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- © 2012 by Neil H. Shubin 77 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 78 Dædalus, the Journal ofthe American Academy of Arts & Sciences 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.