Dark skin

Dark skin is a type of human skin color that are rich in melanin pigments, especially eumelanin.[1][2][3] People with very dark skin are often referred to as "black people",[4] although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.[5][6][7][8]

A woman with dark skin

The evolution of dark skin is believed to have begun around 1.2 million years ago,[9] in light-skinned early hominid species after they moved from the equatorial rainforest to the sunny savannas. In the heat of the savannas, better cooling mechanisms were required, which were achieved through the loss of body hair and development of more efficient perspiration. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate depletion, and to a lesser extent, DNA damage. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to ultraviolet radiation; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal embryogenesis and spermatogenesis led to the selection of dark skin pigmentation. By the time modern Homo sapiens evolved, all humans were dark-skinned.[3][10][11][12][13][14][15]

Humans with dark skin pigmentation have skin naturally rich in melanin (especially eumelanin), and have more melanosomes which provide superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to DNA.[3][16]

Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk—especially in the winter—of vitamin D deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, numerous types of cancers, and possibly cardiovascular disease and low immune system activity.[3][17] However, some recent studies have questioned if the thresholds indicating vitamin D deficiency in light-skinned individuals are relevant for dark-skinned individuals, as they found that, on average, dark-skinned individuals have higher bone density and lower risk of fractures than lighter-skinned individuals with the same levels of vitamin D. This is possibly attributed to lower presence of vitamin D binding agents (and thus its higher bioavailability) in dark-skinned individuals.[18][19]

The global distribution of generally dark-skinned populations is strongly correlated with the high ultraviolet radiation levels of the regions inhabited by them. These populations, with the exception of indigenous Tasmanians almost exclusively live near the equator, in tropical areas with intense sunlight: Australia, Melanesia, New Guinea, South Asia, and Africa. Studies into these populations indicates dark skin is a retention of the pre-existing high UVR-adapted state of modern humans before the out of Africa migration and not a later evolutionary adaptation.[20][21] Due to mass migration and increased mobility of people between geographical regions in the recent past, dark-skinned populations today are found all over the world.[3][22][23]

Evolution

Due to natural selection, people who lived in areas of intense sunlight developed dark skin colouration to protect against ultraviolet (UV) light, mainly to protect their body from folate depletion. Evolutionary pigmentation of the skin was caused by ultraviolet radiation of the sun. As hominids gradually lost their fur between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked and lightly-pigmented skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations' skin reflectance (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo.[24] The effect of sunlight on folic acid levels has been crucial in the development of dark skin.[3][25]

Savannas in East Africa, where most of the hominid evolution of dark skin may have taken place

The earliest primate ancestors of modern humans most likely had light skin, like our closest modern relative—the chimpanzee.[26] About 7 million years ago human and chimpanzee lineages diverged, and between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of East Africa.[22][27] They not only had to cope with more intense sunlight but had to develop a better cooling system. It was harder to get food in the hot savannas and as mammalian brains are prone to overheating—5 or 6 °C rise in temperature can lead to heatstroke—there was a need for the development of better heat regulation. The solution was sweating and loss of body hair.[22]

Sweating dissipated heat through evaporation. Early humans, like chimpanzees now, had few sweat glands, and most of them were located in the palms of the hand and the soles of the feet. At times, individuals with more sweat glands were born. These humans could search for food and hunt for longer periods before being forced back to the shades. The more they could forage, the more and healthier offspring they could produce, and the higher the chance they had to pass on their genes for abundant sweat glands. With less hair, sweat could evaporate more easily and cool the bodies of humans faster. A few million years of evolution later, early humans had sparse body hair and more than 2 million sweat glands in their body.[22][28][29]

Hairless skin, however, is particularly vulnerable to be damaged by ultraviolet light and this proved to be a problem for humans living in areas of intense UV radiation, and the evolutionary result was the development of dark-coloured skin as a protection. Scientists have long assumed that humans evolved melanin in order to absorb or scatter harmful sun radiation. Some researchers assumed that melanin protects against skin cancer. While high UV radiation can cause skin cancer, the development of cancer usually occurs after child-bearing age. As natural selection favours individuals with traits of reproductive success, skin cancer had little effect on the evolution of dark skin. Previous hypotheses suggested that sunburned nipples impeded breastfeeding, but a slight tan is enough to protect mothers against this issue.[22][30][31][32]

A 1978 study examined the effect of sunlight on folate—a vitamin B complex—levels. The study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. Low folate levels are correlated with neural tube defects, such as anencephaly and spina bifida. UV rays can strip away folate, which is important to the development of healthy foetuses. In these abnormalities children are born with an incomplete brain or spinal cord. Nina Jablonski, a professor of anthropology and expert on evolution of human skin coloration,[33] found several cases in which mothers' visits to tanning studios were connected to neural tube defects in early pregnancy. She also found that folate was crucial to sperm development; some male contraception drugs are based on folate inhibition. It has been found that folate may have been the driving force behind the evolution of dark skin.[3][20]

As humans dispersed from equatorial Africa to low UVR areas and higher altitudes sometime between 120,000 and 65,000 years ago, dark skin posed a disadvantage.[34][35] Populations with light skin pigmentation evolved in climates of little sunlight. Light skin pigmentation protects against vitamin D deficiency. It is known that dark-skinned people who have moved to climates of limited sunlight can develop vitamin D-related conditions such as rickets, and different forms of cancer.[3][36]

Earlier hypotheses

The main other hypotheses that have been put forward through history to explain the evolution of dark skin coloration relate to increased mortality due to skin cancers, enhanced fitness as a result of protection against sunburns, and increasing benefits due to antibacterial properties of eumelanin.[3]

