Written in Stone: Evolution, the Fossil Record, and Our Place in Nature

by Brian Switek

  Most beautiful is what we do not comprehend.

  —NICOLAUS STENO, 1673

  In October 1666 a French fishing boat spotted an enormous fish off the shores of Tuscany. It was a great white shark, a toothy monster over a ton in weight, and it was quickly caught and hauled onto the beach. It did not perish easily; just when it seemed to die at last it would suddenly begin thrashing the sand with its crescent-shaped tail. So it was tied to a tree to prevent it from flopping itself back into the sea and escaping.

  Sharks were common in the Mediterranean, but such a prodigious fish was a rare catch. Word of its capture quickly reached the Grand Duke Ferdinando II at the Medici palace in Florence. Nothing would please him more than to have his cadre of anatomists dissect the lamia, but he would have to act quickly. The shark had already begun to rot and its bulk was too great to convey overland to the anatomical theater. There was no option but to cut off the shark’s battered and bloody head, which was loaded onto a cart bound for Florence.

  The question was who would get the honor of dissecting the rare treasure from the sea. Ferdinando had cultivated a garden of scholars able to do the job, but he chose a young naturalist from Denmark named Nicolaus Steno. At twenty-eight Steno was already known for his keen observations and preternatural skill with a scalpel. As soon as the great fish head arrived Steno prepared to study it.

  The dissection was not a quiet affair hidden away in a silent laboratory. The autopsy was a public event, as much a piece of performance art as a scientific deconstruction of a putrid fish. Layer by layer, Steno peeled back the strata of tissues to reveal the primitive inner workings of the sea monster, but the most significant part of the head was among the most obvious: the teeth. Even though Steno made no precise measurements of them, the teeth of this shark would remain on his mind long after he left the anatomical theater.

  For as long as anyone could remember, dark, triangle-shaped stones of varying size had been found throughout the European countryside. No one knew what they were, but there was certainly something special about them. Some scholars, like the classical natural historian Pliny, thought they fell from the sky on moonless nights, and others believed they were the petrified tongues of snakes, called glossopetrae. One of the most popular beliefs was that they were products of the rock trying to imitate life. The earth, through some kind of plastic force, was reproducing bits and pieces of the living world, or in the words of Steno’s contemporary Athanasius Kircher, had been produced by a “lapidifying virtue diffused through the whole body of the geocosm.” Regardless of their origin, though, it was widely agreed that such unique stones held special powers. They were often prescribed (in powdered form) as a cure for a variety of common ailments.

  The sixteenth-century French naturalist Guillaume Rondelet had a different view. The tongue-stones were curiously similar to the teeth of sharks he commonly saw in the fish markets. Perhaps, as Rondelet publicly speculated in 1554, the glossopetrae were not of supernatural origin after all, but had once been arrayed in the mouths of living sharks. This sounded too fantastic to be true, especially since so many of the “shark teeth” were found far inland. It was easier to accept that they were supernatural in origin, and this view still prevailed when the Italian lawyer Fabio Colonna considered the “tongue stones” in 1616. Colonna had been driven to investigate the glossopetrae in detail due to the claims of their medicinal value, but after dissecting them for himself he felt safe in concluding that “nobody is so stupid that he will not affirm at once at the first insight that the teeth are of the nature of bones, not stones.”

  As with Rondelet’s conclusion, however, Colonna’s findings did not gain traction. What they had proposed grated against “common sense.” If an alternate explanation of the origins of the teeth was to catch on it would have to make sense not only of the nature of the teeth but also how they found their way onto dry land.

  Steno, working fifty years later, was sure that the glossopetrae were true teeth like those from the mouth of the shark. He had seen similar stone teeth in the collection of his former professor, Thomas Bartholin, but he ran into the same questions that Rondelet and Colonna faced. Where had the teeth come from? How had teeth turned to stone? And, for that matter, why were the teeth often found among seashells many miles from the nearest shoreline?

  FIGURE 5 - A woodcut of the head of a great white shark, as figured in Steno’s report on its dissection. The resemblance between its teeth and “tongue stones” found around the countryside started Steno thinking about Deep Time.

  As Steno studied the curious petrifactions, he had a startling thought. The fossil teeth had clearly once belonged to living animals, so perhaps the teeth had been shed and deposited in the soft mud of an ancient seabed. They were eventually buried, along with shells and other detritus, by sand and mud that hardened into rock as the years went by. This process entombed the organic productions and eventually transformed them to resemble the rock in which they were found. What had happened to the ancient sea was another matter.

  For Steno there seemed to be only two possibilities: either the ocean had once covered the land or the seafloor had become land. The Bible clearly stated that the earth had twice been entirely covered in water, during the Creation and the later Flood, but observations showed that land once under the sea could be pushed up in the aftermath of earthquakes. Both scenarios could have caused the fossils to come to rest in the countryside.

  Steno published these early findings in his 1667 report on the shark dissection, Canis carcharias dissectum caput, but his work on the subject was only just beginning. The puzzle of the fossils lured him further away from the anatomical theater and more often into the mountains in search of fossil remnants of the ancient seabed. This was a period of emotional agony for him, for he was wrestling with his faith and ultimately converted to Catholicism, but Steno kept up his work and was blindsided by a finding so obvious it was strange that no one had reported on it before.

