One interesting point is that the first recorded Africans to be bought on the Mainland U.S. were not true slaves as later defined by law and custom. Instead, they were assigned status as indentured servants, and some of them were able to work off their debt and become free. There were actually African slave-holders living in Virginia. Now, this is not to say that the distinction might be more of a semantic one -- servants or slaves, their treatment was still brutal. However, the institutionalization of slavery took hold in stages, rather than all at once.