Most mammals, including our closest relatives, have fur coats. Then why did we lose ours?
If an extraterrestrial race were to come to Earth and put humans in a species along with all other primates, one of the first differences they might notice - along with their upright posture and unique way of communicating - would be their apparently furless bodies.
Indeed, compared to other mammals, humans are conspicuously hairless (with the exception of individuals). Only a few other mammals share this property - among them naked moles, rhinoceroses, whales and elephants.
But how did we actually end up so naked? Does it benefit us today? And how do we explain the presence of thick and lush 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 our body surface.
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Almost all follicles on the human body produce gentler pubescence, fine, short, frizzy hairs growing from shallow follicles, different from the deeper and thicker, terminal hairs that we can only find on the head and (after puberty) under the armpits, intimate parts of the body and, mostly in men, on the face.
"Technically speaking, we have hair all over our body, it's just miniaturized follicles," says Tina Lasisi, a biologist and anthropologist at the University of Southern California who specializes in hair and skin science.
"They are so miniaturized that they no longer provide us with functional protection".
Scientists don't know definitively why the change from thicker, coarser fur to vellus hair occurred, and they don't know when it happened. However, some theories point to what could have caused hair loss.
The most dominant opinion among scientists is called the "body cooling" hypothesis, which is also known as the savannah hypothesis. It points to the growing need of early human beings for body thermoregulation, which was the driving force behind the fur loss process.
During the Pleistocene, Homo erectus and later hominins began persistent hunting in the open savannah - chasing prey for hours in order not to tire it and without the need for sophisticated hunting weapons that only later appear in the fossil remains.
This kind of exhaustion was risky for them due to overheating - hence the loss of fur, which allowed them to sweat and cool down more efficiently without having to rest.
Evidence for this theory also comes from studies that have found a trigger for some genes responsible for whether certain cells will develop into sweat glands or follicles.
"All these things are connected in the development path," says Lasisi.
"If we look at it in combination with some processes that we could conclude are related to the genes that increased skin pigmentation, then we can establish with considerable certainty that about one and a half to two million years ago... what happened was that we lost hairiness."
A similar theory from the 80s states that the transition to an upright, bipedal position diminished the benefits of fur due to reflective radiation from our bodies. Since we sweat better without fur, this was relatively more useful than with fur.
But while at first glance this hypothesis seems to make sense and could be taken into account, it does not work in some other areas, according to Mark Pagel, professor of evolutionary biology at the University of Reading.
"When you study your own body temperature over a 24-hour period, you see that we lose a lot more heat at night than we want, so the consequence of losing fur is that we're kind of lacking energy all the time," he says.
He also notes that the majority of the human population has not been running after prey to the point of exhaustion for tens of thousands of years, so its fur has not grown back, no matter how cold the regions of this world we live in.
Lasisi, however, says that hyperthermia - an abnormally high body temperature - would probably be a much bigger problem than hypothermia in equatorial Africa, where humans evolved.
"It seems to me that the desire not to overheat is stronger than the need to be warm."
She also points out that many genetic traits can be channeled—it's complicated to evolve in the opposite direction—and that by the time humans reached colder regions, they developed other technologies that kept them warm, such as fire and clothing. It is also believed that other psychological adaptations to cold have developed, such as adaptation to brown fat, she adds.
In 2003, Pagel and his colleague Walter Bodmer from the University of Oxford put forward another explanation for the early loss of fur in humans and called it the ectoparasitic hypothesis. Their argument was that a primate without fur also had fewer parasites, which was a huge advantage.
"If you look around, ectoparasites are (still) a huge problem in the form of flies that bite and transmit disease," says Pagel.
"And all those flies are trained to land and live in fur and lay their eggs in it...Parasites have probably been the most powerful force in our evolutionary history, and still are."
Pagel says that still "nothing new has come to light to make us reconsider" this hypothesis since he and Bodmer first went public with it.
