1. Why Was Human Evolution So Rapid?

Nowhere in the entire fossil record of life do we find more dramatically accelerated accumulation of evolutionary novelty than we do in the genus Homo. Quite simply, and by whatever criteria you measure it, our species Homo sapiens is more different from its own precursors of two million years ago than is any other species living in the world today. What might account for this unusually rapid rate of evolution? A major influence was almost certainly material culture, though not in the gene-culture co-evolutionary context envisaged by the evolutionary psychologists. Rather, material culture enhances the ability of hominid populations to disperse at times when conditions are favorable for expansion, while incompletely insulating the resulting enlarged populations from environmental stress when circumstances deteriorate. In other words, by facilitating expansion beyond normal physiological limits in good times, culture makes populations more vulnerable to fragmentation in bad ones. Over the course of the Pleistocene, short-term but large-scale local environmental changes became increasingly frequent over large tracts of the Old World, further amplifying the stress-and-response cycle. Since the fixation probabilities of evolutionary novelties of all kinds (as well as of local extinctions) are promoted by population fragmentation and consequent small effective population sizes, we see in the synergy between environmental effects and material culture a sort of ratchet effect which would have acted to leverage rates of accumulating change. This interaction explains the extraordinarily fast tempo of evolution within the genus Homo by invoking perfectly routine evolutionary processes; and it eliminates any need for special pleading in the hominid case, at least in terms of mechanism. Apparent recent diminution in human brain size may result from greater algorithmic efficiency.