The Lost Tribes of Humanity Horizon

We like to think that humans are special.

And we are.

But we're not one of a kind.

The past 30 to 40,000 years in human history are really unique

in that we are alone on the planet.

Earlier, there were always

different forms of humans around that we met,

and would mix with each other.

We now know that there were at least four other distinct types of human

on the planet at the same time as us.

Maybe there were even others that we don't know about yet.

And all of them, except us, are now lost in time.

Studying human origins at this time is very, very exciting,

because the story keeps changing.

We are really writing it now.

More and more, we've found that they were getting feathers

for ornamentation, to wear them in some way.

And we now know that we, homo sapiens,

made contact with these other humans.

When we got the first result that suggested that there had been

interbreeding, we were very sceptical, all of us.

Traditional palaeoanthropology, based on fossils,

is being transformed by genetics.

And right at the vanguard in the hunt for these lost tribes

are scientists digging into modern and ancient DNA.

Over the next few years, we'll be able to convincingly show

that there are multiple forms of human

that we have no fossil data for,

and the only evidence of their existence is in our DNA.

Who were all these other ancient humans?

What did they look like?

And where did they all go?

The answers to these questions strike deep into the core

of who we think we are

and reveal that at least parts of these ancient lost tribes

are still alive today.

Evolution explains the diversity of life on Earth.

But for us, the species who worked it out,

it holds a special message.

We are descended from ancient apes.

And I've been fascinated by this image for as long as I can remember.

It's such an iconic image of human evolution,

starting off with this knuckle-walking ancient ape

and then this ape who is standing a bit more upright.

And then we have the Neanderthal,

still a bit stooped, and then finally, Cro-Magnon man

with his spear here,

and he's well on his way to being a truly modern human.

But this isn't the real picture of human evolution.

Piecing together who our ancestors actually were

has been an extraordinary scientific adventure.

A dramatic story of mystery and revelation.

What we need is a new image to replace that old iconic one.

So, we can start off with a lineage,

which is just one line

that eventually ends up around 200,000 years ago

being us, being modern humans.

Anatomically and genetically, we have arrived as a species.

So did we evolve from Neanderthals, or are they, as some suspected,

a completely separate and distinct human lineage?

It's dawn.

A small hunting party of Neanderthals

are silently stalking their prey.

They are at home in the seemingly endless forest.

But they will, in time,

disappear from the Earth.

Neanderthals were the archetypal cave dwellers...

..thick set, short and muscular,

with a characteristically heavy brow ridge.

They were thought to be much less intelligent than us.

Many people still think of the Neanderthal as

a simple-minded, thuggish people.

But recent, quite often unexpected discoveries,

are forcing us to confront that perception.

Archaeologist Clive Finlayson and his team are convinced that this

simplistic impression of Neanderthals is wrong.

We've been working in these caves in Gibraltar now for over 25 years.

We have dates in these caves now with Neanderthals

as recently as 32,000 years ago.

That would make this the last site where Neanderthals lived.

This, the last known refuge of a lost tribe of Neanderthals,

has produced thousands of bones and artefacts.

It presents us with almost a total picture of Neanderthal life

in this part of the world.

Families were living here,

sleeping here and going out to hunt and forage from here.

In my view, the stereotype of the ape-like Neanderthal

I think is changing. People are realising, even anatomically, that

there may be physical differences between our ancestors and them,

but they're not as huge as we thought, once upon a time.

What is really changing our view is the cultural evidence,

the evidence of the abilities that we thought only we had

and caused our expansion across the globe.

And we're beginning to realise that

Neanderthals were not that different.

In the past, experts suggested that Neanderthals were incapable of

planning, of thinking two or three steps ahead.

That's not true at all. These people knew their environment very well,

they knew what they had here and they exploited it very well.

And that's probably the key to their success and their survival

for such a long time.

