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

  This conclusion was supported by the evolutionary tree Seiffert’s team produced. Not only did Darwinius and A fradapis group closely together on the basis of their shared characteristics, but they were about as distantly related to early anthropoids as it was possible to be. Their closest living relatives are the lemurs and lorises, not monkeys. (Though they actually were most closely related to other forms of primate that are now entirely extinct.) As expected, it was the tarsiers and their extinct relatives that were most closely related to anthropoids. Ida had unceremoniously been dethroned.3

  FIGURE 2 - The lower jaw of Afradapis longicristatus, reconstructed on the basis of multiple specimens. So far, it is all that is known of this fossil primate.

  FIGURE 3 - A family tree of primates as produced by the cladistic analysis run by Seiffert and colleagues. Not only does Darwinius fall near Afradapis, but both are confirmed as extinct relatives of lemurs far removed from anthropoid primates.

  Her backers were not pleased. Distancing himself from the headline-making claims of a few months before, Hurum stated that Darwinius could still belong to a “stem group” from which early anthropoids evolved. After all, the skeleton of Darwinius was much more complete, and according to Hurum it contained some anthropoid characteristics that could not be seen in the incomplete remains of Afradapis. Gingerich was similarly unimpressed. He asserted that the anthropoid traits seen in Darwinius were not convergences at all; Ida had monkey-like traits because she was closely related to monkeys. Though the Afradapis paper presented a much better supported hypothesis for what the primate family tree looks like, it was hardly the last word on the matter, either. Hurum promised that an independent cladistic analysis of Darwinius was already being planned.

  I watched this back-and-forth from the periphery. As a writer there was not much I could directly contribute to the scientific discourse, but I was hooked by the drama surrounding Ida.4 I couldn’t help but wonder why this petrified primate had caused such a fuss. If Ida had been presented in her proper evolutionary position, as a unique relative of living lemurs, this whole media kerfuffle probably would not have happened. Therein was my answer.

  No matter how much we learn about nature there are some questions our species continually grapples with. Why are we here? How did we get to be this way? Where are we going? Maybe these questions sound a bit trite, but if that is so it is only because they are timeless queries that have been difficult to answer. We desperately want to know where we came from, where we are headed, and, as phrased by novelist Douglas Adams, the “Ultimate Answer to Life, the Universe and Everything.”5

  FIGURE 4 - A simplified version of the evolutionary tree produced by Seiffert and colleagues. It shows a deep split among early primates, with Darwinius and Afradapis being on the side that gave rise to lemurs and lorises, not anthropoids.

  The answers to these questions have traditionally been supplied by religion. We have been created and sustained thanks to God’s will, so the story goes, making us the most privileged thing in all Creation. Even if we feel lost and isolated we can still believe that there is an inherent purpose and direction to life, a beginning and an ending.

  But during the past 150 years these existential questions have taken on new inflections. There might not be a universal answer to “Why are we here?” that provides us with a driving sense of purpose, but an understanding of the quirks and contingencies of evolution allows us to meaningfully understand how we came to be as we are. This was made possible by the work of Charles Darwin in the middle of the nineteenth century. He was not the first person to consider evolution, nor was he the only Victorian naturalist to provide evidence for it, but through his 1859 masterwork On the Origin of Species Darwin popularized a new view of life in which a past far beyond the oldest remnants of human history could help us understand our place in nature. We are inextricably tied to what has come before.

  Our preoccupation with origins made the search for fossil ancestors among the most pressing preoccupations of naturalists. If life had truly been transforming over an incalculable amount of time, then the bones of our distant ancestors, as well as forerunners of every other living species, should speak to us from the earth. This hypothesis was a bit of a gamble for Darwin. Geology and paleontology had been essential to the formation of his evolutionary theory, yet the records of deep time had, prior to 1859, failed to provide the continuous, graded chains of fossils linking the present to the past. While Darwin was correct that the fossil record was an archive “imperfectly kept,” full of gaps and discontinuities, ultimately it would have to provide the solid proofs of the theory he had based on observations of living animals.

  The rarity of these fossil proofs of evolution vexed naturalists. In an 1868 address on the evolution of birds from reptiles Darwin’s ally Thomas Henry Huxley likened the state of affairs to a landowner who, despite his claims, could not produce hard evidence that he really owned the property at all:If a landed proprietor is asked to produce the title-deeds of his estate, and is obliged to reply that some of them were destroyed in a fire a century ago, that some were carried off by a dishonest attorney, and that the rest are in a safe somewhere, but that he really cannot lay his hands upon them; he cannot, I think, feel pleasantly secure, though all his allegations may be correct and his ownership indisputable. But a doctrine is a scientific estate, and the holder must always be able to produce his title-deeds, in a way of direct evidence, or take the penalty of that peculiar discomfort to which I have referred.

  Naturalists would have to supply these “title deeds” if the fact of evolution was to be established. The theoretical question of whether evolution was driven by natural selection or some other force would be debated for decades, but the fossil record held the most immediate potential of supplying solid evidence that evolution was real.

