So, once more, what happened? Did the earliest anatomically modern and the earliest behaviorally modern humans represent separate but skeletally identical species, one of which eventually replaced the other? This scenario seems inherently improbable to me, since any such dramatic Old World-wide replacement would have had to have taken place in a very short window of time. The only evident alternative is that the unique human capacity was born with anatomically modern Homo sapiens, and that it lay fallow, as it were, until it was unleashed by some unknown cultural stimulus. This innovation, whatever it was, would then have been able to spread by cultural contact among populations that already possessed the latent ability to acquire it. No wholesale replacement of populations need have been involved.

What might that stimulus have been? Like many others, my best guess is that it was the invention of language, and we must bear in mind that by the time Homo sapiens evolved the peripheral equipment that allows articulate speech had already been around for several hundred thousand years—having clearly evolved initially in other contexts entirely. The archaeological record is but a dim reflection of the full panoply of behaviors of any early hominid, but if it shows us anything at all it is the starkness of the contrast between the torrential outpouring of symbolic behaviors by the Cro-Magnons and the essentially symbol-free behaviors of their predecessors. The fundamental innovation that we see with the Cro-Magnons is that of symbolic thought, and this is something with which language is virtually synonymous. Like thought, language involves forming and manipulating symbols in the mind, and our capacity for symbolic reasoning is almost inconceivable in its absence.