Scent radiator, warmth provider, or chafe protection? The answer to why humans have clumps of hair in private places is still open for debate We may be the naked ape, but on one measure of hairiness humans trump all other primates. While most of them have finer hair around their genitals than on the rest of their body, adult humans sport an impressively thick bush of pubic hair. It has long been assumed that pubic hair is a remnant of a furrier period in our evolutionary history, and that the real question is why the rest of the body lost its hirsuteness. Earlier this year, though, Robin Weiss of University College London pointed out that our pubic hair clearly became thicker than that on the rest of our bodies at some point in our evolution . And this must have happened for a reason. So what drove the evolution of pubic hair? There's no accepted explanation, but many potential advantages have been suggested over the years. Perhaps the most popular is that since thicker hair gathers in regions where we have apocrine (scent) sweat glands as well as eccrine (cooling) ones, it may serve to waft odours that signal sexual maturity. It may also act as a visual signal of adulthood, along with growing breasts and widening hips in girls and deeper chests and beards in boys. Various other benefits could have made it worth keeping. A thick bush not only protects the genitals during sex and at other times - reducing chaffing while walking, for example - it also helps keep our most sensitive regions warm and free of draughts. So when did it evolve? Based on studies of the evolution of pubic lice by David Reed at the Florida Museum of Natural History in Gainesville, Weiss suggests some time around 3.3 million years ago. This is when human lice diverged from a closely related species found on the thick body hair of gorillas. This, Weiss suggests, could signal that while the rest of our bodies were naked by that time, we had evolved thick enough pubic hair for the lice to jump species and take hold.