Of course black slavery started in Africa, you idiot. Just as white slavery started in Europe, Aztec slavery started in Mexico, and American Indian slavery started in North America. Slavery was a common human condition before the 19th century. Sometimes it was called by other names, like feudalism, but it was an economic fact before industrialization.
Black slaves were enslaved by other blacks, and sold primarily to Arabs. Some few made it to America on Dutch and Portuguese ships. The descendants of these few were lucky, because they were born American citizens and live better lives than the ones left behind to live in cardboard boxes like Obama's half-brother.