Revision as of 22:08, 11 August 2013

The origin of the caste-system

Genetics Proves Indian Population Mixture

A new study indicates that population admixture in the pre-caste era occurred, shedding light on our understanding of present-day Indian populations

By DAVID CAMERON

August 8, 2013

Harvard

Between 4,000 and 2,000 years ago, intermarriage in India was rampant. Figure by Thangaraj Kumarasamy

Scientists from Harvard Medical School and the CSIR-Centre for Cellular and Molecular Biology in Hyderabad, India, provide evidence that modern-day India is the result of recent population mixture among divergent demographic groups.

The findings, published August 8 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 David Reich, professor of genetics at Harvard Medical School. “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 admixture 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 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 analyzed.

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,” said co-first author Priya Moorjani, a graduate student in Reich’s lab at Harvard Medical School. “This is likely because the northern groups have multiple mixtures.”

“This genetic datatells us a three-part cultural and historical story,” said Reich, who is also an associate member of the Broad Institute. “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, currently of Banaras Hindu University, in Varanasi, India, and formerly of the CSIR-Centre for Cellular and Molecular Biology. “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, 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,” said co–first author Kumarasamy Thangaraj, of the CSIR-Centre for Cellular and Molecular Biology, Hyderabad, India.**

Mohan Rao, Director, CSIR-CCMB said, “CCMB's continuing efforts over a decade on this field had helped in understanding the complexity of Indian population history and social structure, such as caste systems.”

This study was funded by the NIH (GM100233); NSF (HOMINID grant 1032255); a UKIERI Major Award (RG-4772); the Network Project (GENESIS: BSC0121) fund from the Council of Scientific and Industrial Research, Government of India; a Bhatnagar Fellowship grant from the Council of Scientific and Industrial Research of the Government of India; and a J.C. Bose Fellowship from Department of Science and Technology, Government of India.

• , ** Quotes adapted from American Journal of Human Genetics news release.

There was no genetic influx 3,500 years ago

Indians are not descendants of Aryans, says new study

Dinesh C. Sharma | Mail Today | New Delhi,

December 10, 2011 |

India Today

Widely believed theory of Indo-Aryan invasion, often used to explain early settlements in the Indian subcontinent is a myth, a new study by Indian geneticists says.

The origin of genetic diversity found in South Asia is much older than 3,500 years when the Indo-Aryans were supposed to have migrated to India, a new study led by scientists from the Centre for Cellular and Molecular Biology (CCMB), Hyderabad, says. The study appeared in American Journal of Human Genetics on Friday.

The theory of Indo-Aryan migration was proposed in mid-19th century by German linguist and Sanskrit scholar Max Muller.

He had suggested that 3,500 years ago, a dramatic migration of Indo-European speakers from Central Asia played a key role in shaping contemporary South Asian populations and this was responsible for introduction of the Indo-European language family and the caste system in India.

"Our study clearly shows that there was no genetic influx 3,500 years ago," said Dr Kumarasamy Thangaraj of CCMB, who led the research team, which included scientists from the University of Tartu, Estonia, Chettinad Academy of Research and Education, Chennai and Banaras Hindu University.

"It is high time we re-write India's prehistory based on scientific evidence," said Dr Lalji Singh, former director of CCMB. "There is no genetic evidence that Indo-Aryans invaded or migrated to India or even something such as Aryans existed". Singh, vice-chancellor of BHU, is a coauthor.

Researchers analysed some six lakh bits of genetic information in the form of SNPs drawn from DNA of over 1,300 individuals from 112 populations including 30 ethnic groups in India.

The comparison of this data with genetic data of other populations showed that South Asia harbours two major ancestry components. One is spread in populations of South and West Asia, Middle East, Near East and the Caucasus. The second component is more restricted to South Asia and accounts for more than 50 per cent of the ancestry in Indian populations.

"Both the ancestry components that dominate genetic variation in South Asia demonstrate much greater diversity than those that predominate West Eurasia. This is indicative of a more ancient demographic history and a higher long-term effective population size underlying South Asian genome variation compared to that of West Eurasia," researchers said.

"The genetic component which spread beyond India is significantly higher in India than in any other part of world. This implies that this genetic component originated in India and then spread to West Asia and Caucasus," said Gyaneshwar Chaube of University of Tartu, Estonia.

If any migration from Central Asia to South Asia took place, the study says, it should have introduced apparent signals of East Asian ancestry into India. "Because this ancestry component is absent from the region, we have to conclude that if such an event indeed took place, it occurred before the East Asian ancestry component reached central Asia," it said.