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

Abstract

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.

Fig. 1.

The effect of NPP on population density (individuals per kilometer squared) plotted on a log–log scale. The relationships selected by fitting the model to the current patterns of genetic variation are represented by shading, with darker colors representing more frequently selected values and the median of the relationships shown as a black line. Population densities for modern hunter–gatherers (not used to inform on the parameters of the model) are plotted as blue circles. Inset shows the role of the two model parameters (NPP_low and NPP_high) on the same log–log scale as the figure.

Fig. 2.

Median arrival times (thousands of years ago) as predicted by the model. Histograms show the distribution [frequency (f)] of times for key areas of the world: (A) the Arabian Peninsula (the exit point out of Africa), (B) Southeast Asia, (C) Australia, (D) Europe, (E) North America, and (F) Central and South America. Red arrows show key archaeological findings commonly referred to as the earliest evidence for AMHs in those areas (SI Text, Fig. S7, and Tables S1 and S2). Gray areas were never colonized, either because of extreme climatic conditions or a lack of connections to the mainland (sea voyages of more than 100 km were not allowed in our model). For clarity, arrival times were capped to 80 kya.

Fig. 3.

Routes of gene flow during the expansion represented by the distribution of coalescence events; 100 gene trees were sampled from each of the best-fitting combinations of demographic parameters. Each population was represented by a single, randomly sampled haplotype. The four panels show coalescence events for populations from (A) Europe, (B) East Asia, (C) Australasia, and (D) the Americas.