Melanin has played a significant role in human evolution. In this paper I will discuss the importance of melanin in its role in the human biological system and how it relates to the natural selection of suitable human life according to geography and environment. Human pigmentation is influenced by hemoglobins within blood vessels in the skin, carotene and melanins. Melanin, the basis of pigmentation, can be found in the forms of eumelanin and phaeomelanin. Eumelanin is the brown-black pigment located in the skin, hair, and eyes. Phaeomelanin is a yellow to reddish-brown pigment found in small quantities within the skin, eyes, and red hair. Because of these two pigments, to a greater or lesser degree, we have the variation in human pigmentation that is seen today. Melanin is the product of cells located within the skin called melanocytes.The function of melanocytes is to manufacture the pigments eumelanin and phaeomelanin. Eumelanin, which is more photostable than phaeomelanin, is the main product of the melanocytes. In melanin, various chemicals, under the influence of enzymes, build a very stable compound that is distributed throughout the body of pigmented peoples.
In an experiment by Wasterstrom, 1984, melanin exposed to up to 600 degrees C was shown to retain close to 50% of its original properties. In addition, melanin has been found in 1,000-year-old mummies and in extinct mammoth skins, according to Daniels et al., 1972. After production, melanin is packaged in membrane bodies called melanosomes. Dark skin has large melanosomes that are in a single state and also contains more melanosomes than lighter skin such as in Europeans. Most Asian skin, on the other hand, has smaller concentrated groups of melanosomes, whereas European skin has the smallest melanosomes, which are combined into bound materials. The spacing and amount of melanosomes are what gives the appearance of skin color due to the dispersal of light hitting the skin. A study of Melanesians found that the darker skinned inhabitants possessed large, singly dispersed melanosomes, whereas the lighter skinned inhabitants had smaller melanosomes bound in complexes . This infers that the large single state melanosome is directly related to dark skin.
The size of these melanosomes are not specific to “race”. For example, Africans with light skin contain the smaller melanosome complexes associated with light skin. Skin color is not dependent on the number of melanocytes, because all people have the same number. What does influence skin color variation is the size and distribution pattern of melanosomes as well as other factors. A major function of melanin is to act as a defensive barrier of the skin against radiation damage. Ultraviolet light between the wavelengths 280-320nm (UV-B) causes sunburn and damages the skin on nonpigmented peoples. Exposure to this degree of light causes their skin to become dry, leathery, and wrinkled in appearance similar to that associated with aging. This can be seen in Whites living in Australia and South Africa, where ultraviolet light intensity is high because of the relative latitude or closeness to the equator. Melanin is also said to be linked with hearing.
Melanocytes are present in the inner ear, the eye, and in the membrane that covers the brain and spinal cord. It has been demonstrated that melanin deposits in these areas are proportional to the amount of melanin found in the skin . These areas of the body, similar to the skin, are exposed to high-energy free radicals that can damage the surrounding cells, and thus causing a lower threshold for hearing. Evidence shows that black people, on average, hear better than whites, and that within both races, women surpass men. A partial explanation of the differences may lie in the abundance in the inner ear of melanin pigments. Also, numerous studies have found that people with light eye colors, such as blue, green, and hazel, are more vulnerable to hearing damage than are people with brown or black eyes.
Reasons for the greater susceptibility among light-eyed blacks remain unclear, but melanin according to studies, melanin may have something to do with it. Researchers have determined that the inner ear produces extra melanin when stressed by noise. Melanin, however, may both protect and harm hearing. It sometimes cleans up harmful molecules, but at other times, it contributes to their creation. Melanin is not only linked with hearing, but also with hair color. The smallest amount of melanin in hair will make hair appear black. This is illustrated by the fact that those who are lightly pigmented in the skin can still have black hair. Hair color of light-skinned people ranges from black to brown to blond. However, in dark-skinned people, there appears to be an intense selection for dark hair. The hair color variants of light-skinned people are due to the two melanins–eumelanin and phaeomelanin, just as in the skin. Red hair contains melanosomes that contain phaeomelanin in high concentrations.
Thus hair color is determined by various proportions of both eumelanin and phaeomelanin. Eye color, on the same hand, is determined by the concentration and location of melanin in the iris of the eye. A large amount of melanin is present in the iris of brown- or black-eyed individuals. Light eyes such as blue, green, or hazel are the product of dispersed melanosomes, which are not as concentrated as in dark-eyed individuals. Melanin found in the iris of the eye functions as a protector of cells making up the iris and retina, similar to melanin’s protection of the skin. Iris and retina cells of individuals with light eyes are much more sensitive to bright light than corresponding cells from dark-eyed individuals.