Darkly pigmented, eumelanin-rich skin protects against DNA damage caused by the sunlight.[37] This is associated with lower skin cancer rates among dark-skinned populations.[38][39][40][41][42] The presence of pheomelanin in light skin increases the oxidative stress in melanocytes, and this combined with the limited ability of pheomelanin to absorb UVR contributes to higher skin cancer rates among light-skinned individuals.[43] The damaging effect of UVR on DNA structure and the entailing elevated skin cancer risk is widely recognized.[24][44][45][46][47] However, these cancer types usually affect people at the end or after their reproductive career and could have not been the evolutionary reason behind the development of dark skin pigmentation.[24][31] Of all the major skin cancer types, only malignant melanoma have a major effect in a person's reproductive age. The mortality rates of melanoma has been very low (less than 5 per 100,000) before the mid-20th century. It has been argued that the low melanoma mortality rates during reproductive age cannot be the principal reason behind the development of dark skin pigmentation.[32]

Studies have found that even serious sunburns could not affect sweat gland function and thermoregulation. There are no data or studies that support that sunburn can cause damage so serious it can affect reproductive success.[3]

Another group of hypotheses contended that dark skin pigmentation developed as antibacterial protection against tropical infectious diseases and parasites. Although it is true that eumelanin has antibacterial properties, its importance is secondary to 'physical adsorption' (physisorption) to protect against UVR-induced damage. This hypothesis is not consistent with the evidence that most of the hominid evolution took place in savanna environments and not in tropical rainforests.[48] Humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments.[35] The antimicrobial hypothesis also does not explain why some populations (like the Inuit or Tibetans) who live far from the tropics and are exposed to high UVR have darker skin pigmentation than their surrounding populations.[3]

Biochemistry and genetics
Darkly pigmented skin

Dark-skinned humans have high amounts of melanin found in their skin. Melanin is derivative of the amino acid tyrosine. Eumelanin is the dominant form of melanin found in human skin. Eumelanin protects tissues and DNA from the radiation damage of UV light. Melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis.[49] Melanin is produced inside small membrane-bound packages called melanosomes. People with naturally-occurring dark skin have melanosomes which are clumped, large and full of eumelanin.[50][51] A four-fold difference in naturally-occurring dark skin gives seven- to eight-fold protection against DNA damage,[51] but even the darkest skin colour cannot protect against all damage to DNA.[3]

Dark skin offers great protection against UVR because of its eumelanin content, the UVR-absorbing capabilities of large melanosomes, and because eumelanin can be mobilized faster and brought to the surface of the skin from the depths of the epidermis.[3] For the same body region, light- and dark-skinned individuals have similar numbers of melanocytes (there is considerable variation between different body regions), but pigment-containing organelles, called melanosomes, are larger and more numerous in dark-skinned individuals. Keratocytes from dark skin cocultured with melanocytes give rise to a melanosome distribution pattern characteristic of dark skin.[52][53] Melanosomes are not in aggregated state in darkly pigmented skin compared to lightly pigmented skin. Due to the heavily melanised melanosomes in darkly-pigmented skin, it can absorb more energy from UVR and thus offers better protection against sunburns and by absorption and dispersion UV rays.[24]

Darkly-pigmented skin protects against direct and indirect DNA damage. Photodegration occurs when melanin absorbs photons. Recent research suggest that the photoprotective effect of dark skin is increased by the fact that melanin can capture free radicals, such as hydrogen peroxide, which are created by the interaction of UVR and layers of the skin.[24] Heavily pigmented melanocytes have greater capacity to divide after ultraviolet irradiation, which suggests that they receive less damage to their DNA.[24] Despite this, medium-wave ultraviolet radiation (UVB) damages the immune system even in darker skinned individuals due to its effect on Langerhans cells.[24] The stratum corneum of people with dark or heavily tanned skin is more condensed and contains more cornified cell layers than in lightly-pigmented humans. These qualities of dark skin enhance the barrier protection function of the skin.[24]

Although darkly-pigmented skin absorbs about 30 to 40% more sunlight than lightly-pigmented skin, dark skin does not increase the body's internal heat intake in conditions of intense solar radiation. Solar radiation heats up the body's surface and not the interior. Furthermore, this amount of heat is negligible compared to the heat produced when muscles are actively used during exercise. Regardless of skin colour, humans have excellent capabilities to dissipate heat through sweating.[35] Half of the solar radiation reaching the Earth's surface is in the form of infrared light and is absorbed similarly regardless of skin coloration.[24]

In people with naturally occurring dark skin, the tanning occurs with the dramatic mobilization of melanin upward in the epidermis and continues with the increased production of melanin. This accounts for the fact that dark-skinned people get visibly darker after one or two weeks of sun exposure, and then lose their colour after months when they stay out of the sun. Darkly-pigmented people tend to exhibit fewer signs of aging in their skin than the lightly-pigmented because their dark skin protects them from most photoaging.[35]

Skin colour is a polygenic trait, which means that several different genes are involved in determining a specific phenotype. Many genes work together in complex, additive, and non-additive combinations to determine the skin colour of an individual. The skin colour variations are normally distributed from light to dark, as it is usual for polygenic traits.[54][55]

Data collected from studies on MC1R gene has shown that there is a lack of diversity in dark-skinned African samples in the allele of the gene compared to non-African populations. This is remarkable given that the number of polymorphisms for almost all genes in the human gene pool is greater in African samples than in any other geographic region. So, while the MC1Rf gene does not significantly contribute to variation in skin colour around the world, the allele found in high levels in African populations probably protects against UV radiation and was probably important in the evolution of dark skin.[56][57]

Skin colour seems to vary mostly due to variations in a number of genes of large effect as well as several other genes of small effect (TYR, TYRP1, OCA2, SLC45A2, SLC24A5, MC1R, KITLG and SLC24A4). This does not take into account the effects of epistasis, which would probably increase the number of related genes.[58] Variations in the SLC24A5 gene account for 20–25% of the variation between dark- and light-skinned populations of Africa,[59] and appear to have arisen as recently as within the last 10,000 years.[60] The Ala111Thr or rs1426654 polymorphism in the coding region of the SLC24A5 gene reaches fixation in Europe, and is also common among populations in North Africa, the Horn of Africa, West Asia, Central Asia and South Asia.[61][62][63]

Health implications

Skin pigmentation is an evolutionary adaptation to various UVR levels around the world. As a consequence there are many health implications that are the product of population movements of humans of certain skin pigmentation to new environments with different levels of UVR.[3] Modern humans are often ignorant of their evolutionary history at their peril.[3] Cultural practices that increase problems of conditions among dark-skinned populations are traditional clothing and vitamin D-poor diet.[64]

