Human Skin Colour
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Human skin color ranges from the darkest brown to the lightest hues. Differences in skin color among individuals is caused by variation in
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
ation, which is the result of
genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar wor ...
(inherited from one's
biological parent A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male ...
s and or individual gene alleles), exposure to the sun, natural and sexual selection, or all of these. Differences across
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
s
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation t ...
through
natural Nature, in the broadest sense, is the physical world or universe. "Nature" can refer to the phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. Although humans are p ...
or
sexual selection Sexual selection is a mode of natural selection in which members of one biological sex mate choice, choose mates of the other sex to mating, mate with (intersexual selection), and compete with members of the same sex for access to members of t ...
, because of social norms and differences in environment, as well as regulations of the biochemical effects of
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
penetrating the skin. The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
. Melanin is produced within the skin in cells called
melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
s and it is the main determinant of the skin color of darker-skin humans. The skin color of people with light skin is determined mainly by the bluish-white connective tissue under the
dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
and by the
hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
circulating in the veins of the dermis. The red color underlying the skin becomes more visible, especially in the face, when, as consequence of
physical exercise Exercise is a body activity that enhances or maintains physical fitness and overall health and wellness. It is performed for various reasons, to aid growth and improve strength, develop muscles and the cardiovascular system, hone athletic ...
or
sexual arousal Sexual arousal (also known as sexual excitement) describes the physiological and psychological responses in preparation for sexual intercourse or when exposed to sexual stimuli. A number of physiological responses occur in the body and mind as ...
, or the stimulation of the
nervous system In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes th ...
(
anger Anger, also known as wrath or rage, is an intense emotional state involving a strong uncomfortable and non-cooperative response to a perceived provocation, hurt or threat. A person experiencing anger will often experience physical effects, su ...
,
embarrassment Embarrassment or awkwardness is an emotional state that is associated with mild to severe levels of discomfort, and which is usually experienced when someone commits (or thinks of) a socially unacceptable or frowned-upon act that is witnessed ...
),
arteriole An arteriole is a small-diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries. Arterioles have muscular walls (usually only one to two layers of smooth muscle cells) and are the primar ...
s dilate. Color is not entirely uniform across an individual's skin; for example, the skin of the palm and the sole is lighter than most other skin, and this is especially noticeable in darker-skinned people. There is a direct correlation between the geographic distribution of ultraviolet radiation (UVR) and the distribution of indigenous skin pigmentation around the world. Areas that receive higher amounts of UVR, generally located closer to the equator, tend to have darker-skinned populations. Areas that are far from the tropics and closer to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations. Some researchers suggest that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa as they migrated to different UV zones, and that such major changes in pigmentation may have happened in as little as 100 generations (≈2,500 years) through
selective sweep In genetics, a selective sweep is the process through which a new beneficial mutation that increases its frequency and becomes fixed (i.e., reaches a frequency of 1) in the population leads to the reduction or elimination of genetic variation among ...
s. Natural skin color can also darken as a result of
tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of artificial light in place of the sun **Sunless tanning, application of a stain or dye t ...
due to exposure to
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when t ...
. The leading theory is that skin color adapts to intense sunlight irradiation to provide partial protection against the ultraviolet fraction that produces damage and thus mutations in the DNA of the skin cells. In addition, it has been observed that
females Female (symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females a ...
on average are significantly lighter in skin pigmentation than
males Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...
. Females need more
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...
during
pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...
and
lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
. The body synthesizes
vitamin D Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group are vitamin D3 (c ...
from sunlight, which helps it absorb calcium. Females evolved to have lighter skin so their bodies absorb more calcium. The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to
social status Social status is the level of social value a person is considered to possess. More specifically, it refers to the relative level of respect, honour, assumed competence, and deference accorded to people, groups, and organizations in a society. Stat ...
and
discrimination Discrimination is the act of making unjustified distinctions between people based on the groups, classes, or other categories to which they belong or are perceived to belong. People may be discriminated on the basis of race, gender, age, relig ...
.


Melanin and genes

Melanin is produced by cells called
melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
s in a process called
melanogenesis Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
. Melanin is made within small membrane–bound packages called
melanosome 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 i ...
s. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''str ...
s. Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes. They also regulate melanin production and replication of melanocytes. People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin. The genetic mechanism behind human skin color is mainly regulated by the
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
tyrosinase Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper Mason pathway. Firstly, the hydroxylat ...
, which creates the color of the skin, eyes, and hair shades. Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin. Melanocytes produce two types of melanin. The most common form of
biological Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary in ...
melanin is
eumelanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...
, a brown-black
polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
of dihydroxyindole
carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
s, and their reduced forms. Most are derived from the amino acid
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 non-essential amino acid with a polar side group. The word "tyrosine" is from the Gr ...
. Eumelanin is found in hair,
areola The human areola (''areola mammae'', or ) is the pigmented area on the breast around the nipple. Areola, more generally, is a small circular area on the body with a different histology from the surrounding tissue, or other small circular ar ...
, and skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin.
Pheomelanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the a ...
, a pink to red hue is found in particularly large quantities in
red hair Red hair (also known as orange hair and ginger hair) is a hair color found in one to two percent of the human population, appearing with greater frequency (two to six percent) among people of Northern or Northwestern European ancestry and ...
, the lips, nipples, glans of the penis, and vagina. Both the amount and type of melanin produced is controlled by a number of
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
s that operate under
incomplete dominance In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and ...
. One copy of each of the various genes is inherited from each parent. Each gene can come in several
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s, resulting in the great variety of human skin tones. Melanin controls the amount of
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
(UV) radiation from the sun that penetrates the skin by absorption. While UV radiation can assist in the production of vitamin D, excessive exposure to UV can damage health.


