comes into play. To appreciate this such as a backbone, skull, and centralized approach, we must look back to a time in brain. Mice and people are even more biology when the study of dna and the similar; unlike ½sh, they share hair, warm study of fossils were utterly separate ap- bloodedness, and mammary glands. We proaches to science. could even look at the genes and proteins of each of these animals and come to the Biology is a vast ½eld encompassing same conclusion: ½sh, mice, and people multiple levels of organization, from mol- are more closely related to one another ecules, to genes and cells, to entire ecosys- than any are to clams. Moreover, mice and tems. Work on each of these levels has its people have more in common with each own empirical approach: different tools of other than they do with ½sh.6 By adding microscopy, spectroscopy, and ½eld analy- species and features to this analysis, we sis underlie disciplines as varied as struc- could ultimately develop a hypothesis of tural biology and ecology. In the 1970s, a complete tree of life. biology became increasingly fragmented The important point to draw from this according to level and approach, with a exercise is that we do not need a single fos- number of prominent, comprehensive sil to infer the relationships among living

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 things. By applying the “descent with In the 1940s, no approach helped pale- Neil H. modi½cation” approach to the relation- ontologists understand the origin of Shubin ships discussed above, we can infer that whales. With nostrils modi½ed to become mice and people share a more recent com- blowholes, no hind limbs, and extreme mon ancestor with each other than they modi½cations of the brain case, whales do with ½sh. Armed with a knowledge of were a complete enigma. They were so genes, tissues, and organs of living crea- odd that they could not easily be com- tures, we can infer the hierarchy of life–a pared to any creature–living or extinct. tree of relatedness that shows the relative The problem was so great that the pale- recency of common ancestry. ontologist George Gaylord Simpson in- Far from removing fossils from the pic- serted the group arbitrarily into his classic ture, however, this approach de½nes their 1945 classi½cation of mammals, saying importance. Plotting the relationships that cetaceans are “the most peculiar and between living vertebrates helps us con- aberrant of mammals,” and adding that struct the family tree, for example, by “there is no proper place for them in a demonstrating that turtles and lizards are scale naturae.”9 more closely related to mammals than A few years later, in the late 1940s, two birds are. But this method alone cannot scientists used a crude test to look at the tell us what the ancestors of mammals similarity of proteins in the blood of dif- looked like. When we explore the fossil ferent mammals.10 Using an assay that record from rocks 230 million years old, criminologists employed to discern human we ½nd a number of creatures with reptil- from animal blood at a crime scene, they ian jaws and skulls, but with a dog-like tested the plasma of different species by posture. These creatures have features of looking at how they interacted with anti- the ear, teeth, and skull that reveal inter- bodies. Closely related species should have mediate conditions between so-called more similar antibody reactions than reptilian bodies and those of mammals. more distantly related ones. The scientists Fossils bring to light transitional fea- found that the proteins in the blood of tures, ancient environments, and ecosys- whales were more similar to those of even- tems that have been lost in time.7 toed ungulates–including deer, hippos, Together, genes and fossils provide and goats–than to anything else. But this information that each alone cannot. If you was a puzzling discovery. These creatures, take a tree of relatedness developed from artiodactyls, have a very distinctive ankle genes, or from any kind of data (Figure 1a), bone, consisting of a double pulley joint and map known fossil occurrences onto it that helps them with their running and (Figure 1b), the end result is a clear pic- bounding gaits. Extant whales not only ture of what is unknown (Figure 1c). By had no ankle bones with a pulley joint, looking for these so-called ghost taxa8– but they had no hind limbs whatsoever. So extinct species we infer should be present how could there be a connection? Simp- but are absent–we can concentrate our son’s problem remained. ½eld efforts to ½ll huge gaps in the fossil As new techniques to compare genes record with transitional forms. There is a and proteins emerged in the ensuing de- deep beauty to the idea that comparisons cades, scientists gained a bonanza of new of dna in different species can give us data to compare whales with other mam- clues about where to discover new fossils mals. By the mid-1990s, mitochondrial inside rocks. genes,11 milk casein genes,12 and others not only strengthened the artiodactyl idea

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils Figure 1 Everywhere Filling Gaps

Using genes to explore for fossils, (a) an evolutionary tree can be constructed for living creatures, such as sharks, jawless ½sh, and their closest invertebrate relatives; (b) the fossil representatives of each form can be mapped in time; and (c) merging the tree and the fossil occurrences reveals places in the geological record where fossils are likely missing (indicated by the dotted line and arrow). Source: Figure created by John Westlund, University of Chicago; used here with permission from Westlund.

