Dr. David Reich is like
many people at Harvard, does many, many things. So he is a professor in
the Department of Genetics at the Harvard Medical School,
an investigator of the Howard Hughes Medical Institute,
and a senior associate member of the Broad Institute
of Harvard and MIT. He received a bachelor's degree
in physics from Harvard College and carried out his doctoral
work in statistical methods for learning about evolutionary
history with applications to gene mapping, and that was
at the University of Oxford. His work focuses on
studying population mixture with application to both
medical and human history. In medical genetics,
he's best known for developing and
applying methods to use the history of the
mixture of populations in the history of
African-Americans to find genetic risk
factors that contribute to health disparities. He started the first state
of the art ancient DNA lab in the United States
in 2013, and much of his current research
focuses on using the transformative
power of ancient DNA to gain new insight about
medical and evolutionary genetics. In 2015, in Nature, he was named
one of 10 people who matter-- so that's 10 people
who matter in all of the sciences for
his contribution to transforming ancient DNA
data from quote "a niche pursuit to industrial process." Some of his contributions, we
cannot possibly mention them all, include proving that
interbreeding occurred between Neanderthals and modern humans,
discovering a new archaic human population, the Denisovans,
reconstructing Indian population history and
risk factors for disease, and explaining the elevated
risk for prostate cancer in African-Americans. And the work continues. Five years ago,
scientists had retrieved just one ancient human genome
from the entire Western Hemisphere. And now, the total
has reached 229, enabling important
discoveries about how humans spread through the
Americas thousands of years ago. Why do I mention
this in particular? Well, as reported in the
New York Times just today, David and his team have
published new research in Cell Magazine about new
genetic exchanges between North and South America that have
significant implications for models of the
peopling of the Americas. So you can see it really
is ongoing state of the art research. Dr. Reich has published
extensively on these topics and received many awards,
including the Newcomb Cleveland Prize from the American
Association for the Advancement of Science and the
Dan David Prize in the archaeological
and natural sciences for his computational
discovery of intermixing between Neanderthals
and Homo sapiens. As I mentioned earlier, he's a
member of the Allen Discovery Center for Human brain evolution
at Boston Children's Hospital and Harvard Medical
School together with doctors Michael
Greenberg and Chris Walsh, who some of
you may have heard speak a few weeks ago here. And that center aims
to bring together brain science with
evolutionary genetics to search for the key changes
in the genome that endow humans with our unique abilities
for language, art, culture, and science. I'm sure you're wishing
to get me off the stage, and I can say I'm
just about to leave. Tonight, he will discuss his
New York Times bestselling book, Who We Are and How We Got
Here, Ancient DNA and the New Science of the Human Past. He will also be signing copies
of the book on the table to my right after the lecture. So please join me in welcoming
Professor David Reich. [APPLAUSE] So thank you all for coming. I spoke here in 2011,
the year after the work that we did in collaboration
with Svante Paabo's laboratory in Germany on
Neanderthal intermixing, and it was the best audience
and the best talk I ever gave, so I'm hoping I can
give another good talk, and I hope you enjoy this talk. So the theme of
this talk is really to give a series
of four examples about how genetic
data is showing us that things we
assumed about our past are wrong, fundamentally
wrong, and how quickly, how very quickly, in the
last five or 10 years we've really overturned a lot
of assumptions about the past with this new field
and its potential. So I think if there's
one take-home message it's that mixing is in human
nature, a profound mixing of very, very
different populations, and that no one
population is really even could be pure in any sense. So I'm going to begin by
talking about the disruptive power of ancient DNA. So my lineage intellectually is
as a grandchild, academically, of Luca Cavalli-Sforza, who
died a couple of months ago. And in 1960, he
made a grand bet. And the bet was that it would
be possible to reconstruct the deep past based on analysis
of the tremendous diversity that exists in the world today. So there's people speaking
7,000 or so languages around the world-- incredible, incredible,
wonderful diversity around the world. And his thought was that
by comparing populations around the world to each other
and seeing how closely related they were to each
other genetically, he could learn how
people moved and migrated by seeing who is most
closely related to whom. So the data that was available
was not very good at the time, but what he hoped was that
by using the data that was available, which were
protein polymorphisms, like the ABO blood groups
that you can measure in blood and seeing who, on average,
is most closely related, you can actually reconstruct
which populations are most closely related
to who and create a tree of human relationships. So in this way,
in 1978 initially, and then in this
analysis updated to 1993, he and his colleagues drew
maps of how people are related to each other around the world. And what this is is a principal
component analysis updated to 1993 and redrawn in the
book that I just published, which looks at about 100
places where there's-- 100 protein polymorphisms
like the ABO blood group or the the resist blood
group, where people differ, and based on measuring the
frequency of these types in diverse European
populations and looking at the primary gradients
of genetic variation amongst European
populations, he saw that the primary
gradient in the data was a Southeast to
Northwest gradient, and he interpreted
this as evidence of the spread of farming
from Europe, which we know, based on the archeology, spread
from the far southeast, Turkey and Greece, all the
way up to the Northwest after 9,000 years ago. And so his belief was that the
gradient variation was driven by the archaeologically
documented spread of farming and that that gradient
shown here is, in some way, related to the proportion of
ancestry from first farmers who then diluted that
ancestry as they spread and mixed with local indigenous
European hunter-gatherers. But that turns out to be wrong. The truth is more like this. And so it's that, in fact, the
gradient of farmer ancestry is almost perpendicular
to the one that he drew. And the reason is it's because
people moved too much and more than what was expected. It's not the case that the
movement of farmers into Europe was the last major
movement of people. In fact, there was another
mass migration into Europe that occurred after
9,000 or 8,000 years ago. In fact, it occurred
more between 5,000 and 4,000 years ago and utterly
transformed the ancestry of that subcontinent. And I'm going to tell
you that a little later, what the evidence for that is. So now, I'm going to
introduce this technology by talking about
ancient DNA, which is a powerful new
scientific instrument, much like the microscope was
several hundred years ago when it was first introduced. And the measure of the power
of a new scientific instrument is what happens when you
turn it to something that's never been looked at before. So when the microscope was
used for the first time in the late 1600s to look at
the microscopic world of cells, for example, or
very small things, they discovered cells
for the first time. People discovered structures
that we're just not even envisaged before. And in the same
way, ancient DNA, when it's turned to analyze
human populations that have never been analyzed
before [INAUDIBLE] skeletal remains that are associated
with ancient archaeological cultures. Almost everything that
we see is a surprise when it's looked at
