Light skin

Light skin is a human skin color that has a low level of eumelanin pigmentation as an adaptation to environments of low UV radiation.
Due to migrations of people in recent centuries, light-skinned populations today are found all over the world. Light skin is most commonly found amongst the native populations of Europe, East Asia, West Asia, Central Asia, South Asia, Siberia, and North Africa as measured through skin reflectance. People with light skin pigmentation are often referred to as "white" although these usages can be ambiguous in some countries where they are used to refer specifically to certain ethnic groups or populations.

Humans with light skin pigmentation have skin with low amounts of eumelanin, and possess fewer melanosomes than humans with dark skin pigmentation. Light skin provides better absorption qualities of ultraviolet radiation, which helps the body to synthesize higher amounts of vitamin D for bodily processes such as calcium development. On the other hand, light-skinned people who live near the equator, where there is abundant sunlight, are at an increased risk of folate depletion. As a consequence of folate depletion, they are at a higher risk of DNA damage, birth defects, and numerous types of cancers, especially skin cancer. Humans with darker skin who live further from the tropics may have lower vitamin D levels, which can also lead to health complications, both physical and mental, including miscarriage and a greater risk of developing schizophrenia. These two observations form the "vitamin D–folate hypothesis", which attempts to explain why populations that migrated away from the tropics into areas of low UV radiation evolved to have light skin pigmentation.

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.

Evolution

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. 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. It is widely supposed that light skin pigmentation developed due to the importance of maintaining vitamin D3 production in the skin. Strong selective pressure would be expected for the evolution of light skin in areas of low UV radiation.

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.

In a 2013 study, Canfield et al. established that ''SLC24A5'' sits in a block of haplotypes, one of which (C11) is shared by virtually all chromosomes that bear the ''A111T'' variant. This "equivalence" between C11 and ''A111T'' indicates that all people who carry this skin-lightening allele descend from a common origin: a single carrier who lived most likely "between the Middle East and the Indian subcontinent". Canfield et al. attempted to date the ''A111T'' mutation but only constrained the age range to before the Neolithic. However, a second study from the same year (Basu Mallick et al.) estimated the coalescent age (split date) for this allele to between ~28,000 and ~22,000 years ago.

The second most important skin-lightening factor in West Eurasians is the depigmenting allele ''F374'' of the ''rs16891982'' polymorphism located in the melanin-synthesis gene ''SLC45A2''. From its low haplotype diversity, Yuasa et al. (2006) likewise concluded that this mutation (''L374F'') "occurred only once in the ancestry of Caucasians".

Summarizing these studies, Hanel and Carlberg (2020) decided that the alleles of the two genes ''SLC24A5'' and ''SLC45A2'' which are most associated with lighter skin colour in modern Europeans originated in West Asia about 22,000 to 28,000 years ago and these two mutations each arose in a single carrier. This is consistent with Jones et al. (2015), who reconstructed the relationship between Near Eastern Neolithic farmers and Caucasus Hunter-Gatherers: two populations which carried the light skin variant of ''SLC24A5''. Analysing newly sequenced ancient genomes, Jones et al. estimated the split date at ~24,000 bp and localised the separation to somewhere south of the Caucasus. However, a coalescent analysis of this allele by Crawford et al. (2017) gave a more narrowly constrained, and earlier, split date of ~29,000 years ago (with a 95% confidence window from 28,000 to 31,000 bp).

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. Lighter skin and blond hair also evolved in the Ancient North Eurasian population.

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. 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. In the San people, it was acquired from interactions with Eastern African pastoralists. Meanwhile, in the case of East Asia and the Americas, a variation of the ''MFSD12'' gene is responsible for lighter skin colour. The modern association between skin tone and latitude is thus a relatively recent development.

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. African, South Asian and Australo-Melanesian populations also carry derived alleles for dark skin pigmentation that are not found in Europeans or East Asians. Huang et al. (2021) found the existence of "selective pressure on light pigmentation in the ancestral population of Europeans and East Asians", prior to the divergence of their ancestors. Skin pigmentation was also found to be affected by directional selection towards darker skin among Africans, as well as lighter skin among Eurasians. Crawford et al. (2017) found evidence for the emergence of alleles associated with lighter and darker pigmentation prior to the origin of modern humans at c. 300kya.

The predominance of lighter skin among East Asians can be traced back to the positive selection for the rs1800414-G allele thought to date back to the Late Palaeolithic period (c. 25–30 kya), following the northward migration of modern humans from South/Southeast Asia. This allele is distinct from that observed among West Eurasians.

The ''A111T'' mutation in the ''SLC24A5'' gene predominates in populations with Western Eurasian ancestry. The geographical distribution shows that it is nearly fixed in all of Europe and most of the Middle East, extending east to some populations in present-day Pakistan and Northern India. It shows a latitudinal decline toward the Equator, with high frequencies in North Africa (80%), and intermediate (40−60%) in Ethiopia and Somalia.

Ancient Populations

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."

Europe

A study from 2015 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."

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). 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. 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.

The Western Steppe Herders, an early Bronze Age population are believed to have also contributed to the skin and hair pigmentation in Europe, having a dominant effect on the phenotypes of Northern Europeans in particular.

Bagnasco, G et al. (2024), discovered that the phenotypic traits for a group of Etruscans from 3,000 to 2,700 years ago showed a population with blue-eyes, light to dark brown hair, and pale white to intermediate skin tones.

Middle-East

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).

An individual from the Pre-Pottery Neolithic in Ain' Ghazal, Jordan had both of the major derived 'European' depigmentation alleles ''(AA, SLC45A2: rs16891982 and SLC24A5: rs1426654)'', while another only had one of the ''SLC24A5'' ancestral genotypes ''(GG)''. It indicated evidence of a more northerly origin for this population, possibly indicating an influx from the region of northeastern Anatolia.

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.

Africa

A research paper published in 2017 indicated Egyptians at Abusir el-Meleq from 2,590 to 2,023 years ago, had a derived variant for the ''SLC24A5'' locus, which contributes to lighter skin pigmentation, and was shown to be at high frequency in Neolithic Anatolia, accordant with the sample's ancestral affinities. Parabon NanoLabs (2021) based on this data from Schuenemann et al. (2017) using whole genome sequencing and advanced bioinformatics, further discovered that these ancient Egyptian samples instead had a light brown complexion, but carried the main gene for light skin. They stated the results were highly consistent with Schueneman et al.'s findings.

In the same year, according to phenotype SNP analysis, the precolonial Guanche inhabitants of the Canary Islands were showing consistent traits such as light and medium skin, with dark hair and brown eyes.

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.

Geographic distribution; ultraviolet and vitamin D

In the 1960s, biochemist W. Farnsworth Loomis suggested that skin colour is related to the body's need for vitamin D. The major positive effect of UV radiation in land-living vertebrates is the ability to synthesize vitamin D3 from it. A certain amount of vitamin D helps the body to absorb more calcium which is essential for building and maintaining bones, especially for developing embryos. Vitamin D production depends on exposure to sunlight. Humans living at latitudes far from the equator developed light skin in order to help absorb more vitamin D. People with light ( type II) skin can produce previtamin D3 in their skin at rates 5–10 times faster than dark-skinned ( type V) people.

In 1998, anthropologist Nina Jablonski

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