Human skin color
Human skin color ranges from the
The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment
There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations.
The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination.
Melanin and genes
Melanin is produced by cells called
The genetic mechanism behind human skin color is mainly regulated by the
Both the amount and type of melanin produced is controlled by a number of
Evolution of skin color
Time scale of skin color evolution
With the evolution of hairless skin, abundant sweat glands, and skin rich in melanin, early humans could walk, run, and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species.[9] Research on the MC1R alleles using assumptions about past population size and an absence of population bottlenecks suggests the allele for dark skin present in modern Africans arose at least by 1.2 million years ago.[24]
This was the genotype inherited by
- From the origin of hairlessness and exposure to UV-radiation to less than 100,000 years ago, archaic humans, including archaic Homo sapiens, were dark-skinned.
- As some Homo sapiens 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, although the bulk of humans remained dark-skinned.
- At some point, some 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.
- Subsequent migrations into different UV environments and admixture between populations have resulted in the varied range of skin pigmentations we see today.
The genetic mutations leading to light skin, though partially different among East Asians and Western Europeans,[25] suggest the two groups experienced a similar selective pressure after settlement in northern latitudes.[26]
The theory is partially supported by a study into the
Research by
According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation in African populations appear to have originated more than 300,000 years ago.[31] African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians.[32] Huang et al. 2021 found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to their divergence from each other. Skin pigmentation was also found to be affected by directional selection towards darker skin among Africans, as well as lighter skin among Eurasians.[33] Crawford et al. (2017) similarly found evidence for selection towards light pigmentation prior to the divergence of West Eurasians and East Asians.[32]
Functional considerations
Elias et. al. in 2010 showed a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the
Genetics
To some extent, skin color is determined independently of eye and hair color, as can be seen from variation in skin coloration in human populations. For the evolution of human skin color, see section above.[25][35][36][30][37][38]
For skin color, heritability is very high, even though it can be modified by exposure to sunlight.
A recent systematic study found 169 genes involved in human skin coloration. Most of the genes were involved in melanosome biogenesis, endosomal transport, and gene regulation. Notably, the function of these genes was verified in tissue culture experiments using CRISPR-Cas9 knockouts, showing that these genes are indeed involved in melanin production.[39]
Dark skin
All modern humans share a
MC1R
The melanocortin 1 receptor (MC1R) gene is primarily responsible for determining whether pheomelanin and eumelanin are produced in the human body. Research shows at least 10 differences in MC1R between African and chimpanzee samples and that the gene has probably undergone a strong positive selection (a selective sweep) in early Hominins around 1.2 million years ago.[43] This is consistent with positive selection for the high-eumelanin phenotype seen in Africa and other environments with high UV exposure.[41][42]
Light skin
For the most part, the evolution of light skin has followed different genetic paths in European and East Asian populations. Two genes, however,
KITLG
The
ASIP
Europe
A number of genes have been positively associated with the skin pigmentation difference between European and non-European populations. Mutations in SLC24A5 and SLC45A2 are believed to account for the bulk of this variation and show very strong signs of selection. A variation in TYR has also been identified as a contributor.
Research indicates the selection for the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20,000 years ago and perhaps as recently as 12,000 to 6,000 years ago.
Some authors have expressed caution regarding the skin pigmentation predictions. According to Ju et al. (2021), in a study addressing 40,000 years of modern human history, "we can assess the extent to which they carried the same light pigmentation alleles that are present today", but explain that c. 40,000 BP Early Upper Paleolithic hunter-gatherers "may have carried different alleles that we cannot now detect", and as a result "we cannot confidently make statements about the skin pigmentation of ancient populations.”[59]
SLC24A5
SLC45A2
TYR
The
East Asia
A number of genes known to affect skin color have alleles that show signs of positive selection in East Asian populations. Of these, only OCA2 has been directly related to skin color measurements, while DCT, MC1R and ATRN are marked as candidate genes for future study.
