human skin

skin cancer

Malignant tumour of the skin, including some of the most common human cancers. Though recognizable at an early stage, it has a significant death rate. Light-skinned people have the highest risk but can reduce it by limiting exposure to sunlight and to ionizing radiation. The most common types arise in the epidermis (outer skin layer) and have become more frequent with the thinning of the atmosphere's ozone layer. The most serious form is melanoma, which is frequently fatal if not treated early with surgery. Cancers arising from the dermis are rare; the best-known is Kaposi sarcoma.

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A section through the skin. The tough, dead cells of the outer epidermal surface (corneal layer) elipsis

Surface covering of the body that protects it and receives external sensory stimuli, consisting of an epidermis over a thicker dermis. The epidermis contains cells involved in immune defenses, sensory receptors, pigment cells, and keratin-producing cells. The last harden and migrate to the surface to form a dead, relatively dry outer layer of horny tissue that constantly sloughs away. The dermis contains sensory nerves and blood vessels within connective tissue. Collagen and elastin fibres give skin its tough, elastic quality. Cells scattered through it produce its components and take part in immune and other skin responses. A fat layer under the dermis provides nutritional storage, cushioning, and insulation. Skin disorders range from dermatitis and acne to skin cancer. Changes in skin colour (e.g., jaundice) or texture may be clues to systemic disorders. Seealso dermatology; hair; integument; nail; perspiration; sebaceous gland; sweat gland.

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Human skin color can range from almost black (due to very high concentrations of the dark brown pigment melanin) to nearly colorless (appearing reddish white due to the blood in the skin) in different people. Skin color is determined by the amount and type of melanin, the pigment in the skin. Variation in skin color is largely due to genetics. As a general pattern people with ancestors from tropical regions (hence greater sunlight exposure) have darker skin than people with ancestors from subtropical regions. This is far from a hard and fast rule however, because many light skinned groups have managed to survive at the equator by way of social adaptation. The same can be said of dark skinned groups living at subtropical latitudes.

Melanin and genes

Melanin comes in two types: pheomelanin (red) and eumelanin (very dark brown). Both amount and type are determined by four to six genes which operate under incomplete dominance. One copy of each of those genes is inherited from each parent. Each gene comes in several alleles, resulting in a great variety of different skin tones.

The evolution of the different skin tones is thought to have occurred as follows: the haired primate ancestors of humans, like modern great apes, had light skin under their hair. When Hominids evolved relative hairlessness (the most likely function of which was to facilitate perspiration), they evolved dark skin, needed to prevent low folate levels since they lived in sun-rich Africa. (The skin cancer connection is probably of secondary importance, since skin cancer usually kills only after the reproductive age and therefore does not exert much evolutionary selection pressure.) When humans migrated to less sun-intensive regions in the north, low vitamin D3 levels became a problem and light skin color re-emerged. Sexual selection and diet may have played a part in the evolution of skin tone diversity, as well.

The Inuit and Yupik are special cases: even though they live in an extremely sun-poor environment, they have retained their relatively dark skin. This can be explained by the fact that their traditional fish-based diet provides plenty of vitamin D.

Health related effects

Dark skin (melanin) protects against ultraviolet light; this light causes mutations in skin cells, which in turn cause skin cancers. Light-skinned persons have about a tenfold greater risk of dying from skin cancer under equal sunlight exposure, with redheads having the greatest risk. Furthermore, dark skin prevents radiation of UV-A rays from destroying the essential folic acid, derived from B vitamins. Folic acid (or folate) is needed for the synthesis of DNA in dividing cells and folate deficiency in pregnant women are associated with birth defects.

While dark skin better preserves vitamin B, it can also lead to vitamin D deficiency at higher latitudes which in turn can cause fatal cancers affecting the colon, lung and prostate. Dark-skinned people are also at higher risk for rickets, cardiovascular disease, diabetes and multiple sclerosis..To address this issue, some countries have programs to ensure fortification of milk with vitamin D.

The advantage of light skin at high latitudes is that it allows more sun absorption, leading to increased production of vitamin D3, necessary for calcium absorption and bone growth. The lighter skin of women at higher latitudes most likely results from the higher calcium needs of women during pregnancy and lactation. However, some have postulated that it may also derive from sexual selection (this hypothesis may derive from current social conditions in which light skinned people set the beauty standard).

Albinism is a condition characterized by the absence of melanin, resulting in very light skin, eyes, and hair; it is caused by an inability to synthesize tyrosine, and has a genetic basis.

