Friday, July 10, 2009

Melanin

Melanin

Also called pigment, melanin is a substance that gives the skin and hair its natural color. It also gives color to the iris of the eye, feathers, and scales. In humans, those with darker skin have higher amounts of melanin. By contrast, those with less pigment have lighter or more fair skin coloring.

Melanin, sometimes referred to as a chemical, is formed as part of the process of metabolizing an amino acid called tyrosine. In the skin, melanin is formed by cells called melanocytes. Certain medical conditions, such as albinism, are associated with the lack of melanin. Albinism is a condition marked by the abesnce of a normal amount of pigment in the body. Animals, humans, and even plants can have albinism.

Albinism exists in a number of variations. Depending on the type of albinism, the skin, hair, and eyes may all be affected. In fact, ocular albinism affects not only the color of the eyes, hair, and skin, but also results in poor vision. Additionally, some types of melanin deficiency are associated with increased mortality rates.

Melanin provides many benefits to human beings. One of the most recognized benefits involves ultraviolet rays of the sun. Melanin provides a natural protection against the harmful effects of these rays. However, it does not provide complete protection from the sun, and individuals with darker skin tones are still at risk from the sun's damaging rays.

Generally, those with darker skin tones and more melanin are able to tolerate exposure to the sun for hours without getting sunburn. By contrast, a person with lighter skin may get sunburn after spending only minutes in the midday sun. Skin cancer is directly related to exposure to the sun and the presence of less than optimal amounts of pigment. Sun exposure has even been linked with cataracts.

Melanin is also a mechanism for absorbing heat from the sun. This purposes is of particular importance to cold-blooded animals. Snakes, lizards, certain types of fish, and a wide range of other animals depend on their surroundings, including the rays of the sun, to establish and maintain their body temperatures.

Also important for sharpness of vision, melanin serves to minimalize the number of light beams that enter the eye. It also provides for the absorption of scattered light within the eye. In this way, pigmentation allows for more keen sight.

    The Science

  1. Practically everyone, at one time or another, has experienced the discomfort and itchiness that accompanies sunburn. Your skin turns an unattractive--and painful--shade of red, and when the pain and redness finally go away, your skin starts peeling off! But what causes all this? You can blame your burning pain on the ultraviolet (UV) radiation in the sun's rays. There are three different types of UV rays: UVA, UVB and UVC. Each type of UV ray causes premature aging and destroys vitamin A in the skin. However, UVB rays are the ones responsible for that red hot pain. Because these these rays only reach the top layer of the skin (the only visible layer), they cause cosmetic damage to the skin--otherwise known as sunburn.

    2. Ultraviolet Rays

  2. A sunburn is literally an ultraviolet burn to the skin. UVC rays are the most dangerous type of ultraviolet radiation, but fortunately, these rays are absorbed by the earth's ozone layer and therefore, do not reach us. However, this is not the case for UVA and UVB rays. Although UVA rays are more abundant than UVB rays, they do not cause sunburn. They actually penetrate all the way to the base layer of the skin and cause no visible damage to your skin.

    3. But Why Do I Burn?

  3. UVB rays penetrate only the top layer of your skin, and your body's natural reaction is to protect your skin from damage. To do this, your skin needs a certain amount of melanin--its protective pigment. The amount of melanin in your skin is based on your natural skin color--darker skin indicates higher amounts of melanin. Because the amount of melanin in your skin is determined by your genetic makeup, when your skin is overexposed to UVB rays, your body does not have enough melanin to adequately protect itself. The sun's rays literally damage your skin cells, turning the top layer of your skin red, hot and sun-bruised. Typically, the effects of sunburn are visible about two to six hours after exposure. The damaged skin remains red for three to five days afterward, then rids itself of those damaged cells by peeling.
Human skin color can range from almost black (due to very high concentrations of the dark brown pigment melanin) to nearly colorless (appearing reddish white due to the blood vessels under the skin) in different people. Skin color is determined by the amount and type of melanin, the pigment in the skin. Variation in skin color is largely due to genetics. As a general pattern people with ancestors from tropical regions and higher altitudes (hence greater UV light exposure) have darker skin than people with ancestors from subtropical regions. This is far from a hard and fast rule however, because many light skinned groups have managed to survive at the equator by way of social adaptation. The same can be said of dark skinned groups living at subtropical latitudes.

Melanin and genes

Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in the great variety of different skin tones.


The evolution of the different skin tones is thought to have occurred as follows:[who?] the haired primate ancestors of humans, like modern great apes, had light skin under their hair. When Hominids evolved relative hairlessness (the most likely function of which was to facilitate perspiration), they evolved dark skin, which was needed to prevent low folate levels since they lived in sun-rich Africa. When humans migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity, as well[3].