for the first time. So ancient DNA, the way we
implement it in our laboratory, begins with a human
skeletal remains like this ear bone, which is a
common type of skeletal remains we analyzed. In a clean room,
where the goal is to protect the remain against
the people handling it-- because the people
handling it have tens or hundreds of thousands
or millions of times more DNA in them than the
ancient skeletal remains you're analyzing. So even a little
bit of touching, a little bit of
contamination will overwhelm the ancient
sample you're analyzing. So a large number of
measures, such as face masks, and ultraviolet radiation,
and cleansing with bleach, and positive air pressure are
used to protect the samples. We prepare these ear bones
or other skeletal remains. We remove the parts that we
think are richest in DNA. Here's a cochlea
from the ear bone that is known to be
particularly rich in DNA. We turn that into a powder. We release the DNA from the
powder in a solution that's designed to remove the protein
and the mineral content, and we turn it into a
form that can be sequenced in one of these DNA
sequencers, which came online about one decade
ago, a little bit more than that, and made it
possible to literally reduce the amount of sequencing
by about a million fold since about 20 years ago. So with this set, this
pipeline, we can generate data. So what I did in
2013 was I wanted to focus on trying to
study large numbers of ancient human
skeletal samples, mostly within the last 10,000 years. And I got my training
in ancient DNA from Svante Paabo
in Germany, who developed many of the techniques
of ancient DNA analysis and led the work to sequence
the Neanderthal genome and other archaic genomes
that I've gotten the privilege to be involved in as someone who
was analyzing the data to learn about population
mixture and history. But what they were
doing was analyzing one or two or three or four
extremely important samples. And it was not studying
large numbers of samples. And if we want to study
variation and population history, we need to be able to
study large numbers of samples. And so in 2015, this
very important group, a paper by a group in
Denmark was published, which was really the
largest possible study that one could
conceivably do almost, with this brute-force
technology of sequencing DNA from ancient skeletal remains
and just brute-force sequencing and in the sequencer. The problem is that when
you extract DNA from ancient remain, most of it is not human. So even though the
gene sequencing is relatively inexpensive,
even though the technology has advanced to the point that
we can regularly, successfully get DNA from ancient
skeletal remains, most of the material, often
95% of it or 99% of it is microbial from the
bacteria, and fungi, and other microorganisms
that were around the skeletal remains when the
individual died. And as a result, it's
prohibitively expensive to study large
numbers of samples. And so this study studied
about 100 individuals. We lived about 4,000
or 5,000 years ago from Russia and Europe. And the X-axis, the
bottom, is the amount of sequencing that
was actually done. So about a billion DNA sequences
were generated typically for each of the samples
at a cost of about $10,000 per sample. So this was a $1
million experiment, and it would be hard to scale
it much higher than that. So the approach that
I thought that we were going to invest in when
we started our ancient DNA laboratory here was to take a
trick from medical genomics. And in medical
genomics, the approach that many people
were beginning to use was to isolate the part
of the DNA that was most interesting for analysis. So in medical genomics,
you want to study often all the genes in the genome,
which is only 2% of the genome. So what you do is you wash
your DNA sample over a series of artificially printed
out bait sequences, so 50-DNA-letter-long A, C,
T's, and, G's sequences that are printed out, using a
DNA printer essentially, and put in a liquid. And you can take your
DNA sample and wash it over these baits and
only sequence what stuck to the baits. And so what is typically
done in medical genetics is the baits are all the
genes, and the genomes are fragments of
them, and you end up sequencing just that
part of the genome. So what we decided to
do was to sequence not all the genes in the genome,
but about a million places in the genome or a
little bit more that are known to be informative
about human history and biology because people vary at them. And so we developed a
cocktail, a liquid cocktail of more than a million
positions in the genome that we would wash
our DNA samples over, and we would only
sequence those parts. So we would only be
sequencing human DNA, and that would result in a
10 to 100-fold enrichment, and we would only be sequencing
the parts of the DNA that were useful for us,
our studies of history, and that would be
another major enrichment. So this resulted in 10 to
100 times less sequencing, and it typically produce higher
quality data per individual. And so here is a
paper we published in that same year, where
the cost per sequencing and the amount of sequencing was
far, far less, 10 to 100 times, and the quality of the
data was much higher. And this has made it possible
to sequence many, many samples. So since the laboratory
started in 2013, there has been a 100-fold
increase in the amount of data produced by our
laboratory and others, and the number keeps increasing. And with that much
more data, it's possible to ask and
answer questions about the past that are
simply not possible to answer with much smaller data sets. And I'm going to tell you
four examples of that now. OK, so I'm going to
begin a little bit more by giving you a little bit more
background about how such data reveals about the past. So the type of data we're
looking at in cartoon form can be thought of as follows. So in this cartoon,
which is a picture that I made for the book to try to
explain this with an artist, you start with a cell. So that, on the left, is a cell
with 23 pairs of chromosomes. Your DNA is an approximately
three-billion long sequence of DNA letters-- adenine,
cytosine, guanine, and thymine, that is broken up into
23 pairs of chromosomes, which are the packages on which
those letters are contained. And those are shown in the
nucleus of the cell that then blows up into a pair of
chromosomes, which one of which you get from your
mother, and one of which you get from your father. You get one of each
chromosome from each. And here is a blow up of
a little section where you can see the double helix
of DNA, which is comprised of these four DNA letters-- A, C, T, and G. So the way we learn about
history from this data is that we study the differences
between paternal and maternal and other DNA sequences we have. So the great majority of DNA
sequences between your mother's copy and your father's copy that
they gave to you are identical. The letters are 99.9% identical
where you can line them up, more or less, which is a very
high level of similarity. However, in the
0.1% that differ, since the genome is
three billion bases long, 1,000th of that is 3
million differences. That's a lot of
differences, and we can use that to learn about
the history since your mother and father share
common ancestors at each place along your DNA. And so what we can do is look
at those occasional differences and learn about history. In particular, if you look
at places in the genome where there are not very many
differences, that tells you that in that place
of the genome, your mother and father
are quite closely related because there hasn't
spent a lot of time for the random miscopying
of DNA to accumulate these differences, these
mutations, these random errors; whereas in places where
there are many differences, that's been a place where your
mother and father are typically quite ancient related. The typical time since
any two sequences, for example your
mothers and fathers, share a common
ancestor in humans is one to two million years. But if I'm comparing my sequence
to that of my brother's, it might be only one
generation old. OK, so how do you use this
data to learn about the past? Well, one important
