Human skin color


Human skin color
Variety of skin colors

Human skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath.[1] Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain. According to scientific studies, natural human skin color diversity is highest in Sub-Saharan African populations,[2] 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.[3] The term "range" is loosely defined in this case, as African albinos have obviously not been taken into consideration when calculating the "range".

The natural skin color can be darkened as a result of tanning due to exposure to sunlight. The leading explanation is that skin color adapts to sunlight intensities which produce vitamin D or ultraviolet light damage to folic acid.[4] Other hypotheses include protection from ambient temperature, infections, skin cancer or frostbite, an alteration in food, and sexual selection.[5]

The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and racism.

Contents

Melanin and genes

Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is triggered by an enzyme called tyrosinase, which creates the color of skin, eyes, and hair shades. Melanocytes produce two types of melanin: 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.

Melanin controls the amount of ultraviolet (UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.

Genetics of skin color variation

The KIT ligand (KITLG) gene is involved in the permanent survival, proliferation and migration of melanocytes.[6] A mutation of this gene, A326G (rs642742[7]) has been positively associated with variations of skin color 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.[8] The A326G allele occurs in over 80% of European and Asian samples, compared with less than 10% in African samples.[9]

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 color in humans – rs2424984[10] has been identified as one of three indicators of skin tone in a forensics analysis of human phenotypes[11] and has a frequency of roughly 80% in Europeans, 75% in Asians and 20–25% in Africans.[12] A 2-SNP haplotype (rs4911414[13] and rs1015362[14]) has also been linked with skin color variation within European populations[15] and has a similar frequency distribution.[16]

Solute carrier family 24 member 5 (SLC24A5) regulates calcium in melanocytes and is important in the process of melanogenesis. The Thr111Ala allele (rs1426654[17]) has been shown to be a major factor in the light skin tone of Europeans in a number of studies.[11][18][19][20] It is virtually non-existent in Asian and African populations and is found in about 99.9% of Europeans.[21] It is believed to represent some 25–40% of the difference in skin tone between Europeans and Africans,[22] and appears to have arisen as recently as within the last 10,000 years.[23]

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 color of modern Europeans through its Phe374Leu (rs16891982[24]) variation.[11][15][19][20][25] 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[26]), found solely in 40–50% of Europeans[27] and linked to light-colored skin in studies of mixed-race populations.[19][28]

Oculocutaneous albinism II (OCA2) assists in the regulation of pH in melanocytes. The His615Arg (rs1800414[29]) allele has been shown to account for about 8% of the skin tone difference between African and East Asian populations.[30] It is found in 85% of East Asian samples and is non-existent in European and African samples.[31]

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[32]),[33] melanocortin 1 receptor (MC1R) Arg163Gln (rs885479[34])[35] and attractin (ATRN)[25] have been indicated as potential contributors to the evolution of light skin in East Asian populations.

Mutations in genes can cause also affect skin color 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.[36]

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[37]), Arg160Trp (rs1805008[38]), Asp294Sys (rs1805009[39]), Val60Leu (rs1805005[40]) and Val92Met (rs2228479[41]) have been shown to cause red hair and pale skin that does not tan in a small percentage of the human population.[42] 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.[25][28]

Evolution of skin color

Approximately 1.5 million years ago, the earth endured a megadrought that drove hominids from lush rainforests into arid, open landscapes. This, coupled with the loss of dense body hair, caused early human skin to endure excess UV-B radiation and xeric stress [43]. Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of 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. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair [44]. Over time human hair disappeared to allow better heat dissipation through sweating[3] and the skin tone grew darker to increase the epidermal permeability barrier[43] and protect from folate depletion due to the increased exposure to sunlight.[4] By 1.2 million years ago, around the time of homo ergaster and homo erectus, the ancestors of all people living today had exactly the same receptor protein as modern Africans.[44] 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 it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D.[3] 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 from Africans which had dark skin.
  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,[25] suggest the two groups experienced a similar selective pressure due to settlement in northern latitudes.[5]

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.[45][46] 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[23] which is in line with the earliest evidence of farming.[47]

One of the most recently proposed drivers of the evolution of skin pigmentation in humans is based on research that shows a superior barrier function in darkly pigmented skin [48]. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the stratum corneum (SC). It is therefore logical to surmise that the SC that has undergone the most genetic change since the loss of human body hair. Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermis, because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented tribes.

