Another post about white-skinned native people in America.
Today's post will look into what causes light-colored skin, and address the question of why people can have white skin without having European ancestry (like the Chachapoyas in Peru).
The color of the skin depends on the amount of melanin, a pigment that causes the skin to darken. Melanin plays an important role in protecting the skin from damage caused by ultraviolet (UV) rays from the sun, which can cause burns, and over the years, skin cancer.
Since humans seem to have evolved in the tropical and subtropical zones of Africa, which have high levels of solar radiation, the basal color of human skin was dark as more melanin was necessary to protect the skin in equatorial regions.
But, as our ancestors moved out of Africa and reached temperate and cold areas, which due to their latitude receive lower levels of solar radiation, the dark skin became a problem. Sunlight is necessary to help the body convert a provitamin called 7-dehydrocholesterol, into previtamin D3. This process requires ultraviolet radiaton, and takes place in the skin.
Low levels of vitamin D3 can cause health problems such as ricketts in children, poor calcium absorption (leading to fragile bones), lower immunity, and impaired muscle development. Those who lived in regions with little sunlight and had pale skin produced more vitamin D3 than their dark-skinned counterparts, and had a selective advantage. They survived, mated, and passed this trait to their offspring.
>> Read our Vitamin D fact sheet
So, people who live in Northern Europe inherited a genetic trait that favors sunlight absorption and vitamin D3 production in the overcast, long-winters, short-summers, region they live in which also receives sunlight with a low incidence angle (that weakens it). Their skin is pale.
Now we know that it is due to a mutation unique to European people in the genes SLC24A5, TYR, OCA2, and SLC45A2.
The people living in northern Asia also have pale skin, for the same reasons, but they developed it through a different mutation. This is known as convergent evolution. The same outcome through different genes and adaptations (think about dolphins, and sharks or bats, birds and pterosaurs).
The East Asian gene is still unknown, but rs885479 a site inside the OCA2 gene is thought to play a role in their white skin. But there may be other genes involved. In fact, a paper suggests that this is the case, white skin among Asians may be polygenic, this means that several genes work together to produce this effect.
Below we cite a paper reporting the whitening effects of another gene, the MFSD12 gene in East Asians and Amerindians.
The people who left Africa and lived in areas with high levels of solar radiation (New Guinea, Australia, Tropical India, and also in the Amazon region of South America, all retained the dark-skinned genes, due to selection, as it protects them from sunburns and cancer.
Native Americans
Since Amerindians are said to have originated in Eastern Asia and Siberia, they too should carry the Asian variant in low levels (due to bottle necks and founder effects).
But, as mentioned previously, European discovery and conquest brought diseases that exterminated nearly 90% of the Native Americans, then they introduced millions of African slaves and all groups intermingled causing a genetic mixture that impacted on skin color of the modern populations.
A study conducted in 2023 (by Ang et al.) analyzed the genetics of a group of people known as Kalinago in the Island of Dominica in the Caribbean. These people have a high level of African and Native American ancestry (~87%) so the white skin effect added by their European background is minimized, this helps eliminate it as a confounding factor. It found that European ancestry with the SLC24A5 and SLC45A2 genes reduced skin darkness by only 5 and 4 melanin units while "Native American ancestry had the highest effect on pigmentation and reduced it by more than 20 melanin units." The authors checked the genetics but were not able to identify the genes involved in causing this light-skinned effect (hypopigmentation) in the Amerindians.
The authors used a Melanin Index to classify skin color by measuring the amount of light reflected by the skin on the inside of the upper part of the arm of the subjects. The average value ranged between 20.7 and 79.7 units.
The authors conclude that "Additional Native American hypopigmenting alleles of significant effect size remain to be identified. Previously characterized variants do not explain this difference. It is possible that multiple hypopigmenting variants of small effect sizes are together required to reach Native American and/or East Asian levels of hypopigmentation, individually having insufficient effect to detect in the Kalinago, given our power limitations."
In plain English: they could't identify any genes among the Native ancestry that could account for the skin whitening effect. So, if it is a gene that has a big effect, it hasn't been found, or, if it is the cummulative small effects of many genes, the individual impact is too small to be detected by their methods.
The "previously characterized variants" mentioned above may refer to an earlier paper from 2019 by Adhikari et al., that analyzed sampled of 6,357 Latinamericans and identified a gene linked to lighter-colored skin tone, MFSD12. It is also found in East Asians. The authors suggest that positive selection for this variant began some 10,834 year ago (CI: 5,266–33,801 years ago). Below is the average frequencies of this variant in Asian and Amerindian people, in Africa and West Eurasia it is zero.
The mutation is the SNP rs2240751 on the "the third exon of the major facilitator superfamily domain containing 12 (MFSD12) gene".
The frequency is high in America roughly 30% on average and peaks at 55% among the Quechua. In East Asia it is lower,~18% on average and 37% among Japanese.
Perhaps it was higher among some natives in America leading to whiter skin (like those reported by the Spanish chroniclers in the 1500s as we will see in tomorrow's post).
Dark Skin variants
The 2019 paper also reports a skin-darkening variant: "The derived allele for the index SNP (rs11198112) is associated with darker skin pigmentation, in contrast to the effect of the majority of variants associated with skin pigmentation... The derived allele is segregating at low to moderate frequency across many populations, but reaches its highest frequency (>50%) in Native Amazonians and Melanesians."
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Disclaimer: This post is not intended to promote any notions about racism or discrimination, it is just trying to explain why evolution selected for some pale skin colors in regions with less sunlight and kept dark skin or selected for it in other regions with more sunlight. White, whiteness, dark skin and similar words are used in a scientific manner.
ReplyDeleteIn fact my aim is to separate Europeans from whiteness, and stress that East Asians, Siberians, American Natives also have hypopigmentation genes.