December 7, 2010
Evolution never got much time in my elementary school science classes. When the topic came up, inevitably near the end of the term, the standard, pre-packaged historical overview came along with it. Charles Darwin was the first person to come up with the idea of evolution, and, despite the ravings of religious leaders offended at our relationship to monkeys, the idea that natural selection adapted life into “endless forms most beautiful” quickly became established among the scientists of the day.
Like many textbook stories, the story of evolutionary discovery my classmates and I were presented with was clean, neat and hopelessly flawed. Darwin was not the first naturalist to propose that evolution was a reality; many of his colleagues thought that natural selection was too weak of a force to affect evolution, and for several decades following the 1859 publication of On the Origin of Species, many naturalists preferred alternative evolutionary mechanisms such as large-scale mutations and internal forces driving organisms onward and upward. Darwin was not even the first naturalist to come up with the idea of natural selection. Many naturalists had previously considered it and thought that it could at best preserve life as is and at worst destroy species. (As for Alfred Russel Wallace and the role he played in the development of evolutionary ideas, my classmates and I didn’t have a clue that he existed.)
The significance of Darwin’s work was in his demonstration of how natural selection could modify life and create a branching pattern of diversity over vast expanses of time. He had worked long and hard to collect all the necessary data to support his case. There was no “Newton’s Apple” type moment—another favorite science myth—in which a Galapagos finch perched on Darwin’s shoulder and whispered the secrets of evolution to the previously clueless naturalist. In Darwin’s time evolution was a frequently discussed issue, and the debate over what natural laws drove the change in species continued long after 1859.
Almost every major figure of the emerging field of evolutionary science has been miscast at one time or another. Richard Owen, one of the first evolutionists, has been traditionally portrayed as a brooding creationist for his opposition to natural selection. St. George Jackson Mivart met a similar fate despite the seriousness with which Darwin took his objections. Charles Lyell, on the other hand, became the white knight of geology who did away with the religiously fundamentalist views of catastrophic change popularized by Georges Cuvier (yet another myth). In order to preserve any semblance of the intellectual March of Progress each character must take up their proper place in the historical drama; they must fall along a simple chain of succession from ignorance to understanding. But among the most pernicious myths are those which seek to honor past scholars for the wrong reasons.
In 1996 a single photograph caused quite a stir at the annual Society of Vertebrate Paleontology meeting in New York City. The picture depicted a small dinosaur in the classic death pose—head thrown back and stiff tail levered straight up—but it was covered in a fuzzy coat of rudimentary feathers. Eventually named Sinosauropteryx, this creature was the first feathered dinosaur to be found since the first specimens of Archaeopteryx were chiseled out of German limestone quarries in the late 19th century. It was a stunning confirmation of what many paleontologists had come to suspect on the basis of anatomy alone—that birds had evolved from dinosaurs, and many characteristic avian traits appeared among dinosaurs first. John Ostrom, Bob Bakker and other paleontologists were not the first to support this idea. The hypothesis had once been among the most prominent explanations for the origin and birds, and many authorities credited the Victorian naturalist Thomas Henry Huxley as being the first to propose it.
Huxley is often included among Darwin’s supporting cast. He was a prominent public voice for evolutionary science while Darwin mostly kept track of the discussions and debates about evolution through correspondence. In fact, Huxley was among the first scientists to propose graded lines of descent for birds, whales and horses, but his determination of these evolutionary transitions required a circuitous process of discovery and realization. Huxley’s ideas about bird origins, especially, were not a perfect anticipation of our current knowledge, but a set of nuanced hypotheses which relied on Huxley’s idiosyncratic conception of evolution.
Huxley’s views about evolution were influenced by his friendship with Darwin. According to traditional lore, after reading Darwin’s theory in On the Origin of Species Huxley exclaimed, “How extremely stupid [of me] not to have thought of that!” But, a staunch defender of his friend though he was, Huxley’s reading of Darwin did not inspire him to start thinking about transitions in the history of life. Huxley thought that large-scale mutations—evolutionary jumps termed “saltations”—were more important than variations acted upon by natural selection, and so he did not expect the graded chains of transitional forms Darwin’s theory predicted.
A brilliant anatomist, Huxley was primarily concerned with identifying the common denominators of form among organisms. One association of special interest to Huxley was the correspondence between birds and reptiles. While teaching anatomy at the Royal College of Surgeons in 1863, Huxley taught his students that birds were “so essentially similar to Reptiles in all the most essential features of their organization, that these animals may be said to be merely an extremely modified and aberrant Reptilian type.” Rather than explicitly delineate how such a transition could have taken place, however, Huxley was at this point content to highlight the anatomical similarities alone. Life had most certainly evolved—there could no longer be any reasonable doubt—but Huxley’s preoccupation with form and his ambivalence about natural selection prevented him from digging into the subject to any great depth.
The publication of a different book caused Huxley to change course. In 1866 the German evolutionist Ernst Haeckel published Generelle Morphologie. When Huxley read it he started scribbling hypothetical lines of evolutionary descent in his notebooks. The correspondences Huxley had already recognized as a dedicated anatomist could be used to predict lines of descent, and in 1868 Huxley wrote to Haeckel:
In scientific work the main thing just now about which I am engaged is a revision of the Dinosauria—with an eye to the Descendenz Theorie! The road from Reptiles to Birds is by way of Dinosauria to the Ratitaez—the Bird ‘Phylum’ was Struthious, and wings grew out of rudimentary fore limbs. You see that among other things I have been reading Ernst Haeckel’s Morphologie.
