Sahara pump theory
The Sahara pump theory is a hypothesis that explains how flora and fauna migrated between Eurasia and Africa via a Levantine land bridge. The theory observes that extended periods of abundant rainfall lasting many thousands of years (pluvial periods) in Africa are associated with a "wet Sahara" phase, during which larger lakes and more rivers exist. This causes changes in the flora and fauna found in the area. 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 (mya), the Nile ceased to flow completely and possibly flowed only temporarily in other periods due to the geologic uplift (Nubian Swell) of the Nile River region.
During periods of a wet or "Green Sahara", the Sahara and Arabia become a savanna grassland and African flora and fauna become common.Following inter-pluvial arid periods, the Sahara area then 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 as a result retreat northwards to the Atlas Mountains, southwards into West Africa, or eastwards into the Nile Valley and thence either southeast to the Ethiopian Highlands and Kenya or northeast 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 allopatric speciation.
The Plio-Pleistocene migrations to Africa included the Caprinae in two waves at 3.2 Ma and 2.7-2.5 Ma; Nyctereutes at 2.5 Ma, and Equus at 2.3 Ma. Hippotragus migrated at 2.6 Ma from Africa to the Siwaliks of the Himalayas. Asian bovids moved to Europe and to and from Africa. The primate Theropithecus experienced contraction and its fossils are found only in Europe and Asia, while Homo and Macaca settled wide ranges.
185,000–20,000 years ago
Between about 133 and 122 thousand years ago (kya), the southern parts of the Saharan-Arabian Desert experienced the start of the Abbassia Pluvial, a wet period with increased monsoonal precipitation, around 200–100 mm/year. This allowed Eurasian biota to travel to Africa and vice versa. The growth of speleothems (which requires rainwater) was detected in Hol-Zakh, Ashalim, Even-Sid, Ma'ale-ha-Meyshar, Ktora Cracks, Nagev Tzavoa Cave. In Qafzeh and Es Skuhl caves, where at that time precipitation was 600–1000 mm/year, the remains of Qafzeh-Skhul type anatomically modern humans are dated from this period, but human occupation seems to end in the later arid period.
The Red Sea coastal route was extremely arid before 140 and after 115 kya. Slightly wetter conditions appear at 90–87 kya, but it still was just one tenth the rainfall around 125 kya. Speleothems are detected only in Even-Sid-2.
In the southern Negev Desert speleothems did not grow between 185–140 kya (MIS 6), 110-90 (MIS 5.4-5.2), nor after 85 kya nor during most of the interglacial period (MIS 5.1), the glacial period and Holocene. This suggests that the southern Negev was arid to hyper-arid in these periods.
The coastal route around the western Mediterranean may have been open at times during the last glacial; speleothems grew in Hol-Zakh and in Nagev Tzavoa Caves. Comparison of speleothem formation with calcite horizons suggests that the wet periods were limited to only tens or hundreds of years.
From 60–30 kya there were extremely dry conditions in many parts of Africa.
Last Glacial Maximum
An example of the Saharan pump has occurred after the Last Glacial Maximum (LGM). During the Last Glacial Maximum the Sahara desert was more extensive than it is now with the extent of the tropical forests being greatly reduced. During this period, the lower temperatures reduced the strength of the Hadley Cell whereby rising tropical air of the Inter-Tropical Convergence Zone (ITCZ) brings rain to the tropics, while dry descending air, at about 20 degrees north, flows back to the equator and brings desert conditions to this region. This phase is associated with high rates of wind-blown mineral dust, found in marine cores that come from the north tropical Atlantic.
Around 12,500 BC, the amount of dust in the cores in the Bølling/Allerød phase suddenly plummets and shows a period of much wetter conditions in the Sahara, indicating a Dansgaard-Oeschger (DO) event (a sudden warming followed by a slower cooling of the climate). The moister Saharan conditions had begun about 12,500 BC, with the extension of the ITCZ northward in the northern hemisphere summer, bringing moist wet conditions and a savanna climate to the Sahara, which (apart from a short dry spell associated with the Younger Dryas) peaked during the Holocene thermal maximum climatic phase at 4000 BC when mid-latitude temperatures seem to have been between 2 and 3 degrees warmer than in the recent past. Analysis of Nile River deposited sediments in the delta also shows this period had a higher proportion of sediments coming from the Blue Nile, suggesting higher rainfall also in the Ethiopian Highlands. This was caused principally by a stronger monsoonal circulation throughout the sub-tropical regions, affecting India, Arabia and the Sahara. Lake Victoria only recently became the source of the White Nile and dried out almost completely around 15 kya 
The sudden subsequent movement of the ITCZ southwards with a Heinrich event (a sudden cooling followed by a slower warming), linked to changes with the El Niño-Southern Oscillation cycle, led to a rapid drying out of the Saharan and Arabian regions, which quickly became desert. This is linked to a marked decline in the scale of the Nile floods between 2700 and 2100 BC. 
The Saharan pump has been used to date four waves of human migration from Africa, namely:
- Homo erectus (ssp. ergaster) into Southeast and East Asia
- Homo heidelbergensis into the Middle East and Western Europe
- Homo sapiens sapiens "Out of Africa theory"
- The spread of Afro-Asiatic languages (Berber and Egyptian to North Africa and Semitic to the Arabian Peninsula and Middle East).
The formation of the modern Sahara, as a result of the 5.9 kiloyear event, is also considered to be an effect of the same mechanism.
- van Zinderen-Bakker E. M. (1962-04-14). "A Late-Glacial and Post-Glacial Climatic Correlation between East Africa and Europe". Nature 194 (4824): 201–203. Bibcode:1962Natur.194..201V. doi:10.1038/194201a0.
- "Structural Controls Of The Egyptian Nile".
- Williams, Martin A.J.; Talbot, Michael R. (2009). Late Quaternary Environments in the Nile Basin 89. p. 61. doi:10.1007/978-1-4020-9726-3_4.
- Orbit: Earth's Extraordinary Journey
- Hughesm, Jk; Elton, S; O'Regan, Hj (Jan 2008). "Theropithecus and 'Out of Africa' dispersal in the Plio-Pleistocene". Journal of Human Evolution 54 (1): 43–77. doi:10.1016/j.jhevol.2007.06.004. ISSN 0047-2484. PMID 17868778.
- Vaks, Anton; Bar-Matthews, Miryam; Ayalon, Avner; Matthews, Alan; Halicz, Ludwik; Frumkin, Amos (2007). "Desert speleothems reveal climatic window for African exodus of early modern humans". Geology 35 (9): 831. doi:10.1130/G23794A.1.
- Mellars, P. (Jun 2006). "Why did modern human populations disperse from Africa ca. 60,000 years ago? A new model" (Free full text). Proceedings of the National Academy of Sciences 103 (25): 9381–9386. Bibcode:2006PNAS..103.9381M. doi:10.1073/pnas.0510792103. ISSN 0027-8424. PMC 1480416. PMID 16772383.
- Adams, Jonathan. "Africa during the last 150,000 years". Environmental Sciences Division, ORNL Oak Ridge National Laboratory.
- Stager, J. C.; Johnson, T. C. (2008). "The late Pleistocene desiccation of Lake Victoria and the origin of its endemic biota". Hydrobiologia 596: 5. doi:10.1007/s10750-007-9158-2.
- Burroughs, William J. (2007) "Climate Change in Prehistory: the end of the reign of chaos" (Cambridge University Press)
- Stephen, Stokes. "Chronology, Adaptation and Environment of the Middle Palaeolithic in Northern Africa". Human Evolution, Cambridge University.