more excerpts...

human evolution, like that of other organisms, is not a gradual transformation of form and behavior over time. Rather, like the shifts in the environments in which organisms find themselves, evolutionary change is abrupt, even catastrophic.

http://www.sciam.com/article.cfm?articleID=
0004C78E-D2FB-1D07-8E49809EC588EEDF&pageNumber=1&catID=2

the earth's climate does great flip-flops every few thousand years, and with breathtaking speed. In just a few years, the climate suddenly cools worldwide. With only half the rainfall, severe dust storms whirl across vast areas. Lightning strikes ignite giant forest fires. For most mammals, including our ancestors, populations crash.

Our ancestors lived through hundreds of such abrupt episodes since the more gradual Ice Ages began two and a half million years ago--but abrupt cooling produced a population bottleneck each time, one that eliminated most of their relatives. We are the improbable descendants of those who survived--and later thrived. William H. Calvin's marvelous A Brain for All Seasons argues that such cycles of cool, crash, and burn powered the pump for the enormous increase in brain size and complexity in human beings. Driven by the imperative to adapt within a generation to "whiplash" climate changes where only grass did well for a while, our ancestors learned to cooperate and innovate in hunting large grazing animals.

http://www.amazon.com/exec/obidos/ASIN/0226092011/ref=
ase_scientificameric/104-3176739-9199161