Models of human evolution give quite a bit of attention to the role that climate change may have played in the evolution and dispersal of hominin species, both ancient and modern. A study just published presents evidence of an extreme and prolonged drought in East Africa, spanning 135,000 – 75,000 years ago – the time when the Out of Africa hypothesis suggests that Homo sapiens was moving out into Europe and Asia.
Evidence for the drought comes from a drilling project that obtained sediments from the bottom of Lake Malawi, in east Africa. Researchers recovered around 400m of sediment cores that allowed them to look back at environmental changes over the last million or so years (Scholz et al. 2007).
Analysis of the sediments indicated that between 135,000 and 75,000 years ago, the lake almost disappeared, losing 95% of its water. During this time, surrounding habitats were very dry indeed. The researchers came to this conclusion because the lake's sediments contained hardly any organic material – little or no pollen or charcoal – suggesting that there was hardly any vegetation on the land around the lake. And the organisms that lived in the lake were typical of shallow and alkaline water bodies (today the lake is around 700m deep, at its deepest point). Assuming that these conditions were widespread in Africa, human populations of the time would have found survival extremely difficult and many would have died, leading to a population bottleneck.
Data from Lake Malawi and other lakes show that the climate became wetter again from around 70,000 years ago. If conditions in the Rift Valley – including the lakes and the headwaters of the Nile river – were more hospitable, this could have provided a route for east African humans to disperse northwards, and ultimately out of Africa into the rest of the Old World.
References: C.A. Scholz et al.(2007) East African megadroughts between 135 and 75 thousand years ago and bearing on early-modern human origins. Proceedings of the National Academy of Sciences [advance electronic version]: http://www.pnas.org/cgi/reprint/0703874104v1, access date 15 October 2007.