Our ancestors lived in groups of no more than a few hundred people, and those on the other side of a river or mountain range might as well have been living in a separate world. We developed ethical principles to help us to deal with problems within our community, not to help those outside it. The harms that it was considered wrong to cause were generally clear and well defined. We developed inhibitions against, and emotional responses to, such actions, and these instinctive or emotional reactions still form the basis for much of our moral thinking.
Peter Singer, Practical Ethics
ethics, evolution, history