Hair-raising evolution of hominids

“AND they were both naked, the man and his wife, and were not ashamed’ says Genesis.

But when Adam and Eve misbehaved, “the eyes of them both were opened, and they knew that they were naked; and they sewed fig leaves together and made themselves aprons”. So, according to the Bible, we humans were created largely hairless.

Science gives a different account. The earliest hominids, it says, were hairy but lost their coats. Dressing up is a comparatively recent and local practice. So when did our ancestors lose their hair and why? Writing in the February edition of Scientific American, palaeontologist Nina Jablonski argues that we lost our hair over two million years ago because of climate change.

All mammals have at least some hair and most are covered with it from head to toe. A coat keeps its owner warm and few mammals can afford to do without one. Moles, living underground where cold is not a problem, don’t need hair and smooth bare skin helps them burrow through soil. Nudity is also found among very large creatures; it helps elephants rhinos and hippos stay cool in hot climates. Big creatures lose heat so slowly, they risk brain fever in the stifling conditions of the tropics. Cold not heat threatened mammoths, relatives of the elephant which lived in Ice Age Europe, so they were covered in hair. A northern relative of the rhino, now extinct, was also hairy.

Desmond Morris described us ‘naked apes’; we are the only near-nude members of the entire primate tribe. Tufts remain in our armpits and groins, helping to reduce friction when we move our limbs. Our only significant patch of hair is on the head.

We like to think that becoming human was an overnight event, but Jablonski maintains that humanisation was a long drawn-out process. Fingernails evolved around 54 million years ago. The opposable thumb arrived 25 million years later. The first of our ancestors to walk on two legs lived about 3.5 million years ago. That adaptation left the hands free to manipulate tools. Our forebears at this stage, she claims, still had hair all over their bodies. Descended from tree-dwellers, the early up-right walkers had a semi-arboreal life-style. Their brains, at around 400cc, were about the same size as those of modern chimps.

Then, two and a half million years ago, the climate became colder, tree cover declined and large open savannahs took the place of forests. Like Adam and Eve, the early hominids lost their traditional habitat. Forced to venture into open country, they earned a living by hunting and gathering. Meat-eating was adopted at around this time; new, easily digestible, fast-foods became available. But the new lifestyle demanded that vast distances be covered searching for food and prey. The body hair, Jablonski argues, was shed so that excess heat generated by the muscles was dissipated in the breeze flowing over tall upright bodies.

Nudity enabled an ingenious refinement of the cooling system to develop. Pores carry warm moisture out to the surface of the skin. Converting water into steam requires energy and evaporating sweat drew heat from the body. Humans can produce up to 12 litres of perspiration a day. “In a marathon on a hot day, a human could out-compete a horse”, a colleague of Jablonski’s claims.

The hunter-gatherer life-style demanded intelligence. Catching animals and outwitting predators requires ingenuity. The brain size of the savannah hominids doubled to about 800ccs. A fifth of the energy the modern human body generates goes to power the brain, heat which must be dissipated to keep the organ cool. An upright stance keeps the brain away from the hot ground and the head cools in the breeze. Hair on the crown blocks the sun’s rays.

Efficient cooling made the next great leap in human evolution possible. Between two million and one million years ago, brain size expanded even further. Creating more sophisticated tools and the mastering fire were now possible. An ancestor, whose brain at 1,200cc was the same size as ours, arrived on the scene about half a million years ago. Homo sapiens is about 200,000 years old.