W. H. Calvin (2002) abstract for CSHL talk

ABRUPT CLIMATE CHANGE CREATES A CRASH-BOOM-BOOM CYCLE IN HOMINID (BUT NOT APE) EVOLUTION.

William H. Calvin, University of Washington, Seattle WA 98195-1800

Paleoclimate records show that, superimposed on the gradual Milankovitch effects, there are also a series of abrupt climate changes occurring every few thousand years. Worldwide, the climate flips between a mode which is warm and wet (like today) and an alternate mode which is cool, dry, windy, and dusty (like the ice ages, but often lasting only a few centuries). In both directions, the transitions are so fast that, within only 3-5 years, habitats are transformed. At higher latitudes, the abrupt coolings may be 10-20°C. Near the tropics, the 3-5°C cooling is not the main problem, but rather the dryness; African rain forests may shrink 80%. So the onset is like a drought – but, unlike regional droughts where unstressed peripheral subpopulations can repopulate the stricken area afterward, these droughts are so widespread that even the scattered refugia come under pressure.

The drought and unusual winds would, of course, promote forest and range fires. These would affect most terrestrial mammals, not just our ancestors (though waterhole predators might have some advantages). But the year after a fire, grasslands would greatly expand. Since the surviving grazing animals could double and redouble their populations in just a few years, their predators would also experience a temporary boom time; indeed, there might be few choices in food for a few generations. As brush and forests (suitable to the new climate) developed, the grasslands would become patchy and some isolated populations of grazers and their predators would develop, speeding evolution once again. When the climate abruptly flipped into the warm-and-wet mode a few centuries later, some deserts would become grasslands and there would be a second boom time for grazers and their predators. This crash-boom-boom cycle does not provide evolutionary advantages for the great apes in general, only for those such as Homo erectus, increasingly able to exploit herds of large grazing animals. (See W. H. Calvin, A Brain for All Seasons: Human Evolution and Abrupt Climate Change, Univ. of Chicago Press 2002.)

That this crash-boom-boom cycle repeats every few millennia means that small incremental advantages can compound. Cooperative behaviors are important to such predation; indeed, even if a lone hunter kills a large antelope, it is too much meat for even a single family. The obvious strategy is to give most of it away and count on reciprocity tomorrow. Besides reciprocal altruism, throwing accuracy would also come under selection; during “get set” one must improvise an appropriate-to-the-target orchestration of many muscles. While there are hundreds of ways to throw that would hit a particular target, they are embedded in a search space of millions of wrong ones, any one of which would cause dinner to run away. Thus the crash-boom-boom cycle seems capable of pumping up both cooperative behaviors and advance planning capacities. If the several dozen abrupt climate changes of the last 100,000 years are any indication of the ice ages in general, there may have been hundreds of such worldwide episodes not only serving to select for the relevant abilities, but also causing patchy habitats to develop that augment the chances for peripatric speciation to occur. Thus the crank of natural selection would often have a ratchet to prevent backsliding (dilution via unimproved immigrants is the usual fate of adaptations).

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Talk intro:

This is a talk on biogeography. And what, you might ask, is a physiologist like me doing talking about biogeography? I am sure that, since Guns Germs and Steel, Jared Diamond gets asked the same question all the time, as he is a G-I tract physiologist.

I am, at least, a neurophysiologist, concerned with the neocortical circuitry that it takes to do syntax -- and hoping for some insights from paleoanthropology about how a chimp-like brain could reorganize and enlarge to become human. But that leads one on to evolutionary dynamics and so to paleoclimate. A few years ago, the editor of The Atlantic Monthly persuaded me to write a cover story, “The great climate flip-flop.” By the time that they had edited out all of my references to brains and evolution, leaving a standalone story about paleoclimate and oceanography, even my colleagues were confused, wondering if there was some other Wm H Calvin writing away.

But I’ve been following the abrupt climate change story for 20 years looking for how the implied boom and bust cycles might resonate with evolutionary dynamics of the punctuated equilibria sort. And so today, again mostly stripped of brain references, I thought that I’d cover this search for resonances and pumps. Indeed, if crash-boom—boom weren’t such a good title, I’d have called this a Tale of Two Pumps.

Let’s begin with the standard story and see what the abruptness adds to it.