Cultural effects

Sexual preference of paleness in women by men is a cultural universal, and has been reported from medieval Japan or Aztec Mexico, even before there was significant contact with Western ideology. The effect has even been found in Moorish Spain, where the ruling class was of darker complexion than the conquered natives. Also, preference of lighter-skinned women by black men is reported both in sub-Saharan Africa and in the black diaspora. In his foreword to Peter Frost's 2005 Fair Women, Dark Men, U. of Washington sociologist Pierre L. van den Berghe summarizes:
"Although virtually all cultures express a marked preference for fair female skin, even those with little or no exposure to European imperialism, and even those whose members are heavily pigmented, many are indifferent to male pigmentation or even prefer men to be darker.
A consequence of this is that, since higher-ranking men get to marry the more attractive women, the upper classes of a society generally tend to develop a lighter complexion than the lower classes by sexual selection (see also Fisherian runaway).

Differences in skin tone are the most readily perceptible phenotypical distinction of human populations, and hence has historically lent itself to color terminology for race, often to the effect of darker skin being seen as being of lowest social value, and lighter skin of highest. However, according to classical scholar Frank Snowden, the Egyptians and Greeks (et al.) assigned relatively neutral connotations to skin color variation because conquest rather than skin color was the major determinant of slave status.

Skin tone variability

The tone of human skin can vary from a dark brown to nearly a colorless pigmentation, which may appear reddish due to the blood in the skin. Europeans have lighter skin, hair, and eyes than any other group on Earth. For practical purposes, such as exposure time for sun tanning, six skin types are distinguished following Fitzpatrick (1975), listed in decreasing lightness:

type also called tanning behavior hair and eye color von Luschan scale
I very light, also "Celtic" Often burns, rarely tans. Tends to have freckles, red or fair hair, blue or green eyes. 1-5
II light, or light-skinned European Usually burns, sometimes tans Tends to have light hair, blue or brown eyes. 6-10
III light intermediate, or dark-skinned European or "average Caucasian" Sometimes burns, usually tans. Tends to have brown hair and eyes. 11-15
IV dark intermediate, also "Mediterranean" or "Olive" Sometimes burns, often tans. Tends to have dark brown eyes and hair. 16-20
V dark or "Brown" type Naturally black-brown skin Often has dark brown eyes and hair. 21-28
VI very dark, or "Black" type Naturally black-brown skin Usually has black-brown eyes and hair. 29-36

Types I to IV are all subsumed under "white". Types V and VI do not natively occur in Europe, North Asia, Northern America, or indeed significantly north of the Tropic of Cancer, while types I to IV do not natively occur in Sub-Saharan Africa or Australia. North Africa, Southwest and South Asia natively have intermediate types III to V.

In attempting to discover the mechanisms that have generated such a wide variation in human skin tone, discovered that there is a high correlation between the tone of human skin of indigenous peoples and the average annual ultraviolet (UV) radiation available for skin exposure where the indigenous peoples live. Accordingly, Jablonski and Chaplin plotted the skin tone (W) of indigenous peoples who have stayed in the same geographical area for the last 500 years versus the annual UV available for skin exposure (AUV) for over 200 indigenous persons and found that skin tone lightness W is related to the annual UV available for skin exposure AUV according to

W = 70 - frac{AUV}{10}
where the skin tone lightness W is measured as the percentage of light reflected from the upper inner arm at which location on humans there should be minimal tanning of human skin due to personal exposure to the sun; a lighter skinned human would reflect more light and would have a higher W number. Judging from the above linear fit to the empirical data, the theoretical lightness maximum of human skin would reflect only 70 per cent of incident light for a hypothetical indigenous human-like population that lived where there was zero annual UV available for skin exposure (AUV = 0 in the above formula). Jablonski and Chaplin evaluated average annual UV available for skin exposure AUV from satellite measurements that took into consideration the measured daily variation in the thickness of the ozone layer that blocked UV hitting the Earth, measured daily variation in opacity of cloud cover, and daily change in angle at which the sunlight containing UV radiation strikes the Earth and passes through different thicknesses of Earth's atmosphere at different latitudes for each of the different human indigenous peoples' home areas from 1979 to 1992.

Jablonski and Chaplin proposed an explanation for the observed variation of untanned human skin with annual UV exposure. By Jablonski and Chaplin's explanation, there are two competing forces affecting human skin tone:

  1. the melanin that produces the darker tones of human skin serves as a light filter to protect against too much UV light getting under the human skin where too much UV causes sunburn and disrupts the synthesis of precursors necessary to make human DNA; versus
  2. humans need at least a minimum threshold of UV light to penetrate the epidermis to produce vitamin D, which is essential for building and maintaining the bones of the human skeleton.