Irregular pigmentation

Uneven pigmentation of some sort affects most people, regardless of ethnic background or skin color. Skin may either appear lighter or darker than normal; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Skin pigmentation disorders occur because the body produces either too much or too little melanin.

Increased melanin production, also known as hyperpigmentation, can be:

  • Melasma describes the darkening of the skin.
  • Chloasma describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy.
  • Solar lentigo, also known as "liver spots" or "senile freckles" refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure.

Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes.[49] This is especially true for those with darker skin tones.

The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as liver spots, these pigment problems are not connected with the liver.

On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin.

Exposure to sun

Melanin in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes direct and indirect DNA damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation.

There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin. Secondly, there is an increase in production of melanin (melanogenesis).[50] Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin.

A person's natural skin color has an impact on their reaction to exposure to the sun. 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:[51][52][53]

Type Also called Sunburning Tanning behavior von Luschan scale
I Very light or white, "Celtic" type Often Occasionally 1–5
II Light or light-skinned European Usually Sometimes 6–10
III Light intermediate or dark-skinned European Rarely Usually 11–15
IV Dark intermediate, also "Mediterranean" or "olive skin" and East Asian Rarely Often 16–21
V Dark or "brown" type Very rarely Sometimes darkens 22–28
VI Very dark or "black" type Extremely rarely Naturally black-brown skin 29–36

Dark skin with large concentrations of melanin protects against exposure to ultraviolet light and skin cancers; light-skinned people 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.[54]

In a number 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.[55]

While dark skin offers better protection from intense ultraviolet light, it may result in low vitamin D levels and has led to concern that darker skinned people living at relatively high latitude, such as African Americans, may have inadequate vitamin D levels.[56][57] Research shows that dark-skinned people living in Western societies have lower vitamin D levels.[58][59] The explanation for low vitamin D levels in dark-skinned people is thought to be that melanin in the skin hinders vitamin D synthesis.[60] However, recent studies have found novel evidence that low vitamin D levels among people of African ancestry may be due to other reasons,[61] such as that black women have an increase in serum parathyroid hormone – implicated in adverse cardiovascular outcomes – at a lower vitamin D level than white women.[62] In a large scale association study of the genetic determinants of vitamin D insufficiency in Caucasians no links to pigmentation were found.[63][64]

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.[65] 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.[66] 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.[67]

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.[68] However, any given physical characteristic generally is found in multiple groups,[69] 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.[70]

Social status and racism

French actresses Romane Bohringer and Aïssa Maïga at the Deauville American Film Festival, 2009.
Skin colors according to von Luschan scale

According to classical scholar Frank Snowden, skin color did not determine social status in ancient Egypt, Greece or Rome. Relations between the major power and the subordinate state was viewed as more significant in a person's status than was their skin color.[71]

The preferred skin tone varies by culture and has varied over time. A number of indigenous African groups, such as the Maasai, associated pale skin with being cursed or caused by evil spirits associated with witchcraft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin.[72]

Many cultures have historically favored lighter skin for women. In Europe, before the Industrial Revolution, pale skin was preferred and 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.[73] Women even went as far as to put lead-based cosmetics on their skin to artificially whiten their skin tone.[74] However, when not strictly monitored these cosmetics caused lead poisoning. Achieving a light-skinned appearance was brought about in many other ways, including the use of arsenic to whiten skin, and powders. Other methods included wearing full-length clothes when outdoors, including gloves and parasols.

