Human skin color

"Skin pigmentation" redirects here. For animal skin pigmentation, see Biological pigment.

Human skin color is primarily due to melanin; it ranges from skin almost black in appearance to white with a pinkish tinge due to blood vessels underneath. Variations in skin hue can be caused by tanning due to exposure to sunlight, but are mainly of genetic origin although the evolutionary causes are not completely certain. The leading explanation is that skin colour adapts to sunlight intensities which produce vitamin D deficiency or ultraviolet light damage to folic acid. Other hypotheses include protection from ambient temperature, infections, skin cancer or frostbite, an alteration in food, and sexual selection. According to scientific studies, natural human skin color diversity is highest in Sub-Saharan African populations, with skin reflectance values ranging from 19 to 46 (med. 31) compared with European and East Asian populations which have skin reflectance values of 62 to 69 and 50 to 59 respectively.

Melanin and genes

Main article: Melanin

Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both the amount and type of melanin produced is controlled by a number of genes which operate under incomplete dominance. One copy of each of the various genes is inherited from each parent. Each gene can come in several alleles, resulting in the great variety of human skin tones. By absorbing ultraviolet (UV) radiation from the sun, melanin controls the amount that penetrates the skin. UV radiation is needed to manufacture vitamin D, but excess UV can damage health.

Genetics of skin color variation

See also: Human genetic variation, Race and genetics, and Human genetic clustering

The KIT ligand (KITLG) gene is involved in the permanent survival, proliferation and migration of melanocytes, the cells that produce melanin. A mutation of this gene, A326G (rs642742) has been positively associated with variations of skin colour in African-Americans of mixed West African and European descent and is estimated to account for 15-20% of the melanin difference between African and non-African populations. The A326G allele occurs in over 80% of European and Asian samples, compared with less than 10% in African samples.

Agouti signalling peptide (ASIP) acts as an inverse agonist, binding in place of alpha-MSH and thus inhibiting eumelanin production. Studies have found alleles in the vicinity of ASIP are associated with skin colour in humans - rs2424984 has been identified as one of three indicators of skin tone in a forensics analysis of human phenotypes and has a frequency of roughly 80% in Europeans, 75% in Asians and 20-25% in Africans. A 2-SNP haplotype (rs4911414 and rs1015362) has also been linked with skin colour variation within European populations and has a similar frequency distribution.

Solute carrier family 24 member 5 (SLC24A5) regulates calcium in melanocytes and is important in the process of melanogenesis. The Thr111Ala allele (rs1426654) has been shown to be a major factor in the light skin tone of Europeans in a number of studies. It is virtually non-existant in Asian and African populations and is found in nearly 100% of Europeans. It is believed to represent some 25-40% of the difference in skin tone between Europeans and Africans, and appears to have arisen as recently as within the last 10,000 years.

Solute carrier family 45 member 2 (SLC45A2 or MATP) aids in the transport and processing of tyrosine, a precursor to melanin. It has also been shown to be a major factor in the skin colour of modern Europeans through its Phe374Leu (rs16891982) variation. Like SLC24A5 it is ubiquitous in European populations but extremely rare elsewhere.

The TYR gene encodes the enzyme tyrosinase which is involved in the production of melanin from tyrosine. It has an allele, Ser192Tyr (rs1042602), found solely in 40-50% of Europeans and linked to light coloured skin in studies of mixed-race populations.

Oculocutaneous albinism II (OCA2) assists in the regulation of pH in melanocytes. The His615Arg (rs1800414) allele has been shown to account for about 8% of the skin tone difference between African and East Asian populations. It is found in 85% of East Asian samples and in non-existant in European and African samples.

A number of studies have found a genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Asian populations. While not linked to measurements of skin tone variation directly, dopachrome tautomerase (DCT or TYRP2 rs2031526), melanocortin 1 receptor (MC1R) Arg163Gln (rs885479) and attractin (ATRN) have been indicated as potential contributors to the evolution of light skin in East Asian populations.

Mutations in genes can cause also affect skin colour through oculocutaneous albinism (OCA) - a lack of pigment in the eyes, skin and sometimes hair that occurs occasionally in a very small fraction of the population. The four known types of OCA are caused by mutations of the TYR, OCA2, TYRP1 and SLC45A2 genes.

