In fact, it is difficult to say for sure here, tk. There are many hypotheses.
Here are the most serious and believable …
Immediately it is necessary to make a reservation that the hair has not disappeared anywhere. Their number differs in people of different races (Negroids and Mongoloids have fewer, Caucasians more), their sizes can vary widely. But on average, there is as much hair on the human body as in other tropical mammals of a similar size. In humans, the hair has simply greatly decreased in size.
“- Is it worth reasoning? He will die from the winter rains; the sun will burn him! What harm can a hairless frog do us? Let him run with the flock. " Thus cried the young wolves, looking at little Mowgli. Indeed, one can only envy a wolf or one's own dog when they calmly run through the forest in their "fur coat" at minus twenty … Why did the ancestors of Homo sapiens get rid of this fur coat? When and why, as it is written in one article, the paths of a man and his fur coat diverged?
During the period of loss of wool, people lived where there is no minus twenty. On the territory of Kenya, for example, it is rarely even plus twenty (usually the temperature is higher). And at the time when the distant ancestors of man lived there, the climate was just as hot. Our ancestors, who lost their wool, lived in the savannah - in an open space, under the scorching sun. They mastered Savannah at least 4-5 million years ago. Before that, they lived in the forest in the trees.
Human ancestors became erect, as it is now believed, even during the "arboreal" life. They walked along the branches while standing, holding on to other branches with their hands. And in the savannah, upright walking was very useful to them.
While our distant ancestors (Australopithecus) ate mainly plant foods and small animals, they did not have to run a lot. Apparently, they were covered with wool that protected them from sunburn and scratches. But our closer ancestor - a skilled man - mastered the manufacture of stone chops. With the help of tools, he was able to butcher the corpses of large herbivores killed by predators or died from other causes. Here I had to run: there are many who want to use such a resource as corpses, and they need to get ahead. I had to run during the day so as not to run into a lion or a saber-toothed cat - they are active at dusk. (Later, other species of people who hunted for big game, too, were forced to run after wounded animals. And today, bushmen and other hunters can chase a wounded animal for days.)
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In these conditions, bipedality turned out to be very useful. Calculations show that the resulting total solar radiation in the vertical position of the body is less than about a third; and at noon it is as much as four times less. In addition, in an upright position, the body is better cooled by the wind. But even a bipedal creature, if it is covered with wool, is threatened with overheating during a long run. Therefore, I had to get rid of the fur coat.
In parallel with the loss of hair in humans, the number of sweat glands that secrete watery sweat has increased (in smaller quantities, many primates have such glands). For cooling, it is better to evaporate sweat from the surface of the skin than from a wet coat. This is how the human ancestors got an efficient cooling system. She did not allow her body temperature to rise too much, even with a long run.
Bare skin is at risk of sunburn under the scorching sun, and ultraviolet radiation can cause cancer. Therefore, due to the loss of hair, people's skin darkened. The dark pigment melanin absorbs dangerous ultraviolet radiation. Chimpanzees in most populations have light skin (although this trait is quite variable); it is likely that the common ancestors of humans and chimpanzees, covered with wool, had light skin. All aboriginal peoples of the equatorial zone have dark skin. Many genes are responsible for skin color. The study of variants of one of them showed that the skin darkened, apparently no later than 1.2 million years ago.
Often against all these explanations, a seemingly deadly argument is put forward - why did not the fur disappear from other large mammals of the savannah - zebras, giraffes, antelopes, lions and cheetahs? But it is murderous only at first glance. There are no bipeds among these mammals. Many of them, as I have already noted, are active not during the day, but at dusk. They have no or poorly developed sweat glands that secrete watery sweat - namely, they provide the greatest advantage in cooling the body with bare skin. Finally, some of them have special brain cooling mechanisms that humans do not have. For example, in an antelope, when running, the body temperature can rise by 4-5 ° C; but due to the special arrangement of the network of blood vessels in the head, the temperature of the brain remains almost unchanged. A person does not have such a vascular system, so you have to cool the whole body,so as not to overheat the brain.
There are other explanations for hair reduction in human ancestors. One of them connects a person's “hairlessness” with neoteny and the action of sexual selection. It is known that man is an "underdeveloped monkey". In many ways, we are more like baby chimpanzees than adults. In particular, chimpanzee babies are born with thick hair on their heads, but with an almost naked body. "Childhood" signs in many animals cause an instinctive desire to take care of their owners. It is beneficial for females to have such traits so that the males take better care of them. (Girls not only outwardly resemble children more than men. They often show their boyfriends childish elements of behavior - the need for affection, "begging" for food, etc.). The “selection for hairlessness” among women also led to the loss of hair in men (though not so complete).
Another hypothesis links hair loss with getting rid of parasites. Apparently, a very long time ago, man began to build dwellings and use caves as shelters. Under these conditions, “nesting” parasites (ticks, fleas, bedbugs) could pester our ancestors much more than chimpanzees who build only “disposable” nests. It is easier to search for and remove sucked parasites with bare skin.
It is evident that all these hypotheses do not contradict each other. For example, the hypothesis about the role of parasites with the previous one is connected by the idea that the display of healthy, clear skin could play an important role in choosing a sexual partner. Most likely, a combination of many factors contributed to the loss of hair.
And why is the hair preserved on the head, under the armpits and in the groin? There are different explanations here too
Hair warms the head instead of a hat, protects it from blows, protects it from overheating on a sunny day - especially such thick and curly, like many Negroids. The beard, mustache and sideburns seem to be adornments for men. V. R. Dolnik suggests that a man's lush hair (like the mane of old male baboons) can increase his status in the eyes of his relatives. And Charles Darwin believed that the beard arose from sexual selection, as it serves as an ornament that increases the attractiveness of males. Modern research across cultures (including “primitive” tribes) shows the opposite: women rate beardless faces as more attractive. On the other hand, men consider persons with beards to be more aggressive, and both sexes rate bearded people as having an older age and a higher social status. So the beard obviously plays some kind of signaling role.
Brows ward off sweat running down the forehead from the eyes. Eyelashes cover the eyes from bright light, and also protect them from dust, midges and grains of sand that can damage them.
In the armpits and groin there are special sweat glands that secrete thicker and more oily sweat. It is often assumed that it contains sex pheromones and that these gradually evaporate from the surface of the hair. The presence of sex pheromones in humans has not yet been clearly proven. However, women can distinguish the smell of men's axillary sweat and react to it differently depending on its composition - this has been proven quite reliably (see, for example, here). In addition, in these areas, the skin usually touches the skin, and from puberty (when a person becomes much sweaty) hair protects the skin from chafing and diaper rash. Pubic and armpit hair by itself can serve as a signal of reaching puberty.
Sergey Glagolev
