“Stop it! ” called Tatiana, playfully. Her boyfriend, Zach, was inspecting her skin very carefully. “Look,” he answered her, his voice taking on a more serious tone. “Today a woman walked into the clinic for her annual physical. Everything about her seemed ne. She leads a balanced lifestyle, she eats well, she exercises: she’s healthy! But as she was about to leave, I noticed a mole on her arm.
It had many of the warning signs of skin cancer. So, I removed the mole. is woman now has to wait for the lab results to see if it was cancerous. If it is, maybe we caught it early enough to treat it, and maybe not. Either way, her life is changed. I just want to make sure you don’t have any suspicious moles, okay? ” Tatiana relented and allowed Zach to examine her skin. She asked: “Do only white people get skin cancer? ” “No, people of all skin tone can get skin cancer, but it does occur more frequently in Caucasians. ”
Skin Pigmentation and UV Light
Why are human populations di erently pigmented? What caused the evolution of an array of di erent skin colors? Humans Were Initially Lightly Pigmented About seven million years ago, humans and chimpanzees shared a common ancestor.
Since that time, the two species have evolved independently from one another. It is generally assumed that chimpanzees changed less over that time period than humans—because they have remained in their original environment. Chimpanzees are therefore often used as a surrogate to make inferences about the physical and behavioral attributes of our common ancestors. e skin of chimps is light and covered with hair. From this observation, it has been inferred that our earliest ancestor was also probably light-skinned and covered with hair.
Since humans and chimps diverged, humans left the protection of trees and adapted to a new environment (the open savannah). is change in habitat required several adaptations. Life on the savannah provided little shade and so little protection from the sun, and required a more active lifestyle (i. e. , hunting as opposed to picking fruits). It is also hypothesized that the social interactions and strategizing required for successful hunting favored the development of a large brain, which consumed a lot of energy and generated heat.
An increased number of sweat glands and loss of body hair evolved to dissipate heat. s created a new problem, as the light skin became exposed and vulnerable to the sun’s damaging ultraviolet (UV) radiation. Melanin: Natural Sunscreen UV light is harmful to living organisms because it causes changes (i. e. , mutations) in the DNA sequence. Skin cells that produced a pigment called melanin were advantaged because melanin is a natural sunscreen; it absorbs the energy of UV light and shields cells from the radiation’s harmful e ects. Such cells were favored in evolution and now all human skin cells can produce this pigment.
People vary in their skin tone due to di erences in the distribution, quantity, size, and type of melanin found in their skin cells. As you might suspect, people with dark skin tend to have larger and more numerous melanin-containing particles in their skin. is provides protection from the sun’s UV rays. Many genes are known to a ect the production of melanin and cause skin color variation in humans. While skin color is an inherited characteristic, the fact that many genes code for this trait explains why children do not always exactly match their parents’ skin tone.
Tanning is the process of producing more melanin in the skin in response to ultraviolet exposure, and does not require a change in the genetic code (if a parent gets a tan, the o spring will not be more pigmented). Distribution of UV Light across the Globe e following image represents a map of the world on which the UV-light Index has been superimposed. e latitudes are shown on the left (latitude helps de ne a location on Earth, speci cally how far north or south of the equator a site is).
Natural Selection and Evolution of Skin Color
Based on the information provided so far, it seems reasonable to hypothesize that darker skin evolved to protect against the harmful e ects of UV light. In particular, individuals who lacked optimal pigmentation for tropical latitudes had a greater risk of skin cancer and death. Until fairly recently, this was the leading hypothesis about the evolution of skin color. However, there is a problem with this hypothesis. Let’s see if you can nd it. Here is some basic information on evolution by natural selection. Evolution is a change in the gene pool of a population of organisms from generation to generation. Natural selection is but one of several mechanisms by which evolution can take place. rough natural selection, populations evolve and become adapted to their speci c environment.
Natural selection will occur if the following three conditions are present:
- Variation: e organisms in the population vary with regard to a trait.
- Heredity: Variation in the trait has a genetic component transmissible to o spring.
- Selective Pressure & Di erential Reproductive Success: Some traits increase the odds of surviving to reproductive age and successfully producing and rearing o spring in a given environment. Such traits are more adaptive. ose organisms having the better adapted trait leave more o spring behind—they are “naturally selected. In the next generation, this adaptive (and inherited) trait will increase in frequency and will be represented in a greater proportion of the population.
- At this point, the genetic makeup of the population is di erent from that of the starting population: the population has evolved.
- Evolution is really a “number’s game”: the organisms that reproduce the most “win” because their traits will be disproportionally represented in the next generation. Note also that individuals do not evolve. ey either breed more e ectively or less e ectively, depending on already existing di erences in their traits.
Only populations evolve or change over time.
Folate: A Di erent Way of Looking at It
Since skin cancer tends to occur after age 50, it has little impact on reproductive success. Consequently, skin cancer probably exerted little pressure on the evolution of skin color. Some other factor must explain the range in pigmentation that is observed in the human population. For years, this fact was overlooked by the scienti c community, and the consensus was that dark skin had evolved as protection against skin cancer. In 1991, the anthropologist Nina Jablonski was skimming though scienti c journals when she came upon a 1978 paper by Branda and Eaton. s paper investigated the e ects of sunlight on an essential chemical found in our body: folate or folic acid (one of the B vitamins).
