Final answer:
Skin color variation is due to evolutionary adaptations to ultraviolet radiation and the need for vitamin D synthesis, with melanin levels responding to sunlight exposure levels. These adaptations have influenced the distribution of different skin colors around the world, based on proximity to the equator and UV intensity.
Step-by-step explanation:
People have different colors of skin due to evolutionary adaptations for protection against ultraviolet radiation and vitamin D synthesis, influenced by the levels of exposure to sunshine in various parts of the world. Melanin is the primary substance that determines the color of skin, with darker skin producing more melanin, which provides better protection against UV damage and folic acid breakdown. However, too much melanin can restrict vitamin D production, which is crucial for calcium absorption and bone health. The balance of melanin in skin is therefore a result of evolutionary adaptations to different levels of sunlight, balancing the needs for UV protection and vitamin D production.
There are two types of melanin: eumelanin (which is black or brown) and pheomelanin (which provides a red color). In sunny regions near the equator, higher levels of melanin protect the skin from the sun's damaging effects. However, in areas with less intense solar radiation, like northern Europe, lighter skin allows for more efficient vitamin D synthesis, which is essential for strong bones and immune function.
The geographical location and distance from the equator played significant roles in this evolutionary process. As early humans migrated away from equatorial regions, natural selection favored lighter skin in those areas with less UV radiation to ensure sufficient vitamin D could be synthesized. Vitamin D is made by the skin when exposed to UVB radiation from sunlight, and this production can be filtered by pigments like melanin. Folate, another critical nutrient, is protected from the destruction caused by UV radiation by the presence of melanin in darker skin, indicating a dual role in maintaining health.