My understanding is that 20,000 years ago is that we lived in highly cooperative and egalitarian small groups where the good of the tribe was paramount. As you’ve said, humans are a highly social species. This idea of looking out only for number one is a relatively recent phenomenon. Evolutionary psychologists deeply steeped in their own hierarchical society may not have been able to conceive of anything different, but recent (and old) research says that simply isn’t the case. In a non-hierarchical society, which is what we’ve had for 97% of human history, “me first” thinking would have been unlikely.