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Whatever happened to the "type de Mechta" or the Mechta-Afalou?
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[QUOTE]Originally posted by Supercar: [QB] "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." - Gregory Barsh [IMG]http://biology.plosjournals.org/archive/1545-7885/1/1/figure/10.1371_journal.pbio.0000027.g002-M.jpg[/IMG] ^^Understand that various factors also play into the skin pigmentation variability, in addition to effects of UV irradiation; things like level of access to vitamin D, not to mention when a given group migrated to or else has been living in the said latitudes or environments: "...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 [b]becomes a liability in more polar latitudes due to vitamin D deficiency (Murray 1934).[/b] 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. [b]An oft-cited anecdote in support of the vitamin D hypothesis is that Arctic populations whose skin is relatively [i]dark given their latitude[/i], such as the Inuit and the Lapp, have [i]had a diet that is historically rich in vitamin D.[/i][/b] 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." - Gregory S. Barsh, [i]associate professor of Departments of Genetics and Pediatrics and an associate investigator at the Howard Hughes Medical Institute, Stanford University School of Medicine, Stanford, California, United States.[/i] [IMG]http://biology.plosjournals.org/archive/1545-7885/1/1/figure/10.1371_journal.pbio.0000027.g001-M.jpg?SSImageQuality=Full[/IMG] Above figure: Biochemistry and Histology of Different Skin Types "(A) Activation of the melanocortin 1 receptor (MC1R) promotes the synthesis of eumelanin at the expense of pheomelanin, although oxidation of tyrosine by tyrosinase (TYR) is required for synthesis of both pigment types. The membrane-associated transport protein (MATP) and the pink-eyed dilution protein (P) are melanosomal membrane components that contribute to the extent of pigment synthesis within melanosomes. (B) There is a gradient of melanosome size and number in dark, intermediate, and light skin; in addition, melanosomes of dark skin are more widely dispersed. This diagram is based on one published by Sturm et al. (1998) and summarizes data from Szabo et al. (1969), Toda et al. (1972), and Konrad and Wolff (1973) based on individuals whose recent ancestors were from Africa, Asia, or Europe." - Courtesy of G. Barsh; used in his article. [/QB][/QUOTE]
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