Folate is an essential nutrient for DNA synthesis. Since cells reproduce at a fast pace during pregnancy (and hence, there is a lot of DNA replication), the highest levels of folate are needed during pregnancy. Folate de ciencies during pregnancy can lead to anemia in the mother and malformations of the nervous system (neural tube defects in particular), gastrointestinal system, aorta, kidney, and skeletal system in the fetus. re is also a high rate of miscarriages.
In addition, folate de ciency has been linked to spermatogenesis defects (inability to form sperm) in mice and rats (Mathur et al. , 1977), and anti-folate agents are being investigated as a form of male contraceptive (Cosentino et al. , 1990). Branda & Eaton’s paper measured the folate concentration in two human test groups. e results are shown. One group (called “patients”) was exposed to UV-light, while “normals” were not so exposed.
Vitamin D: Still Another Way of Looking at It
Folate can explain why dark skin evolved, but it cannot account for the evolution of light skin. Another factor must be at play. Vitamin D3 is essential for normal growth, calcium absorption, and skeletal development. It is particularly important in maintaining and repairing healthy bones and teeth.
Its role in calcium absorption makes it essential in maintaining a healthy heart, blood clotting, a stable nervous system, and an e ective immune system. De ciencies manifest themselves as rickets (softening of the bones), osteoporosis, and osteomalacia. It can lead to death, immobilization, or deformities. Women have a higher need for this nutrient during pregnancy and lactation due to their need to absorb calcium to build the fetal skeleton. Humans can obtain vitamin D3 by one of two means. ey can consume it in certain foods ( sh liver oil and to a lesser extent, egg yolk).
Alternatively, skin cells have the ability to synthesize it from a cholesterol-like precursor. However, this process requires the energy of UV radiation. eoretical research on the dose of ultraviolet radiation required to produce vitamin D3 suggests that for moderately to darkly pigmented individuals: • ere is enough sunlight reaching the tropics (approximately 5° north of the Tropic of Cancer to approximately 5° south of the Tropic of Capricorn) to meet all of a human’s requirement for vitamin D3 throughout all months of the year. is is indicated by the dotted area on the map.
Note: Vitamin D3 is not produced to toxic levels when high quantities of sunlight are present. In the area indicated by narrowly-spaced obliques, there is not enough ultraviolet light to synthesize vitamin D3 in human skin for at least 1 month of the year;
Sexual Selection Too?
Sexual selection is a special type of natural selection. Darwin was the rst to recognize that if males or females of a species choose their mate based on a particular trait, then their preference will exert selective pressure on that trait. e result is that the opposite sex will evolve to meet that preference. An obvious example is the peacock’s tail. If females prefer males with the longest and most ornate and iridescent tail as mates, even if having such a tail seems detrimental to the male’s survival, it may be selected for evolution. is is because reproductive success, and not survival per se, is the most important characteristic that drives evolution.
Females might prefer males with elaborate traits because it is a real signal of health and genetic worth: only males with good genes can a ord to have such a handicap (such a large tail). is process is called sexual selection. It occurs when individuals in the population di er in their ability to attract a mate. Research by Nina Jablonski and George Chaplin suggest that women generally produce 3–4% less melanin in their skin than do men in all populations of the world.
- Barsh, G. S. (2003). What controls variation in skin color? PLOS 1(1): 019–022.
- Bodnar, L. M. , Simhan, H. N. , Powers, R. W. , Frank, M. P. , Cooperstein, E. , and Roberts, J. M. (2007). High prevalence of vitamin D insu ciency in black and white pregnant women residing in the northern United States and their neonates. Journal of Nutrition 137(2):447–52.
- Branda, R. F. , and Eaton, J. W. (1978). Skin colour and nutrient photolysis: An evolutionary hypothesis. Science 201: 625–626.
- Calvo, M. S. , and Whiting, S. J. (2003). Prevalence of vitamin D insu ciency in Canada and the United States: Importance to health status and e cacy of current food forti cation and dietary supplement use. Nutrition Reviews 61(3): 107–13.
- Cosentino, M. J. , Pakyz, R. E. , and Fried, J. (1990). Pyrimethamine: An approach to the development of a male contraceptive. Proceedings of the National Academy of Scences. (U. S. A. ) 87, 1431–1435.
- Freeman, S. (2005). Biological Science, 2nd edition. Upper Saddle River, NJ: Pearson Prentice Hall.
- Gozdzik, A. , Barta, J. L. , Wu, H. , Wagner, D. , Cole, D. E. , Vieth, R. , Whiting, S. , and Parra, E. J. (2008). Low wintertime vitamin D levels in a sample of healthy young adults of diverse ancestry living in the Toronto area: Associations with vitamin D intake and skin pigmentation. BMC Public Health 8: e336. Retrieved 22 April 2011 from http://www. biomedcentral. com/1471-2458/8/336#B37.
- Holick, M. F. , and Chen, T. C. (2008). Vitamin D de ciency: A worldwide problem with health consequences. Am J Clin Nutr 2008, 87(4):1080S-1086S.
- Mathur, U. , Datta, S. L. , and Mathur, B. B. (1977). e e ect of aminopterin-induced folic acid de ciency on spermatogenesis. Fertility Sterility 28, 1356–1360.