Bangalore: In one of the boldest attempts to reconstruct the population history of the country, which has so far held that north Indians descended from the Aryans and south Indians from Dravidian speakers, researchers now say nearly all Indians are a mixture of two ancestral groups which predate the arrival of Indo-European and Dravidian speakers in the country.
Map: Sandeep Bhatnagar / Mint
In Thursday’s issue of
Nature, scientists from the Centre for Cellular and Molecular Biology (CCMB) in Hyderabad, Harvard Medical School and Broad Institute at Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, report that different Indian groups have inherited 40-80% of their ancestry from a population which they call Ancestral North Indians (ANI) and the rest from a population called Ancestral South Indians (ASI).
Genetically, ANIs are closest to present-day Europeans, and ASIs to the disappearing Andamanese tribe of Onge, which CCMB scientists, in another study, have shown to be similar to the first humans that migrated out of Africa some 65,000 years ago.
The present research’s claim is indeed grandiose, but the scientific evidence is compelling and it’s a “bold idea”, says Aravinda Chakravarti, a professor at the McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine in Baltimore. “At least other samples will allow acceptance or rejection of this hypothesis.”
Calling the Indian set of ancestries akin to the “many hands that can be dealt from a deck of cards”, Chakravarti says the diversity in Indian population is due to the varying proportions of ancestry and specific genomic content that is inherited.
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The “gradient” in genetic diversity that these researchers have captured, showing the decrease in Central Asian ancestry as one goes from the north to south, corroborates what historians have proposed earlier, but that doesn’t necessarily mean that there’s an ASI population, says Analabha Basu, a scientist at the National Institute of Biomedical Genomics at Kalyani in West Bengal.
“This study is too neat to be true; I’d need more evidence to believe the findings.”
Researchers studied the genomes of 132 Indians from 25 population groups that represented all six language families across 15 states and included traditionally “upper” and “lower” castes and tribal groups.
Analysis of 500,000 genetic markers—random mutations that serve as milestones—using extensive statistical tools, shows that diversity within India is three-four times higher than that seen within Europe.
Based on this, they suggest that many groups in modern India descend from a small number of founding individuals and have been genetically isolated from other groups.
In science, it is called a “founder event”; in plainspeak, this means certain types of genetic diseases, particularly single gene disorders, will be more common, just as breast cancer is more common in the Parsi community due to inbreeding.
In another instance, the Vaishya community in Andhra Pradesh lacks an enzyme that metabolizes anaesthesia and hence people end up with complications, says Kumarasamy Thangaraj, a co-author from CCMB. In Europe, Finns and Ashkenazi Jews have experienced founder events, and have a high rate of single-gene (or recessive) diseases as a result.
The surprise in India is that such a large proportion of the population is expected to have a similar effect, says David Reich, a co-author from the department of genetics at Harvard Medical School. “As far as I know, this is not generally appreciated by geneticists as a major cause of recessive diseases in India, but our findings suggest that it will be a much more important cause of recessive diseases than consanguinity (marriages between close relatives), which is the focus of many Indian genetic studies at present.”
The widespread history of founder events, says co-author Lalji Singh from CCMB, helps explain why the incidence of genetic diseases among Indians is different from the rest. “This also indicates that many drugs tested on the Western population will not be as effective on the Indian population,” says Singh.
For the first time, dealing with prehistoric India through genetic tools, researchers also show that the ANI and ASI theory applies to traditional tribes as well as castes; one cannot be distinguished from the other.
“The genetics proves that they are not systematically different and supports the view that castes grew directly out of tribal-like organizations during the formation of Indian society,” says Thangaraj.
India did not participate in any global human genome diversity project, including the HapMap. If it had, say experts, some of these findings would have come sooner.
“African, East Asian, European, and American populations have participated in these international genetic projects, and as a result are further ahead in terms of medical genetic technology than India,” says Reich.
India occupies centre stage in human evolution, says Basu, and this study will put many things in perspective.
For the researchers though, the next stage will involve going back in history to see when this admixture of ANI and ASI took place.
Reich says in principle the genetic data contains this information, but they haven’t succeeded in cracking it. “One complication is that the ANI-ASI mixture may have occurred at different times in different places in India, and possibly reflects multiple historical mixture events in the history of each group.”