The gene MC1R is primarily responsible for determining whether pheomelanin and eumelanin is produced in humans. Mutations of this very polymorphic gene, such as Arg151Sys (rs1805007), Arg160Trp (rs1805008), Asp294Sys (rs1805009), Val60Leu (rs1805005) and Val92Met (rs2228479) have been shown to cause red hair and pale skin that does not tan in a small percentage of the human population. Although these alleles have differing frequencies across African, European and Asian populations, there is no evidence of positive selection for them and they do not appear to be associated with the evolution of lighter skin in Eurasian populations.

Evolution of skin color

Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences for chimpanzees and humans from various regions of the Earth. Rogers concluded that roughly five million years ago, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Over time the hair disappeared to allow better heat dissipation through sweating and the skin tone grew darker to protect from folate depletion to the increased exposure to sunlight. By 1.2 million years ago, shortly after the final speciation of homo sapiens from homo ergaster, the ancestors of all people living today had exactly the same receptor protein as modern Africans. Evolutionary pressure meant that any gene variations that resulted in lighter skin were unable to survive under the intense African sun, and human skin remained dark for the next 1.1 million years.

Approximately 70,000-100,000 years ago modern humans began to migrate away from the tropics to the north where they were exposed to less intense sunlight, possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin so would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that:-

1. From ~1.2 million years ago to less than 100,000 years ago, the ancestors of all people alive were as dark as modern Africans.
2. As populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance North a population migrated, resulting in a range of skin tones within northern populations.
3. At some point northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations.

The genetic mutations leading to light skin, though different among East Asians and Europeans, suggest the two groups experienced a similar selective pressure due to settlement in northern latitudes.

There is a long-standing hypothesis that the selection for lighter skin due to higher vitamin D absorption occurred soon after the Out of Africa migration sometime before 40,000 years ago. A number of researchers disagree with this and suggest that the northern latitudes permitted enough synthesis of vitamin D combined with food sources from hunting to keep populations healthy, and only when agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D. The theory suggests that the reduction of game meat, fish, and some plants from the diet resulted in skin turning white many thousands of years after settlement in Europe and Asia. This theory is supported by a study into the SLC24A5 gene which found that the allelle associated with light skin in Europe may have originated as recently as 6,000-10,000 years ago which is in line with the earliest evidence of farming.

Health related effects

Dark skin with large concentrations of melanin protects against exposure to ultraviolet light and skin cancers; light-skinned persons have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore, UV-A rays from sunlight are believed to interact with folic acid in ways which may damage health. In a wide range of traditional societies the sun was avoided as far as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap. While dark skin offers superior protection from intense ultraviolet light, it may be the cause of low Vitamin D levels in African Americans and has led to concern that darker skinned people, such as African Americans, living at relatively high latitude may be having inadequate vitamin D levels because of their relatively greater pigmentation. Research shows that dark-skinned people living in Western societies have lower vitamin D levels. The explanation for low vitamin D levels in dark-skinned people is thought to be that melanin in the skin hinders vitamin D synthesis, however recent studies have found novel evidence that low vitamin D levels among people of African ancestry may be due to other reasons, that black women have an increase in serum parathyroid hormone - implicated in adverse cardiovascular outcomes - at a lower vitamin D level than white women, and in a large scale association study of the genetic determinants of vitamin D insufficiency in Caucasians no links to pigmentation were found.

Cultural effects

French actresses Romane Bohringer and Aïssa Maïga at the Deauville American Film Festival, 2009.

Differences in skin tone are the most readily perceptible phenotypical distinction of human populations, however the connotations associated with dark and light skin have varied across cultures and time.

A number of indigenous African groups, such as the Maasai, associated pale skin with being cursed or caused by evil spirits associated with witch craft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin.

Before the Industrial Revolution in Europe, pale skin was a sign of high social status. The poorer classes worked outdoors and got darker skin from exposure to the Sun, while the upper class stayed indoors and had light skin. Light skin became associated with wealth and high position.

According to classical scholar Frank Snowden, the ancient Egyptians and Greeks assigned relatively neutral connotations to skin color variation. This was because conquest rather than skin color was the major determinant of slave status.

