The environmental context for hominin evolution—the environmental characteristics that prevailed in the areas where hominins evolved—fundamentally reflects the interplay between Earth’s orbital parameters, tectonism, and the biogeochemical processes that controlled greenhouse gas concentrations. The tectonic elements set the stage by delineating land and ocean, including oceanic gateways and land bridges, as well as controlling regional and local topography. Earth’s orbital variations determined the amount of solar radiation any location on the Earth’s surface received at a given time and season. Greenhouse gas concentrations controlled the very large scale characteristics of planetary temperature and regional moisture balance, and the state of the cryosphere. Together, these produced the interaction of atmospheric air masses with topographic and oceanic effects that controlled the specific regional climates that impinged on evolving hominins.

The history of East African tectonics and orography is dominated by the development of the East African Rift System (Tiercelin and Lezzar, 2002) (see Figure 2.2). Prior to inception of the East African Rift System, most of northeastern Africa was a low-lying landscape of deeply weathered terrain. Major eruptions of flood basalt and rhyolites created the Ethiopian Plateau around 30 Ma (Wolfenden et al., 2005), marking the onset of rifting along the East African Rift System. By 10 Ma, active rifting had propagated southward for 4,000 km, from the Gulf of Suez to the Mozambique Channel. Rifting in the Turkana basin, the Red Sea, and the Gulf of Aden were all coincident with the onset of Afar volcanism. The rift system has two distinct branches, an older and more volcanically active eastern branch, active since the Oligocene and occupied today by many small alkaline/saline lakes, and a younger (late Miocene) western branch, which has experienced much less extensive volcanic activity and is occupied by large and mostly freshwater lakes. Unlike the eastern branch of the Rift Valley, where southward propagation of rifting is well documented, the western branch appears to have developed along its entire length at about the same time between 8 and 12 Ma. Both rift branches are surrounded by major uplifted mountain ranges, which have acted since the late Miocene to intercept moisture and create rainshadows along their leeward (western) flanks (Sepulchre et al., 2006). This rift valley system and the flanking mountains regulated the extent of aridity in various parts of tropical Africa, as well as the occurrence of water resources upon which early hominins would have relied.

The development of the Nile River system was an additional significant element in the history of hominin evolution on the continent. There is no evidence for a Nile drainage system prior to the early Miocene, when major fluvial deposits began to accumulate south of the present Nile delta (Said, 1993). Today the Nile is fed by both the Blue Nile that drains the Ethiopian Highlands and the White

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