In his path-breaking book Who We Are And How We Got Here, geneticist David Reich writes about “population bottlenecks”, where small numbers of people have a large number of descendants who do not mix with those around them. Usually a factor of geographical barriers of natural catastrophes, Reich explains that ancient DNA points to this also being evidence of the prevalence of caste endogamy in India.

Such conclusions are far from unusual for the Harvard professor, who has been at the forefront of a revolutionary new movement in the last few years that uses ancient genetic matter to answer questions about the distant past. The rapidly developing branch of scientific research has upended traditional approaches to history, while raising thorny issues about the biological differences between populations and their origins and movements over thousands of years. Often, these are politically charged matters, such as the contentious question of the Aryan migration into India and how it ties into a Hindutva agenda.

While the groundbreaking work of Reich and other geneticists in the field has been heralded for its potential to throw new light on many dark patches in the history of human migration, it has also received criticism for making sweeping generalisations and lacking rigour.

Reich spoke to about all this and more at the sidelines of the Jaipur Literature Festival in January. Edited excerpts from the interview:

In the introduction to Who We Are And How We Got Here you write about the underlying reason for the study of ancient DNA, such as its applications in medical science. You conclude, however, by saying that “intrinsic curiosity” should be reason enough. Could you tell us a little bit more about that?
I’m a firm believer in exploration of the world and the Enlightenment perspective of trying to find out about the unknown things in the world. There is importance in doing that for its own sake and it doesn’t always need to be justified by medical applications and for building better iPhones or any of those things. We should learn about knowledge for knowledge’s sake.

What has the response to that argument been? Has there been more interest – more funding or more universities that are keen about this area of research?
In terms of US government funding, which is where I am, there hasn’t been a change. It was zero and it’s still near zero. I think the United States has not chosen, in its funding agencies, to support work in ancient DNA research – it’s still privately funded. In Europe, there continues to be a strong interest in funding ancient DNA and that’s where a lot of the work is being done.

I think there’s an increasing awareness of ancient DNA. I would not say that my book has been solely responsible for that but it has been part of the increased awareness. It has become a high-profile thing where people think of it as an important way of understanding the past that disrupts narratives that people had before, for better or for worse.

You wrote in a hotly debated piece in The New York Times last year about race and genetics, that “well-meaning people” could be “digging themselves into an indefensible position, one that will not survive the onslaught of science”. There are also controversies that have erupted in places such as India, with the Aryan invasion question. How do you navigate such political and social minefields with research that is still new and emerging?
The article is basically a distillation of the first half of Chapter 11 of my book and is in a lot of ways different from the rest of the book, because it’s not about ancient DNA but about genetics and society.

In my work, what I try to do is to get at the truth and try to use ancient DNA to constrain what we can know about the past. And often, when you get DNA from places where there’s never been DNA before, it’s almost always the case – as I say in my book, it’s a recurrent theme – it challenges existing traditional or scientific or archaeological or historical narratives. It’s such a new type of information, such a new type of measurement that it is almost impossible to reconcile. That’s the situation we find ourselves in.

From my perspective, what I’m interested in doing is publishing the work so it can be added to scholarly discussions about these topics and if that means trying to write the work in as neutral language as possible...that’s what we try to do, we try to bounce it off local people who are already immersed in the debates [so] that what we write is not egregiously off-base, it doesn’t grate against people’s understanding of these issues. At the same time, we try to write the work to interact with scholarly debate so it can build upon it in a useful way.

I discussed in the New York Times article [that] people have come to ways of talking about the nature of genetic differences in human populations or stories about our history that are oversimplifications and do not actually capture the nature of true human variation. Sometimes, that’s maybe okay, like it was in the 1970s when some of these simplifications were made and used to talk to people about the nature of relationships between populations.

But now they are inadequate and they don’t describe the research that’s going on, and the public feels like the truth is being hidden from them with regard to what genetics is finding. At such times, we geneticists, rather than repeating that there’s no space for there to be average, meaningful difference across populations, need to provide guidance and say that there actually is space – it’s not much, it’s actually less than the differences that exist between individuals and populations – but there actually is space for there to have been evolutionary differences amongst populations and we need to think about and deal with them because if we don’t, genetic studies will overtake us. We need to be prepared to talk about those things.

Do you think there’s a greater need for scientists and geneticists to collaborate with those from the humanities to contextualise this “new type of information”?
Absolutely. I think that’s the goal, that’s what we’re trying to do. I think in our work, what we’re trying to do is pioneer more and more equal and respectful collaborations amongst geneticists and archaeologists, historians, linguists and anthropologists where multiple perspectives are brought to bear...that’s going to be more and more necessary as the work that we do becomes more fine-grained and detailed and less big picture. For me, that’s one of my primary goals of the coming years – to be part of pioneering studies that bring these communities together.