thing to realize is that you're actually
not one person. You contain within you a
multitude of individuals. The reason is is that your
DNA, your whole sequence, is packaged on 47 chunks of DNA. You're 23 pairs-- so 23
times two, 46 chromosomes, and your mitochondrial
sequence, which is a short bit of DNA
you get from your mother. So you start in your
current generation with 47 chunks of DNA. And when your mother and father
produce an egg or a sperm to produce you,
they break the DNA they got in turn
from their parents, on average, about 70
times per generation. They splice together their own
mothers and fathers chromosomes to send you mixed DNA
to their offspring. That's the process
called "recombination." So these 47 chunks,
thinking back in time, fragment into another 70, so
about 118 one generation back, and another 70, 189
two generations back, so you add about 70, 71 chunks,
on average, every generation going back in time. And these chunks get
diluted into ever larger, exponentially increasing
numbers of ancestors-- 2, 4, 8, 16, 32, 64. And so by the time you're
10 or 11 generations back, you have actually fewer
DNA chunks than ancestors. There are some ancestors that
are not even ancestral to you. They haven't given you any DNA
because your DNA is getting homeopathically diluted over
larger and larger numbers of ancestors. So what this is telling you
is that but for the sections that you do get from your
ancestors, each of them gives you a sample of
ancestry from that ancestor. So you're not one
person; you, in fact, are hundreds or thousands
or, if you go back a few tens of thousands
of years, tens or hundreds of thousands of ancestors. And so with this data, you
can determine exquisitely accurately how closely
your genome related is to another genome
by averaging over all of these independent
combinations. So you might think that
you're only one sample, only one flip of a coin, to measure
the probability of being heads, but, in fact, you're 10 or
100,000 flips of a coin, and you could obtain
exquisite measurements of how closely related two
people are to each other. And that's the
power of the genome, and that's why it's so much
more powerful than what you've probably heard about
for several decades, like mitochondrial DNA sequence
or Y chromosomes, which is just one section of the genome. So this ancient DNA
revolution that's unfolded over just the
last eight or 10 years has really been
powered by the fact that you're looking
not just at one section of the
genome anymore, which is what's been happening
for several decades and has produced a number
of interesting insights about the past, but now we
have this multi-dimension, Technicolor-rich
quarrying that's possible by sequencing
the whole genome, including of ancient samples. OK, so the talk is going to be
based around four lessons that, for me, are lessons in humility,
which is that the assumptions I came to in looking at each
of these problems were wrong, and that's what we see again
and again with DNA analysis. So the first lesson
in humility is that our ancestry of
all people in the world is not all traced back
to sub-Saharan Africa 100,000 years ago. So in the '70s,
and '80s, and '90s, it became increasingly clear
that the great majority of human ancestry comes out of
Africa 50,000 to 100,000 years ago, and we still believe that. So there were archaic
humans outside of Africa for the last two
million years almost. But anatomically, modern
humans, people whose skeletons look like ours,
with globular brain cases like this Cro-Magnon
individual from Europe, have been the longest in
the African skeletal record, going back to 200,000
to 300,000 years. So it was thought, based on DNA
analysis of present-day people, including in my PhD
work in the late 1990s and by Luca Cavalli-Sforza
and many others, that all lineages
traced back to Africa, that non-African
diversity is just a small sample of
African diversity because it's an
out-of-Africa population. However, Neanderthals, which
are archaic humans, who were discovered in the mid-19th
century in parts of Europe and who had brains
as big as ours and made tools as sophisticated
as our own ancestors, primary ancestors did
and were distributed in this distribution in
Europe and Western Asia, these people we know
met modern humans. And so when it became
possible to sequence a Neanderthal genome,
a question was, as modern humans, who looked
liked us skeletally, moved out of Africa 50,000 to
100,000 years ago and moved through
Neanderthal territory, did they interbreed
with Neanderthals? And if they interbred, did
that interbreeding produce descendants who live on today? So the question was,
was there interbreeding? And if it occurred,
did it occur in Europe where the Neanderthals are
densest and most densely distributed? Or did it occur elsewhere, like
in the Near East, where there's a more scattershot
documentation of Neanderthals and modern humans
and not totally clear from the archaeological
record, whether they interacted as it is in Europe? And what about
the archaic humans who are documented less clearly
but still definitely documented further east? So when the Svante
Paabo laboratory succeeded in generating
a whole genome sequence from the Neanderthals
in the late 2000s, my colleague Nick
Patterson and I were brought on to try to test
whether Neanderthals match some humans more than others. And we developed several
tests for interbreeding, and the simplest one
I can show you here. So this is a chromosome
in cartoon form, so that's a structural
element in the chromosome, the centromere, and
these are long arms in the chromosome
on which hundreds of millions or tens of millions
of DNA letters are packaged. If you compare a DNA
sequence from a French person and a Nigerian person and look
at the 0.1% of the letters that differ-- for example, where the French
person has a thymine, a T letter, and the Nigerian person
has a guanine, a G letter, we can then ask,
does the Neanderthal carry the French type
or the Nigerian type? And we did that. And when we did
that analysis, we found that Neanderthals carried
the French type significantly more often, in this case
92,000 matches to the French and 84,000 matches
to the Nigerian. And if it was the case
that Neanderthals separated from the common ancestor
of Nigerians and French before they separated
from each other fully, then they should
match each other we can prove equally often,
but that's not the case. And this and other
lines of evidence made it very clear
that Neanderthals, similar to the one
that we had sequenced, had interbred into the
ancestors of French and the same test
when you replace French with any other
non-African population showed a similar
result. For example, Chinese people also have
ancestry from Neanderthals, even a little more. So we tried to estimate
the proportion of ancestry from Neanderthals in
non-Africans today, and we just repeated
the same test where we look at the Nigerian
and the non-African person, and then we replaced
the non-African person in this analysis with a
second Neanderthal sequence. And you ask the
question, how much is the way of excess
matching is a non-African to when you replace it
with a second Neanderthal. And when you look at the
answer, it's about 2% of the way, which means
that non-Africans today have about 2% of their DNA
from Neanderthals, and it's significantly more in
East Asians than in Europeans, and we now know why
that's the case. I won't explain
that in this talk, but I can explain it to you
some other time for sure. So the next thing
we wanted to do is we wanted to know a date
when this interbreeding occurred between Neanderthals and the
ancestors of non-Africans. And the way we did this is to
develop a statistical technique to estimate date
based on fragmentation of the genome that occurs
generation by generation as you splice together DNA from
your mother and father. OK, so this is, again,
representing two chromosomes. For example, red here
might be modern human, and green here might
be Neanderthal. And a mixed individual will have
one entirely red Neanderthal and one entirely green
modern human sequence. And when you produce
