NOVA

ELLA AL-SHAMAHI: Neanderthals roamed for around 400,000 years.

Why did they go extinct and Homo sapiens survive?

If you were going to place a bet on who would be left standing, you'd probably bet it wouldn't be us.

The traces Neanderthals left behind are transforming our understanding of them.

The Neanderthals were masters of their environment.

They had evolved here for hundreds of thousands of years.

And then they met us.

SILVIU CONSTANTIN: Neanderthal and Homo sapiens interbreeding, people were just not ready to accept that.

AL-SHAMAHI: What really happened to our powerful human cousins?

"Human: Neanderthal Encounters," right now, on "NOVA."

(water flowing) (wind howling) (panting) (yelps) (grunts) (gasping) (grunting) (baby crying) AL-SHAMAHI: Sometime around 20,000 to 30,000 years ago, a child was born into a new world.

The first child born onto a planet in which we, Homo sapiens, were alone.

This was the first time in history that only one species of human walked this Earth.

(bird calling) All the others were now gone.

And in a tale written by the sole survivors, it's actually quite easy to forget that we weren't destined to be the only ones.

And yet here we are.

(animal calling in distance) How this happened is one of the most poignant chapters in the human story.

And it's one that's etched into the DNA of every single one of us alive today.

For hundreds of thousands of years, Homo sapiens evolved in Africa.

60,000 years ago, some dispersed into the Middle East and, over generations, continued onwards as far as Australia.

But our species didn't stop there.

Other groups began to make their way north, into Europe.

For thousands of years, Europe had been out of reach to Homo sapiens, repelled by its icy climate.

But now a shift in conditions opened up a route into this new realm.

(wind howling) And some members of our species left the familiar behind... ...and stepped into the unknown.

We don't really know why they came.

Was it a romantic notion, like pure curiosity, or was it something much more practical?

Say, the need for food.

Or perhaps it was the same forces that drive migrants today, that need for shelter and safety.

We don't know the exact routes they took.

But by following rivers and coasts or wandering along mountain ranges like this... ...they slowly made their way into new lands.

And not long after these migrants reached Europe, they would've encountered something unexpected.

When they got here, they would have discovered that another species had beat them to it.

Two other human species were widespread at the time.

To the east, from Siberia to Southeast Asia, lived the mysterious Denisovans, known only to us from DNA preserved in a few fossils.

Across lands to the west, all the way from Russia to the Atlantic coast of Europe, were the Neanderthals.

Homo sapiens were latecomers to Europe.

It had been home to the Neanderthals for almost 400,000 years before we showed up.

Now, these Homo sapiens venturing into Europe would have met another sort of human.

People who looked a lot like us, but with obvious differences.

(wildlife chirping) We can only imagine what these early migrants would have made of this other species... ...when our two cultures, perhaps just two families, encountered each other for the first time.

Neanderthals were close relatives of Homo sapiens, but we had evolved along separate branches of the human family tree.

There's often this belief that we evolved from Neanderthals, so we came from Neanderthals.

Actually, that's incorrect.

We shared a common ancestor with them, and then, due to chance and the environment, we went on these two really different evolutionary journeys.

So, we evolved-- Homo sapiens-- for Africa.

We ended up taller and leaner.

Now, the Neanderthals evolved for much cooler, more wooded environments.

So, they were shorter.

On average, the males were about five-foot-five.

They had shorter limbs and they seem to have had bigger torsos.

And they used a lot of brute force because they were close-range hunters.

The Neanderthals were masters of their environment.

They had evolved here for hundreds of thousands of years.

If you were going to place a bet on who would be left standing, you'd probably bet on the obvious choice, and it wouldn't be us.

Neanderthals were clearly highly skilled survivors.

(slide projector clicks) Yet, in the 19th century, when the first Neanderthal fossils were unearthed... (slide projector clicks) ...scientists quickly jumped to conclusions... (slide projector clicks) ...that have persisted ever since.

As a result, Neanderthals haven't had the best PR.

If somebody calls you a Neanderthal, it's probably not a compliment, and that stereotype of Neanderthals has been with us since the very beginning.

And it kind of suited us to see ourselves as the pinnacle of evolution and them as these knuckle-dragging ape men.

But partly, that stereotype is actually just a mistake of science.

Paleoanthropology at the time was quite a new science, and when they came to reconstruct this one Neanderthal, called La Chapelle-aux-Saints, they portrayed it as kind of really hunched over and knuckle-dragging, which was just wrong-- this kind of brutish, hairy Neanderthal, looking like it's about to attack.

