Light skin

Source: Wikipedia, the free encyclopedia.
A Norwegian woman with light skin

Light skin is a

eumelanin pigmentation as an adaptation to environments of low UV radiation.[1][2][3] Light skin is most commonly found amongst the native populations of Europe, West Asia, Central Asia, and Northeast Asia as measured through skin reflectance.[4] People with light skin pigmentation are often referred to as "white"[5][6] although these usages can be ambiguous in some countries where they are used to refer specifically to certain ethnic groups or populations.[7]

Humans with light skin pigmentation have skin with low amounts of

evolved to have light skin pigmentation.[3][12][13]

The distribution of light-skinned populations is highly correlated with the low ultraviolet radiation levels of the regions inhabited by them. Historically, light-skinned populations almost exclusively lived far from the equator, in high latitude areas with low sunlight intensity.[14] Due to colonization, imperialism, and increased mobility of people between geographical regions in recent centuries, light-skinned populations today are found all over the world.[3][15]

Evolution

History of human pigmentation in Europe (with Asia geographic extension). Scandinavian hunter-gatherers had higher levels of light pigmentation variants compared to their ancestors from other parts of Europe, suggesting adaptation to low light conditions.[16] Some authors have expressed caution regarding the skin pigmentation predictions.[17]

It is generally accepted that

selective pressure would be expected for the evolution of light skin in areas of low UV radiation.[12]

After the ancestors of West Eurasians and East Eurasians diverged more than 40,000 years ago, lighter skin tones evolved independently in a subset of each of the two populations. In West Eurasians, the A111T allele of the rs1426654 polymorphism in the pigmentation gene SLC24A5 has the largest skin lightening effect and is widespread in Europe, South Asia, Central Asia, the Near East and North Africa.[23]

In a 2013 study, Canfield et al. established that SLC24A5 sits in a block of haplotypes, one of which (C11) is shared by virtually all chromosomes that bear the A111T variant. This "equivalence" between C11 and A111T indicates that all people who carry this skin-lightening allele descend from a common origin: a single carrier who lived most likely "between the Middle East and the Indian subcontinent". Canfield et al. attempted to date the A111T mutation but only constrained the age range to before the Neolithic.[23] However, a second study from the same year (Basu Mallick et al.) estimated the coalescent age (split date) for this allele to between ~28,000 and ~22,000 years ago.[24]

The second most important skin-lightening factor in West Eurasians is the depigmenting allele F374 of the rs16891982 polymorphism located in the melanin-synthesis gene SLC45A2. From its low haplotype diversity, Yuasa et al. (2006) likewise concluded that this mutation (L374F) "occurred only once in the ancestry of Caucasians".[25]

Summarising these studies, Hanel and Carlberg (2020) decided that the alleles of the two genes SLC24A5 and SLC45A2 which are most associated with lighter skin colour in modern Europeans originated in West Asia about 22,000 to 28,000 years ago and these two mutations each arose in a single carrier.[21] This is consistent with Jones et al. (2015), who reconstructed the relationship between Near Eastern Neolithic farmers and Caucasus Hunter-Gatherers: two populations which carried the light skin variant of SLC24A5. Analysing newly sequenced ancient genomes, Jones et al. estimated the split date at ~24,000 bp and localised the separation to somewhere south of the Caucasus.[26] However, a coalescent analysis of this allele by Crawford et al. (2017) gave a more narrowly constrained, and earlier, split date of ~29,000 years ago (with a 95% confidence window from 28,000 to 31,000 bp).[27]

The light skin variants of SLC24A5 and SLC45A2 were present in Anatolia by 9,000 years ago, where they became associated with the Neolithic Revolution. From here, their carriers spread Neolithic farming across Europe.[28] Lighter skin and blond hair also evolved in the Ancient North Eurasian population.[29]

A further wave of lighter-skinned populations across Europe (and elsewhere) is associated with the

