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

by Brian Switek

  There was a geological lesson to learn from the humanitarian disaster. On the shore nearby Darwin saw that the mussel beds had been thrown up several feet above the high-tide line. It was at once apparent that the rise of continents above the waves did not happen in a single catastrophic event; they had been pushed by starts and stops over immense periods of time until what had once been the sea floor became mountains. Geological monuments as impressive as the Andes had been created by small changes over an incalculable amount of time, and as Darwin searched the strata of South America he began to find evidence that the procession of life may have also been precipitated by natural processes.

  Though not a paleontologist, Darwin was familiar with fossils. His education in geology instructed him that the earth as we knew it was only the latest iteration of a succession of fossil worlds once dominated by strange creatures like icthyosaurs and mammoths. Despite the distinctiveness of creatures in each era, however, there were hints of continuity between organisms that spanned the geological divide.

  Darwin knew from reading the books in the Beagle library that fossils of creatures strange, yet familiar, were to be found in South America. Previous accounts told of giant armadillo shells, and everyone knew Cuvier’s description of a giant sloth, a gargantuan version of the living arboreal mammals in the continent’s jungles. Darwin found similar fossils and began to ship them back to England to be studied by expert anatomists. As he did so he wondered what these animals might be and, in a letter to Henslow sent between October and November 1832, Darwin confided that he believed he had found ancient remains of a kind of rodent, some parts of a giant sloth, and the plates of the curious giant “armadillo” that had been previously reported. These latter fossils, especially, suggested a correspondence between the life of the present and the past. The ancient bits of shell plating were not only armadillolike in construction, but were often found alone, just as the empty shells of armadillos were more often found than an entire body. This correspondence was even more apparent among the smaller remains of a fossil agouti. In a field notebook entry made in February 1835, Darwin commented that he hoped that the remains of these mammals would “be one more small instance, of at least a relation of certain genera with certain districts of the earth. This co-relation to my mind renders the gradual birth & death of species more probable.”

  Such a relation was supported by the discovery of another mysterious fossil mammal. In early 1834 Darwin found bones that he initially attributed to a mastodon near Puerto San Julián in Argentina, but, the geological context of the fossil led him to doubt this initial hypothesis. It had been embedded in gravel, not the type of substrate for a lush forest to take root, even in ancient times, and it was more reasonable to think that this animal had lived in a scrubby habitat akin to that favored by the camel-like guanacos Darwin often saw during his journey. Given the essential match between an organism and its environment, it seemed more likely that the “mastodon” was really a creature similar to a guanaco.

  The life of the past was related to that of the present, but Darwin had not yet become a transmutationist. While Darwin saw that the “death” of species might have occurred gradually the appearance of new species in the fossil record still seemed sudden, and Darwin had no idea what kind of creative law might account for the dispersal of related forms through time. As his musings on the Australian riverbank in 1836 showed, he was still convinced that animal forms had been contrived through the genius of one Creator.

  When the Beagle returned to England later that year Darwin was ready to be received as an accomplished naturalist. In addition to helping write up some of the official accounts of the journey, Darwin met with various experts to discover the secrets of the various specimens he had collected. When it came to fossils there was no naturalist more skilled than Richard Owen, a surgeon who applied his knowledge of anatomy to natural history. Although vehemently opposed to wild theorizing about evolution (the socially conservative Owen had just kicked Darwin’s old evolutionist mentor, Robert Grant, out of the Zoological Society), Owen was fascinated by ideas about form and growth being discussed on the continent.

  FIGURE 12 - A portrait of Charles Darwin, painted by George Richmond in the late 1830s.

  As Owen began to work on the fossils he soon found that Darwin had gathered a greater variety of fossil mammals than even the young naturalist had expected. One of the first to be examined was a skull Darwin had purchased from a rancher for eighteen pence that, in life, had belonged to an unusual hoofed mammal the size of a rhinoceros. Owen named it Toxodon, and the naturalists of London were thrilled that Darwin had brought them such a strange beast.

  The mystery of the “armadillo” plates was soon resolved, as well. For years naturalists had debated whether the plates were to be attributed to Megatherium or an entirely different sort of animal. The fossils Darwin brought back allowed Owen to confirm that they did represent a distinct creature, which he named Gylptodon in 1839. It was a close relative of the giant sloths, but one encased in a tough shell of osteoderms.

  FIGURE 13 - The skeleton of Toxodon, the enormous fossil mammal Darwin discovered in Patagonia.

  Toxodon and Glyptodon were among the fossil celebrities that Lyell presented to the Geological Society in February 1837. (The “guanaco” from San Julián would later turn out to be an entirely new kind of mammal, a kind of trunked camel, which Owen would name Macrauchenia.) While Owen focused on their description, Lyell considered their implications, specifically that there was a succession of related types in South America over time. This is what Darwin had suspected since digging them out of the ground, but the question remained: Why did one form succeed another type? Their extinction was a puzzle, but even more confounding was the force that had brought them into existence in the first place.

