The family tree model is a myth | David Reich

0:04 When you can take DNA from people who lived

0:06 in the past and compare it to each other.

0:08 And with people living today,

0:10 you see things that you just didn't even imagine before.

0:13 The step migrations, the expansion of Lapita pottery culture.

0:17 People of East Asian ancestry to the Southwest Pacific.

0:21 The world's full of history that we didn't know about before,

0:24 and it just shows us how little we knew before and how little we still know.

0:28 Every time we get DNA from Africa for more than 3000 years ago,

0:32 it's a huge surprise.

0:33 You look in Malawi and south central Africa,

0:36 the people there 3000 years ago, nothing like the people there today.

0:39 They're a population that simply doesn't exist anymore.

0:42 And you sort of are confronted this data and you say,

0:44 I don't know anything anymore.

0:46 What's really happened is the introduction of a new scientific instrument

0:50 that is able to measure things that have not been measured before.

1:01 Ancient DNA is really a remarkable technology.

1:05 There was no data of any real meaning before 2010,

1:10 and at that time we simply could not look at DNA from the bones of people

1:16 who lived in the past and compare them to each other and people who live today.

1:20 So really, all we could do was to speculate about how people who lived

1:24 at any one place in the past related to other people and to people today.

1:28 And there was a reasonable set of hypotheses that people spread in a simple way,

1:36 moving from one place to the other out of an origin in Africa,

1:40 maybe 50 or 60 or 70,000 years ago, spread to different parts of Eurasia,

1:45 and eventually to Australia and to the Americas.

1:48 And once people got to a place,

1:50 people pretty much stayed there without much change afterward.

1:54 We had this bias that modern humans, as they spread out of Africa,

1:58 had not mixed with the archaic humans they encountered.

2:00 Like the Neanderthals.

2:01 It looked like a simple out of Africa spread.

2:05 Then, after 2010, it became possible to lift the veil

2:09 on the data and see whether that was true.

2:12 And what was really amazing was every time we

2:14 got data from a place that we haven't seen,

2:16 the results were surprising and new and and different from what we expected,

2:20 different from what I expected.

2:22 The biases I had coming into this work,

2:25 when we looked at the data, there was very,

2:27 very clear evidence that Neanderthals that we

2:30 were sequencing from Europe were more

2:32 closely related to people outside of Africa

2:35 today than people inside Africa today.

2:37 And it became absolutely clear that as modern humans moved out of Africa,

2:41 they mixed with the local archaic population in about 2%

2:45 of the DNA from Non-africans today comes from these archaic humans.

2:50 And that was just the first of many,

2:52 many surprises that comes out of this ancient DNA revolution.

2:57 I was working mostly on medical genetics and trying

3:00 to learn about risk factors for disease, and I became very,

3:05 very interested in trying to wed these two areas,

3:08 studies of people living today and studying of ancient DNA.

3:11 And the people who were leading this work in the world were not really

3:14 applying these methodologies at all to people

3:18 living in the last 20,000 or 15,000 years,

3:21 but the last ten or 15 or 5000 years is

3:25 when the world got to be the way it is today,

3:27 when all the languages diversified,

3:29 when the cultural variation that you see appears.

3:32 And so you can look at diverse people around the world

3:35 today and try to understand how that all comes together.

3:38 And so I completely retooled my laboratory.

3:41 We started with a mammoth that had been dredged up

3:44 in Hurricane Sandy on the ocean floor off Cape Cod,

3:49 so that we didn't contaminate it with human DNA and see if

3:52 we could extract DNA from a mammoth tooth that had been dredged up.

3:55 And then we started looking at our first human DNA,

3:57 and we started working with a colleague, David Anthony, who is an archeologist,

4:03 an amateur linguist who had become interested in asking how it is

4:07 that the languages that are spoken across

4:10 a wide swath of Eurasia languages like Sanskrit,

4:13 Indic, Iranian languages, English, which is a Germanic language,

4:17 italic like Latin to Korean that's was in eastern China,

4:21 or young or Hittite, and what's now Turkey and the Anatolian,

4:25 how it is that these languages came to be so closely related.

4:29 He had a theory which was that this might be related

4:32 to people who lived in the steps north of the Caspian Sea,

4:35 a step culture called the Yamnaya.

4:37 Who are the first people to take wheeled vehicles which,

4:41 using wheels that had just been invented out onto the open steppe lands.

4:44 So we sequence some bones from the Yamnaya and from their successor cultures,

4:50 and also some bones from, Germany to see whether the Yamnaya

4:54 might have had an impact on the history of these people.

4:57 And we got this incredible result from this data.

5:00 The Yamnaya had this very distinctive ancestry that is a cocktail of genetically

5:07 recognizable components that are similar to what you see in Europeans today,

5:10 but somehow more extreme.

5:12 And when you look at the farmers who lived in Europe before,

5:15 5000 years ago, for example, in Germany or Hungary.

5:18 They don't have this type of ancestry you see in the Yamnaya.

5:22 And then after 5000 years, it's everywhere.

5:25 People mostly thought that there could not

5:27 have been a big spread from the steppe,

5:28 because how could pastoralists herding sheep

5:31 and cattle make a huge impact demographically

5:34 on a densely settled farming area like Europe was after 5000 years ago?

