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The Genes Behind Human Skin Colour

S u m m a r y :
New genes coding for human skin colour have been discovered by a team of geneticists studying a variety of African groups. The findings are published in the journal Science.

Broad Diversity in Skin Colour

Skin colour elicits many different kind of emotions and responses throughout the different cultures and social groups of the world, and many forget that the beauty of the diversity of skin tones reflects the very nature of this planet—a planet that showcases organisms and landscape features of different hues. What is behind this broad palette of skin colour? One word: genes. However, only few such genes have been unveiled by scientists in studies limited in scope as only European populations participated. So, the recent study, conducted by University of Pennsylvania geneticists, brings in invaluable elements that shed light on the heterogeneity of skin colour.

A researcher measures the skin reflectance of a study participant of Nilo-Saharan ancestry. Photo credits: Tishkoff laboratory.

New Genes Coding For Skin Colour

New genetic variants that are responsible for human skin colour have been identified in a range of African groups—these findings not only help explain the variety of skin colour found in the African continent, but they also bring a new twist to human evolution. Moreover, the research provides a better understanding of genetic risk factors playing a role in conditions like skin cancer.

“Skin color is a classic variable trait in humans, and it’s thought to be adaptive,” says study senior author Sarah Tishkoff. “Analysis of the genetic basis of variation in skin color sheds light on how adaptive traits evolve, including those that play a role in disease risk.”

People might battle all they want over the ‘superiority’ of fair skin over dark skin, but the science behind the diversity points at something else altogether: both light and dark skin tones have their own benefits. For example, while dark skin constitutes a shield against ultraviolet light (UV), light skin is associated with greater synthesis of vitamin D in areas with low UV light exposure.

“We have identified new genetic variants that contribute to the genetic basis of one of the most strikingly variable traits in modern humans,” says Tishkoff.

Genes That Demarcate Light Skin From Dark Skin

Tishkoff explains that African populations display a much broader range of skin tones than one would have thought; while the stereotypical image one has of skin colour in Africa goes along the lines of ‘just having darker skin’, the new results show a “huge amount of variation ranging from skin as light as some Asians to the darkest skin on a global level and everything in between.”

The team reveals genetic variants that are responsible for these traits together with mutations that draw the line between light and dark skin, the latter being an occurrence that has existed since modern humans first originated.

Breaking Down the Range of Skin Colour in Africa

Tishkoff and her team used a colour meter to quantify the reflection of light on the skin of over 2,000 Africans coming from different ethnic and genetically diverse groups, a measurement that allowed them to represent that range of skin pigmentation in Africa. The darkest skin was observed in participants from Nilo-Saharan pastoralist populations in eastern Africa, and the lightest skin was seen in people from San hunter-gatherer populations in southern Africa. The figures also allowed the researchers to determine the levels of skin pigment melanin.

Picture Collage: Alessia Ranciaro and Simon Thompson.

Furthermore, the genetic information of around 1,600 people was obtained, and from this, over 4 million single nucleotide polymorphisms across the genome were analysed; these are regions of the DNA that are different from each other by one nucleotide. The researchers, then, found 4 main areas of the genome where variations thereof correlated with skin colour differences.

The genome piece with the strongest correlations was in and around the gene variant known to be associated with light skin colour in European and some southern Asian populations; this variant is thought to be over 30,000 years of age. It was common in areas of Ethiopia and Tanzania that were once linked with populations of southeast Asia and the Middle East; this suggests that the genetic variant travelled from these regions into Africa, where it was possibly positively selected through evolution.

Interestingly, the next region with the strongest association to skin pigmentation was expressed in low levels in people with vitiligo, a condition characterised by loss of pigmentation in the skin in certain areas.

Mutations in and around this gene that were linked with dark skin colour were also found at high frequencies in people of Nilo-Saharan ancestry, groups who have very dark skin, and in those across sub-Saharan populations, except the San, who tend to have lighter skin. Gene variants associated with dark skin were also found in South Asian Indian and Australo-Melanesian populations, who also have some of the darkest skin, besides Africa.

“The origin of traits such as hair texture, skin color and stature, which are shared between some indigenous populations in Melanesia and Australia and some sub-Saharan Africans, has long been a mystery.” says Tishkoff. “Some have argued it’s because of convergent evolution, that they independently evolved these mutations, but our study finds that, at genes associated with skin color, they have the identical variants associated with dark skin as Africans.

“Our data are consistent with a proposed early migration event of modern humans out of Africa along the southern coast of Asia and into Australo-Melanesia and a secondary migration event into other regions. However, it is also possible that there was a single African source population that contained genetic variants associated with both light and dark skin and that the variants associated with dark pigmentation were maintained only in South Asians and Australo-Melanesians and lost in other Eurasians due to natural selection.”

Also, commenting on skin cancer, Tishkoff explains:

“Africans don’t get melanoma very often. The variants near these genes are highest in populations who live in areas of the highest ultraviolet light intensity, so it makes sense that they may be playing a role in UV protection.”

Tishkoff concludes with the great diversity in Africa.

“Many of the genes and new genetic variants we identified to be associated with skin color may never have been found outside of Africa, because they are not as highly variable,” Tishkoff said. “There is so much diversity in Africa that’s not often appreciated. There’s no such thing as an African race. We show that skin color is extremely variable on the African continent and that it is still evolving. Further, in most cases the genetic variants associated with light skin arose in Africa.”

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