The Middle East has a long history of slavery. It spread to Africa under the muslims, with Africans enslaving other Africans to sell to muslim slave traders. A lot of these men were turned into eunuchs to guard harems. Having an exotic big strong ebony-skinned African man to guard your harem was the thing to do at the time, apparently. Of course they took white slaves from the North to fill those harems. The raids against Europe were as much about filling their harems as it was about conquering new territory. Arabs were the first to bring wholesale slavery (as we know it today) to Africa. It would be another 400 years before the Portuguese began dealing in this slave trade, which had already been well established on the coasts of Africa.