Translator: Michele Gianella Reviewer: Nicoletta Pedrana
This is the genome of a Tuscan:
but it could just as easily be the genome of any of us.
You see the 23 chromosomes that make it up,
one of which is the X chromosome,
one of the sex ones, which determines the sex we are born with.
For each of these chromosomes,
we have a copy from mom and a copy from dad,
represented here with these two sequences of A-G-C-T,
the bases that make up precisely the DNA.
Did you see, I have highlighted little areas,
in this genome in blue?
These are small fragments,
which are quite rare in human populations:
each of these is quite rare, at least.
That's why,
when we have one of these fragments at those positions,
we tend to differ from most of the people around us.
That is why, as you see, this individual's mom and dad
left him with two essentially different chromosomes.
But what is it that really makes these fragments special?
Well, if we go and look at the chromosome
left to this individual by his mom or dad,
it is identical to the Neanderthal genome.
This doesn't just apply to our Tuscan friend,
it applies to all of us:
for some pieces of your genome,
some of these fragments are more similar to those of a Neanderthal
who lived perhaps 50,000 years ago in a cave in Croatia
than to those of your chair neighbour.
You may now be wondering, oh my gosh, what does that mean,
that my mom or dad were Neanderthals?
So, to avoid family drama, let's go and see who the Neanderthals were.
We know Neanderthals by their tools,
which were essentially simpler than Sapiens tools
in the same time period;
and we spot them through the skeletons,
which were essentially massive, with a robust supraorbital torus;
they were kind of short, basically robust, a little bit stocky.
And we can -
Their skeletons are found in Western Eurasia,
up to 50,000 years ago.
In the same time period, we only find Sapiens in Africa.
However, something starts to change around 50,000 years ago:
we start finding more and more Sapiens in Europe, and in Eurasia,
until the Neanderthals disappeared, and then we only find Sapiens.
Now, a little ago,
I kept from you a very interesting and important detail:
we found far less Neanderthals in Africans
than there is in the rest of Eurasia today.
What does this suggest?
That when the Sapiens came out of Africa, and colonized the globe,
they met these Neanderthals and, simply, had sex with them:
and therefore, part of the Neanderthal genome
entered the Sapiens gene pool.
And by colonizing the whole globe, we brought it everywhere,
even where Neanderthals had never been:
to Australia, the Americas.
And even, probably with later migrations,
back to Africa.
So well, maybe your mom, dad weren’t -
in fact, definitely weren't Neanderthals;
but some of your great-great-great, great-great-grandparents certainly were.
Now, all of these discoveries were made possible
by techniques that allow us to sequence ancient DNA today.
These techniques were mainly developed
in the Max Planck laboratory in Leipzig,
led by Svante Pääbo,
which is also the institute where I did research for ten years.
Here we sequenced a lot of Neanderthals;
and in particular, three of them at very high quality,
almost comparable to modern genomes.
But there is perhaps one of the most interesting fossils,
which is seemingly useless, morphologically speaking,
is just a small phalanx of a little finger,
so obviously, seemingly insignificant,
found in a cave in Siberia: Denisova cave.
Now, it’s cold in Siberia; and because of that,
the DNA of this individual was preserved very well.
When researchers went to sequence it,
what they found was that it was yes, quite similar to Neanderthal:
but not even that much.
He was basically farther from Neanderthals,,
than what every human population,
every individual we take we find around the globe today,
is different from each other.
What did that suggest?
That, precisely, there was some other archaic population,
which we did not know from morphology,
living in Eurasia, of which we had no knowledge before.
So how do we tell
where these individuals, now commonly called "Denisovans", used to live?
That's where modern genomes come in handy.
When we go and look, for example,
in the genomes that we find in Papua New Guinea,
and we see that for each of these Papuans-
or Australian Aborigines,
up to five percent of their genome comes from the Denisovan one.
We find slightly less Denisova in Asia.
And in the Far East we find more fragments of Denisova:
in today's Japanese, in the Chinese.
And these fragments are extremely interesting:
for example, in Tibet
the gene that confers altitude adaptation
is precisely of Denisovan origin.
And that makes us think that when we went out,
we encountered all these populations,
and some of them left us genes
that allowed us to adapt to new environments.
By the way, Denisova DNA was recently found in Tibet,
even in sediments.
That gives us a little bit of a better idea
of what the globe was like up to 50,000 years ago.
So the Neanderthals, the Denisova and the Sapiens,
who met all these populations,
and simply had sex with them.
Now, we have seen how useful modern genomes are
in recreating these scenarios:
so, I take you back on your imaginary trip to Europe,
particularly Iceland,
where a huge effort has been made
to characterize the genomes of a great many individuals.
That made it possible for us to do a study, a couple of years ago,
where we used over 25,000 human genomes,
and where we went and characterized the Neanderthal fragments of all of these.
And what we noticed is,
if we put all these little fragments together in a row,
we can reconstruct practically 50 percent of a Neanderthal genome
just by using modern genomes.
