Dr. Kumarasamy Thangaraj Kumarasamy Thangaraj takes a blood sample from an Andaman islander, as part of his research into the genetics of India's castes

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.

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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 nearly all bloodlines back to two ancestral groups, one hailing from Africa, the other from Eurasia. These groups mingled, married and swapped genes. A mixture of their genetic material can be found in nearly every person on the subcontinent today.

But at some mysterious point in history, these braided bloodlines began to fray. The population divided along linguistic, religious and tribal lines, to the point where it separated into 4,635 distinct genetic groups. Europe and Asia look positively homogeneous in comparison, says Thangaraj. He and his collaborators at Harvard Medical School wanted to know when exactly the Indian melting pot stopped melting.

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Their finding, recently published in the American Journal of Human Genetics, made waves when it was revealed that genetic mixing ended 1,900 years ago, around the same time the caste system was being codified in religious texts. The Manusmriti, which forbade intermarriage between castes, was written in the same period, give or take a century.

Thangaraj says the study shows only a correlation between the early caste system and the divergence of bloodlines, and whether one caused the other is a debate better left to historians. Nonetheless, it puts a stake in the ground, marking the moment when the belief that one should marry within one’s own group developed into an active practice.

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He also doesn’t want the early signs of a caste system to overshadow another finding of his study — how completely the population mixed 2,000 years ago. He points to the Paliyar tribe in the foothills of southern India. Their villages are inaccessible by car, and outsiders cannot visit them without a government permit. “They’re still in the forest,” says Thangaraj, “but still they have some affinities with other groups. At some point in time, everybody was mixed.”

It’s a point that he stresses to anyone who wants to turn bloodlines into battle lines. On Aug. 15, on India’s independence day, a mob from the Rajput community in Bihar attacked men, women and children in the Dalit community. They beat them with rods, killing one and injuring 54. “Look, we were all brothers and sisters 2,000 years back,” Thangaraj says of this sort of violence, “why are you fighting now?” Although he did observe one notable outlier from the extended family: the spear-wielding fishermen of the Andaman Islands have no trace of the genetic mix that pervades the mainland. Proof that the only the thing that really could have stopped India’s ancestral populations from mixing was an 840-mile schlep to a remote tropical island.

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