For some reason - am I missing something? - this question has been coming out a lot lately. Have you checked our FAQ section that touches on this, particularly this comment? Also, on your question about African trade, I'll quote from that comment:

The statement that "there was no cultural transfusion and trade between regions that these countries could profit from" and that "isolation" deserves blame for the OP's perceived deficit is dead wrong. Dead, dead wrong. The syncretic and dynamic nature of African societies is well known among historians and anthropologists. The belief that African societies were ahistorically static (the "tribal model") is not supported by even the limited archaeological evidence we have; not even the oral histories, which often exist just to create timeless ideas of legitimacy and lineage, support the concept of isolation. The fact that the societies were largely nonliterate south of the Sahel and west of the Indian Ocean coast doesn't mean that those cultural exchange dynamics did not exist. The wealth of the Sahelian empires derived from trade; those nearer the Akan forest and in the region of Cameroon were in fact sought out actively by Dyula and other mercantile castes seeking to channel an otherwise uncontrolled flow of trade. Kanem-Bornu could not have survived a thousand years (!!!) without trade from the south and headed north; Aksum, Meroë, and other states whose records we do not know well (in the latter case because we can't actually read them, but they're all over the place) were intermediaries and manufacturers between the interior and trans-oceanic trade. So trade and exchange existed, often as the very reason for a state's existence, as did syncretic development and cross-pollination within and beyond Africa. Read An African Classical Age (Ehret), Schoenbrun's The Great Lakes of Africa, and a few other books to get an idea of what we do know and how badly it threatens to overturn this model of African splendid isolation from one another and from the wider world. They weren't that isolated, even if they weren't building ships and landing on other shores. They weren't doing that because traders from other parts of Africa, or the wider world, were coming to them. Internal trade was often (relatively) short-range and involved many intermediaries, but valuable goods still had wide dispersal. The Sahara was a barrier between about 500BCE to 600CE during the extended dry phase, but not otherwise. It also implies that Africa is culturally homogenous to say that it was isolated by sand and sea; nothing could be further from the truth!