Human skin color ranges in variety from the 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, Ebony skin exposure to sun.
In evolution, skin pigmentation in human beings evolved by a Ebony skin 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 Ebony skin most important substance is the pigment melanin.
Melanin is produced within 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 light 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 nervous 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 Ebony skin, generally located closer to the equator, tend to have darker-skinned populations.
Areas that are Ebony skin from the tropics and Ebony skin to the poles have lower intensity of UVR, which is reflected in lighter-skinned populations. 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.
Females need more calcium during pregnancy and lactation. The body synthesizes vitamin D from sunlight, which helps it absorb calcium.
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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 and discrimination.
Melanin is produced by cells called melanocytes in a process called melanogenesis. Melanin is Ebony skin 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 Ebony skina brown-black polymer of dihydroxyindole carboxylic acidsand their reduced forms.
Most are derived from the amino acid tyrosine. Eumelanin is found in hair, areolaand 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.
Each 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 Ebony skin 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 Ebony skin. Such conditions likely caused excess UV-B Ebony skin.
This favored the 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 Ebony skin 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 Ebony skin aspect of human genetic variation. About ,—70, years ago, some anatomically modern humans Homo sapiens began to migrate away from Ebony skin 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 represented a Ebony skin 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 Ebony skin the 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 40, years ago. A number of researchers disagree with this and suggest that the northern latitudes Ebony skin enough synthesis of vitamin D combined with food sources from hunting to keep populations healthy, and only when 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 Ebony skin 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 Ebony skin, even in Ebony skin little as generations.
One of the most recently proposed Ebony skin 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 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 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 Ebony skin 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 Ebony skin 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.
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.
Ebony skin populations have different allele frequencies Ebony skin 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 Ebony skin independently in Europe and East Asia after modern humans had expanded 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 Ebony skin 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 Ebony skin the light-skin alleles of these genes in Europeans is comparatively recent, having occurred later than 20, years ago and perhaps as recently as 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. Ebony skin 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 alleles that show signs of positive selection in East Asian populations. Tanning response in humans is controlled by a variety of genes. Oculocutaneous albinism OCA is 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 Ebony skin born pale, regardless of what Ebony skin 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 Ebony skin 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 Ebony skin 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. 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 Ebony skin the interaction between melanocytes and keratinocytes.
It has been observed that adult human females are consistently Ebony skin in skin pigmentation than males in the same population. 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 and rickets.
Natural selection has led to Ebony skin with lighter skin than males in all indigenous populations because women must get enough vitamin D and calcium to support the development of fetus and nursing infant and to maintain their own health. 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. Uneven pigmentation of some sort affects most people, regardless of Ebony skin 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 jaundicecarotenosisor argyriaskin pigmentation disorders generally occur because the body produces either too much or too Ebony skin melanin. Various types of albinism are a result of genetic mutations affecting different parts of the melanin production pathway.
In a person with albinism, melanocytes can be entirely absent, or fail to produce melanin, or melanosomes can fail to mature and be transferred to keranocytes. The ability to produce melanin in patches around the body is a condition known as Ebony skin. Some types of albinism 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. Albinism is a serious problem in areas of high sunlight intensity, leading to extreme sun sensitivity, skin cancerand 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. Albinos often face social and Ebony skin challenges even threatsas the condition is often a source of Ebony skin, racism, fear, and violence.
Many cultures around the world have developed beliefs regarding people with albinism.
Albinos are persecuted in Tanzania by witchdoctorswho use Ebony skin body parts of albinos as Ebony skin in rituals and potions, as they are thought to possess magical power.
Vitiligo is a condition that causes depigmentation of sections of skin. Snow White's been busy lately. This year alone she's starring in two movies while also appearing in a television series.