OCA2
Candidate genes
A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Chinese and East Asian populations. While not linked to measurements of skin tone variation directly,
Tanning response
Tanning response in humans is controlled by a variety of genes. MC1R variants Arg151Sys (rs1805007
Albinism
Oculocutaneous albinism (OCA) is a lack of pigment in the eyes, skin and sometimes hair that occurs in a very small fraction of the population. The four known types of OCA are caused by mutations in the TYR, OCA2, TYRP1, and SLC45A2 genes.[90]
Age
In
The skin of children becomes darker as they go through
Human skin color fades with age. Humans over the age of thirty experience a decrease in melanin-producing cells by about 10% to 20% per decade as melanocyte stem cells gradually die.[92] The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes. The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between melanocytes and keratinocytes.[9]
Sexual dimorphism
It has been observed that females are found to have lighter skin pigmentation than males in some studied populations.[11] This may be a form of sexual dimorphism due to the requirement in women for high amounts of calcium during pregnancy and lactation. Breastfeeding newborns, whose skeletons are growing, require high amounts of calcium intake from the mother's milk (about 4 times more than during prenatal development),[93] part of which comes from reserves in the mother's skeleton. Adequate vitamin D resources are needed to absorb calcium from the diet, and it has been shown that deficiencies of vitamin D and calcium increase the likelihood of various birth defects such as spina bifida and rickets. Natural selection may have led to females with lighter skin than males in some indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infants and to maintain their own health.[9] However, in some populations such as in Italy, Poland, Ireland, Spain and Portugal men are found to have fairer complexions, and this has been ascribed as a cause to increased melanoma risk in men.[94][95]
The sexes also differ in how they change their skin color with age. Men and women are not born with different skin color, they begin to diverge during puberty with the influence of sex hormones. Women can also change pigmentation in certain parts of their body, such as the areola, during the menstrual cycle and pregnancy and between 50 and 70% of pregnant women will develop the "
Disorders of pigmentation
Uneven pigmentation of some sort affects most people, regardless of bioethnic background or skin color. Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Apart from blood-related conditions such as jaundice, carotenosis, or argyria, skin pigmentation disorders generally occur because the body produces either too much or too little melanin.
Depigmentation
Albinism
Some types of albinism affect only the skin and hair, while other types affect the skin, hair and eyes, and in rare cases only the eyes. All of them are caused by different genetic mutations. Albinism is a recessively inherited trait in humans where both pigmented parents may be carriers of the gene and pass it down to their children. Each child has a 25% chance of being albino and a 75% chance of having normally pigmented skin.[97] One common type of albinism is oculocutaneous albinism or OCA, which has many subtypes caused by different genetic mutations. Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity, skin cancer, and eye damage.[9]
Albinism is more common in some parts of the world than in others, but it is estimated that 1 in 70 humans carry the gene for OCA. The most severe type of albinism is OCA1A, which is characterized by complete, lifelong loss of melanin production, other forms of OCA1B, OCA2, OCA3, OCA4, show some form of melanin accumulation and are less severe.[9] The four known types of OCA are caused by mutations in the TYR, OCA2, TYRP1, and SLC45A2 genes.[90]
Albinos often face social and cultural challenges (even threats), as the condition is often a source of ridicule, racism, fear, and violence. Many cultures around the world have developed beliefs regarding people with albinism.
Vitiligo
Hyperpigmentation
Increased melanin production, also known as hyperpigmentation, can be a few different phenomena:
- Melasma describes the darkening of the skin.