It's incredibly aggressive.

(exhales): I personally love Neanderthals, and the more we learn about them, the more we study them, the more we discover about them, the more we realize that this is actually incredibly incorrect.

This now-outdated image of the simple, brutish caveman is finally being replaced... ...with a picture of a once-vibrant, thriving culture.

We keep finding things at Neanderthal sites that really challenge us.

Things like beaded shells with pigmentation on them, almost like they're being used as necklaces.

Eagle talons, probably for a similar purpose.

And then there's my actual favorite, which is evidence of feathers.

But not just any feathers.

No, the Neanderthals seemed to be really interested in black and colored feathers from things like red kites.

And you've got to wonder, why were they so interested in those particular colors?

And it's presumably 'cause they're high-value, they're beautiful.

You kind of have an impression of them as having these incredible headdresses, or maybe cloaks made of these brilliant, bright feathers.

When you put this all together, you paint a picture of a Neanderthal not as this aggressive creature, standing behind a rock with a massive club, but actually as these beings very interested in adorning themselves.

Interested in looking beautiful, with necklaces and gorgeous colored headdresses.

Suddenly, you're looking at beings who aren't just interested in food and shelter.

They're interested in the way they are seen by the world.

This, all this, makes them tangibly human.

For thousands of years, Homo sapiens and Neanderthals lived in close proximity.

But how close were we?

For decades, most assumed interbreeding between our two species didn't happen.

But in the early 2000s, this was called into question... (slide projector clicking) ...by new finds in Romania, bone fragments dating back to around 40,000 years ago, which were painstakingly reassembled to reveal humans with a mysterious mix of features.

It even smells... (sniffs): ...like a fossil.

This, I assume, is Oase 1.

This one is Oase 1.

And that's Oase 2.

Skull-- that's Oase 2.

This is quite special, 'cause I've read about them, I've studied them.

They're hugely significant fossils, but I've never... I've never seen the originals.

I've never been this close to them, it's... It looks and it is a modern Homo sapiens... Yeah.

But it has some features which, which are more like Neanderthal.

Yeah.

Like this one, it's quite clear it's a mandible of a modern human with this, this chin.

Because it's, there's a chin.

There's a chin, yeah.

And Neanderthals don't have a chin, Neanderthals' chin kind of recedes.

But then you, you see the, the size of the molars.

Yeah.

Which are really huge.

More a Neanderthal feature.

Modern sapiens... Yeah.

...but with Neanderthal teeth.

Yeah, Oase 2 has the same hybrid features.

Mm-hmm.

Like, if you look at the face.

You look at that and you, you do think that's Homo sapiens.

And then it has these features on it, which are more Neanderthal, like this occipital bone here at the back, that bulge at the back of the skull here.

Yeah, that's kind of strange.

It's not a Neanderthal, but it's, it has Neanderthal features, which prompt us to think about some sort of interbreeding.

Neanderthal and Homo sapiens interbreeding, it was pretty controversial.

People were just not ready to accept that.

AL-SHAMAHI: Around a decade after the bones were discovered came a revolutionary breakthrough.

Advances in genetic analysis allowed scientists to extract DNA from ancient fossils.

Proving these two species could and did produce offspring.

How did it feel to be proven right, to be vindicated, especially over something so controversial?

I feeled, felt relieved.

Yeah.

Like, "Okay."

(chuckles) "Now you know."

(chuckles) (laughs silently) Yeah, we were happy to be right.

(chuckles) Yeah.

You know, there's people who spend their whole lives, their whole lives, trying to find a fossil as significant as this, and... Wow, that's just amazing.

(wildlife chittering) AL-SHAMAHI: Since the discovery of Oase 1, evidence has continued to grow, proving hybrids like this were not just possible, but may have been relatively common.

We'll never know the full story of Oase 1 and the other hybrids.

And to be honest, we'll never know the full circumstances under which they were conceived.

For all we know, it could have been non-consensual, or it could have been the result of a romantic notion, like love.

Or it might have been the result of a practical decision, like as part of a trade agreement.

But whatever it was, what must it have been like to have been a hybrid child?

Did these children feel like they belonged or were they teased and ostracized?

We'll never know, but what we do know, because I held Oase 1 in my hands, is that they existed.

And so, somebody loved them, and somebody was raising them to adulthood.

And so, we tangibly know that the Neanderthals and the Homo sapiens, they didn't just meet, they joined.

