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Human skin col­or ranges in vari­ety from the dark­est brown to the light­est pink­ish-white hues. Human skin pig­men­ta­tion is the result of nat­ur­al selec­tion. Skin pig­men­ta­tion in human beings evolved pri­mar­i­ly to reg­u­late the amount of ultra­vi­o­let radi­a­tion pen­e­trat­ing the skin, con­trol­ling its bio­chem­i­cal effects. The actu­al skin col­or of dif­fer­ent humans is affect­ed by many sub­stances, although the sin­gle most impor­tant sub­stance is the pig­ment melanin. Melanin is pro­duced with­in the skin in cells called melanocytes and it is the main deter­mi­nant of the skin col­or of dark­er-skinned humans. The skin col­or of peo­ple with light skin is deter­mined main­ly by the bluish-white con­nec­tive tis­sue under the der­mis and by the hemo­glo­bin cir­cu­lat­ing in the veins of the der­mis. The red col­or under­ly­ing the skin becomes more vis­i­ble, espe­cial­ly in the face, when, as con­se­quence of phys­i­cal exer­cise or the stim­u­la­tion of the ner­vous sys­tem (anger, fear), arte­ri­oles dilate. There is a direct cor­re­la­tion between the geo­graph­ic dis­tri­b­u­tion of UV radi­a­tion (UVR) and the dis­tri­b­u­tion of indige­nous skin pig­men­ta­tion around the world. Areas that receive high­er amounts of UVR, gen­er­al­ly locat­ed clos­er to the equa­tor, tend to have dark­er-skinned pop­u­la­tions. Areas that are far from the trop­ics and clos­er to the poles have low­er con­cen­tra­tion of UVR, which is reflect­ed in lighter-skinned pop­u­la­tions. Researchers sug­gest that human pop­u­la­tions over the past 50,000 years have changed from dark-skinned to light-skinned and vice ver­sa as they migrat­ed to dif­fer­ent UV zones, and that such major changes in pig­men­ta­tion may have hap­pened in as lit­tle as 100 gen­er­a­tions (~2,500 years) through selec­tive sweeps. Nat­ur­al skin col­or can also dark­en as a result of tan­ning due to expo­sure to sun­light. The lead­ing the­o­ry is that skin col­or adapts to intense sun­light irra­di­a­tion to pro­vide par­tial pro­tec­tion against the ultra­vi­o­let frac­tion that pro­duces dam­age and thus muta­tions in the DNA of the skin cells. In addi­tion, it has been observed that adult human females are con­sid­er­ably lighter in skin pig­men­ta­tion than males. Females need more cal­ci­um dur­ing preg­nan­cy and lac­ta­tion. The body syn­the­sizes vit­a­min D from sun­light, which helps it absorb cal­ci­um. Females evolved to have lighter skin so their bod­ies absorb more cal­ci­um. The social sig­nif­i­cance of dif­fer­ences in skin col­or has var­ied across cul­tures and over time, as demon­strat­ed with regard to social sta­tus and discrimination.