- Chloasmadescribes 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.[101] 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 the sun
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, also known as IPD (immediate pigment darkening). Secondly, there is an increase in production of melanin known as
A person's natural skin color affects their reaction to exposure to the sun. Generally, those who start out with darker skin color and more melanin have better abilities to tan. Individuals with very light skin and albinos have no ability to tan.[103] The biggest differences resulting from sun exposure are visible in individuals who start out with moderately pigmented brown skin: the change is dramatically visible as tan lines, where parts of the skin which tanned are delineated from unexposed skin.[9]
Modern lifestyles and mobility have created mismatch between skin color and environment for many individuals. Vitamin D deficiencies and UVR overexposure are concerns for many. It is important for these people individually to adjust their diet and lifestyle according to their skin color, the environment they live in, and the time of year.[9] For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:
Fitzpatrick scale
The following list shows the six categories of the Fitzpatrick scale in relation to the 36 categories of the older von Luschan scale:[104][105]
Type | Also called | Sunburning | Tanning behavior | Von Luschan's chromatic scale |
---|---|---|---|---|
I | Light, pale white | Always | Never | 0–6 |
II | White, fair | Usually | Minimally | 7–13 |
III | Medium white to light brown | Sometimes | Uniformly | 14–20 |
IV | Olive, moderate brown | Rarely | Easily | 21–27 |
V | Brown, dark brown | Very rarely | Very easily | 28–34 |
VI | Very dark brown to black | Never | Rarely | 35–36 |
Dark skin with large concentrations of melanin protects against 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,
Geographic variation
Approximately 10% of the variance in skin color occurs within regions, and approximately 90% occurs between regions.[107] 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 where people from different regions have mixed extensively, the connection between skin color and ancestry has substantially weakened.[108] 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.[109]
In general, people living close to the equator are highly darkly pigmented, and those living near the poles are generally very lightly pigmented. The rest of humanity shows a high degree of skin color variation between these two extremes, generally correlating with UV exposure. The main exception to this rule is in the New World, where people have only lived for about 10,000 to 15,000 years and show a less pronounced degree of skin pigmentation.[9]
In recent times, humans have become increasingly mobile as a consequence of improved technology, domestication, environmental change, strong curiosity, and risk-taking. Migrations over the last 4000 years, and especially the last 400 years, have been the fastest in human history and have led to many people settling in places far away from their ancestral homelands. This means that skin colors today are not as confined to geographical location as they were previously.[9]
Social status, colorism and racism
According to classical scholar Frank Snowden, skin color did not determine social status in ancient Egypt, Greece or Rome. These ancient civilizations viewed relations between the major power and the subordinate state as more significant in a person's status than their skin colors.[110][page needed]
Nevertheless, some social groups favor specific skin coloring. 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.[111]
Many cultures have historically favored lighter skin for women. Before the Industrial Revolution, inhabitants of the continent of Europe preferred pale skin, which they interpreted as 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. Hence light skin became associated with wealth and high position.[112] Women would put lead-based cosmetics on their skin to whiten their skin tone artificially.[113] However, when not strictly monitored, these cosmetics caused lead poisoning. Other methods also aimed at achieving a light-skinned appearance, including the use of arsenic to whiten skin, and powders. Women would wear full-length clothes when outdoors, and would use gloves and parasols to provide shade from the sun.
Colonization and enslavement as carried out by European countries became involved with
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.[117]
A preference for fair or lighter skin continues in some countries, including Latin American countries where whites form a minority.
In
Skin-whitening products have remained popular over time, often due to historical beliefs and perceptions about fair skin. Sales of skin-whitening products across the world grew from $40 billion to $43 billion in 2008.
Significant exceptions to a preference for lighter skin started to appear in Western culture in the mid-20th century.[134] However, a 2010 study found a preference for lighter-skinned women in New Zealand and California.[135] Though sun-tanned skin was once 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 Frenchwoman Coco Chanel (1883–1971) presenting tanned skin as fashionable, healthy, and luxurious.[136] As of 2017[update], though an overall 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 or very dark skin.[137][138][139] Western mass media and popular culture continued[when?] to reinforce negative stereotypes about dark skin,[140] but in some circles 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.[112] Studies have also emerged indicating that the degree of tanning is directly related to how attractive a young woman is.[141][142]
See also
- Afro-textured hair
- Carnation (heraldry)
- Color terminology for race
- Olive skin
- Complexion
- Eye color
- Health effects of sun exposure
- Human hair color
- Human physical appearance
- Human skin
- Race (human classification)
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- ^ Skin whitening big business in Asia Archived 2010-07-26 at the Wayback Machine. Pri.Org. Retrieved 2011-02-27.
- ^ Mowbray, Nicole (4 April 2004). "Japanese girls choose whiter shade of pale". Guardian Unlimited. London. Retrieved 2010-05-24.
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- ^ Color Counts: "... it is evident that differing color holds considerable importance within the black community and is measurably influencing self-esteem, prestige, and marital status." |USA Today (Society for the Advancement of Education) Archived 2014-03-30 at the Wayback Machine. Retrieved 2012-09-25.
