The african tribal leaders sold them into slavery and the first slave owner was another black african.
Slavery was ubiquitous in the old world. Its not just a black thing (however much racists like MtnSnake want to make it so). The difference between chattel slavery as embraced by the South was the idea that slavery was a positive moral good---good for the slave and master. That represented a radical break with antiquity in which slavery was universally regarded as an evil, albeit sometimes a necessary evil.
