Dark skin is a type of

human skin color Human skin color ranges from the Dark skin, darkest brown to the Light skin, lightest hues. Differences in Human skin, skin color among individuals is caused by variation in pigmentation, which is largely the result of genetics (inherited from o ...

that is rich in

melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

s. People with dark skin are often referred to as

black people Black is a racial classification of people, usually a political and skin color-based category for specific populations with a mid- to dark brown complexion. Not all people considered "black" have dark skin and often additional phenotypical ...

, although this usage can be ambiguous in some countries where it is also used to specifically refer to different

ethnic groups An ethnicity or ethnic group is a group of people with shared attributes, which they collectively believe to have, and long-term endogamy. Ethnicities share attributes like language, culture, common sets of ancestry, traditions, society, rel ...

or populations. The

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

of dark skin is believed to have begun around 1.2 million years ago, in light-skinned early

hominid The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...

species after they moved from the equatorial

rainforest Rainforests are forests characterized by a closed and continuous tree Canopy (biology), canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforests can be generally classified as tropi ...

to the sunny

savanna A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) biome and ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach th ...

s. 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 Perspiration, also known as sweat, is the fluid secreted by sweat glands in the skin of mammals. Two types of sweat glands can be found in humans: eccrine glands and Apocrine sweat gland, apocrine glands. The eccrine sweat glands are distribu ...

. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against

folate Folate, also known as vitamin B9 and folacin, is one of the B vitamins. Manufactured folic acid, which is converted into folate by the body, is used as a dietary supplement and in food fortification as it is more stable during processing and ...

(vitamin B9) depletion, and to a lesser extent,

DNA damage DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is constantly modified ...

. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to

ultraviolet radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

and

spermatogenesis Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testicle. This process starts with the Mitosis, mitotic division of the stem cells located close to the basement membrane of ...

led to the selection of dark skin pigmentation. By the time modern ''

Homo sapiens Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

'' evolved, all humans were dark-skinned. Humans with dark skin pigmentation have skin naturally rich in

, especially

eumelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

, and have more

melanosomes A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotection ...

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. Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk—especially in the winter—of

vitamin D Vitamin D is a group of structurally related, fat-soluble compounds responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, along with numerous other biological functions. In humans, the most important compo ...

deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing

rickets Rickets, scientific nomenclature: rachitis (from Greek , meaning 'in or of the spine'), is a condition that results in weak or soft bones in children and may have either dietary deficiency or genetic causes. Symptoms include bowed legs, stun ...

, numerous types of cancers, and possibly cardiovascular disease and low immune system activity. 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. 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 The Aboriginal Tasmanians (palawa kani: ''Palawa'' or ''Pakana'') are the Aboriginal people of the Australian island of Tasmania, located south of the mainland. At the time of European contact, Aboriginal Tasmanians were divided into a numb ...

, almost exclusively live near the equator, in

tropical The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

areas with intense sunlight:

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

Melanesia Melanesia (, ) is a subregion of Oceania in the southwestern Pacific Ocean. It extends from New Guinea in the west to the Fiji Islands in the east, and includes the Arafura Sea. The region includes the four independent countries of Fiji, Vanu ...

South Asia South Asia is the southern Subregion#Asia, subregion of Asia that is defined in both geographical and Ethnicity, ethnic-Culture, cultural terms. South Asia, with a population of 2.04 billion, contains a quarter (25%) of the world's populatio ...

Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

West Asia West Asia (also called Western Asia or Southwest Asia) is the westernmost region of Asia. As defined by most academics, UN bodies and other institutions, the subregion consists of Anatolia, the Arabian Peninsula, Iran, Mesopotamia, the Armenian ...

, and the

Americas The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

. Studies into non-African populations indicates dark skin is not necessarily a retention of the pre-existing high UVR-adapted state of modern humans before the

out of Africa ''Out of Africa'' is a memoir by the Danish people, Danish author Karen Blixen. The book, first published in 1937, recounts events of the eighteen years when Blixen made her home in Kenya, then called East Africa Protectorate, British East Africa ...

migration, but may in fact be a later evolutionary adaptation to tropical rainforest regions. 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.

