From The New York Times:
The researchers set out to test the power of gossip, which has been exalted by theorists in recent decades. Language, according to the anthropologist Robin Dunbar, evolved because gossip is a more efficient version of the “social grooming” essential for animals to live in groups. Apes and other creatures solidify their social bonds by cleaning and stroking one another, but the size of the group is limited because there’s not enough time in the day to groom a large number of animals.
Speech enabled humans to bond with lots of people while going about their hunting and gathering. Instead of spending hours untangling hair, they could bond with friendly conversation (“Your hair looks so unmatted today!”) or by picking apart someone else’s behavior (“Yeah, he was supposed to share the wildebeest, but I heard he kept both haunches”). Gossip also told people whom to trust, and the prospect of a bad reputation discouraged them from acting selfishly, so large groups could peacefully cooperate. At least, that was the theory: gossip promoted the “indirect reciprocity” that made human society possible.