Climate-driven evolution

From a book review by Henry Harpending in PLoS Biology:

There are several rather distinct traditions in the contemporary study of human evolution. One is the detailed study of anatomical differences among fossils. A second emphasizes classical ecology with detailed attention to ancient climates and environments and comparative biogeography. A third has a focus on behavioral ecology (i.e., what used to be called sociobiology). [. . .] The behavioral ecology tradition emphasizes mating competition and intergroup struggles as the important drivers of human evolution. With roots in economics, comparative animal behavior, and the new population genetics of interaction started by W.D. Hamilton in the 1960s, social interactions themselves are the prime drivers of human evolution. [. . .]

The Humans Who Went Extinct is firmly in the [. . .] classical ecological tradition. This admirable book is a summary and interpretation of human evolution from the first bipeds several million years ago to the earliest settled agricultural populations after the end of the Pleistocene. The title does not do justice to the scope of the book: there is only a weak focus on the extinction of the Neanderthals. It is an excellent summary of the classical ecological approach to human evolution with a dazzling portrayal of the relevant paleoclimatology, paleoecology, and biogeography.

These traditions ought to be complementary; instead they are rather isolated from each other. The discipline would be enriched if we acknowledged that the narratives that each tells are models and that models should be evaluated and falsified with data. There are numerous situations where models from the ecological and behavioral traditions compete directly, and such situations ought to be exploited for direct tests of the competing models. [. . .]

A staple of my lecture notes on the modern human diaspora is that when the earth is cold the Middle East is ecologically Europe with its Palearctic fauna, while warming means the return of Africa fauna to the Middle East. Several modern looking skulls from the Middle East (Skuhl, Qafzeh) then simply represent a temporary intrusion of African fauna into the Levant ca. 120 kya. Finlayson destroys this understanding of mine, pointing out that the faunal associations of the early moderns in the Levant, and of the slightly later Neanderthals, were much the same.

Another staple of my lectures is that the appearance of anatomically modern humans in Europe is marked by a new technology called Aurignacian (distinguished by the use of blade tools) brought by the moderns. Finlayson casually destroys this staple too, pointing out that there is no known association between the Aurignacian and modern human remains. The Aurignacian could very well have been made by Neanderthals.

Even worse, I repeat the standard narrative that moderns out-competed Neanderthals and were somehow responsible for their extinction. Not so fast, says Finlayson, suggesting that climate change drove the Neanderthals away and they may never have even encountered the modern human invaders. His candidate for an unequivocal archaeological marker of moderns is the Gravettian cultural tradition with its origin in western Asia, a tradition that was able to exploit open country herd prey on the expanding tundra.

This book is packed with new data, insights, and perspectives, with the dominant theme being about how climate has driven human evolution. The narrative starts with the handful of very early apparent precursors of the Australopithecines and the later appearance of Homo erectus with its larger brain and slightly advanced tool-making technology. The conventional story is that this new version of Homo evolved in Africa and left about one and one half million years ago, but Finlayson points out that there is plenty of room for a hypothesis of an Asian origin of this taxon. [. . .]

As a population geneticist, I must explain why I have not discussed genetics in this review. In the heady times of mitochondrial DNA in the 1980s it seemed that genetics would answer important questions about human evolution. Since then, we seem to know less and less: the mutation rate of mtDNA slows down as larger time intervals are considered, and it has become clear that haploid markers like mtDNA and the Y chromosome are not very reliable markers of origins and ancestry. The most important finding about our genetics from the nuclear genome was the apparent small effective size of our species, on the order of 10,000 individuals. We knew in the 1980s that this was due to some severe bottleneck or bottlenecks in human history, but since then it has become apparent that other demographic phenomena, like waves of advance and selective sweeps of advantages genes and haplotypes, could generate the apparent small effective size. In other words, we know much less today from genetics than we thought we did two decades ago.