Niger-Congo speaking groups. It is inferred that it may have arrived into the region via migration from the Levant, which is also supported by linguistic evidence.[32] In the San people, it was acquired from interactions with Eastern African pastoralists.[33] Meanwhile, in the case of north-east Asia and the Americas, a variation of the MFSD12 gene is responsible for lighter skin colour.[29] The modern association between skin tone and latitude is thus a relatively recent development.[21]

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.”[17]

According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation appear to have originated more than 300,000 years ago.[34] African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians.[30] 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.[35] Crawford et al. (2017) similarly found evidence for selection towards light pigmentation prior to the divergence of West Eurasians and East Asians.[30]

A study conducted by Fregel, Rosa et al. (2018), showed that

Western Eurasian ancestry. The geographical distribution shows that it is nearly fixed in all of Europe and most of the Middle East, extending east to some populations in present-day Pakistan and Northern India. It shows a latitudinal decline toward the Equator, with high frequencies in North Africa (80%), and intermediate (40−60%) in Ethiopia and Somalia.[23]

Geographic distribution; ultraviolet and vitamin D

Skin reflectance vs. latitude
Skin reflectance vs. latitude
Some people in Mongolia and Manchuria have light skin.

In the 1960s, biochemist W. Farnsworth Loomis suggested that skin colour is related to the body's need for

vitamin D3 from it. A certain amount of vitamin D helps the body to absorb more calcium which is essential for building and maintaining bones, especially for developing embryos. Vitamin D production depends on exposure to sunlight. Humans living at latitudes far from the equator developed light skin in order to help absorb more vitamin D. People with light (type II) skin can produce previtamin D3 in their skin at rates 5–10 times faster than dark-skinned (type V) people.[37][38][39][40][41]

In 1998, anthropologist Nina Jablonski and her husband George Chaplin collected spectrometer data to measure UV radiation levels around the world and compared it to published information on the skin colour of indigenous populations of more than 50 countries. The results showed a very high correlation between UV radiation and skin colour; the weaker the sunlight was in a geographic region, the lighter the indigenous people's skin tended to be. Jablonski points out that people living above the latitudes of 50 degrees have the highest chance of developing vitamin D deficiency. She suggests that people living far from the equator developed light skin to produce adequate amounts of vitamin D during winter with low levels of UV radiation. Genetic studies suggest that light-skinned humans have been selected for multiple times.[42][43][44]

Some people in Afghanistan and Pakistan have light skin.

Polar regions, vitamin D, and diet

A light-skinned Assyrian woman.

Polar regions of the Northern Hemisphere receive little UV radiation, and even less vitamin D-producing UVB, for most of the year. These regions were uninhabited by humans until about 12,000 years ago. (In northern Fennoscandia at least, human populations arrived soon after deglaciation.)[45] Areas like Scandinavia and Siberia have very low concentrations of ultraviolet radiation, and indigenous populations are all light-skinned.[3][38]

However, dietary factors may allow vitamin D sufficiency even in dark skinned populations.

Eskimos), retained their dark skin; they ate Vitamin D-rich seafood, such as fish and sea mammal blubber.[49]

Furthermore, these people have been living in the far north for less than 7,000 years. As their founding populations lacked alleles for light skin colour, they may have had insufficient time for significantly lower melanin production to have been

high latitude." Additionally, in the spring, Inuit would receive high levels of UV radiation as reflection from the snow, and their relatively darker skin then protects them from the sunlight.[3][12][8]

Earlier hypotheses

Two other main hypotheses have been put forward to explain the development of light skin pigmentation: resistance to cold injury, and genetic drift; now both of them are considered unlikely to be the main mechanism behind the evolution of light skin.[3]

The resistance to cold injury hypothesis claimed that dark skin was selected against in cold climates far from the equator and in higher altitudes as dark skin was more affected by

capillaries to differences in temperature, and not with pigmentation.[3]

The supposition that dark skin evolved in the absence of selective pressure was put forward by the probable mutation effect hypothesis.

assortive mating[53] and sexual selection contributed to an even lighter pigmentation in females.[55][56] Doubt has been cast on this hypothesis, as more random patterns of skin colouration would be expected in contrast to the observed structural light skin pigmentation in areas of low UV radiation.[44] The clinal (gradual) distribution of skin pigmentation observable in the Eastern hemisphere, and to a lesser extent in the Western hemisphere, is one of the most significant characteristics of human skin pigmentation. Increasingly lighter skinned populations are distributed across areas with incrementally lower levels of UV radiation.[57][58]

Genetic associations

Variations in the KITL gene have been positively associated with about 20% of melanin concentration differences between African and non-African populations. One of the alleles of the gene has an 80% occurrence rate in Eurasian populations.