Advantages of dark skin pigmentation in high sunlight environments

Dark-pigmented people living in high sunlight environments are at an advantage due to the high amounts of melanin produced in their skin. The dark pigmentation protects from DNA damage and absorbs the right amounts of UV radiation needed by the body, as well as protects against folate depletion. Folate is a water-soluble vitamin B complex which naturally occurs in green, leafy vegetables, whole grains, and citrus fruits. Women need folate to maintain healthy eggs, for proper implantation of eggs, and for the normal development of placenta after fertilization. Folate is needed for normal sperm production in men. Furthermore, folate is essential for fetal growth, organ development, and neural tube development. Folate breaks down in high intensity UVR.[35] Dark-skinned women suffer the lowest level of neural tube defects.[35][65] Folate plays an important role in DNA production and gene expression. It is essential for maintaining proper levels of amino acids which make up proteins. Folate is used in the formation of myelin, the sheath that covers nerve cells and makes it possible to send electrical signals quickly. Folate also plays an important role in the development of many neurotransmitters, e.g. serotonin which regulates appetite, sleep, and mood. Serum folate is broken down by UV radiation or alcohol consumption.[35] Because the skin is protected by the melanin, dark-pigmented people have a lower chance of developing skin cancer and conditions related to folate deficiency, such as neural tube defects.[3]

Disadvantages of dark skin pigmentation in low sunlight environments
Rickets is a condition associated with dark skin.

Dark-skinned people living in low sunlight environments have been recorded to be very susceptible to vitamin D deficiency due to reduced vitamin D synthesis. A dark-skinned person requires about six times as much UVB than lightly-pigmented persons. This is not a problem near the equator; however, it can be a problem at higher latitudes.[35] For humans with dark skin in climates of low UVR, it can take about two hours to produce the same amount of vitamin D as humans with light skin produce in 15 minutes. Dark-skinned people having a high body-mass index and not taking vitamin D supplements were associated with vitamin D deficiency.[66][67] Vitamin D plays an important role in regulating the human immune system and chronic deficiencies in vitamin D can make humans susceptible to specific types of cancers and many kinds of infectious diseases.[35][68][69] Vitamin D deficiency increases the risk of developing tuberculosis five-fold and also contributes to the development of breast, prostate, and colorectal cancer.[70]

The most prevalent disease to follow vitamin D deficiency is rickets, the softening of bones in children potentially leading to fractures and deformity. Rickets is caused by reduced vitamin D synthesis that causes an absence of vitamin D, which then causes the dietary calcium to not be properly absorbed. This disease in the past was commonly found among dark-skinned Americans of the southern part of the United States who migrated north into low sunlight environments. The popularity of sugary drinks and decreased time spent outside have contributed to significant rise of developing rickets. Deformities of the female pelvis related to severe rickets impair normal childbirth, which leads to higher mortality of the infant, mother, or both.

Vitamin D deficiency is most common in regions with low sunlight, especially in the winter.[71] Chronic deficiencies in vitamin D may also be linked with breast, prostate, colon, ovarian, and possibly other types of cancers.[22][72][73][74] The relationship between cardiovascular disease and vitamin D deficiency also suggest a link between health of cardiac and smooth muscle.[75][76] Low vitamin D levels have also been linked to impaired immune system and brain functions.[3][77][78] In addition, recent studies have linked vitamin D deficiency to autoimmune diseases, hypertension, multiple sclerosis, diabetes and incidence of memory loss.

Outside the tropics UVR has to penetrate through a thicker layer of atmosphere, which results in most of the intermediate wavelength UVB reflected or destroyed en route; because of this there is less potential for vitamin D biosynthesis in regions far from the equator. Higher amount of vitamin D intake for dark-skinned people living in regions with low levels of sunlight are advised by doctors to follow a vitamin D-rich diet or take vitamin D supplements,[22][79][80][81][82][83] although there is recent evidence that dark-skinned individuals are able to process vitamin D more efficiently than lighter-skinned individuals so may have a lower threshold of sufficiency.[19]

Geographic distribution

There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of skin pigmentation around the world. Areas that have higher amounts of UVR have darker-skinned populations, generally located nearer the equator. Areas that are further away from the equator and generally closer to the poles have a lower concentration of UVR and contain lighter-skinned populations. This is the result of human evolution which contributed to variable melanin content in the skin to adapt to certain environments. A larger percentage of dark-skinned people are found in the Southern Hemisphere because latitudinal land mass distribution is disproportionate.[24] The present distribution of skin colour variation does not completely reflect the correlation of intense UVR and dark skin pigmentation due to mass migration and movement of peoples across continents in the recent past.[24] Dark-skinned populations inhabiting Africa, Australia, Melanesia, Papua New Guinea and South Asia all live in some of the areas with the highest UV radiation in the world, and have evolved very dark skin pigmentations as protection from the sun's harmful rays.[22][24] Evolution has restricted humans with darker skin in tropical latitudes, especially in non-forested regions, where ultraviolet radiation from the sun is usually the most intense. Different dark-skinned populations are not necessarily closely related genetically.[84] Before the modern mass migration, it has been argued that the majority of dark-pigmented people lived within 20° of the equator.[85]

Natives of Buka and Bougainville at the northern Solomon Islands in Melanesia and the Chopi people of Mozambique in the southeast coast of Africa have darker skin than other surrounding populations. (The native people of Bougainville, Papua New Guinea, have some of the darkest skin pigmentation in the world.) Although these people are widely separated they share similar physical environments. In both regions, they experience very high UVR exposure from cloudless skies near the equator which is reflected from water or sand. Water reflects, depending on colour, about 10 to 30% of UVR that falls on it.[35][86] People in these populations spend long hours fishing on the sea. Because it is impractical to wear extensive clothing in a watery environment, culture and technology does little to buffer UVR exposure. The skin takes a very large amount of ultraviolet radiation. These populations are probably near or at the maximum darkness that human skin can achieve.[35]

More recent research has found that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa. Only 100–200 generations ago, the ancestors of most people living today likely also resided in a different place and had a different skin color. According to Nina Jablonski, darkly-pigmented modern populations in South India and Sri Lanka are an example of this, having re-darkened after their ancestors migrated down from areas much farther north. Scientists originally believed that such shifts in pigmentation occurred relatively slowly. However, researchers have since observed that changes in skin coloration can happen in as little as 100 generations (~2,500 years), with no intermarriage required. The speed of change is also affected by clothing, which tends to slow it down.[87]

Australia
An Aboriginal Australian man with dark skin.

