For thousands of years, the forefathers of contemporary Indians interbred among four broad ancestral populations. This interbreeding dipped suddenly around 1,600 years ago, coinciding with the Gupta reign, according to a study at National Institute of Biomedical Genomics (NIBMG) in Kalyani, West Bengal.
It was probably during the Gupta period that rules prohibiting inter-caste marriage were laid down, infer the authors of the study, which analysed DNA samples of 367 unrelated Indians belonging to 20 population groups, different castes and different parts of India.
“Though the caste system originated way before the Gupta reign, the rules that prohibit marriage outside one’s caste were probably laid down during this era, when the Dharamshastra was written,” said Analabha Basu, an assistant professor who, with Neeta Sarkar-Roy and NIBMG director Partha P Majumder, co-authored the study, “Genomic reconstruction of the history of extant populations of India reveals five distinct ancestral components and a complex structure”, which suggests the interbreeding happened mainly among four of these five populations.
The Dharmashastras are ancient books prescribing moral laws and principles for religious duty and righteous conduct for followers of Hinduism. It is difficult, however, to assess how seriously these laws were enforced by the ruling classes among all sections of society, the authors said, going by the complex nature of society in earlier times.
“I think the abrupt dip in the inter-mixture of castes, as seen in our study, was caused by some social, cultural process. We found out this happened about 1,575 years ago, give or take 10 to 15 years. We looked up history and found that the period coincided with the Gupta reign,” said Majumder.
A scholar on this very period had a contrary view. Parjanya Sen, who is pursuing his PhD on the Gupta period from the Centre for Studies on Social Sciences in Kolkata, said the Gupta kings were great patrons of Buddhism during that particular era. “It’s very unlikely that they would have imposed a caste system in all its rigidity as Buddhism doesn’t believe in a caste system,” Sen said.
Majumder and Basu agreed that their data are based on estimates. “I have traced the dip in interbreeding back to 70 generations and I took the span of each generation to be 22.5 years,” Basu said. “But American scientists take it to be 29 years. This might push the timeline back by about 150 years.”
Earlier studies by American and Indian scientists have identified two major ancestral populations, the ancestral North Indians and the ancestral South Indians, as having given birth to the diverse population of present-day India. The new study counts five such populations, the additional three being an ancestral Tibeto-Burman group, an ancestral Austro-Asiatic and an ancestral Andamani group. It is largely among the mainland ancestral groups — North Indian, South Indian, Tibeto-Burman and Austro-Asiatic — that interbreeding took place before the dip, the authors said.
“Indians are primarily endogamous, which means that they marry within a specific ethnic group, but there is a varying degree of marriage outside ethnic groups too,” Basu said. “In this scenario, we needed a vast array of samples to ensure that our study was as foolproof as it could be.”
The study took samples from Khatris of northern India, Brahmins of Bengal and Gujarat, Iyers and other Dravidian speakers of the south, Marathas of Maharashtra and several tribes of central and southern India, as well as Andaman and Nicobar Islands.
“Your genomic makeup will reveal the imprint of your ancestral population,” Basu said. “If you are a Brahmin from North India, your genetic makeup will most likely have footprints of Ancestral North Indians primarily. If you are a tribal from Chhattisgarh, your genetic makeup will primarily have footprints of the Ancestral Austro Asiatic group.”
The study found that ancient North Indian males, who belonged to higher castes, would breed with other population groups, but it rarely happened the other way round. “We have found traces of their footprints in the makeup of people from different population groups. This might be because of patriarchy,” Basu said.
However, he said, most present-day Indians have traces of all four primary ancestral populations. “The proportions may vary,” he said, “but a Brahmin has the same genetic makeup as a Dalit.”