Modern-day Indians may have descended from three ancestral groups related to Iranian and Central Asian cultures, according to a new study.
These groups include new stone age Iranian farmers, Eurasian Steppe pastoralists and South Asian hunter-gatherers, the study published in the journal Cell noted.
“Most individuals in north and south India show evidence of this three-way mixture related to Neolithic Iranian farmers, Eurasian Steppe pastoralists and South Asian hunter-gatherers,” Priya Moorjani, assistant professor at the Center for Computational Biology, University of California, Berkeley, told Down To Earth.
“In India, genetic and linguistic variation often go hand in hand, shaped by ancient migrations and social practices,” lead author Elise Kerdoncuff of UC-Berkeley, said in a statement.
Through this study, Moorjani and colleagues attempted to answer two key questions: When did people first migrate to India from Africa and what is the contribution from the archaic humans, Neanderthals and Denisovans to Indian populations?
The research included 2,762 individuals from 18 states, representing diverse geographic regions. These encompassed rural and urban areas, speakers of various language families (Indo-European, Dravidian and Tibeto-Burman languages) and historically underrepresented communities (scheduled tribes, scheduled castes and other backward class), providing the most comprehensive snapshot of genetic variation in India.
The data was part of the Longitudinal Ageing Study in India Diagnostic Assessment of Dementia, a population-based prospective genomic cohort study of individuals aged 60 years or older. The scientists generated the whole genome sequences from these individuals to reconstruct the evolutionary history of India over the past 50,000 years at fine scale.
The analysis showed that genetic variation among Indians comes from a single major migration out of Africa that occurred around 50,000 years ago, followed by gene flow (the transfer of genetic material from one population to another) from Neanderthals and Denisovans. This gene flow contributed about 1-2 per cent to the genetic makeup of Indians.
The researchers also found the highest variation in Neanderthal ancestry among non-Africans. “When you compare the Neanderthal segments in two individuals, they are often not shared. This allows us to reconstruct a lot more of the Neanderthal and Denisovan genome and uncover its contribution to immune function and adaptation in India,” Moorjani explained.
The study reconstructed around 50 per cent of the Neanderthal genome and 20 per cent of the Denisovan genome from Indian individuals. Indians, Moorjani explained, harbour similar proportions of genome-wide Neanderthal ancestry as Europeans and Americans at an individual level.
“One possibility is that because of the complex history of South Asians, including multiple mixture events over the past 10,000 years, followed by strong bottlenecks in many groups, individuals in India are a very mosaic of different ancestries,” she added.
The study also highlighted that India experienced a major demographic shift toward endogamy, marriages within the community. Within roughly 2,700 people included in the study, the researchers found that each individual has at least one closer relative.
Indians have elevated levels of homozygosity, which is two- to nine-times higher than East Asians and Europeans. Homozygosity refers to inheriting identical versions of a gene from both parents.
Increased homozygosity can raise the prevalence of harmful genetic variants and increase the risk of recessive diseases caused by inheriting two copies of a mutated gene.
Many variants from Indians are associated with congenital (condition from birth) and blood disorders, metabolic diseases and complex conditions such as cognitive decline and dementia.