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Why do we grow more hair on our heads than on our bodies?

a man and woman looking at each other's hair
(Image credit: Westend61 via Getty Images)

Humans are the oddballs of the mammalian class. Hippos and naked mole rats aside, nearly every other mammal has fur covering its body. Humans are practically naked, besides the hair on our heads. So why are people mostly hairless apart from our head hair?

First, it's crucial to understand why mammals have fur in the first place, said Mark Pagel, an evolutionary biologist at the University of Reading in the United Kingdom. Fur keeps animals warm when it's cold at night and protects them from the sun during the day. Human ancestors were able to lose most of their body hair because they had the unique ability to compensate with fire, shelter and clothing.

That explains why our human ancestors could survive without most of their hair, but not why it disappeared over time. Hairlessness must have given humans some sort of evolutionary advantage. There are three main theories about what the advantage could have been, Pagel told Live Science.

Related: Fur, wool, hair: What's the difference?

First, a thick coat of fur could have led ancient humans to overheat in the hot noonday sun. "If you're wearing a great big fur coat in the middle of the African savanna in the hot season, you're going to be way too hot," Pagel said. "Wouldn't it be nice if you could take your great big fur coat off? Which is what we did." Moreover, humans evolved to have more sweat glands than our primate relatives. If we had kept our long body hair, it likely would have gotten soaked with sweat, which would have made it hard for the sweat to evaporate and cool us down, Yana Kamberov, an assistant professor of genetics at the University of Pennsylvania, previously told Live Science.

However, the so-called body-cooling hypothesis fails to explain some aspects of human body hair patterns, such as why men tend to be hairier than women. Of note, humans are covered with tiny and colorless vellus hairs, except on the palms, soles of the feet, lips and nipples, Kamberov previously told Live Science. Hormones that emerge during puberty can transform some of these vellus hairs into longer, colored terminal hairs. But aside from this sometimes scruffy body hair, long hair tends to grow only on our heads.

The second theory, known as the aquatic ape hypothesis, proposes that ancient humans spent a lot of time in water. Fur weighed them down while swimming, so they gradually lost their hair. However, there is no evidence that humans spent a significant amount of time in water during the evolutionary past, so Pagel finds this hypothesis hard to believe. It also fails to explain why humans didn't evolve to gain their fur back after leaving the waterside.

Pagel proposed the third theory, the ectoparasite hypothesis, in a 2003 study published in the journal Proceedings of the Royal Society B: Biological Sciences. Ectoparasites are parasites that live on the outside of the host's body. These parasites — which include lice, ticks and fleas — are a major cause of disease and mortality across species. Ectoparasites may be less attracted to hairless skin, and it may be easier to get rid of them when they're not buried in fur. In turn, having less hair, and thus fewer parasites, may have presented a survival advantage.

But if hair can lead to harm, why did we keep it on our heads?

As bipeds, or animals that walk upright on two legs, our heads are directly exposed to the sun. Near the equator, where humans evolved, sun exposure can be overbearing, and head hair helps people avoid overheating. "It's sort of a built-in hat," Pagel said.

Head hair also helps retain heat at night. "Our brains are relatively small compared to the rest of our bodies, but they're enormously metabolically active," Pagel said. This activity produces heat, and head hair could insulate this area of concentrated warmth.

Sexual selection also may play a role. Humans don't just have head hair; we style it. Ancient people may have, too. Hair doesn't fossilize well, so researchers don't have much direct evidence of this, except for preserved mummies in places such as Egypt and Peru. However, researchers have studied modern-day Indigenous people who haven't had contact with the outside world and found that they also style their hair, suggesting that their ancestors did, too. This hair care may help attract a mate, Pagel said. 

"We don't just have head hair," Pagel said, "but we have it in a form that we can make really attractive to members of the opposite sex."

Originally published on Live Science.