A Russian Orthodox monk, displaying head and facial hair growth. Image by Rklawton

When you think about the differences between humans and other mammals, what comes to mind? You may consider how we walk upright on two legs, or our intellect and culture. However, a feature less often remarked on but nonetheless unusual among mammals is our fur, or lack thereof. Chimpanzees, bonobos, and gorillas all sport a coat of dark fur, but on humans, hair is primarily confined to our heads and groins. So why did humans lose their fur?

Fur is a very useful feature for most mammals. It’s an insulator, and can protect animals from both cold and heat. It is also a barrier, and can keep skin from being scratched or scorched by the sun. For humans to have lost their fur, there must have been very specific pressures that made hairlessness more advantageous. Darwin thought that perhaps ancestral humans found sparser fur more attractive, and thus chose mates that showed more skin, but biologists now think that there may have been one or several more complex factors in play.

Hippos also have very little fur covering their bodies.

Hairless Hypotheses

One idea that used to be popular for explaining human hairlessness is the “aquatic ape” hypothesis. Dolphins, whales, and many other aquatic mammals are predominantly hairless, and it was suggested that ancestral humans may have had an aquatic phase as well, which led to their lack of furry coat. While this idea generated years of interesting scientific dialogue, no evidence was ever found to support it, leading to its general dismissal.

Several more recent hypotheses have emerged. Some scientists suggest that humans lost their fur as a way of reducing their number of lice, fleas, and other parasites. But if that was the only reason, why are other primates and social mammals furry?

Emotional communication may have also played a role in our ancestor’s slow depilation. Humans have three types of cones in their eyes, which allows them to see a finer range of colors in the middle of the spectrum than animals that only have two. Colors on other people’s faces can indicate information about their thoughts or health, such as a blush for embarrassment or arousal, or looking pale or greenish when ill. If human faces were covered in fur, we would be unable to see these color changes. Many other social mammals communicate using ear and tail position. As humans have no tails and our ears are immobile, being able to see each other’s faces clearly has an obvious benefit.

Interestingly, the correlation between color vision and fur loss is seen in other primates as well. Primates that have bald faces (or rumps!) like humans tend to have three types of cones and better color vision, while primates with hairy faces and rear ends usually only have two cones.

2016 U.S. Olympic Team Marathon Trials. Image by Rev Dills.

A final explanation for human furlessness has to do with another trait, a human superpower hiding in plain sight: Our ability to run for long distances.

Humans are pretty unimpressive when it comes to sprinting. Average humans can sprint at around 15 mph, though Usain Bolt can reach the remarkable rate of 27.78 mph. Dogs, on the other hand, can sprint at around 20 mph, cats can sprint 20 – 30 mph, and cheetahs can reach speeds of over 60 mph when chasing prey. We won’t catch up to most quadruped mammals over a short distance, but when the sprint turns into a marathon, humans begin to excel.

Humans can jog or run much longer than most mammals, due in large part to our superior ability to expel heat. We have many more sweat glands than the average mammal, ten times as many as chimpanzees. Sweat is very effective at cooling the body down, but functions best on bare skin. Only a few other animals, like horses, rely on sweating to stay cool. Panting is far more common, and has many benefits that sweating lacks. However, when an animal runs, panting becomes a disadvantage.

A running dog has to simultaneously use its breathing to both cool down and maintain oxygen levels, which limits physiological flexibility. Humans, whose sweating turns the entire body into a cooling unit, can use breathing entirely to maintain oxygen flow. This would have allowed our ancestors to run for a much longer time without overheating than the average mammal, which made them better able to acquire food to fuel their large brains. They could have foraged over a large territory, but also allowed them to access an unusual hunting method: Persistence hunting.

Animals from Chauvet Cave, painted around 30,000 years ago in France. Image by Thomas T.

Persistence hunting

Our ancestors hunted with sharpened stakes of wood, and eventually stone spears and arrowheads, but before the development of weapons, hunting would have been very challenging. If they needed more meat than they could scavenge, early humans may have utilized persistence hunting.

Our human ability to jog for long distances means that we could have run large prey to collapse, either from heat stroke or exhaustion. For cultures without modern medicine, this would have been a much safer way of killing prey than violent means, only necessitating drawing close when the animal was already on its last legs.

While we may never know if this method was actually used by early humans, the skeletons of Homo erectus and other early humans indicate that they were in the habit of running the same amount as Olympic-level long distance runners today. Homo erectus is considered the direct ancestor of modern humans, and had many specialized features that would have made running easier that we still have today. This includes long legs, large attachments for butt muscles, and special ligaments that hold the head in place while running.

It is likely that it was some combination of evolutionary pressures that drove humans to hairlessness, rather than a single one. For now, hairlessness remains a fascinating trait that separates us from most other land-dwelling mammals. And this is not the only interesting question that could be asked about human hair. Why, for example, does the hair on our head grow so long? That is a question for another article, with many other interesting explanations.

Kate Dzikiewicz, Science Curatorial Associate