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

So if you ask the question, a white British person today, and you say, this person, do

they feel 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 homeopathically 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

or being confronted with this data. It's really an untenable picture. It's just

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 East Asians, like 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 no one time that we been able to reconstruct where there a single homogeneous population that ancestral to everybody who comes after Rather it some kind

of constant change over time with mixtures, separations, structuring, coming back together

Some groups disappear, other groups don't. And it's just very complicated. And that's sort of

fundamental nature of who we are. It's not kind of 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 you're 30% Nigerian, and you're 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 fractions 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 could 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. But 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. But the way I think about it is back in time. And if you look

at any one person today, you can look at all your genealogical ancestors. I have two parents

They have two parents. I have four great-grandparents, eight great-great-grandparents, 16 32 64 128 256 512 etc It gets pretty big pretty fast And at any time depth of 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 just comes clear from ancient DNA is that

the people who lived in any one place a thousand 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 and 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 the previous groups, but it's interpolated and separate and 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. In Britain, the whole island is

cleared of people by the last glacial maximum. So that's between 25,000 to 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 begins to people 14,000 or 15,000 years ago

There's hunter-gatherers related to the people on the continent. They hang out there for a very

long time, until about 6,000 years ago. And 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're a completely transformative event. So only 1% of the DNA, if that, of the

people in Britain after this event come from the hunter-gatherers of Britain. So if you want to ask

And I descended from, quote, Cheddar Man, the sort of 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 can genetically predict, had very, very dark skin, almost like what you'd expect from Nigeria or something like this, and blue eyes, like 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 years ago And then flash forward 1 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 100 years, those people

are 90 to 100% displaced by a new stream of people coming from the continent. This time

we now know from an area like the Netherlands associated with this Bell Beaker event

That's another 90% to 100% spread. And then wait a bit more, and about 3,000 years ago, there was a huge stream of people from what's now probably from France, probably related to the spread of Celtic languages

And that displaces half of the DNA of the Britons. And then associated with the Saxon period, maybe 400 or 500 CE, 1,600 years ago, 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's actually relatively protected from the churn of people that has occurred in many

other parts of the world. If you look in Hungary, it's much more extreme and even more frequent disruptions

If you look in Italy, there's many disruptions. It's not just Europe

If you look in the Near East, there's disruption after disruption after disruption. Every place in Africa, 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 one of, for the most part, 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 is instead something to embrace as part of our history