Not to mention that access to land was fiercely defended by highly territorial tribes that fought wars of conquest and destruction against each other. This is not dissimilar to the behaviour of apes and large carnivores today.

Edit: To elaborate, "civilization" the way I see it has simply put the guardrails of due process around these things. We're still nakedly competing with each other, and in-groups are often bent on the destruction of out-groups. We just have laws and courts to make sure things don't get too far out of hand. It's peaceful coexistence not mass cooperation.