Evolution

Due to natural selection, people who lived in areas of intense sunlight developed dark skin colouration to protect against

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

(UV) light, mainly to protect their body from

depletion.

Evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary pigmentation of the skin was caused by ultraviolet radiation of the sun. As

s gradually lost their

fur A fur is a soft, thick growth of hair that covers the skin of almost all mammals. It consists of a combination of oily guard hair on top and thick underfur beneath. The guard hair keeps moisture from reaching the skin; the underfur acts as an ...

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 ''Homo'' () is a genus of great ape (family Hominidae) that emerged from the genus ''Australopithecus'' and encompasses only a single extant species, ''Homo sapiens'' (modern humans), along with a number of extinct species (collectively called ...

''. The effect of sunlight on folic acid levels has been crucial in the development of dark skin. Kiang West savanna

The earliest primate ancestors of modern humans most likely had pale skin, like our closest modern relative—the

chimpanzee The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of Hominidae, great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close rel ...

. About 7 million years ago

human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

and chimpanzee lineages diverged, and between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of

sub-Saharan Africa Sub-Saharan Africa is the area and regions of the continent of Africa that lie south of the Sahara. These include Central Africa, East Africa, Southern Africa, and West Africa. Geopolitically, in addition to the list of sovereign states and ...

. 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. 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. 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 Skin cancers are cancers that arise from the Human skin, skin. They are due to the development of abnormal cells (biology), cells that have the ability to invade or metastasis, spread to other parts of the body. It occurs when skin cells grow ...

. 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. A 1978 study examined the effect of sunlight on

—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 Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. It is a cephalic disorder that results from a neural tube defect that occurs when the rostral (head) end of the neural tube ...

and

spina bifida Spina bifida (SB; ; Latin for 'split spine') is a birth defect in which there is incomplete closing of the vertebral column, spine and the meninges, membranes around the spinal cord during embryonic development, early development in pregnancy. T ...

. UV rays can strip away folate, which is important to the development of healthy

foetus A fetus or foetus (; : fetuses, foetuses, rarely feti or foeti) is the unborn offspring of a viviparous animal that develops from an embryo. Following the embryonic stage, the fetal stage of development takes place. Prenatal development is a ...

es. In these abnormalities children are born with an incomplete brain or spinal cord.

Nina Jablonski Nina Grace Jablonski (born August 20, 1953)
at 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. As humans dispersed from equatorial

to low UVR areas and higher latitudes sometime between 120,000 and 65,000 years ago, dark skin posed a disadvantage. Populations with

light skin Light skin is a human skin color that has a low level of eumelanin pigmentation as an adaptation to environments of low Ultraviolet, UV radiation. Due to migrations of people in recent centuries, light-skinned populations today are found all ov ...

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

, and different forms of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

. A 2022 study revealed that traits such as dark skin show strong signals for

Convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

and selective pressure ( positive Selection).

Other hypotheses

The main other hypotheses that have been put forward through history to explain the evolution of dark skin colouration 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

. Darkly pigmented, eumelanin-rich skin protects against DNA damage caused by the sunlight. This is associated with lower skin cancer rates among dark-skinned populations. The presence of

pheomelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the Biological pigment, pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melan ...

in light skin increases the oxidative stress in

melanocyte Melanocytes are melanin-producing neural-crest, neural crest-derived cell (biology), cells located in the bottom layer (the stratum basale) of the skin's epidermis (skin), epidermis, the middle layer of the eye (the uvea), the inner ear, vagina ...

s, and this combined with the limited ability of pheomelanin to absorb UVR contributes to higher skin cancer rates among light-skinned individuals. The damaging effect of UVR on DNA structure and the entailing elevated skin cancer risk is widely recognized. 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. Of all the major skin cancer types, only malignant

melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...

have a major effect in a person's reproductive age. The mortality rates of melanoma have 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. Studies have found that even serious sunburns could not affect sweat gland function and

thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...

. There are no data or studies that support that sunburn can cause damage so seriously it can affect reproductive success. 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 Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely wikt:perturb, perturbed upon adsorption. Overview The fundamental interacting force of physisorption is Van der Waals ...

