Book excerpt: "John and Mary Gribbin tell the remarkable story of how we came to understand the phenomenon of Ice Ages, focusing on the key personalities obsessed with the search for answers. How frequently do Ice Ages occur? How do astronomical rhythms affect the Earth's climate? Have there always been two polar ice caps? Is it true that tiny changes in the heat balance of the Earth could plunge us back into full Ice Age conditions? With startling new material on how the last major Ice Epoch could have hastened human evolution, Ice Age explains why the Earth was once covered in ice - and how that made us human."--BOOK JACKET.