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Significance

The timing and causes of hominin (pre-Homo sapiens) migrations out of Africa have been of recent interest. Two scenarios, one based on modern genomic data and the other on the chronology of hominin sites, indicate population bottlenecking in the Early Pleistocene. An ice age is invoked as bottleneck trigger in both cases even though they differ in timing, and therefore in the actual event that triggered depopulation. Our assessment of the chronology of key hominin sites in Eurasia leads us to conclude that bottlenecking occurred at the first major ice age of the Pleistocene, ~900,000 y ago, in agreement with the genomic model, and coincided with a major diaspora from Africa into Eurasia when hominins came close to extinction.

Abstract

Two recently published analyses make cases for severe bottlenecking of human populations occurring in the late Early Pleistocene, one case at about 0.9 Mya based on a genomic analysis of modern human populations and the low number of hominin sites of this age in Africa and the other at about 1.1 Mya based on an age inventory of sites of hominin presence in Eurasia. Both models point to climate change as the bottleneck trigger, albeit manifested at very different times, and have implications for human migrations as a mechanism to elude extinction at bottlenecking. Here, we assess the climatic and chronologic components of these models and suggest that the several hundred-thousand-year difference is largely an artifact of biases in the chronostratigraphic record of Eurasian hominin sites. We suggest that the best available data are consistent with the Galerian hypothesis expanded from Europe to Eurasia as a major migration pulse of fauna including hominins in the late Early Pleistocene as a consequence of the opening of land routes from Africa facilitated by a large sea level drop associated with the first major ice age of the Pleistocene and concurrent with widespread aridity across Africa that occurred during marine isotope stage 22 at ~0.9 Mya. This timing agrees with the independently dated bottleneck from genomic analysis of modern human populations and allows speculations about the relative roles of climate forcing on the survival of hominins.