Light-colored Skin May Have Evolved Because It
Kenneth
Light skin is a human skin color that has a low level of eumelanin pigmentation as an adaptation to environments of low UV radiation.[1][2]Due to migrations of people in recent centuries, light-skinned populations today are found all over the world.[2][3] Light skin is most commonly found amongst the native populations of Europe, East Asia,[4][5][6] West Asia, Central Asia, and Siberia as measured through skin reflectance.[7] People with light skin pigmentation are often referred to as "white"[8][9] although these usages can be ambiguous in some countries where they are used to refer specifically to certain ethnic groups or populations.[10]
The distribution of light-skinned populations is highly correlated with the low ultraviolet radiation levels of the regions inhabited by them. Historically, light-skinned populations almost exclusively lived far from the equator, in high latitude areas with low sunlight intensity.[17]
It is generally accepted that dark skin evolved as a protection against the effect of UV radiation; eumelanin protects against both folate depletion and direct damage to DNA.[2][20][21][22] This accounts for the dark skin pigmentation of Homo sapiens during their development in Africa; the major migrations out of Africa to colonize the rest of the world were also dark-skinned.[23] It is widely supposed that light skin pigmentation developed due to the importance of maintaining vitamin D3 production in the skin.[24] Strong selective pressure would be expected for the evolution of light skin in areas of low UV radiation.[15]
After the ancestors of West Eurasians and East Eurasians diverged more than 40,000 years ago, lighter skin tones evolved independently in a subset of each of the two populations. In West Eurasians, the A111T allele of the rs1426654 polymorphism in the pigmentation gene SLC24A5 has the largest skin lightening effect and is widespread in Europe, South Asia, Central Asia, the Near East and North Africa.[25]
The light skin variants of SLC24A5 and SLC45A2 were present in Anatolia by 9,000 years ago, where they became associated with the Neolithic Revolution. From here, their carriers spread Neolithic farming across Europe.[30] Lighter skin and blond hair also evolved in the Ancient North Eurasian population.[31]
A further wave of lighter-skinned populations across Europe (and elsewhere) is associated with the Yamnaya culture and the Indo-European migrations bearing Ancient North Eurasian ancestry and the KITLG allele for blond hair. Furthermore, the SLC24A5 gene linked with light pigmentation in Europeans was introduced into East Africa from Europe over five thousand years ago. These alleles can now be found in the San, Ethiopians, and Tanzanian populations with Afro-Asiatic ancestry.[25][32][33] The SLC24A5 in Ethiopia maintains a substantial frequency with Semitic and Cushitic speaking populations, compared with Omotic, Nilotic or Niger-Congo speaking groups. It is inferred that it may have arrived into the region via migration from the Levant, which is also supported by linguistic evidence.[34] In the San people, it was acquired from interactions with Eastern African pastoralists.[35] Meanwhile, in the case of East Asia and the Americas, a variation of the MFSD12 gene is responsible for lighter skin colour.[31] The modern association between skin tone and latitude is thus a relatively recent development.[23]
According to Crawford et al. (2017), most of the genetic variants associated with light and dark pigmentation appear to have originated more than 300,000 years ago.[36] African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians.[32] Huang et al. (2021) found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to their divergence from each other. Skin pigmentation was also found to be affected by directional selection towards darker skin among Africans, as well as lighter skin among Eurasians.[37] Crawford et al. (2017) similarly found evidence for selection towards light pigmentation prior to the divergence of West Eurasians and East Asians.[32]
Some authors have expressed caution regarding the skin pigmentation SNP predictions in early Paleolithic groups. According to Ju et al. (2021): "Relatively dark skin pigmentation in Early Upper Paleolithic Europe would be consistent with those populations being relatively poorly adapted to high-latitude conditions as a result of having recently migrated from lower latitudes. On the other hand, although we have shown that these populations carried few of the light pigmentation alleles that are segregating in present-day Europe, they may have carried different alleles that we cannot now detect. As an extreme example, Neanderthals and the Altai Denisovan individual show genetic scores that are in a similar range to Early Upper Paleolithic individuals, but it is highly plausible that these populations, who lived at high latitudes for hundreds of thousands of years, would have adapted independently to low UV levels. For this reason, we cannot confidently make statements about the skin pigmentation of ancient populations."[38]
