Thanks for the book tip.
Jared Diamond's book "Guns, Germs and Steel" also touches on the African empires (according to him, the Bantu people conquered and ruled most of Africa), although that is not his main topic.
If our industrial civilization kills itself off in a big nuclear conflagration, what could happen next would be that one group of Africans with greater resistance to radiation damage (there is greater breadth of genetic variation among Africans than in the rest of the human race) starts a new wave of emigration "Out of Africa"... and the whole cycle starts again.