an egg or a sperm, you fragment that DNA one
or two times per generation. We know the rate at which
that fragmentation occurs. And today, far
afterward, you can look at the dice
size, the chunk size, of Neanderthal and
modern human segments, and that tells you
how long it's been since the interbreeding
between these two very different groups happened. So what we did is we looked
at that typical size measured on the x-axis of fragmentation
in Nigerian individuals and the scale at
which these fragments that we'd thought we
might detect occurred were very tiny, consistent
with shared ancestry between Nigerians and
Neanderthals-- very, very anciently. However, if you look
at non-Africans, the scale is much
larger, and that is due to French sharing
ancestry with Neanderthals much more recently. And by the size of
this and knowing how fast this
fragmentation occurs, we were able to estimate a date. We had at particular
luck, several years ago, obtaining DNA sequence in
2014 from a 45,000-year-old individual, a non-African
from Siberia, and there, they have huge fragments
because they haven't-- they were living
pretty close in time to the interbreeding event. And because we know the
date of that individual from radiocarbon
dating and because we can see the big fragments that
tell that the mixing occurred 5,000 to 10,000 years
before the individual lived, we can estimate quite
precisely that the mixture in the common ancestry
of non-Africans today occurred in the narrow band
between 49,000 and 54,000 years ago. So the conclusions of
this set of findings that we had between
2010 and 2014 was that there was Neanderthal
interbreeding, about 2% with the ancestors
of all non-Africans and that that interbred,
that mixed population then spread its ancestry
throughout Eurasia, and that's why all Eurasians
have about 2% ancestry, with interesting ways in which
that's not exactly true that I won't tell you about. So in the same year that
we published this work on the Neanderthal genome as
we were closing this project, my colleagues in Leipzig
obtained DNA sequence, which was completely unexpected
from the following site in South Central Siberia from
Denisova Cave in the Altai mountains near the border
of Mongolia and Kazakhstan. And this individual was-- the DNA was from a
finger bone, which was labeled as
possibly modern human, and it was 70% human DNA. It was a very special
sample that was full of DNA, and they obtained a
high-quality genome sequence from this individual. When we analyzed the data,
we found that Neanderthals, these are four Neanderthals,
were quite closely to each other, the
genome sequences we had. But this Denisova sample
was very distantly related but more closely related
to the Neanderthals than it was to modern humans. So this separation
corresponds to time. It's counting number of
mutations that occurred, and we could show
that this corresponded to a separation of many, many
hundreds of thousands of years. So this was not another
Neanderthal and not a modern human. It was something else entirely. So we also had a second
sample from this population but much poorer
quality from a tooth. And so we, of course,
played the same game. We asked the question,
does this Denisovan genome named after the cave from
which it was excavated match some humans
more than others? And we obtained a
tremendous surprise. When we compared the Denisovan
genome to two East or Southeast Asian populations,
Chinese and New Guineans, we found that the Denisovan
genome match the New Guineans much more often. And this was definitely
a real signal. And when we replayed the same
analysis that I told you about with Neanderthals, we can
estimate that New Guineans and some nearby populations
have about 3% to 6% of their DNA derived from Denisovans--
actually quite distant cousins of the Denisovan from Siberia
separated by about 300,000 years-- another group
altogether, but more closely related to Denisovans
than anything else. So this was a shock where,
in the case of Neanderthals, we actually had a very
well-posed question posed by 150 years of archeology. It was a question of, we
know about the new archeology of Neanderthals, these
very impressive people who left behind
these very impressive archaeological remains. Did they interbreed
with modern humans? That was a question that
genetics could answer. But here, with the
Denisovans, it was reversed. We had no expectation at all. And here, instead of a
fossil in search of a genome, it was a genome in
search of a fossil. And the question was,
who were these people? We still don't know anything
about what they looked like or what tools they had. The only DNA we have
is from this one cave from Siberia, where are we
now have multiple genomes from this cave. Denisovan genomes are widespread
east of the Wallace Line-- so in New Guinea,
in places that are affected by New Guinean
ancestry in Australia and also in the Philippines. And so the conclusions
is that there's 2% Neanderthal
interbreeding, and then further interbreeding
leading to Denisovans. We now know there's a small
tiny seven interbreeding event that also contributed to
East Asians, a different type of Denisovan ancestry. So just published, not by
my group, but by the group in Germany, Svante
Paabo's group, they obtained DNA sequence
just a few months ago from another individual
from Denisova Cave who has a Neanderthal mother
and a Denisovan father. So this is a
first-generation hybrid of Neanderthals and Denisovans
that was actually sequenced. In studies of modern
humans from Romania, where an individual about
40,000 years ago, we found an individual
that was a mixture within four to six
generations of a Neanderthal in modern humans. So we've only studied 10 or
20 individuals from this time period, but what's very
clear is that there were many hybrids of very
different populations, more different from any pair
of human populations today running around
the world at that time. So it was a very
different population. It shows you that humans, even
very distantly related humans, when they encountered
each other, were mixing again, and
again, and again, and again. So what has happened as a
result of these DNA sequences, it's unleashed and opened up
a Pandora's box of mixtures between archaic
humans that we now know about and have glimpses
of from the ancient DNA that were just not expected before. And so this is an
example of humility. I think before this
work, we thought these were distant
groups separated by us from a million
years or a very long time with a simple history
relative to us. But now, we know
they're entwined with our own history,
and each other's history, and lineages we didn't
even know about altogether that we now have
sampled in mixed form in these populations? OK, the second part of my talk,
the second lesson in humility comes closer in time. And this is a short
section of my talk. And it's about how the idea of
white people, "White" people, West Eurasians or
Caucasians, is actually quite misconceived in some way. So for several
hundred years, people have recognized that there's
a large region of homogeneity in terms of how people
look and other traits all the way from
the Atlantic coast of Europe to the Near East,
and Iran, and Central Asia. And it's true genetically. If you actually look at
people across this region, the average difference in the
frequencies of DNA letters are quite small
compared, for example, to the differences between
Europeans and East Asians. So in 2016, we obtain DNA from
people across this region, West Eurasia, early
hunter-gatherers of Europe, who lived before
farmers got there in the east and west
of Europe, farmers from the Levant
in the Near East, and farmers from ancient
Iran further east of that. And when we got DNA from all of
these groups, what we could do is we can measure the frequency
differences between these DNA letters in all of these groups. And we found-- I'm using this
color coding to show ancestry from each of these groups-- we found that the
average frequency difference between
these groups is measured by the average squared frequency
difference of variable DNA letters, where some people
have an adenine, for example, and some people
