Why your ancestry test can't tell you who you really are | David Reich

0:04 So if you ask the question,

0:05 a white British person today and you say this person that they feel

0:09 like they're pure descendants of people who lived in Britain a long time ago,

0:13 nothing could be further from the truth.

0:15 They're like homeopathic,

0:16 diluted compared to the hunter gatherers or the first farmers.

0:20 And this idea that any one people is somehow

0:24 purely descended from the people who lived in the past.

0:27 And it was a common thing that people thought prior to the genetic revolution,

0:33 are being confronted with this data.

0:34 It's really an untenable picture.

0:36 It's just it's every time we look, it's not true.

0:44 The tree of human populations that I learned when I was like,

0:50 learning about how human populations around the world are related to each other,

0:54 has a common ancestor, maybe somewhere in Africa.

0:58 That's based on early data from the 1980s,

1:01 based on mitochondrial sequences, which are inherited from mother to daughter,

1:04 splitting into different African lineages that are represented today,

1:09 and then spreading out of Africa and splitting

1:11 into, like East Asians and Australians and ancestors of Europeans.

1:16 And then maybe Native Americans might split off

1:18 the East Asians like a simple tree like that.

1:21 And that corresponds to an idea that as people move around, they split,

1:25 they don't remix, and they settle in different places around the world.

1:29 But the truth is from genetic data that in every one of the places I mentioned,

1:34 all of those groups are profoundly mixed, of multiple, very divergent groups.

1:38 So the truth is not a simple splitting from a common ancestor,

1:43 but a kind of braided trellis going back into the past.

1:46 And that trellis doesn't stop that.

1:49 50,000 years ago, it just keeps going.

1:52 So as we get Neanderthal sequences or Denisovan sequences

1:55 from these archaic humans and compare them to modern humans,

1:58 we see that they too are ad mixed with different groups of people,

2:01 and modern humans are mixed or have contributed to a mixture of other groups.

2:06 And so there's no one time that we've been able to reconstruct

2:09 where there's a single homogeneous population

2:12 that's ancestral to everybody, who comes after.

2:15 Rather, it's some kind of constant change

2:19 of over some changes over time with mixtures,

2:23 separations, structuring, coming back together.

2:26 Some groups disappear.

2:28 Other groups don't.

2:29 And it's just very complicated.

2:31 And that's sort of the fundamental nature of who we are.

2:34 It's not kind of an unusual thing.

2:36 We used to be that we had this null hypothesis that no mixture occurred,

2:40 and we would test that null hypothesis.

2:42 And that is almost a silly null hypothesis at this point,

2:45 because everybody is mixed.

2:47 Nobody is pure.

2:48 When you send your DNA to a direct consumer ancestry testing company,

2:51 you might get back an answer like you're 15% Irish

2:55 and your 30% Nigerian and you're 20% South Asian or something like this.

3:01 And what you're being told is an answer

3:03 that's related to a very specific moment in time.

3:07 Basically, what you're being told is if you

3:09 roll back 500 years or a thousand years,

3:13 what fraction of your ancestors lived in Ireland?

3:15 What fraction of your ancestors lived in West Africa somewhere?

3:19 What fraction of your ancestors lived in South Asia somewhere?

3:22 And we're making our best guess of that.

3:24 But if you ask a question about another time, 80,000 years ago,

3:28 you can ask another question what are your ancestral components?

3:31 And there it would not be in those particular

3:34 places because those people have moved so much.

3:36 But there the question will be, what fraction of your ancestors,

3:39 if you're a non-African person,

3:41 will live in Europe because they're Neanderthals?

3:43 What fraction of your ancestors will live in sub-Saharan Africa?

3:46 Because they're from the African population

3:48 that expands out of Africa around that time?

3:51 So it's a matter of what time, in the past you're talking about.

3:55 But the way I think about it is back in time.

3:57 And if you look at any one person today,

4:00 you can look at all your genealogical ancestors.

4:02 I have two parents.

4:04 They have two parents, my four great grandparents,

4:06 eight great great grandparents, 1632, 60, 428, 256, 512, etc.

4:12 it gets pretty big, pretty fast and at any

4:15 time depth of number of generations back in the past,

4:18 you can ask what fraction of them lived here, what fraction of them lived there.