Northern Africa, the Horn of Africa, West Asia, Central Asia and South Asia.[64][65][66]

Biochemistry

pheomelanin.[42][68] The concentration of pheomelanin varies highly within populations from individual to individual, but it is more commonly found among lightly pigmented Europeans, East Asians, and Native Americans.[22][69]

For the same body region, individuals, independently of skin colour, have the same amount of melanocytes (however variation between different body parts is substantial), but organelles which contain pigments, called melanosomes, are smaller and less numerous in light-skinned humans.[70]

For people with very light skin, the skin gets most of its colour from the bluish-white connective tissue in the

physical exercise or stimulation of the sympathetic nervous system (usually embarrassment or anger).[71] Up to 50% of UVA can penetrate deeply into the dermis in persons with light skin pigmentation with little protective melanin pigment.[48]

The combination of light skin,

loss-of-function mutation in the melanocortin 1 receptor (MC1R) gene.[72][73] However, variations in the MC1R gene sequence only have considerable influence on pigmentation in populations where red hair and extremely light skin is prevalent.[44] The gene variation's primary effect is to promote eumelanin synthesis at the expense of pheomelanin synthesis, although this contributes to very little variation in skin reflectance between different ethnic groups.[74] Melanocytes from light skin cells cocultured with keratinocytes give rise to a distribution pattern characteristic of light skin.[75]

Freckles usually only occur in people with very lightly pigmented skin. They vary from very dark to brown in colour and develop a random pattern on the skin of the individual.

premature aging and skin cancer.[78][79] The strongly red appearance of lightly pigmented skin as a response to high UV radiation levels is caused by the increased diameter, number, and blood flow of the capillaries.[22]

People with moderately pigmented skin (

DNA damage compared to naturally occurring dark skin,[80][81] however it offers great protection against seasonal variations in UVR. Gradually developed tan in the spring prevents sunburns in the summer. This mechanism is almost certainly the evolutionary reason behind the development of tanning behaviour.[3]

Health implications

Skin pigmentation is an evolutionary adaptation to the various UV radiation levels around the world. There are health implications of light-skinned people living in environments of high UV radiation. Various cultural practices increase problems related to health conditions of light skin, for example

sunbathing among the light-skinned.[3]

Advantages in low sunlight

Humans with light skin pigmentation living in low sunlight environments experience increased vitamin D synthesis compared to humans with dark skin pigmentation due to the ability to absorb more sunlight. Almost every part of the human body, including the skeleton, the immune system, and brain requires vitamin D. Vitamin D production in the skin begins when UV radiation penetrates the skin and interacts with a cholesterol-like molecule produce pre-vitamin D3. This reaction only occurs in the presence of medium length UVR, UVB. Most of the UVB and UVC rays are destroyed or reflected by ozone, oxygen, and dust in the atmosphere. UVB reaches the Earth's surface in the highest amounts when its path is straight and goes through a little layer of atmosphere.

The farther a place is from the equator, the less UVB is received, and the potential to produce of vitamin D is diminished. Some regions far from the equator do not receive UVB radiation at all between autumn and spring.[48] Vitamin D deficiency does not kill its victims quickly, and generally does not kill at all. Rather it weakens the immune system, the bones, and compromises the body's ability to fight uncontrolled cell division which results in cancer. A form of vitamin D is a potent cell growth inhibitor; thus chronic deficiencies of vitamin D seem to be associated with higher risk of certain cancers. This is an active topic of cancer research and is still debated.[48] The vitamin D deficiency associated with dark skin leads to higher levels of schizophrenia in such populations residing in northerly latitudes.[82]

With the increase of vitamin D synthesis, there is a decreased incidence of conditions that are related to common vitamin D deficiency conditions of people with dark skin pigmentation living in environments of low UV radiation:

flu.[3][15] When exposed to UVB, the entire exposed area of body's skin of a relatively light skinned person is able to produce between 10 and 20000 IU of vitamin D.[48]

Disadvantages in high sunlight

Fatal neural tube defect with evident anencephaly.