The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional fish-based diet provides plenty of vitamin D[4].

Brown skin is the likely ancestral (or original) skin color among modern humans (Harding et al. 2000). This is due to modern humanity's common origin in equatorial Africa ~200,000 years ago (Tishkoff, 1996). Dark skin was crucial in this UV rich context given that a thick coat of UV protective body hair had long been selected against by this time (Rogers et al. 2004) most likely in order to facilitate the evaporation of perspiration (ie the cooling of the body). This trait (dark skin) continues to be under strong selection in equatorial regions such as Africa, India, and New Guinea (Harding 2000 p 1355). Geneticists estimate that a relatively small group of humans left Africa ~60,000 years ago, and that the descendants of this group went on to populate the entire non-African world. Those migrants that settled in non-African equatorial regions (such as the mentioned India, New Guinea, and/or Australia) retained most of the ancestral sequence at the MC1R locus (Harding 2000 p 1355), a gene strongly associated with determining skin color. Specifically, Harding et al. (2000 p 1355) found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans (except for a small number of variants at silent sites).

The retention of the ancestral trait at the equator is due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays (Jablonski 2006). Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for Afro-textured hair prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, Afro hair ceased to be under strong selection once dark skin arose ~1 million years ago (Harding 2000) (rather, it remained as a vestigial trait among Africans, Andamanese, and Melanesians and changed to straight in the north for adaptive reasons—see hair texture). In fact, dark skin is so selectively advantageous at the equator that initially light skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.

According to (Norton et al., 2006), light skin observed in Europeans (with deep red and/or yellowish skin tones), non-Indian Southeast Asians, East Asians and North Africa (Maghreb) is due to independent genetic mutations in at least three loci. They concluded that light pigmentation is at least partially due to sexual selection, however Jablonski postulates that the predominant reason revolved around the facilitation of vitamin D production in northern Eurasia (see hair texture).

Health related effects

Dark skin (melanin) protects against ultraviolet light; this light causes mutations in skin cells, which in turn may cause skin cancers. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sunlight exposure. Furthermore, dark skin prevents radiation of UV-A rays from destroying the essential folic acid, derived from B vitamins. Folic acid (or folate) is needed for the synthesis of DNA in dividing cells and folate deficiency in pregnant women are associated with birth defects[5].

While dark skin better preserves vitamin B, it can also lead to vitamin D deficiency at higher latitudes which in turn can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for rickets, cardiovascular disease, diabetes and multiple sclerosis.[6] An American study by the USDA found 87% of African Americans to be Vitamin D deficient.[7] To address this issue, some countries have programs to ensure fortification of milk with vitamin D.

The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation. However, some have postulated that it may also derive from sexual selection[8].

Albinism is a condition characterized by the absence of melanin, resulting in very light skin, eyes, and hair; it is caused by an inability to convert tyrosine to melanin, and has a genetic basis.

Cultural effects

Differences in skin tone are the most readily perceptible phenotypical distinction of human populations. Virtually every society has tended to assign some valuation to skin color differences, especially when these have corresponded with existing political and economic differentiations. Moreover, as variations in skin tone typically correspond with other racial/genetic and associated cultural characteristics, more general trends in perceptions and stereotypes are apparent throughout history and across cultures.

Perhaps the most controversial cross-cultural perception of skin color today is the perception of darker skin color as indicating lower attractiveness, intelligence, and economic status, combined with greater athleticism, impulsivity, and reproductive rates.

Sexual preference of paleness in women by men has been found in certain cultures throughout the world. The effect has been discovered in Moorish Spain, where the ruling class was of darker complexion than the conquered natives.[9] In recent years, several research projects have suggested a general preference for lighter-skinned women by African-American men. In his foreword to Peter Frost's 2005 Fair Women, Dark Men, U. of Washington sociologist Pierre L. van den Berghe summarizes:

"Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented; the trend for men in integrated societies has shown an increasing popularity for men of colour, especially those particular to African descendence. These trends have been recorded in areas such as South America; where in Brazil it was estimated that by 2009, black people of African descendence will be the single most dominant ethnic group. In popular media in the western world "blacks" have been repeatedly surrounded by advantageous stereotypes and myths, that praise their athletic aptitudes amongst many other things; and often depict them as males of superior genetic inheritance."
Differences in skin tone are the most readily perceptible phenotypical distinction of human populations, and hence has historically lent itself to color terminology for race, often to the effect of darker skin being seen as being of lowest social value, and lighter skin of highest. However, according to classical scholar Frank Snowden,[11] the Egyptians and Greeks (et al.) assigned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status.

