The mixing of groups in India predates the caste system, but castes have shaped the genes of modern Indian populations.

The caste system in India has long been a source of controversy, just not for as long as we once imagined, according to a new genetic analysis.

Research published in the American Journal of Human Genetics shows that the mixing of different cultural groups in India occurred between 4,200 and 1,900 years ago but started to decline as people began marrying only within their social castes, a much more recent development. Still, this means modern-day Indians share connections to all the groups that intermarried in the distant past.

“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 study co-senior author Dr. Lalji Singh of Banaras Hindu University in Varanasi, India in a press release. “Thus, the present-day structure of the caste system came into being only relatively recently in Indian history.”

Most Indian groups, as the study explains, are descended from two separate populations: Ancestral North Indians (ANI), who are related to Central Asians, Middle Easterners, Caucasians, and Europeans; and Ancestral South Indians (ASI), whose roots are mostly confined to the subcontinent.

The caste system creates a hierarchy among four social groups in India, called varnas. The varnas, in descending order, are the Brahmanas, Kshatriyas, Vaisyas, and Sudras. The caste system curtailed intermarriage and led to discrimination, especially among the lower castes.

The researchers can’t say for certain what demographic events solidified the caste system, but according to co–first author Priya Moorjani, a graduate student at Harvard Medical School in Boston, there are some clues in Indian literature.

“The caste system grouped people based on occupational roles, so neither linked to genotype or phenotype,” she explained. “Evidence for this comes from the study of ancient Indian texts such as Rig Veda. The bulk of the Rig Veda mentions a society with substantial movement across groups. The four-class system…is first mentioned in the appendix (book 10) of the Rig Veda that was likely composed at a much later time. However, the caste system of endogamous marriages was first mentioned in the Law code of Manu or Manusmriti that forbade marriages across caste groups.”

Researchers used genome-wide data from 571 individuals from 73 ethno-linguistic groups in South Asia, including 71 Indian and two Pakistani groups. All the groups in the study are referred to as Indian.

Because the scientists determined that mixture between the populations did occur before 1,900 years ago, the researchers were able to assert that “all groups in mainland India are admixed,” even if today populations are less diverse because of the eventual increase in endogamy, or marriage only within certain groups.

The researchers also found that the estimated dates aligned with clues in geography and language, with groups that spoke Indo-European languages in the north admixing, or interbreeding, before groups that spoke Dravidian languages in the south.

Because intermarriage declined sharply around 2,000 years ago, Indian people today face specific genetic health issues.

“An important consequence of these results is that the high incidence of genetic and population-specific diseases 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 Dr. Kumarasamy Thangaraj of the Council of Scientific and Industrial Research’s Centre for Cellular and Molecular Biology in a press release.

The genetic analysis does point to some intriguing conclusions about the growth of Indian society, but perhaps even more incredible are the implications on a human level. Be it in Asia or the United States, modern-day humans are the result of thousands of years of group interactions.

“The most remarkable aspect of the ANI-ASI mixture is how pervasive it was,” added Moorjani in a press release. “It affected not just traditionally upper-caste groups, but also traditionally lower-caste and isolated tribal groups, all of whom are united in their history of mixture in the past few thousand years.”

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