  As Steno ambled the hills around Florence in search of fossils he often saw layers of rock stacked one on top of the other like the pages of a great stone book. This had been appreciated by a few earlier naturalists, Leonardo da Vinci among them, but their findings were either lost, kept secret, or forgotten. Just as with the shark teeth, it would be Steno who would have to not only rediscover what others had already found, but explain the phenomenon as well.

  Steno presented his thoughts on both strata and the fossils in his 1669 essay Prodromus to a Dissertation on Solids Naturally Enclosed in Solids. It was only an abstract of a fuller work that he was planning, but an essential part was Steno’s idea of how fossils formed. Steno wrote:Given a substance endowed with a certain shape, and produced according to the laws of nature, to find in the substance itself clues disclosing the place and manner of its production.

  It was not necessary to appeal to “plastic forces” or supernatural phenomena; if you want to understand nature you have to turn to nature itself. Fossils were relics of lost chapters in earth’s history that contained clues as to how they were made. Neither Steno nor anyone else could visit the time when Florence was sunk beneath the shark-patrolled waters, but by studying fossils Steno could discover what had happened during the incredibly remote period when the fresh teeth fell to the seafloor.

  The layers of rock Steno observed were essential to proving the antiquity of the fossil teeth and shells he had collected. The fossils were found in lower layers of rock, and Steno reasoned that as sediment accumulated on the bottom of the primeval sea the layers began to stack up. The youngest layers would be on top and the oldest on the bottom. Tracing the characteristics of the layers from top to bottom allowed one to travel through time. The belief that the rocks had simply been made that way from the beginning could be cast out. The earth was literally wrapped in its own history.

  Frustratingly, though, the geological sections Steno observed were not always arranged in neat, layer-cake sections. Some of them were tilted, which critics argued w
as inconsistent with the whole idea of stratification since the sea itself could not tilt. But Steno knew this was a foolish argument. Strata were always deposited parallel to the surface of the sea in a horizontal manner. Any flipping, dipping, folding, or tilting occurred long after the deposition of the rock. If strata could be tilted, or even flipped, though, how could you tell which side was up? The solution came from the makeup of each layer. Elements in the layer would naturally settle by weight, with the heaviest at the bottom; any subslice of a layer that contained the heaviest parts was the bottom, and the lightest elements would be at the top.

  Strata did not just represent isolated patches. Instead they were continuous formations of rock that stretched out to the side until they were stopped by some other formation or material. In a valley the strata on one side would correspond in the same sequence to those on the other, despite the space between them. If you knew the arrangement of strata, you could successively trace them across such gaps.

  Together these principles provided a basic guide that could be used to map the past, but not everyone was ready for this new interpretation of earth history. There were still some who doubted that fossils were representations of living creatures. Many fossil shells resembled those that could be found at the seashore, but there were others that were clearly different from any modern form. Surely God would not let any part of the Creation disappear, and the notion that life may have changed through the ages was strictly against accepted theology. The Bible could not be controverted, no matter how reasonable Steno’s conclusions otherwise seemed.

  Steno had promised to work out such difficulties in a full dissertation on his discoveries, but that book was never written. Science did not allow Steno to reach the absolute truths he yearned for, and after becoming disaffected with science he became a priest in the Catholic Church in 1675.

  With Steno’s voice withdrawn, those who believed that fossils were spontaneously generated in the bowels of the earth were left to dominate. Though the Englishman Robert Hooke also came to the conclusion that fossils had once been parts of living things after comparing fossilized wood to wood charred in a fire, there was still widespread doubt as to how fossils were made and for what purpose. Despite this reticence, however, it was now reasonable to scientifically discuss the origins of fossils. Naturalists might not have been inclined to accept Steno’s work, but the history of the earth and life could no longer be taken for granted.

  It was especially important to resolve such issues because it was becoming clear that geology and Scripture did not exactly align. If the strata reflected the earth’s history they told a story that was much more complex, and theistic geologists strained to keep the Bible and the rock record in accord. In a scheme presented by English naturalist John Woodward in his 1695 book An Essay toward a Natural History of the Earth, for example, the Flood was an extraordinarily violent event in which the suspension of the laws of gravity by God caused the entire world, organisms and all, to be blended into a slurry. The earth’s rock layers formed out of the murky suspension and fossils were organized by specific gravity with the heaviest on the bottom. Yet this is not what geologists in the field saw. The succession of fossil types across strata could not be understood as being organized by weight or size, and appeals to a single (or rare) catastrophe did not account for the formation of strata or fossils.

  The present would be crucial to unlocking the past. Even on a timescale as short as a human life it was obvious that the appearance of the coastline changed gradually, with shells and sediment being deposited at a very slow rate. Within the fossil shells scattered about the countryside, naturalists could see the same patterns of growth that shells of living invertebrates exhibited. By the beginning of the eighteenth century the idea that nature was in the habit of throwing up perfect stony facsimiles seemed silly. From the natural origin of the fossils to the formation of the strata they were encased in, it seemed that nature worked by way of familiar, imperceptibly slow processes. The narrow, literalist interpretation of Genesis was breaking apart, and a more allegorical approach was needed to reconcile the testimony of the rocks with scripture.

