Many people poorly understand why the human species is filled with so many different skin colors, but there is actually an answer to this question based on evolution, adaptation, and natural selection. I will try to clear up this question for you.
What causes differences in skin color?
Skin color is determined by four to six incomplete dominant genes, which code for skin pigments. Although skin color can be changed, it is primarily a genetic trait. These genes determine how much melanin is in your skin. Melanin is the human skin pigment, which ranges from yellow to brown and brown to black. Lack of melanin produces light skin.
Why does skin color vary?
In general, people with ancestors from tropical and equatorial regions have darker skin and people with ancestors from middle latitudes and polar regions have lighter skin. This is for many survival reasons. Hairy primate ancestors of humans originally had light skin. But when humans came along, we had much less hair on our skin. Because of this, the first humans in Africa needed darker skin to counter the affects that ultraviolet radiation had on their folate levels. When humans migrated to less sunny regions in the north, light skin reemerged because dark skin could not support enough vitamin D3. Still, some peoples living in northern regions, such as the Inuit, retained their darker skin probably because their diet of fish provided enough vitamin D3 to survive. Although dark skin does protect from ultraviolet rays much better than light skin, cancer resulting from sun exposure was not really a major factor in the evolution of human skin color, as death related to ultraviolet radiation does not occur until well after the reproductive age.
Here is a map from von Luschan, showing the skin color of indigenous populations around the world.
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