Skin tone variability

The tone of human skin can vary from a dark brown to nearly a colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans generally have lighter skin, hair, and eyes than any other group on Earth, although this is not always the case. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in decreasing lightness:[12]

It should be noted that while Fitzgerald has distinguished several separate skin tones that culturally fall under "white," or Caucasian, he fails in doing a rigorous study of those on the darker side of the human skin color gradient. There is a remarkable difference, for example, between the skin colour of Nelson Mandela and that of the artist/entertainer known as Seal. Under the Fitzgerald understanding, however, they are both placed under category VI while there arguably exists a less subtle difference between those Fitzgerald has listed under categories I, II, and III.

type also called tanning behavior hair and eye color von Luschan scale example
I Pale skinned, or "Celtic" [13] Often burns, rarely tans. Tends to have freckles, red or blond hair, blue or green or gray eyes. 1-5
(Tyler MacNiven)
II light, or fair-skinned European[13] Usually burns, sometimes tans Tends to have light or dark hair, blue, green, hazel, brown or gray eyes. 6-10
(Jóhanna Vala Jónsdóttir)
III light intermediate, or "average Caucasian"[13] Sometimes burns, usually tans. Usually has brown hair and blue, green, hazel, or brown eyes. 11-15
(Roger Federer)
IV tan, also "Mediterranean" or "Olive skin"[13] Sometimes burns, often tans. Tends to have dark brown hair and eyes. 16-20
(Zinedine Zidane)
V darker tan or "light-Brown people" type Naturally tawny or light-brown skin Often has black-brown hair and eyes. 21-28
(Halle Berry)
VI brown skin, often referred to as "Black" type Naturally dark-brown skin Usually has black-brown hair and eyes. 29-36
(Ati Woman)
VI brown skin, often referred to as "Black" type Naturally dark-brown skin Usually has black-brown hair and eyes. 29-36
(Khosian Man)
VI dark brown skin, often referred to as "Black" type Naturally dark-brown skin Usually has black-brown hair and eyes. 29-36
(Ben Carson)
VI dark brown skin, often referred to as "Black" type Naturally dark-brown skin Usually has black-brown hair and eyes. 29-36
(Kofi Annan)
VI darker brown skin, often referred to as "Black" type Naturally dark-brown skin Usually has black-brown hair and eyes. 29-36
(Michael Jordan)

Environmental factors

In attempting to discover the mechanisms that have generated such a wide variation in human skin tone, Jablonski & Chaplin (2000) discovered that there is a high correlation between the tone of human skin of indigenous peoples and the average annual ultraviolet (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the skin tone (W) of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that skin tone lightness W is related to the annual UV available for skin exposure AUV according to[14]

W = 70 - \frac{AUV}{10}

where the skin tone lightness W is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical lightness maximum of human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the Earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the Earth and passes through different thicknesses of Earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.

Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing forces affecting human skin tone:

  1. the melanin that produces the darker tones of human skin serves as a light filter to protect against too much UV light getting under the human skin where too much UV causes sunburn and disrupts the synthesis of precursors necessary to make human DNA; versus
  2. humans need at least a minimum threshold of UV light to penetrate the epidermis to produce vitamin D, which is essential for building and maintaining the bones of the human skeleton.

Jablonski and Chaplin note that when human indigenous peoples have migrated, they have carried with them a sufficient gene pool so that within a thousand years, the skin of their descendants living today has turned dark or turned light to adapt to fit the formula given above—with the notable exception of dark-skinned peoples moving north, such as to populate the seacoast of Greenland, to live where they have a year-round supply of food rich in vitamin D, such as fish, so that there was no necessity for their skin to lighten to let enough UV under their skin to synthesize the vitamin D that humans need for healthy bones.

In considering the tone of human skin in the long span of human evolution, Jablonski and Chaplin note that there is no empirical evidence to suggest that the hominid ancestors six million years ago had a skin tone different from the skin tone of today's chimpanzees—namely light-skinned under black hair. But as humans evolved to lose their body hair a parallel evolution permitted human populations to turn their base skin tone dark or light to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, prior to Homo sapiens colonization of extra-African territories, humans had dark skin given that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed light skin.(Chaplin)

Genetics of skin color variation

Several genes have been invoked to explain variations of skin tones in humans, including SLC45A2,[15] ASIP, MATP, TYR, and OCA2.[16] A recently discovered gene, SLC24A5 has been shown to account for a substantial fraction of the difference in the average of 30 or so melanin units between Europeans and Africans.

Wide variations in human skin tones have been correlated with mutations in another gene; the MC1R gene [17]. The "MC1R" label for the gene stands for melanocortin 1 receptor, where

  • "melano" refers to black,
  • "melanocortin" refers to the hormone stimulant produced by the pituitary gland that stimulates cells to produce the melanin that makes skin cells black,
  • the "1" in the MC1R gene name specifies the first family of melanocortin genes, and
  • "receptor" indicates that the protein from the gene serves as a signal relay from outside the cell membrane to inside the cell—to the place in the cell where the black melanin is synthesized.

