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Ancient DNA just proved that ‘pure genetics’ don’t exist

Harvard geneticist David Reich has spent years extracting DNA from ancient human remains. What the data shows keeps defying every assumption scientists had built.

Every population ever studied is profoundly mixed. Britain alone has been almost completely replaced four or five times over. The hunter-gatherers who built Stonehenge's foundations? Gone within a century. The idea of a clean lineage connecting any living person to any ancient place is, as Reich puts it, simply untenable.

DAVID REICH: So if you ask the question, a white British person today, and you say this person feels like they’re pure descendants of people who lived in Britain a long time ago, nothing could be further from the truth. They’re like homeopathic, diluted compared to the hunter-gatherers or the first farmers. And this idea that any one people is somehow purely descended from the people who lived in the past — and it was a common thing that people thought prior to the genetic revolution — is being confronted with this data. It’s really an untenable picture. Every time we look, it’s not true.

The tree of human populations that I learned when I was learning about how human populations around the world are related to each other has a common ancestor, maybe somewhere in Africa. That’s based on early data from the 1980s, based on mitochondrial sequences, which are inherited from mother to daughter, splitting into different African lineages that are represented today, and then spreading out of Africa and splitting into East Asians and Australians and ancestors of Europeans. And then maybe Native Americans might split off the East Asians — a simple tree like that. And that corresponds to an idea that as people move around, they split, they don’t remix, and they settle in different places around the world.

But the truth is, from genetic data, that in every one of the places I mentioned, all of those groups are profoundly mixed, of multiple very divergent groups. So the truth is not a simple splitting from a common ancestor, but a kind of braided trellis going back into the past. And that trellis doesn’t stop 50,000 years ago — it just keeps going. So as we get Neanderthal sequences or Denisovan sequences from these archaic humans and compare them to modern humans, we see that they too are admixed with different groups of people, and modern humans are mixed or have contributed to a mixture of other groups.

And so there’s no one time that we’ve been able to reconstruct where there’s a single homogeneous population that’s ancestral to everybody who comes after. Rather, it’s some kind of constant change over time, with mixtures, separations, structuring, and coming back together. Some groups disappear. Other groups don’t. And it’s just very complicated. And that’s the fundamental nature of who we are. It’s not an unusual thing. It used to be that we had this null hypothesis that no mixture occurred, and we would test that null hypothesis. And that is almost a silly null hypothesis at this point, because everybody is mixed. Nobody is pure.

When you send your DNA to a direct-to-consumer ancestry testing company, you might get back an answer like you’re 15% Irish and 30% Nigerian and 20% South Asian, or something like this. And what you’re being told is an answer that’s related to a very specific moment in time. Basically, what you’re being told is: if you roll back 500 years or 1,000 years, what fraction of your ancestors lived in Ireland? What fraction of your ancestors lived in West Africa somewhere? What fraction of your ancestors lived in South Asia somewhere? And we’re making our best guess of that.

But if you ask a question about another time, 80,000 years ago, you can ask another question: what are your ancestral components? And there it would not be in those particular places, because those people have moved so much. There the question will be: what fraction of your ancestors, if you’re a non-African person, will live in Europe because they’re Neanderthals? What fraction of your ancestors will live in sub-Saharan Africa because they’re from the African population that expands out of Africa around that time? So it’s a matter of what time in the past you’re talking about.

The way I think about it is back in time. If you look at any one person today, you can look at all your genealogical ancestors. I have two parents. They have two parents — my four grandparents, eight great-grandparents, sixteen, thirty-two, sixty-four, one hundred twenty-eight, two hundred fifty-six, five hundred twelve, and so on. It gets pretty big, pretty fast. And at any time depth, any number of generations back in the past, you can ask: what fraction of them lived here? What fraction of them lived there?

And the thing that becomes clear from ancient DNA is that the people who lived in any one place 1,000 years ago or 5,000 years ago are almost never directly ancestral to the people who live in that place today. So the strategy used by these direct-to-consumer ancestry testing companies of using modern-day Irish as surrogates for who lived in Ireland in the past — maybe that works in the last thousand years, but it’s not even going to really work very well 2,500 years ago or 3,000 years ago, because there’s just too much mixing and migration.

If you look at the genetic data, what you see is there are periods of continuity where populations descend from previous groups, but it’s interrupted every few hundred years or few thousand years by profound disruptions. I’ll give you one place where we know this particularly intensely, which is the island of Great Britain. The whole island is cleared of people by the Last Glacial Maximum, between 25,000 and 19,000 years ago. Then the glaciers begin to melt. People walk across the lower North Sea, which was just a land bridge, and get into Great Britain. There begin to be people there 14,000 or 15,000 years ago. There are hunter-gatherers related to the people on the continent. They stay there for a very long time, until about 6,000 years ago.