Jablonski and Chaplin note that when human indigenous peoples have migrated, they have carried with them a sufficient gene pool so that within a thousand years, the skin of their descendants living today has turned dark or turned light to adapt to fit the formula given above—with the notable exception of dark-skinned peoples moving north, such as to populate the seacoast of Greenland, to live where they have a year-round supply of food rich in vitamin D, such as fish, so that there was no necessity for their skin to lighten to let enough UV under their skin to synthesize the vitamin D that humans need for healthy bones.

In considering the tone of human skin in the long span of human evolution, Jablonski and Chaplin note that there is no empirical evidence to suggest that the hominid ancestors six million years ago had a skin tone different from the skin tone of today's chimpanzees—namely light-skinned under black hair. But as humans evolved to lose their body hair a parallel evolution permitted human populations to turn their base skin tone dark or light to adjust to the competing demands of 1) increasing eumelanin to protect from UV that was too intense and 2) reducing eumelanin so that enough UV would penetrate to synthesize enough vitamin D. By this explanation, prior to Homo Sapien colonization of extra-African territories, humans had dark skin given that they lived for extended periods of time where the sunlight is intense. As some humans migrated north, over time they developed light skin.

Adaptive origins of skin color variation

Brown skin is the likely ancestral (or original) skin color among modern humans (Harding et al 2000). This is due to modern humanity's common origin in equatorial Africa ~200,000 years ago (Tishkoff, 1996). Dark skin was crucial in this UV rich context given that a thick coat of UV protective body hair had long been selected against by this time (Rogers et al 2004) most likely in order to facilitate the evaporation of perspiration (ie the cooling of the body). This trait (dark skin) continues to be under strong selection in equatorial regions such as Africa, India, and New Guinea (Harding 2000 p 1355). Geneticists estimate that a relatively small group of humans left Africa ~60,000 years ago, and that the descendants of this group went on to populate the entire non-sub-Saharan African world. Those migrants that settled in non-African equatorial regions (such as the mentioned India, New Guinea, and/or Australia) retained most of the ancestral sequence at the MC1R locus (Harding 2000 p 1355), a gene strongly associated with determining skin color. Specifically, Harding et al (2000 p 1355) found that the haplotype sequences for Indians and New Guineans are virtually identical to those of continental sub-Saharan Africans (except for a small number of variants at silent sites).

The retention of the ancestral trait at the equator is due to natural selection for melanin pigment production which serves to protect the body from harmful UV rays (Jablonski 2006). Notably, given that hair is a part of the skin, the retention is also analogous to that which occurred for Natural afro-hair prior to pre-Holocene admixture events among people who settled in India and Australia. However, certain evidence suggests that, unlike skin color, Afro hair ceased to be under strong selection once dark skin arose ~1 million years ago (Harding 2000) (rather, it remained as a vestigial trait among Africans, Andamanese, and Melanesians and changed to straight in the north for adaptive reasons--see hair texture). In fact, dark skin is so selectively advantageous at the equator that initially light skinned native Americans who migrated to Mexico and/or South America experienced renewed selective pressure towards the evolution of dark skin.

According to (Norton et al., 2006), light skin observed in Europeans (with deep Red and/or yellowish skin tones), non-Indian Southeast Asians, East Asians and North Africa (Maghreb) is due to independent genetic mutations in at least three loci. They concluded that light pigmentation is at least partially due to sexual selection, however Jablonski postulates that the predominant reason revolved around the facilitation of vitamin D production in northern Eurasia (see hair texture).

Genetics of skin color variation

Several genes have been invoked to explain variations of skin tones in humans, including SLC45A2, ASIP, MATP, TYR, and OCA2. A recently discovered gene, SLC24A5 has been shown to account for a substantial fraction of the difference in the average of 30 or so melanin units between Europeans and Africans.

Wide variations in human skin tones have been correlated with mutations in another gene; the MC1R gene . The "MC1R" label for the gene stands for melanocortin 1 receptor, where

  • "melano" refers to black,
  • "melanocortin" refers to the hormone stimulant produced by the pituitary gland that stimulates cells to produce the melanin that makes skin cells black,
  • the "1" in the MC1R gene name specifies the first family of melanocortin genes, and
  • "receptor" indicates that the protein from the gene serves as a signal relay from outside the cell membrane to inside the cell—to the place in the cell where the black melanin is synthesized.

