Slavery may have been common among prehistoric peoples even before agriculture / horticulture.
The need for disposable (sold or killed) workforce existed even among hunter-gatherers.
Read "Slaves, chiefs and labour on the northern Northwest Coast", Kenneth M. Ames, in World Archaeology Vol. 33(1): 1-17 The Archaeology of Slavery
http://web.pdx.edu/~amesk/pdfs/slaves_World_Arch.pdf