Why do humans have so much less hair than other primates?

If an alien people were to come to Earth and line humans up next to other primates, the first differences they might notice would be our upright posture, our unique ways of communicating—and our seemingly hairless human body.

Compared to most mammals, humans are generally hairless (with known exceptions of course). A few mammals have this same characteristic, such as the naked rat, the rhinoceros, the whale and the elephant.

But why do we end up hairless? Has bare skin benefited us? And how to explain the presence of dense and dense hair in certain parts of the body?

In fact, it is clear that human beings have a lot of fur. On average, there are about five million hair follicles on the surface of our body.

But most follicles in the human body produce fine, short, fuzzy strands that grow from shallow follicles, as opposed to the deeper, thicker hairs found only on our heads and, after puberty, in our armpits, pubic areas, and, most notably, men, in the face.

“Technically, we have hair all over our bodies, but they’re tiny follicles,” says Tina Lasisi, a bioanthropologist at the University of Southern California in the United States who specializes in skin and hair science. “But they’re miniaturized to the point where, functionally, they don’t protect us anymore.”

Scientists aren’t sure why this change led to this change, turning our thick, curly hair into these light strands. They also don’t know exactly when it happened.

But several theories suggest what might have caused our body hair to fall out.

The prevailing view among scientists is the so-called “body cooling” hypothesis, also known as the “savannah hypothesis”. According to her, the increasing need for thermoregulation of early humans would have led to hair loss.

During the Pleistocene period, the homo erectus and other hominids began to practice persistent hunting in the open savannah, chasing their prey for several hours until they were exhausted. This method eliminated the need for sophisticated hunting tools, which would appear later, according to the fossil record.

This endurance exercise may have put the hominids at risk of overheating. Hence the loss of hair, which would allow them to sweat more efficiently and cool down faster, without having to take breaks to recover energy.

Evidence for this theory comes from studies that have found changes in some of the genes responsible for determining whether certain cells develop to form sweat glands or hair follicles.

“All this has a relative development path”, according to Lasisis.

“If we look at this in conjunction with some inferences we’ve been able to make about genes that increased human skin pigmentation, we can imagine, with some confidence, that 1.5 to 2 million years ago… humans (in the process of evolution) they’ve probably lost their body hair,” he adds.

A related theory, developed in the 1980s, suggests that the shift to an upright bipedal position reduced the benefits of hair in reflecting radiation from our bodies (except the top of the head). Because we can sweat better without hair, it has become relatively less beneficial to humans.

The hypothesis of cooling the body is obviously perfectly reasonable and may have a reason. But it is flawed in some areas, according to Mark Pagal, a professor of evolutionary biology at the University of Reading in the UK.

“When we look at how our body heats up over a 24-hour period, we lose more heat at night than we want,” he says. “So the average effect of hair loss is that we’re constantly in a kind of energy deficit.”

Pagal also notes that there are many human populations that haven’t run for endurance in tens of thousands of years, but their hair hasn’t grown back, even though many of them now live in very cold areas of the planet.

Lasisi, on the other hand, states that hyperthermia – a body temperature higher than normal – would probably have been a much bigger problem than its opposite (hypothermia) in equatorial Africa, where human beings evolved.

“It seems to me that there’s a little bit more pressure to not overheat, not necessarily to get hot,” he says.

Lasisi also points out that many genetic traits can be channeled – further evolving them into different forms is difficult.

And when humans arrived in colder environments, they had already developed other technologies to keep warm, such as fire and clothing.

He adds that humans have likely evolved other physiological adaptations to the cold, such as adapting brown fat, for example.

In 2003, Pagal and his colleague Walter Bodmer, of the University of Oxford, in the United Kingdom, presented another explanation for the loss of human hair. They called it the extraparasite case.

For them, hairless monkeys would have suffered from fewer parasites, which is a major advantage.

“If you look around the world, ectoparasites [ainda] it’s a huge problem, like flies that bite and spread diseases,” explains Pagal. “And these flies specialized in landing and living on hairs and laying their eggs there. … Parasites were probably and still are one of the biggest selective forces in our evolutionary history.”

