[Animals] Why humans, unlike other primates, do not have a completely covered body with hair

[#Steeven.™]

Humans have approximately five million hair follicles on the surface of the body.

If aliens were to come to Earth and put humans in the same line with all other primates, one of the first differences they would notice, along with our upright posture and unique way of communicating, would be our seemingly hairless bodies.

In fact, compared to most mammals, humans are remarkably sparse (with the exception of the occasional individual). A handful of other mammals share this quality, including naked mole-rats, rhinos, whales, and elephants.

But how exactly did we end up in this state? Does it bring us any benefit today? And how do we explain the presence of thick and dense hair on some parts of our body?

Of course, humans actually have a lot of hair: on average, we have approximately five million hair follicles on the surface of our bodies.

But almost all of those hair follicles produce short, fine body hair that grows from superficial follicles, as opposed to the deeper, coarser hairs found only on the head and (after puberty) in the armpits, pubic areas. and, mainly in men, on the face.

“Technically we have hair all over our bodies, they are just miniature hair follicles. But it is miniaturized to the point where it no longer functionally isolates us,” says Tina Lasasi, a biological anthropologist at the University of Southern California who specializes in hair and skin science.

 

Hypothesis for all tastes
Scientists don't know conclusively the reason behind this change from thicker, coarser fur to these softer downs, and they don't know exactly when it happened. Even so, several theories have been put forward as to what might have caused our loss of body hair.

The most dominant opinion among scientists is the so-called “cooling of the body” hypothesis, also known as the “savanna” hypothesis. This points to a growing need for early humans to thermoregulate their bodies as a factor that led to hair loss.

During the Pleistocene, Homo erectus and later hominids began to hunt persistently in the open savannah, pursuing their prey for many hours to exhaustion without the need for sophisticated hunting tools, which later appear in the fossil record. .

This endurance exercise could have put them at risk of overheating, hence the hair loss, which would have allowed them to sweat more efficiently and cool down faster without the need for breaks.

Evidence supporting this theory also comes from studies that have found switches for some genes responsible for determining whether certain cells become sweat glands or hair follicles.

“So all of these things have a related development path. If we look at that in combination with some of the things we can infer about genes that increased human skin pigmentation, then basically we can confidently estimate that 1.5 to 2 million years ago humans probably lost body hair," says Lasasi. .

A related theory elaborated in the 1980s suggested that switching to an upright bipedal position diminished the benefits of fur in reflecting radiation from our bodies (except for the tops of our heads). Since we can sweat better without hair, this became relatively more beneficial than having hair.

But while the body-cooling hypothesis on the surface makes a lot of sense and may have some merit, it fails in some respects, argues Mark Pagal, a professor of evolutionary biology at the University of Reading in the UK.

“When you study our body heat over a 24-hour period, [you notice that] we lose more heat at night than we want to, so the net effect of losing your fur is that we're in sort of an energy deficit. all the time,” says Pagal.

He further highlights that there are many human po[CENSORED]tions that have not done endurance races for tens of thousands of years, yet none have grown their fur back, despite the fact that many now live in very cold regions of the world.

Lasasi, however, says that hyperthermia, an abnormally high body temperature, would likely have been a much bigger problem than hypothermia in equatorial Africa, where humans evolved.

“It seems to me that there's a little stronger pressure not to overheat, rather than one to stay warm,” she says.

He also points out that many genetic traits can become channeled (difficult to re-evolve in different ways) and that when humans arrived in colder environments, they had developed other technologies to keep warm, such as fire and clothing.

In addition, he adds, they probably developed other physiological adaptations to cold as well, such as the adaptation to brown fat.

In 2003, Pagal and his colleague Walter Bodmer of the University of Oxford (UK) proposed another explanation for early fur loss in humans, which they called the ectoparasite hypothesis. They argued that a hairless ape would have suffered from fewer parasites, an important advantage.

“If you look around the world, ectoparasites are [still] a huge problem in the form of biting flies that carry disease,” says Pagal.

“And those flies are all specialized to land and live on the fur and lay their eggs on the fur. Parasites have probably been one of the strongest selective forces in our evolutionary history, and they still are,” he adds.

Likewise, he assures that since he and Bodmer first proposed this hypothesis "nothing has appeared that makes us question it."

Lasasi says he wouldn't rule out the possibility that other factors contribute to hair loss. But “you really have to ask yourself, well, why would this happen in humans and not chimpanzees, bonobos and gorillas?” he says.

“I am inclined to focus on hypotheses that may suggest behaviors or migrations to places that would have differentiated humans from other apes in a way that would have required hair loss,” he says.

