Final answer:
The variation in skin color among humans is due to evolutionary adaptations and chance. While dark skin protects against excessive UV radiation, light skin allows for the absorption of vitamin D in regions with lower levels of UV radiation.
Step-by-step explanation:
Not all humans have dark skin because the amount of melanin, the pigment that determines skin color, varies across populations due to evolutionary adaptations and chance. While dark skin provides protection against excessive UV radiation in sunny regions, it can limit the production of vitamin D in areas with less sunlight. Lighter skin allows for greater absorption of vitamin D in regions with lower levels of UV radiation.