The Neanderthals were not only

an immensely successful European species,

but we're starting to realise that they were cultured in a way

that we thought was the sole preserve of modern humans.

The clues are hidden amongst the many bird bones

buried in these caves.

More and more we've found that the evidence is in the form of cut marks

on the bones and what we realised was that a lot of the marks were on

the wing bones of these birds.

Using stone tools and dead birds found on the shoreline,

Clive's team have been exploring what the Neanderthals might

have been doing.

If you dissect the wing bone of a vulture or an eagle,

what you find is a bone and a tendon

and the feathers attached to the skin. There's no meat,

because you have to be lightweight to be able to soar.

So if you're cutting these wing bones in the birds of prey,

it has to be something to do with feathers and certainly has nothing

to do with meat or food.

In parallel, we realise that a lot of the species of bird of prey being

brought in were birds of prey with dark feathers.

And we showed that there seemed to be a discrimination

in terms of colour.

Neanderthals preferred black, for some reason.

The conclusion that we came to after a lot of analysis is that

they were getting the feathers for ornamentation,

to wear them in some way.

Many cultures across the globe, historically and today,

use bird feathers. North American Indians, for example.

Because of this close relationship that they had with birds,

we decided to call them the Bird People of Europe.

Deeper in these caves,

the team have found evidence suggesting that these Bird People

had the capacity for abstract thought.

Something else which was believed to be exclusive to modern humans.

We have found this incredible engraving on the rock,

made by Neanderthals.

And it is the only one that is known today anywhere.

It took us two years of intensive study and we came to the conclusion

that it took them at least two hours to do,

because we replicated the whole procedure, so it wasn't something...

It wasn't a doodle - they'd actually thought about it.

And it wasn't marks done for butchery, for example,

if you're cutting through skin.

We tried to replicate that with skin and the lines go all over the place.

This was something that wasn't functional,

it was sending a message to somebody, and it was done

very, very deliberately, and it took a long time.

This is not mere survival.

This looks like art.

It's the next step, if you like.

The smoking gun, in terms of Neanderthal behaviour.

So, contrary to the popular perception of them,

we are now realising that Neanderthals exhibited many aspects

of what we call modern human behaviour.

And they are much closer to us in other ways, too.

They are cousins of ours.

We've known that from an anatomical point of view, looking at the bones,

but now we know it from genetics as well.

What we know about them is that they branch from a common ancestor,

500,000 to 700,000 years ago,

and here they are,

almost surviving to the present day, but not quite.

Genetically, Neanderthals were a species distinct from

the modern humans who originated in Africa

and who, much later on, would emerge from that continent

and venture into the Neanderthal's home territory.

As the climate changes,

modern humans are spreading from other parts of the globe

into what will eventually become known as Europe.

Tall and lean bodied, modern humans also skilled hunters.

But they've got to compete for resources...

..with the local Neanderthals.

I find it really interesting to imagine what might have happened

if these two groups of people had come into contact with each other.

The incoming modern humans and the indigenous Neanderthals.

How different did they seem to each other?

Would they have reacted with friendliness and curiosity?

Or would it have inevitably been violent and hostile?

We're faced with an interesting challenge.

If we look at the archaeological record,

at what Neanderthals did and made,

and then we look at what modern humans made,

can we see any suggestion that

there was not only contact between the two groups,

but a fruitful exchange of ideas?

Based at the Natural History Museum in London,

anthropologist Chris Stringer has been investigating ancient humans

for over 30 years.

In particular, the similarities and differences between modern humans

and Neanderthals.

Neanderthals are very evolved humans.

In their own way, as evolved as us, overall.

They walked upright as well as we do

and their brains were as large as ours.

We now know that Neanderthals shared much of their behaviour with us.

They buried their dead,

they were very capable at their stone toolmaking.

This is a typical Neanderthal hand axe.

-It is a beautiful one, isn't it?

-Yeah.

Beautifully shaped.

It was top-quality flint.

