A recent study has revealed that the caste system prevalent in Indian society is the result of a recent population mixture among divergent demographic groups.
Scientists from CSIR-Centre for Cellular and Molecular Biology (CCMB) and Harvard Medical School, Boston, USA have provided evidence that modern-day India is the result of a recent population mixture among divergent demographic groups. It shows evidence that modern-day India is an admixture of various groups.
The findings, published in the American Journal of Human Genetics on Thursday, describe how India transformed itself 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.
In 2009 the same team had published a paper in Nature, based on an analysis of 25 different Indian population groups. The paper described that in the pre-historic India, there were only two ancestral populations; Ancestral North Indians (ANI), who were related to Central Asians, Middle Easterners, Caucasians and Europeans; and Ancestral South Indians (ASI), who were primarily from the subcontinent.
Further, they have demonstrated that all contemporary populations in India show evidence of a genetic admixture of the above two ancestral (ANI and ASI) groups. However, at that point of time, they could not establish the precise date of admixture.
“We now want to establish the clear evidence as to when in history did such admixture occur. For that we have studied about one million genetic markers in 73 additional Indian populations, predominantly represented by Dravidian and Indo-European speakers” Kumarasamy Thangaraj, a senior scientist at CSIR-CCMB said.
The researchers took advantage of the fact that the genomes of Indian people are a mosaic of chromosomal segments of ANI and ASI ancestry. Originally, when the ANI and ASI populations mixed, these segments would have been extremely long, extending the entire lengths of chromosomes. However, after admixture 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 chromosome segments of ANI and ASI ancestry in Indian genomes, the authors were thus able to obtain precise estimates of the age of population mixture, which they infer varied about 1,900 to 4,200 years, depending on the population analysed.
“Only a few thousand years ago was the Indian population structure vastly different from today,” says co-senior author David Reich, professor of genetics at Harvard Medical School.
“The caste system has been around for a long time but not forever. “Prior to about 4,000 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,’’ he said.
“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, currently vice-chancellor of Banaras Hindu University in Varanasi and former director of CCMB. “Thus, the present-day structure of the caste system came into being only relatively recently in Indian history.”
While the findings show that no groups in India are free of such mixture, the researchers did identify a geographic element. “Groups in the north tend to have more recent dates and southern groups have older dates, This is likely because the northern groups have multiple mixtures,” co-first author Priya Moorjani said.
But once established, the caste system became genetically effective, the researchers observed. Mixture across groups became very rare.
“An important consequence of these results is that the high incidence of genetic and population-specific diseases that is characteristic of present-day India is likely to have increased only in the last few thousand years when groups in India started following strict endogamous marriage,” Thangaraj added.