Why don't humans have fur?

Most mammals, including our closest living relatives, have hairy coats. So why did we lose ours?
Jocelyn Timperley

Source - https://www.bbc.com/future/article/20230310-why-dont-humans-have-fur

If an alien race came to Earth and lined up humans in a row alongside all the other primates, one of the first differences they might observe – together with our upright position and unique form of communication – is our apparently furless bodies.

In fact, compared to most mammals, humans are remarkably unhairy (granted, with the exception of the occasional individual). A handful of other mammals do share this quality – naked mole rats, rhinos, whales and elephants among them. But how exactly did we end up in this bare state? Does it bring us any benefits today? And how do we account for the presence of thick, dense hair on some parts of our bodies?

Of course, humans actually do have lots of hair: on average we have approximately five million hair follicles across our body surface. But almost all the hair follicles on human bodies produce vellus hair, fine, short, fuzzy hair which grows from shallow follicles, different to the deeper, thicker terminal hairs only found on our head and (after puberty) our underarms, pubic areas and, mostly among men, faces.

"We technically have hair all over our bodies, it's just miniaturised hair follicles," says Tina Lasisi, a biological anthropologist at the University of Southern California who specialises in the science of hair and skin. "But it's miniaturised to the point where it functionally doesn't insulate us anymore."

The body-cooling hypothesis points to a rising need for early humans to thermoregulate their bodies as a driver for fur loss

Scientists don't definitively know the reason behind this change from thicker, coarser fur to these light vellus hairs, and they also don't know exactly when it happened. Still, several theories have been suggested as to what could have sparked the loss of our hair.

The most dominant view among scientists is the so-called "body-cooling" hypothesis, also known as the "savannah" hypothesis. This points to a rising need for early humans to thermoregulate their bodies as a driver for fur loss.

During the Pleistocene, Homo erectus and later hominins started persistence hunting on the open savannah – pursuing their prey for many hours in order to drive it to exhaustion without the need for sophisticated hunting tools, which appear later in the fossil record.

This endurance exercise could have put them at risk of overheating – ergo the fur loss, which would have allowed them to sweat more efficiently and cool down faster without needing breaks.

Evidence for this theory also comes from studies that have found switches for some genes responsible for determining whether certain cells develop into sweat glands or hair follicles. "So all of these things have a related developmental pathway," says Lasisi. "If we look at that in combination with some of the things we're able to infer about genes that increased human skin pigmentation, then we're able to basically confidently guesstimate that 2-1.5 million years ago… humans probably would have lost their body hair."

A related theory set out in the 1980s suggested that the change to an upright bipedal position decreased the benefits of fur for reflecting radiation off our bodies (bar on the top of our heads). As we can sweat better without fur, this became relatively more beneficial than having fur.

But while the body-cooling hypothesis ostensibly seems to make a lot of sense, and there may be some merit to it, it fails in some realms, argues Mark Pagel, a professor of evolutionary biology at the University of Reading.

"When you study our body heat over a 24-hour period, we lose more heat at night than we want to, and so the net effect of losing your fur is that we're in a sort of energy deficit all of the time," he says. He also notes there are lots of human populations that have not done endurance running for tens of thousands of years, but none have grown their fur back, despite many now living in very cold regions of the world.

Lasisi, however, says that hyperthermia – an abnormally high body temperature – would likely have been a far bigger issue than hypothermia in equatorial Africa, where humans evolved. "It seems to me that there is a bit of a stronger pressure to not overheat, rather than one to necessarily stay warm."

She also notes many genetic traits can become canalised – difficult to re-evolve in different ways – and that by the time humans reached colder environments, they had developed other technologies to keep warm, such as fire and clothing. They also likely developed other physiological adaptations to cold such as brown fat adaptation, she adds.

Parasites are probably one of the strongest selective forces in our evolutionary history – Mark Pagel

In 2003, Pagel and his colleague Walter Bodmer at the University of Oxford put forward another explanation for early human fur loss, which they called the ectoparasite hypothesis. They argued that a furless ape would have suffered from fewer parasites, a major advantage.

