Scientists Trace Effect Of Caste System On Indian Population Genetics
Scientists have found genetic evidence that modern-day India is the result of recent population mixture among divergent demographic groups.
Asian Scientist (Aug. 15, 2013) - Scientists have found genetic evidence that modern-day India is the result of recent population mixture among divergent demographic groups.
The findings, published in the American Journal of Human Genetics, describe how India transformed from a country where mixture between different populations was rampant to one where endogamy — that is, marrying within the local community and a key attribute of the caste system — became the norm.
"Only a few thousand years ago, the Indian population structure was vastly different from today," said co–senior author Professor David Reich.
"The caste system has been around for a long time, but not forever."
In 2009, Reich and colleagues published a paper based on an analysis of 25 different Indian population groups. The paper described how all populations in India show evidence of a genetic mixture of two ancestral groups: Ancestral North Indians (ANI), who are related to Central Asians, Middle Easterners, Caucasians, and Europeans; and Ancestral South Indians (ASI), who are primarily from the subcontinent.
However, the researchers wanted to glean clearer data as to when in history such mixture occurred. For this, the international research team broadened their study pool from 25 to 73 Indian groups.
The researchers took advantage of the fact that the genomes of Indian people are a mosaic of chromosomal segments of ANI and ASI descent. Originally when the ANI and ASI populations mixed, these segments would have been extremely long, extending the entire lengths of chromosomes.
However, after mixture these segments would have broken up at one or two places per chromosome, per generation, recombining the maternal and paternal genetic material that occurs during the production of egg and sperm.
By measuring the lengths of the segments of ANI and ASI ancestry in Indian genomes, the researchers were able to obtain precise estimates of the age of population mixture.
Based on these measurements, they inferred that population mixture occurred about 1,900 to 4,200 years ago, depending on the population analyzed.
While all groups in India showed evidence of mixture, population mixture tended to occur more recently for groups in the north compared to groups in southern India. The researchers believe this is because the northern groups had multiple population mixtures.
"This genetic data tells us a three-part cultural and historical story," said Reich.
"Prior to about 4000 years ago there was no mixture. After that, widespread mixture affected almost every group in India, even the most isolated tribal groups. And finally, endogamy set in and froze everything in place."
"The fact that every population in India evolved from randomly mixed populations suggests that social classifications like the caste system are not likely to have existed in the same way before the mixture," said co–senior author Lalji Singh.
"Thus, the present-day structure of the caste system came into being only relatively recently in Indian history."
But once established, the caste system became genetically effective as mixture across groups became very rare.
The article can be found at: Moorjani et al. (2013) Genetic Evidence For Recent Population Mixture In India.
Source: Harvard Medical School; Photo: kholkute/Flickr/CC.
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