PALGRAVE MACMILLAN TRADE, JUNE 2015Before the mid-1900s, human fossils were for the most part studied and pronounced upon by specialists in human anatomy. Based in medical schools, these researchers were exquisitely attuned to variation within Homo sapiens, but were largely unconcerned with the riotous diversity of species out there in the living world. Ignorant of taxonomic norms, they branded newly discovered hominin fossils with new Latin names, much as they gave each of their children a separate name. In this way, throughout the first half of the 20th century the rapidly expanding paleoanthropological literature became littered with formal names—even though many of the freshly unearthed fossils actually belonged to species that had already been described.
I write about this crucible of discovery and folly in my new book, The Strange Case of the Rickety Cossack.
The haphazard application of species names to every new hominin fossil was a practice that could not continue indefinitely. And in 1950 the ornithologist Ernst Mayr, one of the fathers of the Evolutionary Synthesis, took it upon himself to lecture the paleoanthropologists on the error of their ways. The Synthesis was an elaboration of evolutionary theory that saw most evolutionary change as the gradual accumulation, via natural selection, of small genetic innovations within ancestor-descendant sequences. So Mayr depicted human evolution as the slow modification of a single lineage culminating in Homo sapiens. Among the fifteen hominin genera then described, Mayr said, there was in reality only one: our own genus, Homo. What’s more, the Homo lineage contained only three species: Homo transvaalensis (an early biped) had given rise to Homo erectus, which in turn had evolved into Homo sapiens. Acutely aware that their nomenclatural proliferation lacked any theoretical justification, the paleoanthropologists capitulated. For half a century thereafter, most of them viewed human evolution as a single-minded progression from primitiveness to sapient perfection driven by natural selection: an idea that fit rather well with the undeniable fact that only one hominin exists in the world today.
From the beginning, though, it was obvious that Mayr’s scheme was a huge oversimplification. As fossil discoveries rapidly continued to accumulate, his single lineage began to bulge at the seams. A new image of hominin evolution began to develop.
Extensive additions to the hominin fossil record beginning in the 1960s eventually made it glaringly evident both that many more morphologies are present in that record than could ever be accommodated by Mayr’s minimalist taxonomy, and that the saga of human evolution was immensely more complex than the burnishing of a single lineage.
A hominin family tree that I drew up in 1993 already featured 12 species, spanning the period from 4 million years ago to the present, while one of my recent trees contains twice as many species, scattered over the last 7 million years. Either way, at any one point in the past, several different hominin species typically coexisted, revealing human evolution not as a linear affair but as a process of vigorous and continuing experimentation with the hominin adaptive potential. Homo sapiens is evidently a huge exception in being the sole hominin species on the planet, and its lonely state cannot be taken as a guide to the past. There is something unprecedented about our species that makes it both intolerant of competition and uniquely able to eliminate it.
Almost certainly, this novel element lies in the unusual way in which we process information, whereby a vocabulary of mental symbols makes it possible for us to remake the world in our minds. And both the form of our family tree and the archaeological record make it plain that this unique capacity was acquired not only very recently, but also very abruptly in evolutionary terms. Mayr’s perspective suggests that we were gradually fine-tuned by natural selection, over the eons, to be the kind of creatures we are. But the diversity of the rapidly expanding hominin fossil record strongly argues otherwise. And if that is the case, we are not condemned by our biology to act in any specific ways. Instead, we are responsible for our own individual behaviors.
Ian Tattersall is a curator emeritus in the Division of Anthropology of the American Museum of Natural History in New York City. Read an excerpt from The Strange Case of the Rickety Cossack: And Other Cautionary Tales from Human Evolution.