In 2015, it was discovered that 13,000 year old samples of Caucasus Hunter Gatherers (CHG) from Georgia carried the mutation and derived alleles for very fair skinned pigmentation similar to Early Farmers (EF). This trait was said to have a relatively long history in Eurasia and risen to high frequency during the Neolithic expansion, with its origin probably predating the Last Glacial Maximum (LGM).[39]
In the same year, a study found that genes contributing to fair skin were nearly fixed in the Anatolian Neolithic Farmers: "The second strongest signal in our analysis is at the derived allele of rs16891982 in SLC45A2, which contributes to light skin pigmentation and is almost fixed in present-day Europeans but occurred at much lower frequency in ancient populations. In contrast, the derived allele of SLC24A5 that is the other major determinant of light skin pigmentation in modern Europe appears fixed in the Anatolian Neolithic, suggesting that its rapid increase in frequency to around 0.9 (90%) in Early Neolithic Europe was mostly due to migration."[40]
In 2018, a study was released showing many late Mesolithic Scandinavians from 9,500 years ago in Northern Europe had blonde hair and light skin, which was in contrast to some of their contemporaries, the darker Western Hunter Gatherers (WHG).[41] However, a 2024 paper found that phenotypically most of their studied WHG individuals carried the dark skin and blue eyes characteristic of WHGs, but some other WHGs in France they sequenced also had pale to intermediate skin pigmentation.[42] Another entry in 2018, showed that the Eastern Hunter Gatherers (EHG), Scandinavian Hunter Gatherers (SHG), and the Baltic foragers, all had the derived alleles for light skin pigmentation.[43]
A study on the populations of the Chalcolithic Levant (6,000-7,000 years ago), found that an allele rs1426654 in the SLC24A5 gene which is one of the most important determinants of light pigmentation in West Eurasians, was fixed for the derived variants in all Levant Chalcolithic samples, suggesting that the light skinned phenotype may have been common in the community. The individuals also had high incidence of genomic markers associated with blue-eye color.[44][45]
A paper conducted by Fregel, Rosa et al. (2018) showed that in North Africa, Late Neolithic Moroccans had the European/Caucasus derived SLC24A5 mutation and other alleles and genes that predispose individuals to lighter skin and eye colours.[46]
In 1998, anthropologist Nina Jablonski and her husband George Chaplin collected spectrometer data to measure UV radiation levels around the world and compared it to published information on the skin colour of indigenous populations of more than 50 countries. The results showed a very high correlation between UV radiation and skin colour; the weaker the sunlight was in a geographic region, the lighter the indigenous people's skin tended to be. Jablonski points out that people living above the latitudes of 50 degrees have the highest chance of developing vitamin D deficiency. She suggests that people living far from the equator developed light skin to produce adequate amounts of vitamin D during winter with low levels of UV radiation. Genetic studies suggest that light-skinned humans have been selected for multiple times.[52][53][54]
Polar regions of the Northern Hemisphere receive little UV radiation, and even less vitamin D-producing UVB, for most of the year. These regions were uninhabited by humans until about 12,000 years ago. (In northern Fennoscandia at least, human populations arrived soon after deglaciation.)[55] Areas like Scandinavia and Siberia have very low concentrations of ultraviolet radiation, and indigenous populations are all light-skinned.[2][48]
However, dietary factors may allow vitamin D sufficiency even in dark skinned populations.[56][57] Many indigenous populations across Northern Europe and Northern Asia survive by consuming reindeer, which they follow and herd. Reindeer meat, organs, and fat contain large amounts of vitamin D which the reindeer get from eating substantial amounts of lichen.[58] Some people of the polar regions, like the Inuit (Eskimos), retained their dark skin; they ate Vitamin D-rich seafood, such as fish and sea mammal blubber.[59]
Furthermore, these people have been living in the far north for less than 7,000 years. As their founding populations lacked alleles for light skin colour, they may have had insufficient time for significantly lower melanin production to have been selected for by nature after being introduced by random mutations.[60] "This was one of the last barriers in the history of human settlement," Jablonski states. "Only after humans learned fishing, and therefore had access to food rich in vitamin D, could they settle regions of high latitude." Additionally, in the spring, Inuit would receive high levels of UV radiation as reflection from the snow, and their relatively darker skin then protects them from the sunlight.[2][15][11]
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