And they had a knack of finding that, you know?

Yes. So this is a 60,000-year-old one?

Wonderful, isn't it? Beautiful.

Neanderthals and modern humans were both expert craftsmen

in their own ways,

carefully preparing their flint and able to strike off a usable

flake or blade with a single strike.

I'll try in between the two and see.

-There we are. All that cutting edge down there.

-Fantastic. Look at that.

This is buffalo hide, and that's gone straight through.

-Sharp as a razor.

-Yeah, yeah.

Wonderful.

There are a number of Neanderthal sites across Europe,

where the technology, to some experts,

looks suspiciously similar to that found at modern human sites.

It's tempting to ask whether they each developed the techniques

independently, or whether this represents an exchange of ideas.

There is a view that that perhaps reflects contact between the groups,

that Neanderthals were picking up aspects of behaviour and they are

reflected in these changes that they are making.

They are making tools which we might think typical of modern humans,

they are using, to a much greater extent, bone or ivory.

As well as tools, there are other cultural artefacts which go

far beyond what's been found at other Neanderthal sites.

These sites contain evidence of jewellery, for example,

pierced animal teeth. So, the Neanderthals,

they are making body adornments

which normally would be thought of as modern human features.

'Now, with much better dating techniques,

'we are able to use radiocarbon to show that

'the Neanderthal cultures do go on to perhaps 39,000 years ago,

'maybe a bit younger in places.

'And a modern human arrival certainly'

arrives before 40,000 years ago, in Europe.

So there is an overlap and potentially,

when these groups came together,

we did pick up some things from Neanderthals that were an

advantage to us, about living and surviving

in those colder environments in Europe and Asia.

So the archaeological evidence hints at an exchange of ideas

between Neanderthals and modern humans.

But eight years ago,

a fossil fragment from a cave in southern Siberia was about to drop

what can only be described as a bombshell

into the world of paleoanthropology.

Because it suggested that Neanderthals and modern humans

had neighbours.

Another tribe of humans living right on their doorstep.

Deep in the Altai Mountains,

near to where Kazakhstan will one day meet Mongolia,

winter is coming.

But this mountain-dwelling tribe is at home in their environment.

Their cave is on a migration route...

..which offers herds easy passage through the mountains

to the warmer grasslands further south.

Surprisingly, the eventual discovery of this tribe

would actually be made 3,500 miles away, in Leipzig, Germany.

Svante Paabo is a geneticist from the Max Planck Institute

and he specialises in archaic DNA.

He and his team are collaborating with Russian archaeologists

who are excavating the remote mountain cave.

So, in 2008,

our colleagues found a lot of different bones.

And among those bones,

they found a tiny little piece,

a little fragment of the last phalanx of a little finger.

Just a piece of it.

It was obviously from a child, because it was very, very small.

And it's a bit unclear how old it is.

It's at least in the order of 50,000 years, probably older.

And they sent this, along with some other bones, to us in Leipzig.

They sent it to Svante in the hope that he and his team

could extract some DNA and perhaps determine whether the bone was

Neanderthal or Homo sapiens.

What we do when we get the bone to sample

is to remove the surface and little area,

drill the hole with a dentistry drill,

so we get powder out.

From that, we then isolate the DNA.

We purify it away from all of the other components of the bones at

this end, the minerals are there, the proteins, the fats.

And you then have it in a form

that you can feed into sequencing machines.

The problem when you study ancient DNA is that first of all,

there's very little there, in the remains.

The vast majority of the DNA is not indigenous to the bone,

but comes from bacteria and fungi that have colonised the bone

over tens of thousands of years

when it was deposited in the cave.

And perhaps the biggest problem is the risk of contamination from

present-day human DNA, from ourselves in the laboratory,

in a room like this, where people move around.

A large part of the dust in the air is actually skin fragments

and a single such dust particle can contain a lot more DNA

than we have in our sample of the bone.

