Golden Harvest
Economists realize that the skin color of each indigenous group is not randomly distributed. The darker-skinned ethnic groups are mostly distributed near the equator, while the lighter-skinned ethnic groups are close to the polar regions. For many years, it is generally believed that skin color is deepened through evolution to prevent
However, a series of findings allow us to establish a new framework to understand the reasons for the evolution of skin color.
Humans have gone from hairy to hairless.
The evolution of skin pigment is related to human hairlessness, and the relationship between them can be traced back to human history. At least 7 million years ago, our ancestors and their nearest relatives
, evolved into
This independent species. because
Their evolution speed is slower than that of humans, so they can provide a lot of information about distance.
about
and
Clues to the situation.
He has a light complexion and most of his body is covered with hair. When he was young, his face, hands and feet were reddish. With age, these parts become darker or spotted due to exposure to sunlight. It is almost certain that the earliest humans were light-skinned and covered in hair. Presumably, hair falls off first, and then skin color changes. But this raises a question: When did humans begin to shed their hair?
primitive
(For example, the famous Lucy 3.2 million years ago.
) provides a good clue for understanding the physical structure and lifestyle of our ancestors. The daily activities of primitive people such as Lucy more than 3 million years ago are similar to those of primates on the African prairie today. Every day, before going back to a safe place in the tree to sleep, they spend most of their time searching for food within five or six kilometers of Fiona Fang.
However, about 6.5438+0.6 million years ago, there was evidence that this lifestyle began to change significantly. The famous "Turkana boy"
(belonging to ergaster
), a biped with long legs and striding, may walk a long distance. These active primitive people are faced with the problem of how to cool down to keep their heads from overheating. Liverpool, England
Wheeler of the University of Wales says the problem can be solved by increasing sweat glands and reducing them.
To solve it, once most of it is removed,
Another challenge is to protect the skin from sunlight, especially ultraviolet rays.
The formation of skin shadowing effect.
The skin of the hairless part of chimpanzees contains
Melanin can synthesize and absorb ultraviolet rays. When humans removed most of it,
In the future, the function of skin to produce melanin is very important. Melanin is a natural sunscreen barrier and an organic macromolecule, which can physically and chemically block ultraviolet rays.
The dual function of injury can absorb ultraviolet rays, make them lose energy, and neutralize harmful chemicals produced after ultraviolet rays irradiate the skin.
Golden Harvest
It is generally believed that the high content of melanin in people's skin in tropical areas is to prevent
. For example: University of California, USA
E. Cliff on campus said,
Damaged coloring
Patient, after sun exposure
and
The prevalence of these two diseases has increased significantly, and these two diseases are usually not difficult to treat.
This is a more deadly disease, but the incidence rate only accounts for
4%, and mainly harms light-skinned people. The onset of all skin cancers is relatively late, and most patients get sick after the first delivery. Therefore, the explanation that skin color evolved only to protect the skin cannot fully explain the reason why human skin color deepens. So we started asking questions:
What role does melanin play in this process?
Folic acid in human body
At 199 1, Joblonski, one of the authors of this article, came across an article published at 1978 by Richanl F.Branda and John Wheaton, who are now in.
And the University of Louisville. These researchers found that the blood of light-skinned people exposed to strong sunlight basically
The content of folic acid is abnormally low. They also observed that folic acid would be lost by 50% in 1 hour when human serum was placed in a similar environment.
As we know, colleagues at the University of Western Australia are engaged in research on the main types of birth defects. Then, the significance of our above findings for human reproduction (and evolution) will be obvious. In the late 1980s, Fiana J.Stanley and Carol Bower have confirmed that folic acid deficiency in pregnant women will increase the risk of neurological defects in infants, such as
The vertebral arch can't completely wrap the spinal cord. Many research groups around the world have confirmed the correlation between the two, so people try their best to supplement foods containing folic acid and widely educate women about the importance of this nutrient.
Soon after, we found that folic acid not only leads to neural tube defects, but also plays an important role in many other aspects. Because folic acid is a cell.
