A certain amount of ultraviolet radiation must penetrate the outer skin layer in order for the body to produce vitamin D 3 . This is important because D 3 is needed for the intestines to absorb calcium and phosphorus from food for bone growth and repair. Calcium is also necessary in adults to maintain normal heart action, blood clotting, and a stable nervous system. Pregnant women who are deficient in folic acid are at a higher risk of having babies with neural tube defects . P eople who live in far northern latitudes, where solar radiation is relatively weak most of the year, have an advantage if their skin has little shielding pigmentation.
Nature selects for less melanin when ultraviolet radiation is weak.
Women who had prolonged vitamin D 3 deficiencies as girls have a higher incidence of pelvic deformities that prevent normal delivery of babies. They have moderately heavy skin pigmentation despite the high latitude at which they live. While this is a disadvantage for D 3 production, they apparently made up for it by eating fish and sea mammal blubber that are high in D 3 . B efore the mass human migrations of the last 500 years, dark skin color was mostly concentrated near the equator and light color progressively increased further away, as illustrated in the map below. In fact, the majority of dark pigmented people lived within 20 of the equator.