Today's WSJ (subscription required) has an article: "Animals Seem to Have An Inherent Sense Of Fairness and Justice." It cites experimental examples of monkeys acting according to a code of fairness and reciprocity. For example:

[A tray with food on it] was counterweighted so that both monkeys had to pull a bar to haul in lunch, moving the tray snugly against the cage in such a way that Sammy could reach one cup and Bias the other.

But Sammy was in such a hurry to chow down that after grabbing the apple in her cup, she let go of the tray before Bias could dig into her own. The tray snapped out of reach, causing Bias to scream bloody murder. After half a minute, Sammy understood. She reached out for the tray and helped Bias reel it in.
. . .
[Then the researchers] counterweighted tray so that it required only one monkey to reel it in. In this case, the monkey almost never shares its apple with the monkey who hasn't helped. No work, no pay is fair.

When pulling the tray requires two monkeys' efforts, but only one cup is filled, the lucky monkey often shares its spoils. "Winners were, in effect, compensating their partners for received assistance," Prof. de Waal writes. It was the fair thing to do.
. . . .
It isn't hard to see the survival value of being able to detect inequity. Cooperation requires a grasp of fairness. You need to be able to detect (and punish) freeloaders to keep a cooperative society running. "Fairness counts," she says. "Humans and other animals are able to detect unfairness because doing so is beneficial."

I knew it! The P2P movement represents a step backwards in evolution -- an state less advanced than the capuchin monkey!