When was the first history of slaves?

I tend to believe that slavery is as old as mankind itself. In all kinds of animals living in groups, the practice of the dominant member of the group imposing restriction on freedom on other members of the groups. In some, the practice of actual slavery exist, where insect of one species invade and enslave all members of a group of similar, but slightly different, species. Knowing the selfish nature of humans and existence of practice of slavery even in other living beings with much lower level of development, it is not difficult to believe that slavery among humans is as old as the mankind.

Definite indications and proofs of slavery since ancient times are available through several sources. The first well known case of slavery practiced by civilized people is provided by Sumerians in Mesopotamia. In this civilization, the slaves formed the lowest class of society about 3500 B.C.