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Kumarasamy Thangaraj traveled 840 miles (1,350 km) off of the eastern coast of India by plane, then ship, then six hours by car, then ship again to collect blood samples from an isolated tribe of hunter-gatherers on the Andaman Islands. Their blood, he explained through an interpreter, would help him understand a pivotal moment in India’s genetic history. The tribesmen had never heard of a gene before or an academic study for that matter, and the whole pitch struck them as an interesting diversion from their usual routine of spearfishing.

“They mostly laughed,” Thangaraj says, before they offered up their arms in exchange for food. A few needle pricks later, they returned to their boats to fling short wooden spears into the water with uncanny aim, while Thangaraj made the long journey home to Hyderabad. He deposited the latest samples into a blood bank, alongside another 32,000 samples from his countrymen.

The collective bloodlines at the Centre for Cellular & Molecular Biology, India’s leading genetic-research institute, pose a unique riddle for researchers. On the one hand, geneticists can trace...