Accordingly, the MC1R gene specifies the amino acid sequence in the receptor protein that relays through the cell membrane the hormone signal from the pituitary gland to produce the melanin that makes human skin very dark. Many variations in the amino acid sequence of this receptor protein result in lighter or darker skin.

The human MC1R gene consists of a string of 954 nucleotides, where each nucleotide is one of the four bases Adenine (A), Guanine (G), Thymine (T), or Cytosine (C). But 261 of the nucleotides in the MC1R gene can change with no effect on the amino acid sequence in the receptor protein produced from the gene. For example, the nucleotide triplets GGT, GGC, GGA, and GGG are all synonymous and all produce the amino acid Glycine[18], so a mutation in the third position in the triplet GGT is a "silent mutation" and has no effect on the amino acid produced from the triplet. (Harding et al., 2000, pg.1355) analyzed the amino acid sequences in the receptor proteins from 106 individuals from Africa and 524 individuals from outside Africa to find why the tone of the sampled Africans' skin was dark.

Harding found that there were zero differences among the Africans for the amino acid sequences in their receptor proteins, so the skin of each individual from Africa was dark. In contrast, among certain (European) non-African individuals, there were 18 different amino acid sites in which the receptor proteins differed, and each amino acid that differed from the African receptor protein resulted in skin lighter than the skin of the African (and other equatorial) individuals. Nonetheless, the variations in the 261 silent sites in the MC1R were similar between the Africans and non-Africans, so the basic mutation rates among the Africans and non-Africans were the same. Also, close examination of the haplotype variation among the non-Europeans (including East Asians) suggested that, among most non-European non-Africans, the most common variants were in the silent mutation positions (Harding et al. 2000 p 1355). Thus, at least at this locus, most non-Europeans share the ancestral function. The fact that relatively light skinned east Asians varied little genetically from dark skinned Africans at this locus supports the fact that skin color is a complex trait determined by several genes. Thus light skin among east Asians occurs by way of a different genetic mechanism than that among Europeans.

With regards to Europeans, the next question to ask would be: why were there zero differences and no divergences in the amino acid sequences of the receptor protein among the Africans (and other equatorial groups) while there were 18 differences among the populations in Ireland, England, and Sweden? (Harding et al., 2000, pp.1359–1360) concluded that the intense sun in Africa created an evolutionary constraint that reduced severely the survival of progeny with any difference in the 693 sites of the MC1R gene that resulted in even one small change in the amino acid sequence of the receptor protein—because any variation from the African receptor protein produced significantly lighter skin that gave less protection from the intense African sun. In contrast, in Sweden, for example, the sun was so weak that no mutation in the receptor protein reduced the survival probability of progeny. Indeed, for the individuals from Ireland, England, and Sweden, the mutation variations among the 693 gene sites that caused changes in amino acid sequence was the same as the mutation variations in the 261 gene sites at which silent mutations still produced the same amino acid sequence. Thus, Harding concluded that the intense sun in Africa selectively killed off the progeny of individuals who had a mutation in the MC1R gene that made the skin lighter. However, the mutation rate toward lighter skin in the progeny of those African individuals who had moved North to areas with weaker sun was comparable to the mutation rate of the folks whose ancient ancestors grew up in Sweden. Hence, Harding concluded that the lightness of human skin was a direct result of random mutations in the MC1R gene that were non-lethal at the latitudes of Sweden. Even the mutations that produce red hair with little ability to tan were non-lethal in the northern latitudes.

(Rogers, Iltis & Wooding 2004) examined Harding's data on the variation of MC1R nucleotide sequences for people of different ancestry to determine the most probable progression of the skin tone of human ancestors over the last five million years. Comparing the MC1R nucleotide sequences for chimpanzees and humans in various regions of the Earth, Rogers concluded that the common ancestors of all humans had light skin tone under dark hair—similar to the skin tone and hair color pattern of today's chimpanzees. That is 5 million years ago, the human ancestors' dark hair protected their light skin from the intense African sun so that there was no evolutionary constraint that killed off the progeny of those who had mutations in the MC1R nucleotide sequences that made their skin light. (Sweet 2002) argues that based on cave paintings, Europeans may have been dark as recently as 13,000 years ago. The painters depicted themselves as having darker complexions than the animals they hunted.

However, over 1.2 million years ago, judging from the numbers and spread of variations among human and chimpanzee MC1R nucleotide sequences, the human ancestors in Africa began to lose their hair and they came under increasing evolutionary pressures that killed off the progeny of individuals that retained the inherited lightness of their skin. Folate breakdown in sun-exposed skin is inhibited by the presence of melanin and is essential for human fetal development. It is likely that folate conservation played an important role in the selection of dark skin in the ancient African ancestors of modern humans. By 1.2 million years ago, all people having descendants today had exactly the receptor protein of today's Africans; their skin was dark, and the intense sun killed off the progeny with any lighter skin that resulted from mutational variation in the receptor protein (Rogers, Iltis & Wooding 2004, p. 107).