Colonisation and slavery by European countries led to a belief that dark skin was uncivilised and was to be considered inferior and subordinate to lighter skin, a belief that has continued into modern times for many people. During slavery, lighter-skinned African Americans were perceived as more intelligent, cooperative, and beautiful. They were more likely to work as house slaves and were also given preferential treatment by plantation owners and their henchmen. For example, they had a chance to get an education while darker African Americans worked in the fields and did not get an education.

These racial stereotypes about worth and beauty were still persistent in the last half of the 20th century; the African American journalist Jill Nelson wrote that "to be both prettiest and black was impossible" and elaborated:

As a girl and young woman, hair, body, and color were society's trinity in determining female beauty and identity, the cultural and value-laden gang of three that formed the boundaries and determined the extent of women's visibility, influence, and importance. For the most part, they still are. We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look. As we enter womanhood, the pervasive power of this trinity is demonstrated again and again in how we are treated by the men we meet, the men we work for, the men who wield power, how we treat each other and, most of all, ourselves. For black women, the domination of physical aspects of beauty in women's definition and value render us invisible, partially erased, or obsessed, sometimes for a lifetime, since most of us lack the major talismans of Western beauty. Black women find themselves involved in a lifelong effort to self-define in a culture that provides them no positive reflection.

With the majority of Western mass media and popular culture reinforcing negative stereotypes about dark skin, light skin has become a symbol of wealth and success through much of the developing world. Skin whitening products sales grew from $40 to $43 billion in 2008. Skin whitening is not uncommon in Africa and several research projects have suggested a general preference for lighter-skinned women by African-American men.

Lighter skin is seen as more attractive in Latin America. In Mexico and in Brazil, light skin represents power, as well as attractiveness. A dark-skinned person is more likely to be discriminated against in Brazil. Most South American actors and actresses (as do most other Latin American actors and actresses) have Nordic features - blue eyes, pale skin, and blond hair. A light-skinned person is considered to be more privileged and have a higher social status; a person with light skin is considered more beautiful and it means that the person has more wealth. Skin color is such an obsession in these countries that specific words describe distinct skin tones from "hincha", Puerto Rican slang for "glass of milk" to "morena", literally "brown".

A Vietnamese motorcyclist wears long gloves to block the sun, despite the tropical heat.

In South and East Asian countries, light skin has traditionally been seen as more attractive and a preference for lighter skin remains prevalent. In ancient China and Japan, for example, pale skin can be traced back to ancient drawings depicting women and goddesses with fair skin tones. In ancient China, Japan, and Southeast Asia, pale skin was seen as a sign of wealth. Thus, skin whitening cosmetic products are popular in East Asia. 4 out of 10 women surveyed in Hong Kong, Malaysia, the Philippines and South Korea used a skin-whitening cream, and more than 60 companies globally compete for Asia's estimated $18 billion market. Changes in regulations in the cosmetic industry led to skin care companies like Super Skin Lightener introducing harm free skin lighteners. In India also, pale skin is considered more attractive and skin whitening is prevalent. Most actors and actresses have light skin.

It has been found that, on average, women of a given ancestry have a lighter skin tone than men of the same ancestry and that there is a sexual preference for paleness in women and darkness in men in many cultures throughout the world. 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 ... in integrated societies has [been toward] increasing popularity for men of color, especially those [of] African descent. These trends have been recorded in areas such as South America, where in Brazil it was estimated that by 2009 black people of African descent 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."

In modern Western societies darker skin is becoming more desirable in both men and women. Pale skin has become associated with indoor office workers, while tanned skin shows the increased leisure time, sportiness and good health that comes with wealth and higher social status. A preference for women with tanned skin has emerged with studies find that the degree of tanning is directly related to how attractive a young woman is perceived to be. The election of a dark-skinned US President and the financial success of many African-American singers and actors has led to a number of positive stereotypes associated with darker skin tone, and an increase in the perceived attractiveness of dark-skinned women.

Tanning

At the beginning of the 20th Century, in the United States, lighter-skinned people avoided the sun. Tanned skin was considered lower class, a belief that is still held by many today. Tanned skin has been shown in the United States to be viewed both as more attractive and more healthy than pale skin. Though sun-tanned skin used to be associated with the sun-exposed manual labor of the lower-class, the associations became dramatically reversed in the mid-20th century, a change usually credited to the trendsetting French woman Coco Chanel making tanned skin seem fashionable, healthy, and luxurious.