Evolution of skin color

Loss of body hair in
Hominini The Hominini form a taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (humans) and '' Pan'' (chimpanzees and bonobos) and in standard usage excludes the genus ''Gorilla'' (gorillas). The t ...
species is assumed to be related to the emergence of
bipedalism Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...
some 5 to 7 million years ago. Bipedal hominin body hair may have disappeared gradually to allow better heat dissipation through sweating.. The emergence of skin pigmentation dates to about 1.2 million years ago, under conditions of a
megadrought A megadrought (or mega-drought) is a prolonged drought lasting two decades or longer. Past megadroughts have been associated with persistent multiyear La Niña conditions (cooler than normal water temperatures in the tropical eastern Pacific Oc ...
that drove early humans into arid, open landscapes. Such conditions likely caused excess UV-B radiation. This favored the emergence of skin pigmentation in order to protect from
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 ...
depletion due to the increased exposure to sunlight. A theory that the pigmentation helped counter xeric stress by increasing the epidermal permeability barrier has been disproved. With the evolution of hairless skin, abundant sweat glands, and skin rich in melanin, early humans could walk, run, and forage for food for long periods of time under the hot sun without brain damage due to overheating, giving them an evolutionary advantage over other species. By 1.2 million years ago, around the time of ''
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 ''Homo erectus, H. erectus'' is an ongoin ...
'',
archaic humans A number of varieties of ''Homo'' are grouped into the broad category of archaic humans in the period that precedes and is contemporary to the emergence of the earliest early modern humans (''Homo sapiens'') around 300 ka. Omo-Kibish I (Omo I) f ...
(including the ancestors of ''
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
'') had exactly the same receptor protein as modern sub-Saharan Africans. This was the genotype inherited by
anatomically modern humans Early modern human (EMH) or anatomically modern human (AMH) are terms used to distinguish ''Homo sapiens'' (the only extant Hominina species) that are anatomically consistent with the range of phenotypes seen in contemporary humans from extin ...
, but retained only by part of the extant populations, thus forming an aspect of
human genetic variation Human genetic variation is the genetic differences in and among populations. There may be multiple variants of any given gene in the human population (alleles), a situation called polymorphism. No two humans are genetically identical. Even m ...
. About 100,000–70,000 years ago, some anatomically modern humans (''Homo sapiens'') began to migrate away from the tropics to the north where they were exposed to less intense sunlight. This was possibly in part due to the need for greater use of clothing to protect against the colder climate. Under these conditions there was less photodestruction of folate and so the evolutionary pressure working against the survival of lighter-skinned gene variants was reduced. In addition, lighter skin is able to generate more
vitamin D Vitamin D is a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, and many other biological effects. In humans, the most important compounds in this group are vitamin D3 (c ...
(cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. Hence the leading hypothesis for the evolution of human skin color proposes that: # From about 1.2 million years ago to less than 100,000 years ago,
archaic humans A number of varieties of ''Homo'' are grouped into the broad category of archaic humans in the period that precedes and is contemporary to the emergence of the earliest early modern humans (''Homo sapiens'') around 300 ka. Omo-Kibish I (Omo I) f ...
, including archaic ''Homo sapiens'', were dark-skinned. # As ''Homo sapiens'' populations began to migrate, the evolutionary constraint keeping skin dark decreased proportionally to the distance north a population migrated, resulting in a range of skin tones within northern populations. # At some point, some northern populations experienced positive selection for lighter skin due to the increased production of vitamin D from sunlight and the genes for darker skin disappeared from these populations. # Subsequent migrations into different UV environments and admixture between populations have resulted in the varied range of skin pigmentations we see today. The genetic mutations leading to light skin, though partially different among
East Asia East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea and ...
ns and
Western Europe Western Europe is the western region of Europe. The region's countries and territories vary depending on context. The concept of "the West" appeared in Europe in juxtaposition to "the East" and originally applied to the ancient Mediterranean ...
ans, suggest the two groups experienced a similar selective pressure after settlement in northern latitudes. The theory is partially supported by a study into the
SLC24A5 SLC may refer to: Places * Salt Lake City, Utah * Salt Lake City International Airport, IATA Airport Code Education * Sarah Lawrence College, NY * School Leaving Certificate (Nepal) * St. Lawrence College, Ontario, Canada * Small Learning Comm ...
gene which found that the allele associated with light skin in Europe "determined that 18,000 years had passed since the light-skin allele was fixed in Europeans" but may have originated as recently as 12,000–6,000 years ago "given the imprecision of method" , which is in line with the earliest evidence of farming. Research by
Nina Jablonski Nina G. Jablonski (born ) is an American anthropologist and palaeobiologist, known for her research into the evolution of skin color in humans. She is engaged in public education about human evolution, human diversity, and racism. In 2021, she ...
suggests that an estimated time of about 10,000 to 20,000 years is enough for human populations to achieve optimal skin pigmentation in a particular geographic area but that development of ideal skin coloration may happen faster if the evolutionary pressure is stronger, even in as little as 100 generations. The length of time is also affected by cultural practices such as food intake, clothing, body coverings, and shelter usage which can alter the ways in which the environment affects populations. One of the most recently proposed drivers of the evolution of skin pigmentation in humans is based on research that shows a superior barrier function in darkly pigmented skin. Most protective functions of the skin, including the permeability barrier and the antimicrobial barrier, reside in the
stratum corneum The stratum corneum (Latin for 'horny layer') is the outermost layer of the epidermis. The human stratum corneum comprises several levels of flattened corneocytes that are divided into two layers: the ''stratum disjunctum'' and ''stratum compact ...
(SC) and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair.
Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
, because it improves barrier function as compared to non-pigmented skin. In lush rainforests, however, where UV-B radiation and xeric stress were not in excess, light pigmentation would not have been nearly as detrimental. This explains the side-by-side residence of lightly pigmented and darkly pigmented peoples. Population and admixture studies suggest a three-way model for the evolution of human skin color, with dark skin evolving in early
hominids 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 east ...
in
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
and light skin evolving partly separately at least two times after modern humans had expanded
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 seventeen years when Blixen made her home in Kenya, then called East Africa Protectorate, British East Afr ...
. For the most part, the evolution of light skin has followed different genetic paths in Western and Eastern Eurasian populations. Two genes however, KITLG and ASIP, have mutations associated with lighter skin that have high frequencies in Eurasian populations and have estimated origin dates after humans spread out of Africa but before the divergence of the two lineages.


Genetics

The understanding of the genetic mechanisms underlying human skin color variation is still incomplete; however, genetic studies have discovered a number of genes that affect human skin color in specific populations, and have shown that this happens independently of other physical features such as eye and hair color. Different populations have different allele frequencies of these genes, and it is the combination of these allele variations that bring about the complex, continuous variation in skin coloration we can observe today in modern humans. Population and admixture studies suggest a 3-way model for the evolution of human skin color, with dark skin evolving in early
hominids 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 east ...
in
sub-Saharan Africa Sub-Saharan Africa is, geographically, the area and regions of the continent of Africa that lies south of the Sahara. These include West Africa, East Africa, Central Africa, and Southern Africa. Geopolitically, in addition to the List of sov ...
and light skin evolving independently in
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
and
East Asia East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea and ...
after modern humans had expanded
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 seventeen years when Blixen made her home in Kenya, then called East Africa Protectorate, British East Afr ...
. For skin color, the broad sense heritability (defined as the overall effect of genetic vs. nongenetic factors) is very high, provided one is able to control for the most important nongenetic factor, exposure to sunlight. Many aspects of the evolution of human skin and skin color can be reconstructed using comparative anatomy, physiology, and genomics. Enhancement of thermal sweating was a key innovation in human evolution that allowed maintenance of homeostasis (including constant brain temperature) during sustained physical activity in hot environments. Dark skin evolved ''
pari passu ''Pari passu'' is a Latin phrase that literally means "with an equal step" or "on equal footing". It is sometimes translated as "ranking equally", "hand-in-hand", "with equal force", or "moving together", and by extension, "fairly", "without pa ...
'' with the loss of body hair and was the original state for the genus Homo. Melanin pigmentation is adaptive and has been maintained by natural selection. In recent prehistory, humans became adept at protecting themselves from the environment through clothing and shelter, thus reducing the scope for the action of natural selection on human skin. Credit for describing the relationship between latitude and skin color in modern humans is usually ascribed to an Italian geographer, Renato Basutti, whose widely reproduced "skin color maps" illustrate the correlation of darker skin with equatorial proximity. More recent studies by physical anthropologists have substantiated and extended these observations; a recent review and analysis of data from more than 100 populations (Relethford 1997) found that skin reflectance is lowest at the equator, then gradually increases, about 8% per 10° of latitude in the Northern Hemisphere and about 4% per 10° of latitude in the Southern Hemisphere. This pattern is inversely correlated with levels of UV irradiation, which are greater in the Southern than in the Northern Hemisphere. An important caveat is that we do not know how patterns of UV irradiation have changed over time; more importantly, we do not know when skin color is likely to have evolved, with multiple migrations out of Africa and extensive genetic interchange over the last 500,000 years (Templeton 2002).Regardless, most anthropologists accept the notion that differences in UV irradiation have driven selection for dark human skin at the equator and for light human skin at greater latitudes. What remains controversial are the exact mechanisms of selection. The most popular theory posits that protection offered by dark skin from UV irradiation becomes a liability in more polar latitudes due to vitamin D deficiency (Murray 1934). UVB (short-wavelength UV) converts 7-dehydrocholesterol into an essential precursor of cholecaliferol (vitamin D3); when not otherwise provided by dietary supplements, deficiency for vitamin D causes rickets, a characteristic pattern of growth abnormalities and bony deformities. An oft-cited anecdote in support of the vitamin D hypothesis is that Arctic populations whose skin is relatively dark given their latitude, such as the Inuit and the Lapp, have had a diet that is historically rich in vitamin D. Sensitivity of modern humans to vitamin D deficiency is evident from the widespread occurrence of rickets in 19th-century industrial Europe, but whether dark-skinned humans migrating to polar latitudes tens or hundreds of thousands of years ago experienced similar problems is open to question. In any case, a risk for vitamin D deficiency can only explain selection for light skin. Among several mechanisms suggested to provide a selective advantage for dark skin in conditions of high UV irradiation (Loomis 1967; Robins 1991; Jablonski and Chaplin 2000), the most tenable are protection from sunburn and skin cancer due to the physical barrier imposed by epidermal melanin.


