I'm sorry Brospierre, but who claims 'Libyans' (from North Africa?) enslaved 'Bushmen' (from southern Africa? 3,000miles or 4,500km away?) in 8000BC?
Our earliest records (from settled literate civilisations eg Egypt, Mesopotamia, the Phoenician cities) go back about 5,000 years (ie to about 3000BC); and these are pretty sketchy.
Before that (and indeed oftern after that) it is really really hard to have any real idea about what is going on, except in a general technical sense - some people doing this (eg farmning wheat and herding cows) with this sort of technology (eg stone axes and pottery) are living here while nearby some people doing this (eg gathering nuts and hunting deer) with this sort of technology (eg flint flake blades).
What those people might have looked like is pretty much beyond us. Different 'racial' groups, if they exist, remain ellusive. Culture is easier to determine - 'these people used shell ornaments and buried their dead under heaps of stones, those people used bone ornaments and burned their dead on pyres' - but culture isn't in the last analysis an indicator of biology anyway (or I wouldn't have eaten Chinese food last night).
Sorry. I call bullshit. The earliest firm evidence for slavery will be found in the earliest literate civilisations. In pre-literate societies, you might get evidence suggestive of slavery but that's all.
If you can demonstrate that there is a group that lives in settlements with another, different group (in terms of physical appearance as well as culture), and one of the groups seems to be consistently disadvantaged relative to the other (evidence of worse diet, lower life-expectancy, more traumatic injuries to skeletons, and their material culture seems to be 'poorer' which is a subjective judgement anyway), then you have evidence that might suggest slavery.
But there might be other explanations. The two populations might live together quite happily, but one culture might, for historical reasons (eg religion), have a poor diet and knowledge of dangerous technology, whilre their material goods looked 'poorer' for other cultural reasons. Maybe one group eats lots of processed grains and does lots of logging, while they bury their dead without grave goods, because their religion tells them that it's sinful to eat fish and adorn the dead. They're not very robust but have lots of trauma inguries and have 'poor' gaves. The other group eats fruit and fish and buries their dead in graves with lots of ornaments and other grave goods so they look healthy and 'rich', because their religious code tells them that fruit and fish are cool and you should honour your departed ancestors; neither group enslaves the other, but you might think that one group did.
To be honest, in Europe it might look (on some indicators at least) like the Mesolithic hunter-gatherers (who tend to be pretty robust) had 'enslaved' the neolithic farmers (who tend to be smaller and less muscular). But we ('archaeologists' that is) don't think this is the case, and for the same reasons I'd be dubious about claims about other cultures.



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