The Aborigines of Australia, as with all humans, are descendants of African migrants, and their ancestors may have been among the first major groups to leave Africa around 50,000 years ago. Despite early migrations, genetic evidence has pointed out that the indigenous peoples of Australia are genetically very dissimilar to the dark-skinned populations of Africa and that they are more closely related to Eurasian populations.[88]

The term black initially has been applied as a reference to the skin pigmentation of the aborigines of Australia; today it has been embraced by aboriginal activists as a term for shared culture and identity, regardless of skin colour.[89][90]

Melanesia
Dark-skinned blond boy from Vanuatu, Melanesia

Melanesia, a subregion of Oceania, whose name means "black islands", have several islands that are inhabited by people with dark skin pigmentation. The islands of Melanesia are located immediately north and northeast of Australia as well as east coast of Papua New Guinea.[91] The western end of Melanesia from New Guinea through the Solomon Islands were first colonized by humans about 40,000 to 29,000 years ago.[92][93]

In the world, blond hair is exceptionally rare outside Europe and Southwest Asia, especially among dark-skinned populations. However, Melanesians are one of the dark-skinned human populations known to have naturally-occurring blond hair.[94][95]

New Guinea
Buka boys from Bougainville Island, Papua New Guinea. People from Bougainville have some of the darkest skin tones among humans.

The indigenous Papuan people of New Guinea have dark skin pigmentation and have inhabited the island for at least 40,000 years. Due to their similar phenotype and the location of New Guinea being in the migration route taken by Indigenous Australians, it was generally believed that Papuans and Aboriginal Australians shared a common origin. However, a 1999 study failed to find clear indications of a single shared genetic origin between the two populations, suggesting multiple waves of migration into Sahul with distinct ancestries.[96]

Sub-Saharan Africa
Women from South Sudan

Sub-Saharan Africa is the region in Africa situated south of the Sahara where a large number of dark-skinned populations live.[97][98] Dark-skinned groups on the continent have the same receptor protein as Homo ergaster and Homo erectus had.[99] According to scientific studies, populations in Africa also have the highest skin colour diversity.[100] High levels of skin colour variation exists between different populations in Sub-Saharan Africa. These differences depend in part on general distance from the equator, illustrating the complex interactions of evolutionary forces which have contributed to the geographic distribution of skin color at any point of time.[35]

Due to frequently differing ancestry among dark-skinned populations, the presence of dark skin in general is not a reliable genetic marker, including among groups in Africa. For example, Wilson et al. (2001) found that most of their Ethiopian samples showed closer genetic affinities with lighter-skinned Armenians than with dark-skinned Bantu populations.[101] Mohamoud (2006) likewise observed that their Somali samples were genetically more similar to Arab populations than to other African populations.[102]

South Asia
Fisherman from Chennai, Tamil Nadu in southern India.

South Asia has some of the greatest skin color diversity outside of Africa. Skin color among South Indians is on average darker than North Indians. This is mainly because of the weather conditions in South Asia—higher UV indices are in the south.[103] Several genetic surveys of South Asian populations in different regions have found a weak negative correlation between social status and skin darkness, represented by the melanin index. A study of caste populations in the Gangetic Plain found an association between the proportion of dark skin and ranking in the caste hierarchy. Dalits had, on average, the darkest skin.[104] A pan-India study of Telugu and North Indian castes found a similar correlation between skin color and caste association, linked to the absence of the rs1426654-A variant of the SLC245-A gene, but are also linked to mutations overriding these variants.[105]

Americas
A dark-skinned Wayuu couple from Colombia. Many other Indigenous peoples of tropical areas or subtropical of the Americas have dark skin.

Relatively dark skin remains among the Inuit and other Arctic populations. A combination of protein-heavy diets and summer snow reflection have been speculated as favouring the retention of pigmented skin.[3][100][25]

Earliest European colonial descriptions of North American populations include terms such as "brown", "tawny" or "olive", though some populations were also described as "light-skinned".[106] Most North American indigenous populations rank similar to African and Oceanian populations in regards to the presence of the allele Ala111.[107]

Native South Americans and Mesoamericans are also typically considered dark-skinned, ranking similarly to African and Oceanian populations in regards to Ala111 presence.[107] High ultraviolet radiation levels occur throughout the Andes region of Peru, Bolivia, Chile, and Argentina.[108]

Genetic tests show significant Australian influence,[109] theorizing that Amazonian Indians and Australians both diverged from common ancestor. Scientists tested the ancient and present-day Genome-wide analysis of 49 Central and South Americans up to 11,000 years old from Belize, Brazil, Peru, and the Southern Cone (Chile and Argentina).[110]

Culture
Main article: Discrimination based on skin color

The preference or disfavour for darker skin has varied depending on geographical area and time. Today, darker skin is viewed as fashionable and as a sign of well-being in some societies. This resulted in the development of tanning industry in several countries. However, in some countries, dark skin is not seen as highly desirable or indicative of higher class, especially among women.[24]

