What's the effect of staying in the tropics for a long time on your health?

You can find it online, hehe (the netizen above answered very well, too! Among primates, only human skin is mostly naked and has different colors. For a long time, geographers and anthropologists have realized that the skin color of various indigenous groups 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 can prevent skin cancer through evolution and deepening, but a series of findings have enabled 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 parted ways with their closest relatives, chimpanzees, and evolved into an independent ape species. Because chimpanzees evolved more slowly than humans, they can provide many clues about the anatomy and physiology of ancient humans. Chimpanzees have light skin color and most of their bodies are covered with hair. When they were young, their faces, hands and feet were light red. With the increase of age, these parts darken or produce spots due to sun exposure. 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?

The remains of primitive people (such as the famous remains of Lucy 3.2 million years ago) provide good clues 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 remains of the famous "Turkana Boy" (belonging to ergaster Homo sapiens) are bipedal creatures with long legs. It is possible to walk in big strides and long distances. These active primitive people are faced with the problem of how to cool down to keep their heads from overheating. Wheeler of Johns Mures University said that this problem can be solved by increasing sweat glands and reducing body hair, and once most body hair is removed, primitive humans face another challenge: protecting their skin from sunlight, especially ultraviolet rays.

The formation of skin shadowing effect.

The skin of hairless parts of chimpanzees contains melanocytes, which can synthesize melanin and absorb ultraviolet rays. When most body hair is removed, the function of skin to produce melanin becomes very important. Melanin is a natural sunscreen barrier and an organic macromolecule, which has the dual functions of physically and chemically preventing ultraviolet rays from damaging, absorbing ultraviolet rays, making them lose energy and neutralizing harmful chemical free radicals produced after ultraviolet rays irradiate the skin.

Anthropologists and biologists generally believe that people in tropical areas have high melanin content in their skin, which can prevent skin cancer. For example, e.cleaver of the University of California, San Francisco said that the incidence of squamous cell carcinoma and basal cell carcinoma in patients with xeroderma pigmentosum whose melanocytes were destroyed increased significantly after sun exposure. These two diseases are usually not difficult to treat. Malignant melanoma is a more deadly disease, but its incidence is only 4% of skin cancer, and it mainly harms people with lighter skin color. 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 began to ask questions: What is the role of melanin in human evolution?

Folic acid in human body

At 199 1, joblonski, one of the authors of this article, came across an article published in 1978 by richanl f.branda and John Wheaton, who are now working in university of vermont and Louisville universities respectively. These researchers found that the blood level of basic B vitamin folic acid in light-skinned people exposed to strong sunlight was 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 spina bifida and the vertebral arch can not 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 necessary to synthesize dna during cell mitosis. Male mice with chemical-induced folic acid deficiency have reduced sperm production and no reproductive ability. Although no similar human experiments have been done, 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. Our idea is supported by an article published in 1996 by pablo lapunzina, an Argentine pediatrician. 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 Africans go out of the equatorial region in search of various development opportunities. They encountered an environment where the annual ultraviolet radiation was significantly reduced. In this case, the high concentration of natural sunscreen in their skin may be unfavorable. Because there are too many melanin in dark skin, there are few ultraviolet rays, especially short-wave ultraviolet rays (uvb). 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 melanocytes in the skin for residents who migrated to the northern region.

The relationship between melanin and vitamin D synthesis is expounded in 1967, and 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 the help of Elizabeth Weatherhead from the Institute of Environmental Science Cooperation of the University of Colorado at Boulder. 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 prone to sunburn.

In the study, an interesting problem is that the actual skin color distribution does not strictly follow the above model. Eskimos in Alaska and northern Canada have darker skin colors than we predicted based on ultraviolet levels. 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 marine mammals. These foods make up for the problem that their skin color can't fully synthesize vitamin D in the northern region, and make their skin 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). Scientists often think about the reasons, and most of them think that it is caused by sexual selection-most men prefer women with lighter skin color. We believe that although this may be part of the reason, it is by no means the root cause. Women need more calcium than men in life, especially during pregnancy and lactation, 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 penetrating through the skin, thus improving the ability to produce vitamin D. In areas with strong light in the world, women have indeed experienced the severe test of natural selection, which not only protects the skin to the greatest extent, but also ensures 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. Africans born and bred have the longest history of skin color adapting to the environment, because anatomically speaking, modern people began to evolve there. Humans moved 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 people), which 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. Descendants of these colored people (originally called hottentots) can still be found in South Africa, and their skin color is obviously lighter than that of African aborigines near the equator. This is a remarkable adaptive change to the relatively low sunshine level at the southern tip of Africa.