Slavery is an extremely old concept, a concept meaning that a human being can be owned by another human being, just like any other item. People have held slaves since ancient times - I think there has been slaves since man settled down for the first time; and that is a long time ago. It probably all started with two cavemen-tribes having a fight. Then some caveman got knocked out by a club, and when he woke up he found himself in the wrong camp! Well, then his captors probably wondered what to do with their prisoner of war (I suppose on such occasions they sometimes ate their prisoners, but lets presume these cavemen were a little civilised). And then some smart caveman suddenly got the great idea that they could keep the other caveman to work for them instead of Ogor, who got his head bashed in during the fight! And so slavery was born.
Maybe later the cavemen started to knock out people in fights on purpose, because they liked having people to work for them (yes, they were lazy in the stone-age too!). And then they started going to other tribes on the purpose of knocking out people to use as slaves. And then maybe they started trading these slaves for other nice things (you know, flint axes and such). And so slave trade was invented.