DNA is very fragile and it doesn't preserve easily.

But amazingly, despite lying for thousands of years in a cave,

the DNA in that tiny bone had hardly degraded at all.

This little part of the little finger was so well-preserved,

we were able to sequence, actually, the entire genome to a very high

quality, as high a quality as you would sequence

your or my genome today.

It's an astonishing achievement

to get a complete genome from a 50,000-year-old bone.

But the biggest shock came when they tried to identify

what species of human it was.

We were very surprised to find that it was not a Neanderthal.

It was very, very distant from other Neanderthal genomes

we had looked at.

So we immediately realised this was something very special.

It was not a Neanderthal, it was not a modern human,

it was something new.

We had found a new form of extinct human.

This was completely unexpected.

A human that was related to all of us,

but genetically distinct.

Another group of cousins.

So we had a lot of discussions among ourselves and our Russian colleagues

what we'd call this group of humans.

And after a lot of back and forth,

we agreed to call them the Denisovans.

Just like Neanderthals are called Neanderthals

after the Neander Valley,

where the first remains of Neanderthals were found,

Denisovans are called after the first place where they were found.

We know very little of Denisovans, it's very frustrating.

We do know that they lived in this cave and that they lived

intermittently with Neanderthals there.

There seem to have been Neanderthals there early,

there were then later Denisovans

and later again, Neanderthals.

And it's, to me, a really amazing place in the world,

because it's the only place we know

that at least three different forms of humans have lived.

We can tell that Denisovans were more closely related to Neanderthals

than to us.

Whereas the ancestors of Neanderthals and modern humans

split around 600,000 years ago,

Denisovans split from a common ancestor with Neanderthals,

about 400,000 years ago.

And really, we only know about this species from their genetics.

We don't know much about their anatomy at all,

the fossil record is so fragmentary.

Further excavations in the cave have uncovered two Denisovan teeth

and so far, that is all the physical evidence we have

of this new lost tribe of mountain-dwelling humans.

The burning question that remains, though,

is did Denisovans ever meet our own wandering ancestors?

It is well accepted that our species, Homo sapiens,

originated in Africa.

But just how and when our ancestors left Africa to colonise the globe

has been a matter of considerable debate.

And each new piece of genetic and fossil evidence

changes our understanding.

Some very recent finds have really pushed the boundaries of how far

and how early modern humans strayed from their African homeland.

And the earlier we left Africa,

the more chance we had of overlapping in time

and in the same parts of the world with our other human cousins.

To understand the story of

what really happened to us,

we really need to take into account the whole map. And China

is a very, very important part of this puzzle.

Archaeologist Maria Martinon-Torres, from University College London,

has teamed up with colleagues from the Chinese Academy of Sciences

to study a treasure trove of human teeth found in a cave

in Daoxian province in south-east China.

Some people might think that all teeth look the same,

but they do not.

Teeth are like little landscapes in miniature.

These teeth are clearly modern human, Homo sapiens,

rather than from an archaic hominin, like a Denisovan or a Neanderthal.

Homo sapiens have very simple teeth and they are really very slender.

The roots are very thin, they are really narrow towards the tip.

However, if you look at the roots of an archaic hominin,

they have really stout and robust roots,

like columns that really diverge and open very strongly.

So it's really a big difference.

The teeth had been perfectly preserved in the limestone cave

underneath a layer of calcite.

The Daoxian teeth were found in the calcite layer, like a gravestone,

that is really like sealing all the layer where the fossils were found.

We dated the stalagmites that really grow on top of that layer

and the stalagmites have been dated 80,000 years ago.

Meaning that everything that is accumulated below has to be at least

80,000 years ago.

This new evidence and dramatically overturns previous theories

about human movement around the world.

It shows us just how well-travelled these early modern humans were.

The Daoxian teeth are very important,

because they represent the earliest and soundest evidence we have

of our own species, Homo sapiens, being outside Africa.