have a cytosine, was as large as between Europeans
and East Asians in all four of these groups. So this was 8,000,
10,000 years ago. And in that region, there
were at least four groups as different from each other
as Europeans and East Asians are today. So if you were able to
go back in a time machine and try to reconstruct
the population structure of the world
10,000 years ago, it would not look at
all like it does today, with its supposedly relatively
homogeneous group that we see today. Instead, it would be
broken up into many groups. This is where we know it best. But presumably,
it's also that way in East Asia, and South
Asia, and parts of Africa, and other parts
of the world, too. So how did this region of great
heterogeneity, which was not anticipated based
on present-day data, get to be the way
where it is today? Well, we learned that
from studying later people before these hunter-gatherer
and early farmers. And what happened is
that by 6,000 years ago, none of these four
groups disappeared. They all mixed with each other. They expanded and
mixed with each other. And that mixing has caused a
homogenization of these groups, just like when you mix
ingredients to bake a cake. And there was a three-fold
reduction in differentiation by 6,000 years ago. And by 4,000 years ago,
at the beginning or middle of the Bronze Age,
the population had reached its present-day
low level of differentiation. So white people are, in
fact, a recent phenomenon. They're not an age-old thing
that existed for a long time. They're a product
of profound mixture of multiple groups as different
from each other as Europeans and East Asians
that came together due to processes in
this part of the world in the last 10,000 years. Third lesson in
humility, which was that that event that explains
why Luca Cavalli-Sforza was wrong about his assumption that
the main gradient of variation in Europe that he was
measuring reflects the movement of farmers into Europe. So prior to three years
ago, there was an assumption that farming was the only
economic transformation of the last 10,000 years
in Europe large enough to make a substantial
demographic dent on it. The idea was that
prior to farming, there were
hunter-gatherers in Europe. They were not exploiting the
environment very efficiently compared to farmers. And so when farming was
invented in the Near East, as it was 11,000 to
12,000 years ago, those farmers would
be able to move into hunter-gatherer territory
with their new technology, and bring people into
Europe, and displace or mix with the local
hunter-gatherers, and that would have achieved a
substantial population transformation and turnover. But once there was a densely
settled farming population in Europe, it would be
very difficult to make a dent in Europe. For the same reason, the Mughals
and the British politically controlled India for
many hundreds of years, but neither of them has made
much of a dent demographically on India. However, that turns out
to be wrong for Europe. And the data shows
that, and I'm going to show you the
evidence for that. So with ancient DNA
in 2014 and 2015, almost all the DNA
available at that time was older than 5,000 years ago. And if you estimated what
proportion of ancestry, what proportion of
ancestors, people 5,000 years ago had from
the farmers of Anatolia, which is the source
population of farming-- the source population that
brought farming technology into Europe, as we now know
from ancient DNA correlation to the actual remains at
archaeologically sites-- so it's shown in blue
here-- and hunter-gatherers, people were mixed of
these two sources, mostly farmer ancestry, but
some hunter-gatherer ancestry with variable proportions
in different individuals. That was the state of
knowledge in 2014 and 2015. However, today, if you look
at the ancestry of people in Europe, there is
a third ancestry, which, in many groups, is
the predominant ancestry, especially in Northern Europe,
this red ancestry, which, for example in Northeast Europe,
is more than half the ancestry. So when did this
third ancestry arrive? It must have been sometime
between $5,000 years ago and today. And so we and others sought
to try to figure that out. So in the two or three
years before this, we had had an
important observation in our laboratory
based only on analysis of modern individualism,
and it went as follows. So we developed a
statistical test for whether a
population is mixed when you analyzed
genome-wide data, and the test works as follows. We analyzed hundreds
of thousands of places in the genome where
people are variables, where some people, for example,
have an adenine, an A, a DNA letter, and some
people have a cytosine. And so we look at the frequency
of those variable positions, here it's shown in a pie chart,
in different populations-- for example, in Northern
European population, in Native American population,
and in Sardinian population, which is a Southern European
isolated island population. And in this example, the
Northern European frequency of the adenine, the
A, or the thymine might be intermediate
between the Native Americans and Sardinians. So what we do is we
averaged the frequency difference over all 600,000
positions we're analyzing, and we ask the
question, on average, are Northern
Europeans intermediate in frequency between two
comparison populations? And when they are, on average,
intermediate frequency, it's provable that
Northern Europeans are mixed between
two groups related, maybe very distantly
in the past, to the two comparison
populations. So we have this
statistical test in hand, and we applied it to all
sorts of populations. So for example, for
Northern Europeans, we took and applied it
to all possible pairs of other populations. We get a huge signal
in Northern Europeans, which is maximized when one of
the populations is Sardinians. And the reason we think
it's maximizing Sardinians, we now know, is
because Sardinians are a relatively isolated
population descended from the first
farmers of Europe, who have not been affected
by much subsequent movement. So it's a mixture of
farmers and something else. And the second population of
all people was Native Americans. And that was a real shock to us. It was definitely
Native Americans. It was not South
Asians or Siberians. It was not East Asians. And we, of course, didn't
think that Native Americans crossed the Atlantic and
moved into the Americas. Instead, we proposed a
new model, a hypothesis, which was that Northern
Europeans today are a mixture of ancient farmers
from the Near East and a group that we called Ancient
North Eurasian, which no longer exists in the world. It no longer exists
in unmixed form, but was somewhere
in Northern Eurasia before 15,000 years ago. Sometime before 15,000
years ago, descendants of it crossed the Bering land
bridge into the Americas and contributed to the
ancestry of Native Americans. And sometime after
5,000 years ago, presumably, they also
contributed some ancestry to Europeans. So we proposed this population. It wasn't sampled
in any modern data, but it was a statistical
reconstruction, what we call a ghost
population, something that is statistically a
figment of our imagination, but we predict exists. So a year and a
half later, a group working in Denmark,
the same group that produced that important
paper I showed you earlier, obtained DNA from
this ghost population. They found it, and it was in
this East Central Siberian sample, a 24,000-year-old little
boy buried near the shores of Lake Baikal in Siberia,
and this shows his affinities genetically. He has a strong affinity
to Native Americans, shown in this heat map, and
also affinity to Europeans, but relatively little
relatedness to people who live in that
same region today, to indigenous people, because
present-day indigenous people are largely post-ice
age migrants from the south back into that region. So this was very exciting. And since the discovery
of this ghost population in ancient DNA, there
have been discoveries of many additional
ghost populations. So now, I'm going to show
you a still movie that reveals how the ancestry of
these ancient North Eurasians, which is the third ancestral
population of Europe, got in to Northern