) 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. Humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments. The antimicrobial hypothesis also does not explain why some populations (like the

Inuit Inuit (singular: Inuk) are a group of culturally and historically similar Indigenous peoples traditionally inhabiting the Arctic and Subarctic regions of North America and Russia, including Greenland, Labrador, Quebec, Nunavut, the Northwe ...

Tibetans Tibetans () are an East Asian ethnic group native to Tibet. Their current population is estimated to be around 7.7 million. In addition to the majority living in the Tibet Autonomous Region of China, significant numbers of Tibetans live in t ...

) who live far from the tropics and are exposed to high UVR have darker skin pigmentation than their surrounding populations.

Biochemistry and genetics

Dark-skinned humans have high amounts of

found in their skin. Melanin is derivative of the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

Eumelanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

is the dominant form of melanin found in human skin. Eumelanin protects tissues and

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

from the radiation damage of UV light. Melanin is produced in specialized cells called

s, which are found at the lowest level of the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and Subcutaneous tissue, hypodermis. The epidermal layer provides a barrier to infection from environmental pathogens and regulates the ...

. Melanin is produced inside small membrane-bound packages called

. People with naturally-occurring dark skin have melanosomes which are clumped, large and full of eumelanin. A four-fold difference in naturally-occurring dark skin gives seven- to eight-fold protection against DNA damage, but even the darkest skin colour cannot protect against all damage to DNA. 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. 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. 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. 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 In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...

, such as

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscosity, viscous than Properties of water, water. It is used as an oxidizer, bleaching agent, and antiseptic, usua ...

, which are created by the interaction of UVR and layers of the skin. Heavily pigmented melanocytes have greater capacity to divide after ultraviolet irradiation, which suggests that they receive less damage to their DNA. Despite this, medium-wave ultraviolet radiation ( UVB) damages the immune system even in darker skinned individuals due to its effect on

Langerhans cell A Langerhans cell (LC) is a tissue-resident macrophage of the skin once thought to be a resident dendritic cell. These cells contain organelles called Birbeck granules. They are present in all layers of the epidermis and are most prominent in t ...

s. The

stratum corneum The stratum corneum (Latin language, Latin for 'horny layer') is the outermost layer of the epidermis (skin), epidermis. Consisting of dead tissue, it protects underlying tissue from infection, dehydration, chemicals and mechanical stress. It is ...

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. 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. Half of the solar radiation reaching the Earth's surface is in the form of

infrared light Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those o ...

and is absorbed similarly regardless of skin colouration. 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 Photoaging or photoageing (also known as "dermatoheliosis") is a term used for the characteristic changes to skin induced by chronic Ultraviolet, UVA and UVB exposure. Effects of UV light Molecular and genetic changes UVB rays are a primary ...

. Skin colour is a

polygenic A polygene is a member of a group of non- epistatic genes that interact additively to influence a phenotypic trait, thus contributing to multiple-gene inheritance (polygenic inheritance, multigenic inheritance, quantitative inheritance), a type ...

trait, which means that several different

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s are involved in determining a specific

phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...

. 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 In probability theory and statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real number, real-valued random variable. The general form of its probability density function is f(x ...

from light to dark, as it is usual for polygenic traits. 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 ''MC1R''f 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. 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 Tyrosinase-related protein 1, also known as TYRP1, is an intermembrane enzyme which in humans is encoded by the ''TYRP1'' gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is ...

'', '' OCA2'', '' SLC45A2'', '' SLC24A5'', ''MC1R'', '' KITLG'' and ''SLC24A4''). This does not take into account the effects of

epistasis Epistasis is a phenomenon in genetics in which the effect of a gene mutation is dependent on the presence or absence of mutations in one or more other genes, respectively termed modifier genes. In other words, the effect of the mutation is depe ...

, which would probably increase the number of related genes. Variations in the ''SLC24A5'' gene account for 20–25% of the variation between dark- and light-skinned populations of Africa, and appear to have arisen as recently as within the last 10,000 years. The Ala111Thr or rs1426654 polymorphism in the coding region of the SLC24A5 gene reaches fixation in

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, and is also common among populations in

North Africa North Africa (sometimes Northern Africa) is a region encompassing the northern portion of the African continent. There is no singularly accepted scope for the region. However, it is sometimes defined as stretching from the Atlantic shores of t ...

