Apes and monkeys, humanity's closest kin, differ from other animals in the intensity of their social relationships. All their grooming is not so much about hygiene as it is about cementing bonds, making friends, and influencing fellow primates. But for early humans, grooming as a way to social success posed a problem: given their large social groups of 150 or so, our earliest ancestors would have had to spend almost half their time grooming one another -- an impossible burden. What Dunbar sugges...