And they were outside Africa much earlier than expected.

So in order for Homo sapiens to reach China so long ago,

we must have left Africa around 100,000 years ago, possibly earlier.

That means modern humans must have spent thousands of years roaming the

planet and sharing it with the other two human tribes.

And this is very exciting for a scientist, with the Daoxian teeth,

we have a lot of new questions, like

did the hominins we found in Daoxian ever meet Neanderthals?

Did they ever meet these mysterious Denisovans?

So there are a lot of new questions we have to understand about

who we were about 100,000 years ago.

The answers to some of these questions will no doubt be found

somewhere in Asia,

a vast continent sure to contain many anthropological surprises.

In fact, the South East Asian islands have already thrown up one

of the most startling and controversial the discoveries

of recent years in paleoanthropology.

Evidence of a lost tribe that is human, but almost unrecognisably so.

Living on an island isolated from the rest of the world...

..is a group of miniature humans.

Adults are only one metre or 3ft tall.

Living off dwarf elephants

and using technology as advanced as other human groups around the world.

Anthropologists Laura Shackelford and Fabrice Demeter

study human fossils across south-east Asia,

but they hadn't anticipated that in 2003,

a group from Australia would discover a hobbit-sized human

in a cave on the island of Flores in Indonesia.

When the fossils were discovered on Flores, it was a very exciting find,

because no-one knew quite what they were.

When Flores came out to the public,

it was a big shock for the scientific community.

This was a species that looked nothing like anything

that had been discovered before.

Because it has very specific features.

Particularly its short stature, only about 3ft tall.

And what was even more interesting about the species

was that they had a very small brain,

a brain that was about the size of a chimpanzee's brain.

So this was a new species that no-one really knew what to do with.

Initially, there was scepticism.

Fears that this was just a single deformed modern human.

But now, with accurate dating and several individuals uncovered,

it's accepted that these were a distinct species of human.

They've been named Homo floresiensis,

after the island they were found on.

But more popularly, they are known as the hobbits.

I don't think it is nice to say something like this about a human,

but they are weird.

You know, and they are weird in their aspect, in their age,

in their location, in their history.

The riddle of the unusual size of Homo floresiensis

could actually be answered because of the island they lived on.

100,000 years ago, the sea level was very, very low.

So these islands were connected to the continent.

But Flores was always an isolated island.

Recently-discovered fossils show

that the ancestors of the hobbit-sized humans

were already small 700,000 years ago.

It does seem that the island itself may have been to blame.

Since they've always been isolated, then they were probably subject to

a phenomenon that is called island dwarfism.

And this is a phenomenon where

when a species of any sort, not just human, but animals as well,

when they are isolated on an island,

then often, the species will be reduced in size.

And this may be because there are fewer resources on an island,

so the species may get smaller to conserve resources.

The discovery of the hobbit-sized humans has raised many more

questions about human origins than it has answered.

Homo floresiensis is such a puzzle.

It was a startling discovery,

and we are still struggling to make sense of it.

These tiny people seem to have evolved from ancestors who go back,

way back in the family tree of humans to perhaps

2 million or maybe even 3 million years ago.

What is really fascinating about this is that in contrast to today,

where we are the only human species on the planet,

you don't have to go back that far to find all of these others.

The diminutive Homo floresiensis,

the surprisingly cultured Neanderthals,

the enigmatic Denisovans,

all living alongside modern humans 100,000 years ago.

But 30,000 years ago, we were still there...

..and all these others had gone extinct.

The past 30,000 to 40,000 years in human history are really unique

in that there are no other closely-related forms around.

We are alone on the planet.

So why did Homo sapiens survive

when all those other human tribes died out?

The Neanderthals were, as far as we know,

the last of our cousins to disappear.

And the latest research suggests

there are many reasons for their demise,

but perhaps the most inescapable was the fluctuating climate.

