Q&A with David Reich

The Harvard geneticist talks about the disruptive effects of ancient DNA.

By Katherine Lagomarsino featured image Photograph by Bizu Tesfaye (Howard Hughes Medical Institute)

What, exactly, does a population geneticist do?

A population geneticist studies differences between peoples’ DNA sequences to learn about how people relate to each other and how our biology has changed over time.

The genomes of any two people today are remarkably similar — about 99.9 percent of the positions are the same. But the differences that population geneticists study — rare as they may be — are far from trivial. The human genome is so large that there are about 3 million such differences between any pair of genomes, and it is these differences that are responsible for most of the extraordinary human diversity we see around us.


How has DNA has become the ultimate disruptor when it comes to the ways in which we understand human populations and our past?

The experience of the last decade has been one of repeated surprise — of ancient DNA studies overthrowing assumptions about the past.

A remarkable example is the finding in 2015 by several research groups, including my own, that beginning around 4,500 years ago, there was a mass migration from the eastern European steppe into central Europe. This migration replaced at least 70 percent of the central European population and probably brought the Indo-European languages that are ubiquitous in the region today. Before ancient DNA, archaeologists knew that the period around 4,500 years ago was one of great cultural transformation. But no mainstream scientist dreamt of a population turnover remotely as large as to what happened. We still don’t know how, but it happened. A cattle-herding population from eastern Europe spread thinly over the ground largely replaced the densely settled farming population of Europe. So we need to rethink our archaeological theories.

The example of the steppe migration into Europe around 4,500 years ago is in fact only a specific example of how ancient DNA is disrupting our understanding of the deep past. It is revealing mass migrations that no one had expected, and it is revealing that instead of being rare great mixtures of anciently separated populations were common. The ubiquity of population mixture — something we now know about from ancient DNA — shows that we are not living in particular special times viewed in the broad sweep of human history. This epiphany makes it much harder to sustain racist worldviews.


In one interview you likened population geneticists to barbarians, who have stormed the fields of archeology, linguistics, and history, armed with information and technology that wasn’t available before. Are some threatened by this new genomic data?

The field most affected by the ancient DNA revolution is archaeology. Archaeologists are fundamentally scientists, thirsty for any new source of information about episodes of the past that could not previously be studied.

This means that archaeologists are fundamentally friendly to ancient DNA, even if the theories of individual archaeologists might be challenged by its findings, and even if the new papers presenting the findings of the ancient DNA revolution may sometimes sound naïve to archaeologists. Once archaeologists are able to master ancient DNA, they will be able to add nuance to its interpretation that we geneticists — with only amateur knowledge of the ancient cultures that are being discussed — cannot provide.


You’ve described Luca Cavilli-Sforza as your academic grandparent (i.e. your professor’s professor). Can you talk a little bit about him and how he influenced your work?

Luca Cavalli-Sforza supervised the Ph.D. of my own supervisor David Goldstein. When I started in this field in 1997, I began work on datasets that Luca had collected, and his prescient vision of the power of DNA to shed light on the past remains my guiding light. In 2011, when he was 89, I made a pilgrimage to see him in Pavia, Italy. He is my academic hero, and it was one of the great privileges of my career to get a chance to talk with him in person and share several meals and discussions with him.


You worked with Svante Pääbo in the Neanderthal genome project. How did that project change your worldview about the history of human origin?

My work with Svante Pääbo, which began in 2007, can only be described as a second post-doc.

What I mean by this is that the work I did with Svante and others in the consortium was not of the type that one might expect a person who ran his own laboratory to do. I started as a faculty at Harvard in 2003, which meant that I already had many students and post-docs of my own by 2007. In the normal course of events, I would have sent a student to work with Svante. However, when Svante invited me to join his consortium, it was clear to me that this was the most important data in the world, and I greedily wanted to have the pleasure of thinking about and analyzing the data with my own eyes and hands and brain and computer programs.

Over the next seven years working with Svante — a period that involved more than a dozen week-long trips to Leipzig in Germany — I learned all about ancient DNA and also imbibed his deeply thoughtful approach to science. These perspectives have fundamentally shaped my own work.


In your book, Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past, you mention how interbreeding with Neanderthals and Denisovans has helped give modern humans an edge, biologically speaking. Can you give some examples?

One striking example is a mutation in a gene that allows Tibetans to thrive at high altitudes. Rasmus Nielsen and colleagues at the University of California at Berkeley found that this mutation almost certainly arose in Denisovans, for whom it may have been useful in some way, allowing them to survive better in the environments in which they lived. The ancestors of Tibetans picked up this mutation through interbreeding with Denisovans more than 40,000 years ago, and it rose to high frequency after they began living at high altitudes.


How accurate are direct-to-consumer genetic tests in determining your origins or, say, the amount of Neanderthal DNA you have in your ancestry?

Unfortunately these tests are rather deceptive — even if they are popular. For example, the 23andMe test purports to tell people how many Neanderthal-derived mutations they have inherited. But the statistical uncertainty around this estimate is extremely large — more than 10 times larger than the true variation in Neanderthal DNA that we know exists across individuals in Europeans. This means that if 23andMe reports a person as being in the 99th percentile with regard to the number of Neanderthal mutations they carry, the true probability that they carry more than the average is hardly more than 50 percent.


What is the biggest surprise DNA has uncovered about human history?

The big, persistent surprise is that we are not who we think we are. There is almost no human population in the world today that is directly descended without mixture from the human population that lived in the same place 10,000 years before. We are all profoundly mixed up. To give just one example, white Europeans today are the result of a convulsive mixture of three groups that 10,000 years ago were each as different from each other as Europeans and East Asians are today. This provides a new critique of concepts of race that is very different from the traditional critiques.


How does your work studying past populations inform the way you see humanity today? How does it influence your views on issues like nationalism and racism?

The findings have particular resonance when it comes to the current nasty discussions in both the United States and in Europe about immigration. Immigration and population mixture are fundamental to who we are and without them we would not be here. There are good reasons to try to limit immigration to avoid too much strain on an economy or on civil society. But people who reject immigrants on the grounds that they are disrupting a kind of mythical purity are flying in the face of who we are as a species.


In your book, you mention that your lab grinds through the bones of hundreds of ancient peoples every month, and how you have considered that, in all likelihood, some of these people would not have wanted their remains used in this way. How did your uncle help you resolve this issue? 

My uncle is a strict orthodox rabbi, but he is also open to the modern world; for example, he led the establishment of a school that trains women to be rabbis, something that is not strictly forbidden by Jewish law but is entirely new in a culture where for thousand of years, women have been relegated to secondary liturgical roles.

I asked my uncle what he thought of the ethics of opening up so many ancient graves, including of people whose religious beliefs surely would not have welcomed this treatment. His “Psak Din” — the judgment that a rabbi gives when asked a question on a topic about which there is no precedent — is that as long as the work that is being done has the potential to overcome racism and misunderstanding, then there is an argument to be made for it.

When I do my research now, I always have this Psak Din in my mind. Every time my lab grinds a new bone, I am aware that these ancient people were once as alive and as real as us. These people are deeply connected to us in so many ways — this is what I learn again and again from analyzing the data from these ancient peoples — and we need to treat them with the same extraordinary respect we would use for the remains of our families.