I've been looking at the spread of crops OTL, and I think Jared Diamond underestimates the connectivity between Africa & Europe/Asia. Middle Eastern crops arrived in parts of East Africa (Egypt, Nubia, Ethiopia, the Sahel, the southern Sudan) fairly rapidly. Even the early east Asian crops, like millet, were in Africa well before 1 AD, and had made it all the way to the Niger basin by the fall of the Roman Empire. Plus, the Niger basin had its own native rice domesticate. Really, looking historically, East Africa has been no less isolated from the major east-west axis of Old World culture than India is - and India is one of the great centers of world civilization! So I think that without the Tsetse fly, the more fertile parts of subsaharan Africa, especially in East & Northeast Africa, have the definite capacity for sustaining quite substantial cultures & nations. Of course, turning this into a "Black man's burden" scenario is still pretty ASB, barring some major climatological or geological POD. Europe is just too fertile and well-connected by trade to be turned into as much of a backwater as subsaharan Africa was OTL.