CLIMATE CHANGES DURING HOMINID EVOLUTION

As hominids evolved in African woodlands and woodland margins, several climatic factors were in operation at tectonic and orbital time scales during the last 6 million years. Monsoons were the main control of sub-Saharan moisture. At orbital time scales, monsoon peaks have been driven by summer insolation changes at the precession cycle. The presence of sapropels in Mediterranean sediments indicates these 23,000-year cycles have persisted for 15 million years or more, consistent with the geographic stability of the African plate and negligible changes in Sun strength during that time.

Northern hemisphere Ice sheets primarily affected North African climate during winter when strong atmospheric dynamics transmit changes far from the ice. As seasonal drying culminated in spring, dust was lifted and transported to the ocean. Burial fluxes in ocean sediments have increased over the last 3 million years as ice sheets have grown larger. Claims of ‘glacial aridity’ in Africa based on dry conditions at the last ice-sheet maximum are complicated (compromised) by the fact that monsoons reached a synchronous minimum 20,000 years ago. And lake levels are very low today, even without ice sheets present.

Atmospheric CO2 levels fell over the last 6 million years and at glacial maxima, as shown by C4 grass replacement of C3 trees and shrubs. Long-term vegetation change caused by falling CO2 levels complicates inferences of a gradual drying of African climate.

Despite changes in monsoons, ice sheets, and CO2, both woodlands and savannahs must have always existed somewhere along north-south and mountain-slope moisture gradients in Africa. Small migrations would have enabled hominids to stay within whatever vegetation setting they favored. From a climatic perspective, hypotheses based on a long-term drying that forced hominids from woodlands into tree savannah seem suspect.