  This want of ancestors is what allowed the Darwinius-for-ancestor lobby to enthrall the public. The fossil record does not contain a complete roll of every living thing that ever lived. It is rare that a living thing dies in circumstances amenable to fossilization, and even among this fossil pool the remains of many organisms are destroyed by geological processes. Of this fraction of a fraction only a very few specimens exist in rocks accessible to scientists, and of that tiny slice fewer still are collected and studied. The discovery of any fossil with transitional features that helps us understand the transformation of one form into another is cause for celebration, and most celebrated of all are those that connect familiar animals to their extinct forerunners.

  The fossil forms which bridge the gap between one group of organisms and another have popularly been called “missing links” (and this is especially true of the search for our own ancestors). This is an unfortunate misnomer that reveals the ancient origin of the phrase as well as the biases that run though it. Indeed, the idea of missing links originally did not contain any evolutionary significance at all. During the Middle Ages Christian scholars thought that life was organized according to a hierarchical scale of natural productions ranked from “lower” to “higher.” This was the Great Chain of Being, and it was a static arrangement that reflected the virtues of Creation: plentitude, continuity, and gradation.

  Since God was benevolent and omnipotent He had created everything that was possible.6 Ours was the best of all possible worlds, one of magnificent plenitude, but there was an order to the diversity of nature. In the continuous, unbroken hierarchy anything in nature could be linked to another by recognizing their shared characteristics. A rock had existence, while a plant had both existence and life, and the ability of animals to move around on their own placed them above plants. And so the rankings went, all the way from pebbles up to the Almighty, with humans representing the highest point of the “animal Creation.” Our kind was a step above other animals but one below angels, beings possessed of a heavenly infused soul but still subject to animal urges.

  Despite the certainty that God had ordered creation according to these laws, however, there were breaks i
n the chain. Among the most troublesome was the one between humans and the vulgar monkeys (which, for many medieval Christians, represented what a life of sin could lead to). Monkeys were clearly similar to humans but far too low to be on the rung right below us. Between us and them there should have been a humanlike being that lacked a soul, but for centuries this missing link remained elusive.

  This view of nature was later co-opted into ideas about evolution. By the beginning of the nineteenth century the Great Chain of Being ceased to be a useful concept to organize nature, but vestiges of it still remained. The vertical dimension of the hierarchy, rather than representing only the rank of living organisms, was impressed onto the geological timeline. Fish appeared before amphibians, which preceded reptiles, which in turn gave way to an Age of Mammals capped by the appearance of our own species. The story of evolution still presented a chain of beings connected through a series of intermediate links, and it was among fossil vertebrates that the first of these intermediate forms were found. In his 1870 address as president of London’s Geological Society, Huxley stated that “when we turn to the higher Vertebrata, the results of recent investigations, however we may sift and criticize them, seem to me to leave a clear balance in favour of the doctrine of the evolution of living forms one from another.” Fossil vertebrates provided some of the most compelling evidence for evolutionary change, and it was not surprising that some scientists interpreted the succession of these forms to represent life’s progress.

  This underlying thread has given rise to some of our most iconic evolutionary images. The March of Progress from early primate to human is one, but the same imagery has been employed for the evolution of horses, elephants, the earliest terrestrial vertebrates, early mammals, birds, and whales. As transitional forms have been found they have been strung up in temporal sequences to show the progressive transformation of the archaic into the modern. This interpretation might not be explicit, and perhaps it is even outright denied by the presenters of these diagrams, but such illustrations leave little doubt that the biases inherent in the Great Chain of Being remain with us even today.

  And this drive toward progress implies the question of what might come next, particularly for our own species. What might our descendants be like a thousand, a million, or ten million years from now? If the past presents us with a tale of progress from “primitive” to “advanced,” then what might the future hold for us? What is the next evolutionary step? There is no way to tell. It is impossible to predict how our species might be adapted, but the annals of science fiction reveal our expectations. It is no coincidence that in popular culture, from Hollywood films to discussion boards run by UFO conspiracy nuts, technologically superior aliens are envisaged as having large heads stuffed with enormous brains and frail humanoid bodies .7 They are species that have advanced to the point where body is sublimated to mind, and they act as proxies for what many expect our species to become given enough time. As hypothetical creatures that live more in the mental realm than the physical, they occupy the place once inhabited by angels, above humans but below God, on the Great Chain of Being.

  The irony of this view is that Darwin envisioned evolution as producing a wildly branching tree of life with no predetermined path or endpoint. It is significant that the only illustration in On the Origin of Species is not a revised version of the Great Chain of Being, but a series of branches embedded within greater branches, all connected by common ancestry. With a sufficiently complete fossil record it is possible to trace the evolution of particular forms according to direct lines of descent, but doing so requires that neighboring branches containing close relatives be lopped off. And the further back in time we go, the more relatives we have to ignore.

