Final answer:
The mutation in the SLC24A5 gene for light skin occurred between 1875 to 225 years ago in Europeans. Dark skin is adaptive in equatorial regions as it protects against sun's UV radiation and prevents excessive vitamin D synthesis.
Step-by-step explanation:
The mutation in the SLC24A5 gene that produces exceptionally light skin in most Europeans occurred before the appearance of HIV, between 1875 to 225 years ago. The mutation is most prevalent in northern European populations and may have been selected for due to exposure to diseases such as plague and smallpox. It is believed that the mutation may have been spread through Viking invasions from Northern Europe.
In equatorial regions, dark skin is adaptive because it provides protection against the damaging effects of the sun's ultraviolet (UV) radiation. Dark skin has high levels of melanin, which acts as a natural sunscreen and helps prevent sunburn and DNA damage. Additionally, dark skin helps prevent the synthesis of excessive amounts of vitamin D in areas where sunlight is strong, which could be harmful.