Melanin pigmentation plays an important part in adaptation to the environment. It protects against the carcinogenic effects of actinic radiation. As man migrated away from the tropics, the lightening of the species might have been due to a decreased need for this protection as well as to the need of a lighter skin to allow proper synthesis of vitamin D. New adaptations to sunnier climates might be accompanied by reverse darkening. The reason for the darkening of hair from infancy into adulthood needs studying as well as the differences in texture and coloring between scalp and beard hair. The sociological implications of skin pigmentation are serious. Sufferers from pigmentary disturbances need strong emotional support in addition to the standard forms of treatment.
Allende MF. The Enigmas of Pigmentation. JAMA. 1972;220(11):1443-1447. doi:10.1001/jama.1972.03200110025005