, the

Horn of Africa The Horn of Africa (HoA), also known as the Somali Peninsula, is a large peninsula and geopolitical region in East Africa.Robert Stock, ''Africa South of the Sahara, Second Edition: A Geographical Interpretation'', (The Guilford Press; 2004), ...

Central Asia Central Asia is a region of Asia consisting of Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, and Uzbekistan. The countries as a group are also colloquially referred to as the "-stans" as all have names ending with the Persian language, Pers ...

and

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. Modern humans are often ignorant of their evolutionary history at their peril. Cultural practices that increase problems of conditions among dark-skinned populations are traditional

clothing Clothing (also known as clothes, garments, dress, apparel, or attire) is any item worn on a human human body, body. Typically, clothing is made of fabrics or textiles, but over time it has included garments made from animal skin and other thin s ...

and

-poor diet.

Advantages in high sunlight

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. Folate is essential for fetal growth, organ development, and neural tube development. Folate breaks down in high intensity UVR. Dark-skinned women suffer the lowest level of neural tube defects. 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 Serotonin (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...

which regulates appetite, sleep, and mood. Serum folate is broken down by UV radiation or alcohol consumption. Because the skin is protected by the melanin, dark-pigmented people have a lower chance of developing

and conditions related to

folate deficiency Folate deficiency, also known as vitamin B9 deficiency, is a low level of folate and derivatives in the body. This may result in megaloblastic anemia in which red blood cells become abnormally large, and folate deficiency anemia is the term given ...

, such as

neural tube defects Neural tube defects (NTDs) are a group of birth defects in which an opening in the spine or cranium remains from early in human development. In the third week of pregnancy called gastrulation, specialized cells on the dorsal side of the embry ...

Disadvantages in low sunlight

Dark-skinned people living in low sunlight environments have been recorded to be very susceptible to

vitamin D deficiency Vitamin D deficiency or hypovitaminosis D is a vitamin D level that is below normal. It most commonly occurs in people when they have inadequate exposure to sunlight, particularly sunlight with adequate ultraviolet B rays (UVB). Vitamin D def ...

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. 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. Vitamin D plays an important role in regulating the human immune system. Chronic deficiencies in vitamin D can make humans susceptible to specific types of cancers and many kinds of infectious diseases. Vitamin D deficiency increases the risk of developing tuberculosis five-fold and also contributes to the development of breast, prostate, and colourectal cancer. The most prevalent disease to follow vitamin D deficiency is

, 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. Chronic deficiencies in vitamin D may also be linked with

breast The breasts are two prominences located on the upper ventral region of the torso among humans and other primates. Both sexes develop breasts from the same embryology, embryological tissues. The relative size and development of the breasts is ...

prostate The prostate is an male accessory gland, accessory gland of the male reproductive system and a muscle-driven mechanical switch between urination and ejaculation. It is found in all male mammals. It differs between species anatomically, chemica ...

, colon, ovarian, and possibly other types of

s. The relationship between

cardiovascular In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart a ...

disease and vitamin D deficiency also suggest a link between health of cardiac and smooth muscle. Low vitamin D levels have also been linked to impaired

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

and brain functions. In addition, recent studies have linked vitamin D deficiency to

autoimmune disease An autoimmune disease is a condition that results from an anomalous response of the adaptive immune system, wherein it mistakenly targets and attacks healthy, functioning parts of the body as if they were foreign organisms. It is estimated tha ...

hypertension Hypertension, also known as high blood pressure, is a Chronic condition, long-term Disease, medical condition in which the blood pressure in the artery, arteries is persistently elevated. High blood pressure usually does not cause symptoms i ...

multiple sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

diabetes Diabetes mellitus, commonly known as diabetes, is a group of common endocrine diseases characterized by sustained high blood sugar levels. Diabetes is due to either the pancreas not producing enough of the hormone insulin, or the cells of th ...

and incidence of

memory loss Amnesia is a deficit in memory caused by brain damage or brain diseases,Gazzaniga, M., Ivry, R., & Mangun, G. (2009) Cognitive Neuroscience: The biology of the mind. New York: W.W. Norton & Company. but it can also be temporarily caused by t ...

