Among pre-1500 sub-Saharan states, traces of slavery are only occasionally clear. Evidence of slavery in Aksum and the Christian kingdoms of Nubia, for example, is scarce. For the ancient West African empires of Ghana and Mali, the written record makes only an occasional reference to slave status. For the Islamic empire of Songhai, on the other hand, there are clearer indications of significant numbers of persons held in slave status by the monarchy and by lords of the realm. To the east of Songhai, the kingdom of Kanem-Bornu may also have had substantial numbers of slaves. In East Africa, slaves were important to the labor of the Islamic Swahili states along the coast as well as to the greater Indian Ocean regional economy. In the states of Ife, Oyo, and Benin in West Africa, Kongo in Central Africa, and Munhumutapa in what is now Zimbabwe, slave populations took form around powerful monarchs.
             It is difficult to assess the extent of slavery outside of these major states prior to 1500 because of the lack of data. But the earliest written reports by visitors from Europe and North Africa and the later anthropological records suggest that various kinds of slavery existed in smaller as in larger polities.
             It is also difficult to assess the nature of servility during this era, and to know whether it was equivalent to chattel slavery. By the beginning of the twentieth century, according to the descriptions of European writers, African societies had developed many different types of servitude. But some of this variation may have developed over time, in response to the imposition of chattel slavery in the plantations and mines of the Americas.
             During the Transatlantic Slave Trade, 1450-1850
             Portuguese and Spanish holdings of African slaves expanded with the maritime voyages of the fifteenth century, then grew moderately until, after 1650, the transatlantic trade exceeded the slave trade across the Sahara and Red Sea. Portuguese and th...