However, the progeny of those humans who migrated North away from the intense African sun had another evolutionary constraint: vitamin D availability. Human requirements for vitamin D (cholecalciferol) are in part met through photoconversion of a precursor to vitamin D3. As humans migrated north from the equator, they were exposed to less intense sunlight, in part because of the need for greater use of clothing to protect against the colder climate. Thus, under these conditions, evolutionary pressures would tend to select for lighter-skinned humans as there was less photodestruction of folate and a greater need for photogeneration of cholecalciferol. Tracking back the statistical patterns in variations in DNA among all known people sampled who are alive on the Earth today, it appears that

  1. From ~1.2 million years ago for at least ~1.35 million years, the ancestors of all people alive were as dark as today's Africans.
  2. The descendants of any pre-historic people who migrate North from the equator will mutate to become light over time because the evolutionary constraint keeping Africans' skin dark decreases generally the further North a people migrates[19]. This also occurs as a result of selection for light skin due to the need to produce vitamin D by way of the penetration of sunlight into the skin (the exception being if dietary sources of vitamin D are available—see the Inuit).
  3. The genetic mutations leading to light skin among East Asians are different from those of Europeans, suggesting that, following the migration out of Africa, the two groups became distinct populations that experienced a similar selective pressure due to settlement in northern latitudes.

Footnotes

  1. ^ Frost, Peter (2006). "Why Do Europeans Have So Many Hair and Eye Colors?". University of California – Los Angeles. http://cogweb.ucla.edu/ep/Frost_06.html. Retrieved on 2007-10-15.
  2. ^ Norton et al., 2006
  3. ^ http://pages.globetrotter.net/peter_frost61z/European-skin-color.htm
  4. ^ http://www.snowwowl.com/peopleinuit5.html
  5. ^ http://www.nichd.nih.gov/news/releases/miscarriage_risk.cfm
  6. ^ Vitamin D, generated in the skin by the UV radiation in sunlight, is essential for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease - Holick 80 (6): 1678S - American Journal of Clinical Nutrition
  7. ^ http://www.ars.usda.gov/IS/pr/2006/060525.htm
  8. ^ http://answers.yahoo.com/question/index?qid=20080913055751AACeYIj
  9. ^ Peter Frost "Fair Women, Dark Men: The Forgotten Roots of Color Prejudice," (2005).
  10. ^ Lyang 2006)
  11. ^ Snowden, Frank. Blacks in antiquity: Ethiopians in the Greco-Roman experience. Harvard University Press, 1970.
  12. ^ Weller, Richard; John Hunter, John Savin, Mark Dahl (2008). Clinical Dermatology (4th ed.). Malden, Massachusetts, USA: Blackwell Publishing. pp. 268. ISBN 978-1-4051-4663-0. ; Cancer Research UK, sample images
  13. ^ a b c d these are commonly encountered names for the types, e.g. US Army "Healthy Skin Campaign" goldnbrown.co.uk, hautzone.ch etc.
  14. ^ (Jablonski & Chaplin 2000, p. 67), formula coefficients have been rounded to one-figure accuracy
  15. ^ SpringerLink - Journal Article
  16. ^ http://backintyme.com/admixture/shriver01.pdf
  17. ^ Harding et al., 2000, pg.1351
  18. ^ See DNA Codon Table
  19. ^ (Rogers, Iltis & Wooding 2004), (Jablonski 2004)

[edit] References
  • Harding, Rosalind M.; Eugene Healy, Amanda J. Ray, Nichola S. Ellis, Niamh Flanagan, Carol Todd, Craig Dixon, Antti Sajantila, Ian J. Jackson, Mark A. Birch-Machin, and Jonathan L. Rees (2000). "Evidence for variable selective pressures at MC1R". American Journal of Human Genetics 66: 1351–1361. doi:10.1086/302863.
  • Rogers, Alan R.; Iltis, David; Wooding, Stephen (2004), "Genetic variation at the MC1R locus and the time since loss of human body hair", Current Anthropology 45 (1): 105–108, doi:10.1086/381006
  • Sweet, Frank W. (2002), The Paleo-Etiology of Human Skin Tone, http://www.backintyme.com/Essay021215.htm
    • Proposes that the advent of agriculture and a grain diet low in vitamin D gave Northern Europeans their very pale skin.
    • Argues that skin tone is regulated by five genes and suggests Native Americans lost some genes in passage through the Arctic, preventing them from evolving very dark skin in equatorial America.
    • Gives some history of global skin tone maps, noting that Biasutti map is out of date.
  • Jablonski, Nina G., and George Chaplin (2002). "Skin deep." Scientific American 287 (4) (October): 74-82.
  • Lamason RL, Mohideen MA, Mest JR, Wong AC, Norton HL, Aros MC, Jurynec MJ, Mao X, Humphreville VR, Humbert JE, Sinha S, Moore JL, Jagadeeswaran P, Zhao W, Ning G, Makalowska I, McKeigue PM, O'Donnell D, Kittles R, Parra EJ, Mangini NJ, Grunwald DJ, Shriver MD, Canfield VA, Cheng KC (2005). SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans. Science 310 (5755): 1782-6. PMID 16357253
  • Rees, J.L., and N. Flanagan (1999). "Pigmentation, melanocortins, and red hair." Q. J. Med." 92: 125-131.
  • Robins, A.H. 1991. Biological Perspectives on Human Pigmentation. Cambridge University Press.

