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

by Brian Switek

  Darwin addressed this problem in two chapters: “On the Imperfection of the Geological Record” and “On the Geological Succession of Organic Beings.” “Geology assuredly does not reveal any such finely-graduated organic chain,” Darwin admitted, “and this, perhaps, is the most obvious and gravest objection which can be urged against my theory.” Paleontologists were always working from an incomplete record, of which Darwin wrote:For my part, following out Lyell’s metaphor, I look at the natural geological record, as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating only to two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines. Each word of the slowly-changing language, in which the history is supposed to be written, being more or less different in the interrupted succession of chapters, may represent the apparently abruptly changed forms of life, entombed in our consecutive, but widely separated, formations.

  Yet Darwin could do more than emphasize the contingencies of the fossil record. As he pointed out, naturalists already recognized that species appeared and disappeared in a piecemeal fashion over time. This constant flux of forms in competition with each other was most beautifully expressed in the last paragraph of the book:It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the largest sense, being Growth with Reproduction; Inheritance which is almost implied by reproduction; Variability from the indirect and direct action of the external conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of Character and the Extinction of less-improved forms. Thus, from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.

  When the book went up for sale on November 24, 1859, Darwin was both elated and anxious. That morning he wrote to the ornithologist T. C. Eyton, “My Book will horrify & disgust you,” but he hardly could have expected the response of his old geology mentor, Adam Sedgwick. Sedgwick, not holding back, admitted that he read Darwin’s book with “more pain than pleasure,” continuing, “Parts of it I admired greatly; parts I laughed at till my sides were almost sore; other parts I read with absolute sorrow; because I think them utterly false & grievously mischievous.”

  This deeply distressed Darwin. Still, it was the opinion of his close friends Hooker, Lyell, and Huxley that mattered most. Darwin almost pleaded for Huxley, especially, to tell him what he thought of the book, and was greatly relieved when Huxley stated that it was the greatest work on natural history he had read in nearly a decade. More than that, Huxley, ever opposed to the hold of the supernatural in the sciences, was ready to defend Darwin’s evolutionary theory in public. “I am sharpening up my claws & beak in readiness,” Huxley wrote, for he knew that opposition to Darwin’s theory would be manifested in more than just personal letters from naturalists of the old guard.

  One of the first attacks appeared in the Edinburgh Review and, despite being published anonymously, was immediately recognizable by its self-referential style as having been written by Richard Owen. While he praised On the Origin of Species for fully considering an evolutionary mechanism, Owen asserted Darwin had not approached the topic of the “mystery of mysteries” with due reverence. On the Origin of Species contained a few observational gems, but these tidbits were awash in a sea of speculation. For Owen, Darwin’s jottings on the fossil record were the most embarrassing sections of all.

  Lasting and fruitful conclusions have, indeed, hitherto been based only on the possession of knowledge; now we are called upon to accept an hypothesis on the plea of want of knowledge. The geological record, it is averred, is so imperfect! But what human record is not? Especially must the record of past organisms be much less perfect than of present ones. We freely admit it. But when Mr. Darwin, in reference to the absence of the intermediate fossil forms required by his hypothesis—and only the zootomical zoologist can approximatively appreciate their immense numbers—the countless hosts of transitional links which, on “natural selection,” must certainly have existed at one period or another of the world’s history—when Mr. Darwin exclaims what may be, or what may not be, the forms yet forthcoming out of the graveyards of strata, we would reply, that our only ground for prophesying of what may come, is by the analogy of what has come to light.

  Even worse, the fossil species that were known seemed to show no change. Extinct creatures such as the Ichthyosaurus, Megatherium, mastodon, and dinosaurs appeared suddenly in the fossil record, showed no sign of transforming into another type, and were entirely wiped out. Citing himself, Owen ambiguously maintained “that perhaps the most important and significant result of palaeontological research has been the establishment of the axiom of the continuous operation of the ordained becoming of living things.”

  The Oxford geologist John Phillips also used geology to attack On the Origin of Species. In an 1860 volume called Life on Earth, Phillips maintained that the fossil record was filled with the rapid appearance of species followed by little change, not the continuous replacement of older forms by new species derived from them. This opposition was as theologically based as it was scientific, and it could have been just as easily written by Paley or Buckland.

  Phillips, like Owen and others, was worried that Darwin’s evolutionary view undermined our special place in nature. Natural selection was like a dark mirror image of natural theology. Rather than being an expression of divine benevolence, Darwin’s “entangled bank” was a warzone in which each generation did battle on a Golgotha made of perished species. This was “Nature, red in tooth and claw” that Tennyson feared, and Darwin’s view presented little solace for those that wished to “stretch lame hands of faith . . . And faintly trust the larger hope.”

