It's the same today. We still cooperate and share with about a few hundred closest people, and for everyone else, we have capitalism. Whenever the two meet, we shock-horror a bit about the callousness of others, and then we go back to treating everyone outside our Dunbar number as an object to extract resources from.

I don't want to suggest that it's biologically inevitable, but that we aren't going to overcome it until we start recognizing the blind spots in our cognition and empathy.

That and, as far as I understand, when they went hunting, only a few get lucky, and those couldn't eat all the meat themselves before it spoiled.