Some countries, like India, tend to be more defensive about ancient DNA findings than others. Do you find that there’s a pattern or reason you’ve observed about where this defensiveness comes from?
I think it’s quite interesting to contrast India to Pakistan. In’s primarily a Muslim country...and people who are Muslims don’t fully feel like their history is rooted in Pakistan, because there are important things that happened in Muslim culture that are not from Pakistan, they’re from Arabia and elsewhere. I feel people are more able to think about the past in objective and scientific ways. So talking to Pakistani archaeologists about people who lived a very long time ago is a different experience from talking to Indian archaeologists or Native Americans or religious Jews who have different perspectives on the time-depth of human variations where it is relevant to their own cultures.

What I’ve noticed is a sense of how deep people in a country feel the connection is of their present-day culture to ancient-day culture. When people feel they’re part of cultures that have changed dramatically over time, there’s not as much of an emotional connection to what may or may not be true about the deep past. In Europe, there’s almost no emotionality at all about the ancient farmers or Bronze Age people or hunter-gatherers. There’s in fact, no emotion about the dead. So it’s very different, for example, amongst Native Americans where there are strong connections to the remains of the dead and their connection to place.

What do you make of the policy that countries such as India have, to keep the ancient DNA within their borders and not share it?
I think a free market for scientific research is beneficial to everybody. Obviously, that’s to some extent a self-serving perspective. For example, in Britain, much of the ancient DNA work has been done by non-British people. British archaeologists are very happy to send their samples to our laboratory and other laboratories outside Britain. So the story of their ancient DNA is being told by non-British people to a large extent, in strong collaboration with British archaeologists. I think that Britain has been ahead of a lot of the rest of the world in its scientific research on its own past and its progress in thinking about its own past, which has been quite profound over the last few years because of its openness to participation.

I would use that experience as a positive experience to suggest that in South Asia as well, open collaboration with foreign scientists can be helpful. I also [want to] sort of turn the conversation a little bit to work on disease genetics...there are a lot of resources to do genetic work outside of India, essentially free DNA sequencing. There are a lot of passionate non-Indian and Indian biologists, geneticists and medical doctors who would love to be able to help study people with different diseases and traits in India – work that would help Indians more than anyone else.

Arguably, India is the single best place to study human genetics in the world. The reason being that India is not a large population. It’s an extremely large number of small populations, whereas China is really best thought of as a single, very large population. And that creates a laboratory for trying to understand differential risk for disease that affects each of these groups in different ways that can be systematically studied and a lot of the expertise for doing that is through passionate scientists, not only in India but outside India. You know, I hope that India would consider involving those people in research on Indian variation. I think it could be valuable.

A piece in the New York Times recently called your work too quick and expansive, based on a race to publish, without sufficient peer review. The piece argues that a race for ancient bones has been set off by the ancient DNA revolution. How would you respond to these claims?
So, the main argument of that piece – there really wasn’t a coherent argument – but the main one was that ancient DNA specialists – and I was the most highlighted individual there – are generalising overly simplistic pictures of the past very quickly based on two small sample sizes.

So, based on just a few samples in some cases, for example, in the case of specific islands that the article was talking about, that we made statements that were grandiose and final claims about the history of a vast region. And that is untrue. In those papers we identified a very small number of these samples that were completely outside the current theories that were prevailing about the origins of people in the Pacific – the first people in two different island groups in the Pacific had less New Guinean identity than anyone in the Pacific has today. That was not predicted by the prevailing theory. So even a small sample size is sufficient to force a change in the prevailing theory.

We were very careful not to make any general statement about that. All that meant was that people got from East Asia, skirting New Guinea, got to the first islands of the Pacific and are not the only ancestors of people in the Pacific today. There must have been later movements. That’s exactly what we said in the last sentence of that paper: “Further work and larger sample sizes are necessary to figure out the details of what happened.” The article instead said that we were making grand, sweeping simplistic statements, and suggested to say, have the final word, which is emphatically, what we did not do.

A similar oversimplification, not just oversimplification but simply wrong statement, in that article was what they characterised about our work in Europe. They said that we argued that the population of Europe was entirely replaced, or nearly entirely replaced, by people from the Steppe after 5,000 years ago. But in fact, what our data paper showed was that it was not a replacement at all but a mixture process that partly affected Southern Europe and even in Northern Europe, in most groups, didn’t contribute more than half of the ancestry.

It’s a profound event, it’s an important event but it’s very far from simplification. In fact, it’s making the situation more complex. What the piece was trying to argue was that the genetic data is painting a narrative of simplicity, of population replacement, of generalisation from small sample sizes, whereas instead it’s disrupting existing theories, it’s constraining models and it’s opening up the ability to develop more complex models. So that entire critique was not based on fact. The author never checked these critiques with me and if he had, they would have completely fallen apart.