As you will notice, some chunks of these chromosomes
are a little bit empty of this blue, of this Neanderthal fragment;
and I will come back to that later.
But I would like to dwell on an extremely interesting detail:
when we go and look at so many of these genomes of Icelanders,
we also find some Denisova.
Even in Europeans, so even in your genomes,
there is some Denisova.
Little, much less than what we find
in the genome of people living in Papua today:
putting all these fragments together
we reconstruct about three percent of the Denisova genome.
Basically, though,
we all share part of our genome
with these archaic populations.
Now, we don't know exactly
how this Denisova genome came back to the Icelanders.
But we do know
with all the events, migrations and gene exchange,
they probably came back in some way
through human migrations.
Now, you're probably wondering
if it's true that wherever we went,
we had sex with whoever we found.
And the truth is, we see that we were not the only ones:
an interesting clue, in this regard, is another bone,
also quite insignificant if one looks at it that way,
found again in Denisova Cave.
When the researchers initially sequenced it,
they began with mitochondrial genome.
Now, the mitochondrial genome is left to us by our mother;
and therefore, it just tells us the history of the maternal line.
And so, when it was sequenced, initially,
they saw that the genome of mother's side was Neanderthal,
so they thought it was Neanderthal like everybody else.
However, when we sequenced the nuclear genome,
what we saw was that for each chromosome
there was one Denisovan copy and one Neanderthal copy.
And what does this mean?
This scenario is compatible with only one possibility:
this was the daughter-because precisely it belonged to a woman -
of a Neanderthal mom and a Denisova dad.
Incidentally, from the bone - although non-diagnostic,
we cannot really tell which bone it belonged to-
from the thickness we can tell that this woman lived at least 14 years.
This suggests, then, that these individuals,
children of these early mixed couples,
were able to survive to a certain age;
and therefore, probably had some form of family units.
But this is not the only, ancient mixed pair that we find:
if we go to Eastern Europe, to the caves, to Romania,
or to Bulgaria, to the Bacho Kiro cave,
we have recently found individuals,
which date more or less 45,000 years ago,
which is the time when Neanderthals disappeared,
and the Sapiens start to be in Europe.
And what we find in these genomes
is that there are really, really long Neanderthal fragments.
Now, there is a process called recombination, in the genome,
that causes the genome, in each generation, to mix a little bit.
This is the same phenomenon
whereby we all have a copy of a maternal chromosome,
and a copy of a chromosome from our dad.
We only have bits from our grandparents, though,
and even smaller bits from our great-grandparents.
Using this property,
we can then roughly date
how many generations passed
after the ancestors of these Neanderthals lived,
the actual Neanderthals of these individuals;
and therefore, we can identify
that four or six generations before these individuals died
and left our bones,
there was some Neanderthal remote ancestor.
So we catch them almost in the act.
Now, something extremely interesting is that, as I told you,
there are parts of the genome, in modern genomes,
that do not have Neanderthals.
We see that even in these individuals,
these areas do not have Neanderthal fragments;
and that suggests something extremely interesting,
it gives us a window into these populations' life.
Somehow,
the individuals that had some Neanderthal in these areas of the genome
probably did not live well;
and in the early generations,
so these offspring of these mixed pairs,
were only breeding so much.
Maybe they lived less, we don't know;
but it gives us an idea, it must have been a particularly intense period.
And this is not entirely surprising.
Today we see that some of these Neanderthal variants
leave us with diseases:
and even Covid,
specifically the variant that makes us more susceptible,
that some people have,
is of Neanderthal origin.
This differs, I would like to say,
from what we observe for modern populations,
where there are no such reproductive barriers:
as we have seen, we're all essentially African,
our genetic variability is all, essentially, of African origin.
And we only differ for some very external traits
that interface with the environment,
like skin color and some things mostly related to metabolism:
however, we are essentially the same.
So, what does that suggest to us?
Definitely, that there was a lot of sex between populations.
Although I put cupids before,
because there was sex,
but we don't really know the nature of these interactions.
Instead, we know that so many species, when we came out of Africa,
went extinct, from Americas to Australia.
So we cannot rule out that there was violence,
that this sex was not entirely consensual,
that there was rape.
We don't know what they went through:
after all, we don't see them walking around us today.
And potentially, we may have been effective competitors,
taking prey and lands away from them.
Much like what may have happened in the case of the saber-toothed tiger.
One thing we do know, though:
in our past, we have always mixed a lot,
so much that anthropologists today agree
that the concept of the human races is a construct, in fact it does not exist,
and there are wonderful TED and TEDx Talks
by Guido Barbujani and Svante Pääbo on this.
So I would like to leave you with an image, which is this.
For small parts of your genome,
you are closer to a Neanderthal,
who lived perhaps 100,000 years ago in Siberia,
than to your friends or relatives
or neighbor.
Thank you.
(Applause)