It’s easy to forget that humans are primates, but indeed we are. Taxonomic classification puts humans in the order Primates (apes, monkeys, and lemurs) and in the family Hominidae (great apes), which, in addition to humans, includes chimpanzees, bonobos, gorillas, and orangutans. Humans are different from other primates in several ways, our facility with spoken and written language being one of the most obvious.
But another obvious difference is that somewhere along the evolutionary path, humans lost most of their body hair. When you’re bundling up in wool sweaters and down comforters to get through the winter, you may wonder why you can’t just rely on a snug, natural coat like your hominid cousins — and most other mammals.
When We Shed Our Fur
A good coat of fur can certainly be useful. It provides insulation and also protects the skin from the sun. It offers some protection from wounds and insect bites and a bit of a cushion when you fall. A full coat of hair makes an animal look bigger, which might help attract a mate or present a fierce appearance to an enemy. Hair can even stand on end, making an animal seem even larger, signaling potential enemies that they’d be wiser to back off.
But humans traded the advantages of fur for the benefits of bald bodies. This happened probably around two million years ago when humans became active runners and walkers in hot, open environments in equatorial Africa, says Nina Jablonski, Atherton Professor and Evan Pugh University Professor of Anthropology, Emerita, at The Pennsylvania State University.
So that’s roughly when we shed our fur. But why?
Read More: How Similar Are Humans and Monkeys?
The Benefit of a Bald Body
Over the years, several hypotheses have been proposed. In 2003, Mark Pagel and Walter Bodmer resurrected and refined a theory that had been around in one form or another since Darwin. It’s the ecoparasite hypothesis. It holds that humans evolved a mostly hairless body because naked bodies provide fewer hiding places for ectoparasites, parasites that live on the body rather than inside it. Once we started wearing clothes and living in houses, we could abandon our natural coats for clothes that can be washed and houses that can be cleaned.
Others have suggested that humans went bald when we started wrapping ourselves in the fur of other animals and no longer needed to grow our own fur. However, evidence has shown that we lost our body hair before we started wearing clothes. Another hypothesis suggests that when humans began walking upright, we no longer needed body hair to protect us from solar radiation, though this does not account for the fact that the rest of the body of a naked ape is exposed to the sun even though the top of the head is not.
Today, the most widely accepted idea is the thermoregulatory theory. This holds that our ancestors lost their fur when they moved from the forests to the sunny savanna. When keeping cool became more important than keeping warm, we took off our coats. Though we lost hair, we gained sweat glands, which allowed body heat to dissipate through the evaporation of sweat without too much hair getting in the way, explains Jablonski, who studies the evolution and function of body hair.
Read More: Why Do Humans Go Bald?
The Hair That Remains
Body hair wasn’t completely lost, Jablonski points out. Instead, it became very fine, almost invisible, except in a few places where it’s thicker: our heads, armpits, and pubic areas, and in some people, mostly males, our faces. But why didn’t we lose hair everywhere? Why go mostly naked but keep those seemingly random patches of hair?
The reasons are different depending on which patch of hair we’re talking about. Scalp hair keeps our brains from getting too hot and lessens water loss from the top of the head, Jablonski says. Her research has shown that dark, tightly coiled hair provides better protection than do other types of hair. Softer, flowing locks evolved only after humans spread from the African savanna to more moderate climates.
Hair in the armpits and groin was likely kept because hair in those places is an excellent means of propagating human pheromones, she adds.
Eyelashes, of course, act as a filter, protecting our eyes from contaminants. And eyebrows? Well, they’re useful for communicating, but please don’t raise an eyebrow at these theories.
Read More: Chimpanzees: Understanding Our Closest Relatives in the Animal Kingdom
Article Sources
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Avery Hurt is a freelance science journalist. In addition to writing for Discover, she writes regularly for a variety of outlets, both print and online, including National Geographic, Science News Explores, Medscape, and WebMD. She’s the author of Bullet With Your Name on It: What You Will Probably Die From and What You Can Do About It, Clerisy Press 2007, as well as several books for young readers. Avery got her start in journalism while attending university, writing for the school newspaper and editing the student non-fiction magazine. Though she writes about all areas of science, she is particularly interested in neuroscience, the science of consciousness, and AI–interests she developed while earning a degree in philosophy.
