to 40% more sunlight than lightly-pigmented skin, dark skin does not increase the body's internal heat intake in conditions of intense solar radiation. solar radiation heats up the body's surface and not the interior. furthermore, this amount of heat is negligible compared to the heat produced when muscles are actively used during exercise. regardless of skin colour, humans have excellent capabilities to dissipate heat through sweating. half of the solar radiation reaching the earth's surface is in the form of infrared light and is absorbed similarly regardless of skin coloration. in people with naturally occurring dark skin, the tanning occurs with the dramatic mobilization of melanin upward in the epidermis and continues with the increased production of melanin. this accounts for the fact that dark-skinned people get visibly darker after one or two weeks of sun exposure, and then lose their colour after months when they stay out of the sun. darkly-pigmented people tend to exhibit fewer signs of aging in their skin than the lightly-pigmented because their dark skin protects them from most photoaging. skin colour is a polygenic trait, which means that several different genes are involved in determining a specific phenotype. many genes work together in complex, additive, and non-additive combinations to determine the skin colour of an individual. the skin colour variations are normally distributed from light to dark, as it is usual for polygenic traits. data collected from studies on mc1r gene has shown that there is a lack of diversity in dark-skinned african samples in the allele of the gene compared to non-african populations. this is remarkable given that the number of polymorphisms for almost all genes in the human gene pool is greater in african samples than in any other geographic region. so, while the mc1rf gene does not significantly contribute to variation in skin colour around the world, the allele found in high levels in african populations probably protects against uv radiation and was probably important in the evolution of dark skin. skin colour seems to vary mostly due to variations in a number of genes of large effect as well as several other genes of small effect (tyr, tyrp1, oca2, slc45a2, slc24a5, mc1r, kitlg and slc24a4). this does not take into account the effects of epistasis, which would probably increase the number of related genes. variations in the slc24a5 gene account for 20–25% of the variation between dark- and light-skinned populations of africa, and appear to have arisen as recently as within the last 10,000 years. the ala111thr or rs1426654 polymorphism in the coding region of the slc24a5 gene reaches fixation in europe, and is also common among populations in north africa, the horn of africa, west asia, central asia and south asia. health implications skin pigmentation is an evolutionary adaptation to various uvr levels around the world. as a consequence there are many health implications that are the product of population movements of humans of certain skin pigmentation to new environments with different levels of uvr. modern humans are often ignorant of their evolutionary history at their peril. cultural practices that increase problems of conditions among dark-skinned populations are traditional clothing and vitamin d-poor diet. advantages in high sunlight dark-pigmented people living in high sunlight environments are at an advantage due to the high amounts of melanin produced in their skin. the dark pigmentation protects from dna damage and absorbs the right amounts of uv radiation needed by the body, as well as protects against folate depletion. folate is a water-soluble vitamin b complex which naturally occurs in green, leafy vegetables, whole grains, and citrus fruits. women need folate to maintain healthy eggs, for proper implantation of eggs, and for the normal development of placenta after fertilization. folate is needed for normal sperm production in men. furthermore, folate is essential for fetal growth, organ development, and neural tube development. folate breaks down in high intensity uvr. dark-skinned women suffer the lowest level of neural tube defects. folate plays an important role in dna production and gene expression. it is essential for maintaining proper levels of amino acids which make up proteins. folate is used in the formation of myelin, the sheath that covers nerve cells and makes it possible to send electrical signals quickly. folate also plays an important role in the development of many neurotransmitters, e.g. serotonin which regulates appetite, sleep, and mood. serum folate is broken down by uv radiation or alcohol consumption. because the skin is protected by the melanin, dark-pigmented people have a lower chance of developing skin cancer and conditions related to folate deficiency, such as neural tube defects. disadvantages in low sunlight rickets is a condition associated with dark skin. dark-skinned people living in low sunlight environments have been recorded to be very susceptible to vitamin d deficiency due to reduced vitamin d synthesis. a dark-skinned person requires about six times as much uvb than lightly-pigmented persons. this is not a problem near the equator; however, it can be a problem at higher latitudes. for humans with dark skin in climates of low uvr, it can take about two hours to produce the same amount of vitamin d as humans with light skin produce in 15 minutes. dark-skinned people having a high body-mass index and not taking vitamin d supplements were associated with vitamin d deficiency. vitamin d plays an important role in regulating the human immune system and chronic deficiencies in vitamin d can make humans susceptible to specific types of cancers and many kinds of infectious diseases. vitamin d deficiency increases the risk of developing tuberculosis five-fold and also contributes to the development of breast, prostate, and colorectal cancer. the most prevalent disease to follow vitamin d deficiency is rickets, the softening of bones in children potentially leading to fractures and deformity. rickets is caused by reduced vitamin d synthesis that causes an absence of vitamin d, which then causes the dietary calcium to not be properly absorbed. this disease in the past was commonly found among dark-skinned americans of the southern part of the united states who migrated north into low sunlight environments. the popularity of sugary drinks and decreased time spent outside have contributed to significant rise of developing rickets. deformities of the female