Human skin color is quite variable around the world. It ranges from a very dark brown among some Africans and Australians to a near yellowish pink among some northwest Europeans. There are no people who actually have true black, white, red, or yellow skin. These are commonly used terms that do not reflect biological reality. Skin color is due primarily to the presence of a pigment called melanin . Both light and dark complexioned people have this pigment. However, two forms are produced - one is red to yellow in color, and the other is brown to black. In addition, individuals differ in the number and size of melanin particles. These two variables are more important in determining skin color than the percentages of the different kinds of melanin. In lighter skin, color is also affected by red cells in blood flowing close to the skin. Melanin is normally located in the epidermis, or outer skin layer. It is produced at the base of the epidermis by specialized cells called melanocytes .
Melanoma is a serious threat to life. In the United States, approximately 54,000 people get this disease every year and nearly 8,000 of them die from it. Those at highest risk are European Americans. They have a 10 times higher risk than African Americans. Ultraviolet radiation reaching the earth usually increases in summer and decreases in winter. Tanning is primarily an increase in the number and size of melanin granules due to the stimulation of ultraviolet exposure. While skin tanning is often most noticeable on light complexioned people, even those with very dark brown skin can tan as a result of prolonged exposure to the sun. However, some northwest Europeans have lost the ability to tan as a result of relaxed natural selection. Their skin burns and peels rather than tans. They are at a distinct disadvantage in tropical and subtropical environments. Not only do they suffer the discomfort of readily burning, but they are at a much higher risk for skin cancer. It would be harmful if melanin acted as a complete shield. A certain amount of shortwave ultraviolet radiation (UVB) must penetrate the outer skin layer in order for the body to produce vitamin D3. Approximately 90% of this vitamin in people normally is synthesized in the skin with the help of ultraviolet radiation. The remaining 10% comes from foods such as fatty fish. Vitamin D3 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. However, too much ultraviolet radiation penetrating the skin may cause the break down of folic acid, which can cause anemia. Pregnant women who are deficient in folic acid are at a higher risk of having babies with neural tube defects. Because folic acid is needed for DNA replication in dividing cells, its absence can have an effect on many body processes, including the production of sperm cells. It may be that the ability to produce melanin was selected for in our early human ancestors because it helped preserve the body's folic acid supply in addition to reducing the chances of developing skin cancer. People 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. In such an environment, very dark skin is a disadvantage because it can prevent people from producing enough vitamin D3, potentially resulting in rickets disease in children and osteoporosis in adults. Women who had prolonged vitamin D3 deficiencies as girls have a higher incidence of pelvic deformities that prevent normal delivery of babies. In the United States and other developed nations, milk is now usually fortified with vitamins D3 and A in order to prevent developmental problems such as those described above. However, the popularity of soft drinks and other alternatives to milk among children along with a decrease in the amount of time spent outdoors under the sun has led to a considerable rise in the rate of rickets disease.
Use information from the reading to select the best answer for each question below.
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Before 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 at the left. In fact, the majority of dark pigmented people lived within 20o of the equator. Most of the lighter pigmented people lived in the northern hemisphere north of 20o latitude.
Nature has selected for people with darker skin in tropical latitudes, especially in nonforested regions, where ultraviolet radiation from the sun is usually the most intense. Melanin acts as a protective biological shield against ultraviolet radiation. By doing this, it helps to prevent sunburn damage that could result in DNA changes and, subsequently, melanoma - a cancer of the skin.