  FIGURE 14 - Restorations of many of the peculiar fossil mammals which lived in South America during the Pleistocene. During his journey to Patagonia, Charles Darwin collected the bones of many of these animals. Clockwise from the top left: Doedicurus, Glyptodon, Hippidion, Toxodon, Mylodon, Megatherium, Macrauchenia (dog included for scale).

  The birds Darwin and FitzRoy collected during a stop in the Galapagos Islands provided a crucial clue. During his visit to the archipelago Darwin did not think much about the birds. He simply collected them and stuffed them, often not even bothering to record the island from which they had come. Among the few that Darwin took the trouble to label by island were four mockingbirds, all relatively similar to each other. As he began to jot down notes about these specimens in his “Ornithological notes,” however, he thought that each type might have been a variety suited to its particular island, not specially made but slightly altered to a different habitat:If there is the slightest foundation for these remarks the zoology of Archipelagoes—will be well worth examining; for such facts would undermine the stability of Species.

  The popular view at the time was that species radiated outward from a “center of creation.” When a new species came into existence, by whatever mechanism, it spread out from this central spot to occupy suitable habitats nearby. If this were true, however, all of the mockingbirds Darwin found should have been the same from island to island. They were not that far from each other, after all, and the only way to describe the differences between them would be to assume that the Creator had called a slightly different bird into existence on each island. That made a mockery of the Almighty, and it was more sensible to think that each of the birds was a slightly modified type derived from an aboriginal species.

  When ornithologist John Gould had a look at the specimens, however, he found that they were not just variations. Each mockingbird was a distinct species. This was an even more powerful expression of what Darwin was already starting to think about. Perhaps species were not as immutable as had been supposed.

  Discoveries made by other naturalists also fueled Darwin’s speculations. In the spring of 1837 two fossil primates, one from India and the other France, were discovered, and
Lyell joked that they would provide transformists like Lamarck “many thousand centuries for their tails to wear off, and the transformation to men to take place.” Darwin was becoming less willing to dismiss such thoughts with a laugh. His own view was that such species were like individual creatures in that they had a set time of growth, senescence, and death. This would perfectly explain the extinction of species like the Glyptodon and Macrauchenia he found in South America as there did not seem to be any sign of a catastrophic event that wiped them out. Their numbers simply seemed to dwindle before they disappeared altogether, like the withering stem of a plant. Species appeared to have a lifespan just as individuals did, but the same old problem remained of where these animals came from in the first place.

  For a time Darwin thought that the production of “monsters” might provide an answer. Maybe some ancient Glyptodon literally gave birth to a more modern-looking armadillo, and so the production of species would result from the birth of individuals vastly different from their parents. This would explain why there was not a series of connecting “links” between living and fossil animals, but this was not a steadfast rule. The numerous species of finch that Gould cataloged contradicted this trend by exhibiting a graded series of closely related types.

  The questions surrounding such evolutionary changes enthralled Darwin. Other naturalists shied away from the topic, lest their discoveries bestialize humanity in the process, but Darwin could not stop wondering about what had produced both the living and fossil species he had collected aboard the Beagle voyage. Species were not static, that much was clear, but how did they change? Were there limits to transformation? How long did it take and by what natural force could it be achieved? These intellectual obstacles had to be overcome. As a young naturalist looking to solidify his career he could not be too careful.18 The reception Lamarck and Grant received made it clear that open speculation on such subjects would not be taken lightly. Instead, in the middle of July 1837, Darwin started a new “transmutation notebook” to privately record his thoughts.

  Rather than build on his previous musings on new species as mutants, though, Darwin began to consider how circumstances might favor particular forms over others. It was clear that individuals of a species varied, and that variations were tied to the intricacies of environmental change, but the exact nature of the connection was unclear. Perhaps the environment stimulated the birth of new forms, or maybe some other natural force was at work.

  The pattern of life’s history also demanded an explanation. To Darwin’s eye there did not seem to be a linear progression of the “primitive” to the “advanced.” If organisms changed according to the conditions of their environment then no such linear form of evolution made sense, and Darwin instead conceived of evolution as producing a tree that was united by common ancestry but cut back by extinction. The extinct mammals of South America were some of the withered limbs. In his notebook Darwin wrote, “We may look at Megatherium, armadillos, and sloths as all offsprings of some still older type some of the branches dying out.”

  Darwin had now moved beyond merely recognizing the succession of types to positing common ancestry. The living and extinct animals he had collected were all derived from still older creatures, each bearing adaptations to the environment it inhabited. Without a solid mechanism to constrain the growth of the tree, however, these isolated observations and bits of data could just as well be shoehorned into the old views of centers of creation, as Lyell and others had done. Darwin had recognized that adaptation to local conditions would lead to the origin of new species and the extinction of those unable to change, but what drove this process was another question.