Repeatedly, the Neanderthal populations were hit

by very rapid climate changes. They could never stabilise and

grow their numbers for any length of time.

Every time they expanded in the better conditions, suddenly,

a tremendous drop in temperature very rapidly,

and a lot of those populations died out.

Between each period of relative warmth,

which can last for thousands of years, the cold would come back,

sometimes engulfing Europe within a matter of a fewer decades.

And the Northern Neanderthal groups would have been wiped out again.

For me, it is a matter of luck.

Had modern humans and Neanderthals been in that place at a time when

instead of the climate getting colder and drier,

it got warmer and wetter,

then we might well be Neanderthals today

discussing why the others went extinct.

I think the Neanderthals were already an endangered species,

before modern humans came out of Africa

and started overlapping with them.

Then I think even by a small amount,

modern humans were more effective at hunting and gathering

and I think the Neanderthals just got displaced

by that growing success of modern humans.

The lesson that the Neanderthals tell us is that you can be an

intelligent, large-brained hominid and still go extinct.

The changing climate would have affected human populations across

the globe and evidence of the impact of that is written into ancient DNA.

Janet Kelso, a colleague of Svante Paabo at the Max Planck Institute,

has been studying the history of humans

through comparative genetics.

In terms of diversity, in the Neanderthal population,

it was much lower than what we see in present-day populations.

So if you look around this train carriage,

there's more diversity in this train carriage, perhaps,

than there was in the population of Neanderthals that we know about.

We think that that's probably because they were living

in rather small populations.

When they got the first really high-quality Neanderthal genome,

one surprise was where the two versions of the genome

that this individual had inherited from its mother and its father

had no differences between them.

And this obviously suggests that the mother and father of these

individuals were closely related.

You're on the level of half-siblings,

or double first cousins.

We don't know if this is typical of Neanderthal societies at that time,

or if it's something special that happened in this cave

at this point in time.

What's certain is that low genetic diversity

is not good for a population.

It is known that populations that are more integrated,

that have less diversity,

have less ability to respond to, say, a new pathogen

or a changing environment.

So this diversity kind of provides a buffer for the environment,

and if the Neanderthals were less diverse, it's quite possible

that may have contributed to why they died out.

But it turns out that

Neanderthals may not have died out completely after all.

Genetic research can tell us a great deal about who we are and where

we come from and it is completely reshaping our idea

of what it is to be a modern human.

While our Homo sapien's ancestors

were sharing the planet with the various lost tribes,

there was, of course, the chance

that they might have interbred with them.

This meant that there was a possibility,

however unlikely it seems,

that there might be a remnant of some archaic DNA

hanging around in the genomes of present-day humans.

So the geneticists in Leipzig went digging for it.

What we wanted to look at was whether there was interbreeding from

Neanderthals into modern humans.

And our expectation was that there was none.

And when we got the first result that suggested that there had been

interbreeding, we were very sceptical, all of us.

Comparing modern human DNA to the detailed Neanderthal genome

seemed to suggest some sharing of genes.

We all kind of spent a lot of time looking at the data,

trying to figure out how this could be an error.

But the evidence was there.

Certain modern human genomes from different parts of the world

contained sections that matched the Neanderthal DNA.

So what we're looking at here is a region of chromosome 4.

At the top, we have two Neanderthals,

and then we have a Denisovan,

and here we have Asian individuals,

European individuals,

South American individuals,

Papuans, Australians and then a group of Africans.

And if you look down, you can see that in most positions,

everyone is very similar,

and that's because we are very closely related, right?

Modern humans and Neanderthals are not that different from one another,

which makes it really hard to

work out where are the interesting differences.

And so what we do is, instead, we mask out,

we remove all the sites where everyone is the same.

And we only look at the sites where individuals differ

from the referenced genome.

And so what you see here now is the Neanderthals and the Denisovan

carry a difference from the modern human reference at this position

and here and there and there.