Pagal claims that “nothing has come up to make us question” that case since he and Bodmer first made it.

Lasisi says she doesn’t rule out the possibility that other factors contributed to human hair loss.

“You really have to ask yourself why this would happen to humans and not chimpanzees or bonobos or gorillas,” he says.

“I tend to focus on hypotheses that might suggest behaviors or migrations to places that would separate humans from other primates, such that hair loss was necessary,” Lasisi explains.

Another theory – this one, improbable – is the aquatic primate hypothesis, first proposed in the 1960s and now practically abandoned.

According to this theory, the chimpanzees that eventually became humans were different from other primates in that they were adapted to spend significant time in water.

And the resulting adaptations explain some features of modern humans, such as hairlessness and bipedalism.

The problem with this hypothesis is that “anthropologically, there is not a shred of evidence that we evolved on beaches or near water, [nem que] we had a water phase,” according to Pagal. “Its a shame.”

Other scientists point out that semi-aquatic mammals, such as otters and marsh waders, have a lot of fur. So why would people have lost their fur for this reason?

One factor in the shedding of human hair may have been the development of clothing made from the skins of other animals, which humans could remove and wash as needed.

In this case, we could put the time of hair loss at 100,000 to 200,000 years ago, much later than the body cooling hypothesis suggests. This estimate is based on the time when the first human body lice, which live only on clothes, appeared.

Pagal says he’s led to believe this timeline is the most likely for most human hair loss, though “nobody really knows” since hair rarely fossilizes.

Charles Darwin believed that our hair loss was due to sexual selection – our ancestors simply preferred less furry mates. But most researchers today reject this possibility as the main cause of hair loss.

When we think about the lack of hair in humans, an obvious question always arises: why do we still have hair on our heads, pubic areas and armpits?

“What seems to make sense is that people may have maintained hair on their heads, even growing longer and particularly wavy hair, to minimize heat gain from solar radiation,” says Lasisi. She studied this topic in her PhD thesis and the results are expected to be published soon.

In particular, human hair, when tightly coiled, forms a complex structure with open pockets. This structure allows heat to dissipate very efficiently, minimizing the amount of heat reaching the scalp, according to Lasisi.

“The more space you put between the part that gets hit by the sun (the top of the head) and the part you want to protect (which is the scalp), the better you look,” she explains.

As for the pubic area and armpits, Lasisi thinks it could be a so-called “spandru” – a by-product of the evolution of another trait – or, possibly, a holdover from primate ancestors who used pheromones to communicate with each other. There is no good evidence that people are using pheromones today.

Pheromones are chemical substances produced internally and which are expelled to the outside of the body as a stimulus for communication between elements of the same or different species.

Regardless of the causes of human hair loss, it is highly likely that it coincided with darker skin pigmentation in early humans, where body hair was once needed for UV protection.

“It is the logical conclusion we can draw”, according to Lasisi. “It could be that some people ended up being born with completely hairless bodies and this then became a common adaptation with some of these people developing darker skin.”

“Or maybe there was a slightly more gradual decline in hair that happened with the slightly more gradual increase in skin pigmentation,” he explains.

Although it is interesting to see how we lose our hair, it may seem less relevant to our lives today. However, research suggests that our increased understanding could have implications for people with unwanted hair loss, such as baldness, chemotherapy or disorders that cause hair loss.

In early 2023, geneticist Nathan Clark, of the University of Utah, along with his colleagues Amanda Kowalczyk and Maria Chikina, of the University of Pittsburgh, both in the United States, searched the genes of 62 mammals – including humans – looking for genetic changes present in hairless mammals but lacking in their furry cousins.

They concluded that humans apparently possess the genes that provide full body hair coverage, but our genomic regulation currently prevents their expression.

They also found that when a species loses hair, it is due to repeated changes in the same set of genes, and they found several new genes involved in the process.

“Some of these genes [novos] they hadn’t really been characterized before because people hadn’t done a lot of genetic testing for the presence and absence of hair in the past,” says Clark. “Looks like [eles] maybe they are master controllers that can be manipulated in the future if people want to stimulate hair growth.”