. Outside the tropics UVR has to penetrate through a thicker layer of

atmosphere An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, 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, 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.

Geographic distribution

There is a correlation between the geographic distribution of

UV radiation Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of t ...

(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 The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

. 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. 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. Dark-skinned populations inhabiting

and

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

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.

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. Before the modern mass migration, it has been argued that the majority of dark-pigmented people lived within 20° of the equator. Ati woman

Natives of Buka and Bougainville at the northern

Solomon Islands Solomon Islands, also known simply as the Solomons,John Prados, ''Islands of Destiny'', Dutton Caliber, 2012, p,20 and passim is an island country consisting of six major islands and over 1000 smaller islands in Melanesia, part of Oceania, t ...

in Melanesia and the

Chopi people The Chopi are a Bantu ethnic group of Mozambique. They have lived primarily in the Zavala region of southern Mozambique, in the Inhambane Province. They traditionally lived a life of subsistence agriculture, traditionally living a rural existen ...

Mozambique Mozambique, officially the Republic of Mozambique, is a country located in Southeast Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west, and Eswatini and South Afr ...

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. 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. 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 colour. According to Nina Jablonski, darkly-pigmented modern populations in South India and

Sri Lanka Sri Lanka, officially the Democratic Socialist Republic of Sri Lanka, also known historically as Ceylon, is an island country in South Asia. It lies in the Indian Ocean, southwest of the Bay of Bengal, separated from the Indian subcontinent, ...

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 colouration 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.

Australia

Indigenous Australians Indigenous Australians are people with familial heritage from, or recognised membership of, the various ethnic groups living within the territory of contemporary Australia prior to History of Australia (1788–1850), British colonisation. The ...

, as with all other populations outside of Africa, are descendants of the Out-of-Africa wave. Genetic evidence has pointed out that the Indigenous peoples of Australia are genetically dissimilar to the dark-skinned populations of Africa and that they are more closely related to other non-African populations. The term black initially has been applied as a reference to the skin pigmentation of Indigenous Australians; today it has been embraced by Aboriginal activists as a term for shared

culture Culture ( ) is a concept that encompasses the social behavior, institutions, and Social norm, norms found in human societies, as well as the knowledge, beliefs, arts, laws, Social norm, customs, capabilities, Attitude (psychology), attitudes ...

and identity, regardless of skin colour.

Melanesia

, a subregion of

Oceania Oceania ( , ) is a region, geographical region including Australasia, Melanesia, Micronesia, and Polynesia. Outside of the English-speaking world, Oceania is generally considered a continent, while Mainland Australia is regarded as its co ...

, whose name means "black islands", have several islands that are inhabited by

people The term "the people" refers to the public or Common people, common mass of people of a polity. As such it is a concept of human rights law, international law as well as constitutional law, particularly used for claims of popular sovereignty. I ...

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. 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. In the world,

blond Blond () or blonde (), also referred to as fair hair, is a human hair color characterized by low levels of eumelanin, the dark pigment. The resultant visible hue depends on various factors, but always has some yellowish color. The color can be ...

hair is exceptionally rare outside Europe, North Africa and West Asia, especially among dark-skinned populations. However, Melanesians are one of the dark-skinned human populations known to have naturally-occurring blond hair.

New Guinea

Buka boys performing at a Buin folk festival

The indigenous

Papuan people Papuans may refer to: * Indonesian Papuans – the Native Indonesians of Papua-origin * Papua New Guineans – the nationals of Papua New Guinea * Indigenous people of New Guinea {{Disambiguation Language and nationality disambiguation page ...

New Guinea New Guinea (; Hiri Motu: ''Niu Gini''; , fossilized , also known as Papua or historically ) is the List of islands by area, world's second-largest island, with an area of . Located in Melanesia in the southwestern Pacific Ocean, the island is ...

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

, 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 __NOTOC__ Sahul (), also called Sahul-land, Meganesia, Papualand and Greater Australia, was a paleocontinent that encompassed the modern-day landmasses of mainland Australia, Tasmania, New Guinea, and the Aru Islands. Sahul was in the south- ...

with distinct ancestries.