Dark skin

All modern humans share a
common ancestor Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. All living beings are in fact descendants of a unique ancestor commonly referred to as the last universal comm ...
who lived around 200,000 years ago in Africa. Comparisons between known skin pigmentation genes in
chimpanzees The chimpanzee (''Pan troglodytes''), also known as simply the chimp, is a species of great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. When its close relative the ...
and modern Africans show that dark skin evolved along with the loss of body hair about 1.2 million years ago and that this common ancestor had dark skin. Investigations into dark-skinned populations in
South Asia South Asia is the southern subregion of Asia, which is defined in both geographical and ethno-cultural terms. The region consists of the countries of Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka.;;;;;;;; ...
and
Melanesia Melanesia (, ) is a subregion of Oceania in the southwestern Pacific Ocean. It extends from Indonesia's New Guinea in the west to Fiji in the east, and includes the Arafura Sea. The region includes the four independent countries of Fiji, Va ...
indicate that skin pigmentation in these populations is due to the preservation of this ancestral state and not due to new variations on a previously lightened population. ; MC1R :The
melanocortin 1 receptor The melanocortin 1 receptor (MC1R), also known as melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor, is a G protein–coupled receptor that binds to a class of pituitary peptide hormones ...
(MC1R) gene is primarily responsible for determining whether pheomelanin and eumelanin are produced in the human body. Research shows at least 10 differences in MC1R between African and chimpanzee samples and that the gene has probably undergone a strong
positive selection In population genetics, directional selection, is a mode of negative natural selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. Under dir ...
(a
selective sweep In genetics, a selective sweep is the process through which a new beneficial mutation that increases its frequency and becomes fixed (i.e., reaches a frequency of 1) in the population leads to the reduction or elimination of genetic variation among ...
) in early Hominins around 1.2 million years ago. This is consistent with positive selection for the high-eumelanin
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
seen in Africa and other environments with high UV exposure.


Light skin

For the most part, the evolution of light skin has followed different genetic paths in European and East Asian populations. Two genes, however, KITLG and ASIP, have mutations associated with lighter skin that have high frequencies in both European and East Asian populations. They are thought to have originated after humans spread out of Africa but before the divergence of the European and Asian lineages around 30,000 years ago. Two subsequent
genome-wide association studies In genomics, a genome-wide association study (GWA study, or GWAS), also known as whole genome association study (WGA study, or WGAS), is an observational study of a genome-wide set of genetic variants in different individuals to see if any varian ...
found no significant correlation between these genes and skin color, and suggest that the earlier findings may have been the result of incorrect correction methods and small panel sizes, or that the genes have an effect too small to be detected by the larger studies. ; KITLG :The
KIT ligand Stem cell factor (also known as SCF, KIT-ligand, KL, or steel factor) is a cytokine that binds to the c-KIT receptor ( CD117). SCF can exist both as a transmembrane protein and a soluble protein. This cytokine plays an important role in hematopoi ...
(KITLG) gene is involved in the permanent survival, proliferation and migration of melanocytes. A mutation in this gene, ''A326G'' (rs642742), has been positively associated with variations of skin color in African-Americans of mixed West African and European descent and is estimated to account for 15–20% of the melanin difference between African and European populations. This allele shows signs of strong positive selection outside Africa and occurs in over 80% of European and Asian samples, compared with less than 10% in African samples. ; ASIP :
Agouti signalling peptide Agouti-signaling protein is a protein that in humans is encoded by the ASIP gene. It is responsible for the distribution of melanin pigment in mammals. Agouti interacts with the melanocortin 1 receptor to determine whether the melanocyte (pigment ...
(ASIP) acts as an
inverse agonist In pharmacology, an inverse agonist is a drug that binds to the same receptor as an agonist but induces a pharmacological response opposite to that of the agonist. A neutral antagonist has no activity in the absence of an agonist or inverse agon ...
, binding in place of
alpha-MSH α-Melanocyte-stimulating hormone (α-MSH) is an endogenous peptide hormone and neuropeptide of the melanocortin family, with a tridecapeptide structure and the amino acid sequence Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. It ...
and thus inhibiting eumelanin production. Studies have found two alleles in the vicinity of ASIP are associated with skin color variation in humans. One, rs2424984, has been identified as an indicator of skin reflectance in a forensics analysis of human phenotypes across Caucasian, African-American, South Asian, East Asian, Hispanic and Native American populations and is about three times more common in non-African populations than in Africa. The other allele, ''8188G'' (rs6058017) is significantly associated with skin color variation in African-Americans and the ancestral version occurs in only 12% of European and 28% of East Asian samples compared with 80% of West African samples.


