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HUMAN EVOLUTION:
Out of the Chattering Ice

Historians have not yet sorted out the causes of all this attention to calamity. It could have something to do with those apocalyptic scenarios fretted over in the Reagan era (e.g., nuclear winter), or with social critics of the 1960s and 1970s gaining professorships (Stephen Jay Gould used to call himself a "dialectical materialist"), or even with the fact that catastrophes make good television. One also has to reckon, though, with the blunt fact that nature has been reasserting itself against the blinders of gradualistic prejudices, as Gould and others began to stress more than 20 years ago. Some catastrophes are just plain real.

The first two-thirds of William Calvin's A Brain for All Seasons is a creative attempt to incorporate abrupt climate change into theories of human origins. A neurobiologist at the University of Washington School of Medicine, Calvin builds on the discovery, a decade or so ago, that the temperature swings we have experienced over the past 8000 years are small compared with fluctuations in the deeper past. Contrary to previous assumptions of slow and stately coolings extending over millennia, ice cores from northern Greenland and elsewhere have shown that ice ages can begin quite suddenly, perhaps even within a space of only a few years. Climatologists believe that interruptions in the flow of the Gulf Stream may be one of the causes of such abrupt coolings: If, for some reason, the warming waters flowing from the south cannot reach higher latitudes in the North Atlantic, glaciers will start growing, reflecting more and more sunlight into space, causing runaway cooling.

Calvin argues that these repeated coolings had profound consequences for human evolution. Abrupt coolings were accompanied by prolonged droughts at lower latitudes, reducing herbivore populations and shrinking the numbers of predators eating those herbivores. In Africa circa 2 to 3 million years ago, when bipedal primates first began making and using stone tools, abrupt climate change rewarded those creatures able to improve their hunting powers. "Whiplash" climate fluctuations forced hominid populations through hundreds of severe-drought bottlenecks, during which those with the better survival skills (including bigger brains and greater intelligence) survived to flourish when the good times began again. Calvin claims that selection did not work on braininess per se, but rather on the faculties responsible for things like the neuromotor skills involved in accurate throwing, which had a long "learning curve." Tool-using hominids had invented a novel techno-niche not available to other predators (like the big cats), and when proto-human brains began to expand to capitalize on the selective advantages of accurate throwing, other faculties were dragged along. The net result was cerebral modernity, a flexible and capacious hominid brain "for all seasons."

This is a variant on the "man the hunter" thesis, but with a couple of new and intriguing twists: Calvin holds that the improvement was not slow and steady but episodic, according to what he calls "catastrophic gradualism." He proposes that these warm-to-cold, boom-to-bust cycles (limiting access to hoofed prey) augmented brain function in tiny spurts, as ice-age oscillations ("chatterings") and the resulting bottlenecks sharpened hunting skills. Calvin also elaborates his theory of how chimp-like australopithecines first began to hunt with tools: Stones or sticks, he says, may originally have been thrown into herds gathered at water holes, in the hope that the commotion might cause an animal or two to be trampled. The predators eventually learned that sharper stones worked better than dull stones and that, by battering the edges, they could be made sharper still. Certain shapes were then found to fly farther and hurt more than others, whence the origin of all those enigmatic Acheulian "handaxes."

The last third of the book strays a bit, offering nonspecialist readers an excellent overview of the geophysics behind abrupt climate change. Calvin focuses on the forces involved in stabilizing the Gulf Stream and North Atlantic Current--the "Achilles heel" in much recent climate modeling (though itself perhaps only an index of some larger process). The flow of the book is interrupted here to a certain extent, as human evolution is put on a back burner and our attention is drawn to the dangers posed to modern civilization should global warming launch us into cooling mode. Calvin gets graphic: We might first feel the effects of drought, including crop failure, massive fires from lightning, and dust storms kicked up by loss of vegetation. Populations would crash, and wars would break out over control of resources. Calvin is clearly aware of the uncertainties in such prognostications, but he also reminds us that if history (as preserved in the ice cores) is any guide, we are due for a cooling. Prior to the relatively mild climate of the past eight millennia, there were catastrophic cooling events like the Younger Dryas (12,900 to 11,600 years ago), when North Atlantic temperatures dropped by about 8ºC. If such a cooling were to repeat today, it would devastate most of the world's agriculture and then some.

Readers may be disappointed to find certain topics missing from the text: Calvin talks about bombing ice jams in Scandinavian fjords to prevent catastrophic releases of fresh water into the North Atlantic, and speculates on the value of reopening the trans-Panama oceanic throughway closed off 3 to 4 million years ago, but we hear little or nothing about some of the more obvious ways to reduce warming--and therefore the threat of abrupt cooling--such as conservation, curbing greenhouse gases, and a serious shift to globally responsible fuels.

Calvin does make it clear, though, that too much focus on global warming in itself--i.e., the heat--could obscure this larger danger of catastrophic cooling. He aptly cites Ray Pierrehumbert's caution that if one pulls on a sleeping dragon's tail without knowing how much it takes to waken it, one had better be prepared for the unexpected.

Summary of this Article Reprint (PDF) Version of this Article dEbates: Submit a response to this article Similar articles found in: SCIENCE Online Search Medline for articles by: Proctor, R. N. Alert me when: new articles cite this article Download to Citation Manager     Collections under which this article appears: Anthropology Atmospheric Science