The standard explanation for lighter skin in Europe is the vitamin D hypothesis: at higher latitudes, where ultraviolet radiation is weaker and more seasonal than in equatorial environments, individuals with lighter skin synthesize vitamin D more efficiently than those with darker skin. Over many generations, natural selection would favor lighter skin in low-UV environments because of the health costs of vitamin D deficiency.
This explanation is described by Mark Thomas as “the most robust theory” for skin pigmentation change, but it has complications. If lighter skin were so strongly advantageous in the low-UV conditions of northern Europe, why did Western Hunter-Gatherers apparently maintain darker pigmentation for thousands of years after first arriving in Europe? This is the puzzle that the ancient DNA data pose.
Several hypotheses have been advanced. One possibility is that the Western Hunter-Gatherers’ diet provided adequate vitamin D through food sources — marine fish, fat from terrestrial animals, and other items rich in the vitamin — reducing the selection pressure for lighter skin. Hunter-gatherer diets are often rich in fat-soluble vitamins including vitamin D, and if dietary intake was sufficient, the selection pressure for cutaneous synthesis might have been weaker than assumed.
Another possibility is that lighter skin variants, though they existed in some European populations (including Scandinavian hunter-gatherers and some groups in the east), were not yet present in or strongly selected in the Western Hunter-Gatherer populations of southwestern Europe. The genetic variants most strongly associated with lighter European skin today are substantially more common in agricultural and steppe-herder populations than in hunter-gatherers, suggesting that the spread of lighter skin in western Europe was primarily driven by the demographic changes of the Neolithic and Bronze Age rather than by selection within the hunter-gatherer population.