Late Pleistocene climate change and the global expansion of anatomically modern humans

The extent to which past climate change has dictated the pattern

and timing of the out-of-Africa expansion by anatomically modern

humans is currently unclear [Stewart JR, Stringer CB (2012) Science

335:1317–1321]. In particular, the incompleteness of the fossil record

makes it difficult to quantify the effect of climate. Here, we

take a different approach to this problem; rather than relying on

the appearance of fossils or archaeological evidence to determine

arrival times in different parts of the world, we use patterns of

genetic variation in modern human populations to determine the

plausibility of past demographic parameters.We develop a spatially

explicit model of the expansion of anatomically modern humans

and use climate reconstructions over the past 120 ky based on the

Hadley Centre global climate model HadCM3 to quantify the possible

effects of climate on human demography. The combinations

of demographic parameters compatible with the current genetic

makeup of worldwide populations indicate a clear effect of climate

on past population densities. Our estimates of this effect,

based on population genetics, capture the observed relationship

between current climate and population density in modern hunter–

gatherers worldwide, providing supporting evidence for the realism

of our approach. Furthermore, although we did not use any

archaeological and anthropological data to inform the model, the

arrival times in different continents predicted by our model are

also broadly consistent with the fossil and archaeological records.

Our framework provides the most accurate spatiotemporal reconstruction

of human demographic history available at present and

will allow for a greater integration of genetic and archaeological

evidence.