If you want to ask about the most bizarre organ changes that have appeared in the process of human evolution, , that is, why does our skin degenerate and lose hair?
You must know that the ancestors of mammals worked hard for 200 million years to evolve hair to maintain a constant body temperature . As a result, humans spent 
million years degenerating the hair all over their bodies to only a little .
Scientists still cannot fully explain why humans became " naked apes ", so there is the hypothesis of " sea ape evolution ".
Why is it said that human skin is very weird ? Because you can find it by comparing it with other hairless mammals in nature.
Human skin
Weird skin
Let me make it clear that people do not have body hair, but that body hair has degraded to the point where cannot cover the whole body's skin , which is equivalent to being "naked".
There are also mammals in nature that are hairless like humans, but their life patterns are completely different from humans.
First of all, it is marine mammals . For example, cetaceans have the most obvious hair degradation. They have no body hair, so cannot go ashore at all , and cannot leave the water .
Cetaceans cousins hippos also have no hair protection, so dare not bask in the sun on the shore in broad daylight and have to soak in the water.
Like those pinniped animals that can go ashore to bask in the sun, such as sea lions , fur seals , etc., they still have hair on their bodies.
Whales have lost their hair.
Human beings are different. We do not need to live in the water , but live on the shore.
Human skin does not need to use water as a protection . On the contrary, humans can live well even in dry environment.
The next one is naked mole rat . It is just like a human being, skin is exposed and only a few hairs are on the surface.
They cannot leave their underground lair , otherwise they will be sunburned by the sunlight from the outside.
Naked Mole Rat
Although sun exposure will increase the risk of skin cancer in humans, human skin is not afraid of sunlight and we can live in the sun.
It stands to reason that human skin is actually not that thick. Compared with elephants' thick skin that resists direct sunlight, human skin is simply unreasonable.
What is so magical about our skin?
Elephant skin is super thick
Magical skin
The thickness of human skin is only 0.5 to 4 mm. It is really very thin. A slight scratch may cause skin rupture and bleeding..
Compared to the elephant .8 cm , bullets cannot penetrate the skin of , and our skin looks very fragile.
Human skin is divided into epidermis and dermis two parts. The main component of the epidermis is cuticle , which are dead cells of , and these cells come from the germinal layer of the epidermis.
The germinal layer keeps dividing cells , adding to the stratum corneum.
The epidermis is just a layer of flat epithelial tissue , and the dermis is a dense connective tissue .
The structure of the skin
There are capillaries and nerves in the dermis, and pain can be felt. Sometimes when a person gets a scratch, it only hurts the surface of the skin, so there is no bleeding, no scarring, or even much pain.
Once the dermis is injured, not only bleeding and pain may be left with scars.
More importantly, the dermis has rich capillaries , and infection will occur if not handled properly.
The dermal layer also has hair follicles , which secrete oil and adhere to the epidermis for protection.
So the bath mud that we wash off in the shower is mostly exfoliated cutin from the epidermis, also called dead skin .
Hair follicles on the skin
In addition, the skin also has a part of subcutaneous tissue , which is mostly muscle and fat . After the dermis is destroyed, it serves as a buffer to prevent damage to deeper tissues.
The total weight of the skin is about accounting for 16% of the body weight. The skin area is about .2 to 2.0 square meters . The thickness of the skin in different parts is different, but it is difficult to exceed 4 mm.
Skin is the first line of defense of our immune system . Although it is very thin, is very tough . General viruses and bacteria cannot break through the skin and enter the human body.
The first line of defense of human immunity-skin
Because the epidermal layer of the skin is basically composed of dead cells, germs cannot infect the dead cells, and they are helpless with the stratum corneum of the epidermis.
But once skin is injured, germs will enter from the wound, follow the capillaries in the dermis, and reach the whole body.
So, how do such thin cells resist the sun’s ultraviolet rays?
Sunbathing
Melanin
The epidermal layer of the skin is divided into two parts, stratum corneum occupies 80%, and the remaining 20% of is germinal layer .