To put it another way, Huxley saw a potential evolutionary pathway from small dinosaurs—such as the recently-discovered Compsognathus—through flightless birds and on to flying forms. But this was only an outline of a more nuanced view of evolutionary change Huxley was just beginning to bring together. In 1859 Huxley had presented his view that most major evolutionary transitions must have occurred during “non-geologic time,” or at a date so remote that there were no longer any rocks to record it. The transitional fossils that recorded the evolution of birds would forever be out of the reach of paleontologists, and so the known forms from the fossil record represented only long-lived lineages—“persistent types”—which were records of those earlier changes. Furthermore, even the bird-like dinosaur Compsognathus was found in the same strata as the earliest bird, Archaeopteryx, meaning that the actual transition must have occurred at some earlier time. Dinosaurs, Huxley proposed, could not have been ancestral to birds, but they did represent the form of those ancestors.
Huxley formally presented his ideas to his colleagues the same year that he wrote Haeckel, but his work on the subject was only just beginning. With an eye towards confirming a dinosaur-like ancestor of birds, Huxley pointed out avian traits in the skull of the large predator Megalosaurus and in the hips of the small herbivore Hypsilophodon. He also approached the question from the other side, citing the resemblance between the leg of an embryonic chick and the legs of dinosaurs.
Though only a handful of dinosaur taxa were known, from relative scraps of material, by the late 1860s—certainly far less than the over 1,000 genera known today—Huxley was able to point out bird-like traits in many of them. (Some of those resemblances turned out to be red herrings, e.g. the hip of Hypsilophodon only superficially looked like that of a bird. This dinosaur was not a bird ancestor, but at the time of its discovery it was the only dinosaur with complete hips and Huxley took it to be representative of the group.) Huxley stressed that the creatures included in his transitional sequence from dinosaurs to birds represented the forms of the true ancestors of modern birds. They were, in his words, “intercalary types” which were more like evolutionary “uncles and nephews,” and given the spotty nature of the fossil record the odds were against finding a well-documented series of true ancestors.
By 1870, however, Huxley’s work on the subject slowed. Unlike Darwin, he did not have enough money to retire to the life of a gentleman naturalist and had to write, teach and lecture to make a living. Between all his responsibilities and appointments, Huxley was nearly working himself to death, and in 1872 his wife Nettie sent him on a vacation to Egypt to recuperate. When Huxley returned he threw himself back into science, but in a different way. He largely eschewed paleontology in favor of laboratory anatomy, though he did not abandon the subject of bird origins all together.
In 1876 Huxley began a grand lecture tour of the United States, and among the subjects he had selected for the series was the evolution of birds. The Yale paleontologist O.C. Marsh had just discovered toothed birds from the Cretaceous sediments of Kansas the previous year—a finding that added a little more nuance to the transition Huxley was proposing—and the fossil evidence then known still indicated that birds originated from something akin to small, predatory dinosaurs. Huxley even went as far to say: “There is no evidence that Compsognathus possessed feathers; but, if it did, it would be hard indeed to say whether it should be called a reptilian bird or an avian reptile.”
Huxley did not perfectly anticipate our modern understanding that birds evolved from feathered maniraptoran dinosaurs. Using the rather paltry evidence that was available to him, he proposed a plausible scenario for bird ancestry which was meant to break down any potential anatomical barriers to such a change. Especially during the end of his career, Huxley pointed to his work on bird origins as an indication that evolution was a reality and could be supported with hard evidence from the fossil record even if the actual phases of the transition had not yet been found. Other naturalists such as E.D. Cope, Carl Gegenbaur and Andreas Wagner had also recognized the resemblance between dinosaurs and birds, but it was Huxley who turned these similarities into compelling evidence for evolution by means of natural selection. During a time when the fossil record appeared to be at odds with Darwin’s theory, Huxley endeavored to find examples of transitional forms and he found just that in the evolution of birds from reptiles.
I have no doubt that some readers may be disappointed by the dissolution of a favorite story. Huxley came tantalizingly close to predicting our present understanding but came up short. Yet, though reading Huxley’s original works, I think I am even more impressed by his work. He marshaled a wide array of evidence to create a framework for one of the major transitions in the fossil record but always kept in mind what remained unknown. Huxley’s insistence that we distinguish between direct ancestors and creatures which represent the expected form of those ancestors was especially ahead of its time—to this day paleontologists remind themselves to be careful when drawing out ancestors. Nevertheless, I can’t help but wonder what Huxley would think of all we have learned since his time. There are now scores of feathered dinosaur specimens that unquestionably show that many traits we once thought were unique to birds appeared in dinosaurs first. Rather than dinosaurs being bird-like, we should say birds are dinosaur-like. Call me presumptuous if you like, but I think Huxley would be delighted.
For more details, please see Chapter 5 of Written in Stone and my recently-published paper “Thomas Henry Huxley and the reptile to bird transition.”
Switek, B. (2010). Thomas Henry Huxley and the reptile to bird transition Geological Society, London, Special Publications, 343 (1), 251-263 DOI: 10.1144/SP343.15
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