Rapid changes in nature demanded quick adaptation and well-functioning brains, which in turn demanded steady blood temperature. Most of the hair disappeared, and sweat glands, which are distinctive to humans, were developed. These, along with blood vessels in the naked skin surface, were easily damaged by the hot sun of the equator, and thus dark skin began to evolve. It is able to withstand ten times more sun than white skin before the skin burns. Blood contains many Vitamin B substances, among them folates, which are best protected in dark skin.
Folates are needed in vital processes during cell division. A deficiency of folates, for example, more frequently causes women to give birth to children with spina bifida, and causes men to be less fertile. It has been proven that Vitamin B substances are particularly sensitive to solar radiation, but other vital substances in the blood can also be broken down by a large amount of sunlight. Dark skin also evolved to protect the blood vessels in the skin as well as the substances in the blood.
Human migration can be mapped by examining genes. Some humans migrated eastward from Africa towards Indonesia, continually remaining in the tropics, and kept their dark skin colour. Others migrated north. This occurred several times, and each time their skin colour became lighter. This happened so that they would be able to get enough Vitamin D. White skin needs only one-sixth as much sun as black skin to make enough Vitamin D.
The sun's effect on health is underscored by the fact that several different mutations have yielded light skin. Light-skinned Asians have other mutations than Europeans have. Dark-skinned immigrants even today encounter the same problem when they come to our unsunny country. The seriousness of Vitamin D deficiency is revealed by the fact that rickets, «the English disease», came in the wake of the Industrial Revolution and was a scourge on cities where there was little sunlight and where children and adults worked indoors and in mines.
Up to 80 per cent of the children in the cities became ill. Skin colour maintains a balance between Vitamin D production and folate breakdown. Generally speaking, skin colour owes to a population group's having solar radiation where they live, and to the length of time they have lived there. We are able to follow skin colour changes during human migration northward towards Eastern Asia, across the Bering Strait and south towards the equator on the American continent.
Here, skin colour has not yet become completely dark. The 15,—20, years that the American Indians have been there is too short a time. Eskimos stopped along the way, but kept some of their dark skin colour because they ate Vitamin D-rich seafood. Their diet made completely white skin unnecessary for them to survive.
Jablonski and Chaplin predicted the skin colors of indigenous people across the globe based on how much ultraviolet light different areas receive. Graphic by Matt Zang, adapted from the data of N. Jablonski and G. Jablonski later came across three documented cases in which children's neural-tube defects were linked to their mothers' visits to tanning studios during early pregnancy.
Moreover, she found that folate is crucial to sperm development -- so much so that a folate inhibitor was developed as a male contraceptive. But why do some people have light skin? As far back as the s, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body's need for vitamin D. The vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. The need for vitamin D during pregnancy may explain why women around the globe tend to have lighter skin than men.
Unlike folate, vitamin D depends on ultraviolet light for its production in the body. Loomis believed that people who live in the north, where daylight is weakest, evolved fair skin to help absorb more ultraviolet light and that people in the tropics evolved dark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations. By the time Jablonski did her research, Loomis's hypothesis had been partially disproved.
The next step was to find some hard data correlating skin color to light levels. Until the s, researchers could only estimate how much ultraviolet radiation reaches Earth's surface.
Three years ago, Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. To their delight, there was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin. Jablonski went on to show that people living above 50 degrees latitude have the highest risk of vitamin D deficiency.
Humans have spent most of their history moving around. To do that, they've had to adapt their tools, clothes, housing, and eating habits to each new climate and landscape. But Jablonski's work indicates that our adaptations go much further. In addition, the Inuit have been in the far north for only about 5, years. This may not have been enough time for significantly lower melanin production to have been selected for by nature.
In the United States and other developed nations, milk is now usually fortified with vitamins D and A in order to prevent developmental problems such as those described above. However, the popularity of carbonated soft drinks and other alternatives to milk along with a decrease in the amount of time spent outdoors has led to a considerable rise in the rate of rickets disease. Not surprisingly, vitamin D deficiency is most acute in the winter in temperate and colder zones , especially among people of African ancestry.
There is a growing epidemic of vitamin D deficiency in the United States. The rate for African Americans in particular was far worse. There is also a strong correlation between the amount of sunlight that children are exposed to and whether or not they will develop multiple sclerosis as adults.
Most cases of this degenerative neural disorder are in the temperate regions of the world where the sunlight is rarely intense. Children growing up in tropical and subtropical regions rarely develop MS regardless of where their ancestors came from. This protection apparently continues for those who move to far northern or far southern regions after 16 years of age.
What processes are responsible for this protection from MS and its possible relationship to skin color are unknown. They suggest that this is probably due to the fact that women have far higher calcium requirements during their reproductive years.
Mate selection preference and other cultural practices may also be partly responsible for this gender difference in skin coloration. Skin Color Distribution Around the World.
Before the mass global migrations of people during the last years, dark skin color was mostly concentrated in the southern hemisphere near the equator and light color progressively increased f a rther away, as illustrated in the map below.
Data for native populations collected by R. Biasutti prior to Such a non-random distribution pattern of human skin color was predicted by Constantin Wilhelm Lambert Gloger , a 19th century German zoologist. In , he observed that heavily pigmented animals are to be found mostly in hot climates where there is intense sunshine.
Conversely, those in cold climates closer to the poles commonly have light pigmentation. Presumably, t he relative intensity of solar radiation is largely responsible for this distribution pattern. There are exceptions to Gloger's rule in the animal kingdom. In some species the survival value of having a camouflaged body can be more important than the selective pressures of ultraviolet radiation.
This is the case with arctic hares, which are white like snow in the winter and mottled black, brown, and gray similar to the rocky ground in their region after the snow melts in the summer. These color changes make it more difficult for arctic foxes to prey on them. Among humans, mate selection preferences may counter some of the evolutionary trend in skin color predicted by Gloger. Ultraviolet radiation is also blocked to some degree by hair, clothing, smog, fog, smoke, clouds, and trees.
The Inuit case described earlier suggests that diet may additionally be a significant factor in some societies. NEWS: In the February issue of the Journal of Nutrition , a research team led by Lisa Bodnar of the University of Pittsburgh School of Public Health reported that among first-time pregnant women from that city who participated in their study, only Tests of umbilical cord blood indicated that only 7. The authors concluded that prenatal vitamins should include higher doses of vitamin D, especially in the northern regions of the U.
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