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AnthroNotes Volume 32 No. 1 Spring 2011 WHY HUMAN SKIN COMES IN COLORS by Nina G. JabIon ski pigmentation provides one of the best examples Skin of evolution by natural selection acting on the human The fact that skin color has been so responsive body. to evolutionary forces tant for is fascinating, modern human and one that is impor- its value as a unique maker of identity is eliminated and the race so defined is rendered nonsensical. We are all "hue-mans"! As modern humans moved around greater numbers and over longer distances tween 50,000 and 10,000 years ago, a lot the world in in the time be- of "fine tuning" occurred in the evolution of skin pigmentation. As popu- moved lations levels, to parts of the world with different UVR they underwent genetic changes that modified their skin pigmentation. from Asia, lations entering high changes which ning is the As people moved for instance, we in some popu- for them to tan easily. Tan- develop temporary melanin pigmenta- tion in the skin in response to merous times tensity UVR and has evolved nu- peoples living under highly seasonal pat- terns of sunshine (Jablonski and Chaplin 2010). When we all skin pigmentation, we people are varying shades of brown. The in- of their brownness and their ability to tan is related UVR in the place where their ancestors came from. to the In the last 1 0,000 years, we have gotten better and better at protecting ourselves against the extremes of UVR by cultural means. Sewn clothing and constructed shelters now protect us from strong sunlight and augment the protection afforded by natural melanin pigmentation. In far northern environments, diets composed of vitamin D-rich foods like oily fish and marine mammals supplement the vitamin D we can make in our skin under low UVR con- ditions. The major problem we able to travel so far so face today is that we are Many people today live or take fast. vacations far away from the lands of their ancestors. This means levels that, often, we ing in low all our skin color is mismatched to the UVR Darkly pigmented people are experiencing. liv- UVR environments and people working indoors of the time are at high risk of developing vitamin D UVR deficiencies. Lightly pigmented people environments are high risk of developing skin cancers. UVR environments underwent genetic We made it possible ability to into the Americas see evidence that map of predicted human at a find that societies to understand. Similar — both dark and light — have evolved independently multiple times in human history. When we think of how races have been defined in the past using skin color, we can immediately see the problem. When the same skin color has evolved many times independently in different skin colors places, look at must recognize these living in high issues in order to avoid major health problems today. Ultraviolet Radiation and Skin Color Human skin colors. Some people have mostly hairless and comes in a range of is black, while others Most other people have in between. Skin color from place very dark skin that to place, skin that has a color is almost somewhere remarkably variable in people and differences be noticed thousands of years ago traveling widely is have very pale skin that is nearly white. in skin color began to when people and engaging in long-distance started trade. Ob- who lived under intense sun close had dark skin and those who lived under servers noted that people to the equator weaker sun away from the equator had light Map But why? skin. Many centuries ago, some early Greek and Roman philosophers speculated that skin color and other feaof model. human Map skin color predicted by George Chaplin. by multiple regression tures were associated with climate. According to them, dark skin tones were produced by excessive heat and light Page 7 inthroNotes Volume 32 No.l Spring 2011 Annual average ultraviolet radiation at the earth's surface, based on data from the NASA Total Ozone Mapping Spectrometer 7 Satellite. tones by excessive cold. the By the mid-1 700s Map naturalists like American Samuel Stanhope Smith observed pigmentation showed to latitude, He poles. a by George Chaplin. that skin pronounced gradient according from dark near the equator to light toward the at different latitudes: "This general uniformity in the effect," Smith wrote, "indicates an influ- ence in climate, under the same circumstances, that, always operate in the same manner." But was it will the heat caused by the sun or something else in the sunshine that skin was responding to? the middle of the 20 th century, observers de- ultraviolet radiation (UVR) from the sun. UVR, in fact, 87% of the variation in human skin color. So how can it be shown that human skin pigmentation is accounts for over an actual evolutionary adaptation to terms, an adaptation it is to reproduce UVR? In