Again, I'm talking about relations involving people from different groups, not within a group. Also, I'm not suggesting that slavery was widespread, only that it was not unknown.

As for "evidence" of slavery among foragers, I can't site any specific source, but it is something I have encountered in my reading--which has included a fair amount of anthropology. One specific reference that has stuck in my mind was Lewis and Clarke mentioning in passing an indigenous group that they encountered having a "slave" from a different 'tribe' among them. The next nonfiction book I'll be reading is Native Nations, by Kathleen DuVal, so maybe I'll get more information from her.

It is certainly the case that, unlike civilization (up to the 'Industrial Revolution'), no economy of any forager group has ever depended on the existence of (bondage) slavery. (Since that transformation civilization has depended on wage slavery: people being paid to be used as machines--or draft animals--for the material aggrandizement of others.) Yet, the very existence of civilization ('the existence of cities', not a normative thing) has depended on (or at least very soon in its history came to depend on) the institution of (bondage) slavery.

Again, the most important thing is doing away with, as much as ever we can, all of the heinous effects of 'other'ness henceforth.