It is now well established that hunter-gatherer societies relied on relationships of equality and group co-operation to survive. Individuals who acted in selfish ways could be ostracized and would not be likely to survive if they did not find another group to join.
Fear of being seen as selfish and of being rejected by others is innate in humans who are not psychologically damaged. It was partly tool-making that resulted in humans evolving to favour the survival of those among them who were more inclined to be egalitarian and one particular group of tools may have been key: weapons. Without weapons, the largest and fittest tend to dominate. With weapons, even the smallest can be a killer.
