Final answer:
Humans have different skin colors due to evolutionary adaptations to sunlight exposure, with melanin levels in skin varying according to geographical location to protect against UV radiation or to facilitate vitamin D production.
Step-by-step explanation:
Humans exhibit different skin colors due to evolutionary adaptations to varying levels of ultraviolet (UV) rays from the sun based on geographical location. The primary substance determining the color of human skin is melanin, which is present in the epidermis. Melanin provides protection against UV radiation; populations living nearer the equator where sunlight is more intense typically have higher levels of melanin and therefore darker skin, as this helps protect them from the harmful effects of the sun. Conversely, in regions with weaker sunlight, lighter skin evolved to facilitate the production of vitamin D beneath the skin.
This variation in skin pigmentation occurred as early humans migrated from Africa and settled in different parts of the world, with skin color changing gradually and forming a spectrum rather than discrete categories. This has shown that biological races as social constructs do not align with the biological reality of human traits. Thus, the differences in skin pigmentation are a reflection of human adaptation to the environment, and not markers of distinct races.