Europe

A number of genes have been positively associated with the skin pigmentation difference between European and non-European populations. Mutations in SLC24A5 and SLC45A2 are believed to account for the bulk of this variation and show very strong signs of selection. A variation in TYR has also been identified as a contributor. Research indicates the selection for the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20,000 years ago and perhaps as recently as 12,000 to 6,000 years ago. In the 1970s, Luca Cavalli-Sforza suggested that the selective sweep that rendered light skin ubiquitous in Europe might be correlated with the advent of farming and thus have taken place only around 6,000 years ago; "a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza … that the early immigrants to Europe … survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6,000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin." This scenario found support in a 2014 analysis of mesolithic (7,000 years old)
hunter-gatherer A traditional hunter-gatherer or forager is a human living an ancestrally derived lifestyle in which most or all food is obtained by foraging, that is, by gathering food from local sources, especially edible wild plants but also insects, fungi, ...
DNA from
La Braña LA most frequently refers to Los Angeles, the second largest city in the United States. La, LA, or L.A. may also refer to: Arts and entertainment Music * La (musical note), or A, the sixth note * "L.A.", a song by Elliott Smith on ''Figure ...
, Spain, which showed a version of these genes not corresponding with light skin color. In 2015 researchers analysed for light skin genes in the DNA of 94 ancient skeletons ranging from 8,000 to 3,000 years old from Europe and Russia. They found c. 8,000-year-old hunter-gatherers in Spain, Luxembourg, and Hungary were dark skinned while similarly aged hunter gatherers in Sweden were light skinned (having predominately derived alleles of SLC24A5, SLC45A2 and also HERC2/OCA2). Neolithic farmers entering Europe at around the same time were intermediate, being nearly fixed for the derived SLC24A5 variant but only having the derived SLC45A2 allele in low frequencies. The SLC24A5 variant spread very rapidly throughout central and southern Europe from about 8,000 years ago, whereas the light skin variant of SLC45A2 spread throughout Europe after 5,800 years ago. ; SLC24A5 : Solute carrier family 24 member 5 (SLC24A5) regulates calcium in melanocytes and is important in the process of
melanogenesis Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
. The SLC24A5 gene's derived ''Ala111Thr'' allele (rs1426654) has been shown to be a major factor in light skin pigmentation and is common in Western
Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelago a ...
. Recent studies have found that the variant represents as much as 25–40% of the average skin tone difference between Europeans and West Africans. This derived allele is a reliable predictor of phenotype across a range of populations. It has been the subject of recent selection in Western Eurasia, and is fixed in European populations. ; SLC45A2 : Solute carrier family 45 member 2 (SLC45A2 or MATP) aids in the transport and processing of tyrosine, a precursor to melanin. It has also been shown to be one of the significant components of the skin color of modern Europeans through its ''Phe374Leu'' (
rs16891982 In genetics, rs16891982, also known as F374L, is the name for a single nucleotide polymorphism found in the SLC45A2 gene. The SNP consists of two alleles: C (cytosine) and G (guanine).It is associated with skin tone and hair/eye color. It is a typ ...
) allele that has been directly correlated with skin color variation across a range of populations. This variation is ubiquitous in European populations but extremely rare elsewhere and shows strong signs of selection. ; TYR :The TYR gene encodes the enzyme tyrosinase, which is involved in the production of melanin from tyrosine. It has an allele, ''Ser192Tyr'' (rs1042602), found solely in 40–50% of Europeans and linked to light-colored skin in studies of South Asian and African-American populations.


East Asia

A number of genes known to affect skin color have alleles that show signs of positive selection in East Asian populations. Of these, only OCA2 has been directly related to skin color measurements, while DCT, MC1R and ATRN are marked as
candidate gene The candidate gene approach to conducting genetic association studies focuses on associations between genetic variation within pre-specified genes of interest, and phenotypes or disease states. This is in contrast to genome-wide association studies ...
s for future study. ; OCA2 : Oculocutaneous albinism II (OCA2) assists in the regulation of pH in melanocytes. The OCA2 gene's derived ''His615Arg'' (rs1800414) allele has been shown to account for about 8% of the skin tone difference between African and East Asian populations in studies of an East Asian population living in Toronto and a Chinese
Han Han may refer to: Ethnic groups * Han Chinese, or Han People (): the name for the largest ethnic group in China, which also constitutes the world's largest ethnic group. ** Han Taiwanese (): the name for the ethnic group of the Taiwanese p ...
population. This variant is essentially restricted to East Asia, with highest frequencies in Eastern East Asia (49–63%), midrange frequencies in Southeast Asia, and the lowest frequencies in Western China and some Eastern European populations. ; Candidate genes :A number of studies have found genes linked to human skin pigmentation that have alleles with statistically significant frequencies in Chinese and East Asian populations. While not linked to measurements of skin tone variation directly,
dopachrome tautomerase Dopachrome tautomerase (dopachrome delta-isomerase, tyrosine-related protein 2), also known as DCT, is a human gene. Its expression is regulated by the microphthalmia-associated transcription factor (MITF). See also * Tyrosinase-related prote ...
(DCT or TYRP2 rs2031526),
melanocortin 1 receptor The melanocortin 1 receptor (MC1R), also known as melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor, is a G protein–coupled receptor that binds to a class of pituitary peptide hormones ...
(MC1R) ''Arg163Gln'' (rs885479) and attractin (ATRN) have been indicated as potential contributors to the evolution of light skin in East Asian populations.


Tanning response

Tanning response in humans is controlled by a variety of genes. ''MC1R'' variants ''Arg151Sys'' (rs1805007), ''Arg160Trp'' (rs1805008), ''Asp294Sys'' (rs1805009), ''Val60Leu'' (rs1805005) and ''Val92Met'' (rs2228479) have been associated with reduced tanning response in European and/or East Asian populations. These alleles show no signs of positive selection and only occur in relatively small numbers, reaching a peak in Europe with around 28% of the population having at least one allele of one of the variations. A study of self-reported tanning ability and skin type in American non-
Hispanic The term ''Hispanic'' ( es, hispano) refers to people, Spanish culture, cultures, or countries related to Spain, the Spanish language, or Hispanidad. The term commonly applies to countries with a cultural and historical link to Spain and to Vic ...
Caucasians found that ''SLC24A5 Phe374Leu'' is significantly associated with reduced tanning ability and also associated ''TYR Arg402Gln'' (rs1126809), ''OCA2 Arg305Trp'' (rs1800401) and a 2- SNP
haplotype A haplotype ( haploid genotype) is a group of alleles in an organism that are inherited together from a single parent. Many organisms contain genetic material ( DNA) which is inherited from two parents. Normally these organisms have their DNA or ...
in ''ASIP'' (rs4911414 and rs1015362) to skin type variation within a "fair/medium/olive" context.