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 but led to the proposition that one group, mon ancestor with other mammals, their Neil H. hippos, are the closest living relative of close relatives must have been quadripeds. Shubin whales. Yet fossil data spoke to a different Then, as one of Gingerich’s graduate theory of whale relationships, although students was cleaning a fossil whale skele- not conclusively. Comparison of the teeth ton in preparation for its extraction back and skulls of whales to other mammals to the lab, a small, pulley-shaped bone suggested a relationship to an extinct appeared to poke out of the rock. Once group of terrestrial, four-legged creatures removed and cleaned, the bone was clear- known as mesonychids. In fact, everything ly identi½able as an ankle bone. And this known of the anatomy suggested that was not just any ankle bone, but one from artiodactyls are only distantly related to the double-pulleyed ankle of an artiodac- whales. As the authors of one of the genet- tyl. Armed with the new fossils showing ic studies noted, “paleontological infor- the transformational character of evolu- mation is grossly inconsistent with [the tion, we are now in a position to under- artiodactyl] hypothesis.”13 stand how the whale’s unique body plan About ten years before this flurry of arose and what the ecosystems it lived in molecular work, Philip Gingerich and his looked like during the change. A predic- colleagues were investigating fossil expo- tion, born of blood samples and extended sures in Pakistan. Gingerich had followed to proteins and genes, was con½rmed the paleontological rulebook: the rocks, at inside ancient rocks.15 about forty-seven million years old, were We are accustomed to thinking of a rev- the right age (they reflected the interval olution in gene sequencing and molecular when the diverse orders of mammals technology, but we are also experiencing came about); were the right type (they one in the ½eld of paleontology. Whales were mapped as ancient stream deposits); with legs are one of a number of creatures and were well exposed. Gingerich, how- that tell us of the great transformations in ever, was working from an inaccurate map, the history of life. Using the paleontologi- and once on-site, he realized that instead cal playbook, expeditions have discovered of stream beds, he was standing on an worms with heads,16 ½shes with elbows, ancient ocean. That setback did not stop wrists, and necks,17 feathered dinosaurs,18 him from looking for fossils anyway. He and human precursors,19 to name only a and his team found a number of new fos- few. Indeed, in the last twenty years, we sils, including pelvic bones that the team have discovered more creatures informa- jokingly called “walking whales.” A few tive of evolutionary transitions than in years later, the rocks yielded whale fossils the previous millennium. in the form of some isolated skulls.14 But those pelvic bones remained enigmatic. Exploratory paleontologists such as Phil With whale origins now on his mind, Gingerich use knowledge of evolutionary Gingerich shifted his focus to Egypt, the history and the geological record to ½nd home of well-exposed marine rocks from evidence of ancient transitions. Another a slightly younger age. Sure enough, the record altogether can provide clues. In team discovered whales. In addition, and the late 1990s, David Kingsley and Katie ½ttingly for the Darwinian theory, these Peichel began a hunt for the ideal species whales had hind limbs. This was a grati- to study the way traits and genes evolve in fying and important discovery, but not natural populations.20 Ever since the days entirely unexpected under Darwinian of T. H. Morgan, biologists have used so- thinking. Because whales share a com- called model organisms, such as fruit flies,