Then farmers stream in from the continent. They are related to people who are from what’s now Turkey, who brought farming to Europe. They first get to Europe about 8,500 years ago. They spread across the continent. They get to Britain 2,500 years later, and they are a completely transformative event. So only 1% of the DNA, if that, of the people in Britain after this event comes from the hunter-gatherers of Britain.

So if you want to ask, “Am I descended from Cheddar Man?” — this individual who was excavated in Cheddar Gorge, where cheddar cheese is made, and we have their DNA, and their DNA corresponds to someone who you could genetically predict had very, very dark skin, almost like what you would expect from Nigeria or something like this, and blue eyes, a combination that’s very rare today — how much DNA do people in Great Britain have from this person? No more than 1%, because of what happened 6,000 years ago.

Then flash forward 1,500 more years, and the last big stones at Stonehenge go up. They are put up by the descendants of these first farmers. There has been continuity for 1,500 years in Great Britain, and then within a hundred years, those people are 90 to 100% displaced by a new stream of people coming from the continent. At this time, we now know from an area like the Netherlands, associated with this Bell Beaker event. That’s another 90 to 100% replacement.

Then wait a bit more. About 3,000 years ago, there’s a huge stream of people from what’s now probably France, probably related to the spread of Celtic languages. And that displaces half of the DNA of the Britons. Then, associated with the Saxon period, maybe around 400 or 500 C.E., 1,600 or 1,500 years ago, there’s another third ancestry. And then there’s continued flow into Britain after that.

And that’s not unusual. Britain is actually relatively protected from the churn of people that has occurred in many other parts of the world. If you look at Hungary, it’s much more extreme, with even more frequent disruptions. If you look at Italy, there are many disruptions. And it’s not just Europe. Everywhere there is disruption after disruption after disruption. Every place in Africa where we’ve had data to look at, there’s disruption after disruption after disruption.

There are interesting exceptional areas. There actually seems to be an unusually high amount of continuity over many thousands of years in many parts of the Americas, which is really fascinating. But there too, there are disruptions as well. So the pattern seems to be, for the most part, that there are periodic profound movements of people coming from somewhere else, either completely displacing or mixing with the people who were there before.

So the thing that I keep coming back to is that everybody is mixed, nobody is pure. Mixing is happening again and again in the past, and is not something to deny or to fight against, but instead something to embrace as part of our history.

Harvard geneticist David Reich has spent years extracting DNA from ancient human remains. What the data shows keeps defying every assumption scientists had built.

Every population ever studied is profoundly mixed. Britain alone has been almost completely replaced four or five times over. The hunter-gatherers who built Stonehenge's foundations? Gone within a century. The idea of a clean lineage connecting any living person to any ancient place is, as Reich puts it, simply untenable.

DAVID REICH: So if you ask the question, a white British person today, and you say this person feels like they’re pure descendants of people who lived in Britain a long time ago, nothing could be further from the truth. They’re like homeopathic, diluted compared to the hunter-gatherers or the first farmers. And this idea that any one people is somehow purely descended from the people who lived in the past — and it was a common thing that people thought prior to the genetic revolution — is being confronted with this data. It’s really an untenable picture. Every time we look, it’s not true.

The tree of human populations that I learned when I was learning about how human populations around the world are related to each other has a common ancestor, maybe somewhere in Africa. That’s based on early data from the 1980s, based on mitochondrial sequences, which are inherited from mother to daughter, splitting into different African lineages that are represented today, and then spreading out of Africa and splitting into East Asians and Australians and ancestors of Europeans. And then maybe Native Americans might split off the East Asians — a simple tree like that. And that corresponds to an idea that as people move around, they split, they don’t remix, and they settle in different places around the world.

But the truth is, from genetic data, that in every one of the places I mentioned, all of those groups are profoundly mixed, of multiple very divergent groups. So the truth is not a simple splitting from a common ancestor, but a kind of braided trellis going back into the past. And that trellis doesn’t stop 50,000 years ago — it just keeps going. So as we get Neanderthal sequences or Denisovan sequences from these archaic humans and compare them to modern humans, we see that they too are admixed with different groups of people, and modern humans are mixed or have contributed to a mixture of other groups.

And so there’s no one time that we’ve been able to reconstruct where there’s a single homogeneous population that’s ancestral to everybody who comes after. Rather, it’s some kind of constant change over time, with mixtures, separations, structuring, and coming back together. Some groups disappear. Other groups don’t. And it’s just very complicated. And that’s the fundamental nature of who we are. It’s not an unusual thing. It used to be that we had this null hypothesis that no mixture occurred, and we would test that null hypothesis. And that is almost a silly null hypothesis at this point, because everybody is mixed. Nobody is pure.