Albinism

Oculocutaneous albinism Oculocutaneous albinism is a form of albinism involving the eyes ('' oculo-''), the skin (''-cutaneous''), and the hair. Overall, an estimated 1 in 20,000 people worldwide are born with oculocutaneous albinism. OCA is caused by mutations in seve ...
(OCA) is a lack of pigment in the eyes, skin and sometimes hair that occurs in a very small fraction of the population. The four known types of ''OCA'' are caused by mutations in the ''TYR'', ''OCA2'', ''
TYRP1 Tyrosinase-related protein 1, also known as TYRP1, is an intermembrane enzyme which in humans is encoded by the ''TYRP1'' gene. Function Tyrp1 is a melanocyte-specific gene product involved in melanin synthesis within melanosomes. Most Tyrp1 p ...
'', and ''SLC45A2'' genes.ALBINISM, OCULOCUTANEOUS, TYPE IA; OCA1A
, Johns Hopkins University


Age

In
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 east ...
s, the parts of the body not covered with hair, like the face and the back of the hands, start out pale in infants and turn darker as the skin is exposed to more sun. All human babies are born pale, regardless of what their adult color will be. In humans,
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
production does not peak until after puberty. The skin of children becomes darker as they go through
puberty Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testes in a boy. ...
and experience the effects of sex hormones. This darkening is especially noticeable in the skin of the
nipple The nipple is a raised region of tissue on the surface of the breast from which, in females, milk leaves the breast through the lactiferous ducts to feed an infant. The milk can flow through the nipple passively or it can be ejected by smooth mu ...
s, the
areola The human areola (''areola mammae'', or ) is the pigmented area on the breast around the nipple. Areola, more generally, is a small circular area on the body with a different histology from the surrounding tissue, or other small circular ar ...
of the nipples, the
labia majora The labia majora (singular: ''labium majus'') are two prominent longitudinal cutaneous folds that extend downward and backward from the mons pubis to the perineum. Together with the labia minora they form the labia of the vulva. The labia majo ...
in females, and the
scrotum The scrotum or scrotal sac is an anatomical male reproductive structure located at the base of the penis that consists of a suspended dual-chambered sac of skin and smooth muscle. It is present in most terrestrial male mammals. The scrotum cont ...
in males. In some people, the
armpits The axilla (also, armpit, underarm or oxter) is the area on the human body directly under the shoulder joint. It includes the axillary space, an anatomical space within the shoulder girdle between the arm and the thoracic cage, bounded superio ...
become slightly darker during puberty. The interaction of genetic, hormonal, and environmental factors on skin coloration with age is still not adequately understood, but it is known that men are at their darkest baseline skin color around the age of 30, without considering the effects of tanning. Around the same age, women experience darkening of some areas of their skin. Human skin color fades with age. Humans over the age of thirty experience a decrease in melanin-producing cells by about 10% to 20% per decade as melanocyte stem cells gradually die. The skin of face and hands has about twice the amount of pigment cells as unexposed areas of the body, as chronic exposure to the sun continues to stimulate melanocytes. The blotchy appearance of skin color in the face and hands of older people is due to the uneven distribution of pigment cells and to changes in the interaction between
melanocyte Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
s and
keratinocyte Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells. Basal cells in the stratum basale, basal layer (''str ...
s.


Sexual dimorphism

It has been observed that
females Female (symbol: ♀) is the sex of an organism that produces the large non-motile ova (egg cells), the type of gamete (sex cell) that fuses with the male gamete during sexual reproduction. A female has larger gametes than a male. Females a ...
are found to have lighter skin pigmentation than
males Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot reproduce sexually without access to ...
in some studied populations. This may be a form of
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
due to the requirement in women for high amounts of calcium during
pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ca ...
and
lactation Lactation describes the secretion of milk from the mammary glands and the period of time that a mother lactates to feed her young. The process naturally occurs with all sexually mature female mammals, although it may predate mammals. The proces ...
. Breastfeeding newborns, whose skeletons are growing, require high amounts of calcium intake from the mother's milk (about 4 times more than during prenatal development), part of which comes from reserves in the mother's skeleton. Adequate vitamin D resources are needed to absorb calcium from the diet, and it has been shown that deficiencies of vitamin D and calcium increase the likelihood of various birth defects such as
spina bifida Spina bifida (Latin for 'split spine'; SB) is a birth defect in which there is incomplete closing of the spine and the membranes around the spinal cord during early development in pregnancy. There are three main types: spina bifida occulta, me ...
and
rickets Rickets is a condition that results in weak or soft bones in children, and is caused by either dietary deficiency or genetic causes. Symptoms include bowed legs, stunted growth, bone pain, large forehead, and trouble sleeping. Complications may ...
. Natural selection may have led to females with lighter skin than males in some indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infants and to maintain their own health. However, in some populations such as in Italy, Poland, Ireland, Spain and Portugal men are found to have fairer complexions, and this has been ascribed as a cause to increased melanoma risk in men. Similarly, studies done in the late 19th Century/early 20th Century in Europe also conflicted with the notion at least in regards to Northern Europeans. The studies found that in
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
women tend to have darker hair, eyes, and skin complexation than men, and in particular women darken in relation to men during puberty. A study in Germany during this period showed that German men were more likely to have lighter skin, blond hair, and lighter eyes, while German women had darker hair, eyes and skin tone on average. The sexes also differ in how they change their skin color with age. Men and women are not born with different skin color, they begin to diverge during puberty with the influence of sex hormones. Women can also change pigmentation in certain parts of their body, such as the areola, during the menstrual cycle and pregnancy and between 50 and 70% of pregnant women will develop the "
mask of pregnancy A mask is an object normally worn on the face, typically for protection, disguise, performance, or entertainment and often they have been employed for rituals and rights. Masks have been used since antiquity for both ceremonial and practica ...
" (melasma or chloasma) in the cheeks, upper lips, forehead, and chin. This is caused by increases in the female hormones estrogen and progesterone and it can develop in women who take birth control pills or participate in hormone replacement therapy.


Disorders of pigmentation

Uneven pigmentation of some sort affects most people, regardless of bioethnic background or skin color. Skin may either appear lighter, or darker than normal, or lack pigmentation at all; there may be blotchy, uneven areas, patches of brown to gray discoloration or freckling. Apart from blood-related conditions such as
jaundice Jaundice, also known as icterus, is a yellowish or greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving abnormal heme meta ...
,
carotenosis Carotenosis is a benign and reversible medical condition where an excess of dietary carotenoids results in orange discoloration of the outermost skin layer. The discoloration is most easily observed in light-skinned people and may be mistaken for ...
, or
argyria Argyria or argyrosis is a condition caused by excessive exposure to chemical compounds of the element silver, or to silver dust. The most dramatic symptom of argyria is that the skin turns blue or blue-grey. It may take the form of ''generalize ...
, skin pigmentation disorders generally occur because the body produces either too much or too little melanin.


