A mother sacrificing her life for one of her children. A sterile worker bee helping to raise its siblings. A wolf attacking another wolf that is invading its territory. Respecting the rights of other people, so long as they respect your own rights. A mother and father cooperating in the raising and care of their children.
Human children are unusually dependent for a long period of time.
The search for shelter during the Ice Ages fractured larger bands into small units. Sexual exclusivity enhances the probability that the father's investment in children perpetuates his own genes. The introduction of hunting as a source of protein led to the specialization of the functions of the two sexes. Continual sexual receptivity created stronger bonds between mates, encouraging long-term relationships. D. de Beauvoir 18.