Imagine going to the first meeting of a course you'd long waited to enroll in. You sit down at your computer, open an e-mail message from your professor, in this case the author William H. Calvin, and get your first lesson. Your professor is thousands of miles away. In fact, he's at 51.4oN, 0.1oE. Where? Why, Charles Darwin's home in Kent, England, of course, the famous Down House. So begins Calvin's journey through evolution, particularly human evolution, as he leads his "class" from the home of the man many would call the father of evolution to various locales that provide fodder for his ultimate message: 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. A neurobiologist by training (he is at the University of Washington School of Medicine), Calvin leads us along a trail that links sudden worldwide coolings to the origin of our large brains and modern human behavior. By modern behavior, he is thinking not just of sophisticated toolmaking; he includes such social behavior as pair bonding and, ultimately, language, a sense of the aesthetic, and "abstract thinking, planning depth, innovation, and symbolic behavior." The sudden coolings, Calvin tells us, reduced rainfall, induced dust storms and fires, and produced bottlenecks in the populations of our forebears. The few survivors had to adapt within one generation to, for example, a climate in which only grass grew well, spurring them to develop innovative techniques for hunting the large grazing animals that converted the grass into edible energy. Thus, he concludes, the cycles of "cool, crash, and burn" drove increased brain size and complexity. I think it unlikely that the climatic shifts were behind changes in the physical size and complexity of the brain, but these sudden jolts could certainly have spurred early humans to exploit the existing potential of the brain. To make his points, Calvin takes us, his class, on a peripatetic journey as he visits museums, attends conferences, pays homage to a variety of African human fossil sites, and flies over huge African expanses and the vast Nordic seas. As one might expect, this approach is not always successful, but if you forget the formatting at the beginning of each brief chapter (a nod toward an e-mail message, but one without typos, code abbreviations and non sequiturs), the read flows a bit better. Calvin's premise--that human evolution is correlated with climatic swings--is, of course, not new. Indeed, the traditional Darwinian view holds that evolution proceeds through organisms tracking their environments. And well over a decade ago paleontologist Elisabeth Vrba proposed that changes in species representation over time, as evidenced especially in the South African fossil record of antelopes and early hominids (such as Australopithecus and Paranthropus), were rapid and correlated with shifts between wetter and drier conditions. But Calvin's presentation differs from the others in that it really is an attempt to think globally about past and present climatic change and its possible effects on creatures and their evolution. As one of the authors whose work on human evolution he cites as recommended reading, I found his discussion of the fossils less engaging than the climate-related information. The book definitely picks up steam when he moves away from trying to discuss human fossils and digs into issues of global warming, shrinking polar ice caps, and oceanic currents. (This may be because much of this section had already been published as "The Great Climate Flip-Flop" in the Atlantic Monthly.) Here he seems to have more fun, getting across an image, for example, of subsurface oceanic water behavior by describing what happens when you pour very cold heavy cream over a spoon into a cup of hot coffee (it sinks as a column) and explaining North Atlantic Ocean current movements by way of a story about incorrectly hooking up a hot-water tank with a toilet that then acted as a radiator. Heading back home to Seattle on the long, great-circle-route flight from Nairobi, over the Gulf Stream and Greenland, Calvin muses on the present global warming brought about by human activities. It could, he says, paradoxically trigger another episode of sudden cooling. The accumulation of greenhouse gases in the atmosphere could induce an abrupt shutting down of the oceanic "conveyor belt" that sends warmer waters into the North Atlantic, plunging much of the earth into a deep chill. But he doubts that another boom-then-bust cycle will jack up our brain power. We're now smart, he concludes, "in ways that owe little to our present brain power, but rather to the accumulated experience of the people that have lived since the last ice age ended. Education. Writing. Technology. Science." And he suggests that if we're really smart, our accumulated experience may just help us find a way to avoid this looming threat.

Jeffrey H. Schwartz teaches physical anthropology at the University of Pittsburgh and is author of Sudden Origins: Fossils, Genes and the Origin of Species (Wiley, 1999).