evolutionary a characteristic more of an organism that successfully under certain en- vironmental conditions than other organisms, which do not have the characteristic. exactly what UVR's Harmful UVR We UVR is and what it first need to understand Effects ing a highly energetic and it is rarely fatal and it mostly Other harmful effects of UVR have potentially much greater effects on reproductive success. Some wavelengths of UVR break down other important biological some forms of molecules, such as is a port metabolism. cell Without adequate of folate in the body. Folate B vitamin, which is needed to produce DNA and sup- We normally get folate from green folate, and whole grains in our diet. we can't make sufficient amounts DNA to maintain normal levels of cell division in our body. Cell division is needed to maintain the function of organs and tissues in our body and is especially important in tissues with a high turnover, like the lining of the gut and the lining of the mouth. Cell division also occurs rapidly in the early the first embryo and in the production of sperm. During few weeks of embryonic development, rapid and precise cell division leads to the establishment of the basic body plan of the body and the development of the early circulation. If cell division critical time, serious or even is slowed fatal birth defects can occur. Protection of the body's folate supplies invisible UV rays by our atmosphere, but some UVR Page 8 UVR dam- people after their child-bearing years. or inhibited at this form of solar radiation that is capable of causing a lot of damage to living organisms. Life on earth is mostly protected from damagis affects bad, but is nervous system and does. effects. DNA, and this activity can eventually cause skin canSkin cancer cer. leafy vegetables, citrus fruits, termined that skin color was most strongly correlated with allows ages related this mainly to differences in sunshine experienced by people By through and has powerful biological still gets is therefore important for successful reproduction (Lucock 2000). And successful reproduction is what evolution is all about. How then, was this ensured? AnthroNotes Volume 32 No. immune Natural Sunscreens When it comes to protection against harmful UVR, many biological systems have evolved natural sunscreens. Most which reduce UVR UV rays. The pigment natural sunscreens are special molecules damage by absorbing or scattering called melanin — and especially the most common type in human skin called eumelanin — is one of the most effective Eumelanin is intensely dark and has the natural sunscreens. ability to absorb potentially damaging UVR Only of making vitamin N is much less and it falls in a highly And dark skin with lots of sun-protective down the process of making vitamin D in and 23.5°S) there seasonal pattern. the skin. So these circumstances posed another challenge to our ancestors. How posure. pathways or structures that are already in existence. Ancestors of the human lineage had the eumelanin in the naked skin on they were exposed to ability to their faces produce and hands when UVR. When our ancestors lost most of their body hair, there was evolutionary pressure to protect exposed skin from the harmful Qablonski 2004). The solution to this effects of problem was to make who carried the genetic changes more or mutations leading to the production of tive eumelanin pigment those who didn't. left more offspring behind than a gene called This gene regulates the production of a protein called the melanocortin-1 receptor that plays role in protec- Genetic studies have shown that some of the most important changes occurred in MC/R. normal pigmentation. All an important modern humans nated from darkly pigmented ancestors origi- UVR- rich sunshine of equatorial Africa. Vitamin D ern Africa, Asia, and Europe, they encountered lower levless potential dam- age to their bodies from harmful radiation, but there was also a downside. UVR is not a universally bad thing: The one important good thing it does of making vitamin D in the skin. is to initiate the process Vitamin D sorption of calcium from the foods that we UVB of Depigmentation moving away from very sunny individuals When people started places with high levels of who had lighter skin were able to stay healthier and leave more offspring. Evolution was The individuals with lighter skin had enough vitamin D, bones don't develop properly and work specific genetic having less natural sunscreen in their skin. again. muta- and so These new pat- terns of genetic variation were very successful. We see evi- dence, in fact, that "selective sweeps" — greatly accelerated periods of evolution by natural selection — led to genes for lighter skin becoming fixed in the population over the course of just a few thousand years. One of the most interesting and important things about the depigmentation