High- and low-latitude forcing of Plio-Pleistocene East African climate and human evolution

We propose that Plio-Pleistocene East African climate is forced by both high latitude climate changes and local orbital forcing, but is independent of glacial-interglacial cycles. Paleo-records show that the late Cenozoic climate of East African is punctuated by episodes of short, alternating periods of extreme wetness and aridity, superimposed on a long-term drying trend. The periods of extreme climate variability are characterised by the precession-forced appearance and disappearance of large, deep lakes in the East African Rift Valley, paralleled by low and high wind-driven dust loads reaching the adjacent ocean basins. Over the last 3 million years such periods only occur at the times of major global climatic transitions, such as the Onset of Northern Hemisphere Glaciation (2.7-2.5 Ma), intensification of the Walker circulation (1.9-1.7 Ma) and the Mid- Pleistocene Revolution (1-0.7 Ma). High latitude forcing in both Hemispheres is required to compress the Inter- Tropical Convergence Zone so that East Africa becomes locally sensitive to precessional forcing, resulting in rapid shifts from wet to dry conditions. Between 0.5 Ma and 5.0 Ma the periods of highly variable East African climate, oscillating from very dry to very wet, occupied less than a third of the total time. In contrast, 12 out of the 15 hominin species i.e., four fifths, first appeared in one of these extreme `wet-dry' periods. We suggest that ephemeral lakes, expanding and contracting on precessional timescales, would have evoked a wide- spread, regional-scale, rapid, and extreme environmental variability, required by the Variability Selection Hypothesis of human evolution.