Why humans lost their body hair: to stop their brains from overheating as we evolved

Losing heat through sweating would have been important for early humans walking on two legs and developing larger brains
  • @SteveAConnor

The need to keep a cool head is why man became a naked ape according to scientists who believe they can finally explain why humans are the only primate to lose their body fur.

Bare skin allows body heat to be lost through sweating which would have been important when early humans started to walk on two legs and began to  develop larger brains than their ape-like ancestors, scientists said.

Overheating of the brain would have been especially dangerous which meant there was a strong evolutionary pressure to develop a way of losing excess body heat through the evaporation of sweat on the skin, said Nina Jablonski, professor of anthropology at Pennsylvania State University.

“We can now make a very good case that this was the primary reason for our loss of hair well over 1 million years ago,” Professor Jablonski said.

Human skin is particularly rich in eccrine sweat glands, which produce a thin, watery solution that quickly evaporates, causing heat to be lost from the body in the process. Thick body hair would quickly become matted and so limit heat loss, she said.

“Probably the most tenable hypothesis is that we lost most of our body hair as an adaptation to being better at losing heat from our body, in other words for thermal regulation,” Professor Jablonski said.

“We became very good sweaters as a result. We lost most of our hair and increased the number of eccrine sweat glands on our body and became prodigiously good sweaters,” she told the American Association for the Advancement of Science meeting in Boston.

Studies of the anatomy of early humans, such as the skeleton of “Turkana boy” living in east Africa more than 1 million years ago, suggest that walking and running on two legs and being relatively active for much of the day would have generated excessive body heat that would have to be controlled, she said.

“When we look at the anatomy of Turkana boy, who lived 1.3 or 1.4 million years ago, we see a very modern skeleton that is adapted to high-speed running and walking locomotion,” Professor Jabloski said.

“For people to live with this type of body frame and to forage for food successfully they had to liberate excess body heat very quickly and this in primates is done through sweating,” she said.

“The loss of our body hair was part of our adaptation to becoming very active bipeds in open environments in equatorial Africa...They would have had to have had naked skin in order to survive and keep their brains cool,” she told the meeting.

Naked skin, however, would be more vulnerable to damage from sunlight compared to fur-covered skin. Dark pigmentation to protect against sun damage would have almost certainly evolved at the same time as body hair was lost, Professor Jablonski said.

It was only when the first modern humans emerged from Africa about 70,000 years ago to colonise more northerly regions in Europe and Asia that dark skin was replaced by lighter skin, which would have aided the manufacture of vitamin D by the skin.

“We actually see the depigmentation of the skin in the ancestors of western Europeans and eastern Asian independently as a result of independent genetic mutations. It allowed people to make vitamin D more easily from ultraviolet radiation,” Professor Jablonksi said.

“We lost our hair and with it we lost most of our hairy UV protection, and we gained permanent skin pigmentation,” she said.