Depigmentation


Albinism

Some types of
albinism Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino. Varied use and interpretation of the term ...
affect only the skin and hair, while other types affect the skin, hair and eyes, and in rare cases only the eyes. All of them are caused by different genetic mutations. Albinism is a recessively inherited trait in humans where both pigmented parents may be carriers of the gene and pass it down to their children. Each child has a 25% chance of being albino and a 75% chance of having normally pigmented skin. One common type of albinism is
oculocutaneous albinism Oculocutaneous albinism is a form of albinism involving the eyes ('' oculo-''), the skin (''-cutaneous''), and the hair. Overall, an estimated 1 in 20,000 people worldwide are born with oculocutaneous albinism. OCA is caused by mutations in seve ...
or OCA, which has many subtypes caused by different genetic mutations. Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity,
skin cancer Skin cancers are cancers that arise from the skin. They are due to the development of abnormal cells that have the ability to invade or spread to other parts of the body. There are three main types of skin cancers: basal-cell skin cancer (BCC) ...
, and eye damage. Albinism is more common in some parts of the world than in others, but it is estimated that 1 in 70 humans carry the gene for OCA. The most severe type of albinism is OCA1A, which is characterized by complete, lifelong loss of melanin production, other forms of OCA1B, OCA2, OCA3, OCA4, show some form of melanin accumulation and are less severe. The four known types of ''OCA'' are caused by mutations in the ''TYR'', ''OCA2'', ''
TYRP1 Tyrosinase-related protein 1, also known as TYRP1, is an intermembrane enzyme which in humans is encoded by the ''TYRP1'' gene. Function Tyrp1 is a melanocyte-specific gene product involved in melanin synthesis within melanosomes. Most Tyrp1 p ...
'', and ''SLC45A2'' genes. Albinos often face social and cultural challenges (even threats), as the condition is often a source of ridicule, racism, fear, and violence. Many cultures around the world have developed beliefs regarding people with albinism. Albinos are persecuted in
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
by
witchdoctor A witch doctor (also spelled witch-doctor) was originally a type of healer who treated ailments believed to be caused by witchcraft. The term is now more commonly used to refer to healers, particularly in regions which use traditional healing ...
s, who use the body parts of albinos as ingredients in rituals and potions, as they are thought to possess magical power.


Vitiligo

Vitiligo Vitiligo is a disorder that causes the skin to lose its color. Specific causes are unknown but studies suggest a link to immune system changes. Signs and symptoms The only sign of vitiligo is the presence of pale patchy areas of depigmen ...
is a condition that causes depigmentation of sections of skin. It occurs when melanocytes die or are unable to function. The cause of vitiligo is unknown, but research suggests that it may arise from
autoimmune In immunology, autoimmunity is the system of immune responses of an organism against its own healthy cells, tissues and other normal body constituents. Any disease resulting from this type of immune response is termed an "autoimmune disease". P ...
, genetic,
oxidative stress Oxidative stress reflects an imbalance between the systemic manifestation of reactive oxygen species and a biological system's ability to readily Detoxification, detoxify the reactive intermediates or to repair the resulting damage. Disturbances ...
, neural, or viral causes. The incidence worldwide is less than 1%. Individuals affected by vitiligo sometimes suffer psychological discomfort because of their appearance.


Hyperpigmentation

Increased melanin production, also known as
hyperpigmentation Hyperpigmentation is the darkening of an area of skin or nails caused by increased melanin. Causes Hyperpigmentation can be caused by sun damage, inflammation, or other skin injuries, including those related to acne vulgaris.James, William; Ber ...
, can be a few different phenomena: *
Melasma Melasma (also known as chloasma faciei,James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews' Diseases of the Skin: Clinical Dermatology''. (10th ed.). Saunders. . or the mask of pregnancy when present in pregnant women) is a tan or dark ...
describes the darkening of the skin. *
Chloasma Melasma (also known as chloasma faciei,James, William; Berger, Timothy; Elston, Dirk (2005). ''Andrews' Diseases of the Skin: Clinical Dermatology''. (10th ed.). Saunders. . or the mask of pregnancy when present in pregnant women) is a tan or dark ...
describes skin discolorations caused by hormones. These hormonal changes are usually the result of pregnancy, birth control pills or estrogen replacement therapy. *
Solar lentigo Solar may refer to: Astronomy * Of or relating to the Sun ** Solar telescope, a special purpose telescope used to observe the Sun ** A device that utilizes solar energy (e.g. "solar panels") ** Solar calendar, a calendar whose dates indicat ...
, also known as "liver spots" or "senile freckles", refers to darkened spots on the skin caused by aging and the sun. These spots are quite common in adults with a long history of unprotected sun exposure. Aside from sun exposure and hormones, hyperpigmentation can be caused by skin damage, such as remnants of blemishes, wounds or rashes. This is especially true for those with darker skin tones. The most typical cause of darkened areas of skin, brown spots or areas of discoloration is unprotected sun exposure. Once incorrectly referred to as
liver spot The liver is a major Organ (anatomy), organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of proteins and biochemicals necessary for ...
s, these pigment problems are not connected with the liver. On lighter to medium skin tones, solar lentigenes emerge as small- to medium-sized brown patches of freckling that can grow and accumulate over time on areas of the body that receive the most unprotected sun exposure, such as the back of the hands, forearms, chest, and face. For those with darker skin colors, these discolorations can appear as patches or areas of ashen-gray skin.


Exposure to the Sun

Melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
in the skin protects the body by absorbing solar radiation. In general, the more melanin there is in the skin the more solar radiation can be absorbed. Excessive solar radiation causes
direct Direct may refer to: Mathematics * Directed set, in order theory * Direct limit of (pre), sheaves * Direct sum of modules, a construction in abstract algebra which combines several vector spaces Computing * Direct access (disambiguation), a ...
and
indirect DNA damage Indirect, the opposite of direct, may refer to: * Indirect approach, a battle strategy * Indirect DNA damage, caused by UV-photons *Indirect agonist or indirect-acting agonist, a substance that enhances the release or action of an endogenous neurot ...
to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens, but can also cause sunburn. The tanning process can also be created by artificial UV radiation. There are two different mechanisms involved. Firstly, the UVA-radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin, also known as IPD (immediate pigment darkening). Secondly, there is an increase in production of melanin known as
melanogenesis Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart. ...
. Melanogenesis leads to delayed tanning and first becomes visible about 72 hours after exposure. The tan that is created by an increased melanogenesis lasts much longer than the one that is caused by oxidation of existing melanin. Tanning involves not just the increased melanin production in response to UV radiation but the thickening of the top layer of the epidermis, the stratum corneum. A person's natural skin color affects their reaction to exposure to the sun. Generally, those who start out with darker skin color and more melanin have better abilities to tan. Individuals with very light skin and albinos have no ability to tan. The biggest differences resulting from sun exposure are visible in individuals who start out with moderately pigmented brown skin: the change is dramatically visible as tan lines, where parts of the skin which tanned are delineated from unexposed skin. Modern lifestyles and mobility have created mismatch between skin color and environment for many individuals. Vitamin D deficiencies and UVR overexposure are concerns for many. It is important for these people individually to adjust their diet and lifestyle according to their skin color, the environment they live in, and the time of year. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in order of decreasing lightness:


Fitzpatrick scale

The following list shows the six categories of the Fitzpatrick scale in relation to the 36 categories of the older von Luschan scale: Dark skin with large concentrations of melanin protects against ultraviolet light and skin cancers; light-skinned people have about a tenfold greater risk of dying from skin cancer, compared with dark-skinned persons, under equal sunlight exposure. Furthermore,
UV-A Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...
rays from sunlight are believed to interact with folic acid in ways that may damage health. In a number of traditional societies the sun was avoided as much as possible, especially around noon when the ultraviolet radiation in sunlight is at its most intense. Midday was a time when people stayed in the shade and had the main meal followed by a nap, a practice similar to the modern
siesta A ''siesta'' (from Spanish, pronounced and meaning "nap") is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those in warm-weather zones. The ...
.


