Excerpted from The Strange Case of the Rickety Cossack, and Other Cautionary Tales from Human Evolution, by Ian Tattersall. Palgrave Macmillan Trade, 2015. Copyright © 2015. Reprinted with permission. (Scientific American is part of Macmillan Publishers.)

If I had to opt for one single year as the most momentous in the twentieth-century intellectual history of paleoanthropology, I would unhesitatingly choose 1950. Theodosius Dobzhansky had, of course, already put the Synthesis cat among the paleoanthropological pigeons back in 1944, but it was wartime, and nobody seems to have taken much immediate notice. Nonetheless, Dobzhansky’s take on human evolution pointed to the future, and in many ways the end of World War II, the year after his article appeared, also marked the passing of the old guard in paleoanthropology. In 1948 the aged though still-industrious Arthur Keith published a volume entitled A New Theory of Human Evolution, but the book actually did little to deliver on its title. It is mostly remembered, if at all, for its vaguely anti-Semitic stance. The time had come for a new cast of characters to step onto the paleoanthropological stage.

A leader among the new generation of biological anthropologists was Sherwood Washburn. Rather conventionally trained at Harvard during the1930s, Washburn enthusiastically embraced the New Evolutionary Synthesis after joining Dobzhansky on the Columbia faculty in 1940. And it was with this energetic convert that the Synthesis at last acquired a conduit into paleoanthropology. In 1950 Washburn (by then at the University of Chicago) and Dobzhansky jointly organized a conference hosted by Long Island’s Cold Spring Harbor Laboratory. Grandly titled “The Origin and Evolution of Man,” this international meeting brought together numerous luminaries of paleoanthropology and adjacent sciences, including all three of the giants of the Synthesis. It was thus loaded with star power, but in retrospect, one contribution stands out not only as the most newsworthy presentation at the conference, but also as one of the most influential benchmarks ever in paleoanthropology. Significantly, it was not made by a paleoanthropologist. It was made by the ornithologist Ernst Mayr.

As forceful on the printed page as in oratory—although his published version bears all the marks of haste in preparation—Mayr didn’t bother to mince his words. In no uncertain terms, he informed the assembled multitude that the picture of complexity in human evolution implied by all those hominid species and genera was just plain wrong. To begin with, he declared, both the theoretical and the morphological yardsticks by which the anatomists had differentiated them were entirely inappropriate. For example, if you took a couple of fruit fly species and blew them up to human size, they would look much more different from one another than the members of any pair of living primate species do. And the same went in spades for fossil hominids.

Spectacularly irrelevant as the metaphor was, it resonated with an audience that was uncomfortably aware of the thin theoretical ice on which it skated. And it primed that audience for Mayr’s more specific claim, that the supposed diversity of hominid genera and species just didn’t exist. What was more, Mayr continued, even in principle there was no way in which that diversity could have existed, because the possession of material culture so remarkably broadened the ecological niche of tool-wielding hominids that there would never have been enough ecological space in the world for more than one human species at a time.

Put together, Mayr said, these various practical and theoretical considerations dictated that every one of the human fossils known should be placed within a single evolving polytypic lineage. And not only was there a mere three species recognizable within that lineage, but every one of those species belonged to a single genus: Homo. As Mayr saw it, Homo transvaalensis (the australopiths) had given rise to H. erectus (including Pithecanthropus, Sinanthropus, and so forth), which in turn evolved into H. sapiens (including the Neanderthals). And that was it.

Still—as if he somehow felt that things couldn’t have been quite this simple—Mayr inquired explicitly why, unlike virtually any other successful mammal family, Hominidae had not thrown off a whole array of species. “What,” he asked, “is the cause of this puzzling trait of the hominid stock to stop speciating in spite of its eminent evolutionary success?” His ingenious answer to this excellent question brought him right back to “man’s great ecological diversity.” Humans, Mayr declared, had “specialized in despecialization.” What was more, “Man occupies more ecological niches than any known animal. If the single species man occupies all the niches that are open for a Homo-like creature, it is obvious that he cannot speciate” (emphasis mine). Mayr also noted something else very special about “man,” something that, in his view at least, supported his reconstruction of human phylogeny as an infinite recession of today’s ubiquitous Homo sapiens back into the past: “Man is apparently particularly intolerant of competitors...the elimination of Neanderthal man by the invading Cro-Magnon man is only one example.”

Mayr took questions at the end of his presentation. When asked (not by a paleoanthropologist, of course) about how the notable morphological differences found among fossil hominids could all be compressed into a single genus, he finessed his answer by responding that “since there are no absolute generic characters, it is impossible to define and delimit genera on a purely morphological basis.” Nobody at the time saw fit to call him on this. Nobody pointed out the obvious: that morphology was the only thing that paleontologists had to work with, and that, while he might technically have been right about the nonexistence of “absolute generic characters”—whatever exactly that meant—fossil genera had to be recognized from their morphology. Nor did anybody suggest that intolerance of competition might be specifically a feature of Homo sapiens, distinguishing it from even its closest relatives. And neither did anyone question any other of Mayr’s sweeping and hugely speculative declarations—either at the time, or in the couple of years following the appearance in print of his provocative comments.

Almost certainly, the reason for this supine acceptance of his many-sided criticisms of their field is that Mayr’s broadside had shocked the tiny elite of paleoanthropologists into some long-overdue introspection. They finally began to realize that they and their predecessors had been operating in a theoretical vacuum, in which nobody—except perhaps Franz Weidenreich—had bothered to think much either about the processes that might have underwritten the stories they were telling about their fossils, or about how their operating assumptions fit in with what was known about how the rest of Nature had evolved. And here was Mayr, the self-assured architect of the Synthesis, with an eloquent and comprehensive analysis of their science: an analysis that combined a nod to morphology with considerations of evolutionary process, systematics, speciation theory, and ecology—all those key factors that paleoanthropologists were now beginning to feel guilty about having largely ignored—to produce a cogent and coherent statement about human evolution. Without an intellectual fallback position, what could they do but capitulate? Caught in this uncomfortable epistemological situation, hardly anybody seemed to mind that Mayr’s scenario was far from firmly anchored in the study of the fossils themselves.

The major English-speaking exception to this instant surrender was Robert Broom’s younger associate John Robinson, who pointed out at some length that the morphological heterogeneity among the gracile and robust australopiths—and some similarities he saw between some South African and early Javan material—indicated at least two coexisting hominid lineages in the Pliocene or early Pleistocene. And although Mayr’s grudging admission that Robinson indeed had a valid point was buried in a pile of notes published in a journal that paleoanthropologists didn’t read, once Robinson had pointed this out most of his colleagues came to agree that the robust australopiths were best excluded from Mayr’s linear scheme. The genus name Australopithecus continued to be used for all the gracile australopiths (and for some paleoanthropologists, mainly Robinson, the robust offshoot continued to be called Paranthropus). Robinson himself also continued to use the name “Telanthropus” (his quotes) for the mysterious, very lightly built hominid fossils from Swartkrans—and by then also from one section of Sterkfontein, which by the mid-1950s also had begun to produce some crudely flaked stone tools.