In the spirit of discussion, here's something else related that you might find interesting.
Besides the fact that up until a few thousand years ago our ancestors lived in small nomadic forager groups with a lot of kin in neighboring bands, the socially and economically cooperative nature of those cultures does not tolerate infighting or disharmony except in very limited responses to recognized breaches of the social contract of the group. It is really only with sedentism related to the agricultural revolution that these egalitarian and largely peaceful cultures began to change in favor of greater hierarchy, oppression, and scarcity which drove aggression and widespread killing.
“Destabilization of the social resource network decreases group stability and efficiency and lowers the average fitness benefit derived from cooperation. When group stability is important for individual advantage, selection will favor active peacemaking and cooperation in our closest relatives and ourselves.” (p. 401)
