‘Light skin’ gene mirrors socio-cultural boundaries in Indian population

The genetic mutation in SLC24A5 is known to be pivotal in the evolution of light skin, and is responsible for a significant part of the skin colour differences between Europeans and Africans.

Now, a new study has examined for the first time a large, uniform genetic sample collected directly in south India, and suggests that natural selection is not the sole factor in skin tone variation across the Indian sub-continent, and that cultural and linguistic traits still delineate this skin pigment genetic mutation. 

The results show that the gene is found with much higher frequency in Indo-European speaking groups that are more prevalent in the north-west of the country.

But the mutation is also high in populations groups known to have migrated north to south, such as the Saurashtrians, who - while native to Gujarat in north-west India - are now predominantly found in the Madurai district in its southernmost tip.

Researchers say that the study, published last week in the journal PLoS Genetics, shows that the genetic mutation in SLC24A5 has a common origin between Europeans and Indians.

But while the complete dominance of the gene in Europeans is likely to be solely down to natural selection, they say, the rich diversity of this genetic variant in India - high in some populations while non-existent in others, even neighbouring ones - has some correlation with factors of language, ancestral migration and distinct social practices such as limiting marriage partners to those with specific criteria.

The researchers say the findings display an “intriguing interplay” between natural selection and the “unique history and structure” of populations inhabiting the Indian subcontinent.         

“In India, this genetic variant doesn’t just follow a ‘classical’ theory of natural selection - that it’s lower in the south where darker skin protects against fiercer sunlight,” said study co-author Mircea Iliescu from Cambridge’s Biological Anthropology Division.

“The distribution of the SLC24A5 genetic variant in India follows patterns very much influenced by population. Understanding the genetic architecture behind the remarkable skin colour variation found today in the populations of India has the potential to shed light on the wider mechanisms responsible for creating diversity throughout human evolution,” Iliescu said.

In the 1950s it was proposed that there was a massive wave of European migration into northern India a few thousand years ago, described as the ‘Aryan invasion’, which led to the collapse of the Harrapan Civilisation - a Bronze-Age Civilisation.

This theory, now considered widely discredited by many researchers due to the lack of archaeological evidence, is still a hugely debated issue in contemporary Indian politics - invoked by political parties in the southern states of India who claim that the southern populations, described by some as the ‘Dravidians’, are the truly indigenous people of India.

The researchers say that, while speculative, they find it “hard to imagine” a large-scale population migration at a single point in history based on this study – since the presence of this genetic mutation is too widespread, with an average frequency of 53%, including the Austroasiatic language groups thought to have originated in southeast Asia.

They say the wide variation and complex pattern hints at the possibility of multiple “gene flows” into the sub-continent over a much longer period of time, some of which might be linked to the spread of agriculture; although the study does show higher frequencies of SLC24A5 in Indo-European speaking groups compared to so-called Dravidian populations.

The researchers suggest that aspects of ‘social selection’, such as high levels of ‘endogamy’ - marriage within a particular group in accordance with custom - as a result of the caste system, has created a “mosaic pattern” for this skin pigmentation mutation across Indian populations.

“This study helps us to understand various other mechanisms that could have contributed or shaped the existing biological spectrum of human skin colour besides natural selection - driven by ultraviolet rays - and further understanding of this complex phenotypic trait,” said Chandana Basu Mallick, a co-author on the study from the University of Tartu in Estonia.

“We are taking gradual steps towards understanding the evolutionary history of this adaptive trait, and the journey of our ancestors from fur to the diverse skin tones of the present day.”

“Our work addresses human diversity, diversity which should be celebrated,” added Iliescu. “It tries to explain the origins and history of this diversity - opening up a window into a different kind of history, not just a history of places and objects, but a living history which helps us to better understand ourselves.”

“Studies on Indian populations have been under-represented in the genomic era, and the understanding of Indian genetics is still at a very early stage. With this study, we hope we’ve brought valuable new understanding to the evolutionary genetics of Indian populations.”