Extinct Humans.

The human family tree has long been envisioned as a straight line progression from bipedal apes to Homo habilis, to Homo erectus, to Neanderthals, and to us, Homo sapi­ens. But this model of a single species at a time is suspi­ciously unlike the pattern of multiple branchings and extinctions known for other groups of organisms, and it fails to confront adequately the variation evident in the hominid fossil record itself. Eschewing preconceived models of evolution, some scientists started to look anew to the morphology of the fossils to see what story they tell. It is a story of great variation, repeated speciation and extinction, played out over the millions of years of hominid history.

One of the most recent books on this subject, Extinct Humans by Ian Tattersall and Jeffrey H. Schwartz (Westview Press, New York: 2000), is based on a careful reexamination of virtually every known hominid fossil in collections around the world. The authors offer a radical reinterpretation of human evolution. They demonstrate that there have been multiple coexisting human species throughout hominid history, even as recently as 25,000 years ago.

Related hominid species lived together over time and space, possibly peaceably, but possibly in direct or indirect competition with one another. Since the mid-twentieth century, for example, it has been evident that two species of australopithecines existed at one time in South Africa, one of which, a specialized vegetarian, went extinct without descendants. Early members of our genus, Homo, existed side by side with australopithecines, complicating the pic­ture further. The recent redating of Asian Homo erectus fossils implies that Java Man might have been a contemporary of European Neanderthals and even of modern humans, cast­ing serious doubt on the longstanding belief that this wide­spread hominid was our direct ancestor. It is increasingly clear as well that the Neanderthals were not directly ances­tral to modern humans but were in fact a side branch whose extinction coincided with the arrival of modern humans to Europe 40,000 years ago.

According to the new evidence, over 15 different species of humans have existed over time, with multiple human species coexisting simultaneously up until only 25,000 years ago. How did our fellow humans differ from us? Which were direct ancestors to us and which represent dead branches on our family tree? Perhaps most provocatively, why are we the lone remaining human species?

Certainly up until the origin of our species, this approach shows us that our pattern has essentially been one of business as usual for the natural world: a story of repeated evolutionary experimentation, diversification, and, ultimate­ly, extinction. And it was clearly in the context of such experimentation, rather than out of con­stant fine-tuning by natural selection over the eons, that our own amazing species appeared on Earth. However, in the end, there was a difference: unlike even our closest relatives, Homo sapiens is not simply an extrapolation or improvement of what went before it. The book concludes that our species is an entirely unprecedented entity in the living world. This cen­tral fact of human uniqueness is one with which we urgently need to come to terms, because evo­lution has done nothing to prepare the biota that not only surrounds but also supports us to cope with this new element on the landscape.