Folic acid is necessary for the synthesis of DNA. Male mice with chemical-induced folic acid deficiency have reduced sperm production and no reproductive ability. Although I haven't done anything like this.
However, scholars at the Nijmegen Medical Center in the Netherlands recently reported that folic acid therapy can increase the sperm count of men with fertility problems.
Under the guidance of these findings, we put forward the hypothesis that the evolution of skin blackening is to prevent folic acid stored in the body from being destroyed. We got our idea from Argentina.
Pablo Lapunzina published an article in 1996. He found that three young and healthy women gave birth to babies with neural tube defects after being tanned by the sun in the early pregnancy. The evidence that folic acid is destroyed after irradiation strongly supports the known statement that ultraviolet radiation is harmful to DNA (leading to skin cancer).
The evolution of human skin
From120,000 years ago to10,000 years ago, human beings in Africa evolved and their skin deepened to adapt to the intense sunlight and heat near the equator. Modern Times
Going out of the equatorial region to look for various development opportunities, we encounter an environment where the annual ultraviolet radiation is obviously reduced. In this case, the high concentration of natural sunscreen in their skin may be unfavorable. Because there is too much melanin in dark skin, ultraviolet rays that penetrate the skin, especially ultraviolet rays with short wavelength.
Ultraviolet rays (UVB) are rare. Although the role of UVB is harmful, it undertakes an indispensable task-promoting the production of vitamin D in the skin. Black people in the equatorial region receive enough light all the year round, so enough UVB enters the skin to produce vitamin D, which is different outside the equator. After a long period of evolution, the solution to the problem is to reduce the skin of the residents who moved to the northern region.
About the relationship between melanin and vitamin D synthesis, in
It has been expounded in detail that scholars have confirmed the importance of vitamin D to the successful reproduction of human beings. Vitamin D can promote the absorption of calcium in the small intestine, thus ensuring the normal development of bones and maintaining the health of the immune system. In the past 20 years, Micheal Holick's research in Postan University School of Medicine has further confirmed the significance of vitamin D to development and immunity. His research team also confirmed that not all sunlight environments can have enough UVB to stimulate the synthesis of vitamin D. For example, in Postan near 42 degrees north latitude, there is not enough UVB in winter, and vitamin D can only be synthesized by skin after mid-March. We think this is another strong evidence of the evolution of human skin color.
In the early 1990s, we tried to find the source of the actual ultraviolet radiation level on the earth's surface, but there was no result. 1996, we got it.
Boulder campus
With the help of Elizabeth Weatherhead of the Cooperative Institute. She provided a database of surface ultraviolet radiation measurements taken by NASA's global positioning scanning satellite. Subsequently, we made a distribution model of ultraviolet radiation on the surface and satellite data of the amount of UVB needed to synthesize vitamin D necessary for human body.
We found that according to the synthesis of vitamin D, the earth's surface can be divided into three regions: tropical region, subtropical region and temperate region, and circumpolar region south of 45 degrees north latitude. The first area has enough UVB all year round, and people have enough opportunities to synthesize vitamin D. The second area has insufficient UVB for at least one month every year, and the last area can't meet the requirements all year round. This distribution can explain why the local people in the equatorial region have darker skin color, while people in the subtropical and temperate regions have lighter skin color and can get a tan, while people around the polar regions have lighter skin color and are easy to get a tan.
In the study, an interesting problem is that the actual skin color distribution does not strictly follow the above model.
And northern Canada
Our skin color is darker than we predicted according to the ultraviolet level. There may be two reasons for this: first, they arrived there for a relatively short time and moved to North America about 5,000 years ago; Another reason is that they are used to eating foods rich in vitamin D, especially fish and fish.
. These foods make up for their skin color.
The problem that vitamin D can't be synthesized completely makes its color darker.