Skin tone variability

The tone of human skin can vary from a dark brown to a nearly 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, although this is not always the case. Africans generally have darker skin, hair, and eyes, although this too is not universal. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:

type	also called	tanning behavior	von Luschan scale
I	very light, "Celtic" type.	Often burns, occasionally tans.	1-5
II	light, or light-skinned European.	Usually burns, sometimes tans.	6-10
III	light intermediate, or dark-skinned European.	Rarely burns, usually tans.	11-15
IV	dark intermediate, also "Mediterranean" or "olive skin".	Rarely burns, often tans.	16-21
V	dark or "brown" type.	Naturally brown skin, sometimes darkens.	22-28
VI	very dark, or "black" type.	Naturally black-brown skin.	29-36

Geographic variation

Map of indigenous skin color distribution in the world based on Von Luschan's chromatic scale.

Approximately 10% of the variance in skin color occurs within groups, and ~90% occurs between groups. Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness, populations with similar pigmentation may be genetically no more similar than other widely separated groups. Furthermore, in some parts of the world in which people from different regions have mixed extensively, the connection between skin color and ancestry has been substantially weakened. In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups.

Considerable speculation has surrounded the possible adaptive value of other physical features characteristic of groups, such as the constellation of facial features observed in many eastern and northeastern Asians. However, any given physical characteristic generally is found in multiple groups, and demonstrating that environmental selective pressures shaped specific physical features will be difficult, since such features may have resulted from sexual selection for individuals with certain appearances or from genetic drift.

See also

• Afro-textured hair
• Albinism
• Argyria
• Color terminology for race
• Complexion
• Human migration
• Hyperpigmentation
• Pigmentocracy
• Race
• Risks and benefits of sun exposure
• Sunlight
• Sun tanning
• Von Luschan's chromatic scale

Other:

• Eye color
• Human hair color
• Human physical appearance

References

• Frost, P (2005). Fair Women, Dark Men: The Forgotten Roots of Color Prejudice. {{cite book}}: Invalid |ref=harv (help)
• Frost, P (2006). "Why Do Europeans Have So Many Hair and Eye Colors?". University of California – Los Angeles. Retrieved 2007-10-15.
• {{cite journal |pages=1351–1361 |doi=10.1086/302863} |ref=CITEREFHarding_et_al2000}
• Holick, Michael F. PMID 15585788 http://www.ajcn.org/cgi/pmidlookup?view=long&pmid=15585788. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
• "The evolution of human skin coloration" (PDF): 57–106. doi:10.1006/jhev.2000.0403. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help)
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• (3): 407–412. doi:10.1111/j.1365-2230.2006.02128.x. PMID 16681590. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
• : 103–5. doi:10.1111/j.1365-4632.2006.03068.x. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help); Missing or empty |title= (help)
• : 710–22. doi:10.1093/molbev/msl203. {{cite journal}}: Cite journal requires |journal= (help); Missing or empty |title= (help)
• : 125–31. doi:10.1093/qjmed/92.3.125. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help); Missing or empty |title= (help)
• . PMID 11126724. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help); Missing or empty |title= (help)
• Robins, AH (1991). Biological Perspectives on Human Pigmentation. Cambridge University Press. ISBN 0521365147. {{cite book}}: Invalid |ref=harv (help)
• : 105–8. doi:10.1086/381006. {{cite journal}}: Cite journal requires |journal= (help); Invalid |ref=harv (help); Missing or empty |title= (help)
• Snowden, F (1970). Blacks in antiquity: Ethiopians in the Greco-Roman experience. Harvard University Press. {{cite book}}: Invalid |ref=harv (help)
• Sweet, FW (2002). "The Paleo-Etiology of Human Skin Tone". {{cite web}}: Invalid |ref=harv (help)
  • 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.
• Walters, KA; Roberts, MS (2008). Dermatologic, cosmeceutic, and cosmetic development: Therapeutic and novel approaches. New York: Informa Healthcare. {{cite book}}: Invalid |ref=harv (help)
• Weller, R (2008). Clinical Dermatology (4th ed.). Malden, Massachusetts, USA: Blackwell Publishing. p. 268. ISBN 978-1-4051-4663-0. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Further reading

• N 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

• Computer Vision : Skin Color Based Face Tracking Examples of a face tracking in videos using a non parametric skin color model.
• Excel spreadsheet with spectrophotometry of skins of several races