People have different colored skin because our ancient ancestors migrated to various regions of the world where their skin adapted to protect them from the intensity of ultraviolet radiation intrinsic to the area where they moved to. In any given location, the amount of ultraviolet radiation that penetrates the atmosphere to reach that site depends on how far from the equator the site is situated. The amount peaks at the equator and is lowest at the north and south poles and decreases gradually in between these two extremes.
One’s skin color is an evolutionary adaptation that reflects the position on the planet where one’s ancient ancestors once lived for numerous generations. People with mixed heritage have a complexion that reflects their mixed ancestry. Since adaptation to a regions level of ultraviolet radiation requires many generations, appropriate skin pigmentation for a given region mostly occurs in populations that have lived in the same region for thousands of years.
In fact, it takes about 10,000 to 20,000 years of natural selection for changes in skin pigmentation to provide the appropriate level of protection for the intensity of ultraviolet radiation in that region. A good illustration of this is the lack of a wide range of skin tones in the Natives of the Americas. These Natives arrived in North America from Asia about 15,000 years ago. Later, from Alaska, they migrated south, populating every region of the Americas. Their relative short span of time in some regions of the Americas has limited their range of skin tones. In other words, their skin tone has not had time to sufficiently adjust to the different latitudes of the continent. Consider Haiti for example, the Natives only arrived there about 2000 years ago. As fairly newcomers to the Americas, when compared to people in other regions of the world, their skin tones do not vary as much between those who live closer to the poles versus those who live near the equator. Native Americans who live in the tropics are lighter in complexion than equatorial Africans who are the most pigmented people in the world. Equatorial Africans have been living in the tropics for at least 100,000 years. The skin tone of equatorial Africans had enough time to adapt to the intensity of ultraviolet radiation bombarding the tropics. Most Haitians are recent descendants of tropical Africans explaining why most of us are dark-skinned.
The darker one’s skin tone is, the more sun protection it offers. Dark skin also protects from developing skin cancers like melanomas. It also protects against some forms of infertility, as well as against some forms of fetal anomalies, particularly anencephaly.
Lighter skin also has its advantages. Light skin facilitates the absorption of calcium, an essential mineral for bone health and for such physiologic actions, like nerve conduction and muscle contraction, among numerous other enzymatic reactions for which calcium is a co-factor. Vitamin D is the hormone that regulates calcium absorption and it also regulates the activity of at least 5% of the human genome. For these reasons, adequate vitamin D level is essential for preventing numerous medical conditions. Nowadays, doctors routinely measure their patients vitamin D levels to help safeguard their health.
While advantageous for vitamin D production, light skin facilitates the destruction of folic acid by ultraviolet radiation that readily penetrates the bodies of light skinned people. Nonetheless, people living in cold climates do not have to worry about the detrimental effects of the little ultraviolet light radiation in their region. That detrimental effect becomes a concern when they move to the tropics where they risk suffering from folic acid deficiency.
Among people living in cold climates, Europeans have the lightest colored skin. Even Asians who live on the same latitude as Europeans do not have skin as light. European skin is so light because when their ancestors migrated out of Africa 50,000 years ago, these Africans first settled in the Middle East and developed a diet based on grains and less on animal fat, a rich source of vitamin D. Their low dietary source of Vitamin D resulted in their skins becoming even lighter to further enhance their own production of Vitamin D. As a result, Western Europeans are lighter than expected for their latitude.
This dependence on grains, occurred after the Agricultural Revolution and for that reason, it is believed that their very light skin was acquired within the past 10,000 years. Some sources put it at less than 5,000 years. A study of genes from 8000-year-old skeletons in Spain and Hungary found that these remains were of dark-skinned people. On the other hand, skeletal samples of similar remains from Sweden showed them to have had light skin. In other words, by 8000 years ago, light skin was not yet ubiquitous in Europe. One important variant of the gene (SLC24A5) for light skin in Europe arose about 30,000 years ago in the Middle East but did not become common in Europe until the past few thousand years.
The skin color of people native to an area is the result of a delicate balance between such things as having adequate bone strength while avoiding the detrimental effects of having too much ultraviolet light radiation penetrate the skin, destroying folic acid and harming cells. When there is a mismatch in a person’s skin tone with the region where that person lives, that person becomes susceptible to problems arising from not producing enough vitamin D or from destroying too much folic acid. It is for this reason that the US government encourages light-skinned people to wear sun protection when they are in the tropics. Equally important, the US government fortifies foods with Vitamin D to aid dark- skinned people living in regions with low sunlight exposure. It is crucial that we understand the health issues related to our skin tones so that we may take appropriate measures depending on where we live.
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