  Any paleontologist worth their salt knows this well. Yes, it is possible to line up a series of forms representing what our direct ancestors looked like at different points over the last six million years or so, but to do so would require that we ignore other types of early humans that lived alongside our ancestors such as the heavy-jawed robust australopithecines and our sister species, the Neanderthals. Even before that, our anthropoid ancestors were just one twig of a more diverse evolutionary bush that coexisted with other kinds of primates such as Afradapis and tarsiers. To focus solely upon our ancestors is to blind ourselves to our own evolutionary context.

  But why consider fossils at all? In the introductory chapters of his 2004 tome The Ancestor’s Tale Richard Dawkins stressed that “dead men tell no tales.” We might be just as well off in our understanding of evolution if not a single fossil even existed:In spite of the fascination of fossils, it is surprising how much we would still know about our evolutionary past without them. If every fossil were magicked away, the comparative study of modern organisms, of how their patterns of resemblances, especially of their genetic sequences, are distributed among species, and of how species are distributed among continents and islands, would still demonstrate, beyond all sane doubt, that our history is evolutionary. Fossils are a bonus. A welcome bonus, to be sure, but not an essential one.

  But this dim view of paleontology is not accurate.8 During the past thirty years scientists have seen the emergence of a new, synthetic paleontology that is giving us an unprecedented look at the machinations of evolution.

  Scientists such as Stephen Jay Gould, Niles Eldredge, Steven Stanley, Elisabeth Vrba, David Raup, and Jack Sepkoski led the charge. Starting in the 1970s these paleontologists questioned the popular interpretation of evolution as a slow and steady process in which species were constantly evolving by tiny steps. Their research was not in conflict with evolution by means of natural selection, but the patterns of the fossil record were far more haphazard than had been expected on the basis of genetics. This precipitated a 1980 conference in Chicago where some of these ideas could be hashed out with biologists, such as John Maynard Smith, who favored a smoother evolutionary pattern. The tension was felt by all. After a presentation by embryologist George Oster about how developmental quirks constrain the forms organisms can take, Maynard Smith responded that scientists like himself had already considered the idea and dispensed with it as being of little importance. The paleontologists and other biologists who were questioning what was commonly accepted were only reinventing the wheel. To this Oster retorted, “You may have had the wheel, John, but you didn’t ride away on it.”

  Paleontologists were ready to hop on and see where they could go, and while relations between paleontologists and neontologists (biologists who work with living organisms) were tense to start with, the debates between them started to feed cross-disciplinary collaborations. Slowly, paleontologists began to incorporate discoveries from molecular biology, genetics, and embryology into their work. This allowed paleontologists not only identify to patterns of change, but to begin to understand how such changes in form might have been caused. The discovery of preserved soft tissues from prehistoric creatures from Neanderthals to mammoths to Tyrannosaurus have even opened a new field of study centered on the recovery and study of ancient molecular materials. Comparative anatomy and geology still form the core of paleontology, but the science has embraced information and techniques from a variety of disciplines, thus allowing scientists to test their ideas about life’s history through the combination of multiple lines of evidence.

  The coalescence of this new paleobiological synthesis coincided with the discovery of many new transitional fossils and the reappraisal of many old ones. Fossils that scientists knew had to exist but had been missing, such as land-dwelling whale ancestors and feathered dinosaurs, were found, while well-known lineages, such as horses and elephants, were revealed to show a wildly branching pattern of diversity rather than a straight line of progress. Even among our own ancestors, what had once been supposed to be a single chain of ancient humans was suddenly split and split again by new discoveries, so much so that at the turn of the twenty-first century no less than three fossils were in competition for the designation “earliest human.” With the development
of the new paleobiology and more complete collections of transitional fossils, paleontologists began to piece together a better understanding for how life changed through time. Paleontology is not just a bonus; it is among the most essential of evolutionary disciplines.

  Fossils do not speak for themselves, however, and the history of science fleshes out the context in which new discoveries have been made and interpreted. The standard story that Charles Darwin’s theory of evolution by means of natural selection was so brilliant that everyone but religious zealots agreed with him is only a caricature of the truth. Darwin’s 1859 book proposed more questions than it provided answers, and the scientific endeavor to answer some of those questions has been affected just as much by contingency and chance as the history of life. The places paleontologists looked for fossils and how those fossils have been interpreted have been influenced by politics and culture, reminding us that while there is a reality that science allows us to approach the process of science is a human endeavor.

  The following pages tie together the complementary narratives of life’s history and our changing understanding of that history. Walking whales, amphibious elephants, feathered dinosaurs, land-dwelling fish, mammals that listened with their jaws, multi-toed horses, and upright apes will be presented through the eyes of the scientists who puzzled over their origins, culminating in what we now understand about the evolution of such creatures. The perspective these stories provide has changed how we interpret the past, and leads us to question some of our most cherished beliefs about our place in the universe.

  The Living Rock

  Beautiful is what we see,

  More beautiful is what we understand,