One of the most shocking realizations of all time has slowly been dawning on us: 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.

Calvin's book is structured as a travelogue that takes us around the globe and back in time. Beginning at Darwin's home in England, Calvin sits under an oak tree and muses on what controls the speed of evolutionary "progress." The Kalahari desert and the Sterkfontein caves in South Africa serve as the backdrop for a discussion of our ancestors' changing diets. A drought-shrunken lake in Kenya shows how grassy mudflats become great magnets for grazing animals. And in Copenhagen, we learn what ice cores have told us about abrupt jumps in past climates.

Perhaps the most dramatic discovery of all, though, awaits us as we fly with Calvin over the Gulf Stream and Greenland: global warming caused by human-made pollution could paradoxically trigger another sudden episode of global cooling. Because of the accumulation of greenhouse gases in the atmosphere, the oceanic "conveyor belt" that sends warmer waters into the North Atlantic could abruptly shut down. If that happens again, much of the Earth could be plunged into a deep chill within a few years. Europe would become as cold and dry as Siberia. Agriculture could not adapt quickly enough to avoid worldwide famines and wars over the dwindling food supplies--a crash from which it would take us many centuries to recover.

With this warning, Calvin connects us directly to evolution and the surprises it holds. Highly illustrated, conversational, and learned, A Brain for All Seasons is a fascinating view of where we came from, and where we're going.