Europeans today. So here what I'm
showing you is something called a "principal
component analysis." So this data is data from
about 1,000 present-day West Eurasians drawn from
the locations here. That's Spain. That's Italy. That's the Black Sea-- to just give you orientation,
and the symbols show you where each sample are from. So what you actually
would do in this analysis is you should think of
your data as follows. I'm going to tell you how
this analysis is done. So the data consist
of a grid, a table, which has about 600,000
rows corresponding to all the positions
we analyzed-- the variable positions. And it's about 1,000
columns corresponding to all the individuals
we analyzed. So it's 1,000 by 600,000 table. And in each cell of the table
is a 0, 1, or 2 corresponding to whether you have zero, one,
or two copies of a variable DNA letter, like a thymine-- adenine or thymine,
so this person at this cell in their table will
have zero, one, or two copies. So then, you multiply
this table by itself, and you'll get 1,000 by
1,000 matrix table measuring how closely related
every pair of individuals is to each other averaged
over these 600,000 positions. So with this table, which
shows how closely related each of these 1,000
individuals are to each other, we can then carry out
principle component analysis on the data, which is a
mathematical technique for finding the
combination of DNA letters that most efficiently separates
the samples from each other. So a person's
position on the X-axis here might be 0.1 times
the value of DNA letter 1 plus 0.1 times the
value of DNA letter 2 minus 0.1 times the value
of DNA letter 3, and so on. And so you can look at the
position of every sample. And the second y-axis is the
second most informative way to separate the samples. And when you analyze
the data this way, something magical happens,
which is, West Eurasians break into two parallel gradients. On one side is the Near East. On the other side is Europe. On the top is
non-Mediterranean populations. And on the bottom is
Mediterranean populations. There's a pretty big gap
in between filled by groups with known recent contact
or plausible recent contact between Europe and
the Mediterranean, like Jewish populations
or island Mediterranean populations. Right, so this is the
present-day samples. And now, I'm going to
gray out these points. So these are the
present-day samples, and I'm going to plot where
the ancient samples fall. We know where they fall
because we can use that 0.1 times the value of position 2-- 1 minus 0.12 times the value
of position 2, et cetera, to just see where they fall. So if you look at the
hunter-gatherers of Europe, people who live
8,000 years ago, you see they fall beyond
Europe in the direction of European differentiation
from the Near East, and that's because
Europeans today are a mixture of
hunter-gatherers and Near Easterners. But these people no longer
exist in unmixed form in Europe because they mixed
with the people from the Near East who
came in with farming. Then, the first farmers come,
piling up on top of Sardinian. Because Sardinians,
you can think of as a relatively
unmixed descendant of those first farmers, with
relatively little influx since that time. But this is where most
Europeans are today. And at that time, 8,000
years ago to 5,000 years ago, you still don't see
people whose ancestry looks like Europeans today. Meanwhile, in Far
Eastern Europe, you see a group called
the Yamnaya, who are a archeologically
well-documented group that I'll tell you a little
bit more about in a, minute who are pastoralists. They're the first
people who went out into the open steppe
lands far away from the rivers to graze their cattle,
and sheep, and goats. But you still don't see
people like Europeans today. That only happens after
5,000 years ago very suddenly in association with
some very well-documented archaeological cultures who
made certain types of pots. And after 5,000 years ago,
you see people with ancestry like Europeans today. So that's when this third
ancestry, which is coming in through these Yamnaya,
got into Europe, who, in turn, got it from
the ancient North Eurasians deeper in time. So a summary is that Europe
has been massively transformed by two mass migrations
in the last 10,000 years. The first, after
9,000 years ago, bringing farmers from Anatolia
from present-day Turkey into Europe, and this
is the proportions of ancestry from first
farmers and hunter-gatherers and a variety of ancient sample. And the second is the mass
migration from the Steppe north of the Black and Caspian Sea. And the Yamnaya, these ancient
culture and the many samples we now have for them, are
an excellent surrogate statistically for this source,
and they bring in this ancestry initially. It's a 70% population
replacement in some parts of Europe. And then, over time-- this
is moving forward in time, there's a rebound where farmer
populations mix back in, and you get populations
which are primarily Yamnaya in many cases, but
still have major other portions of their ancestry. So who are these Yamnaya people? So as I mentioned, this
is a skeleton of one of the people we sequenced. This is a big, scary copper
mace that this individual had. His head was bashed in. He was basically killed
in a violent incident. And this is common for many
of the Yamnaya individuals. So the Yamyana, as I said, were
a very impressive and unique archaeological culture. They were the first
people to take advantage of two recent inventions. They first spread
over the steppe after about 5,300 years ago. And they took advantage
of two recent inventions. One was the recently
domesticated horse, which was a profoundly
important innovation, and the other was the
newly invented wheel. They used horses
hitched to wagons to bring their supplies out into
the open steppe lands far away from the river valleys,
including water, and to exploit the
rich grasslands that were not exploited before. Prior to the Yamnaya, there were
many small different cultures spread over this region,
which had little settlements after the Yamnaya, very
few of those settlements persisted, and all you
see left are big graves. And the interpretation
of many archaeologists is that these people were
living in ancient versions of mobile homes and
moving around the steppe. They were very successful. They expanded from
where they initially originated, all the
way from Hungary in Central Europe, all the
way to the Altai Mountains on the boundary of Mongolia. And many of the groups that had
been there before disappeared. So I'm going to
now show you what happened with the
spread of Yamnaya ancestry in different
places in the periphery. So here's Britain. So from 6,000 years
ago when farming first got to Britain from the
continent to 4,500 years ago, here is the proportion of
farmer ancestry on the Y-axis. And then, bang! 4,400 years ago, this people
from ancestry from the East get into Britain. It's a 90% population
replacement. Stonehenge is built a little
bit before 4,500 years ago by people who, presumably
based on this plot, had entirely farmer ancestry. The big stones had just gone up. And the people in Britain
today who primarily descend from people like this with
90% ancestry from the east are not the people who
built these monuments. This is what happened in Iberia. So a very similar pattern, where
6,000 years ago, there's farmer ancestry up till
about 4,500 years ago. Then, in Iberia, we can
document a period of overlap for a few hundred
years, where there are farmers and also people with
ancestry from the east living side by side. And then, after a
few hundred years, they mixed together and achieved
an intermediate proportion. In each case, it's
only about 40% ancestry from the east, a
less dramatic replacement of population than
you see in Britain. However, if you look at these
circles and the coloring of the circles, you'll see
something very important. So the open circles are
females, and the field circles are males. So for males, you can determine
the Y chromosome sequence, which is the sequence
you get from your father. And if you're a male-- and you can determine
whether it's typical of the