 

The aquatic ape hypothesis

Another unlikely theory stems from the largely dismissed aquatic ape hypothesis first proposed in the 1960s.

According to this theory, the apes that eventually became humans differed from other great apes by adapting to spend significant time in water. The adaptations that occurred because of this explained features of modern humans, such as our lack of hair and being bipedal.

What would be the problem with this idea? “Anthropologically, there is simply not a shred of evidence that we evolved on beaches or near water, [or] we had an aquatic phase. It's unfortunate,” says Pagal.

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?

One factor here might have been the development of clothing made from the skins of other animals, which could be removed and washed. This would date hair loss to as recently as one to two thousand years ago, much later than the body-cooling hypothesis suggests, based on when lice first appeared on the human body, which only They live in clothes.

Pagal says that he is inclined to believe that this timeline is the most likely for most hair loss, although "nobody really knows" since hair rarely fossilizes.

In 2003, Pagal and his colleague Walter Bodmer of the University of Oxford (UK) proposed another explanation for early fur loss in humans, which they called the ectoparasite hypothesis. They argued that a hairless ape would have suffered from fewer parasites, an important advantage.

“If you look around the world, ectoparasites are [still] a huge problem in the form of biting flies that carry disease,” says Pagal.

“And those flies are all specialized to land and live on the fur and lay their eggs on the fur. Parasites have probably been one of the strongest selective forces in our evolutionary history, and they still are,” he adds.

Likewise, he assures that since he and Bodmer first proposed this hypothesis "nothing has appeared that makes us question it."

Lasasi says he wouldn't rule out the possibility that other factors contribute to hair loss. But “you really have to ask yourself, well, why would this happen in humans and not chimpanzees, bonobos and gorillas?” he says.

“I am inclined to focus on hypotheses that may suggest behaviors or migrations to places that would have differentiated humans from other apes in a way that would have required hair loss,” he says.

The aquatic ape hypothesis
Another unlikely theory stems from the largely dismissed aquatic ape hypothesis first proposed in the 1960s.

According to this theory, the apes that eventually became humans differed from other great apes by adapting to spend significant time in water. The adaptations that occurred because of this explained features of modern humans, such as our lack of hair and being bipedal.

What would be the problem with this idea? “Anthropologically, there is simply not a shred of evidence that we evolved on beaches or near water, [or] we had an aquatic phase. It's unfortunate,” says Pagal.

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?

One factor here might have been the development of clothing made from the skins of other animals, which could be removed and washed. This would date hair loss to as recently as one to two thousand years ago, much later than the body-cooling hypothesis suggests, based on when lice first appeared on the human body, which only They live in clothes.

Pagal says that he is inclined to believe that this timeline is the most likely for most hair loss, although "nobody really knows" since hair rarely fossilizes.

Regarding the pubic and armpit hair, Lasasi considers that it could be either what is known as adhesive tape -a by-product of the evolution of another characteristic- or a possible remnant of the primate ancestors that used pheromones to communicate with each other ( there is currently no convincing evidence that humans use pheromones).

 

Less hair, more color
Whatever the reason for the loss of human fur, one thing is highly likely: it coincided with the appearance of darker skin pigmentation in early humans, where body hair previously existed as necessary protection against UV radiation.

"It's the logical deduction that we can make," says Lasasi.

“It could be that some humans ended up being born without body hair and that became an adaptation and some of those humans acquired darker skin. Or it could be that there was a slightly more gradual reduction in hair along with a slightly more gradual increase in skin pigmentation,” he adds.

Although it is interesting to consider how we lost our fur, it may seem irrelevant to our lives today. But research has indicated that a greater understanding could even have implications for people with unwanted hair loss today due to baldness, chemotherapy, or disorders that cause hair loss.

In the US, 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, studied the genes of 62 mammals, including humans, to find the genetic changes that hairless mammals shared with each other, excluding their furry cousins.

They also found that humans appear to have the genes for a full layer of body hair, but regulation of our genome currently prevents them from being expressed.

They also found that when a species loses hair, it does so through repeated changes in the same set of genes, and discovered several new genes involved in this process.

“Some of those [new] genes hadn't really been characterized at all, because people hadn't done a lot of genetic testing for the presence and absence of hair in the past,” Clark says.

“They seem to be master controllers that could be mani[CENSORED]ted in the future if you wanted to stimulate hair growth,” he concludes.

 

https://www.eluniverso.com/larevista/salud/por-que-los-humanos-a-diferencia-de-otros-primates-no-tenemos-el-cuerpo-completamete-cubierto-de-pelo-nota/