5:39 But they did.

5:40 The genetic data shows they did it,

5:41 and it's the main ancestral population of Europeans

5:44 throughout northern Europe and even much of southern Europe.

5:46 Today, something huge happened.

5:48 And again, it's the kind of surprise that this field yields.

5:52 When we first started collecting DNA from ancient humans,

5:56 we started seeing these transformative changes

5:59 that happened radical shifts in ancestry,

6:02 huge mixture proportions off sex biased,

6:05 where the two sources of ancestry that would come together in any mixture event,

6:09 for example, European hunter gatherers and European farmers,

6:12 or Papuans and people of East Asian ancestry

6:15 in the Southwest Pacific would come together in very asymmetrical ways,

6:19 with men from one group participating

6:21 and women from another group participating.

6:23 And this is telling you about events from the past.

6:26 In some cases, it's evidence of probably extreme unhappiness and displacement.

6:32 So one of the examples that we know from pre-history is in Spain,

6:36 about 4000 to 4500 years ago.

6:40 There's a movement of these steppe pastoralist derived people into Spain.

6:44 It's about 40% of the ancestry gets displaced by people coming from the east,

6:48 but 100% of the Y chromosomes come from east,

6:51 which means that the local men completely fail to reproduce.

6:55 These were disruptive events.

6:56 These were events that completely changed the people

6:59 who lived at different places in the past,

7:01 and often were much more extreme or different from, or completely

7:04 unexpected compared to what people thought about before the tree

7:10 of human populations that I learned when I was like learning

7:15 about how human populations around the world are related to each other,

7:18 has a common ancestor, maybe somewhere in Africa,

7:21 based on mitochondrial sequences, which are inherited from mother to daughter,

7:25 splitting into different African lineages that are represented today,

7:30 and then spreading out of Africa and splitting

7:32 into, like East Asians and Australians and ancestors of Europeans.

7:36 And then maybe Native Americans might split off

7:39 East Asians like a simple tree like that.

7:41 And that corresponds to an idea that as people move around, they split,

7:45 they don't remix, and they settle in different places around the world.

7:49 But the truth is from genetic data that in every one of the places I mentioned,

7:54 all of those groups are profoundly mixed, of multiple, very divergent groups.

7:58 So the truth is not a simple splitting from a common ancestor,

8:03 but a kind of braided trellis going back into the past.

8:07 When you send your DNA to a direct consumer ancestry testing company,

8:10 you might get back and answer like you're 15% Irish and you're 30% Nigerian,

8:17 and you're 20% South Asian or something like this.

8:20 And what you're being told is an answer

8:23 that's related to a very specific moment in time.

8:26 Basically, what you're being told is if you

8:28 roll back 500 years or a thousand years,

8:31 what fraction of your ancestors lived in Ireland?

8:34 What fraction of your ancestors lived in West Africa somewhere?

8:37 What fraction of your ancestors lived in South Asia somewhere?

8:39 And we're making our best guess of that.

8:42 But if you ask a question about another time,

8:44 80,000 years ago, you can ask another question.

8:46 What are your ancestral components?

8:48 And the thing that just comes clear from ancient DNA

8:51 is that the people who lived in any one place

8:54 a thousand years ago or 5000 years ago are almost never

8:57 directly ancestral to the people who live in that place today.

9:01 The pattern that we found in each case,

9:05 when we get a new ancient genome sequence is again, and again,

9:09 you find that this individual descends from a mixture of at least two,

9:13 sometimes more than two ancestral populations.

9:16 So when it's the case that every time you collect new data,

9:19 you find evidence that this group is mixed.

9:21 The inevitable conclusion from induction is that everybody's mixed,

9:26 you know, at every time depth.

9:28 And so what this is produced is an amazing dialog between genetics,

9:32 which provides this unambiguous information about what happened

9:35 in the past in archeology and sometimes linguistics,

9:39 which provide other sorts of information.

9:41 And it forces a new way of talking

9:43 and discussing and thinking about these events in the past.

9:47 The truth is that the world's a mystery

9:49 and that we've carved out areas of clarity.

9:53 That certainly gives us a lot of perspective.

9:55 I think that it should give us humility about

9:58 whether our story about ourselves is is the correct story.

10:02 Ancient DNA has disrupted a number of assumptions about who we are.

10:07 Simple migration out of Africa, displacing the archaic humans who were there.

10:11 That's out the door now.

10:13 And it has also changed our understanding

10:16 of the pattern of human evolution as one

10:19 of simple branching to one that's definitely

10:21 a trellis of a mixing and mixing us again.

10:24 Over time, it's provided some little insight about the nature

10:30 of how natural selection has shaped our species over time.

10:33 It's showed that our genome is malleable in response to environmental changes,

10:38 like the ones that have occurred in the last ten or 15 or 5000 years.

10:41 And so I think it adds nuance

10:44 to our understanding of how our species is changing.

10:48 It makes it clear that it's continuing to change going forward.

10:51 And it sort of shows how we're all connected in myriad ways.

10:54 We never imagined.