In early September, the journals Cell and Science published two long-awaited papers based on the science of ancient DNA. The papers confirmed and expanded on what has been reported in the recent past about human settlement in South Asia. The study in Cell is based on the DNA from a single sample, a female, from a 4,500-year-old burial site in the Harappan city of Rakhigarhi. A summary of the results includes three key findings: the individual was from a population that is the largest source of ancestry for South Asians; Iranian-related ancestry in South Asia split from the Iranian-plateau lineages more than twelve thousand years ago; and the first farmers of the Fertile Crescent—a region that was the cradle of the Egyptian, Phoenician, Assyrian and Mesopotamian civilisations—contributed little to no ancestry to later South Asians. This indicates that not only were that the greater part of most South Asians’ genome is derived from the Harappan people, but also that farming may well have an independent origin in the region. Most South Asians carry some ancestry derived from steppe pastoralists, ranging from less than ten percent to a little over twenty percent. This ancestry is entirely absent in the Harappan genome, suggesting that the steppe pastoralists migrated to the subcontinent in substantial numbers after the decline of the Harappan civilisation. Both papers clearly spell out the likelihood that the steppe pastoralists brought the Indo-European languages to the subcontinent.
Hartosh Singh Bal, the political editor of The Caravan, spoke to Vagheesh Narasimhan, the lead author of the Science paper. Narasimhan is a post-doctoral fellow at the Reich Laboratory, at Harvard Medical School. His research is focused on using ancient DNA to understand human migration and evolution. They discussed the research methodologies that led to the laboratory’s findings, possible correlations between languages and genetics, and what the results say about when the steppe pastoralists arrived in India. The geneticists’ work so far, Narasimhan said, maps the settling of the subcontinent till 1000 BC. But a similar study of more recent samples is necessary to answer questions central to an understanding of Indian society today, including the origins and the evolution of the caste system.
Hartosh Singh Bal: Could you tell us about the work on human migration and ancient DNA that has been going on at the Reich lab?
Vagheesh Narasimhan: We’ve been working with ancient DNA for the past five years or so, and prior to that we’ve been working with modern data. Revolutionary new technology with the ability to sequence ancient samples emerged around ten or so years ago, with the sequencing of first Neanderthal genome in 2010. Since then we’ve started utilising this technology not just to study very deep ancient history but also to study more recent events. In 2009, our lab first began examining Indian genetic history. We sequenced a large number of samples from modern India and tried to reconstruct population history from these. We know today that’s a very challenging thing to do, and it leaves a lot of ambiguity about what happened in the past. Directly having ancient DNA sequences—that is, genomes of individuals who were buried in the ground thousands, if not tens of thousands, years ago— allows us to examine how humans moved through space and time. Along with radiocarbon data which gives very precise temporal information of the samples, we can then examine exactly how human migrations happened. So that’s the history of the field and how we started to look at India.
HSB: Sampling ancient DNA poses real challenges, and particularly so in the subcontinent. Why is this?
VN: For two reasons. One is technical: ancient DNA degrades. You have DNA that lies in the soil or skeletons and this degrades just by natural processes—you have less and less of it. The length of the DNA fragments that remain becomes shorter and shorter, and it becomes so short that it is no longer valuable to use them in analysis. Warm and humid climates provide additional challenges as the rate of degredation increases under such conditions. The second issue is that we hadn’t thus far had access to the kind of material that’s been available say, in Europe or Central Asia, or even the Americas. But based on the success that we’ve had with several of these projects, we hope to put in proposals with relevant authorities including the Anthropological [Survey of India] and the Archaeological Survey of India. We have also been getting access to hundreds of samples from Pakistan, so it would be really great if we can get access to samples from India as well.