Cooperation is as old as time. There are single celled organisms living symbiotically on your skin right now.

The mitochondria in my cells are also symbiotes but thats just because whatever ancestor ate then found they were hard to digest.

The naturalistic fallacy is still a fallacy.

The bacteria that are most related to mithocondria are intracelular parasits, so they were probably not eaten while roaming arround pacefully, they are probably nasty parasits that got lazy.

But humans aren't living in the "untamed wilds". We figured out that it's possible to cooperate, even in large numbers, in many-thousands-of-years-BC. Since then we're been scaling up the level of cooperation. The last century provides many examples of successful cooperation between even states -- e.g. various nuclear test ban treaties. Why pretend that now of all times it's somehow impossible for us to cooperate?