Those with pigmented skin have the highest counts of melanocytes in the genitalia and nipples. The pigmentation in these areas can be influenced by sex hormones like estrogens and androgens. During pregnancy, the nipples, face, and abdominal wall become darkened. These areas of increased pigmentation during pregnancy are due to the increase in the production of estrogens. Anthropologists try to account for the fact that the genitalia are darker by suggesting that protection is needed against ultraviolet radiation damage, and has been thus evolved through natural selection.. This means that protection of the genitalia against forces that may prevent reproduction was needed. Similarly, in this view, the pigmentation of the nipples during pregnancy would be a defense against ultraviolet radiation damage to the nipples.
Supposedly, this type of protection would be needed while breast feeding during the early evolution of humans in Africa. According to Robins, 1991, darker pigmentation found in the genitals, may have evolved for the “protection of reproductive capacity”, in that the pigmentation protects gametes within the genitalia from ultraviolet radiation damage. Variation of pigmentation within populations is sometimes greater than variation between populations. Comparisons of the European populations who inhabit the Mediterranean and those who reside in Scandinavia vary from dark skin, hair, and eyes to white pale skin, blue eyes, and red hair, respectively. These variations throughout the world are due to factors caused by the sun. The angle of sunlight hitting the earth varies with latitude.
The higher the latitude, the lower the UV. Thus, in northern latitudes such as in Asia, there is lower UV radiation, and in southern latitudes, such as in Southeast Asia, Australia and India, there is higher UV radiation. Geography and climate has also been thought of as having factors in pigmentation variation. According to some, the cold weather in the temperate regions of the earth may have been a selective force against survival of some human groups, according to Steegman, 1967, and may have been one of several factors responsible for the evolution of white skin. Reports on Senegalese troops during World War I and Ethiopian troops during the Korean War showed higher rates of frostbite and frozen feet in the African soldiers than their European counterparts. The laboratory work of the researchers also suggested that black skin tissue was more prone to cold injury; however, white skin also exhibited some damage. It was discovered that these samples of black skin tissue were less vulnerable to cold damage than the lighter European tissue.
Anthropologists believe that there are no substantial genetic differences between human populations, but biochemically, Caucasians have a higher concentration of enzyme inhibitors that suppress melanin production, according to Halprin & Ohkawara, 1966. Asians also have enzymes that inhibit melanin production, but significantly less than Caucasians. Therefore, there are genetic differences that have evolved to allow for the many colors expressed in human skin. Human skin is responsible for the synthesis of vitamin D. Ultraviolet radiation absorption by the skin converts 7-dehydrocholesterol (7-DHC) to vitamin D. Vitamin D is essential for normal growth and skeletal development. Vitamin D deficiency causes immobilization, pelvic deformities, tickets, and even death. The dark-skinned African migrants in Europe during the late Pleistocene, whom produce Vitamin D with the assistance of the sun received less UV light due to the cloudy, cold climate.
Their protective clothing and shelters blocked sunlight from entering the skin, causing less vitamin D production. Due to the resulting vitamin D deficiency, extreme cases of bone deformalities may have ensued. These deformities significantly reduced reproductive efficiency, adding to natural selection. Pelvic deformities, as a result of rickets, may have reduced the life expectancy of women due to birthing problems. Children may have been born brain damaged due to deliverance through a deformed pelvis. Children and adults with painfully weak bones and muscles or stunted growth would be disadvantaged for their type of environment. In short, the early African migrants in northern latitude such as Europe may have experienced high child mortality rates and low life expectancy because of a vitamin D deficiency caused by lack of sunlight.
Vitamin D deficiency may have had considerable evolutionary significance due to major selection occurring in the population of Africans in Eurasia. Dark-skinned individuals and even light-skinned individuals who covered themselves with clothing may have been selected out. The only individuals who survived were those with very low or no pigmentation such as albinos. Albinism is due to mutations that affect the function of the enzyme responsible for the production of melanin, causing the absence or extremely low level of pigmentation. Present-day populations of Homo sapiens reflect a thorough mixture of assorted genetics due to gene flow, selective pressures, drift, and random mutations. Environmental pressures such as ozone depletion and global warming may be the selective pressures of the future. Ozone depletion will increase the ultraviolet radiation and cause major problems for light-skinned individuals.
Modern technology such as vitamin supplements and sun block creams have allowed individuals to stay healthy in many parts of the world, causing the adaptive qualities of skin and hair pigmentation to be minimized. In conclusion, melanin production has played a considerably important role in human evolution. Not only does it influence color pigmentation through its protective role of defending against harmful UV rays, but also determines detrimental features such as eye-sight and hearing. Furthermore, melanin production and its evolutionary adaptions mark an important presence upon our biological systems to this day. Therefore, in the process of furthering human evolution, melanin production has played an enormous role in human evolution by selecting for several features that allow for particular adaptions according to the human’s geographical location and environment.
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