Sahara pump theory
Regardless of the aridity of the greater Sahara, migration along the river corridor was halted when, during a desert phase 1.8-0.8 million years ago, the Nile ceased to flow completely and possibly flowed only temporarily in other periods due to Nubian Swell uplift.
During the periods of a wet Sahara, the Sahara and Arabia become a savanna grassland and African flora and fauna become common. During the following dry period, the Sahara reverts to desert conditions usually as a result of the retreat of the West African Monsoon southwards. Evaporation exceeds precipitation, the level of water in lakes like Lake Chad falls, and rivers become dry wadis.
Flora and fauna previously widespread retreat northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley and thence either south-east to the Ethiopian Highlands and Kenya or north-east across the Sinai into Asia. This separates populations of some of the species in areas with different climates, forcing them to adapt, possibly giving rise to speciation (species splitting).
The Saharan pump has been used to date four waves of human emigration from Africa, namely:
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- Stephen, Stokes. "Chronology, Adaptation and Environment of the Middle Palaeolithic in Northern Africa". Human Evolution, Cambridge University.