EARLY LATE-PLEISTOCENE MEGADROUGHTS IN AFRICA AND THEIR ECOLOGICAL CONSEQUENCES FOR HOMO SAPIENS

Recent drill core results from the Lake Malawi drilling project, coupled with records from elsewhere in Africa, demonstrate the occurrence of extraordinarily severe and protracted episodes of drought during the Early Late Pleistocene. These arid intervals far exceeded in magnitude the aridity of the Last Glacial Maximum in at least parts of tropical/subtropical Africa, and were accompanied by major changes in terrestrial and lacustrine ecosystems. Within the Lake Malawi watershed Zambezian-type woodland was replaced by semi-desert, where vegetation cover and fuel load was insufficient to maintain fire. Lake Malawi itself, presently a freshwater, deep (~700m) lake housing hundreds of endemic fish and invertebrate species, was transformed into a saline/alkaline lake, a fraction of its current size and ~100m deep.

These drought episodes and their termination ~90ka provide a context for understanding both an hypothesized human population bottleneck suggested by molecular genetic evidence to have occurred in the Late-Middle or Early-Late Pleistocene in Africa, and the subsequent rebound of human populations and their expansion out of Africa. Archaeological data shows that whereas occupation sites in both North and South Africa are common during the megadrought period, in tropical Africa such sites are rare and restricted to high elevations. Furthermore, archaeological sites become much more common in the immediate aftermath of the worst episodes of megadrought after 90ka. The combined data suggests a most probable period for expansion of anatomically modern humans out of Africa occurring between ~90-75ka.