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South Asia is home to one of the world’s most diversified populations where around 1.5 billion people, a mash-up of many ethnic identities, languages, religions, castes, and customs reside. The scientists have now revealed a thorough analysis of this population’s evolutionary history.
According to a preprint published on bioRxiv last month as reported by Science.com, the whole-genome study from South Asia provides fresh insights into the origins of India’s Iranian ancestors and the timing of their settlement by prehistoric hunter-gatherers. The study also revealed the high diversity of genes from Neanderthals and Denisovans. Due to the lack of relics of these extinct human ancestors in India, scientists are theorising as to how these genes got there and why they persisted.
According to population geneticist Kelsey Witt of Clemson University, global genetic sequencing efforts have largely ignored India. “We’re learning a lot about populations that we didn’t know much about,” he said.
Three ancestral populations make up the majority of Indians: farmers with Iranian ancestry who arrived between 4700 and 3000 BCE, hunters and gatherers who inhabited the area for tens of thousands of years, and herders from the central Eurasian steppe region who swept into the area about 3000 BCE.
Priya Moorjani, a population geneticist at the University of California, Berkeley, who co-led the earlier study, and her colleagues confirmed the identities of the ancestral groups.
The team sequenced over 2700 modern Indian genomes using data from the Longitudinal Aging Study in India–Diagnostic Assessment of Dementia (LASI-DAD). These genomes included speakers of nearly every major language group, individuals from all tribes and castes, and people from nearly every geographic region.
The researchers also examined the previously retrieved ancient DNA from Iranian-ancestry groups that existed before the genetic pulse into India. Further, they carried out simulations to determine whose genes most closely matched the genetic patterns observed in modern Indians. Farmers from Sarazm, an old agricultural hub in the northwest of what is now Tajikistan, were the most compatible. These farmers reared livestock, farmed wheat and barley, and engaged in significant commerce with other countries in Eurasia.
Michael Frachetti, an archaeologist at Washington University in St. Louis, who wasn’t engaged in the new research, said that he was highly compelled by the study.
He mentioned that for a long time that Sarazm would have been a crucial hub for the spread of domestic animals, farming, and human genes into Kashmir and northwest India. “A very important story is being told here. Most people underestimate how interconnected societies were throughout prehistoric times,” he said.
However, other ancestral populations remain somewhat “vague,” according to biological anthropologist Gyaneshwer Chaubey at Banaras Hindu University (BHU). He says the relative paucity of ancient DNA samples from India means other, ancient source populations could be missing from the mix.
Further, researchers argued about whether stone tools discovered in India that dates back to about 80,000 years were made by modern humans, and if so, whether this means that present populations still have their genetic heritage. However, scientists haven’t been able to identify the makers of these tools because there are no remains connected to them.
According to the current study, those early toolmakers only left their mark on living people. By estimating the amount of genetic mutation that occurs between generations and calculating how long it would have taken for India’s modern population to reach its current state of variation, the researchers claimed that the settlers who gave rise to modern Indians were part of a single migration out of Africa approximately 50,000 years ago.
In addition, the researchers discovered that the modern people in the sample had 1% to 2% of their ancestry from Neanderthals and their close relatives, the Denisovans—about the same as Europeans. However, as compared to other global populations, Indians together possess an astounding diversity of these ancient genes.
The 2700 Indian genomes contained over 90% of all known Neanderthal genes that have infiltrated human populations. That is roughly half of what was found in a comparable study of Neanderthal DNA in Icelanders that analysed more than 27,000 genomes. The researchers also identified several new candidates for Neanderthal- and Denisovan-inherited genes that may have given their descendants some evolutionary advantage, though it’s too early to say what those boons might have been.
Moorjani’s team said that the ancient humans might have encountered and mated with a relatively large, genetically diverse population of the archaic cousins living on the subcontinent—although no related fossils have been found.
Another possibility is that India’s vast geographical boundaries and close kin–marrying traditions preserved different segments of Neanderthal DNA than on other continents.
(With inputs from Science.com)
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