These improved data are already prompting anthropologists to re-evaluate their assessment of the environmental and cultural transformations that shaped human evolution. In an accompanying commentary in Nature, John Kappelman of the University of Texas at Austin offers some intriguing speculations along these lines. The long, dry spell of constant brain size suggests to him that among our ancestors, as in modern apes, competition among males for access to females may have created an evolutionary pressure favoring continued large bodies. Behavior that was "more dependent on brawn than brains," Kappleman writes, evidently was successful enough that there was little evolutionary pressure toward a bigger cranium.
In considering the new reconstructions of Homo over the past 90,000 years, Kappelman is struck less by the roughly constant brain size than by the rapid decrease in body size, which runs quite counter to the earlier steady or upward trends. He suggests that this decrease in overall bulk was favored "by a social structure that relied on more cooperative foraging and better communication skills." At the same time, a better and more reliable food supply could support the metabolic demands of a large brain. "The increase in relative brain size of modern humans may then be, in part, an effect of selection for smaller body mass," Kappelman rather ignominiously concludes.
So this is what it has come to. The favored son of the Garden of Eden has been demoted to the incredible shrinking human.