Geographic variation

Approximately 10% of the variance in skin color occurs within regions, and approximately 90% occurs between regions. Because skin color has been under strong selective pressure, similar skin colors can result from convergent adaptation rather than from genetic relatedness; populations with similar pigmentation may be genetically no more similar than other widely separated groups. Furthermore, in some parts of the world where people from different regions have mixed extensively, the connection between skin color and ancestry has substantially weakened. In Brazil, for example, skin color is not closely associated with the percentage of recent African ancestors a person has, as estimated from an analysis of genetic variants differing in frequency among continent groups. In general, people living close to the equator are highly darkly pigmented, and those living near the poles are generally very lightly pigmented. The rest of humanity shows a high degree of skin color variation between these two extremes, generally correlating with UV exposure. The main exception to this rule is in the New World, where people have only lived for about 10,000 to 15,000 years and show a less pronounced degree of skin pigmentation. In recent times, humans have become increasingly mobile as a consequence of improved technology, domestication, environmental change, strong curiosity, and risk-taking. Migrations over the last 4000 years, and especially the last 400 years, have been the fastest in human history and have led to many people settling in places far away from their ancestral homelands. This means that skin colors today are not as confined to geographical location as they were previously.


Social status, colorism and racism

According to classical scholar Frank Snowden, skin color did not determine
social status Social status is the level of social value a person is considered to possess. More specifically, it refers to the relative level of respect, honour, assumed competence, and deference accorded to people, groups, and organizations in a society. Stat ...
in ancient
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Mediter ...
,
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...
or
Rome , established_title = Founded , established_date = 753 BC , founder = King Romulus (legendary) , image_map = Map of comune of Rome (metropolitan city of Capital Rome, region Lazio, Italy).svg , map_caption ...
. These ancient civilizations viewed relations between the major power and the subordinate state as more significant in a person's status than their skin colors. Nevertheless, some social groups favor specific skin coloring. The preferred skin tone varies by culture and has varied over time. A number of indigenous African groups, such as the
Maasai Maasai may refer to: * Maasai people *Maasai language * Maasai mythology * MAASAI (band) See also * Masai (disambiguation) * Massai Massai (also known as: Masai, Massey, Massi, Mah–sii, Massa, Wasse, Wassil or by the nickname "Big Foot" Mas ...
, associated pale skin with being cursed or caused by evil spirits associated with witchcraft. They would abandon their children born with conditions such as albinism and showed a sexual preference for darker skin. Many cultures have historically favored lighter skin for women. Before the
Industrial Revolution The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
, inhabitants of the continent of Europe preferred pale skin, which they interpreted as a sign of high social status. The poorer classes worked outdoors and got darker skin from exposure to the sun, while the upper class stayed indoors and had light skin. Hence light skin became associated with wealth and high position. Women would put lead-based cosmetics on their skin to whiten their skin tone artificially. However, when not strictly monitored, these cosmetics caused
lead poisoning Lead poisoning, also known as plumbism and saturnism, is a type of metal poisoning caused by lead in the body. The brain is the most sensitive. Symptoms may include abdominal pain, constipation, headaches, irritability, memory problems, inferti ...
. Other methods also aimed at achieving a light-skinned appearance, including the use of
arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, but ...
to whiten skin, and powders. Women would wear full-length clothes when outdoors, and would use gloves and parasols to provide shade from the sun. Colonization and enslavement as carried out by European countries became involved with
colorism Discrimination based on skin color, also known as colorism, or shadeism, is a form of prejudice and/or discrimination in which people who share similar ethnicity traits or perceived race are treated differently based on the social implications th ...
and
racism Racism is the belief that groups of humans possess different behavioral traits corresponding to inherited attributes and can be divided based on the superiority of one race over another. It may also mean prejudice, discrimination, or antagonism ...
, associated with the belief that people with dark skin were uncivilized, inferior, and should be subordinate to lighter-skinned invaders. This belief exists to an extent in modern times as well. Institutionalized
slavery Slavery and enslavement are both the state and the condition of being a slave—someone forbidden to quit one's service for an enslaver, and who is treated by the enslaver as property. Slavery typically involves slaves being made to perf ...
in North America led people to perceive lighter-skinned African-Americans as more intelligent, cooperative, and beautiful. Such lighter-skinned individuals had a greater likelihood of working as
house slave A house slave was a slave who worked, and often lived, in the house of the slave-owner, performing domestic labor. House slaves performed largely the same duties as all domestic workers throughout history, such as cooking, cleaning, serving meals, ...
s and of receiving preferential treatment from plantation owners and from overseers. For example, they had a chance to get an education. The preference for fair skin remained prominent until the end of the
Gilded Age In United States history, the Gilded Age was an era extending roughly from 1877 to 1900, which was sandwiched between the Reconstruction era and the Progressive Era. It was a time of rapid economic growth, especially in the Northern and Weste ...
, but
racial stereotypes An ethnic stereotype, racial stereotype or cultural stereotype involves part of a system of beliefs about typical characteristics of members of a given ethnic group, their status, societal and cultural norms. A national stereotype, or nation ...
about worth and beauty persisted in the last half of the 20th century and continue in the present day. African-American journalist
Jill Nelson Jill Nelson (born June 14, 1952) is a prominent African-American journalist and novelist. She has written several books, including the autobiographical ''Volunteer Slavery: My Authentic Negro Experience'', which won an American Book Award. She wa ...
wrote that, "To be both prettiest and black was impossible," and elaborated:
We learn as girls that in ways both subtle and obvious, personal and political, our value as females is largely determined by how we look. ... For black women, the domination of physical aspects of beauty in women's definition and value render us invisible, partially erased, or obsessed, sometimes for a lifetime, since most of us lack the major talismans of Western beauty. Black women find themselves involved in a lifelong effort to self-define in a culture that provides them no positive reflection.
A preference for fair or lighter skin continues in some countries, including Latin American countries where whites form a minority. In Brazil, a dark-skinned person is more likely to experience discrimination. Many actors and actresses in Latin America have European features—
blond hair Blond (male) or blonde (female), also referred to as fair hair, is a hair color characterized by low levels of the dark pigment eumelanin. The resultant visible hue depends on various factors, but always has some yellowish color. The color can ...
,
blue eyes Eye color is a polygene, polygenic phenotypic character determined by two distinct factors: the pigmentation of the eye's Iris (anatomy), iris and the frequency-dependence of the scattering of light by the Turbidity, turbid medium in the Stro ...
, and pale skin. A light-skinned person is more privileged and has a higher social status; a person with light skin is considered more beautiful and lighter skin suggests that the person has more wealth. Skin color is such an obsession in some countries that specific words describe distinct skin tones - from (for example) "''jincha''", Puerto Rican slang for "glass of milk" to "''morena''", literally "brown". In
South Asia South Asia is the southern subregion of Asia, which is defined in both geographical and ethno-cultural terms. The region consists of the countries of Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka.;;;;;;;; ...
, society regards pale skin as more attractive and associates dark skin with lower class status; this results in a massive market for
skin-whitening Skin whitening, also known as skin lightening and skin bleaching, is the practice of using chemical substances in an attempt to lighten the skin or provide an even skin color by reducing the melanin concentration in the skin. Several chemicals ha ...
creams. Fairer skin-tones also correlate to higher caste-status in the Hindu social order—although the system is not based on skin tone. Actors and actresses in Indian cinema tend to have light skin tones, and Indian cinematographers have used graphics and intense lighting to achieve more "desirable" skin tones. Fair skin tones are advertised as an asset in Indian marketing. Skin-whitening products have remained popular over time, often due to historical beliefs and perceptions about fair skin. Sales of skin-whitening products across the world grew from $40 billion to $43 billion in 2008. In
South South is one of the cardinal directions or Points of the compass, compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Pro ...
and
East Asian East Asia is the eastern region of Asia, which is defined in both geographical and ethno-cultural terms. The modern states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. China, North Korea, South Korea a ...
countries, people have traditionally seen light skin as more attractive, and a preference for lighter skin remains prevalent. In ancient China and Japan, for example, pale skin can be traced back to ancient drawings depicting women and goddesses with fair skin tones. In ancient China, Japan, and Southeast Asia, pale skin was seen as a sign of wealth. Thus skin-whitening cosmetic products are popular in East Asia. Four out of ten women surveyed in
Hong Kong Hong Kong ( (US) or (UK); , ), officially the Hong Kong Special Administrative Region of the People's Republic of China ( abbr. Hong Kong SAR or HKSAR), is a city and special administrative region of China on the eastern Pearl River Delt ...
,
Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...
, the
Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...
and
South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korea, Korean Peninsula and sharing a Korean Demilitarized Zone, land border with North Korea. Its western border is formed ...
used a skin-whitening cream, and more than 60 companies globally compete for Asia's estimated $18 billion market. Changes in regulations in the cosmetic industry led to skin-care companies introducing harm-free skin lighteners. In
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
, the
geisha {{Culture of Japan, Traditions, Geisha {{nihongo, Geisha, 芸者 ({{IPAc-en, ˈ, ɡ, eɪ, ʃ, ə; {{IPA-ja, ɡeːɕa, lang), also known as {{nihongo, , 芸子, geiko (in Kyoto and Kanazawa) or {{nihongo, , 芸妓, geigi, are a class of female ...
have a reputation for their white-painted faces, and the appeal of the , or "beautiful white", ideal leads many Japanese women to avoid any form of tanning. There are exceptions to this, with Japanese fashion trends such as
ganguro is a fashion trend among young Japanese women that started in the mid-1990s, distinguished by a dark tan and contrasting make-up liberally applied by fashionistas. The Shibuya and Ikebukuro districts of Tokyo were the centres of ''ganguro'' fash ...
emphasizing tanned skin. Skin whitening is also not uncommon in
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, and several research projects have suggested a general preference for lighter skin in the African-American community. In contrast, one study on men of the Bikosso tribe in Cameroon found no preference for attractiveness of females based on lighter skin color, bringing into question the universality of earlier studies that had exclusively focused on skin-color preferences among non-African populations. Significant exceptions to a preference for lighter skin started to appear in Western culture in the mid-20th century. However a 2010 study found a preference for lighter-skinned women in New Zealand and California. Though sun-tanned skin was once associated with the sun-exposed manual labor of the lower class, the associations became dramatically reversed during this time—a change usually credited to the trendsetting Frenchwoman
Coco Chanel Gabrielle Bonheur "Coco" Chanel ( , ; 19 August 1883 – 10 January 1971) was a French fashion designer and businesswoman. The founder and namesake of the Chanel brand, she was credited in the post-World War I era with popularizing a sporty, c ...
(1883–1971) presenting tanned skin as fashionable, healthy, and luxurious. , though an overall preference for lighter skin remains prevalent in the United States, many within the country regard tanned skin as both more attractive and healthier than pale or very dark skin. Western mass media and popular culture continued to reinforce negative stereotypes about dark skin, but in some circles pale skin has become associated with indoor office-work while tanned skin has become associated with increased leisure time, sportiness and good health that comes with wealth and higher social status. Studies have also emerged indicating that the degree of tanning is directly related to how attractive a young woman is.