We now know that for a time, at least, Homo sapiens and Neanderthals managed to live alongside one another.

But survival in this region meant braving challenging conditions that would push both species to their limits.

(wind howling) Because it's likely Homo sapiens arrived here during a brief thaw in the weather.

And by doing so, they had walked into a trap.

Within just a few centuries, Europe's winters became colder, drier, and more hostile.

Unrecognizable to us today.

Rainfall in some areas fell to half its modern level and much of the continent became tundra, a vast, inhospitable plain.

Homo sapiens had to try to adjust to this colder world.

There's no way for them to have known it, but before the first Homo sapiens arrived, most of Europe would've been in the depths of winter.

Ice sheets like this one would have spread from here all the way down to Britain.

It's currently minus-eight degrees.

I am wearing so many layers, it's actually ridiculous.

And yet, I am still completely miserable.

It is so cold, I can't feel parts of my face.

These families, they were here, and they were trying to keep young children alive.

These conditions would have been life-threatening.

While Homo sapiens had originally adapted to the warmer weather in Africa... ...Neanderthals had evolved to survive in colder climates over almost 400,000 years.

It's thought they stored more brown fat than Homo sapiens.

This burns more calories and generates more heat, conserving energy in the cold.

And their larger nasal passages provided increased surface area, warming and moistening the icy air before it reached the lungs.

Without the Neanderthals' adaptations or knowledge, these early European Homo sapiens would have been doing everything they could just to cling on.

And yet, the bitter cold was just the beginning.

This glacier is a remnant of one of the great ice sheets that have grown and shrunk from the poles for millennia.

Deep within are clues about the world these Homo sapiens would have found themselves in.

ANN ROWAN: We're working in mountain glaciers like Folgefonna because we can use the evidence of how the glaciers have changed in the past to understand how they behaved in response to climate change.

Many of the places we live in now, where I live in Bergen, would have been underneath a kilometer of ice.

AL-SHAMAHI: Yeah, I mean, there were times when Britain was part of that.

ROWAN: So, this is where we're drilling the ice core.

AL-SHAMAHI: Yeah.

So, it's manual drilling, then?

ROWAN: Yes, exactly, and there's blades at the bottom that are cutting through the ice that would... How tough is that?

That's, it's quite, it can be quite hard work.

Yes.

ROWAN: And then we lift it out and we bring it over here.

You can see... AL-SHAMAHI: Look at that.

If we hold it up to the light, we can see the air bubbles.

So basically, this is effectively a time capsule.

Yes.

And this is young ice from Folgefonna glacier.

(slide projector clicking) ROWAN: But if this was deep, old ice core, those air bubbles would tell us about what the atmosphere was like in the past.

We can look at what we see... Mm-hmm.

...from the ice cores in Greenland.

This shows us how the climate changed through that period in the North Atlantic region.

Yeah.

There was a relatively cold but stable climate... Mm-hmm.

...from 70,000 to 60,000 years ago.

And then, between 60,000 and 30,000 years ago, the climate in this region jumped by eight to ten degrees warmer... Mm-hmm.

...over maybe one or two decades.

That's quite a lot, yeah.

It's huge, it's huge.

And that cycle is repeated all through that period.

And then it cooled again, and then jumped, and this carried on, and we see then a cold but slightly more stable climate before we then warm into the present day.

I mean, how do you exist if the climate changes like that, in such an extreme fashion?

Well, it's very challenging.

It's, it's maybe not even possible, because everything you know about how to live, how to... Yeah.

...raise children becomes, in ten, 20 years, totally changes.

Obsolete.

Yeah, yeah, yeah.

Totally changes.

AL-SHAMAHI: Over just a few hundred years, entire ecosystems transformed.

Forests became barren plains... ...lakes dried up, and rivers froze over.

The real enemy wasn't just cold, it was change.

As landscapes shifted, herds of animals disappeared... ...and sources of food grew scarce.

(reindeer grunting) Imagine what it would be like for our ancestors to live in this world where the land of their grandparents was not the land of their grandchildren.

And when that happens, intergenerational knowledge, knowledge that's passed on from one generation to the next, that's so important for survival in these environments, suddenly, that knowledge isn't actually very useful, because the plants, the animals, the landscape, it's all different.

To survive, Homo sapiens had to be adaptable... ...roaming farther in search of dwindling resources that might not be there.

Homo sapiens and Neanderthals would have been forced to look for better conditions wherever they could... ...and seek refuge in the few habitable places they could find.

