It might seem hard to believe, but we have about the same number of hairs on our bodies as a chimpanzee, it's just that our hairs are useless, so fine they are almost invisible. We aren't sure quite why we lost our protective fur. It has been suggested that it may have been to help early humans sweat more easily, or to make life harder for parasites such as lice and ticks, or even because our ancestors were partly aquatic.
But perhaps the most attractive idea is that early humans needed to co-operate more when they moved out of the trees into the savanna. When animals are bred for co-operation, as we once did with wolves to produce dogs, they become more like their infants. In a fascinating 40-year experiment starting in the 1950s, Russian foxes were bred for docility. Over the period, adult foxes become more and more like large cubs, spending more time playing, and developing drooping ears, floppy tails and patterned coats. Humans similarly have some characteristics of infantile apes – large heads, small mouths and, significantly here, finer body hair.