After the death of Muhammad in 632, Islam spread through Africa, beginning in northern Africa. This spread resulted in the conversion of leaders and the beginnings of unified states as peoples embraced the unifying religion of Islam. This demographic trend enlarged so that eventually peoples forming large groups resulted in the emergence of two empires, one in West Africa and one at the southern edge of the Sahara. Demographics were changed as peoples changed their locations and their economic bases due to these two interrelated trends.
Sorry to be picky, but what do you mean by general Africa? Africa is a massive continent, and so if you are looking for information on demographics, you might find it much more useful to specify a particular area or zone in order to help you gain more information about it. This will greatly help your research, because I doubt there is much information about the demographics of Africa at this time due to lack of written records and ability to keep numbers.