Lasisi says she would not rule out the possibility of other factors contributing to this loss.
"But we should really ask why this only happened to humans and not to chimpanzees, bonobos or gorillas?" she says.
"I'm more inclined to focus on a hypothesis that would be able to explain to us the behavior or migrations that led to the separation of humans from other apes and why they lost their fur."
One of the factors could be the clothing made from the fur of other animals that they could wash and remove as needed. This could place the loss of fur in the period 100 - 200 thousand years ago, much later than the cooling hypothesis, which is based on the first appearance of human lice living only in clothing, leads us to believe.
Pagel says he's inclined to believe this time estimate applies to most fur, though "no one really knows," because hair fossilizes very rarely.
Charles Darwin, meanwhile, thought that the loss of fur occurred due to sexual selection - our ancestors simply preferred their wives with less hair. Nowadays, most scientists reject this theory as the primary reason for fur loss.
But when dealing with the issue of human hair, one obvious question keeps coming up: why do we still have hair on our heads, private parts and underarms?
"What seems to make sense is that people kept their hair and even kept it long and curly to reduce the heat of the sun's rays," says Lasisi, who studied the topic in her doctoral dissertation (her work will soon be published ).
More precisely, tightly coiled human hair has an intricate structure that leaves air pockets open and allows for very effective heat dissipation and a reduction in the amount of heat reaching the skull itself, she says.
"The more space left between the place that is first exposed to solar radiation, i.e. the top of your hair, and what you want to protect, which is your skull - the better for you."
The water monkey hypothesis
Another amazing theory comes through the water monkey hypothesis, which first appeared in 1960.
According to this theory, the apes that eventually became humans separated from other primates by adapting to living in water.
Adaptations that have emerged as a result of this process in modern humans are hair loss and bipedalism.
What is the problem here?
"Anthropologically, there is not a shred of evidence that we evolved on beaches or near water, that is, that we had this aquatic phase at all," says Pagel.
"Unfortunately".
Other scientists point out that semi-aquatic mammals, such as the otter or the water rat, have extremely thick fur, so the question then arises, why would humans lose their fur in such conditions?
As for pubic hair, as well as underarm hair, Lasisi believes that it is either a by-product of evolution or a potential remnant that we inherited from primate ancestors who used pheromones to communicate with each other.
Pheromones are chemicals that animals secrete, like secretions, and thus influence the behavior of other individuals, there are alarm or sex pheromones, but there is no solid evidence that people use them today.
Regardless of what caused the loss of human fur, one thing is very likely: it happened at the same time that early humans acquired dark skin pigmentation, where earlier body hair was a necessary protection against UV radiation.
"That's the only logical conclusion," says Lasisi.
"It is possible that some people were simply born with bodies that were hairless, and that adaptation followed in tandem with the acquisition of darker skin in some people. And it is also possible that there was a gradual reduction of hairiness that took place in accordance with a gradual increase in skin pigmentation."
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While it's interesting to consider how we lost our fur, it's becoming less and less relevant to the lives we lead today. However, research shows that such knowledge may have implications for those people who are dealing with unwanted hair loss - baldness, hair loss due to chemotherapy or other disorders that cause hair loss.
Earlier this year, Nathan Clark, a geneticist at the University of Utah, and his colleagues Amanda Kovalczyk and Maria Chikina at the University of Pittsburgh, looked at 62 mammals, including humans, to find genetic changes that the furless mammals shared with each other until they lost their fur.
They found that humans have genes for fur that covers the entire body, but that our gene regulation currently prevents their expression.
They also found that species that lose hair do so persistently by changing the same set of genes, as well as a few new genes that participate in the process.
"Some of those (new) genes haven't been addressed yet, because people haven't done much genetic testing for the presence or absence of hairiness in the past," Clark says.
"They could be the main controllers that, through some manipulation, could enable the stimulation of hair growth in the future."
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