Bitch modern humans have been on this earth for six hundred thousand years creating an enormous number of distinct and unique cultures which shape the people living under them. The absolute arrogance to think you can generalize a hundred billion people's worth of experience based on what you've experienced in your one tiny lifetime. Humans are so fucking diverse there is VERY little you can say is "human nature." But no please tell me more about how people are just selfish by default despite all the fucking evidence to the contrary.
Consider that their environment, I'm assuming you're from :amerikkka: or at least the west, has drilled into them since childhood that rugged individualism is what's morally correct. The concept of the superstructure and the base, that the superstructure is the combination of education/media/culture that drives into people repeatedly the message that competition is good and on a subtler level might makes right. The culture which valorizes success as material wealth and demonizes poverty is what leads people to be selfish and uncooperative. It is the sense of compassion and cooperation which has been killed by society and the elite have worked very hard to make it this way.
I mean, for sure I think people are only shitty because of our broken capitalist society, but if you need a culture that encourages compassion and cooperation it seems like those are just as much cultivated by the environment as more toxic traits. I think this is mostly just a semantic difference though, it seems like our opinions are pretty dang close otherwise.
I guess what I'm saying is that early humans were definitely inherently cooperative. That on the plains of Africa, we advanced as a species because of our ability to cooperate rather than a drive to compete.