The amount of UV light depends on the latitude: the highest concentration is spread throughout the equator, while the areas close to the Earth’s poles had it in scarcity. However, the South appears to have an irregular zone that does not follow the patterns.
The most lightly pigmented people are expected to be in North America (Alaska, Canada), Greenland, Northern Europe (Scandinavia, the United Kingdom), and Asia (mostly the Russian part and China). On the other hand, the most darkly pigmented people are might be found in Central Africa, South America (such countries as Brazil, Bolivia, and Venezuela), Southeast Asia (for example, Indonesia), and Oceanic states.
The distribution can be explained by the fact that people with much melanin are protected from UV rays, meaning that they are suitable to live in high-exposure areas. Meanwhile, lightly pigmented populations are historically accustomed to regions with a low UV index due to insufficient protection.
It is not randomly distributed: the lowest percentage of skin reflectance is associated with the 0° longitude (the equator), and the value increases for anything lower and higher than that. Dark skin tends to absorb UV light, while light skin reflects it. Genes can greatly impact one’s skin color regardless of the location, which explains albino people, so those populations may have an irregular amount of melanin for genetic reasons. UV exposure decreases blood folate levels, which is evident in the “patients” group.
High levels of folate guarantee conception and normal fetus development, which directly impact a population’s survival and adaptation. A dark skin tone is likely to be correlated with higher folate levels due to its protection mechanism against UV light.
High vitamin D levels prevent many diseases, especially the ones affecting the skeletal system, and facilitates the immune system. Thus, people were healthy and more fit to adapt to the environment. A light skin tone allows one to maintain a high level of vitamin D due to their skin not being very guarded against UV radiation, which is necessary for its synthesis. Both folate and vitamin D contributed to the survival and spread of people with dark and light skin tone, respectively. Eventually, those factors became less relevant, as a person can live anywhere regardless of their skin color nowadays.