Darwin pointed this out as a possible explanation for the emergence of altruism and morality. He didn't like the idea of group selection, so he suggested that h

Darwin pointed this out as a possible explanation for the emergence of altruism and morality. He didn't like the idea of group selection, so he suggested that humans might have started to reason that if they helped others, others might be inclined to help them in the future. As I mentioned in my other comment this probably emerged as a side-effect of the increased memory/reasoning-buffer required for language processing.