What is melanin and why do skin tones vary?

The answer

Melanin is a pigment produced by melanocyte cells in the basal layer of the epidermis. All humans have roughly the same number of melanocytes (about 1,000 to 2,000 per square millimeter), but genetic variations determine how much melanin is produced, what type (eumelanin producing brown/black tones versus pheomelanin producing red/yellow tones), and how it is distributed in surrounding skin cells. Melanin's primary function is protecting DNA from UV radiation damage by absorbing and scattering UV photons. Human skin color evolved as a balance between two selective pressures: darker skin in equatorial regions protects against UV-induced folate destruction and DNA damage, while lighter skin at higher latitudes allows sufficient UVB penetration for vitamin D synthesis. This is why indigenous populations closest to the equator tend to have the darkest skin, with a gradient toward lighter skin at higher latitudes.