See also

References
  1. dark-skinned Princeton University "naturally having skin of a dark color"
  2. "Dark-skinned". thefreedictionary.com. Retrieved 24 January 2017. a person or race having skin of a dark colour
  3. Muehlenbein, Michael (2010). Human Evolutionary Biology. Cambridge University Press. pp. 192–213.
  4. Oxford Dictionaries. April 2010. Oxford University Press. "belonging to or denoting any human group having dark-coloured skin" "black" (accessed 6 August 2012).
  5. Dictionary.com: black 3.a "a member of any of various dark-skinned peoples" 21.a"specifically the dark-skinned peoples of Africa, Oceania, or Australia."
  6. "Global Census". American Anthropological Association. Archived from the original on 14 September 2018. Retrieved 10 December 2012.
  7. Oxford Dictionaries. April 2010. Oxford University Press. "especially of African or Australian Aboriginal ancestry" "black" (accessed 6 August 2012).
  8. James, Mackers (8 November 1828). "Proclamation". Classified Advertising. Trove. Retrieved 10 December 2012.
  9. Nina, Jablonski (2004). "The evolution of human skin and skin color". Annual Review of Anthropology. 33: 585–623. doi:10.1146/annurev.anthro.33.070203.143955. S2CID 53481281. genetic evidence [demonstrate] that strong levels of natural selection acted about 1.2 mya to produce darkly pigmented skin in early members of the genus Homo
  10. Bower, C.; Stanley (1992). "The role of nutritional factors in the aetiology of neural tube defects". Journal of Paediatrics and Child Health. 28 (1): 12–16. doi:10.1111/j.1440-1754.1992.tb02610.x. PMID 1554510. S2CID 45104826.
  11. Minns, R.A. (1996). "Folic acid and neural tube defects". Spinal Cord. 34 (8): 460–465. doi:10.1038/sc.1996.79. PMID 8856852.
  12. Copp; et al. (1998). "Embryonic mechanisms underlying the prevenetion of neural tube defects by vitamins". Mental Retardation and Developmental Disabilities Research Reviews. 4 (4): 264–268. doi:10.1002/(sici)1098-2779(1998)4:4<264::aid-mrdd5>3.0.co;2-g.
  13. Molloy; Mills, J. L.; Kirke, P. N.; Weir, D. G.; Scott, J. M.; et al. (1999). "Folate status and neural tube defects". BioFactors. 10 (2–3): 291–294. doi:10.1002/biof.5520100230. PMID 10609896. S2CID 20128738.
  14. Lucock, M. (2000). "Folic acid: nutritional biochemistry, molecular biology, and role in disease processes". Molecular Genetics and Metabolism. 71 (1–2): 121–138. doi:10.1006/mgme.2000.3027. PMID 11001804.
  15. William; Rasmussen, S. A.; Flores, A; Kirby, R. S.; Edmonds, L. D.; et al. (2005). "Decline in the prevalence of spina bifida and anencephaly by race/ethnicity:1995–2002". Pediatrics. 116 (3): 580–586. doi:10.1542/peds.2005-0592. PMID 16140696. S2CID 12765407.
  16. Nielsen; et al. (2006). "The importance of the depth distribution of melanin in skin for DNA protection and other photobiological processes". Journal of Photochemistry and Photobiology B: Biology. 82 (3): 194–198. doi:10.1016/j.jphotobiol.2005.11.008. PMID 16388960.
  17. Jane, Higdon. "Vitamin D". Micronutrient Information Center. Linus Pauling Institute. Retrieved 10 December 2012.
  18. Holick, Michael F. (21 November 2013). "Bioavailability of Vitamin D and Its Metabolites in Black and White Adults". The New England Journal of Medicine. 369 (21): 2047–2048. doi:10.1056/NEJMe1312291. PMID 24256384.
  19. DeVita Raeburn, Elizabeth (20 November 2013). "Bone Density Higher in Blacks, Vitamin D Lower". MedPage Today. Retrieved 19 June 2014.
  20. Jablonski, N.G.; Chaplin (2000). "The evolution of human skin coloration". Journal of Human Evolution. 39 (1): 57–106. doi:10.1006/jhev.2000.0403. PMID 10896812. S2CID 38445385.
  21. Harding, R; Healy, E; Ray, A; Ellis, N; Flanagan, N; Todd, C; Dixon, C; Sajantila, A; et al. (2000). "Evidence for Variable Selective Pressures at MC1R". The American Journal of Human Genetics. 66 (4): 1351–61. doi:10.1086/302863. PMC 1288200. PMID 10733465.
  22. 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.
  23. O'Neil, Dennis. "Overview". Modern Human Variation. Palomer. Archived from the original on 5 November 2012. Retrieved 10 December 2012.
  24. Nina, Jablonski (2004). "The evolution of human skin and skin color". Annual Review of Anthropology. 33: 585–623. doi:10.1146/annurev.anthro.33.070203.143955. S2CID 53481281.
  25. Gina, Kirchweger. "The Biology of Skin Color: Black and White". Evolution Library. PBS. Retrieved 10 December 2012.
  26. Jablonski, N.G. (2006). Skin: a Natural History. Berkeley: University of California Press.
  27. Dawkins, Richard (2004). The Ancestor's Tale.
  28. Montagna, W. "The consequences of having naked skin". Birth Defects: Original Article Series. 17: 1–7.
  29. Langbein; Rogers, M. A.; Praetzel, S; Cribier, B; Peltre, B; Gassler, N; Schweizer, J; et al. (2005). "Characterization of a novel human type II epithelial keratin K1b, specifically expressed in eccrine sweat glands". Journal of Investigative Dermatology. 125 (3): 428–444. doi:10.1111/j.0022-202X.2005.23860.x. PMID 16117782.
  30. Blum, H.F. (1961). "Does the melanin pigment of human skin have adaptive value?". Quarterly Review of Biology. 36: 50–63. doi:10.1086/403275. PMID 13870200. S2CID 30402725.