Light-skinned people living in high sunlight environments are more susceptible to the harmful UV rays of sunlight because of the lack of

basal cell carcinoma, which is a common form of skin cancer
.

Another health implication is the

embryogenesis and spermatogenesis.[3][15][38]

Individuals with lightly pigmented skin who are repeatedly exposed to strong UV radiation, experience faster aging of the skin, which shows in increased wrinkling and anomalies of pigmentation. Oxidative damage causes the degradation of protective tissue in the dermis, which confers the strength of the skin.[22] It has been postulated that white women may develop wrinkles faster than black women after menopause because white women are more susceptible to sun damage throughout life. Dr. Hugh S. Taylor, of Yale School of Medicine, concluded that the study could not prove the findings but they suspect the underlying cause. Light-coloured skin has been suspected to be one of the contributing factors that promote wrinkling.[84][85]

See also

References

  1. ^ light-skinned Princeton University
  2. ^ "Light-skinned". thefreedictionary.com. Retrieved 24 January 2017.
  3. ^ a b c d e f g h i j k l m n o Muehlenbein, Michael (2010). Human Evolutionary Biology. Cambridge University Press. pp. 192–213.
  4. ISBN 978-1559346672
    .
  5. ^ Oxford Dictionaries. April 2010. Oxford University Press. "belonging to or denoting a human group having light-coloured skin" "white" (accessed 6 August 2012).
  6. ^ Dictionary.com: white 3.a "marked by slight pigmentation of the skin"
  7. ^ "Global Census". American Anthropological Association. Archived from the original on 14 September 2018. Retrieved 10 December 2012.
  8. ^ a b Kirchweger, Gina. "The Biology of Skin Color: Black and White". Evolution Library. PBS. Retrieved 22 September 2018.
  9. PMC 6766707
    .
  10. PMID 33500553
    .
  11. PMID 22552074
    .
  12. ^
    PMID 11126724
    .
  13. PMID 29710859
    .
  14. ^ "Modern human variation: overview". Archived from the original on 5 November 2012.
  15. ^ a b c O'Neil, Dennis. "Skin Color Adaptation". Human Biological Adaptability: Skin Color as an Adaptation. Palomar. Archived from the original on 18 December 2012. Retrieved 10 December 2012.
  16. PMID 29315301
    . However, for all three well-characterized skin and eye-color associated SNPs, the SHGs display a frequency that is greater for the light-skin variants and the blue-eye variant than can be expected from a mixture of WHGs and EHGs. This observation indicates that the frequencies may have increased due to continued adaptation to a low light conditions.
  17. ^
    PMID 33443182
    . Relatively dark skin pigmentation in Early Upper Paleolithic Europe would be consistent with those populations being relatively poorly adapted to high-latitude conditions as a result of having recently migrated from lower latitudes. On the other hand, although we have shown that these populations carried few of the light pigmentation alleles that are segregating in present-day Europe, they may have carried different alleles that we cannot now detect.
  18. ISBN 978-0-306-47767-6
    .
  19. PMID 17209171
    .
  20. S2CID 57808076
    .
  21. ^
    S2CID 220335539
    .
  22. ^
    ISSN 0084-6570
    .
  23. ^
    PMID 24048645
    .
  24. PMID 24244186
    .
  25. PMID 17044855
    .
  26. PMID 26567969
    .
  27. PMID 29025994