  Similar appraisals were made by paleontologists abroad, as well. Like Owen, the German paleontologist Heinrich-Georg Bronn thought Darwin’s proposal to be the most coherently argued mechanism for evolution hitherto forwarded, but he could not accept it as the answer to the mystery of the origin of species. The Swiss authority François-Jules Pictet thought similarly, convinced that evolution was a worthwhile question to consider but disagreeing with Darwin’s hypothesis that all life was connected by common ancestry. The general outline of what was known of the fossil record suggested that evolution was a reality, but Darwin’s explanations in On the Origin of Species seemed both too violent and too weak to drive the changes paleontologists observed. Even Huxley and Lyell, among Darwin’s staunchest supporters, took issue with their colleague’s evolutionary formulation. Lyell was still unsettled by the prospect of human evolution, and Huxley preferred large-scale jumps by mutation and other non-Darwinian hypotheses to make sense of evolution over the whole of earth’s history.

  This state of affairs deeply frustrated Darwin. In an 1861 letter to the American paleontologist Joseph Leidy, who positively referenced Darwin’s views, Darwin wrote, “Most palaeontologists (with some few good exceptions) entirely despise my work; consequently approbation from you has gratified me much.—All the older geologists (with the one exception of Lyell, whom I look at as a host in himself) are even more vehement against the modification of species than are even the palaeontologists.”

  Even if oth
er naturalists could not accept natural selection, however, Darwin’s book, unlike those of Lamarck or Chambers, had made evolution a question that could no longer be ignored. Darwin would continue to investigate and publish on evolution and the major effects of small, cumulative changes for years to come, from the details of animal and plant domestication to orchids and the lives of worms in soil, but determining the fossil record’s contribution to our understanding of evolution would largely fall to others.

  Oddly enough, such work had already been underway while Darwin was beginning to sketch his ideas on natural selection. In the controversy over Darwin’s evolutionary mechanism it was seemingly forgotten that, just a few years prior to the publication of On the Origin of Species, several creatures were proposed as embodying a transition of vertebrates from water to the land. This groundwork had been laid by Richard Owen, who in spite of his opposition to Darwin’s view of life, provided some of the most compelling evidence for his rival’s evolutionary theory.

  From Fins to Fingers

  “The arm of the Man is the fore-leg of the Beast, the wing of the Bird, and pectoral fin of the Fish.”

  —RICHARD OWEN, On the Nature of Limbs, 1849

  It was an ugly fish. Its snub-nosed head topped a sinuous, eel-like body, and in place of fins it had only wispy filaments. Nor did it behave like a proper fish; occasionally it came to the surface to gulp air before returning to the bottom to search for snails and other tasty morsels. It had none of the beauty or charm of its smaller, brightly colored relatives that darted about the same Amazonian rivers and swamps, and its unusual form deeply puzzled Johann Natterer.

  Natterer was an Austrian naturalist who accompanied thirteen other explorers to Brazil in 1817. Commissioned in honor of the marriage of the daughter of Austria’s Emperor Franz II to the crown prince of Portugal, the expedition was believed by the Austrian nobility to be a perfect opportunity to catalog the natural riches of the tropical Portuguese colony. At that time natural science, pressed into the service of empire, involved shooting, trapping, and pickling as many specimens as possible. The detailed study of the specimens had to wait until scholars back in Europe could pick through the spoils.

  By the time Natterer returned to Austria in 1835 he had amassed an impressive collection of tens of thousands of specimens starkly different from the fauna of his native country: jewel-like butterflies, garishly colored frogs, and many other intricately beautiful creatures. But one of the most vexing finds was the ugly brown fish he had pulled from the depths of the Amazon. He brought his specimens to Leopold Fitzinger, an expert on reptiles at the Imperial Museum in Vienna.

  Natterer and Fitzinger’s dissection of the preserved specimen only made things more confusing. As they opened it up and picked among its olive-green bones they found that, in addition to the expected gills, the creature possessed rudimentary lungs. What was a fish doing with the organs of a “higher” class of animals? Though Natterer believed the new species to be an aberrant fish, Fitzinger thought it was a particularly piscine amphibian because of its lungs. The fish threatened to breach one of the great divisions in nature, and to underscore its tenuous place between worlds it was given the name Lepidosiren paradoxus, the “paradoxical, scaled salamander.”

  FIGURE 19 - A caricature of Richard Owen “riding his hobby” (a Megatherium).

  At about the same time that Natterer and Fitzinger were scrutinizing the anatomy of their Amazonian lungfish, Richard Owen received a similar specimen from Africa. It had been found wrapped in a ball of mud and mucus about a foot below the scorched African soil in Gambia, a small nation on the great “ear” of the elephant-shaped continent. Having no idea that his Austrian peers had described a similar fish, Owen issued a brief description, calling it Protopterus anguilliform, in honor of its eel-like appearance.

