An eloquent, spirited look at the relationship between climate and human evolution, by the author of The Throwing Madonna: From Nervous Cells to Hominid Brain (1983). Calvin (Neurophysiology/Univ. of Washington) has one basic point to make: that ""matching wits with the fickle environment is how we became human."" He explores this thesis in large part by poking around past ice ages, seeing how glaciers, toolmaking, and larger brains (the ""Great Encephalization"" of the human stock) ""bootstrapped"" each other--that is, mutually interacted--to produce George Bush, Madonna, et al. For the most part, he argues, evolutionary leaps (i.e., punctuated equilibrium) rather than agonizingly slow change led to modern Homo sapiens, with climatic fluctuations as the ""pumping mechanism."" The upshot? A creature especially adapted to function well in diverse climates; it is our flexibility on icecap or desert that insures our survival. En route to this happy conclusion, Calvin presents a scenario of the next ice age, salutes Neanderthal man, describes an airline flightover the North Pole, and attends a convention in Hungary probing extraterrestrial intelligence. A clear, tightly organized entry in the spate of recent books about nature that double as high literature; further evidence that this may well be the golden age of science writing.