The Pulsed Climate Variability Hypothesis

In summary, new paleoclimate data suggest that the long-term drying trend in East Africa was punctuated by episodes

Fig. 13.3 Three theoretical models of lake changes in East Africa during the Plio-Pleistocene. Model 1: 'smooth' and relatively slow transitions from deep to no lake conditions, which would imply that either high energy wet conditions or prolonged aridity may have influenced human evolution. Model 2: 'threshold' rapid transitions from deep to no lake conditions, which would imply that rapid transition may have influenced human evolution, or the high energy wet conditions or prolonged aridity as suggested by Model 1. Model 3: 'extreme variability,' with high variability during the transitions between deep and no lake conditions, which implies variability influenced human evolution or, again, either high energy wet conditions or prolonged aridity.

Mid Pleistocene Climate Transition

of short, alternating periods of extreme humidity and aridity. These periods of 'pulsed climate variability' are characterized by the precession-forced appearance and disappearance of large, deep lakes in the East African Rift Valley, and are paralleled by low and high wind-driven dust loads reaching the adjacent ocean basins. During the last 3 million years, such periods only occur at the times of major global climatic transitions, such as the intensification of Northern Hemisphere Glaciation (2.7-2.5 Ma), development of the Walker circulation (1.9-1.7 Ma), and the Mid-Pleistocene Revolution (1.0-0.7 Ma). We suggest that 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. Building on Potts' (1998) variability selection hypothesis, we suggest that these periods of pulsed climate variability may have provided a catalyst for evolutionary change, and driven key speciation and dispersal events amongst mammals and hominins in Africa. Hominin species, in particular the early species attributed to the genus Homo, seem to differentially originate and go extinct during periods of extreme climate variability. Results presented in this paper may represent the basis of a new theory of early human evolution in Africa.

Acknowledgments This project was funded by three grants to M.T. and Prof. Manfred Strecker by the German Research Foundation (DFG), and a UCL Graduate School grant to M.M. Moreover this paper could not have been written without the excellent fieldwork of the Potsdam research group. We also thank the government of Kenya and the Kenya Wildlife Service for research permits and support. The authors thank L. Aellio, A. Bergner, P. Blisniuk, B. Christensen, P. deMenocal, A. Deino, M. Dühnforth, F. Gasse, F. Grine, P. Hopley, J. Kingston, R. Leakey, A. Lister and G. Muchemi for inspiring discussions and the anonymous reviewers for their helpful suggestions. We also thank S. Higgins, S. Kabingu, T. Schlüter and M. Ibs-von Seht for logistical support. M.M. would like to dedicate this paper to memory of Nick Shackleton, friend and mentor.

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