When you send your DNA to a direct-to-consumer ancestry testing company, you might get back an answer like you’re 15% Irish and 30% Nigerian and 20% South Asian, or something like this. And what you’re being told is an answer that’s related to a very specific moment in time. Basically, what you’re being told is: if you roll back 500 years or 1,000 years, what fraction of your ancestors lived in Ireland? What fraction of your ancestors lived in West Africa somewhere? What fraction of your ancestors lived in South Asia somewhere? And we’re making our best guess of that.

But if you ask a question about another time, 80,000 years ago, you can ask another question: what are your ancestral components? And there it would not be in those particular places, because those people have moved so much. There the question will be: what fraction of your ancestors, if you’re a non-African person, will live in Europe because they’re Neanderthals? What fraction of your ancestors will live in sub-Saharan Africa because they’re from the African population that expands out of Africa around that time? So it’s a matter of what time in the past you’re talking about.

The way I think about it is back in time. If you look at any one person today, you can look at all your genealogical ancestors. I have two parents. They have two parents — my four grandparents, eight great-grandparents, sixteen, thirty-two, sixty-four, one hundred twenty-eight, two hundred fifty-six, five hundred twelve, and so on. It gets pretty big, pretty fast. And at any time depth, any number of generations back in the past, you can ask: what fraction of them lived here? What fraction of them lived there?

And the thing that becomes clear from ancient DNA is that the people who lived in any one place 1,000 years ago or 5,000 years ago are almost never directly ancestral to the people who live in that place today. So the strategy used by these direct-to-consumer ancestry testing companies of using modern-day Irish as surrogates for who lived in Ireland in the past — maybe that works in the last thousand years, but it’s not even going to really work very well 2,500 years ago or 3,000 years ago, because there’s just too much mixing and migration.

If you look at the genetic data, what you see is there are periods of continuity where populations descend from previous groups, but it’s interrupted every few hundred years or few thousand years by profound disruptions. I’ll give you one place where we know this particularly intensely, which is the island of Great Britain. The whole island is cleared of people by the Last Glacial Maximum, between 25,000 and 19,000 years ago. Then the glaciers begin to melt. People walk across the lower North Sea, which was just a land bridge, and get into Great Britain. There begin to be people there 14,000 or 15,000 years ago. There are hunter-gatherers related to the people on the continent. They stay there for a very long time, until about 6,000 years ago.

Then farmers stream in from the continent. They are related to people who are from what’s now Turkey, who brought farming to Europe. They first get to Europe about 8,500 years ago. They spread across the continent. They get to Britain 2,500 years later, and they are a completely transformative event. So only 1% of the DNA, if that, of the people in Britain after this event comes from the hunter-gatherers of Britain.

So if you want to ask, “Am I descended from Cheddar Man?” — this individual who was excavated in Cheddar Gorge, where cheddar cheese is made, and we have their DNA, and their DNA corresponds to someone who you could genetically predict had very, very dark skin, almost like what you would expect from Nigeria or something like this, and blue eyes, a combination that’s very rare today — how much DNA do people in Great Britain have from this person? No more than 1%, because of what happened 6,000 years ago.

Then flash forward 1,500 more years, and the last big stones at Stonehenge go up. They are put up by the descendants of these first farmers. There has been continuity for 1,500 years in Great Britain, and then within a hundred years, those people are 90 to 100% displaced by a new stream of people coming from the continent. At this time, we now know from an area like the Netherlands, associated with this Bell Beaker event. That’s another 90 to 100% replacement.

Then wait a bit more. About 3,000 years ago, there’s a huge stream of people from what’s now probably France, probably related to the spread of Celtic languages. And that displaces half of the DNA of the Britons. Then, associated with the Saxon period, maybe around 400 or 500 C.E., 1,600 or 1,500 years ago, there’s another third ancestry. And then there’s continued flow into Britain after that.

And that’s not unusual. Britain is actually relatively protected from the churn of people that has occurred in many other parts of the world. If you look at Hungary, it’s much more extreme, with even more frequent disruptions. If you look at Italy, there are many disruptions. And it’s not just Europe. Everywhere there is disruption after disruption after disruption. Every place in Africa where we’ve had data to look at, there’s disruption after disruption after disruption.

There are interesting exceptional areas. There actually seems to be an unusually high amount of continuity over many thousands of years in many parts of the Americas, which is really fascinating. But there too, there are disruptions as well. So the pattern seems to be, for the most part, that there are periodic profound movements of people coming from somewhere else, either completely displacing or mixing with the people who were there before.

So the thing that I keep coming back to is that everybody is mixed, nobody is pure. Mixing is happening again and again in the past, and is not something to deny or to fight against, but instead something to embrace as part of our history.

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