  31. Rigel, D.S. (2008). "Cutaneous ultraviolet exposure and its relationship to the development of skin cancer". Journal of the American Academy of Dermatology. 58 (5): S129–S132. doi:10.1016/j.jaad.2007.04.034. PMID 18410798.
  32. Jemal; et al. (2001). "Recent trends in cutaneous melanoma incidence among white in the United States". Journal of the National Cancer Institute. 93 (9): 678–683. doi:10.1093/jnci/93.9.678. PMID 11333289.
  33. Jablonski, Nina. "Department of Anthropology at Penn State". Penn State University. Archived from the original on 16 January 2013. Retrieved 14 December 2012.
  34. Appenzeller Tim (2012). "Human migrations: Eastern odyssey". Nature. 485 (7396): 24–26. Bibcode:2012Natur.485...24A. doi:10.1038/485024a. PMID 22552074.
  35. Jablonski, Nina (2012). Living Color. Berkeley, Los Angeles, London: University of California Press. ISBN 978-0-520-25153-3.
  36. "Effects of Ecology and Climate on Human Physical Variations". Retrieved 10 December 2012.
  37. Miyamura; et al. (2007). "Regulation of human skin pigmentation and responses to ultraviolet radiation". BioFactors. 20 (1): 2–13. doi:10.1111/j.1600-0749.2006.00358.x. PMID 17250543.
  38. Saraiya; Glanz, K; Briss, P. A.; Nichols, P; White, C; Das, D; Smith, S. J.; Tannor, B; Hutchinson, A. B.; Wilson, K. M.; Gandhi, N; Lee, N. C.; Rimer, B; Coates, R. C.; Kerner, J. F.; Hiatt, R. A.; Buffler, P; Rochester, P; et al. (2004). "Interventions to prevent skin cancer by reducing exposure to ultraviolet radiation: a systematic review". American Journal of Preventive Medicine. 27 (5): 422–466. doi:10.1016/j.amepre.2004.08.009. PMID 15556744.
  39. Agar, N.; Young, A. R. (2005). "Melanogenesis: a photoprotective response to DNA damage?". Mutation Research. 571 (1–2): 121–132. doi:10.1016/j.mrfmmm.2004.11.016. PMID 15748643.
  40. Pfeifer; You, Y. H.; Besaratinia, A; et al. (2005). "Mutations induced by ultraviolet light". Mutation Research. 571 (1–2): 19–31. doi:10.1016/j.mrfmmm.2004.06.057. PMID 15748635.
  41. Rouzaud; et al. (2005). "MC1R and the response of melanocytes to ultraviolet radiation". Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis. 133–152. 571 (1–2): 133–52. doi:10.1016/j.mrfmmm.2004.09.014. PMID 15748644.
  42. Brenner, M.; Hearing, V. J. (2008). "The protective role of melanin against UV damage in human skin". Photochemistry and Photobiology. 84 (3): 539–549. doi:10.1111/j.1751-1097.2007.00226.x. PMC 2671032. PMID 18435612.
  43. Van Nieuwpoort; Smit, N. P.; Kolb, R; Van Der Meulen, H; Koerten, H; Pavel, S; et al. (2004). "Tyrosine-induced melanogenesis shows differences in morphologic and melanogenic preferences of melanosomes from light and dark skin types". Journal of Investigative Dermatology. 122 (5): 1251–1255. doi:10.1111/j.0022-202X.2004.22533.x. PMID 15140229.
  44. Kielbassa; Epe, B; et al. (2000). "DNA damaged induced by ultraviolet and visible light and its wavelength dependence". Methods in Enzymology. 319: 436–445. doi:10.1016/s0076-6879(00)19041-x. PMID 10907532.
  45. Cleaver and Crowely (2002). "UV damage, DNA repair and skin carcinogenesis". Frontiers in Bioscience. 7 (1–3): 1024–1043. doi:10.2741/cleaver. PMID 11897551. S2CID 10115842.
  46. Sinha; et al. (2002). "UV-induced DNA damage and repair: a review". Photochemical & Photobiological Sciences. 1 (4): 225–236. doi:10.1039/b201230h. PMID 12661961. S2CID 11348959.
  47. Schreier, W. J.; Schrader, T. E.; Koller, F. O.; Gilch, P; Crespo-Hernández, C. E.; Swaminathan, V. N.; Carell, T; Zinth, W; Kohler, B; et al. (2007). "Thymine dimerization in DNA is an ultrafast photoreaction". Science. 315 (5812): 625–629. Bibcode:2007Sci...315..625S. doi:10.1126/science.1135428. PMC 2792699. PMID 17272716.
  48. Epel; et al. (1999). "Development in the floating world: defenses of eggs and embryos against damage from UV radiation". American Zoologist. 39 (2): 271–278. doi:10.1093/icb/39.2.271.
  49. Haass, Nikolas K.; Smalley, Keiran S. M.; Li, Ling; Herlyn, Meenhard (1 June 2005). "Adhesion, migration and communication in melanocytes and melanoma". Pigment Cell Research. 18 (3): 150–159. doi:10.1111/j.1600-0749.2005.00235.x. ISSN 0893-5785. PMID 15892711.
  50. Thong, H.Y.; et al. (2003). "The patterns of melanosome distribution in keratinocytes of human skin as one determining factor of skin colour". British Journal of Dermatology. 149 (3): 498–505. doi:10.1046/j.1365-2133.2003.05473.x. PMID 14510981. S2CID 43355316.
  51. Tadokoro, T; et al. (2005). "Mechanisms of skin tanning in different racial/ethnic groups in response to ultraviolet radiation". Journal of Investigative Dermatology. 124 (6): 1326–1332. doi:10.1111/j.0022-202X.2005.23760.x. PMID 15955111.
  52. Minwala, S; et al. (2001). "Keratinocytes Play a Role in Regulating Distribution Patterns of Recipient Melanosomes in Vitro". Journal of Investigative Dermatology. 117 (2): 341–347. doi:10.1046/j.0022-202x.2001.01411.x. PMID 11511313.
  53. Szabo, G; et al. (1969). "Racial differences in the fate of melanosomes in human epidermis". Nature. 222 (5198): 1081–1082. Bibcode:1969Natur.222.1081S. doi:10.1038/2221081a0. PMID 5787098. S2CID 4223552.