  But the earth did not always speak in a clear voice. Fossils and strata recorded earth’s history, but just how far back that history extended was unknown. On the basis of Abrahamic religion it was assumed that the world had a definite beginning and was traveling toward an end, but what if the world was infinitely old? This was the question asked by the eighteenth-century Scottish geologist James Hutton.

  Hutton was an eccentric polymath who did most of his work during the intellectual revival known as the Scottish Enlightenment, which took place in and near Edinburgh. Although initially trained as a physician, he was interested in a variety of sciences, and around 1753 he turned to geology. The stones that littered his farm inspired him to ask where they had come from and, in turn, how the formations underneath the countryside had formed.

  Hutton mulled over his geological speculations for decades, and it was not until 1785 that he presented an abstract of his theory of the earth to the Royal Society of Edinburgh. In Hutton’s view, the land as we know it was cobbled together from the disassociated parts of an earlier world that had been assembled according to natural laws still in operation, and the oceanic abyss was the birthing ground of our present continents. During prehistory, when plants and animals (represented by fossils) lived on the land, sediment and bits of rock fell to the bottom of the sea and hardened. Over time these layers were thrust up, becoming land, and even now there were new future continents forming on the bottom of the sea. The earth was not decaying from a “perfect” state but could restore itself through natural mechanisms.

  This made it nearly impossible to calculate how old the earth might be. The world was not struck by catastrophes that suddenly changed its face, but changed slowly and incrementally through processes that could still be observed. Further research would confirm Hutton’s ideas of deposition and uplift, but in 1788 he could only state that the strata of the world showed “no vestige of a beginning, no prospect of an end.” He would follow these remarks with a full explication of his ideas in the Theory of the Earth published in 1795.

  Two factors impeded the acceptance of Hutton’s view. The first was that his prose was so dense that almost no one could stand to read it. It would only be much later, when his work was summarized by his friend John Playfair, that Hutton’s ideas would gain wider acceptance. Second, Hutton’s cyclical hypothesis required too much time to be crammed into the traditional Creation narrative, and such historical repetitiveness questioned the linear unfolding of God’s work so many Christians believed in. While religious concerns were not as much of an issue in the academic centers of France and Germany, countries at the forefront of science at the time, in England the corroboration of science and scripture was still a major concern. It would be a quirk of translation that would provide succor to those who desired such a reconciliation; it would also provide a target for those who would later carry on Hutton’s legacy.

  During the late eighteenth century one of the most celebrated French scientists was the anatomist Georges Cuvier. Though of humble social origins, Cuvier quickly impressed his family and teachers at a young age with his ability to retain massive amounts of information about history and nature. He attended lectures and studied science when he could, but it was a chance meeting with a naturalist fleeing the post-French Revolutionary Terror that jumpstarted the young naturalist’s scientific career. Scientists were not exempt from the violent purge, and among those who took refuge in rural districts was Henri- Alexandre Tessier, a physician and expert on agriculture who delivered lectures anonymously in the town of Valmont.

  FIGURE 6 - A portrait of Georges Cuvier painted in 1798 by Mathieu-Ignace van Bree.

  When the young Cuvier heard these lectures he was struck by their similarity to the works of Tessier, and Cuvier called the naturalist by name. Tessier was shocked. He feared that if his identity became known he would face e
xecution, but Cuvier assured him that he would not reveal the naturalist’s identity. This confidence was returned by Tessier, and when the flow of blood ebbed in 1794 Tessier introduced Cuvier to other Parisian naturalists. He fit in perfectly. By 1795 the twenty-six-year-old Cuvier had landed a position as an assistant to the comparative anatomist Jean-Claude Mertrud at the prestigious Jardin des Plantes and was almost immediately elected to the newly formed Academy of Sciences.

  Employing the scientific methods developed by colleagues such as the venerable Louis-Jean-Marie Daubenton, Cuvier married geology to comparative anatomy to clear away much of the speculation that surrounded fossil animals. In 1796 he delivered two papers that would frame the debate about fossil animals ever after. The first, on several kinds of fossil elephants found in North America and Siberia, demonstrated that the species of the past were distinct from those living now. What had been suspected by some naturalists was now a reality; species could become extinct. This was backed up by his description of a skeleton transported to Spain from Buenos Aires, Argentina, that Cuvier called Megatherium. It was an immense sloth, unlike any living creature, and further proof that the strata of the earth contained the ruins of a lost world that had been destroyed.

  As the study of fossils became systematized more resolution could be achieved. It had been recognized by some naturalists that certain kinds of fossils only appeared in particular rock formations. Thus, fossils, to a broad degree, could be used to demarcate successive stages of earth history. Invertebrate fossils were the best for this task, as they were so plentiful and the same types generally persisted for relatively short intervals in the geological column. William Smith, an English geologist who was becoming acquainted with geology during the same time, noticed this trend while working as a surveyor, and traced specific fossils, such as species of the shelled cephalopods called ammonites, across the English countryside.