This alone suggests that there must be some powerful countervailing advantage in the human conformation of the vocal tract, but the ability to speak, unfortunately, is not it. We know this because the roof of the vocal tract is also the base of the skull. Thus, where this region is preserved in fossils, we can reconstruct in general terms what the vocal tract had looked like in life. The low larynxhigh pharynx combination betrays itself in a flexion of the bones of the skull base. We begin to see some evidence of such flexion in Homo ergaster, almost 2 myr ago, and a skull of Homo heidelbergensis from Ethiopia shows that it had reached virtually its modern degree by about 600 kyr ago. A vocal tract capable of producing the sounds of articulate speech had thus been achieved among humans well over half a million years before we have any independent evidence that our forebears were using language or speaking.
Clearly, then, the adult human vocal tract cannot in origin have been an adaptation for modern speech--though it might have conferred some advantage in the context of a prelinguistic form of vocal communication. So what, then, is it for? Inevitably we have to come back to exaptation. Despite its disadvantages, basicranial flexion appeared, and it then persisted for a very long time before being capitalized upon for its linguistic qualities. Maybe over that long period it did indeed bestow certain advantages in the production of more archaic forms of speech--forms that we are hardly in a position to characterize. Or maybe it conferred some kind of benefit in terms of respiration, which is an issue that is still very poorly understood among extinct hominids. Still, whatever the case, we have to conclude that the appearance of language and its anatomical correlates was not driven by natural selection, however beneficial these innovations may appear in hindsight to have been.
At present, then, there is no way we can come up with any even modestly convincing scenario of what happened in the origination of the extraordinary creature we are, without invoking the humble process of exaptation. Clearly, we are not the result of a constant and careful fine-tuning process over the millennia, and much of our history has been a matter of chance and hazard. Nature never intended us to occupy the position of dominance in the living world that, for whatever reasons, we find ourselves in. To a remarkable extent, we are accidental tourists as we cruise through Nature in our bizarre ways. But, of course, we are nonetheless remarkable for that. And still less are we free of responsibility.
IAN TATTERSALL is a curator in the division of anthropology at the American Museum of Natural History in New York City. This article is excerpted from The Monkey in the Mirror: Essays on the Science of What Makes Us Human (Harcourt, 2002). His other books include Becoming Human: Evolution and Human Uniqueness (Harcourt Brace, 1998), The Last Neanderthal: The Rise, Success and Mysterious Extinction of Our Closest Human Relatives (Westview, 1999, revised) and Extinct Humans, with Jeffrey Schwartz (Westview, 2000).