Russian steppe or not. And if it is, it's red. And what you see in Iberia
is it's 100% male population replacement despite being only
a 40% overall DNA replacement. And what that's telling you
is that these male individuals coming in from the east
had preferential access to local females
again and again, displacing local males
generation after generation. And it's telling you something
very profound about the nature of this population
replacement that happened through people who descended
from these Yamnaya. We also now know
something profound about how Yamnaya
ancestry spread east. So here's a paper that
we have that we're trying to bring to publication
now where we report data from more than 500 individuals
from these squared points. Here's Kazakhstan. Here's Iran. Here's Pakistan. And we compared it to many,
many modern populations from South Asia to learn how
Yamnaya ancestry spreads east. So with this huge sample
size of ancient DNA, which makes it possible to carry
out population studies, we can do something
that's very difficult to do with small
sample size, which is we can look at variability. So for example,
if you're looking at a population and many
samples from the city of Tokyo for example, if you
only had three samples, it perhaps wouldn't
look very interesting. They would all perhaps
look genetically the same. But if you have 80 samples
or something like that, you're going to see occasional
outliers, individuals who have very different ancestry. They might be European,
or they might be Chinese, or they might be Korean. And they're telling
you about the groups with which people in that
city are in cultural contact. That's what we're
seeing in towns like this that were spread to
the north of present-day India and Pakistan 4,000 years ago. Here's one of these
towns which is one of the first
great civilizations of the ancient world called the
Bactria-Margiana Archaeological Complex. And we have 80 individuals
from this town, and we can study the
outliers at these sites. So most of them are genetically
similar to contemporary farmers from ancient Iran that
we also have data from. However, prior to
4,000 years ago, we see occasional
outliers, which are similar to hunter-gatherers
from that region. After 4,000 years ago, they
have ancestry from the Steppe, ultimately descended from these
Yamnaya Steppe pastoralists. And then, in individuals from
Pakistan from 3,000 years ago, from Northern Pakistan,
the Swat Valley, we see through
these chunks of DNA that have fragmented that the
Steppe ancestry has been there for at least 500 years. So we now can limit when this
Steppe ancestry got injected into South Asia to a relatively
narrow window between 4,000 to 3,500 years ago. And it explains,
today, Steppe ancestry ranges between 0% to 30% of the
ancestry in South Asia today, and it somehow got pushed
through in this time. We also find amongst these
sites of this ancient Bactria-Margiana complex
and other neighboring sites in Eastern Iran,
14 individuals who have a different
type of ancestry, not from hunter-gatherers
from the North, not from Yamnaya, but
rather South Asian related admixture related to present-day
Southern Indians or Southeast Asians, people more similar to
Southern people in India today. And we interpret
these individuals as migrants from the
South and Southeast, and probably from the
Indus Valley Civilisation, which was a civilization further
to the south contemporary to the Bactria-Margiana
Archeological Complex. So what we can see
in South Asia is a history of three layers
of population mixture. Prior to 4,000 years
to today in South Asia, people are a mixture of a
Steppe-ancestry related source, an Iranian
farmer-related surface, and a Southeast
Asian-related source. And people in
India are a mixture of two mixed populations,
which in a paper in 2009 we called the "ancestral North
Indians and the ancestral South Indians," and these are
individual populations. Now, how did this gradient form? Well, more than 4,000
years ago, these samples who are outliers at these towns
are mixtures with no Steppe ancestry, but from an
Iranian-related source and a Southeast
Asian-related source, and we see variable
proportions of it. The Ancestral South
Indians are just a point along that
gradient that we think remained from that time. After 4,000 years ago, this
Steppe-pastoralists group mixes in, and we have multiple
samples along that gradient, and the ancestral
North Indians are a point along that gradient. And mixtures of those mixed
populations after 3,000 years ago or so form South
Asian ancestry today. So three gradients. Looking about it geographically,
here's another way to see it. So farming was developed
in the Near East 11,000 to 12,000 years ago and
explodes both West and East, after 9,000 years ago. Into the west into Europe
from Anatolia, into the East into the Indus Valley from
Iran after 9,000 years ago, it spreads across these two
subcontinent of Eurasia, which are about equal in size
and, typically, historically have had about equal
populations to each other over several thousand years. It takes a few thousand
years for farming to spread because
the farmers need to adapt their crops to the
new ecological conditions, different temperatures,
different rainfall patterns in each place. And so meanwhile, the Steppe-- and the Steppe shown here
in yellow, the Yamnaya form. They spread to the peripheries
of each of these regions, and then mixtures of these
mixed populations of the Steppe people, who then, the farmers
they mixed with in each place, forms the primary gradient
in each region today. So people across this
region from Europe and also India and Iran speak
very related languages, Indo-European languages, which
are almost homogeneously, with some important
exceptions, spoken in Europe and spoken in Northern
India, and Iran, and also in a few other places. And it's now highly likely
that these spoken Indo-European languages are reflecting the
spread of Yamnaya ancestry across into Europe
and into Central Asia and through successor
cultures further on into each of these regions, corresponding
to the times documented by the ancient DNA. So the final part of my
talk is this work today, which was published today
about ancestry in the Americas. So in 2012, we
published a paper which analyzed data from diverse
present-day Native Americans. And here's a lot of present-day
populations we analyzed. And what we showed, and what
subsequent work has also shown much more richly, is that
people in the Americas can be-- the deepest separation
amongst Native Americans today is one that's given rise to
some Northeast Native Americans, like Algonquin-speaking groups
from Northeastern United States and Canada, and Southern Native
Americans, who's everybody in Central and South America. And it might have been
a simple radiation from that ancestral population
leading to the populations today. So in 2015, we did some more
analysis of present-day data because we didn't have
any ancient DNA data yet. And Pontus Scotland, a postdoc
that I work closely with, did an analysis where
we played the same game and looked at whether New
Guineans, Australians, and other people
from Southeast Asia are more closely related
to Native Americans from the Amazon and Native
Americans from Mexico. So you might think this
is a weird analysis, and it is a weird analysis. However, there's a
very strong signal of Native Americans
from Amazonia being more closely related
to New Guineans, Australians, and other Southeast
Asian indigenous groups than Mexicans. And so here is the
degree of sharing. And you have a group of
samples from Amazonia having this sharing with
these Australasia groups. So what we argued is
that this is evidence for two founding
populations of the Americas, that somehow there was a
group that contributed more to these groups in Amazonia
than to other Native Americans, and that it was not a
single-source population, but a mixture of at
least two populations coming early on
into the Americas, and that the primary
ancestry of Native Americans is from a group that's
less closely related to Australasians. So in this paper
in 2015, actually, this is our model that
Amazonians and Native Americans have a different sources