    . On the basis of coalescent analysis with sequence data from the Simons Genomic Diversity Project (SGDP), the time to most recent common ancestor (TMRCA) of most Eurasian lineages containing the rs1426654 (A) allele is 29 thousand years ago (ka) [95% critical interval (CI), 28 to 31 ka], consistent with previous studies.
  28. PMID 24927591
    .
  29. ^ a b Downes, Natasha (21 January 2019). "Genetic study provides novel insights into the evolution of skin colour". UCL News (Press release). University College London. Retrieved 4 December 2021.
  30. ^
    PMID 29025994
    .
  31. PMID 33438000
    .
  32. PMID 22726845
    .
  33. PMID 30530665
    .
  34. ^ Baillie, Katherine Unger (12 October 2017). "Genes responsible for diversity of human skin colors identified". Penn Today (Press release). University of Pennsylvania. Republished by ScienceDaily.
  35. PMID 33495209
    .
  36. PMID 29895688
    .
  37. S2CID 41818974
    .
  38. ^
    PMID 10896812
    .
  39. PMID 16766240
    .
  40. PMID 17637484
    .
  41. PMID 17254541
    .
  42. ^
    S2CID 2245002
    .
  43. S2CID 10087710
    .
  44. ^
    PMID 17182896
    .
  45. S2CID 129136655
    .
  46. PMID 10850004
    .
  47. S2CID 15728496
    .
  48. ^
    ISBN 978-0-520-25153-3
    .
  49. ^ Why Skin Colours Differ Department of Physics: The Faculty of Mathematics and Natural Sciences By Johan Moan, Asta Juzeniene
  50. ^ "Human Biological Adaptability: Skin Color as an Adaptation". www2.palomar.edu.
  51. PMID 1126703
    .
  52. PMID 6056270
    .
  53. ^
    S2CID 145636646
    .
  54. S2CID 85732039
    .
  55. S2CID 36144428
    .
  56. S2CID 22703861
    .
  57. PMID 9453695
    .
  58. PMID 9786336
    .
  59. PMID 18083106
    .
  60. ^ HapMap: SNP report for rs642742. Hapmap.ncbi.nlm.nih.gov (19 October 2009). Retrieved on 2011-02-27.
  61. ^ "SNP report for rs2424984". International HapMap project. US National Center for Biotechnology Information. Retrieved 11 December 2012.
  62. S2CID 2245002
    .
  63. S2CID 43290419
    .
  64. ^ "Graphical display of Allele Frequencies for Ala111Thr". Allele Frequency Database. Retrieved 10 October 2012.
  65. ^ "ALFRED – Polymorphism Information – Ala111Thr". Allele Frequency Database. Retrieved 22 September 2018.
  66. PMID 22726845
    .
  67. ^ Haas et al., 2005.
  68. S2CID 43355316
    .
  69. ISBN 9783319431574, retrieved 6 April 2020{{citation}}: CS1 maint: location missing publisher (link
    )
  70. S2CID 4223552
    .
  71. ^ Jablonski, N.G. (2006). Skin: a Natural History. Berkeley, CA: University of California Press.
  72. S2CID 6156245
    .
  73. PMID 14616056
    .
  74. PMID 11936268
    .
  75. PMID 11511313
    .
  76. PMID 2004316
    .
  77. ^ Fitzpatrick, T. B.; Ortonne, J. P. (2003). "Normal skin color and general considerations of pigmentary disorders". In Fitzpatrick's Dermatology in General Medicine. 6: 819–825.
  78. PMID 11897551
    .
  79. PMID 15020192
    .
  80. PMID 15955111
    .
  81. PMID 16388960
    .
  82. PMID 33500553
    . A separate observation that the offspring of migrants with dark skin who migrate to cold climates have an increased risk of schizophrenia may also be due to low vitamin D during gestation and early life as dark skin requires greater sunlight exposure to make adequate levels of the vitamin D prehormone.
  83. PMID 18021918
    .
  84. ^ Norton, Amy (10 November 2010). "White women's skin may show wrinkles sooner". Reuters. Retrieved 22 September 2018.
  85. ^ Cole, Gary. "Wrinkles". MedicineNet.com. Retrieved 22 September 2018.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Light_skin&oldid=1220249941"