Equatorial dark skin evolved to block UV light from destroying folate.

Evidence

Much evidence now indicates that the shades of skin color found among different populations—from dark to light—across the globe represent a genetic adaptation to the intensity and frequency of exposure to ultraviolet light, including both UVA and UVB. Near the equator, where the sun is intense year-round, natural selection favors darker skin, as seen in populations near the equator in Africa, New Guinea, and Australia. This is because both UVA and UVB light can dismantle the folate present in our skin if not impeded or blocked by melanin. Folate is crucial during pregnancy, and inadequate levels can result in severe birth defects like spina bifida. This is why pregnant women are told by their physicians to take folic acid. In men, folate is important in sperm production. Preventing the loss of this reproductively valuable folate means adding protective melanin to our epidermis, which has the side effect of darkening our skin. — Joseph Henrich, The Secret of Our Success, 279

Categorization

• XREF: Connects to the folate-protection and vitamin D tradeoff literature; evolutionary-medicine framing familiar from adaptationist accounts of skin color. — connects to: Baltic agriculture led to genetic selection for very light skin — Joseph Henrich, The Secret of Our Success: How Culture…, 281; Mosses grow black and crusty in sun because anthocyanin shields them from UV — Robin Wall Kimmerer, Gathering Moss: A Natural and Cultural…, 518

Broader Connections

• Mosses grow black and crusty in sun because anthocyanin shields them from UV #exemplifies (0.85) — Robin Wall Kimmerer, Gathering Moss, 518
• Baltic agriculture led to genetic selection for very light skin #qualifies (0.78) — Joseph Henrich, The Secret of Our Success, 281