Academy Scientists Offer New Theory on the Development of Human Skin Color

SAN FRANCISCO (July 2000) For years, scientists have thought that human skin color is related to the amount of ultraviolet radiation (UVR) from the sun that people are exposed to. It has long been recognized that indigenous peoples with darker skins inhabit areas with higher UVR and people with lighter skins inhabit areas with lower UVR. But, up to this time, the evolutionary explanation for this pattern has not been clear. In the current issue of the Journal of Human Evolution, Nina Jablonski, Curator and Irvine Chair of Anthropology at the California Academy of Sciences, and Academy Research Associate George Chaplin put forth a new theory that the present tapestry of human skin color was produced as the result of a complex biological balancing act of environmental factors that influence reproductive success and survival. Jablonski and Chaplin’s new evidence shows the amount of pigment in the skin regulates how much UVR will affect the body’s physiology. UVR has two very specific effects in the body that influence health and reproductive success. First, it can break down folate (a component of the vitamin B complex), which is essential for early development of the nervous system in human embryos and production of normal sperm. Second, it stimulates the body’s own production of vitamin D3, which is essential for the growth and maintenance of healthy bones and a normal immune system. According to Jablonski and Chaplin, the amount of pigment in the skin is an adaptation to UVR levels. Skin pigmentation in humans has evolved over time to permit just enough UVR to enter the body to stimulate production of vitamin D3, but not so much as to destroy necessary folate. Using clinical data and satellite data collected by NASA that measured UVR levels at the Earth’s surface, Jablonski and Chaplin created a map of UVR at different latitudes that demarcates three human skin tone zones. Zone 1, in the tropics, contains peoples with high levels of melanin, a dark skin pigment that acts as a natural sunblock. Zone 2 includes most of the United States and southern Europe. Residents of this zone historically have moderately pigmented skin that is easily altered through tanning. Zone 2 residents increase melanin levels to prevent folate loss and lighten their tan to take advantage of dimmer, briefer days during winter months. Residents of Zone 3, high-latitude and polar regions, face the greatest risk of vitamin D3 deficiency due to diminished UV exposure and compensate for this by eating vitamin D3-rich foods. Females tend to have lighter skin than males in every examined population, a "significant biological message" that Jablonski attributes to the need for women to generate extra vitamin D3 at critical times in their lives, particularly while pregnant or nursing an infant, despite the possible risk of folate loss from UV overexposure. As our dark-skinned ancestors migrated from the sun-drenched tropics into higher latitudes with different climates and environments, their skin had to adapt to changing sunlight levels. Jablonski points out that today’s globe-trotting population still needs to be aware of the health implications of moving between skin tone zones and to take appropriate precautions. People with light skin who are exposed to lots of sunlight face the risk of folate loss, while those with darker skin who are not exposed to enough sunlight face problems related to vitamin D3 deficiency. Women should take folic acid before and during a pregnancy because lack of folate may cause neural tube defects that can destroy or harm a developing fetus. Too little sunlight, however, restricts the body’s production of Vitamin D3, which is essential for absorbing calcium and preventing bone diseases. "Human skin color is the kind of the balancing act that evolution is great at perfecting," says Jablonski. But, she cautions, "this ancient and finely tuned geographic pattern of human skin color is being disrupted as we move to places distant from the lands of our ancestors and as we adopt practices (such as sunbathing) which our ancestors would never have dreamed of."

Having made a constructive contribution to the socially charged subject of skin color, Jablonski plans to pursue this work further alongside her paleontological research on the evolution or apes and monkeys in Asia and Africa.