Yet when a certain amount of UV rays penetrates the skin, it helps the human body use vitamin D to absorb the calcium necessary for strong bones. This delicate balancing act explains why the peoples that migrated to colder geographic zones with less sunlight developed lighter skin color. As people moved to areas farther from the equator with lower UV levels, natural selection favored lighter skin which allowed UV rays to penetrate and produce essential vitamin D.
The darker skin of peoples who lived closer to the equator was important in preventing folate deficiency. Measures of skin reflectance, a way to quantify skin color by measuring the amount of light it reflects, in people around the world support this idea. While UV rays can cause skin cancer, because skin cancer usually affects people after they have had children, it likely had little effect on the evolution of skin color because evolution favors changes that improve reproductive success.
There is also a third factor which affects skin color: coastal peoples who eat diets rich in seafood enjoy this alternate source of vitamin D. In studies comparing dark and light-skinned residents of northern cities, paler people had higher vitamin D levels throughout the year. Their less pigmented skin let in more rays. A range of skin colors evolved at different times, in different populations, as human spread across the globe.
In addition to these genetic biological changes , groups have also developed cultural adaptations to deal with variable sunlight. For instance, we can consume diets rich in folate and vitamin D. We can also build shelters, wear clothing and slather sunscreen to block UV rays. Skin color is one of the most obvious and literally superficial ways humans differ.
But the evolutionary story behind this variation is shared: Over the course of human evolution, complexion evolved from light to dark to a continuous gradient, mediated by geography, genes and cultural practices. Register or Log In. The Magazine Shop. Login Register Stay Curious Subscribe. Planet Earth. Studies show that new skin colors evolved as humans moved north and south from equatorial Africa toward higher latitudes.
In a paper by two American medical researchers, Jablonski found evidence linking exposure to strong sunlight with low levels of folate, an essential B vitamin, in the blood.
Other research tied folate deficiency in pregnant women to various birth defects. In men, she learned, folate is vital for sperm production. At the outset, then, living near the equator, all humans would have had dark skin. Why and how did lightly pigmented skin come about? The answer, Jablonski reasoned, involves another key vitamin—and the history of human migration.
And this evolution has implications for our health. Vitamin D, as most of us learned in elementary school, is critical for strong bones and healthy teeth. More recent studies show its value in immune function and for fighting off certain cancers and even heart disease. In tropical climates, enough UV penetrates even dark skin to provide an adequate dose of vitamin D.
However, as our forebears began to migrate, wandering far from the equatorial sun, not enough UV could make its way through the protective melanin. One possible motivation of the skin depigmentation in prehistoric Eurasia is agriculturalization, which led to a switch from vitamin D-rich hunter-gatherer diet to a vitamin D-poor agriculturalist diet, together with the increased danger of folic acid deficiency at higher latitudes [ 14 , ].
Moreover, the selective pressures have kept operating for a long time after they initiated the adaptation of skin color, as some ancestral pigmentations alleles were identified in a Mesolithic European BP , and some adaptive alleles under selection in the ancient Eurasians are still evolving in modern humans [ 98 , 99 , ]. Recent studies on archaic hominins e. Other introgressive genes related to skin phenotypes include HYAL genes, which are associated with cellular responses to UV and are under strong positive selection in East Asians [ ], and those involved in keratin filaments formation [ ].
Although these genes are not direct determinants of skin pigmentation, they, like those pigmentation-related genes, possibly helped modern humans adapt to non-African environments. When drawing conclusions of adaptive introgression, we are actually claiming that Neanderthals could be light-complexioned. This inference is just based on some pigmentation-associated genes or alleles identified in existing modern human populations, since visible phenotypes of Neanderthals and other extinct species are not available.
However, when using some other priory genes as potential clues, different results can be obtained. For instance, the derived state of MC1R , which is responsible for pale skin, presents in Neanderthal individuals from Italy and Spain but is missing in Croatian Neanderthals and Denisova [ ], suggesting skin color variation in the archaic hominins.
In addition, the light skin in Neanderthals and modern Eurasians could also result from convergent evolution, rather than adaptive introgression [ ].
The hypothesis of adaptive introgression seems to predate when modern human became pale — long before the late Mesolithic age, as Neanderthals went extinct around 28, years ago. However, we should reconsider whether the genes affecting skin color in archaic hominins indeed determined skin color in modern humans. Even if this is the case, it is also possible that modern human retained these introgressive variants until they showed some phenotypic effects under some specific strong selective pressures.
Thus, more data resources and analyses are necessary to address this issue in the future. As one of the most obvious changes in the environment after modern human migrated out of Africa to higher latitudes, UV has exerted considerable selective pressures on human skin pigmentation, which can be reflected by selection coefficients of the pigmentation-related genes.
The estimation of selection coefficients largely depends on the genes considered and the methodologies. Beleza et al. For example, the estimates are 0. The selection coefficients estimated for pigmentation genes are best understood in the context of estimates for other recently selected loci. The selection advantages are inferred to be 0.
The selection coefficients for pigmentation genes are among the most strongly selected genes in the human genome, indicating a severe selective pressure caused by UV or some other environmental changes in non-African regions. Although a large number of genes have been identified to contribute to skin color variation, how much could they explain the skin color variation in modern humans?
Is there a gene or variant that has a dominant effect on the skin color? Some genes could possibly play a major role in determining skin color in specific populations. However, there are relatively more genes and variants with smaller effects.
The inheritance mode of skin pigmentation follows an additive model, or at least an incomplete additive model [ 16 , 17 , 47 , 56 , 75 ]. Overall, human skin color is a highly variable and complex trait as a consequence of strong selection pressure and is controlled by multiple genetic loci summarized in Table 1. Skin color adaptation is a complex process because different populations have shared and independent genetic mechanisms involving a large number of genes with different effect advantages on the phenotype.
Skin color adaptation is also a long evolutionary process influenced by various historical, even pre-historical, population genetic events. Current studies provide comprehensive insights into the natural selection process and mechanisms of human skin color variation. A richer resource of high-coverage whole-genome sequences and phenotype data may provide opportunities to further speculate an accurate model of genetic architecture and gene-environment effects, and advance our understanding of skin pigmentation in certain minor ethnic groups, such as hunter-gatherers and highlanders.
We believe that these studies may greatly enrich our knowledge of human evolution history and elucidate the genetic basis of complex traits in humans. Ito S, Wakamatsu K. Quantitative analysis of eumelanin and pheomelanin in humans, mice, and other animals: a comparative review. Pigment Cell Res. PubMed Article Google Scholar.
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