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

The primate laugh is given in playful contexts, and as such has a strong similarity to the human laugh.

When we are bad, we are worse than any primate that I know. And when we are good, we are actually better and more altruistic than any primate that I know.

Studies of reconciliation in primates have demonstrated that if the relationship value increases between two parties they are more willing to make peace.

When it comes to social interaction, the chimpanzees appear to be just as intelligent as we are.

Empathy probably started out as a mechanism to improve maternal care. Mammalian mothers who were attentive to their young's needs were more likely to rear successful offspring.

Armies are a purely human invention. Most soldiers who go to war nowadays don't even do it because they're inherently aggressive.