Accordingly, the MC1R gene specifies the amino acid sequence in the receptor protein that relays through the cell membrane the hormone signal from the pituitary gland to produce the melanin that makes human skin very dark. Many variations in the amino acid sequence of this receptor protein result in lighter or darker skin.

The human MC1R gene consists of a string of 954 nucleotides, where each nucleotide is one of the four bases Adenine (A), Guanine (G), Thymine (T), or Cytosine (C). But 261 of the nucleotides in the MC1R gene can change with no effect on the amino acid sequence in the receptor protein produced from the gene. For example, the nucleotide triplets GGT, GGC, GGA, and GGG are all synonymous and all produce the amino acid Glycine, so a mutation in the third position in the triplet GGT is a "silent mutation" and has no effect on the amino acid produced from the triplet. (Harding et al., 2000, pg.1355) analyzed the amino acid sequences in the receptor proteins from 106 individuals from Africa and 524 individuals from outside Africa to find why the tone of the sampled Africans' skin was dark.

Harding found that there were zero differences among the Africans for the amino acid sequences in their receptor proteins, so the skin of each individual from Africa was dark. In contrast, among certain (European) non-African individuals, there were 18 different amino acid sites in which the receptor proteins differed, and each amino acid that differed from the African receptor protein resulted in skin lighter than the skin of the African (and other equatorial) individuals. Nonetheless, the variations in the 261 silent sites in the MC1R were similar between the Africans and non-Africans, so the basic mutation rates among the Africans and non-Africans were the same. Also, close examination of the haplotype variation among the non-Europeans (including East Asians) suggested that, among most non-European non-Africans, the most common variants were in the silent mutation positions (Harding et al 2000 p 1355). Thus, at least at this locus, most non-Europeans share the ancestral function. The fact that relatively light skinned east Asians varied little genetically from dark skinned Africans at this locus supports the fact that skin color is a complex trait determined by several genes. Thus light skin among east Asians occurs by way of a different genetic mechanism than that among Europeans.

With regards to Europeans, the next question to ask would be: why were there zero differences and no divergences in the amino acid sequences of the receptor protein among the Africans (and other equatorial groups) while there were 18 differences among the populations in Ireland, England, and Sweden? (Harding et al., 2000, pp.1359-1360) concluded that the intense sun in Africa created an evolutionary constraint that reduced severely the survival of progeny with any difference in the 693 sites of the MC1R gene that resulted in even one small change in the amino acid sequence of the receptor protein—because any variation from the African receptor protein produced significantly lighter skin that gave less protection from the intense African sun. In contrast, in Sweden, for example, the sun was so weak that no mutation in the receptor protein reduced the survival probability of progeny. Indeed, for the individuals from Ireland, England, and Sweden, the mutation variations among the 693 gene sites that caused changes in amino acid sequence was the same as the mutation variations in the 261 gene sites at which silent mutations still produced the same amino acid sequence. Thus, Harding concluded that the intense sun in Africa selectively killed off the progeny of individuals who had a mutation in the MC1R gene that made the skin lighter. However, the mutation rate toward lighter skin in the progeny of those African individuals who had moved North to areas with weaker sun was comparable to the mutation rate of the folks whose ancient ancestors grew up in Sweden. Hence, Harding concluded that the lightness of human skin was a direct result of random mutations in the MC1R gene that were non-lethal at the latitudes of Sweden. Even the mutations that produce red hair with little ability to tan were non-lethal in the northern latitudes. examined Harding's data on the variation of MC1R nucleotide sequences for people of different ancestry to determine the most probable progression of the skin tone of human ancestors over the last five million years. Comparing the MC1R nucleotide sequences for chimpanzees and humans in various regions of the Earth, Rogers concluded that the common ancestors of all humans had light skin tone under dark hair—similar to the skin tone and hair color pattern of today's chimpanzees. That is 5 million years ago, the human ancestors' dark hair protected their light skin from the intense African sun so that there was no evolutionary constraint that killed off the progeny of those who had mutations in the MC1R nucleotide sequences that made their skin light. argues that based on cave paintings, Europeans may have been dark as recently as 13,000 years ago. The painters depicted themselves as having darker complexions than the animals they hunted.