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Harris investigated the history of the parasol... everywhere ordinary people were forbidden to protect themselves with such devices "pallid skin became a marker of upper-class status". At the beginning of the 20th Century, in the United States, lighter-skinned people avoided the sun... Tanned skin was considered lower class.
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men expressed preferences for lighter skinned female figures in New Zealand and California
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In 1920s France, the caramel-skinned entertainer Josephine Baker became a Parisian idol. Concurrently, fashion designer Coco Chanel was "bronzed" while cruising on a yacht. A winter tan became a symbol of the leisure class and showed you could afford to travel to exotic climates.
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Further reading
- Harding, R; Healy, E; Ray, A; Ellis, N; Flanagan, N; Todd, C; Dixon, C; Sajantila, A; Jackson, I; Birch-Machin, MA; Rees, JL (2000). "Evidence for Variable Selective Pressures at MC1R". The American Journal of Human Genetics. 66 (4): 1351–61. PMID 10733465.
- Holick, Michael F. (2004). "Sunlight and vitamin D for bone health and prevention of autoimmune diseases, cancers, and cardiovascular disease". The American Journal of Clinical Nutrition. 80 (6 Suppl): 1678S–88S. PMID 15585788.
- Jablonski, Nina G. (10 January 2014). Living Color: The Biological and Social Meaning of Skin Color. University of California Press. JSTOR 10.1525/j.ctt1pn64b.
- Jablonski, Nina G. (2004). "The Evolution of Human Skin and Skin Color". Annual Review of Anthropology. 33: 585–623. .
- Jablonski, Nina G.; Chaplin, George (2002). "Skin Deep". Scientific American. 287 (4): 74–82. PMID 12271527.
- Lamason, RL; Mohideen, MA; Mest, JR; Wong, AC; Norton, HL; Aros, MC; Jurynec, MJ; Mao, X; Humphreville, VR; Humbert, J. E.; Sinha, S; Moore, J. L.; Jagadeeswaran, P; Zhao, W; Ning, G; Makalowska, I; McKeigue, P. M.; O'Donnell, D; Kittles, R; Parra, E. J.; Mangini, N. J.; Grunwald, D. J.; Shriver, M. D.; Canfield, V. A.; Cheng, K. C. (2005). "SLC24A5, a Putative Cation Exchanger, Affects Pigmentation in Zebrafish and Humans". Science. 310 (5755): 1782–6. S2CID 2245002.
- Millington, G. W. M. (2006). "Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors". Clinical and Experimental Dermatology. 31 (3): 407–412. S2CID 25213876.
- Millington, George W. M.; Levell, Nick J. (2007). "From genesis to gene sequencing: historical progress in the understanding of skin color". International Journal of Dermatology. 46 (1): 103–5. S2CID 6699141.
- Rees, J.L.; Flanagan, N (1999). "Pigmentation, melanocortins and red hair". QJM. 92 (3): 125–31. PMID 10326071.
- Relethford, JH (2000). "Human skin color diversity is highest in sub-Saharan African populations". Human Biology; an International Record of Research. 72 (5): 773–80. PMID 11126724.
- Robins, AH (1991). Biological Perspectives on Human Pigmentation. ISBN 0-521-36514-7.
- Wade, Nicholas (August 19, 2003). "Why Humans and Their Fur Parted Ways". The New York Times. p. F1. 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 when humans invented clothing.
- Walters, KA; Roberts, MS (2008). Dermatologic, Cosmeceutic, and Cosmetic Development: Therapeutic and Novel Approaches. New York: Informa Healthcare. ISBN 978-0-8493-7589-7.
- Weller, R; Hunter, J; Savin, J; Dahl, M (2008). Clinical Dermatology (4th ed.). Malden, MA: Blackwell Publishing. p. 268. ISBN 978-1-4051-4663-0.
External links
- "Key gene 'controls skin colour'", BBC News. SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry
- "The Biology of Skin Color: Black and White"—PBS
- "The Biology of Skin Color — HHMI BioInteractive Video"—YouTube