Sunday, November 21, 2010

Human skin color

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 color are mainly of genetic origin and are associated with sunlight, but the evolutionary causes are not completely certain. The leading explanation is they are adaptations 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 black or Sub-Saharan African populations.
Social relations have a significant impact on the perception of skin color. In traditional African society, light skin was often seen as unhealthy, and there was no overall preference for it. When people were of a similar ethnic background and were not exposed to intense sunlight, the main difference in skin color was the slight one between men and women which led to the old European stereotype of the "fair sex" and the "tall, dark, and handsome man". Where there was a historical context of slavery in which light-skinned people had been given preferential treatment compared to darker-skinned people in their peer group, it became confused with perceived physical attractiveness and desirable qualities such as social and intellectual competence, probity, and even mental stability. Through the power of mass media, skin tone has often become a standard for evaluating ability and prettiness and has had more of a bearing on women's self-image than that of men. A preference for women with tanned skin has emerged in the modern West; studies find that the degree of tanning is directly related to how attractive a young woman is perceived to be.

Melanin and genes

Main article: Melanin
Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type genes which operate under incomplete dominance. One copy of each of the various genes is inherited from each parent. Each gene comes 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
 Human genetic variation, Race and genetics, and Human genetic clustering
Several genes have been invoked to explain variations of skin tones in humans, including SLC45A2, ASIP, TYR, and OCA2. SLC24A5 has been shown to account for a substantial fraction of the difference in melanin units between Europeans and Africans, variations in human skin tones have been correlated with mutations in another gene; the MC1R gene. Harding found no differences among Africans for the amino acid sequences in their receptor proteins while among certain European non-African individuals there were.
Examination of the variation in 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 and 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. 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.
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, 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. Hence the leading hypothesis for the evolution of human skin color proposes that:-
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.
The descendants of any prehistoric people who migrated North from the equator mutated to become light over time because the evolutionary constraint keeping Africans' skin dark decreased generally the further North a people migrated. 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, as is the case among the Inuit).
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.
A variation of the vitamin D argument is that humans lived in Europe for several thousand years without their skin lightening and that it only became white after they adopted agriculture. It is suggested that in Europe the latitude permitted enough synthesis of vitamin D combined with hunting for health, only when agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D , therefore it is suggested the elimination of game meat, fish, and some plants from the diet resulted in skin turning white several thousand years after modern human settlement in Europe.

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. According to classical scholar Frank Snowden, the Egyptians and Greeks assigned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status. Moreover, as variations in skin tone typically correspond with other social and cultural characteristics, more general trends in perceptions and stereotypes tend to be propagated throughout history and across cultures.
Sexual preference for paleness in women by men has been found in cultures throughout the world. 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 ... in integrated societies has  increasing popularity for men of color, especially those  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."
However, there is an abundance of negative stereotypes as well. In Western societies, persons with dark coloring, particularly those of African origin, were viewed negatively for centuries. Such stereotypes almost always worked to the advantage of the dominant (white) culture. While there is evidence that these prejudices are fading, the wide level of variance between positive and negative stereotypes for the same skin tone indicates both the power and the arbitrariness of such issues.
By contrast, in other cultures, particularly those without much exposure to modern society and the social biases created towards skin color, there exist a negative cultural reaction to pale skin and it is looked upon unfavorably. A number of indigenous African groups such as the Masai, would abandon their children born with conditions such as albinism and there exists sexual preference for darker skin. Pale skin has been associated with being cursed or evil spirits associated with witch craft in such societies.
Skin tone preference varies by culture. Many historically favored and continue to favor lighter skin in women. In his foreword to Peter Frost's 2005 Fair Women, Dark Men, University of Washington sociologist Pierre L. van den Berghe writes: "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." A consequence of this is that, since higher-ranking men get to marry the perceived more attractive women, the upper classes of a society generally tend to develop a lighter complexion than the lower classes by sexual selection (see also Fisherian runaway).
Studies have shown that lighter skin is generally preferred in all cultures and races. Today, and in much of the West, though tanned skin remains desirable, lighter skin is often seen as more attractive. Skin whitening products sales grew from $40 to $43 billion in 2008. In Africa, skin whitening is not uncommon, but in the African American community, lighter skin is generally considered more attractive than darker skin. During slavery, light-skinned African Americans were perceived as 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. Dark African Americans worked in the fields and did not get an education.
Additionally, 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 have European features - blue eyes and pale skin. 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."
In South Asian countries, light skin is seen as more attractive. 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 lightners. In India also, pale skin is considered more attractive and skin whitening is prevalent. Most actors and actresses have light skin.
The perception of beauty can be influenced by racial stereotypes about skin color; 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.

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 hair and eye color von Luschan scale
I very light, or "nordic" or "celtic"  Often burns, occasionally tans. Tends to have freckles, red, brown, auburn, chestnut, or blond hair; blue, hazel, green or grey eyes. 1-5
II light, or light-skinned European Usually burns, sometimes tans Tends to have light or dark hair; blue, green, hazel, brown or grey eyes. 6-10
III light intermediate, or dark-skinned European  Rarely burns, usually tans. Usually has brown hair; blue, green, hazel, brown, or, rarely, dark brown eyes. 11-15
IV dark intermediate, also "Mediterranean" or "olive skin" Rarely burns, often tans. Tends to have black to dark brown hair or may have lighter hair; blue, green, hazel, brown or dark brown eyes. 16-21
V dark or "brown" type Naturally brown skin Black hair; brown or hazel eyes. 22-28
VI very dark, or "black" type Naturally black-brown skin Black hair; dark brown eyes, with minor variations. 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.


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