55,000 years ago, the south of France was still in the thick of the ice age.

Yet compared to the deep freeze of the north, it was one of the more bearable places in an otherwise hostile landscape.

And here, at Grotte Mandrin, archaeologists have spent over three decades unearthing its secrets... ...layer by layer... ...revealing a place that was home to Neanderthals for more than 80,000 years.

Shoes off.

Yes.

There we go.

LUDOVIC SLIMAK: And we are barefoot because if there was a flint or a bone below your feet, you will immediately feel it.

SLIMAK: It's a pretty slow work-- as you can see, we only excavated something like 50 to 60 centimeters for 33 years.

So here we have a very nice section where we can understand what happened.

So you have yellow, black... Yeah.

...orange.

So it's like a barcode.

And so right there, about there, we are at 42,000.

By here, we are at 50,000.

Uh-huh.

And there, this step here of yellow sands, we are at 54,000.

So what you're seeing here is phases of occupation over 80,000 years.

Yeah.

And because you've got incredible resolution, you can really hone in on that.

SLIMAK: There we have all the records of all the societies, how they were living.

You can precisely reconstruct how these people were living in the landscape.

AL-SHAMAHI: For millennia, this cave was home to Neanderthals.

But one layer stood out, containing finely crafted tools.

Small and precise.

Techniques that suggested they were made not by Neanderthals, but by Homo sapiens.

A suspicion confirmed when the earth revealed another treasure.

So, this is a molar... Mm-hmm.

...from a child, from a sapiens, dated at 54,000.

AL-SHAMAHI: That, then, is conclusive evidence that that layer, with those strange, unusual stone tools... Yeah.

...is definitely a Homo sapien layer.

Yes.

These elements that allow us to rewrite a large part of the history of Europe.

AL-SHAMAHI: These discoveries tell us a story of one group of Homo sapiens.

Among the first to come to Europe.

They had ventured into Neanderthal territory... ...seeking refuge in this cave in the depths of the ice age.

When we imagine the past, we often don't imagine children.

We imagine, well, a man, a caveman.

Yeah.

(laughs): Right?

But actually, these were "cavechildren."

But imagine to have been born, the first of your people to, to turn up there.

And we don't know-- they might have been born somewhere else.

Yeah, but the kids, we don't... But it's fascinating.

Fantastic, yes.

Wow.

Suddenly, it became something very concrete.

AL-SHAMAHI: Using advanced dating techniques, the team were able to uncover even more precise details about the people who lived here.

When you see this dark line here, all what is dark here is due to the burning wood, burning bones.

It's when people come in the cave, they make fire, and the roof became black.

AL-SHAMAHI: So, people were building fires.

Yeah.

The fire created soot.

That would end up on the roof.

Exactly.

And then bits of the roof would collapse and end up in your archaeological layers.

(chuckling) SLIMAK: Yes.

AL-SHAMAHI: It's literally telling you when they're using this place.

SLIMAK: Exactly.

AL-SHAMAHI: By counting the microscopic layers of soot deposited on the cave ceiling, the team could tell how often these people came here.

SLIMAK: The soot allow us to know that they came once a year during 40 years.

40 years, it's a lifetime!

But what happened to them?

We know that after 40 years, suddenly, the population are no longer here.

Did they die?

Did they move on another territory?

We just don't know.

AL-SHAMAHI: This exceptional site tells the story of a pioneering group of Homo sapiens who lived here in a break between tens of thousands of years of Neanderthal occupation.

But then, all traces of them vanished.

It's one small but very important chapter in our bigger story.

We don't know what happened to that particular group of Homo sapiens from Grotte Mandrin, but it's likely that their story reflects what was unfolding across the continent.

This wave of Homo sapiens was lured into Europe during a warmer spell.

They were pioneers, for sure, but they were trying to survive in a brand-new environment as best as they could, as best as they knew how, really.

Finding temporary places to shelter before, in the blink of an eye, moving on, or worse, dying out completely.

Because that band of Homo sapiens from Grotte Mandrin would be the last of our species found on this continent for thousands of years.

Perhaps unprepared for the harsh environment they faced, this early wave of Homo sapiens in Europe did not survive.

Once again, and for thousands of years, it became exclusively Neanderthal territory.

Neanderthals had survived, while Homo sapiens died out in Europe.

Yet today, we're the only ones left.

How did our stories end so differently?

Part of the answer lies deep within the forests of Northern Spain.

