There's actually a lot of evidence in primates and other animals that they return favors.

The evolutionary struggle for survival is really a self-serving series of blows and stabs, and yet it can lead to extremely social animals like dolphins, wolves or, for that matter, primates.

Darwin wasn't just provocative in saying that we descend from the apes - he didn't go far enough. We are apes in every way, from our long arms and tailless bodies to our habits and temperament.

Why not assume that our humanity, including the self-control needed for livable societies, is built into us? Does anyone truly believe that our ancestors lacked social norms before they had religion? Did they never assist others in need, or complain about an unfair deal? Humans must have worried about the functioning of their communities well before the current religions arose, which is only a few thousand years ago.

Humans have a lot of pro-social tendencies.

There is little evidence that other animals judge the appropriateness of actions that do not directly affect themselves.

Most men probably wouldn't want to live the lives of bonobos. They're constantly clinging to their mothers' apron strings. They lack the ability to make decisions about their own fates, something that we and male chimpanzees practically consider our birthright.