4:23 And the thing that just comes clear from ancient DNA

4:27 is that the people who lived in any one place

4:29 a thousand years ago or 5000 years ago are almost never

4:32 directly ancestral to the people who live in that place today.

4:35 So the strategy used by these direct to consumer ancestry and testing companies

4:39 of using modern day Irish as surrogates for who lived in Ireland in the past,

4:43 maybe that works in the last thousand years,

4:45 but it's not even going to really work very well.

4:48 2500 years ago or 3000 years ago,

4:51 because there's just too much mixing and migration.

4:54 If you look at the genetic data, what you see is there are periods

4:57 of continuity where populations descend from the previous groups,

5:01 but it's interpolated and separate,

5:03 and every few hundred years or few thousand years by profound disruptions.

5:08 I'll give you one place where we know this particularly intensely,

5:11 which is the island of Great Britain in Britain.

5:14 The whole island is cleared of people by the last glacial maximum.

5:18 So that's between 25,000 to 19,000 years ago.

5:21 Then the glaciers begin to melt.

5:23 People walk across the lower North Sea,

5:26 which was just a land bridge, and get into great Britain.

5:29 There begins to be people 14 or 15,000 years ago.

5:32 There's hunter gatherers related to the people on the continent.

5:35 They hang out there for a very long time, until about 6000 years ago.

5:39 And then farmers stream in from the continent.

5:41 They are related to people who are

5:43 from what's now Turkey who brought farming to Europe.

5:46 They first get to Europe about 80, 500 years ago.

5:49 They spread across the continent.

5:50 They get to Britain 2500 years later,

5:52 and they are completely transformative event.

5:54 So only 1% of the DNA, if that of the people in Britain after

5:59 this event comes from the hunter gatherers of Britain.

6:01 So if you want to ask them, I decided from, quote, cheddar man,

6:04 this sort of individual who has excavated in Cheddar

6:07 Gorge where cheddar cheese is made and we

6:10 have their DNA and their DNA is corresponds

6:13 to someone who you could genetically predict had very,

6:15 very dark skin, almost like what you

6:17 would expect from Nigeria or something like this.

6:20 And blue eyes, like a combination that's very rare today.

6:22 How much DNA do people in Great Britain have from this person?

6:25 No more than 1%, because of what happened 6000 years ago.

6:28 And then flash forward 1500 more years

6:31 and the last big stones at Stonehenge go up.

6:33 They are put up by the descendants of these first farmers.

6:37 There has been continuity for 1500 years in Great Britain,

6:40 and then within a hundred years, those people are 90 to 100% displaced

6:44 by a new stream of people coming from the continent.

6:46 At this time, we now know from an area

6:48 like the Netherlands associated with his Bell Beaker event.

6:51 That's another 90 to 100% spread.

6:54 And then wait a bit more.

6:56 And about 3000 years ago,

6:57 there's a huge stream of people from what's now what's probably from France,

7:02 probably related to the spread of Celtic languages.

7:05 And that displaces half of the DNA

7:08 of the Britons and then associated with the Saxon period,

7:11 maybe 4 or 500 C.E., 1600 years ago, 1500 years ago.

7:16 There's another third ancestry,

7:17 and then there's continued flowed into Britain, after that.

7:21 And that's not unusual.

7:22 Britain is actually relatively protected from the churn of people

7:26 that has occurred in many other parts of the world.

7:28 If you look at Hungary,

7:29 it's much more extreme and even more frequent disruptions.

7:32 If you look in Italy, there's many disruptions.

7:34 If you look, it's not just Europe.

7:35 If you look at the the arrest,

7:37 there's disruption after disruption after disruption.

7:40 Every place in Africa, we've had data to look at.

7:42 There's disruption after disruption after disruption.

7:45 There are interesting exceptional areas.

7:47 There's actually seems to be an unusually high amount of continuity

7:50 over many thousands of years in many parts of the Americas,

7:54 which is really fascinating.

7:55 But there too, there are disruptions as well.

7:57 So the pattern seems to be one of for the most part,

8:02 there are periodic profound movements of people coming from somewhere else,

8:07 either completely displacing or mixing with the people who were there before.

8:12 So the thing that I keep coming back to is that everybody is mixed,

8:18 nobody is pure mixing is happening again and again in the past

8:22 and is not something to deny or to fight against,

8:25 but it's it's instead something to embrace as part of our history.

8:37 And.