  54. Lewis, Ricki (2012). Human genetics : concepts and applications (10th ed.). New York, NY: McGraw-Hill Co. pp. 135–136. ISBN 978-0-07-352530-3.
  55. "Skin Color" (PDF).
  56. Rana, B. K.; Hewett-Emmett, D.; Jin, L.; Chang, B. H.; Sambuughin, N.; Lin, M.; Watkins, S.; Bamshad, M.; Jorde, L. B. (1 April 1999). "High polymorphism at the human melanocortin 1 receptor locus". Genetics. 151 (4): 1547–1557. ISSN 0016-6731. PMC 1460552. PMID 10101176.
  57. "Effects of Ecology and Climate on Human Physical Variations". www.culturechange.org.
  58. Khan, Razib (2009). "Genetics of human pigmentation: Gene expression". Discover Magazine. Retrieved 11 December 2012.
  59. Lamason, R. L.; Mohideen, MA; Mest, JR; Wong, AC; Norton, HL; Aros, MC; Jurynec, MJ; Mao, X; et al. (2005). "SLC24A5, a Putative Cation Exchanger, Affects Pigmentation in Zebrafish and Humans". Science. 310 (5755): 1782–17886. Bibcode:2005Sci...310.1782L. doi:10.1126/science.1116238. PMID 16357253. S2CID 2245002.
  60. Gibbons, A. (2007). "AMERICAN ASSOCIATION OF PHYSICAL ANTHROPOLOGISTS MEETING: European Skin Turned Pale Only Recently, Gene Suggests". Science. 316 (5823): 364. doi:10.1126/science.316.5823.364a. PMID 17446367. S2CID 43290419.
  61. "Graphical display of Allele Frequencies for Ala111Thr". Allele Frequency Database. Retrieved 10 October 2012.
  62. "ALFRED – Polymorphism Information – Ala111Thr". Allele Frequency Database. Retrieved 10 October 2012.
  63. Pagani L, Kivisild T, Tarekegn A, Ekong R, Plaster C, Romero IG, Ayub Q, Mehdi SQ, Thomas MG, Luiselli D, Bekele E, Bradman N, Balding DJ, Tyler-Smith C (21 June 2012). "Ethiopian Genetic Diversity Reveals Linguistic Stratification and Complex Influences on the Ethiopian Gene Pool". The American Journal of Human Genetics. 91 (1): 83–96. doi:10.1016/j.ajhg.2012.05.015. PMC 3397267. PMID 22726845.
  64. "Dark-skinned immigrant urged to take vitamin D". CBC News. Retrieved 10 December 2012.
  65. Buccimazza SS, Molteno CD, Dunnem TT, Viljoen DL (1994). "Prevalence of neural tube defects in Cape Town, South Africa". Teratology. 50 (3): 194–199. doi:10.1002/tera.1420500304. PMID 7871483.
  66. "Dark-skinned immigrants urged to take vitamin D". CBC News.
  67. Oglesby, Erika. "Darker Skin? More Vitamin D, Please!". Care2. Retrieved 1 January 2013.
  68. Murray, F. G. (1934). "Pigmentation, sunlight, and nutritional disease". American Anthropologist. 36 (3): 438–445. doi:10.1525/aa.1934.36.3.02a00100.
  69. Loomis, W. F. (1967). "Skin-pigment regulation of vitamin-D biosynthesis in man". Science. 157 (3788): 501–506. Bibcode:1967Sci...157..501F. doi:10.1126/science.157.3788.501. PMID 6028915. S2CID 41681581.
  70. Chaplin G, Jablonski NG (2009). "Vitamin D and the evolution of human depigmentation". American Journal of Physical Anthropology. 139 (4): 451–461. doi:10.1002/ajpa.21079. PMID 19425101.
  71. Vieth, R (2003). In Bone Loss and Osteoporosis: an Anthropological Perspective. Kluwer Academic/Plenum Press. pp. 135–150.
  72. Garland CF, Garland FC, Gorham ED, et al. (2006). "The Role of Vitamin D in Cancer Prevention". American Journal of Public Health. 96 (2): 252–261. doi:10.2105/ajph.2004.045260. PMC 1470481. PMID 16380576.
  73. Fleet, J.C. (2008). "Molecular actions of vitamin D contributing to cancer prevention". Molecular Aspects of Medicine. 29 (6): 388–396. doi:10.1016/j.mam.2008.07.003. PMC 2613446. PMID 18755215.
  74. Grant, W.B. (2008). "Solar Ultraviolet Irradiance and Cancer Incidence and Mortality". Solar ultraviolet irradiance and cancer incidence and morality. Advances in Experimental Medicine and Biology. 624. pp. 16–30. doi:10.1007/978-0-387-77574-6_2. ISBN 978-0-387-77573-9. PMID 18348444.
  75. Chen, T.C.; et al. (2007). "Factors that influence the cutaneous synthesis and dietary sources of vitamin D". Archives of Biochemistry and Biophysics. 460 (2): 213–217. doi:10.1016/j.abb.2006.12.017. PMC 2698590. PMID 17254541.
  76. Kim, Dae Hyun; Sabour, Siamak; Sagar, Utpal N.; Adams, Suzanne; Whellan, David J. (1 December 2008). "Prevalence of hypovitaminosis D in cardiovascular diseases (from the National Health and Nutrition Examination Survey 2001 to 2004)". The American Journal of Cardiology. 102 (11): 1540–1544. doi:10.1016/j.amjcard.2008.06.067. ISSN 1879-1913. PMID 19026311.
  77. McGrath, J.J.; et al. (2004). "Vitamin D – implications for brain development". Journal of Steroid Biochemistry and Molecular Biology. 89–90 (1–5): 557–560. doi:10.1016/j.jsbmb.2004.03.070. PMID 15225838. S2CID 19993730.
  78. Harms, M.; et al. (2008). "Developmental vitamin D deficiency alters adult behaviour in 129/SvJ and C57BL/6J mice". Behavioural Brain Research. 187 (2): 343–350. doi:10.1016/j.bbr.2007.09.032. PMID 17996959. S2CID 8107153.
  79. "How to get your vitamin D". Archived from the original on 13 January 2013. Retrieved 31 December 2012.
  80. Painter, Kim (19 April 2009). "Your Health". USA Today.
  81. "Vitamin D deficiency and skin sun exposure". Chicago Tribune. 26 October 2011.
  82. Villarosa, Linda. "Why Black People Need More Vitamin D". The Root. Archived from the original on 17 November 2012. Retrieved 1 January 2013.
  83. "Micronutrient Information Center". Linus Pauling. Retrieved 1 January 2013.
