To know what happened, you have to piece together strands of evidence from widely separated records, and from traditions, language families, and genetic families across a fairly large area of Africa—and I am not in any position to do that myself.
I greeted the appearance of Between the Kola Forest and the Salty Sea with enthusiasm, and bought a copy. After reading the introduction, I'm starting to wonder. He seems to have his references to Ham a little mixed (may not be entirely his fault--references are inconsistent), doesn't seem to have a good handle on the history of slavery in Africa (all European and uniquely evil), conflates "blue-eyed Aryans" with all Europeans, and approvingly cites the "black Egyptian" claims. Sorry, I read Herodotus too, and can look at the pictures the Egyptians painted, and read some of their folk-tales—ancient Egyptians were not of sub-Saharan descent. Recent DNA studies bear out the obvious—modern Egyptians have more sub-Sarahan ancestry than ancient ones. That Nubia conquered the north a time or three I can believe--I'd be surprised if they didn't. That Nubians were the ancient Egyptians isn't possible.
If he can't get the well-documented stuff right, I wonder what admixture of imagination is going into the less-documented material? I shall continue—I expect to learn something—but I am less happy.