Millington GWM. (2006) Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors. Clin Exp Dermatol 31: 407-412.

Millington GWM, Levell NJ. (2007) From genesis to gene-sequencing: historical progress in the understanding of skin color. Intl J Dermatol 46: 103-105.

Further reading

  • Nicholas Wade (August 19 2003), "Why Humans and Their Fur Parted Ways" New York Times (Science Times). Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time at which humans invented clothing.
  • Key gene 'controls skin tone' SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry

External links

_________________________________________________

In most societies and by some anthropologists, color terminology was used to label races, sometimes in addition to a non-color term for the same race. Identifying races in terms of their skin color has been common since at least the Physiognomica falsely attributed to Aristotle.

Other scientists were more cautious about such categorization, and Charles Darwin argued that the number of categories, or in this case the number of different colors, is completely arbitrary and subjective. For example, some claimed three distinct colors, some four, and others have claimed even more. In contrast, Darwin argued that there are gradations, or degrees between the numbers of categories claimed, and not distinct categories, or colors.[1]

File:Map of skin hue equi3.png

Western classifications

Johann Friedrich Blumenbach
Further information: Scientific racism

Johann Friedrich Blumenbach (1752-1840), one of the founders of what some call scientific racism theories, came up with the five color typology for humans: white people (the Caucasian or White race), more or less black people (the Ethiopian or black race), yellow people (the Mongolian or yellow race), cinnamon-brown or flame colored people (the American race or red race), and brown people (the Malay or brown race). Blumenbach listed the "races" in a hierarchic order of physical similarities: Caucasian, followed by American, followed by Mongolian, followed by Malayan, followed by Ethiopian.

The concept of “black” as a metaphor for race was first[dubious – discuss] used at the end of the 17th century when a French doctor named François Bernier (1625-1688), an early proponent of scientific racism, divided up humanity based on facial appearance and body type. He proposed four categories: Europeans, Far Easterners, Lapps, and Blacks.[2] The first major scientific model was created in 18th century when Carolus Linnaeus recognized four main races: Europeanus which he labeled the white race, Asiatic, which he labeled the yellow race, Americanus, which he labeled the red race, and Africanus, which he labeled the black race.[3] Linnaeus' protégé, anthropology founder Johann Blumenbach completed the model by adding the brown race, which he called "Malay" for Polynesians and Melanesians of Pacific Islands, and for aborigines of Australia.[4] Rand McNally's map of races describes Amerindians as being the copper race or copper people. [5]

A 1920 map shows the races of the world classified by color: White (shown colored red), black (shown colored gray), yellow, brown, and red (shown colored light orange) (It was understood that Amerindians were the red race.); areas of mixed black and white populations are shown as purple. Later maps of the 1930s through 1960s brought the colors portrayed into line with the accepted definitions of the various races at that time according to physical anthropology (rather than simple external skin color alone as in this 1920 map) and one later map classified North Africans, Middle Easterners, the Iranian peoples, and South Asians as being white, Turkish peoples as being yellow, and restricted the brown category to those now called Austronesians, who were termed the Malayan race. [6]

According to conservative writer Dinesh D'Souza, "Blumenbach's classification had a lasting influence in part because his categories neatly broke down into familiar tones and colors: white, black, yellow, red, and brown."[7][8]

