Skin color is a complex, genetically-determined trait, which is visible due to the production of a pigment called melanin. Two types of melanin exist — pheomelanin (red-yellow), and eumelanin (black-brown). The skin color of a person depends on not only the ratio of the types of melanin to each other, but also to the amount of overall melanin produced in their cells. Melanin is important because it provides protection from the ultraviolet (UV) radiation that comes from the sun. UV radiation is the most intense near the equator and less intense towards the poles, as shown in Figure 1. This radiation enters cells and can damage DNA, which can lead to various forms of skin cancer, such as melanoma. As seen in Figure 2, this type of skin cancer is found all over the world.
Scientists are interested in understanding the genetics and evolutionary selective factors that impact skin color. They use this information to determine how those factors led to the color distribution patterns that we see today. One area that scientists study is the relationship between skin color and latitude. A geographer collected data to create skin color maps. One such map (Figure 3A) compares the skin color distributions of native populations at various latitudes. Another researcher analyzed over 100 populations and created skin reflectance graphs, as the one shown in Figure 3B. Skin reflectance is a measure that quantifies how much light is reflected off the skin. Darker skin has higher absorbance and lower reflectance. Conversely, lighter skin has less absorbance and higher reflectance.
In addition to UV exposure, vitamin D is another factor that seems to impact the variation of skin color by latitude. Vitamin D is made available to the body in two ways: through diet (for example, foods like fish, cheese, egg yolks, and vitamin D supplements are high in vitamin D), and through sun exposure. Although UV radiation can be harmful to skin cells and the DNA located within, penetration of UV light into those cells is actually necessary to help naturally synthesize vitamin D for use within the body. Vitamin D is an essential nutrient needed for proper calcium absorption and healthy bone growth. Low levels of vitamin D can lead to bone issues, such as rickets in children and osteoporosis in adults.
Figure 1 shows the ultraviolet radiation levels for a year. Values near the high end of the scale (pink/purple side, closest to 18) indicate high levels of UV radiation, whereas values near the low end of the scale (blue side, closest to 0) indicate the lowest levels of UV radiation.
Figure 2 shows the incidence rate of melanoma of the skin per 100,000 inhabitants in 2008.
Figure 3A shows skin color distribution and latitude, based on native populations prior to 1940. Additionally, Figure 3B depicts skin reflectance as a function of latitude.
The Inuit, a group of people who live at high northern latitudes with low UV radiation, have very dark skin. Despite this fact, the ancestral Inuit of the 16th century experienced very low rates of rickets. What is the most likely explanation for this?