Human skin color reflects an evolutionary balancing act tens of thousands of years in the making. There’s a convincing explanation for why human skin tone varies as a global gradient, with the darkest populations around the equator and the lightest ones near the poles. Put simply, dark complexion is advantageous in sunnier places, whereas fair skin fairs better in regions with less sun.
… But actually, humanity’s color gradient probably has little to do with sunburn, or even skin cancer. Instead, complexion has been shaped by conflicting demands from two essential vitamins: folate and vitamin D. Folate is destroyed by the sun’s ultraviolent (UV) radiation. Whereas the skin kickstarts production of vitamin D after being exposed to those same rays.
Although the exact timing and causes are debated, many researchers agree that when humans lost their fur, it helped us stay cool while foraging as upright-walking bipeds in the sunny, open habitats of equatorial Africa. The tradeoff, however, was bare skin that was exposed to intense, year-round UV rays. In this context — roughly 1 to 2 million years ago — darker skin was likely better to protect folate stores.
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