Historical sources on the origins of slavery of slavery are sparse, speculative and difficult to evaluate. Slavery seldom, if ever, occurs in hunting/gathering societies but appears in widely separated regions and periods with the advent of pastoralism, and later agriculture, urbanization, and state formation. Most authorities have concluded that slavery derives from war and conquest. The sources of slavery commonly cited are: capture in warfare; punishment for a crime; sale by family members; self-sale for debt and debt bondage. Slavery is the first institutionalized form of hierarchical dominance in human history; it is connected to the establishment of a market economy, hierarchies, and the state. However oppressive and brutal it undoubtedly was for those victimized by it, it represented an essential advance in the process of economic organization, an advance upon which the development of ancient civilization rested. Thus, we can justifiably speak of “the invention of slavery” as a crucial watershed for humanity.
Source: The Creation of Patriarchy (1986)
Read more here.