Dark skin ( a result of increased melanin production in equatorial populations), is likely a response to ultraviolet radiation because UV radiation causes:

Skin cancer

Explanation:

Melanin is a pigment derived from an amino called acid tyrosine. The most common form of melanin is called eumelanin, which is a polymer of dihydroxyindole carboxylic acids and their reduced forms. When a person is exposed to the ultraviolet light (UV) from the sun, the melanocytes will produce eumelanin to prevent the skin from burning and damage to the cell nuclei (where DNA is found) of the epidermis. This melanin production causes the skin to darken. The eumelanin in the skin then acts as a natural sunscreen by blocking the damaging effects of sunlight. So, skin darkens when exposed UV light, thus providing greater protection when needed by producing more eumelanin, but it also becomes more likely to develop melanoma, which is a type of skin cancer. This is because UV rays damage the DNA of skin cells. The DNA (deoxyribonucleic acid) is the genetic material that has the instructios to the growth and functioning of an organisms). Skin cancers begin when eumelanin protection is not sufficient and this damage affects the DNA of the genes that control the growth of skin cells. This results in a tumor, which is the uncontrolled growth of cells (in this case, skin cells) because there will be a mutation in DNA that affects the function of the cells.

most community's take hundreds of years to become stable