doi:10.1038/nindia.2013.116 Published online 29 August 2013
The Indian population got divided into castes and other similar categories around 1900 years before present, according to new genetic research, which makes the interesting inference that till then these people mixed freely and allowed intermarriages within their groups .
At 1.2 billion today, India's population is a collection of numerous groups separated by geography, caste, creed and language. Societal pressures and religious beliefs have ensured that these groups remain largely segregated, especially when it comes to marriage.
The new finding by a group of scientists from India and the US have shown that this segregation was not the norm always. Their genetic analysis reveals that 1900-4200 years before the present time, the Indian population was an intermingling lot. It was only around 1900 years that castes and categories in the population got established and disallowed marriages outside the community. Scientists call this practice of marrying within the group endogamy.
Led by David Reich of Harvard Medical School's genetics department and Lalji Singh, Vice Chancellor of the Banaras Hindu University in Varanasi, the group has been studying genetic ancestry of the Indian population for a while .
In 2009, they found that inhabitants of the Indian mainland had two progenitors or founding populations, called Ancient North Indians(ANI) and Ancient South Indians(ASI). Interestingly, "genetic composition of none of the present day Indians is pure ANI or ASI. Instead, they are a mixture of the two", says Kumarasamy Thangaraj, Deputy Director of Hyderabad-based Centre for Cell and Molecular Biology and one of the authors of the study. The genetic contribution of each founding population to various ethnic groups may vary. The scientists were keen to learn how and when exactly the mixing happened.
The ability of being able to point out which segment of an individual's genome is derived from which founding population helped the scientists date these mixtures. Priya Moorjani, a Ph.D student involved in the work at Reich's lab says, "Originally, when the ANI and ASI populations would have mixed, these segments would have been extremely long, extending the entire lengths of the chromosome. After mixture, these segments would have broken up at a few places per chromosome, per generation during production of sperm and eggs. By measuring the lengths of the ANI and ASI segments in the Indian genomes, we were able to infer the time of the mixture."
Genome samples were collected from 73 diverse ethnic groups in India. "Sampling was done to include many geographical locations, diverse castes and religious groups and individuals belonging to all major language groups," she told Nature India.
For all 73 groups, the date of mixture was found between 1900 and 4200 years before present. Even remote and isolated tribal groups such as Bhil, Chamar and Kallar did not escape this mixing. The scientists say that the mixing was so 'pervasive' that nearly every group in India was affected by it.
After nearly 2000 years of mixing freely, parochialism seems to have set in. Since that time, endogamy has been a prominent feature of the society in India. Some groups like the Vysya from Andhra Pradesh have not experienced any gene flow from outside their group for about 3000 years. Such situations lead to inbreeding which lead to population specific diseases. One such example is the "inability of digesting anesthesia during surgical procedures by the Vysyas", Thangaraj explains.
The researchers are hoping to draw more such correlations to understand why some diseases are more common in certain groups than others. They also hope to understand what brought the ANI and ASI together by studying DNA from ancient human remains. Such studies could be particularly revealing about the geographic distribution of the ANI and ASI prior to their admixture.