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    Genetic study looks for clues to origin of caste system

    A large genetic study of hundreds of people in South Asia has allowed scientists to probe important transition points in the population’s history, pinpointing a signature of cultural changes that occurred as the caste system was put in place in India.

    Researchers have long known that, at some point in history, South Asia was a melting pot for two different groups of people. The clues have been scattered in various fields: the history, language, and ancient farming traditions of South Asia all bore the imprint of different origins.

    But when did these two populations mix, and when did they stop?

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    Harvard Medical School professor of genetics David Reich specializes in analyzing genetic information from modern people to understand how populations mixed in the past.

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    Now, in a partnership with researchers in Hyderabad, India, Reich has examined hundreds of thousands of regions in people’s genomes and found evidence that the northern and southern populations mixed around 1,900 to 4,200 years ago. That period was well after the arrival of agriculture in the region and around the same time as Indo-European languages began to be used, the researchers reported Thursday in the American Journal of Human Genetics.

    “From genetic data, remarkably, you see this picture emerging of cultural change,” Reich said. The population mixture didn’t happen in pockets — it was a profound mixing that has left traces in the DNA of people in all areas of India today. But that came to an abrupt halt around 2,000 years ago, likely due to the implementation of the caste system, Reich said.

    Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.