One of the most visible differences between humans and other primatesour extremely reduced body hairmay result from an evolutionary battle with parasitic bugs. "The fleas, ticks, biting flies, and other ectoparasites that live in an animal's fur cause irritation, carry disease, and so on. If an animal could shed its fur, it would free itself from a lot of these pests," says Mark Pagel, an evolutionary biologist at the University of Reading in England. Once humans mastered fire, shelter, and clothing, thick body hair simply became an evolutionary disadvantage. To Pagel and his colleague Walter Bodmer of John Radcliffe Hospital in Headington, this theory makes more sense than the older explanation that a bald body helped humans shed excess body heat. But why do we still sprout abundant hair in the armpits, the pubic regions, and the top of the head? Charles Darwin speculated that a thick mane might make us more attractive to potential mates. "Pubic hair and hair under our arms are more fun and difficult to explain," Pagel says. "We think hair there creates a hot, humid environment that promotes pheromonal signaling between the sexes."