The reliance on hunting limited population density in horticultural societies, and there is some archeological evidence that over-exploitation of the hunting grounds could lead to social collapse and depopulation of a region. On the other hand, farming required a lot of labor and labor supplies were short, so slavery is found among horticultural peoples. It seems likely that pure nomads would not take captives in war, or would kill the captives they took. Horticultural peoples and agricultural peoples might instead let the captive live -- as a slave. Thus, slavery originates as (from one point of view) an act of mercy.
Among horticultural peoples, however, there still is very little surplus over what is necessary for survival, mostly because of the reliance on hunting. As a result, heirarchy and subordination exist mostly within the family group, in which the old may benefit from the labor of the young, and the barrier between slaves and younger family members is slight. Slaves may make that transition and be adopted. This does not mean that the horticultural people are better people, but that slavery is at its early stages and is still marginal to their economy: similarly, when one nation defeats another in war and demands tribute, the defeated nation are adopted as "nephews."
In the old world and in the Andes in South America, we see true agriculture -- the cultivation of plant crops combined with animal husbandry. This means that the society can be wholly settled -- with men staying home and working in the fields, as well as women -- and this allows for the development of what we know as "civilization." Agricultural societies can produce enough surplus food to allow a class to exist that does not work. For the most part, the non-working class fights. Fighting is profitable in two senses. When one village defeats another, in war, the winner can exact a tribute, which means that the non-working class of the victor village can have more luxuries and bigger numbers, so that they can conquer still more villages, and so on. Second, it can provide a supply of slaves, by capturing enemy soldiers and kidnapping farmers from defeated villages. The non-working class consists of slave-owners and the fighting citizens of towns and cities that gain tribute from other towns and cities. In practice, the citizen who benefits from tribute may also be the owner of a large family farm worked by slaves. This aspect (which was important in late Roman society) particularly impressed Marx and Engels, so they called this the Slave System; but it might be more accurate to call it the Extortion System, since the surplus is extracted from the worker by threat of violence.