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils house mice, and African clawed frogs, to ing or losing hind limbs, among a host of Everywhere provide insights into basic questions of other new traits. genetics, cell biology, and development. The key point is that the differences Laboratory species have features that between freshwater and marine stickle- make studying their basic biology acces- backs are so large that, for all intents and sible: they typically breed rapidly and eas- purposes, they could be characterized as ily, have anatomical or behavioral features different species. However, although they that might provide general insights, and are often reproductively isolated in the are tractable to study using molecular, wild, these very different kinds of stickle- microscopic, and cellular methods. Kings- backs can still be coaxed to interbreed ley and Peichel had an additional goal: under the right conditions in the labora- they wanted to ½nd a creature that would tory. Thus, the team could interbreed the allow them to trace the genes involved in animals and identify the chromosomal the origin of new organs, physiological regions responsible for the differences processes, and behaviors. Their search re- among natural populations. By breeding vealed the potential of a ½sh, ranging from the different kinds of ½sh and analyzing one to four inches long, called the three- their genetic structure, Kingsley, Peichel, spine stickleback. and their colleagues could trace how The threespine stickleback is an ordi- changes at the genetic level were associat- nary-looking ½sh with a long history of ed with dramatic changes in the body and study. The famed Dutch ethologist Niko physiology of the new kinds of stickleback. Tinbergen won a Nobel Prize in part for One of the novelties that distinguishes his work on them. Ecologists and paleon- many freshwater from marine stickle- tologists have had their turn at the species, backs is a reduction in the pelvis and the too, producing a vast literature that con- pelvic spines that attach to it. Marine tains thousands of scienti½c papers and sticklebacks live with a number of preda- analyses. To Kingsley and Peichel, the tors, and the presence of big pelvic spines stickleback had all the characteristics of is one defense to avoid being eaten. Fresh- an excellent genetic system: the creatures water sticklebacks, on the other hand, breed easily and develop relatively rapidly. frequently evolve in environments that But most interesting was the tremendous lack the soft-mouthed predators found in variety of subspecies of threespine stickle- the ocean. Moreover, because ½n skeletons backs that have evolved since the glaciers are metabolically expensive to develop, retreated ½fteen thousand years ago. As the freshwater ½sh often have smaller pel- the ice gave way, new lakes and streams vises and spines, or they lose these fea- emerged. From their ancestral marine tures altogether. With this information as range, migratory ocean sticklebacks in- their inspiration, Kingsley, Peichel, and vaded or became restricted to different colleagues set off to collect sticklebacks streams and lakes, often becoming isolat- for breeding experiments that would ed and evolving a number of important identify the genetic region responsible characteristics. The ecological and physi- for the loss of the pelvis and spines in dif- ological environment of freshwater forms ferent populations. Much like Gingerich is so different from the denizens of the homing in on sites to ½nd fossil whales, oceans that the freshwater sticklebacks Kingsley and colleagues chose the right evolved a number of new features–losing places on Earth to obtain the sticklebacks. their protective armor, changing their The resulting genetic analysis revealed feeding structures, and sometimes reduc- a number of chromosomes involved in the

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 reduction of the pelvic appendage. But in short stretch of dna that serves as a reg- Neil H. terms of relevant data, one site reigned ulatory switch controlling Pitx1’s expres- Shubin supreme: the region responsible for most sion only in hind ½ns.23 A mutation in of the limb loss21 contained the famous this region–the “thermostat” for a single Pitx1 gene. Pitx1 was known in mammals location in the body–leads to loss of ½n and ½sh to be involved in the develop- development in the hind ½n while pre- ment of tissues across the body, from serving other functions of Pitx1. The dif- heads to appendages. ference between freshwater ½sh that lack When the team looked at the differences pelvic ½ns and their marine cousins that in the gene itself, they found that the retain them lies largely in the stretches of dna sequence of Pitx1 was largely un- dna that control gene activity. This makes changed between marine and freshwater sense: a change in the structure or se- ½sh.22 At ½rst glance this ½nding seems quence of the gene would affect every tis- utterly strange: how can Pitx1 be involved sue in which the gene is active. Given that with a major anatomical change like loss Pitx1 has global effects, a change is likely of the pelvis if the gene itself does not to be harmful, if not lethal. The tissue- have any recognizable differences among speci½c changes in gene activity mean the different ½sh? If the structure for the that ½ns can change independently of the gene is not the culprit, perhaps the loss of rest of the body. pelvic ½ns relates to a change in the ele- Not only can this area of regulatory dna ments that control the activity of the gene. be identi½ed and its function mapped, Genes often have one or more outside but it can be swapped between different elements that serve as a kind of switch kinds of stickleback.24 When the Kings- determining when and where the gene is ley group took the Pitx1 regulatory ele- active. Some of these regions, known as ment from a stickleback with a complete regulatory elements, are highly speci½c pelvis and inserted it into an individual to one organ or tissue. Changes to the from a population that had lost the regulatory elements can bring about a pelvis, something remarkable happened: modular change speci½c to one region. like a ghost from the past, the pelvis By contrast, a change in the sequence in appeared.25 Kingsley and his colleagues the gene itself could have an effect every- swapped genes to make a fossil of sorts. where the gene is active. Imagine a house Little sticklebacks may open a window with one furnace but different thermo- to great transformations, perhaps even to stats in each room. A change to the furnace the hind-limb loss we see in fossils like would affect the entire house, a change to those discovered by Gingerich. The more a thermostat only a single room. The same we look, the more we ½nd similarities in is true with the genes and their regulato- the regulatory genes that underlie the ry elements. development of tissues, organs, and the Detecting regulatory elements is dif½- architecture of the bodies of diverse ani- cult and involves fusing dna sequences mals. Pitx1 is no different; it is seen in with visible labels in order to determine mammals, ½sh, lizards, and birds. And the where particular sequences are active, gene leaves a signature of its activity in manipulating the dna to see what hap- the limb, showing a preference for one pens when a region is deleted or changed, side of the body over the other. Given this and sometimes swapping dna between clue, Kingsley and his coauthors suppose species and individuals. With this tool kit, that modi½cations of the regulation of Kingsley’s group identi½ed a relatively this gene may underlie limb reduction in

