Humans lost thick body hair as our ancestors evolved to walk and run long distances under the African sun. Bare skin and millions of sweat glands let us dump heat efficiently. The DNA of body lice suggests we were largely naked by at least a million years ago.
Among the primates, we are freaks: nearly hairless. We still have as many hair follicles as a chimpanzee, but our body hair is fine and wispy instead of a thick coat. Why did our ancestors shed their fur when thousands of other primates kept theirs?
Keeping cool on the savanna
The leading explanation is thermoregulation. As early Homo moved into open, sun-baked country and began walking — and especially running — long distances, overheating became a deadly risk. Bare skin loaded with sweat glands (we have two to four million) sheds heat through evaporation far better than a furry animal that must pant.
The endurance connection
This fits the ‘endurance’ picture of human evolution. A hairless, sweating body may have let our ancestors chase prey through the midday heat until the animals collapsed from heatstroke — so-called persistence hunting. Fur loss, sweating and upright walking form a package for life in a hot, open landscape.
The clue hidden in lice
When did it happen? A wonderfully indirect clue comes from lice. Humans host different lice on the head and body, and body lice can only live in clothing. The genetics of how these lice split imply our ancestors had lost most of their body hair — exposing bare skin — by at least a million years ago, well before clothing appeared.
Why we kept hair where we did
We did not go completely bare. We kept hair on the head (shading the scalp from direct sun) and, after puberty, in the armpits and groin (possibly for scent signalling). The pattern of what we kept is itself a clue to why we lost the rest.
Why it matters
Losing our fur was no trivial cosmetic change; it reshaped our biology. It is tied to how we move, how we cool down, when we started wearing clothes, and even the evolution of skin colour — because bare skin exposed to the sun then had to evolve protective pigment.
From fur to sweat to upright walking — see how the human body came together on the timeline.
Explore the family tree →Frequently asked questions
Why did humans lose their body hair?
The main reason is cooling. As our ancestors walked and ran long distances in the open African heat, bare skin packed with sweat glands let them shed heat by evaporation far better than fur would.
When did humans become hairless?
Genetic studies of lice — body lice need clothing and bare skin — suggest human ancestors had lost most of their body hair by at least a million years ago, long before clothing was invented.
If we lost our hair, why do we still have some?
We kept hair where it is useful: on the head to shade the scalp, and in the armpits and groin, possibly for scent signalling. We also still have as many follicles as a chimp — they just grow fine, wispy hair.
- Kittler, R., Kayser, M. & Stoneking, M. (2003). 'Molecular evolution of Pediculus humanus and the origin of clothing.' Current Biology 13. cell.com
- Jablonski, N. G. (2010). 'The naked truth: why humans have no fur.' Scientific American 302. scientificamerican.com
- Lieberman, D. E. (2015). 'Human locomotion and heat loss: an evolutionary perspective.' Comprehensive Physiology 5. wiley.com