  Darwin scoured the biological literature to find more evidence for what he was considering, but in late September 1838 he turned to Essay on the Principle of Population by Thomas Malthus for a break from his research. It was a book with a bleak message. Human population was growing rapidly, placing an increased strain on available resources, but this growth did not go unchecked. In addition to the struggle for resources, war, famine, disease, and other factors reduced human populations when competition became too severe. Even though the absolute size of the population still grew, the total potential population size was never realized because of the action of these agents.

  Darwin was already familiar with this Malthusian view. It had been debated for years in the context of whether the poor should be given handouts or left to their own devices. Yet when he read Malthus’s book it struck him in a new way. Organisms in the wild, too, seemed to be able to produce more offspring than would actually survive. There were only so much space and food to go around, and thus organisms were in a constant struggle with each other to make a living. The few best able to acquire these resources would survive and reproduce, while those not as well equipped would perish.

  What made the difference in this struggle were inherited variations. Individuals that possessed variations that gave them an advantage in the struggle to survive would pass those traits on to their offspring, so small changes would accumulate to cause the origination of entirely new species. This was a far cry from the beneficent nature that had enchanted Darwin when he read Paley’s work. Instead, as counterintuitive as it seemed, the death of many would allow the few to thrive.

  Hence Darwin’s evolutionary hypothesis was based upon several observations that could not be refuted. First, individuals of any species vary, and these variations are heritable. Second, these variations provided an advantage to some individuals in the competition for resources. Third, these variations made them not only more likely to survive but, more importantly, to leave more offspring with those advantageous traits. Other naturalists, such as James Hutton, had stumbled upon a similar idea before, but they had generally thought it worked within established limits and could not institute change on a grand scale. Darwin thought differently. Given these conditions life had to evolve, and this process would leave behind a branching pattern of organisms all connected by common ancestry.19

  Darwin knew that he would require a mountain of evidence to support his ideas. Even if he constructed a well-argued case some naturalists might still reject his ideas on the grounds that it stripped all power from the Creator. Yet just as Darwin was secretly beginning to identify the rudiments of his evolutionary vision, naturalists were beginning to reject well-worn arguments about divine intervention in the creation of life.

  The growing dissatisfaction with the answers provided by natural theology was epitomized by the reaction to the eight-volume series The Bridgewater Treatises, published between 1833 and 1840. The series had been commissioned by Francis Henry Egerton, a British naturalist whose deathbed act of contrition was funding a series of books to remind the people that God still spoke to them though nature. Each of the eight parts was written by a different authority, from William Whewell’s Astronomy and General Physics Considered with Reference to Natural Theology to the final entry in the series, Chemistry, Meteorology, and the Function of Digestion, Considered with Reference to Natural Theology by William Prout.

  William Buckland, the Oxford geologist who had solved the mystery of Kirkdale Cave, was tapped to write the sixth volume, which appeared in 1836. It was an ode to the benevolent Creator from the perspective of paleontology. In considering the Megalosaurus—an immense fossil reptile that Buckland described in 1824 on the basis of a few skeletal fragments and believed to be akin to a monstrous terrestrial crocodile (and, in 1842, deemed to be a dinosaur by Richard Owen)—the naturalist insisted that the recurved fangs of the reptile were signs of God’s wisdom.

  In a former chapter I endeavoured to show that the establishment of carnivorous races throughout the animal kingdom tends materially to diminish the aggregate amount of animal suffering. The provision of teeth and jaws, adapted to effect the work of death most speedily, is highly subsidiary to the accomplishment of this desirable end. We act ourselves on this conviction, under the impulse of pure humanity, when we provide the most efficient instruments to pro
duce the instantaneous, and most easy death, of the innumerable animals that are daily slaughtered for the supply of human food.

  If there had to be death and suffering, it was better that death come quickly and efficiently. God was not so cruel as to give the Megalosaurus the appetite for flesh and make it such a bumbling predator that tortured its victims to death.

  The Ichthyosaurus also embodied a divine lesson. A marine reptile known from several complete skeletons, many of which had been found by one of the earliest fossil hunters, Mary Anning, it had paddles similar to those of a whale, a crocodile-like head, vertebrae like those of a fish, and a sternum like that of the enigmatic creature from Australia, the platypus. A scoffer might look at this collection of parts and think that God made a mistake, but Buckland insisted that the fact that Ichthyosaurus combined parts from various animals showed that God used the same parts in the designs for His creatures over and over again:The introduction to these animals, of such aberrations from the type of their respective orders to accommodate deviations from the usual habits of these orders, exhibits a union of compensative contrivances, so similar in their relations, so identical in their objects, and so perfect in the adaptation of each subordinate part, to the harmony and perfection of the whole; that we cannot but recognise throughout them all, the workings of one and the same eternal principle of Wisdom and Intelligence, presiding from first to last over the total fabric of Creation.

  Such arguments generated more derision than agreement. The series, called the “Bilgewater Treatises” by critics, was an anachronism better suited to Paley’s time. Buckland’s plea seemed more of an effort to hold back liberal threats to conservative orthodoxy than to understand nature as it truly was.