  84. Marks, Jonathan. "Interview with Jonathan Marks". Race – The Power of an Illusion. PBS. Retrieved 3 January 2013. Certainly dark skin is present all over the world in different populations. Indigenous Australians, indigenous peoples of India, indigenous peoples of Africa are all very darkly pigmented even though they're not particularly closely related.
  85. "Modern human variation: overview". Archived from the original on 18 December 2012.
  86. Chadysiene R, Girgzdys A (2008). "Ultraviolet radiation albedo of natural surfaces". Journal of Environmental Engineering and Landscape Management. 16 (2): 83–88. doi:10.3846/1648-6897.2008.16.83-88.
  87. Krulwich, Robert (2 February 2009). "Your Family May Once Have Been A Different Color". NPR. Retrieved 4 July 2013.
  88. "Aboriginal Genome" (PDF).
  89. "Classified Advertising". 8 November 1828. p. 1 – via Trove.
  90. "Aboriginal identity goes beyond skin colour". The Sydney Morning Herald.
  91. "Papua Web" (PDF).
  92. Matisoo-Smith E, Robins JH (2004). "Origins and dispersals of Pacific peoples: Evidence from mtDNA phylogenesis of the pacific rat". Proceedings of the National Academy of Sciences. 101 (24): 9167–9172. doi:10.1073/pnas.0403120101. PMC 428491. PMID 15184658.
  93. Norton HL, Friedlaender JS, Merriwether DA, Koki G, Mgone CS, Shriver MD (2006). "Skin and hair pigmentation variation in island Melanesia". American Journal of Physical Anthropology. 130 (2): 254–268. doi:10.1002/ajpa.20343. PMID 16374866.
  94. Sindya N. Bhanoo (3 May 2012). "Another Genetic Quirk of the Solomon Islands: Blond Hair". The New York Times. Retrieved 3 May 2012.
  95. Dupree, L. "Afghānistān: (iv.) ethnography". In Ehsan Yarshater (ed.). Encyclopædia Iranica (Online ed.). United States: Columbia University. Retrieved 5 November 2011.
  96. Redd AJ, Stoneking M (1999). "Peopling of Sahul: mtDNA Variation in Aboriginal Australian and Papua New Guinean Populations". American Journal of Human Genetics. 65 (3): 808–828. doi:10.1086/302533. PMC 1377989. PMID 10441589.
  97. "Modern human variation: overview". Archived from the original on 5 November 2012.
  98. "black Africa | Definition of black Africa in English by Lexico Dictionaries". Lexico Dictionaries | English.
  99. Rogers, Alan R.; Iltis, David; Wooding, Stephen (1 February 2004). "Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair". Current Anthropology. 45 (1): 105–108. doi:10.1086/381006. ISSN 0011-3204.
  100. 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.
  101. Wilson, James F.; Weale, Michael E.; Smith, Alice C.; Gratrix, Fiona; Fletcher, Benjamin; Thomas, Mark G.; Bradman, Neil; Goldstein, David B. (2001). "Population genetic structure of variable drug response". Nature Genetics. 29 (3): 265–9. doi:10.1038/ng761. PMID 11685208. S2CID 25627134. 62% of the Ethiopians fall in the first cluster, which encompasses the majority of the Jews, Norwegians and Armenians, indicating that placement of these individuals in a 'Black' cluster would be an inaccurate reflection of the genetic structure. Only 24% of the Ethiopians are placed in the cluster with the Bantu
  102. Mohamoud, A. M. (October 2006). "P52 Characteristics of HLA Class I and Class II Antigens of the Somali Population". Transfusion Medicine. 16 (Supplement s1): 47. doi:10.1111/j.1365-3148.2006.00694_52.x. S2CID 70655900.
  103. Michael F. Holick (2010). Vitamin D: Physiology, Molecular Biology, and Clinical Applications. Springer. p. 531. ISBN 9781603273039.
  104. Mishra, Anshuman; Nizammuddin, Sheikh; Mallick, Chandana Basu; Singh, Sakshi; Prakash, Satya; Siddiqui, Niyamat Ali; Rai, Niraj; Carlus, S. Justin; Sudhakar, Digumarthi V. S.; Tripathi, Vishnu P.; Möls, Märt (1 March 2017). "Genotype-Phenotype Study of the Middle Gangetic Plain in India Shows Association of rs2470102 with Skin Pigmentation". Journal of Investigative Dermatology. 137 (3): 670–677. doi:10.1016/j.jid.2016.10.043. ISSN 0022-202X. PMID 27866970.
  105. Iliescu, Florin Mircea; Chaplin, George; Rai, Niraj; Jacobs, Guy S.; Mallick, Chandana Basu; Mishra, Anshuman; Thangaraj, Kumarasamy; Jablonski, Nina G. (2018). "The influences of genes, the environment, and social factors on the evolution of skin color diversity in India". American Journal of Human Biology. 30 (5): e23170. doi:10.1002/ajhb.23170. hdl:20.500.11820/435c03a5-a3ca-4046-aa50-c81c38d08645. ISSN 1520-6300. PMID 30099804. S2CID 51966049.
  106. Vaughan, Alden T. (1 October 1982). "From White Man to Redskin: Changing Anglo-American Perceptions of the American Indian". The American Historical Review. 87 (4): 918. doi:10.2307/1857900. ISSN 0002-8762. JSTOR 1857900.
  107. Reference SNP(refSNP) Cluster Report: rs1426654 **clinically associated** Archived 5 December 2017 at the Wayback Machine.. Ncbi.nlm.nih.gov (30 December 2008). Retrieved 2011-02-27.
  108. "High UV occurs throughout the Altiplano region" (PDF).
  109. Thompson, Helen. "A DNA Search for the First Americans Links Amazon Groups to Indigenous Australians". Smithsonian.
  110. "Reconstructing the Deep Population History of Central and South America".
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.