  When news of Natterer’s Lepidosiren reached him, Owen was struck by how similar it was to his Protopterus, which he reassessed accordingly. (Some less charitable authorities even suggested that Natterer’s collection from Brazil had gotten mixed up with one from Africa. That there were two such fish on opposite sides of the Atlantic was almost too fantastic to believe.) In 1841 Owen announced that both the African and South American forms belonged to the same genus. The lungfish from the Amazon would keep its name while Protopterus was renamed Lepidosiren annectens, the “connecting, scaled salamander.”20 Arriving at this seemingly simple conclusion had been no easy task:It may truly be said that since the discovery of the Ornithorhynchus paradoxus [the duck-billed platypus], there has not been submitted to naturalists an animal which proves more forcibly than the Lepidosiren the necessity of a knowledge of its whole organization, both external and internal, in order to arrive at a correct view of its real nature and affinities.

  Superficially, both species looked like fish, complete with scales and rudimentary fins, but their internal anatomy complicated the matter. In addition to their simple lungs, other organs of the Lepidosiren, such as aspects of the circulatory system, resembled their counterparts in amphibians. Owen was faced with Natterer and Fitzinger’s dilemma: was Lepidosiren an amphibianlike fish or a fishlike amphibian?

  In most fish, the nasal openings are used for smell and do not play any role in respiration, and Owen noted that both species of Lepidosiren had nasal openings that did not connect to their lungs. Though Owen would later be shown to be incorrect on this point, the overall constellation of “fish traits” that he cataloged supported his decision that Lepidosiren was a fish.

  Owen’s decision to call Lepidosiren a fish has traditionally been taken as a signal of his distaste for all things evolutionary. A staunch opponent of Charles Darwin’s view of evolution, Owen often injected awkward, pious prose into his scientific papers. This led later generations of naturalists to brand him a villainous creationist who did everything in his power to stifle evolutionary ideas.

  Owen would have been shocked by this modern caricature, just as he was mortified by Darwin’s assertion that he was a defender of the religiously charged concept that species were fixed entities which could never change. In On the Origin of Species Darwin wrote that Owen was among the eminent experts of paleontology who ferociously maintained the “immutability of species.” Owen’s reaction to this was recorded in a letter Darwin sent to his friend and mentor, Charles Lyell, less than a month after Darwin’s book was published:I have [had a] very long interview with Owen, which perhaps you would like to hear about, but please repeat nothing. Under garb of great civility, he was inclined to be most bitter & sneering against me. Yet I infer from several expressions, that at bottom he goes immense way with us.—He was quite savage & crimson at my having put his name with defenders of immutability.

  Owen had a right to be cross with Darwin. While Darwin was still privately mulling over that “mystery of mysteries” Owen was doing the same in public. The species name he chose for the African lungfish, Lepidosiren annectens, revealed his transformist speculations.

  At the time Lepidosiren was discovered, naturalists recognized several distinct vertebrate groups: fish, reptiles (including amphibians), birds, and mammals. Thanks to the intellectual baggage carried over from the God-ordained Great Chain of Being each succeeding group was generally thought to be “higher” than the one before it, with humans crowning all organisms. The seemingly clear boundaries between these divisions were muddied by the Lepidosiren.

  When Owen examined the preserved corpses of the lungfish he found that they possessed a mix of archaic and advanced traits. It was as if they had been made of spare parts. In the formation of its gills, for example, Owen wrote that the Lepidosiren exemplified “a most interesting and hitherto unexampled transitional structure” between bony and cartilaginous fish while its fins, on the other hand, resembled the embryonic limbs of amphibians. In this way an adult Lepidosiren could be thought of as an overgrown embryo from a “higher” class of vertebrate, a true transitional form between fish and amphibians.

  The anatomical pattern Owen
saw in his Lepidosiren annectens was consistent with what he had seen elsewhere in his studies. All vertebrate skeletons appeared to have been built on a general body plan, an anatomical blueprint, from which any form could be derived. To see this pattern, however, the generalized blueprint had to be distinguished from products created from it.

  Looking at the diversity of forms in nature it quickly becomes apparent that many different organisms share similar anatomical structures. The pinching claws of a crab, for example, superficially resemble our own arms, in that the crab appears to have shoulders, elbows, wrists, and hands. Yet a crab’s “arm” consists of a soft interior surrounded by a tough, chitinous exoskeleton; there are no bones in it. It is an analogous structure, or a case of convergence, in which the crab’s appendage is similar in form and function as our own arms but made of entirely different parts.

  Homology, the possession of similar traits shared due to ancestry, is the counterpoint to anatomical analogy. The arms of a bat, a frog, a human, and a crocodile are all slightly different modifications of one shared anatomical construction. There is an upper-arm bone (the humerus), articulated with two lower-arm bones (the radius and ulna) at the elbow, connected to the wrist where the bones of the hand, including the fingers, are placed. The arms may have different functions but they are all modified versions of the same basic arm structure; thus, the humerus in our arms is homologus with those in the upper arms of a crocodile, our fingers are homologus with those a bat uses to support its wings, and so on. Owen underscored this connection with a beautiful comparison inspired by a statue of the goddess Victory about to slay a bull in the frontispiece to his 1849 essay On the Nature of Limbs. Owen imagined the bones inside both the angel and the sacrifice and numbered them to show the homolgous limb bones in both.