One of the most shocking realizations of all time has slowly been dawning onus: the earth's climate does great flip-flops every few thousand years, andwith breathtaking speed. William Calvin ma argues that such cycles of cool,crash, and burn powered the pump for the enormous increase in brain size andcomplexity in human beings. Driven by the imperative to adapt within ageneration to "whiplash" climate changes where only grass did well for awhile, our ancestors learned to cooperate and innovate in hunting largegrazing animals. William H. Calvin is affiliate professor of psychiatry andbehavioral sciences at the University of Washington School of Medicine inSeattle.