See also

*
Afro-textured hair Afro-textured hair, or kinky hair, is a human hair texture originating from sub-Saharan Africa. Each strand of this hair type grows in a tiny, angle-like helix shape. The overall effect is such that, contrasted with straight, wavy, or curly hai ...
* Carnation (heraldry) *
Color terminology for race Identifying human races in terms of skin color, at least as one among several physiological characteristics, has been common since antiquity. Such divisions appeared in rabbinical literature and in early modern scholarship, usually dividing huma ...
*
Olive skin Olive skin is a human skin colour spectrum. It is often associated with pigmentation in the Type III to Type IV and Type V ranges of the Fitzpatrick scale. It generally refers to light or moderate tan skin, and it is often described as having ...
*
Complexion Complexion in humans is the natural color, texture, and appearance of the skin, especially on the face. History The word "complexion" is derived from the Late Latin ''complexi'', which initially referred in general terms to a combination of thi ...
*
Eye color Eye color is a polygenic phenotypic character determined by two distinct factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris. In humans, the pi ...
*
Health effects of sun exposure The ultraviolet radiation in sunlight has both positive and negative health effects, as it is required for the synthesis of vitamin D3 and is a mutagen. A dietary supplement can supply vitamin D without this mutagenic effect, but supplementati ...
*
Human hair color Human hair color is the pigmentation of human hair follicles due to two types of melanin: eumelanin and pheomelanin. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter. The t ...
*
Human physical appearance Human physical appearance is the outward phenotype or look of human beings. There are infinite variations in human phenotypes, though society reduces the variability to distinct categories. The physical appearance of humans, in particular those a ...
*
Human skin The human skin is the outer covering of the body and is the largest organ of the integumentary system. The skin has up to seven layers of ectodermal tissue guarding muscles, bones, ligaments and internal organs. Human skin is similar to most ...
*
Race (human classification) A race is a categorization of humans based on shared physical or social qualities into groups generally viewed as distinct within a given society. The term came into common usage during the 1500s, when it was used to refer to groups of variou ...


References


Further reading

* * * * * * * * * * * * Summary of clues to the saga in which humans evolved to lose their hair and had to adjust, including turning from light skin to dark skin, together with an estimation of the time when humans invented clothing. * *


External links


"Key gene 'controls skin colour
BBC News BBC News is an operational business division of the British Broadcasting Corporation (BBC) responsible for the gathering and broadcasting of news and current affairs in the UK and around the world. The department is the world's largest broadca ...
. SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry
"The Biology of Skin Color: Black and White"
PBS
"The Biology of Skin Color — HHMI BioInteractive Video"
YouTube YouTube is a global online video platform, online video sharing and social media, social media platform headquartered in San Bruno, California. It was launched on February 14, 2005, by Steve Chen, Chad Hurley, and Jawed Karim. It is owned by ...
{{Authority control Human genetics Human physiology
color Color (American English) or colour (British English) is the visual perceptual property deriving from the spectrum of light interacting with the photoreceptor cells of the eyes. Color categories and physical specifications of color are associ ...
Skin physiology