Read more: “10 biggest puzzles of human evolution“
MAMMALS expend huge amounts of energy just keeping warm. A pelt is nature’s insulation. Why would we forgo that benefit? The most imaginative explanation is that our ancestors went through an aquatic phase millions of years ago and jettisoned their fur, which is a poor insulator in water, just as cetaceans have done. Critics say that if you want to keep warm in water you need to be round and lardy, not long and limby. Worse, the “aquatic ape” theory lacks fossil evidence to back it up.
More popular is the idea that we lost our fur when overheating, not cooling, became the biggest risk. “We don’t pant or have large ears like elephants,” says Chris Stringer of London’s Natural History Museum. “Our only way to cool down is to sweat, and with thick fur that’s inefficient.” This wouldn’t have been a problem in the shady forest, but when our ancestors moved to more open ground, natural selection would have favoured individuals with very fine hair to help cooling air circulate around their sweaty bodies (). But sweating requires a large fluid intake, which means living near rivers or steams, whose banks tend to be wooded and shady – thus reducing the need to sweat. What’s more, the Pleistocene ice age set in around 1.6 million years ago and even in Africa the nights would have been chilly.
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Mark Pagel at the University of Reading, UK, points out that other animals on the savannah have hung on to their fur. He argues that we did not shed our pelts until we were smart enough to deal with the consequences, which was probably after modern humans evolved, about 200,000 years ago. “We can make things to compensate for fur loss such as clothing, shelter and fire.” Then, Pagel contends, natural selection favoured less hairy individuals because fur harbours parasites that spread disease. Later, sexual selection lent a hand, as people with clear, unblemished skin advertising their good health became the most desirable sexual partners and passed on more genes ().
To confuse things still further, circumstantial evidence points to a very early denuding. The pubic louse evolved around 3.3 million years ago, says Mark Stoneking at the Max Plank Institute of Evolutionary Anthropology in Leipzig, Germany, and it could not have done so until ancestral humans lost their body fur, creating its niche (BMC Biology, ). What’s more, he has dated the evolution of body lice, which live in clothing, to around 70,000 years ago (). So it looks like our ancestors wandered around stark naked for a very long time.
