"Climatic cycles acted as a pump, alternately attracting African peoples onto the Sahara, then expelling them as the aridity returned (Keita 1990). Specialists in predynastic archaeology have recently proposed that the last climate-driven expulsion impelled the Saharans...into the Nile Valley ca. 5000-4500 BCE, where they intermingled with indigenous hunter-fisher-gatherer people already there (Hassan 1989; Wetterstorm 1993). Such was the origin of the distinct Egyptian populace, with its mix of agriculture/pastoralism and hunting/fishing. The resulting Badarian people, who developed the earliest Predynastic Egyptian culture, already exhibited the mix of North African and Sub-Saharan physical traits that have typified Egyptians ever since (Hassan 1985, Yurco 1989; Trigger 1978; Keita 1990; Brace et al. 1993)... Language research suggests that this Saharan-Nilotic population became speakers of the Afro-Asiatic languages.... Semitic was evidently spoken by Saharans who crossed the Red Sea into Arabia and became ancestors of the Semitic speakers there, possibly around 7000 BC... In summary we may say that Egypt was a distinct North African culture rooted in the Nile Valley and on the Sahara."[94]
The 5.9 kiloyear event was one of the most intense aridification events during the Holocene Epoch. It occurred around 3900 BC (5,900 years BP), ending the Neolithic Subpluvial and probably initiated the most recent desiccation of the Sahara desert.
Thus, it also triggered worldwide migration to river valleys, such as from central North Africa to the Nile valley, which eventually led to the emergence of the first complex, highly organised, state-level societies in the 4th millennium BC.[1] It is associated with the last round of the Sahara pump theory.
Someone mentioned the pump theory. So basically, you got humans already living all across Africa and that includes the Sahara which was rich with vegetation and life. That shyt dried up and a portion of that population decided to either go West to the Niger river or East to the Nile river and integrate with the population already there. Berber populations stayed of course.
I'll end this with a scene from 2001 a space odyssey of our evolutionary ancestors in the rift valleys of East Africa