Sub-Saharan Africa

Sub-Saharan Africa Sub-Saharan Africa is the area and regions of the continent of Africa that lie south of the Sahara. These include Central Africa, East Africa, Southern Africa, and West Africa. Geopolitically, in addition to the list of sovereign states and ...

is the region in Africa situated south of the Sahara where a large number of dark-skinned populations live. Dark-skinned groups on the continent have the same receptor protein as ''

Homo ergaster ''Homo ergaster'' is an extinct species or subspecies of archaic humans who lived in Africa in the Early Pleistocene. Whether ''H. ergaster'' constitutes a species of its own or should be subsumed into '' H. erectus'' is an ongoing and unresol ...

'' and ''Homo erectus'' had. According to scientific studies, populations in Africa also have the highest skin colour diversity. 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 colour at any point of time. Due to frequently differing ancestry among dark-skinned populations, the presence of dark skin in general is not a reliable

genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can ...

, including among groups in Africa. For example, Wilson et al. (2001) found that most of their

Ethiopia Ethiopia, officially the Federal Democratic Republic of Ethiopia, is a landlocked country located in the Horn of Africa region of East Africa. It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Ken ...

n samples showed closer genetic affinities with lighter-skinned

Armenians Armenians (, ) are an ethnic group indigenous to the Armenian highlands of West Asia.Robert Hewsen, Hewsen, Robert H. "The Geography of Armenia" in ''The Armenian People From Ancient to Modern Times Volume I: The Dynastic Periods: From Antiq ...

than with darker-skinned Bantu populations. Mohamoud (2006) likewise observed that their Somali samples were genetically more similar to

Arab Arabs (, , ; , , ) are an ethnic group mainly inhabiting the Arab world in West Asia and North Africa. A significant Arab diaspora is present in various parts of the world. Arabs have been in the Fertile Crescent for thousands of years ...

populations than to other African populations.

South Asia

South Asia has some of the greatest skin colour diversity outside of Africa. Skin colour 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. 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 The Indo-Gangetic Plain, also known as the Northern Plain or North Indian River Plain, is a fertile plain spanning across the northern and north-eastern part of the Indian subcontinent. It encompasses northern and eastern India, eastern Pakist ...

found an association between the proportion of dark skin and ranking in the caste hierarchy.

Dalit Dalit ( from meaning "broken/scattered") is a term used for untouchables and outcasts, who represented the lowest stratum of the castes in the Indian subcontinent. They are also called Harijans. Dalits were excluded from the fourfold var ...

s had, on average, the darkest skin. A pan-India study of Telugu and North Indian castes found a similar correlation between skin colour 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.

Americas

Relatively dark skin remains among the

and other Arctic populations. A combination of protein-heavy diets and summer snow reflection have been speculated as favouring the retention of pigmented skin. 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". Most North American indigenous populations rank similar to African and Oceanian populations in regards to the presence of the allele ''Ala111''.Reference SNP(refSNP) Cluster Report: rs1426654 **clinically associated** Archived 5 December 2017 at the Wayback Machine.. Ncbi.nlm.nih.gov (30 December 2008). Retrieved 27 February 2011. Native South Americans and

Mesoamerican Mesoamerica is a historical region and cultural area that begins in the southern part of North America and extends to the Pacific coast of Central America, thus comprising the lands of central and southern Mexico, all of Belize, Guatemala, El S ...

s are also typically considered dark-skinned. High ultraviolet radiation levels occur throughout the Andes region of Peru, Bolivia, Chile, and Argentina. File:Indigenous mother and daughter from Peru.png, An Indigenous mother and daughter from the Urus islands. File:Amerindian man from Peru.png, An indigenous man from

Bolivia Bolivia, officially the Plurinational State of Bolivia, is a landlocked country located in central South America. The country features diverse geography, including vast Amazonian plains, tropical lowlands, mountains, the Gran Chaco Province, w ...

. File:Amerindian man in his reed boat, on lake titicaca.png, An indigenous fisherman from

Lake Titicaca Lake Titicaca (; ; ) is a large freshwater lake in the Andes mountains on the border of Bolivia and Peru. It is often called the highest navigable lake in the world. Titicaca is the largest lake in South America, both in terms of the volume of ...

Culture

The preference or disfavour for darker skin has varied depending on geographical area and time.