pelvis related to severe rickets impair normal childbirth, which leads to higher mortality of the infant, mother, or both. vitamin d deficiency is most common in regions with low sunlight, especially in the winter. chronic deficiencies in vitamin d may also be linked with breast, prostate, colon, ovarian, and possibly other types of cancers. the relationship between cardiovascular disease and vitamin d deficiency also suggest a link between health of cardiac and smooth muscle. low vitamin d levels have also been linked to impaired immune system and brain functions. in addition, recent studies have linked vitamin d deficiency to autoimmune diseases, hypertension, multiple sclerosis, diabetes and incidence of memory loss. outside the tropics uvr has to penetrate through a thicker layer of atmosphere, which results in most of the intermediate wavelength uvb reflected or destroyed en route; because of this there is less potential for vitamin d biosynthesis in regions far from the equator. higher amount of vitamin d intake for dark-skinned people living in regions with low levels of sunlight are advised by doctors to follow a vitamin d-rich diet or take vitamin d supplements, although there is recent evidence that dark-skinned individuals are able to process vitamin d more efficiently than lighter-skinned individuals so may have a lower threshold of sufficiency. geographic distribution there is a correlation between the geographic distribution of uv radiation (uvr) and the distribution of skin pigmentation around the world. areas that have higher amounts of uvr have darker-skinned populations, generally located nearer the equator. areas that are further away from the equator and generally closer to the poles have a lower concentration of uvr and contain lighter-skinned populations. this is the result of human evolution which contributed to variable melanin content in the skin to adapt to certain environments. a larger percentage of dark-skinned people are found in the southern hemisphere because latitudinal land mass distribution is disproportionate. the present distribution of skin colour variation does not completely reflect the correlation of intense uvr and dark skin pigmentation due to mass migration and movement of peoples across continents in the recent past. dark-skinned populations inhabiting africa, australia, melanesia, papua new guinea and south asia all live in some of the areas with the highest uv radiation in the world, and have evolved very dark skin pigmentations as protection from the sun's harmful rays. evolution has restricted humans with darker skin in tropical latitudes, especially in non-forested regions, where ultraviolet radiation from the sun is usually the most intense. different dark-skinned populations are not necessarily closely related genetically. before the modern mass migration, it has been argued that the majority of dark-pigmented people lived within 20° of the equator. natives of buka and bougainville at the northern solomon islands in melanesia and the chopi people of mozambique in the southeast coast of africa have darker skin than other surrounding populations. (the native people of bougainville, papua new guinea, have some of the darkest skin pigmentation in the world.) although these people are widely separated they share similar physical environments. in both regions, they experience very high uvr exposure from cloudless skies near the equator which is reflected from water or sand. water reflects, depending on colour, about 10 to 30% of uvr that falls on it. people in these populations spend long hours fishing on the sea. because it is impractical to wear extensive clothing in a watery environment, culture and technology does little to buffer uvr exposure. the skin takes a very large amount of ultraviolet radiation. these populations are probably near or at the maximum darkness that human skin can achieve. more recent research has found that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa. only 100–200 generations ago, the ancestors of most people living today likely also resided in a different place and had a different skin color. according to nina jablonski, darkly-pigmented modern populations in south india and sri lanka are an example of this, having re-darkened after their ancestors migrated down from areas much farther north. scientists originally believed that such shifts in pigmentation occurred relatively slowly. however, researchers have since observed that changes in skin coloration can happen in as little as 100 generations (~2,500 years), with no intermarriage required. the speed of change is also affected by clothing, which tends to slow it down. australia an aboriginal australian man with dark skin the aborigines of australia, as with all other populations outside of africa, are descendants of the out-of-africa wave. genetic evidence has pointed out that the indigenous peoples of australia are genetically dissimilar to the dark-skinned populations of africa and that they are more closely related to other non-african populations. the term black initially has been applied as a reference to the skin pigmentation of the aborigines of australia; today it has been embraced by aboriginal activists as a term for shared culture and identity, regardless of skin colour. melanesia dark-skinned blond boy from vanuatu, melanesia melanesia, a subregion of oceania, whose name means "black islands", have several islands that are inhabited by people with dark skin pigmentation. the islands of melanesia are located immediately north and northeast of australia as well as east coast of papua new guinea. the western end of melanesia from new guinea through the solomon islands were first colonized by humans about 40,000 to 29,000 years ago. in the world, blond hair is exceptionally rare outside europe and west asia, especially among dark-skinned populations. however, melanesians are one of the dark-skinned human populations known to have naturally-occurring blond hair. new guinea buka boys from bougainville island, papua new guinea. people from bougainville have some of the darkest skin tones among humans. the indigenous papuan people of new guinea have dark skin pigmentation and have inhabited the island for at least 40,000 years. due to their similar phenotype and the location of new guinea being in the migration route taken by indigenous australians, it was generally believed that papuans and aboriginal australians shared a common origin. however, a 1999 study failed to find clear indications of a single