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.
99.99% of the human nature argument is shitty people trying to absolve themselves for shitty behavior by imagining anyone else would have done the same in their circumstances.
Everything about our existence from the cohabitation of gut flora, or the fossilized remains of ancestors with mended bone fractures, to the way our brain processes "Truth" points to a cooperative & collaberative "nature"; but sure go off about how being a selfish dickhead is "natural" behavior to you I guess.
E: This is aside from the fact that it betrays a certain cowardice. Like, even if the Human nature-ist is correct they are not enslaved to this behavior. To the contrary, working to transcend this "natural instinct" would be seen as honorable; rising above the rabble or whatever. But they're too cowardly, stupid, or lazy to even see things on these terms; content instead to shrug and say "Anyone would have sold out their family for some paper."