Today, India is home to 1.24 billion people living in at least 700 ethnic groups and hierarchies which largely keep themselves to themselves and rarely marry outsiders. However, a new genetic study has revealed that intermarriage was once so widespread across the entire subcontinent it has left an imprint in the modern genomes of even the most isolated tribes.
An earlier study in 2009 found that most modern Indians descend from 2 major population groups: Ancestral North Indians (ANI), who likely migrated into the subcontinent at least 8,000 more years ago from Eurasia; and Ancestral South Indians (ASI), who were indigenous to the region.
The latest study took a more detailed look at the modern genomes of 571 people representing 73 ethnic groups across the subcontinent (71 from India and two from Pakistan). They found that, from around 4,200 years ago, previously isolated ANI and ASI populations began a period of extensive intermarriage lasting around 2,300 years. This suddenly ended around 1,900 years ago, when local populations became entrenched and endogamous.
The study found this pattern is measurable across the genomes of most modern Indian ethnic groups, even in those considered the most endogamous. For example, the percentage of DNA inherited from ANI populations ranges from 71% in the Pathan people of northern India, to 17% in the Paniya-speaking people of southwest India.
The study’s lead author, geneticist Priya Moorjani, said: “The most remarkable aspect of the ANI-ASI mixture is how pervasive it was, in the sense that it has left its mark on nearly every group in India.”
The results reflect a period of great social upheaval in India, from the decline of the ancient Indus Valley Civilization from around 4,000 years ago, leading to mass internal migration and the eventual rise of the Vedic religion, the historical predecessor of modern Hinduism. India’s rigid caste system appears to become the cultural norm from 1,900 years ago.