slightly related to Eurasia. So today, we and
several other groups published new DNA from the
Americas, an ancient DNA. And so to give you a sense
of how this is affecting the literature, the amount of
ancient DNA from the Americas was really only eight
high-quality samples until 2017. But just this year,
there's now a big jump of almost 100 new individuals,
most of which are from today. In South America, it's
even more dramatic. There was only one
individual until today. And now, there's 51 individuals. And so with this tool,
we can look, and ask, and answer questions about
the past that were simply not possible to answer before. There were three papers
about this today. I'll tell you about
ours, which I know best. So in our paper, we
analyzed data from 49 newly reported Central and South
American individuals, which correspond to the squares
here, from four regions of the Americas-- Belize, the Central
Andes, Brazil-- that's not Brazil; that's
the Southern Cone-- and each of these gradients
starts, at least, 9,000 and up to 11,000
years ago and goes until, in some of the cases,
until the last 1,000 years. So when we analyze a heat
map measuring how these very ancient samples like this
11,000-year old individual from Chile relates to present-day
Native Americans, you see they're not
particularly closely related, this individual in Chile
in green, triangle, to people from that
same region today. So that individual doesn't
have any obvious evidence of particular
relatedness to people who live in the same region today. Similarly, this 10,000-year
old individual from Brazil doesn't have any obvious
relatedness to people in Brazil today. Similarly, this 7,500-year
old individual from Belize doesn't have any obvious
relatedness more to Belizeans than to other Central
and South Americans. However, between 9,000 to 6,000
years ago, that relatedness developed. So here's a 7,700-years
individual from Argentina, and there's clear relatedness
to present-day Southern Cone populations. Here's a 6,000-year old
individual from Brazil, which begins to show relatedness
to present-day Brazilians. Here's an almost 9,000-year
old individual from Peru, which is clearly more closely
related to Peruvians today than after that time-- than before that time. So what that shows you is that
beginning around these times, there began to be
established, in each of these regions, groups that
were clearly contributing to people later today. So we developed a
very simple model for how these ancient
samples diverged from a common ancestral
population, and it a sample splitting model,
with stem Native Americans, ancient Alaskans,
ancient northern Native Americans, I told you before,
southern Native Americans, and then a rapid radiation
of lineages in Central South America. So quickly, it was
as if they were moving into an empty continent. However, when we compared
them to this individual, which had been published a few
years ago by another group, 13,000-year-old
individual from Montana, from what's known as
the Clovis culture, we found that our three
earliest samples from Belize, from Chile, and from Brazil were
distinctly more closely related to it than the later samples. And so that told us
that these groups had ancestry from a group
related to that that was then largely displaced. So that group is associated
with the Clovis culture, which is the first widely dispersed
culture of North America and dates to about a little
bit before 13,000 years ago and made these very
distinctive points. And there's always
been a question of, was the spread of
Clovis, an event that also impacted Southern America? Of course the culture didn't
move into that region, but maybe spreads of people that
occurred around the same time and maybe were
related to each other might have impacted some
contemporary sites associated, for example, with fishtail
culture in South America. So our data answered that in
showing that spreads of people associate with the spread
of the Clovis culture, at least that one
sample, also impacted South and Central America. So with this data in
hand, we can actually come up with a more complicated,
and rich, and informative model of what happened. And I didn't have time,
because it was just today, to make us a pretty slide,
so this is a little busy. So here is this model
with the Clovis individual and how it's related. And what you can see
with these dotted lines is the mixture events. So blue shows the individuals
who have ancestry from it, our three earliest individual. And the earliest samples in
Chile, Brazil, and Belize have specific relatedness
to this individual, suggesting that a
spread of people that propelled the
Clovis expansion also had a larger impact
across the Americas. So after that, beginning
around 9,000 years ago, that affinity to Clovis
genome disappears. And it implies that a previously
unknown large-scale population turnover that
nobody had predicted occurred and affected Chile
and affected parts of Brazil, and maybe other places besides. And so what that's telling you
is a really important event that it would be very
interesting to see what, archaeologically, it
might correlate with. We also have data from
previously published work from the California
Channel Islands, and they allow us to show that
a another movement between South and North America also spread
widely over the central Andes about 4,000 years ago. Oops. And finally, we don't detect
population Y ancestry related to these Amazonians, but
another paper published today seems to detect that. So that's a fourth source
of ancestry, probably. It's really interesting
to wonder which-- this data answers some
questions, such as, did the spread of Clovis
also have an impact in terms of people on Southern America,
but it raises more questions than it answers. What were the
archaeological events associated with these
major population turnovers? What happened? And which, if any,
were the Americans who we know documented from sites
before Clovis 14,500 years ago or more, for example, in Chile? So I think what's going to
happen in the coming years is we're going to obtain DNA
from more and more people. So this is the DNA
our laboratory has generated and published so far. I told you about the
new work in India. But what you notice here is
that it's very Eurocentric. Almost all the samples are from
a corner of the world, which is not the most important or
the only corner of the world, but what we've published
is similar to mostly to what other groups
have published in terms of its distribution. We're trying to rectify that. So here's one of the
papers I told you about. Here is the paper
we published today. And I think that we're obtaining
more and more data over time, with, essentially, a
doubling every year in terms of the amount of
data that we're generating. And I know this is happening
in other laboratories, too, so it's making it
possible to imagine building an atlas of
human transformations all over the world
in this period. So in summary, ancient
DNA is teaching us that much of what
we thought we knew is wrong because every time
we use this amazing technology to look at the past,
just like a microscope, it shows us profound things
we didn't know about before; that we're all mixed. No one is pure. And I think anybody who claims
that and pays any attention to this data can no longer think
that purity is something that's possible or ever existed. And it's an unusual field
where scratching the surface is guaranteed to surprise. I was asked many times to
write a book five or six years and seven and eight
years ago about this work, and I'm sure many other
people in my field were. And we weren't doing so
because, in our field, the currency of
scientific papers-- and we don't write books-- but as my colleagues
came to include more and more archaeologists,
and linguists, and interested people, I felt that those
people needed a book in order to understand what's going
on, so I wrote this book that tries to explain
in a serious way but a comprehensible,
jargon-free way what's going on because I think it's very
important to understand this work. It has impacts on
lots of other areas. Thank you very much. [APPLAUSE]
