Finally, humans had to evolve what no other primate had: a sense of identity with a group so powerful that it could swallow up the individual identity, at least to a large degree. Humans couldn't have this deep, self-sacrificing loyalty to more than one or two communities at a time. Thus communities were inevitably in conflict with each other, competing for the loyalty of their members. The tribe had to break down the solidarity of the family: religion had to compete with the nation for loyalty. But once a community had that loyalty, the most ardent members would gladly die for it. Not for the other individuals directly, but for the interests of the group as a whole, because in the human mind, that group was the self, and the individual was able to regard himself as merely one iteration of the pattern of the whole. Humans, in order to rise above animals, had learned to convert themselves into nothing more than organs or limbs or even the disposable fingernails and hair of a larger metaphorical organism."
Orson Scott Card, my favorite science fiction writer, from Homecoming Earth.