So, I admit having posted this before, but I couldn’t find the whole item. Now I have.
To assess the extent to which the Nile River Valley has been a corridor for human migrations between Egypt and sub-Saharan Africa, we analyzed mtDNA variation in 224 individuals from various locations along the river. Sequences of the first hypervariable segment (HV1) of the mtDNA control region and a polymorphic HpaI site at position 3592 allowed us to designate each mtDNA as being of “northern” or “southern” affiliation. Proportions of northern and southern mtDNA differed significantly between Egypt, Nubia, and the southern Sudan. At slowly evolving sites within HV1, northern-mtDNA diversity was highest in Egypt and lowest in the southern Sudan, and southern-mtDNA diversity was highest in the southern Sudan and lowest in Egypt, indicating that migrations had occurred bidirectionally along the Nile River Valley. Egypt and Nubia have low and similar amounts of divergence for both mtDNA types, which is consistent with historical evidence for long-term interactions between Egypt and Nubia. Spatial autocorrelation analysis demonstrates a smooth gradient of decreasing genetic similarity of mtDNA types as geographic distance between sampling localities increases, strongly suggesting gene flow along the Nile, with no evident barriers. We conclude that these migrations probably occurred within the past few hundred to few thousand years and that the migration from north to south was either earlier or lesser in the extent of gene flow than the migration from south to north.
This is a good guide to how Eurasian ancestry tails off as you move South down the Nile. As it can be seen, it’s at about 20% in the Southern Sudan, roughly even in Nubia. This is a classic example of a clinal gradient in DNA.
The conclusion of the study was
Nonetheless, we can infer that the migration of northern mtDNA types to the south is older than the migration of southern mtDNA types to the north (or that there has been less gene flow from north to south than from south to north along the Nile River Valley) and that Egypt and Nubia have had more genetic contact than either has had with the southern Sudan. Moreover, we can tentatively infer that these migrations occurred recently enough to fall within the period of the documented historical record of human populations in the Nile River Valley. Thus, it is tempting to try to relate these migrations to specific historical events (Shaw and Nicholson 1995). For example, the migration from north to south may coincide with the Pharaonic colonization of Nubia, which occurred initially during the Middle Kingdom (12th Dynasty, 1991–1785 B.C.) and more permanently during the New Kingdom, from the reign of Thotmosis III (1490–1437 B.C.). The migration from south to north may coincide with the 25th Dynasty (730–655 B.C.), when kings from Napata in Nubia conquered Egypt. Of course, additional migrations documented during the Ptolemeic, Roman, and Arabic times are also likely to have contributed to the current distribution of mtDNA types along the Nile River Valley.