Dark skin

Dark skin is a naturally occurring human skin colour rich in eumelanin pigments and having a dark colour.^[1][2][3] People with relatively dark skin are referred to as brown,^[4] and those with very dark skin are often referred to as black,^[5] although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.^[6][7][8][9]

Evolution of dark skin pigmentation began around 1.2 million years ago^[10] 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 by the loss of body hair and development of more efficient sweating. 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][11][12][13][14][15][16]}

Humans with dark skin pigmentation have skin naturally rich in melanin (especially eumelanin), and have more melanosomes which provide a superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to the DNA.^[3][17]

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, and numerous types of cancers, and possibly cardiovascular disease and low immune system activity.^[3][18]

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 almost exclusively live near the equator, in tropical areas with intense sunlight: Australia, Melanesia, New Guinea, South Asia and Sub-Saharan Africa. Studies into these populations indicates that dark skin is a retention of the pre-existing high UV adapted state of modern humans before the Out of Africa migration and not a later evolutionary adaptation.^[19][20] 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][21][22]

Evolution

People who lived in areas of intense sunlight developed dark skin colouration to protect against ultraviolet light and to protect their body mainly from folate depletion. Evolutionary pigmentation of the skin was caused by ultraviolet (UV) radiation of the sun. As hominids gradually lost their fur (between 4.5 and 2 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.^[23] The effect of sunlight on folic acid levels has been crucial in the development of dark skin.^[3][24]

Humans' earliest ancestors, just like their closest relatives – chimpanzees – today, had light skin.^[25] 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.^[21][26] 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 – so there was a need for the development of better heat regulation. The solution was sweating and loss of body hair.^[21]

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 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, more and healthier offspring they could produce, and 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 body of humans faster. But this less-hairy skin proved to be a problem for humans living in areas of intense UV radiation. As heavy UV radiation has harmful effects, the solution was to develop dark skin as a protection. A few million years of evolution later, early humans had sparse body hair and more than 2 million sweat glands in their body.^[21][27][28]

Hairlessness, however, is particularly vulnerable to be damaged by ultraviolet light. 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.^{[21][29][30][31]}

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 incomplete brain or spinal cord. Nina Jablonski, a professor of anthropology and expert on evolution of human skin coloration,^[32] found several cases in which mother’s 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][19]

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 as a disadvantage.^[33][34] 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][35]

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 skin cancers, enhanced fitness as a result of protection against sunburns, and increasing benefits of due to antibacterial properties of eumelanin.^[3]

Darkly pigmented, eumelanin-rich skin protects against DNA damage caused by the sunlight.^[36] This is associated with lower skin cancer rates among dark-skinned populations.^{[37][38][39][40][41]} 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.^[42] The damaging effect of UVR on DNA structure and the entailing elevated skin cancer risk is widely recognized.^{[23][43][44][45][46]} 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.^[23][30] 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.^[31]

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 that 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 as a physical absorbed to protect against UVR induced damage. This hypothesis is not consistent with the evidence that most of the hominid evolution took place in savanna environment and not in tropical rainforests.^[47] Humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments.^[34] The antimicrobial hypothesis also does not explain why some populations (like the Inuits 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

Dark-skinned humans have high amount 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 radiation damage of UV light. Melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis.^[48] 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.^[49][50] A four-fold difference in naturally occurring dark skin gives seven to eightfold protection against DNA damage,^[50] 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 contect, its superb UVR-absorbing capabilites of large melanosomes, and because eumelanin can be mobilized faster and brought to the surface of the skin from the deeps 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.^[51][52] 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.^[23] 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.^[23] Heavily pigmented melanocytes have greater capacity to divide after UVR irradiation, which suggests that they receive less damage to their DNA.^[23] Despite this, UVB damages the immune system even in darker skinned individuals due to its effect on Langerhans cells.^[23] 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.^[23]

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 rather the body's surface and not the interior. Furthermore, this amount of heat is negligible compared the heat produced when muscles are actively used during exercise. Regardless of skin colour, humans have excellent capabilities to dissipate heat through sweating.^[34] 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.^[23]

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 less signs of aging in their skin than the lightly pigmented because their dark skin protects them from most photoaging.^[34]

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.^[53][54]

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.^[55][56]

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.^[57] Variations in the SLC24A5 gene account for 20–25% of the variation between dark and light skinned populations of Africa,^[58] and appear to have arisen as recently as within the last 10,000 years.^[59] 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.^[60][61][62]

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.^[63]

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 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 intense UVR.^[34] Dark-skinned women suffer the lowest level of neural tube defects.^[34][64] 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 the covers nerve cells and makes it possible to send electrical signals quickly. Folate is 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.^[34] 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

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.^[34] 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.^[65][66] 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.^[34][67][68] Vitamin D deficiency increases the risk of the developing tuberculosis fivefold and also contributes to the development of breast, prostate, and colorectal cancer.^[69] 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.^[70] Chronic deficiencies in vitamin D may also be linked with breast, prostate, colon, ovarian, and possibly other types of cancers.^{[21][71][72][73]} The casual relationship between cardiovascular disease and vitamin D deficiency also suggest link between health of cardiac and smooth muscle.^[74][75] Low vitamin D levels have also been linked to impaired immune system and brain functions.^[3][76][77] 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 UVB reflected or destructed 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 vitamin D rich diet or take vitamin D supplements.^{[21][78][79][80][81][82]}

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 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.^[23] 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.^[23]

Dark-skinned populations inhabiting South Asia, Africa, Melanesia, Papua New Guinea and Australia 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 harmful sun rays.^[21][23] 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.^[83] Before the modern mass migration, it is has been argued that the majority of dark pigmented people lived within 20º of the equator.^[84]

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.^[34][85] 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 UVR radiation. These populations are probably near or at the maximum darkness that human skin can achieve.^[34]

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 redarkened 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.^[86]

Australia

The Aborigines of Australia, as with all humans, are descendents 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.^[87]

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.^[88][89]

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.^[90] 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.^[91][92]

In the world, blond hair is exceptionally rare outside Europe, and Southwest Asia, especially among dark-skinned populations. However, Melanesians are one of dark-skinned human populations known to have naturally occurring blond hair.^[93][94]

New Guinea

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.^[95]

Sub-Saharan Africa

Sub-Saharan Africa is the region in Africa situated south of the Sahara where a large number of dark-skinned populations live.^[96][97] Dark-skinned groups on the continent have the same receptor protein as Homo ergaster and Homo erectus had.^[98] According to scientific studies, populations in Africa also have the highest skin colour diversity.^[99] 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.^[34]

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 affinity with light-skinned Armenians and Norwegians than with dark-skinned Bantu populations.^[100] Mohamoud (2006) likewise observed that their Somali samples were genetically more similar to Arab populations than to other African populations.^[101]

Culture

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.^[23]

See also

• Black people
• Brown (racial classification)

References

v t e Human skin color Fitzpatrick scale Albino (I.) Light/White/Fair (I–III.) Brown/Beige/Pardo/Olive (IV–V.) Dark/Black (V–VI.) Color terminology for race White Yellow/Amarelo Red/Copper/Caboclo Brown/Pardo Black