Why did human beings lose their fur?

10 October 2024

There are lots of similarities between us and our nearest relatives – but a few differences too. The most striking of these is that, compared to gorillas, chimpanzees and orang-utans, we’re pretty hairless at birth. But why did our ancestors, unlike every other ape and most mammals, get rid of their furry coat?

If you were to line them up next to a female gorilla, even the hairiest of men would look strikingly naked. This is because human body hairs are much thinner, rendering them less visible. They also grow much less luxuriantly than those of other primates and the predecessors of homo sapiens.

Researchers have been trying for a long time to work out when and why human beings lost their fur. Clear answers to this question have proven elusive to date. Bones found during archaeological digs have made it possible to reconstruct the physiques of our ancestors with a high level of accuracy and to show how they lived and moved around on two legs. But no impressions of fur or skin have yet been unearthed. Which is why science has thus far only been able to propose hypotheses to explain why we humans are so lacking in hair.

One of these hypotheses essentially sees a correspondence with the migration of early humans onto open terrain: Some 1.6 million years ago, Homo erectus left the shade of the forest to live as hunters on the hot savannas of the African continent. As he already had a lot more sweat glands than his ape relatives, this enabled him to pursue antelopes, for instance, over long periods, even during hot weather, while his prey, itself devoid of sweat glands, would succumb to heat exhaustion. Temperature regulation care of the sweat glands was not yet fully optimal at this point, however. Fur saturated with sweat did not allow perspiration to evaporate directly from the skin, reducing the cooling effect. It was only when the fur disappeared that the cooling effect of sweat would achieve its full potential.

But this new nakedness brought its own disadvantages: Human skin was now exposed more fully to harmful UV radiation. And more fluid was lost through the skin. The compensatory response of the early humans was to develop dark skin pigmentation as a means of protection against the harsh African sun. According to the science, this explains why we were all originally black. The bodies of the early humans also needed less water: According to a study conducted by English researchers, the modern human needs between 30 and 50 percent less fluid than chimpanzees and gorillas and other primates, even though the latter sleep more and have an easy life and we humans have ten times as many sweat glands.

The second theory of human hairlessness has to do with the unwanted guests that live on our skin: According to this theory, human beings lost their fur in order to protect themselves better from lice, fleas and other parasites. This is because these freeloaders find it much easier to nest in fur than on naked skin. Now, vermin like these might at first glance appear to be a marginal reason to shed a lovely coat of fur. But bloodsucking parasites tend to transmit serious diseases, putting their hosts at serious risk.

And, what’s more, if you have to spend less time delousing, you have more capacity for extended hunts and quests for fruit and berries. But if a life freer of vermin simply requires you to lose your fur, why haven’t other species hit on the same idea? Science also has an answer to this question: Simply put, early humans had an even bigger problem with vermin. When they started hunting, they developed “homes” to which they would keep returning. Places where humans sleep are a paradise for parasites, and camps and caves offer them ideal conditions to reproduce. But without fur, these tiny bloodsuckers couldn’t get a grip so easily. Once early humans had learnt to make fire and clothing, all-over body hair was no longer required to keep them alive during cold nights or the winter season.

Researchers believe that there are various reasons why we aren’t completely naked, like naked mole rats, for instance. Body hair protects our precious and delicate brain from heat, cold and skin injuries. Biologists theorise that our pubic hair is there to protect our genitals, which are likewise very sensitive, as well as to visually highlight them for potential sexual partners. Pubic hair also amplifies the effect of pheromones, sexual fragrances that we humans still produce in small quantities, and which linger more effectively in hair than on naked skin.

Nor has the residual hair on our arms and legs simply been forgotten by evolution. It also fulfils a function, as researchers at Sheffield University in the UK revealed. In their experiment, they shaved one arm of each of the test subjects and released bedbugs onto both shaved and unshaved arms. And what do you know: The bugs took longer to bite on the hairy arms. And the hairs on their arms allowed their human hosts to detect them earlier as they crawled around.

So, it clearly makes sense to keep some residual body hair – at least if you don’t want to lose your body’s own early warning system against bedbugs and other bloodsucking insects.