Most civilizations have practiced some form of slavery in their development. Famine or fear of stronger enemies might force one tribe to ask another to help and give themselves in a type of bondage in exchange that was similar to the European serf system.
Arabs also had slave trading, they exchanged slaves for goods from other parts of the world.
Until that moment, slavery was not linked to color, it was mostly linked to war and economic opportunities.
Slavery became a matter of color when Portugal started to explore the West Coast of Africa in 1444. Because African slaves were identified by their skin color since then, slavery became a matter of color and not economic opportunities or war. Europeans settled in Brazil, Caribbean, and North America and developed a system of racially based slavery.