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils many other creatures, including aquatic precision and by reconstructing evolu- Everywhere mammals such as manatees.26 Indeed, mu- tion’s effects, either in part or in full, in tations in Pitx1 activity cause a range of the laboratory. limb malformations in mammals, such as The layers of crust on Earth, like the clubfoot in human infants.27 genes, cells, and dna of every living thing, are chronicles of history. But rocks, bod- This story is more general than Pitx1, ies, and genes are not independent records manatees, whale fossils, or even human of time; they are linked by billions of years skeletons. By leveraging the genetic and of planetary and biological evolution. geological record to discover fossils, and Every living thing is the most extreme tip moreover, by using molecular biology to of a branch of an almost boundless tree of isolate genes underlying evolutionary life; and all living creatures contain arti- change and test their effects in the labo- facts of a history nearly as ancient as the ratory, the study of great transformations planet. There is something almost poetic in the history of life can look forward to a to the notion that 3.5 billion years of future as a predictive science. In the com- change has brought one of these species ing years, it is not unlikely that we will be to a moment when it can see its own past able to study evolution in the distant past and grasp the deep interconnections both by ½nding fossils with increasing embedded in the world around it.

endnotes 1 Barbara R. Jasny and Laura M. Zahn, “A Celebration of the Genome, Part IV”; Eric S. Lander, “The Accelerator”; Peter Donnelly, “Making Sense of the Data”; David Botstein, “Fruits of Genome Sequencing for Biology”; Yijun Ruan, “Presenting the Human Genome: Now in 3D!”; Steven E. Hyman, “The Meaning of the Human Genome Project for Neuropsychiatric Disorders”; Mary-Claire King, “A Healthy Son”; Vololona Rabeharisoa, “Socializing Genetic Diseases”; Liz Lerman, “The Genome Dances”; and Steve Gano and Ro Kinzler, “Bringing the Museum into the Classroom,” all in “Essays on Science and Society,” Science 331 (2011): 1024–1029. 2 Martin J.S. Rudwick, The Meaning of Fossils: Episodes in the History of Paleontology (Chicago: University of Chicago Press, 1985). 3 Ibid. 4 Ibid. 5 Willi Henning, Phylogenetic Systematics (Champaign: University of Illinois Press, 2000). 6 Ian J. Kitching, Peter L. Forey, Christopher J. Humphries, and David M. Williams, Cladistics: Theory and Practice of Parsimony Analysis (Oxford: Oxford University Press, 1998). 7 Michael J. Donoghue, James A. Doyle, Jacques Gauthier, Arnold G. Kluge, and Timothy Rowe, “The Importance of Fossils in Phylogeny Reconstruction,” Annual Review of Ecology and Sys- tematics 20 (1) (1989): 431–460. See also, Philip C.J. Donoghue and M. Paul Smith, eds., Telling the Evolutionary Time: Molecular Clocks and the Fossil Record (Boca Raton, Fla.: crc Press, 2003), chap. 5. 8 Ibid. 9 George G. Simpson, “The Principles of Classi½cation and a Classi½cation of Mammals,” Bul- letin of the American Museum of Natural History 85 (1945): 1–350.

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Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/DAED_a_00163 by guest on 26 September 2021 Fossils 26Michael D. Shapiro, Michael A. Bell, and David M. Kingsley, “Parallel Genetic Origins of Pelvic Everywhere Reduction in Vertebrates,” Proceedings of the National Academy of Sciences USA 103 (37) (2006): 13753–13758. 27 Christina A. Gurnett, Farhang Alaee, Lisa M. Kruse, David M. Desruisseau, Jaquline T. Hecht, Carol A. Wise, Anne M. Bowcock, and Matthew B. Dobbs, “Asymmetric Lower-Limb Mal- formations in Individuals with Homeobox PITX1 Gene Mutation,” The American Journal of Human Genetics 83 (2008): 616–622.

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