process is that it didn't happen Genetic evidence shows that the ancestors of eastern Asians the ancestors of modern underwent independent genetic changes lead- ing to the evolution of lighter skin (Norton, Kitties et 2007). al. These changes involved different genetic mutations, which then were favored by natural selection. In other words, depigmentation evolved independently in both of the lineages of modern humans that began to inhabit higher of the Northern Hemisphere. We also know from the examination of ancient Without at tions that resulted in their producing less eumelanin helps us to eat. under low lighter skin Qablonski Light skin is actually depigmented skin. latitudes build and maintain a strong skeleton by regulating the ab- weak. Vitamin The Role just once. from the most intensely sunny parts of Africa into southof UVR. This meant that they faced living — with modern western Europeans and Benefits When some of our modern human ancestors moved away els is production be and Chaplin 2000; Chaplin 2004). who evolved per- manent eumelanin pigmentation in their skin to protect them from the The answer D UVR UVB to less sunny places with lower levels of UVB, those dark pigmentation permanent. This was accomplished by natural selection. Individuals conditions? could vitamin who were maintained in people Evolution often works by modifying biochemical of are capable D in the skin, and these are in the UVB range. The equator receives a lot of UVB year round, but north and south of the tropics (23.5° neu- UVR ex- UVR certain wavelengths of starting the process melanin slows as well as harmful chemical byproducts caused by tralize we don't get enough vitamin D, our become physically weak and susceptible to dis- systems. If bodies can ease. Spring 2011 1 mentation as a result DNA distant, extinct cousins, the neanderthalensis), who that loss of skin pig- of natural selection occurred in our inhabited Neanderthals {Ho wo much of eastern Europe are D also helps to maintain the health of our Page 9 Volume 32 No.l Spring 2011 inthroNotes and the region around the Mediterranean during the ice age (Lalueza-Fox, Rompler et al. last 2007). In Study of the evolution of skin pigmentation important part of the study of human diversity. is evolutionary time: an Different > degrees of skin darkness and lightness evolved multiple times in humans and adapted to as we local II,* W V environmental conditions. Today, people are moving over much greater distances, Migration: Environment: much -UV radiation - faster V A. 7 have dispersed to different places -Length of migration levels -Recency of migration Precipitation than ever before, and our adaptations are mostly cultural and social, not biological. These cultural and social we still have much how we as individuals and as human societ- adaptations have been imperfect, and to learn about ies can stay healthy in environments far distant from our ancestral homelands. Other cultural adaptations to climate: Diet: -Clothes wearing - Vitamin - Shade seeking - Folate content • Timing of daily Further Reading content Shelter activities Chaplin, G. 2004. "Geographic distribution of environ- mental factors influencing human skin coloration." Ameri- During a human o lifetime: can Journal of PhysicalAnthropology 125(3): 292-302. Endogenous Jablonski, N. G. 2004. "The evolution of human skin and Influences: -UV radiation exposure skin color." Annual Re view of Anthropology 33: 585-623. Jablonski, N. human G, and G. Chaplin. 2000. skin coloration." Journal of - Genetic, esp. Application of tanning MC1R locus or bleaching agents Age "The evolution of Human Evolution 39(1): 57-106. Jablonski, N. G., and G. Chaplin. 2010. mentation the as UV radiation." Proceedings of an adaptation to National Academy of Sciences 107 (Supplement 2): 8962- 8968. Many factors affect human skin pigmentation over different time scales. As people receptor C, H. Rompler, allele et al. 2007. "A melanocortin suggests varying pigmentation are mostly cultural, not biological. and operate move around more rapidly today, our adaptations to different Lalueza-Fox, 1 Ju "Human skin pig- environments Figure by Jennifer Kane. among Neanderthals." Science 318(5855): 1453-1455. Lucock, M. 2000. "Folic acid: Nutritional biochemistry, molecular biology, and role in disease processes." Molecular Genetics and Metabolism 7 1 Norton, H. L., R. A. (1 -2) : Kittles, et for the convergent evolution 1 2 1 - 1 38. al. the 2007. "Genetic evidence of light skin in Page 10 is based on an essay written for a forthcom- RACE: Are We So Different? to be published by American Anthropological Association as a compan- ion volume to the RACE exhibit.] Europeans and East Asians." Molecular Biology and Evolution 24(3): 710722. [This article ing book Nina G. Jablonski is a professor in the Department of Anthropology at The Pennsylvania State University.