As mentioned earlier, the germinal layer mainly provides dead cells for the stratum corneum, but the function of the germinal layer is more than that. It contains a special substance- melanin .
Melanin is not actually unique to humans. Almost any animal or plant has , which is a biological pigment .
But there are very few creatures that can evolve the use of melanin to the ultimate like humans, so that even if we don't have hair all over our bodies, we are not afraid of the sun.
The birthplace of melanin
If cats and dogs are shaved off and then placed in the sun, cats and dogs will get sunburned, indicating that the hair of cats and dogs has a certain sun protection effect.
But people will get tanned if they stand in the sun. Even if they get sunburned, they can heal quickly.
This is because our melanin is playing an important role.
Under the induction of ultraviolet rays, melanocytes secrete melanin , which plays the role of absorbing ultraviolet . If exposed to the sun for a long time, melanin will be produced in large quantities, so skin will turn black .
Being tanned
Generally speaking, melanin will fade away , as long as does not expose to the sun for a period of time .
For example, every year in military training, students are very tanned, but after one semester, everyone gradually becomes fairer.
However, some humans have produced a gene , which can still produce melanin even when is not exposed to sunlight , so that human skin is always black . This gene produces black people .
Because black people have lived in Africa for a long time, through natural selection, they are born with melanin for sun protection.
Of course, melanin, as a pigment secreted by cells, is not necessarily affected by sunlight. The human body itself will also secrete melanin without ultraviolet rays, such as dark circles .
Black people
Dark circles under the eyes are caused by staying up late, which leads to endocrine disordersthe appearance of melanin precipitation.
and have spots , which is because locally secretes too much melanin . These are all related to human health .
In short, the skin of human beings does not appear to be exposed to the sun but it is the strongest sun-resistant skin in the biological world , which puzzles scientists very much.
Because there will be a hairless variant of in chimpanzees, but it is difficult for it to survive in the wild because it will be exposed to the sun directly and get sick .
The genetic mutation of with hair all over the body will also appear in humans. Such children are called " hairy children ". People often think that this is the atavism of and .
Scientists have different opinions on why humans have evolved to lose all their hair, but none of them can stand up to careful consideration.
Hairless Orangutan
The Evolution of Skin
Some scholars mentioned that the reason why human skin is in the naked form it is today may be because humans lived in the sea at a certain period.
So our skin morphology is far from that of land mammals , and more is biased towards marine mammals .
Human fossils have a blank fault between .2 million and 70,000 years ago, which is considered to be the sea ape period.
And some people believe that some sea apes did not return to land, but continued to live in the water , and it is very likely that they differentiated into " mermaids ".
The hypothesis of the sea ape has not been supported by the scientific community, but it can well explain the origin of human skin.
Mermaids are considered to be descendants of sea apes
However, there is one thing that the sea ape hypothesis cannot explain, that is, according to this evolutionary route, human skin will be as fragile as cetaceans.
However, it turns out that human skin is very powerful , far more than that of whales and dolphins, which cannot leave the water and be exposed to the sun.
A theory currently supported by the scientific community is that humans shed their hair and evolved bare skin in order to better dissipate heat .
This is vividly reflected in African Maasai people . Animals in Africa are usually nocturnal. They will not hunt when the sun is high because they do not have a powerful heat dissipation system.
Animals on the African grassland lie dormant day and night.
African Maasai people are able to look for lions on the grassland at noon, because the lion's defensive ability is the weakest at this time.
It can be seen that our ancestors also used this method to hunt in Africa, and thus became the top of the food chain.
After leaving Africa, the latitude of the migration route increased. At this time, humans had degenerated and lost a lot of hair, so could only wear clothes to keep out the cold.
In fact, as humans migrated northward their hair grew back to a certain extent. For example, white Europeans have thicker body hair than black Africans .
But compared to other land mammals, it is still equivalent to being naked.
Humans put on clothes
It is precisely due to evolution that human skin has developed a strong sun protection ability, which allows us to avoid the sun without using hair.