(birds chirping) Where evidence hints that even without competition from Homo sapiens, the Neanderthals were failing to cope with the changing conditions.

A struggle uncovered in a cave known as the Tunnel of Bones.

Oh, wow.

So, this is the famous El Sidrn Cave.

MARCO DE LA RASILLA VIVES: It is, yes, yes.

AL-SHAMAHI: It's got more character than I was expecting, actually.

(chuckles) DE LA RASILLA: Yeah.

DE LA RASILLA: Here is where we found 2,500 Neanderthal bones.

We found 13 Neanderthal individuals of different ages and sexes.

(slide projector clicks) AL-SHAMAHI: Such a, a diverse group in terms of individuals, all found in one spot.

Genetic studies told us that they are related.

So it's a family group.

We know, for example, that one female-- we are not completely sure-- genetic information said was, uh, uh, red hair.

And you know when you say that one of those people had red hair, it, it suddenly brings what were just fossils, really, to life.

AL-SHAMAHI: It's a cave that's filled with ghosts.

We think these, these people were killed by other group.

AL-SHAMAHI: This was not a natural death.

(slide projector clicks) Cracked skulls and precise cuts on the bones... (slide projector clicks) ...suggest that this was a brutal massacre.

13 people killed by another Neanderthal group.

But closer analysis of their remains... (slide projector clicks) ...revealed an even darker truth.

So what do the bones actually tell us?

The first thing the bones tell us is that this Neanderthal group cannibalized another group.

They were really eating these 13 individuals?

Yeah, yeah, yeah, yeah.

So how do we know that this was cannibalism, as opposed to just straight-up murder, or, for that matter, an animal coming?

Well, because we have seen, on the bones, what we call "cut marks."

And also, we have found a lot of bones broken just to get the marrow.

Yeah-- oh!

So, there are the, both things.

We have cut marks, and then... (exhales) ...they broke the bones to get the marrow.

Yeah, if you're getting bone marrow, that, that is... That's, that's... ...an indication of cannibalism, sure.

Yes, yes, sure.

AL-SHAMAHI: This murder-cannibalism of 13 members of a family group isn't the only dark thing that's happening here.

DE LA RASILLA: No, we have other informations in El Sidrn bones.

For example, here.

Uh, bone defects.

We have found that one adult and one adolescent retains the baby teeth, in, in this case, the canine.

That's unusual-- that's a congenital anomaly.

DE LA RASILLA: And then the atlas vertebra, there is a, a hole down there that also is not normal.

And congenital anomalies and defects that tells us maybe that there are inbreeding between related persons.

AL-SHAMAHI: So basically, you've got an inbred population.

DE LA RASILLA: Inbreeding and the consequences of that inbreeding, but different generations, and this number of anomalies is, is high.

(blows through lips) It's painting a picture, isn't it?

Of those, those final thousands... Yes.

...thousands of years before they... Yes.

...eventually became extinct.

I think this is a silent problem.

Silent pathology, you know?

Yeah, it's a silent killer, you're right.

It's, goes, goes, goes down.

Yeah.

But it's continuously, next generation, next generation.

Illnesses, you know, all those things.

At the end, Neanderthals are extinct.

AL-SHAMAHI: The El Sidrn bones hint at more than the suffering of one family.

Because this pattern of starvation, cannibalism, and violence was happening across Europe.

This was a species in free fall.

This is a haunting place.

It's not exactly Neanderthals in their heyday, is it?

If anything, it's kind of like the end of days for them.

They've been driven into this evolutionary cul-de-sac, reduced to eating each other and having children with their relatives, and that inbreeding would have made them more susceptible to disease.

If, on the evolutionary timescale, 12:00 midnight represents extinction for the Neanderthals, this site is past 11:30.

This once-resilient species was now reduced to just a few isolated groups... ...turning on one other.

But any chance Neanderthals may have had of weathering this storm... ...was shattered by the return of another species.

Homo sapiens.

Several thousand years after Homo sapiens had disappeared from Europe, our species would return.

Waves of new settlers, finding their way into Europe for the first time.

But even though the climate was as volatile as ever... ...they were not deterred.

Innovations like weaving and the ability to make warmer clothing likely increased infant survival, even in the harshest months.

Each advance, however small, added up, giving Homo sapiens the one thing Neanderthals lacked: strength in numbers.

The Neanderthals had existed for around 400,000 years.

Developing a rich culture.

And withstanding brutal conditions.

