Indian ancestry revealed by Elie Dolgin (Nature)
"[A] team led by David Reich of the Broad Institute in Cambridge, Massachusetts, and Lalji Singh of the Centre for Cellular and Molecular Biology in Hyderabad, India, has probed more than 560,000 SNPs across the genomes of 132 Indian individuals from 25 diverse ethnic and tribal groups dotted all over India.
"The researchers showed that most Indian populations are genetic admixtures of two ancient, genetically divergent groups, which each contributed around 40-60% of the DNA to most present-day populations. One ancestral lineage — which is genetically similar to Middle Eastern, Central Asian and European populations — was higher in upper-caste individuals and speakers of Indo-European languages such as Hindi, the researchers found. The other lineage was not close to any group outside the subcontinent, and was most common in people indigenous to the Andaman Islands, a remote archipelago in the Bay of Bengal.
"The researchers also found that Indian populations were much more highly subdivided than European populations. But whereas European ancestry is mostly carved up by geography, Indian segregation was driven largely by caste. 'There are populations that have lived in the same town and same village for thousands of years without exchanging genes,' says Reich.
"Indian populations, although currently huge in number, were also founded by relatively small bands of individuals, the study suggests. Overall, the picture that emerges is of ancient genetic mixture, says Reich, followed by fragmentation into small, isolated ethnic groups, which were then kept distinct for thousands of years because of limited intermarriage — a practice also known as endogamy.
"This genetic evidence refutes the claim that the Indian caste structure was a modern invention of British colonialism, the authors say."
The study, published as "Reconstructing Indian Population History" in Nature, also addressed the question "Is the ancestry of tribal groups systematically different from castes?" and concluded that it is not. According to a report in Reuters (September 23, 2009):
"'It is impossible to distinguish castes from tribes using the data,' Kumarasamy Thangaraj of the Center for Cellular and Molecular Biology in Hyderabad, India, who worked on the study, said in a statement.
"'This supports the view that castes grew directly out of tribal-like organizations during the formation of Indian society.'"
In their paper, the researchers say they "find significantly more ANI [Ancestral North Indian] ancestry in traditionally upper than in lower or middle caste groups [...] and find that traditional caste level is significantly correlated to ANI ancestry even after controlling for language [...], suggesting a relationship between the history of caste formation in India and ANI–ASI ["ASI"=Ancestral South Indian] mixture." However, one blogger points out that "that is only with 'all things equal.' Note that upper caste South Indian groups clearly have more ANI than lower caste South Indians, but they have a lower proportion than some North Indian lower castes, and are in the range of one North Indian tribal group." And no group except the isolated Andaman Islanders displayed an unmixed ancestry.
In a commentary in Nature accompanying the study, the geneticist Aravinda Chakravarti writes that this finding "provides a model of how diversity within India came about. As such, its details are imperfect and will surely be contested, revised and improved; but its implications are significant."
The authors of the study themselves make clear:
"We warn that 'models' in population genetics should be treated with caution. Although they provide an important framework for testing historical hypotheses, they are oversimplifications. For example, the true ancestral populations of India were probably not homogeneous as we assume in our model, but instead were probably formed by clusters of related groups that mixed at different times. However, modelling them as homogeneous fits the data and seems to capture meaningful features of history."
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