In chapter 3 of The Happiness Hypothesis by Jonathan Haidt (who has an interesting web site), he talks about the solely human virtue of reciprocity and why it's an evolutionary advantage. He begins by talking about how the law of ultrasociality - living in large cooperative societies in which hundreds or thousands of individuals reap the benefits of an extensive division of labor - has evolved at least 4 times in the animal kingdom: among "hymenoptera" (ants, bees and wasps), among termites, naked mole rats, and among humans. What may seem counterintuitive to the concept of "survival of the fittest" actually makes a lot of sense if you consider the risks. The only way for a species to continue to survive, reproduce and evolve, if you will, is to leave surviving copies of their genes. Which means making sure that everyone carrying a copy of your genes not only survives but prospers. That means siblings (which share 50% of your genes), as well as your children. Nephews and nieces carry a quarter of your genes and your cousins one-eighth. So, obviously, on those terms, reciprocity would play a vital role in the continuation of the species.
Haidt goes on to explain, though, how this concept only really works in manageable groups of a few dozen, or perhaps a hundred. Beyond that, you've moved too far out of the gene pool to risk your own genes to help. Human brains, apparently, have evolved to live in groups of roughly 150 other people. Beyond that, the number is too large to be manageable or beneficial. And this is where size comes in. Brain size that is. Humans have the largest social circle of all social animals. Chimpanzees, our closest ancestors, live in groups of around 30 and spend "emormous amounts of time grooming each other." According to what scientists have deduced from the human brain size (which Haidt explains, but it's complicated and would take up too much time, so just take my word for it that he offers the science), it's capable of roughly the 150 number. Which, when further research into city dwellers' social lives investigated, is the approximate number of friends and acquaintances that the average urbanite claims (or can know directly, by name and face, and how everyone is related to everyone else). Obviously, humans can't spend all their time grooming 150 other people in their social circles, and this, hypothesizes Haidt, is where language came in; as a replacement for physical grooming. Which I would also love to get into, and why gossip, Haidt argues, is also an evolutionary act, which he spends the next part of the chapter getting into.
I don't know, maybe this stuff is totally apparent to everyone else, but when I start reading this evolution stuff, I just find it utterly fascinating. It's not something I've given a lot of thought to in the past, and maybe there's some deep, dark recess of my psyche that doesn't believe it, so when I read about bold-faced evidence of not only how humans evolved, but why, it kind of blows my mind.