Human skin color ranges in variety from Ebony skin darkest brown to the lightest hues. An individual's skin pigmentation is the result of geneticsbeing the product of both of the individual's biological parents ' genetic makeup, and exposure to sun.
In evolution, skin pigmentation in human beings evolved by a process of natural selection primarily to regulate the amount of ultraviolet radiation penetrating the skin, controlling its biochemical effects.
The actual skin color of different humans is affected by many substances, although the single most important substance is the pigment melanin. Melanin is produced Ebony skin the skin in cells called melanocytes and it is the main determinant of the skin color of darker-skinned humans. The skin color of people with Ebony skin skin is determined mainly by the bluish-white connective tissue under the dermis and by the hemoglobin 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 or the stimulation of the Ebony skin system anger, feararterioles dilate.
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 Ebony skin equator, tend to have darker-skinned populations.
Areas that are far from the tropics and closer to the poles Ebony skin lower intensity of UVR, which is reflected in lighter-skinned populations. The leading theory is Ebony skin 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.
Females need more calcium during pregnancy and lactation. The body synthesizes vitamin D from sunlight, which helps it absorb calcium. Females evolved to have lighter skin so their bodies Ebony skin more calcium.
The social significance of differences in skin color has varied across cultures and over time, as demonstrated with regard to social status and discrimination. Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is made within small membrane—bound packages called melanosomes. As they become full of melanin, they move into the slender arms of melanocytes, from where they are transferred to the keratinocytes.
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. The genetic mechanism behind human skin color is mainly regulated by the enzyme tyrosinasewhich creates the color of the skin, eyes, and hair shades.
The most common form of biological melanin is eumelanina brown-black polymer of dihydroxyindole carboxylic acidsand their reduced forms. Most are derived from the amino acid tyrosine. Eumelanin is Ebony skin in hair, Ebony skinand skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin. Pheomelanina pink to red hue is found in particularly large quantities in red hair the lips, nipples, glans of the penis, and vagina.
Both the amount and type of melanin produced is controlled by a number of genes that operate under incomplete dominance. Ebony skin gene can come in several allelesresulting in the great variety of human skin tones.
Melanin controls the amount of ultraviolet 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. Loss of body hair in Hominini species is assumed to be related to the emergence of bipedalism some 5 to 7 million years ago.
The emergence of skin pigmentation dates to at about 1. Such Ebony skin likely caused excess UV-B radiation. This favored Ebony skin emergence of skin pigmentation in order to protect from folate depletion due to the increased exposure to sunlight. 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.
This was the genotype inherited by anatomically modern humansbut retained only by part of the extant populations, thus forming an aspect Ebony skin human genetic variation.
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About ,—70, years ago, some Ebony skin 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 cholecalciferol than darker skin, so it would have Ebony skin a health benefit in reduced sunlight if there were limited sources of vitamin D.
The genetic mutations leading to light skin, though partially different among East Asians and Western Europeans suggest Ebony skin two groups experienced a similar selective pressure after settlement in northern latitudes.
There is a long-standing hypothesis that the selection for lighter skin due to higher vitamin D absorption occurred soon after the Out of Africa migration some time before Ebony skin, years ago. A number of researchers disagree with this and suggest that the northern latitudes permitted enough synthesis of vitamin D combined with food sources from hunting to keep populations healthy, and only Ebony skin agriculture was adopted was there a need for lighter skin to maximize the synthesis of vitamin D.
The theory suggests that the reduction of game meat, fish, and some plants from the diet resulted in skin turning light many thousands of years after settlement in Eurasia. Research by Nina Jablonski suggests that an estimated time of about 10, to 20, 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 generations.
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 Ebony skin skin. Most protective functions of the skin, including Ebony skin permeability barrier and the antimicrobial barrier, reside in the stratum corneum SC and the researchers surmise that the SC has undergone the most genetic change since the loss of human body hair.
Natural selection would have favored mutations that protect this essential barrier; one such protective adaptation is the pigmentation of interfollicular epidermisbecause 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 Ebony skin and darkly pigmented peoples. Population and admixture studies suggest a three-way model for the evolution Ebony skin human skin color, with dark skin evolving in early hominids in Africa and light skin evolving partly separately at least two times after modern humans had expanded out of Africa. For the most part, the evolution of light skin has followed different genetic paths in Western and Eastern Eurasian populations.
Two genes however, Ebony skin 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.
The understanding of the genetic Ebony skin 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.
Ebony skin 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 in sub-Saharan Africa and light skin evolving independently in Europe and East Asia after modern Ebony skin had Ebony skin out of Africa. All modern humans share a common ancestor who lived aroundyears ago in Africa. For the most part, the evolution of light skin has followed different genetic paths in 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, years ago. A number of genes have been positively associated with the skin pigmentation difference between European and non-European populations.
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, years ago and perhaps as recently Ebony skin 12, to 6, years ago. 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, years ago, whereas the light skin variant of SLC45A2 spread throughout Europe after 5, years ago. A number of genes known to affect skin color have Ebony skin that show signs of Ebony skin selection in East Asian populations.
Tanning response in humans is controlled by a variety of genes. Oculocutaneous albinism OCA Ebony skin a lack of pigment in the eyes, skin and sometimes hair that occurs in a very small fraction of the population. In hominidsthe 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 production does not peak until after puberty. The skin of children becomes darker as they go through puberty and experience the effects of sex hormones.
In some people, the armpits 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 Ebony skin around the age of 30, without considering the effects of Ebony skin.
Around the same age, women experience darkening of some areas of their skin. Human skin color fades with age.