How Did Humans Lose Their Body Hair?

An adult human has roughly five million hair follicles, about the density you would expect for an ape of our body size. The difference is what grows from them. Over most of our bodies the follicles produce short, thin vellus hairs instead of the coarse terminal hairs that make a chimpanzee's coat.

So the question of how humans lose their body hair is really a question about why evolution turned that coat down to almost nothing. Three lines of evidence help: our sweat glands, our skin-colour genes, and the family trees of the lice that live on us.

Sweat: the human cooling system

Humans have between two and four million eccrine sweat glands, the kind that release watery sweat directly onto the skin. Most other mammals rely more on panting or on apocrine glands tied to hair follicles. Comparative counts by Yana Kamberov and colleagues found that humans have roughly ten times the eccrine gland density of chimpanzees and macaques.

Sweat cools by evaporation, and evaporation works best on bare skin. A thick coat traps a layer of humid air and slows that process. Fur still has advantages: it shields skin from direct sun and insulates at night. For an animal generating a lot of heat through its own muscles, though, the trade-off shifts towards bare, wet skin.

Walking, running and the heat budget

That is where human locomotion comes in. In 2004, Dennis Bramble and Daniel Lieberman argued in Nature that early Homo was built for endurance running. Their evidence included long legs, springy Achilles tendons, a large gluteus maximus, and a ligament that steadies the head while running. Running hard in the heat produces far more body heat than walking, and an efficient cooling system becomes essential.

The skeleton fits the timing. The 1.5-million-year-old Turkana Boy (KNM-WT 15000) from Kenya has the long legs and narrow body of a hot-climate walker (see Homo erectus). Earlier australopiths, such as Lucy, had shorter legs and a more ape-like upper body.

The endurance-running idea has critics. Some argue that long-distance walking, not running, was the main selective pressure, and that "persistence hunting" (chasing an animal until it overheats) was rare. Both camps agree on one point: early Homo was active for long periods in hot, open places, and shedding heat mattered. You can read more about the origins of upright walking in why humans started walking upright.

When did humans lose their body hair?

Hair rarely fossilises, so the date has to be inferred. The best estimate comes from skin colour, not hair.

Chimpanzees have pale skin under dark fur. Once hair thinned, early humans in Africa would have needed dark, protective skin. In 2004, Alan Rogers, David Iltis and Stephen Wooding studied the MC1R pigment gene, which shows almost no variation in African populations. That pattern points to strong selection keeping skin dark. From the gene's history, they estimated that this selection, and so largely bare skin, dates back at least about 1.2 million years.

The estimate depends on assumptions about mutation rates and population size, so treat it as a rough marker rather than a precise date. It does fit the anatomy of Homo erectus.

The clue hidden in lice

Lice are picky parasites, and their family trees record changes in their hosts' bodies.

• Pubic lice. Our pubic louse, Pthirus pubis, is most closely related to the gorilla louse. In 2007, David Reed and colleagues estimated that human ancestors picked it up from gorillas, or from a gorilla-like ancestor, about 3.3 million years ago. A separate pubic-louse niche only makes sense once body hair had thinned enough to split head hair from groin hair. That makes 3.3 million years a possible early marker of hair thinning, though the authors were cautious about the interpretation.
• Clothing lice. Body lice live in clothing and lay eggs in fabric. A 2011 study by Melissa Toups and colleagues dated their split from head lice to between about 83,000 and 170,000 years ago. That tells us when regular clothing began, not when hair was lost. It does show that humans went bare for hundreds of thousands of years before they dressed.

Other explanations

Cooling is the leading hypothesis, but it is not the only one.

• Parasites. Mark Pagel and Walter Bodmer proposed in 2003 that losing fur reduced ticks, fleas and lice and the diseases they carry. This could have worked alongside cooling, and becomes more plausible once people lived in shelters with fire and clothing.
• Sexual selection. Darwin thought bare skin was mainly a preference that spread through mate choice. It may have reinforced the trend, but it struggles to explain why the change began.
• The "aquatic ape" idea. This claims we lost hair during a semi-aquatic phase. Most anthropologists reject it, because the fossil and environmental record shows no such phase, and many aquatic mammals keep dense fur.

Why we kept hair where we did

The hair we kept is a clue to why we lost the rest. Scalp hair shades the hottest, most sun-exposed part of an upright body. A 2023 study by Tina Lasisi and colleagues used a thermal manikin with human wigs and found that tightly curled hair cut solar heat gain to the scalp the most, while costing little extra sweat. That fits the idea that head hair is itself a heat adaptation from equatorial Africa.

Underarm and pubic hair appear at puberty, near apocrine scent glands. They may help spread scent or reduce friction, but neither idea is firmly tested. Eyebrows help keep sweat and rain out of the eyes, and probably play a big role in facial expression.

Genes behind the change

Researchers are starting to find the DNA behind these traits. Work led by Kamberov identified a human-specific stretch of regulatory DNA near the EN1 gene that increases eccrine gland numbers when tested in mice. The genes that shrank body hair are less well understood. At least one hair-keratin gene that works in chimpanzees has become a non-functional pseudogene in humans. As with most human traits, the change looks like many small genetic steps rather than one switch.

Why it matters

Losing our fur was more than a cosmetic change. It is tied to how we move, how we cool down, our skin colour, and eventually our dependence on clothing and fire. It also shaped later events: as humans moved into weaker sunlight, skin colour changed again, a story told in the evolution of human skin colour and, for our closest cousins, in whether Neanderthals had red hair. Follow these changes species by species on the interactive family tree.