However, over 1.2 million years ago, judging from the numbers and spread of variations among human and chimpanzee MC1R nucleotide sequences, the human ancestors in Africa began to lose their hair and they came under increasing evolutionary pressures that killed off the progeny of individuals that retained the inherited lightness of their skin. Folate breakdown in sun-exposed skin is inhibited by the presence of melanin and is essential for human fetal development. It is likely that folate conservation played an important role in the selection of dark skin in the ancient African ancestors of modern humans. By 1.2 million years ago, all people having descendants today had exactly the receptor protein of today's Africans; their skin was dark, and the intense sun killed off the progeny with any lighter skin that resulted from mutational variation in the receptor protein .

However, the progeny of those humans who migrated North away from the intense African sun had another evolutionary constraint: vitamin D availability. Human requirements for vitamin D (cholecalciferol) are in part met through photoconversion of a precursor to vitamin D3. As humans migrated north from the equator, they were exposed to less intense sunlight, in part because of the need for greater use of clothing to protect against the colder climate. Thus, under these conditions, evolutionary pressures would tend to select for lighter-skinned humans as there was less photodestruction of folate and a greater need for photogeneration of cholecalciferol. Tracking back the statistical patterns in variations in DNA among all known people sampled who are alive on the Earth today, it appears that

  1. From ~1.2 million years ago for at least ~1.35 million years, the ancestors of all people alive were as dark as today's Africans.
  2. The descendants of any pre-historic people who migrate North from the equator will mutate to become light over time because the evolutionary constraint keeping Africans' skin dark decreases generally the further North a people migrates. This also occurs as a result of selection for light skin due to the need to produce vitamin D by way of the penetration of sunlight into the skin (the exception being if dietary sources of vitamin D are available--see the inuit).
  3. The genetic mutations leading to light skin among East Asians are different from those of Europeans, suggesting that, following the migration out of Africa, the two groups became distinct populations that experienced a similar selective pressure due to settlement in northern latitudes.

See also

Footnotes

References

  • Harding, Rosalind M.; Eugene Healy, Amanda J. Ray, Nichola S. Ellis, Niamh Flanagan, Carol Todd, Craig Dixon, Antti Sajantila, Ian J. Jackson, Mark A. Birch-Machin, and Jonathan L. Rees (2000). "Evidence for variable selective pressures at MC1R". American Journal of Human Genetics 66 1351–1361.

  • Norton, Heather L.; Rick A. Kittles, Esteban Parra, Paul McKeigue, Xianyun Mao, Keith Cheng, Victor A. Canfield, Daniel G. Bradley, Brian McEvoy and Mark D. Shriver (2006). "Genetic Evidence for the Convergent Evolution of Light Skin in Europeans and East Asians". Oxford Journals
    • Proposes that the advent of agriculture and a grain diet low in vitamin D gave Northern Europeans their very pale skin.
    • Argues that skin tone is regulated by five genes and suggests Native Americans lost some genes in passage through the Arctic, preventing them from evolving very dark skin in equatorial America.
    • Gives some history of global skin tone maps, noting that Biasutti map is out of date.

  • Jablonski, Nina G., and George Chaplin (2002). "Skin deep." Scientific American 287 (4) (October): 74-82.
  • Lamason RL, Mohideen MA, Mest JR, Wong AC, Norton HL, Aros MC, Jurynec MJ, Mao X, Humphreville VR, Humbert JE, Sinha S, Moore JL, Jagadeeswaran P, Zhao W, Ning G, Makalowska I, McKeigue PM, O'donnell D, Kittles R, Parra EJ, Mangini NJ, Grunwald DJ, Shriver MD, Canfield VA, Cheng KC (2005). SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans. Science 310 (5755): 1782-6. PMID 16357253
  • Rees, J.L., and N. Flanagan (1999). "Pigmentation, melanocortins, and red hair." ''Q. J. Med." 92: 125-131.
  • Robins, A.H. 1991. Biological Perspectives on Human Pigmentation Cambridge University Press.

Millington GWM. (2006) Proopiomelanocortin (POMC): the cutaneous roles of its melanocortin products and receptors. Clin Exp Dermatol 31: 407-412.

Millington GWM, Levell NJ. (2007) From genesis to gene-sequencing: historical progress in the understanding of skin color. Intl J Dermatol 46: 103-105.

Further reading

  • Nicholas Wade (August 19 2003), " Why Humans and Their Fur Parted Ways" New York Times (Science Times). 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 at which humans invented clothing.
  • Key gene 'controls skin tone' SLC24A5 gene controls up to 38% of the tonal range in people with mixed European and West African ancestry

External links


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