Different Races

Two historical anthropologists favored a binary racial classification system that divided people into a light skin and dark skin categories. 18th century anthropologist Christoph Meiners, who first defined the Caucasian race, posited a "binary racial scheme" of two races with the Caucasian whose racial purity was exemplified by the "venerated... ancient Germans", although he considered some Europeans as impure "dirty whites"; and "Mongolians", who consisted of everyone else.[9] Meiners did not include the Jews as Caucasians and ascribed them a "permanently degenerate nature".[10] Hannah Franzieka identified 19th c. writers who believed in the "Caucasian hypothesis" and noted that "Jean-Julien Virey and Louis Antoine Desmoulines were well-known supports of the idea that Europeans came from Mount Caucasus."[11] In his political history of racial identity, Bruce Baum wrote,"Jean-Joseph Virey (1774-1847), a follower of Chistoph Meiners, claimed that "the human races... may divided... into those who are fair and white and those who are dark or black."[12] Later, another binary racial classification system took hold de facto in the United States based on light skin people and dark skin or black people. According to Stephen Saris, in the United States there are two big racial divides. "First, there is the black-white kind, which is basically anti-black.". The second racial divide is the one is "between whites and everyone else" with whites being "narrowly construed" and everyone else being called "people of color".[13]

Understanding of melanin

Racial classification according to skin color became more complex when anthropologists added other, less obvious characteristics, in their attempt to achieve a scientific classification of races. It was later found that skin color depended essentially from the amount of melanin, and could vary widely in the same community. Thus, it could not provide a satisfying way to classify ethnic groups, much less "races." Following World War II and the discredit of such racial classifications, more and more biologists and anthropologists began to question the concept itself of "race." Thus, The Race Question statement by the UNESCO, in the 1950s, proposed to substitute the term "ethnic groups" to the concept of "race," arguing that human communities were defined as much by cultural traits (language, religion, etc.) than by biological characteristics (skin color being only one of them, along with blood types, which did not recover previous racial classifications, etc.).

Symbolism and uses of color terminology

The Martinique-born French Frantz Fanon and African-American writers Langston Hughes, Maya Angelou, and Ralph Ellison, among others, wrote that negative symbolisms surrounding the word "black" outnumber positive ones. They argued that the good vs. bad dualism associated with white and black unconsciously frame prejudiced colloquialisms. In the 1970s the term black replaced Negro in the United States.[14]

Tone gradations

In some societies people can be sensitive to gradations of skin tone, which may be due to intermarriage or to albinism and which can affect power and prestige. In 1930s Harlem Slang such gradations were described by a tonescale of "high yaller [yellow], yaller, high brown, vaseline brown, seal brown, low brown, dark brown".[15] These terms were sometimes referred to in blues music, both in the words of songs and in the names of performers. In 1920s Georgia, Willie Perryman followed his older brother Rufus in becoming a blues piano player: both were albino Negroes with pale skin, reddish hair and poor eyesight. Rufus was already well established as "Speckled Red", Willie became "Piano Red".[16] The piano player and guitarist Tampa Red from the same state developed his career in Chicago, Illinois, at that time: his name may have come from his light skin tone, or possibly reddish hair.

More recently such categorisation has been noted in the Caribbean. It is reported that skin tones play an important role in defining how Barbadians view one another, and they use terms such as "brown skin, light skin, fair skin, high brown, red, and mulatto".[17] An assessment of racism in Trinidad notes people often being described by their skin tone, with the gradations being "HIGH RED – part White, part Black but ‘clearer’ than Brown-skin: HIGH BROWN – More white than Black, light skinned: DOUGLA –part Indian and part Black: LIGHT SKINNED, or CLEAR SKINNED Some Black, but more White: TRINI WHITE – Perhaps not all White, behaves like others but skin White".[18] In Jamaica albinism has been stigmatised, but the albino dancehall singer Yellowman took his stage name in protest against such prejudice and has helped to end this stereotype. The West Indian region uses the term "coolie" for all people of east Indian descent. In Trinidad, however, use of the term is considered extremely offensive.

Russia

In Russia, persons of Caucasus or Kazakh descent are sometimes referred to as Black. "White", apart from its racial meaning, is also a term denoting opponents of the Bolsheviks during the Russian Civil War (see White movement for this usage).

Sometimes, Belarus and Belarusians have been referred to (in Western languages, not Russian) as "White Russia" and "White Russians", which can be misleading; see those articles for discussion in more depth.

China

Huang (yellow) is a common surname, but does not refer to the East Asian race as was popular in Western languages until recently. However, the Yellow Emperor was a legendary founder of China. Yellow is also identified with the "center" cardinal direction (blue-east, red-south, white-west, black-north) while China is known as Zhongguo "central country".

White (白 bai) means "plain" or "free of charge" in many common expressions and was not traditionally used to refer to Europeans or descendants, who were usually identified as "people from [across the] ocean" or some variety of "barbarians" with reddish or pinkish skin colors (eg. Minnan ang mo, "red-haired"). Contemporary Chinese, has, however, adopted Western usage to a large extent. Black (黑 hei) is typically applied to those of African race today. However, the term "black resident" (黑户) also refers to unregistered rural migrants in cities (as in black market).