But the relentless climate, dwindling resources... ...and another species growing in strength all may have pushed this once-resilient species to the brink.

But what delivered the final blow?

How does an entire species of human disappear from the face of the Earth?

Part of the answer may lie in the smallest of things: the genes we exchanged in the form of our hybrid children.

I'm going to try and do a demo to explain genetics.

So, let's see how this goes.

Let's say that these are the Neanderthals and these are the Homo sapiens, and they interbreed.

We don't know where the hybrid children ended up.

Did they end up with the Neanderthals or did they end up with the Homo sapiens?

So, let's just say they went back 50-50.

And we see a little Homo sapiens DNA in the Neanderthal group.

And a little Neanderthal DNA in the Homo sapiens group.

The Neanderthals lived in small, isolated populations, but the Homo sapiens were probably a little bit better at keeping their kids alive.

And also, importantly, they were constantly replenishing from source populations in the Middle East, Africa, and elsewhere.

Numbers made all the difference.

As more Homo sapiens migrated into Europe, Neanderthals were already declining.

So, when the two interbred, the impact on Neanderthals was far greater.

If you're a huge population, that interbreeding doesn't have the same impact as it does on the much smaller Neanderthal population.

It's already a little bit interbred.

Perhaps they were simply absorbed into the larger Homo sapiens population that just kept on replenishing.

Over time, Neanderthal DNA became increasingly diluted by the much larger Homo sapiens population.

So it doesn't actually need to be this big act of aggression.

It might just be the fact that we were there, that we were interbreeding with them, and that we had large population sizes.

Perhaps that was enough to push the Neanderthals to extinction.

It was a perfect storm for Neanderthals.

By around 40,000 years ago, their gene pool was diminishing.

Until only a handful of distinct Neanderthal populations remained.

Hanging on in just a few isolated enclaves.

We don't know where the last Neanderthal outpost was.

It was likely a very remote part of Europe or Asia.

But around 40,000 years ago, that place probably acted as a refuge to the very, very last of their kind.

Archaeologists have pieced together what may be among the final moments of Neanderthal extinction... (slide projector clicks) ...uncovering remains of what could be the last surviving Neanderthal groups.

(slide projector clicks) Some of that evidence has been discovered in coastal caves in Southern Spain.

We don't know what truly happened in those final moments or who was left at the end.

But there was an ending.

Because after that, Neanderthals, who had existed for around 400,000 years... ...vanish from the archaeological record completely.

Our remarkable abilities to connect, innovate, and explore have led to our success.

But time and time again, that seems to come at the expense of those around us.

Homo sapiens' arrival in Europe triggered a slow, unwitting war of attrition against our human cousins until they simply faded away.

But this wasn't the only ending.

After the last Neanderthals, the Denisovans, the species who once spanned much of Asia, may have survived for another 10,000 to 20,000 years until they, too, were likely overwhelmed by Homo sapiens.

This story starts with three species, but it ends with one, and it's part of a wider pattern that always goes the same way.

The survival of our species leading to the demise of everyone else.

Today, these events have faded from memory.

But it's not quite the end of the story.

Because we carry a piece of this history within us.

One of the most striking revelations over the last few years is that everybody from outside of Sub-Saharan Africa has about two percent Neanderthal DNA.

And that DNA is associated with negative things like Crohn's disease.

But it's also associated with all kinds of positives, like being better adapted to the cold.

And now we know that Denisovan DNA has been found in Homo sapiens populations.

It could be as high as five percent in the Philippines.

And it's associated with things like being able to survive better at high altitude.

And if you think about it, it actually makes perfect sense, because when we were leaving Africa, the Neanderthals and the Denisovans had already spent hundreds of thousands of years adapting and evolving to their local environments and pathogens.

And so, what we were doing by interbreeding with them was effectively a quick fix.

We were adopting adaptations that would ultimately aid our survival.

No matter where you're from, it's likely you have traces of Neanderthal or Denisovan DNA within you.

An echo of the human story connecting us to this long line of distant ghosts.

Two percent might not sound like a lot, but my two percent is different from your two percent.

And collectively, all of that Neanderthal DNA that exists within humans living today would make up at least half of the Neanderthal genome.

And so, in a very real sense, Neanderthals and Denisovans have been assimilated into our bodies.

And it's just the loveliest thought, isn't it?

That they live on and exist within us.

Our planet was once home to many human species.

Bit by bit, they've all disappeared, leaving only one to carry on their legacy.