Colonization and slavery by European countries inspired racism, led by the belief that people with dark skin were uncivilized and were to be considered inferior and subordinate to the "white races", which has continued to be perpetuated in modern times.[75] During slavery, lighter-skinned African Americans were perceived as more intelligent, cooperative, and beautiful.[76] They were more likely to work as house slaves and were also given preferential treatment by plantation owners and the overseers. For example, they had a chance to get an education[77] while darker African Americans worked in the fields and did not get an education.[78] The preference for fair-skin remained prominent until the end of the Victorian era, but the racial stereotypes about worth and beauty were still persistent in the last half of the 20th century. African American journalist Jill Nelson wrote that "to be both prettiest and black was impossible"[79] and elaborated:

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... ... 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.[79]

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

In some countries, there continues to be a preference for fair or lighter skin, even in Latin American countries where whites are a minority.[80] In Mexico, light skin is associated with power, as well as attractiveness.[81] A dark-skinned person is more likely to be discriminated against in Brazil.[82] Most Latin American actors and actresses have Caucasian features—blond hair, blue eyes, and pale skin.[83][84][85][86][87][88][89] 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 some countries that specific words describe distinct skin tones from "hincha", Puerto Rican slang for "glass of milk" to "morena", literally "brown".[89]

Skin whitening products have remained prominent over time, often due to historical beliefs and perceptions about fair skin. 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.[90] 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.[91] Changes in regulations in the cosmetic industry led to skin care companies introducing harm free skin lighteners. This also occurs in South Asian countries, and in India, pale skin is considered more attractive and skin whitening is common. Most actors and actresses have light skin.[92] In Japan, the geisha was well known for their white painted faces, and the appeal of the bihaku (美白?), or "beautiful white", ideal leads many Japanese women to avoid any form of tanning.[93] There are exceptions to this, with Japanese fashion trends such as ganguro emphasizing almost black skin, while, in India, dark skin is sometimes associated with a lower class status and some people resort to skin bleaching to achieve a skin color they consider more socially desirable.[94] Skin whitening products sales across the world grew from $40 to $43 billion in 2008.[95] Skin whitening is also not uncommon in Africa,[96][97] and several research projects have suggested a general preference for lighter skin in the African-American community.[98] In contrast, one study on men of the Bikosso tribe in Cameroon found no preference for attractiveness of females based on lighter skin color, bringing into question the universality of earlier studies that had exclusively focused on skin color preferences among non-African populations.[99]

It has been found that, on average, women of a given ancestry have a lighter skin tone than men of the same ancestry[3] 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,[100] University of Washington sociologist Pierre L. van den Berghe stated, "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, many are indifferent to male pigmentation or even prefer men to be darker."[101] He elaborated that popular media in the Western world has repeatedly associated blacks with "advantageous stereotypes," as much as negative ones, such as "myths that praise their athletic aptitudes amongst many other things, and often depict them as males of superior genetic inheritance".[100]

Significant exceptions to a preference for lighter skin started to appear in Western culture mid-20th century.[102] Though sun-tanned skin used to be associated with the sun-exposed manual labor of the lower-class, the associations became dramatically reversed during this time – a change usually credited to the trendsetting French woman Coco Chanel making tanned skin seem fashionable, healthy, and luxurious.[103] Today, though a preference for lighter skin remains prevalent in the United States, many within the country regard tanned skin as both more attractive and healthier than pale skin.[104][105][106][107] Western mass media and popular culture continued to reinforce negative stereotypes about dark skin,[108] but pale skin has become associated with indoor office work while tanned skin has become associated with increased leisure time, sportiness and good health that comes with wealth and higher social status.[73] Studies indicating that the degree of tanning is directly related to how attractive a young woman is have also emerged.[109][110] The election of a dark-skinned United States President and the financial success of many African-American singers and actors has challenged a number of stereotypes associated with darker skin tones.[111] There has also been an increase in the perceived attractiveness of dark-skinned women.[112]

See also

Other:

Footnotes

  1. ^ Walters & Roberts 2008, p. 61.
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References

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

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