Names of ethnic minorities sometimes contain colors, not to indicate skin color, but simply for identification, possibly based on traditional clothing or geographical direction.

  • Red, Black, Blue/Green, White, Flowery (multicolored) Miao (Hmong)
  • the Bai (literally White) are a sedentary lowland people of Yunnan
  • Black Bone and White Bone Yi
  • The Qing dynasty Manchu military were divided into Eight Banners identified by color and with ethnic associations

The Five Races Under One Union theory of national unity can be visualised through an old ROC flag and a variant which emphasised Han administration while de-emphasising the top-to-bottom hierarchy found in the original flag. Red - Han, Yellow - Manchu, Blue - Mongol, White - Hui and Black - Tibetan.

Korea

The word, 인종 in-jong, is used when describing a person's race, which also incorporates his or her skin color. White 백 baek, used with 인 in to make 백인, baek-in, literally means white-person in Korean, cognate to Chinese bairen 白人 and Japanese hakujin 白人. 흑 heug is used to describe persons of African descent, (i.e. 흑인, cognate to Chinese heiren 黑人 and Japanese kokujin 黒人).

Central Asia

The five cardinal directions were historically identified with colors. This was common to the Central Asian cultural area and was carried west by the westward migration of the Turks. These directional color terms were applied both to geographic features and sometimes to populations as well.

North America

Among the natives of North America, stylised medicine wheels are typically depicted with four colors associated with not only cardinal direction, but also with color of race:

  • North: White
  • South: Red
  • East: Yellow
  • West: Black

2000 US Census

In the 2000 United States Census, two of the five enumerated races are labeled by a color.[19] In the 2000 US Census, the white race, refers to a "person having origins in any of the original peoples of Europe, the Middle East, or North Africa."[19] In the 2000 US Census, the black race, also called "African American" on the US Census, refers to a "person having origins in any of the Black racial groups of Africa."[19] The other three races the are not labeled by color.[19] Respondents who reported their race in the write-in section as "colored" or "negro'" (Spanish for the color black) were automatically categorized as part of the black race.[20]

Flag of the Races

At college campuses during the 1960s, some people demonstrated for world peace by carrying a Flag of the Races (also called the Flag of the Human Race) with five horizontal stripes (from top to bottom they were red, black, brown, yellow, and white). Gilbert Baker is said to have gotten the idea for the rainbow flag from this flag. [21]

Notes and references

  1. ^ "It may be doubted whether any character can be named which is distinctive of a race and is constant.", Charles Darwin, The Descent of Man p225,
    The Mis-portrayal of Darwin as a Racist
  2. ^ The End of Racism by Dinesh D'Souza, pg 123, 1995
  3. ^ The Importance of “Whiteness” in American Legal History
  4. ^ The Mismeasure of Man by Stephen Jay Gould, pg 402, 1996
  5. ^ See: Rand McNally’s World Atlas International Edition Chicago:1944 Rand McNally Map: "Races of Mankind" Pages 278-279--In the explanatory section below the map, the American Indian Race is described as being "copper-colored"
  6. ^ For an example of a later color race map, see: Rand McNally’s World Atlas International Edition Chicago:1944 Rand McNally Map: "Races of Mankind" Pages 278-279 (See explanatory section below map)
  7. ^ The End of Racism by Dinesh D'Souza, pg 124, 1995
  8. ^ Race, Identity and Africanity: A Reply to Eboussi Boulaga
  9. ^ Painter, Nell Irvin. Yale University. "Why White People are Called Caucasian?" 2003. September 27, 2007. [1]
  10. ^ Eigen, Sara. The German Invention of Race. Suny Press:New York, 2006. ISBN 0-79146-677-9 p.205
  11. ^ Franzieka, Hannah. Berghahn Books: 2004. ISBN 157181857X James Cowles Prichard's Anthropology: Remaking the Science of Man in Early
  12. ^ Baum, Bruce David. The Rise and Fall of the Caucasian Race: A Political History of Racial Identity. New York University: 2006. ISBN 0814798926
  13. ^ Zack, Naomi. American Mixed Race: The Culture of Microdiversity 1995
  14. ^ Black, black, or African American?, Aly Colón
  15. ^ Zora Neale Hurston's - Glossary of Harlem Slang "Tonescale"
  16. ^ The Blues Collection issue 68, Piano Red, Contribution by Tony Russell, 1996
  17. ^ Barbados - Post Report - eDiplomat
  18. ^ RACISM IN TRINIDAD (pdf)
  19. ^ a b c d US Census Bureau. Race. 2000. Accessed July 14, 2008. [2]
  20. ^ University of Virginia Library. 1990 PUMS Ancestry Codes. 2003. [3]
  21. ^ Carleton College—“Symbols of Pride of the LGBTQ community”:

See also
Posted by at

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)