Our analysis of synthetic vitamin D also makes us understand another feature about skin color: usually women's skin color is slightly lighter than men's (data show that women's skin color is 3-4% lighter than men's). Classic of scientists
Most of the reasons come down to sexual selection-most men prefer lighter-skinned women. We believe that although this may be part of the reason, it is by no means the root cause. In a woman's life, especially in
And most of these calcium must be obtained from food. Therefore, we suggest that women's lighter skin color than men is conducive to more UVB passing through the skin, thus improving the ability to produce vitamin D. In areas with strong light in the world, women have indeed experienced it.
The severe test is to protect the skin to the greatest extent and ensure the synthesis of vitamin D.
The dual role of culture and biology
About 654.38 million years ago, when people moved to different parts of the world, their skin color adapted to different environmental conditions. Native-born locals
Skin color has the longest history of adapting to the environment, because from
Technically, modern people began to evolve from there. Humans migrated from one continent to another, first in Asia, then in Australia, and finally in America, and their skin color changed to some extent. However, it is very important that at that time, human beings can already protect themselves with clothes and shelter. In some places, they have been able to obtain foods rich in vitamin D (such as Inuit), and these two factors have greatly affected the speed and degree of human skin color evolution.
Africa is a continent with complex environment. The earliest immigrants left the equator and moved to South Africa. these
Descendants of the Toisan people (formerly known as hottentots) can still be found in South Africa, and their skin color is better than that in Africa near the equator.
Obviously shallow. This is a remarkable adaptive change to the relatively low sunshine level at the southern tip of Africa.
Interestingly, in South Africa, people's skin color is not uniform. Nowadays, the skin color of Bikwasan language family, a Bantu language family in South Africa, is obviously darker. From the history of South Africa, we know that Bantu people lived in South Africa for a short time, and only moved from places near the equator in the past 1000 years.
Move here. Kwasan and Bantu, such as
The difference of people's skin color shows that the length of settlement in an area is very important to the evolution of skin color.
In modern human history, cultural habits may also have an important impact on the evolution of skin color. from
East and west banks
This effect can be seen from.
People in the West Bank speak the so-called Niha language.
Nile-hammett people think they have settled here for 6000 years. These residents have dark skin, slender figure and developed limbs, and have excellent biological adaptability to heat dissipation and strong ultraviolet radiation. By contrast,
East coast
The farmers and herdsmen settled there only in 2000. These early Arab immigrants from Europe wore well-protected clothes and designed lightweight awnings to shade the sun. Through this unique cultural custom, they also adapted to the environment similar to the other shore. Without such clothes, their skin should also start to turn black. Generally speaking, the more modern immigrants are, the more they adapt to the environment of the relocation place through cultural changes rather than biological changes.
Risks brought by immigration
Although the overall health of human beings has been greatly improved in the last century, there have been some previously rare diseases or diseases in some ethnic groups.
One of them is skin cancer. In light-skinned groups,
and
The most common. There is also rickets caused by the lack of vitamin D in black people. Why is this happening?
People move from an area in one sunlight mode to an area in another.
Its biological and cultural adaptability will not keep pace immediately. in the U.S
Or northern Australia, where light-colored people from northern Europe are getting stronger.
Pay the price—
And skin cancer, not to mention the unknown cost of folic acid loss to human life. On the contrary, some dark forces in South Asia and Africa have arrived.
In Britain, northern Europe or the northeastern United States, due to the lack of sunlight and vitamin D, the incidence of rickets and other diseases related to vitamin D deficiency is high.
The ability of skin color to adapt to various migration environments through long-term evolution reflects the importance of skin color to human survival. But this instability of skin color also makes it one of the most worthless attributes that determine the evolutionary relationship between people. Early western scientists used skin color to describe human race wrongly, but the beauty of science is that it can correct its own mistakes. Our current knowledge about skin evolution shows that, just like most physiological attributes of human beings, the difference of skin color can also be explained by human adaptation.
The result of adapting to the environment. We expect that one day, mankind will erase the traces of mistakes in ancient science and replace them.
With the deepening understanding of evolution, our different skin colors will be solemnly regarded as proof of the evolution of our species.
reference data