RIFT VALLEY SPRINGS AND WETLANDS, A LIFELINE FOR HUMAN MIGRATION OUT OF AFRICA

There is general agreement that the ancestors of humans evolved in Africa and migrated into Asia and Europe, perhaps as early as 1.8 Ma. A small, but significant number of fauna (e.g.. Hippopotamus and Oryx) also migrated northward. But, there is little consensus on whether migrations occurred in waves or by a slow, but steady “drift” northwards. There is also little agreement if the paleoclimatic or paleoenvironmental conditions were an impetus for hominins to leave. The reasons for migration may be linked to the rhythmic expansion and contraction of the northern hemisphere ice sheets and the associated climate changes in the tropics, as well as the large fluctuations of sea level. Whatever the drivers for migration were, the routes taken required dependable potable water and landscapes with habitats yielding, at the least, edible plants for humans, standing freshwater for hippos and grazing and browsing opportunities for animals, in general.

The East African Rift System (eastern and western branches) would have been ideal corridors for animal migration because of the potential for groundwater-fed water sources within the chain of rift basins that extend over 5000 km from Malawi through the Afar Triangle and to the Levant region. Today the high topography of rift block highlands and volcanoes trap moisture from the Indian Ocean. Water enters the ground and moves under hydraulic head to lower elevations within these endorheic basins. Groundwater seeps and springs (fed by aquifers) occur today at base of slopes or along rift-related fractures and provide an analogue for paleoenvironments, such as groundwater-fed forests, woodlands and wetlands. Groundwater is protected from evaporation and tends to move slowly (a few meters a year) so aquifers have longevity and are excellent buffers against climate change. Shorelines along Red and Mediterranean Seas may have developed “coastal oases” when sea level was low (e.g. MIS 2, 6, 8, 10, and 12) exposing continental shelves and providing the hydrologic conditions to develop new habitats. The model proposed here is that East Africa Rift System may have served as a “lifeline out of Africa” for humans and other animals by providing a continuous chain of freshwater habitats that could support life even during extended droughts.