"If you look around the world, ectoparasites are [still] an enormous problem in the form of biting flies that carry disease," says Pagel. "And those flies are all specialised to land on and live in fur and deposit their eggs in fur… Parasites are probably one of the strongest selective forces in our evolutionary history, and still are." Pagel says "nothing's come along to make us question" this hypothesis since he and Bodmer first came up with it.

Lasisi says that she wouldn't exclude the possibility of other factors contributing to fur loss. But "you really have to ask yourself, well, why would this happen in humans and not chimpanzees, not bonobos, not gorillas?" she says.

"I'm inclined to focus on hypotheses that are able to suggest behaviours or migrations into places that would have set humans apart from other apes in a way that would have required hair loss."

One factor here could have been the development of clothing made of other animals' fur, which they could remove and wash. This would date fur loss as recently as 100,000-200,000 years ago, far later than the body cooling hypothesis suggests, based on when human body lice, which only live in clothing, first appeared.

Pagel says he is inclined to believe this timeline is the most likely for the largest part of fur loss, although "no-one really knows" since hair rarely fossilises.

Charles Darwin, meanwhile, thought our fur loss was due to sexual selection – our ancestors simply preferred less hairy mates. Most researchers today dismiss this as a primary cause of fur loss.

But when thinking about human hairlessness there's one obvious question that keeps coming up: why do we still have hair on our heads, public regions and armpits?

"What seems to make sense is that humans may have kept their head hair and in fact have grown longer and especially curlier head hair to minimise heat gain from solar radiation," says Lasisi, who studied the topic in her PhD thesis (her findings are due to be published soon). 

In particular, tightly coiled human hair has an intricate structure that leaves air pockets open, allowing it to dissipate heat very effectively while minimising how much heat comes down to the scalp, she says.

"The more space you can put between where solar radiation is hitting, so the top of the hair, and what you want to protect, which is your scalp, the better off you are."

As for pubic and underarm hair Lasisi considers this could either be a so-called spandrel – a byproduct of the evolution of another characteristic – or potentially a leftover from primate ancestors that used pheromones to communicate with each other (there's no good evidence humans use pheromones today).

No matter what prompted the loss of human fur, one thing is extremely likely: it coincided with early humans gaining a darker skin pigmentation where body hair previously would have been as necessary protection from UV radiation.

"It's the logical inference that we can make," says Lasisi. "It could be that some humans just ended up being born with entirely hairless bodies, and then that became an adaptation in tandem with some of those humans evolving darker skin. Or it could be that there was a slightly more gradual reduction in hair that was happening with a slightly more gradual increase in skin pigmentation."

While it's interesting to consider how we lost our fur, it may seem less than relevant to our lives today. But research has indicated that increased understanding could even have implications for people with unwanted hair loss today due to balding, chemotherapy or disorders that cause hair loss.

In early 2023, Nathan Clark, a geneticist at the University of Utah, and his colleagues Amanda Kowalczyk and Maria Chikina at the University of Pittsburgh, surveyed the genes of 62 mammals including humans to find the genetic shifts hairless mammals shared with each other to the exclusion of their furry cousins. They found that humans seemed to have the genes for a full coat of body hair, but our genome regulation currently stops them from being expressed.

They also found that when a species loses hair, they do it by changes to the same set of genes repeatedly, and uncovered several new genes involved in this process.

"Some of those [new] genes hadn't been really characterised at all, because people hadn't done many genetic screens on presence and absence of hair in the past," says Clark. "[They] seem to maybe be master controllers that might be manipulated in the future if people wanted to stimulate hair growth."

THE AQUATIC APE HYPOTHESIS

Another, improbable, theory comes from the largely dismissed aquatic ape hypothesis, which was first proposed in 1960. According to this theory, the apes that eventually turned into humans diverged from other Great Apes by adapting to spend significant time in water. The adaptations that occurred due to this explained characteristics of modern humans such as our hairlessness and bipedalism. 

The problem with this? "Anthropologically, there just isn't a shred of evidence that we evolved on beaches or near the water, [or] had an aquatic phase," says Pagel. "It's unfortunate." Other scientists have pointed out that semi-aquatic mammals like otters and water voles are extremely furry, so why would humans have lost their fur for this reason?