Memes

off the top of my head there's the ancient remains found multiple times of disabled and/or badly injured hominids who were treated (signs of healing) and lived long into adulthood despite requiring extensive care from others, the fact an extended childhood in our species means that our young are vulnerable for a far longer period than any other animal (a necessity since you can't fit a fully formed adult brain through a human pelvis) and require cooperation with others to raise and continue the species, the fact we have developed specialised skillsets (that are shared between us rather than developing and being held isolate and then lost when the person who holds then dies).

When you have a group that works together go up against one that doesn't, the former comes out on top. When this competition is for resources and survival, it becomes an evolutionary pressure.

If you do a quick googs you should find scores of whitepapers - but the egoistic argument falls flat on its face out of the gate because we have the word 'sociopath' and it's not considered something to emulate. Neither is 'egotistical'. We've literally got coded into our language that isolation, self-absorption and 'self serving at the cost to others' are bad concepts. Being a self absorbed shithead is documented as wrong as far back as our tales can possibly go.