I was listening to an interview with David Reich the geneticist who runs the ancient DNA lab at Harvard and he said something about the Indian caste system that stopped me because it is one of those things that you think you understand until you realize the actual evidence is sitting in everybody’s DNA and it tells a story that is nothing like what you were taught. The standard model of Indian history is that the caste system is ancient and maybe even eternal and that it grew organically out of the Vedic period and has been slowly evolving for three or four thousand years but the genetic data says something much more specific and much more startling. What Reich and his team found is that almost everyone in South Asia today sits on a gradient between two ancestral populations which they call Ancestral North Indians and Ancestral South Indians and if you plot everybody on this gradient it looks exactly like what you would see if you sampled African Americans because African Americans are also a population in the middle of mixing between two very different source populations with some people having more European ancestry and some having more West African ancestry and the gradient tells you that mixing happened recently and then stopped. In India the mixing happened and then it froze.

The data shows that the three primary ancestral groups of India came together around the time of the decline of the Harappan civilization about 3800 years ago and there was a local hunter-gatherer population and a farming population and then the steppe pastoralists from the Yamnaya culture who had migrated across Eurasia and when these three groups met there was a period of convulsive mixing where they formed the two mixed populations we call Ancestral North Indians and Ancestral South Indians and then the mixing stopped about 2000 to 3000 years ago and the reason it stopped was the caste system which locked people into endogamous groups that did not intermarry across boundaries and this freezing is visible in the genome because instead of collapsing to a single blended population like what happened in Europe after similar mixing events you see a preserved gradient that has been stable for two thousand years. The DNA gives you a snapshot of the moment the system crystallized and that snapshot has not changed since because people who were in different caste groups simply stopped exchanging genes with each other and the proof is that the gradient is still there just as sharp as it would have been two millennia ago.

The caste system is not a slowly evolving organic thing in this reading and it is not an ancient unchanging tradition either because what the genome shows is a specific historical moment when a society that was mixing freely suddenly locked itself into a rigid structure and preserved the genetic snapshot of that moment for thousands of years and you can read it in the DNA of every Indian alive today even people who think caste does not matter anymore because the signature is in the frequencies and the proportions and the way the ancestry clusters and it does not lie. The same data also reveals a strange exception where the Patel community from Gujarat sits off the main gradient because they have additional ancestry from Central Asia which is a reminder that the freeze was not absolute for every group and that the history of South Asia is even more complicated than the already complicated story the genomes tell.

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