And some of those differences are shared by a number of populations,

including the African individuals.

And some of those differences are shared just by a few individuals,

so here, and there, and there.

The genomes of certain individuals contained evidence

that our wandering ancestors bred with Neanderthal tribes

and had offspring who survived,

passing on their genes down the generations.

And it wasn't just evident in a couple of individuals.

When we went through the genomes of Africans

and of Europeans and Asians,

and counted how many DNA bases in each of the genomes matched the

Neanderthal genome,

we saw very consistently that all the groups outside of Africa

shared more Neanderthal DNA than the groups inside of Africa.

People with African heritage don't generally have Neanderthal DNA.

But virtually everyone else does.

Including me.

Well, I've had my own genome analysed and I am 2.7% Neanderthal.

And what is interesting is that my bit of Neanderthal DNA will be

different to the bit of Neanderthal DNA in another person's genome.

All people whose roots are outside Africa

carry a bit of Neanderthal DNA.

On average, each person carries somewhere between 1% and 2%.

If we now look at these different pieces,

that different people carry today,

how much of the Neanderthal genome is still running around today

on two legs in different individuals?

How much do we get if we add it up?

And the jury is still a bit out on that.

But in the order of at least half of the Neanderthal genome

still exists today in people.

And that number just keeps rising, the more people they analyse.

It's incredible to think that

around about half the Neanderthal genome is still alive and well

in us today. And going back just ten years ago,

we had no idea that any of us had Neanderthal DNA.

Now we know that practically everybody of predominantly

non-African heritage has Neanderthal DNA in their genome.

We interpreted that to mean that the mixture from the Neanderthals into

the ancestors of those individuals had to have been very early.

At the point very close to when they exited Africa,

perhaps somewhere in the Middle East,

when they were still a single population.

And then as they spread out,

they carry with them that Neanderthal DNA to wherever they go.

And Denisovans also left their mark in the DNA of living people.

Some more than others.

So if your origins are in Papua New Guinea or aboriginal Australians,

for example, they have in the order of 5% of their DNA from Denisovans,

so three or four times more than they have,

or people in Europe would have, from Neanderthals.

Traces of Denisovan DNA can also be found in populations across Asia,

in India, the Himalayas and China,

and it's estimated that there could be as much as 60-80%

of the Denisovan genome still in existence,

spread around the world's populations.

So, clearly, the wandering tribes of early modern humans

readily mixed with our human cousins.

Where they come into contact,

they are doing some interbreeding where they overlap.

And it could have included things like adopting babies,

finding orphaned Neanderthals, Denisovan babies,

taking them into the group and then bringing them up in your group.

So that could have been part of it, too.

At other times, it might have been raiding groups

and stealing women, because a group lacked females.

They would actually go and steal some.

And this happens today in chimpanzee groups

and it happens today in some hunter-gatherer groups,

so I'm sure that was happening as well.

So I think we've got a whole range of possibilities

of how these matings actually happened.

We have realised that these groups of humans are not totally extinct,

if you like. They live on a little bit in people today.

The most fascinating and surprising secrets

of our long-lost past are finally being uncovered.

The idea of modern humans crushing all other species

in a brief and dramatic conquest of the globe

has to give way to a process of assimilation

that has truly shaped who we are today.

But that's not the end of the story.

As genetic techniques get better

and as we delve deeper into our DNA,

a new secret has recently been revealed.

On the grasslands of present-day Africa,

hidden deep in the genomes of certain people,

are the remnants of a fifth human population,

a lost tribe of whom all other traces have disappeared.

The plains of central Africa are a harsh and unforgiving environment.

And yet, an isolated hominim tribe doggedly clings to survival,

leaving no trace in the landscape.

This tribe was actually discovered 9,000 miles away in Seattle,

on the West Coast of America.

Geneticist Joshua Akey has been analysing the genes

of certain modern African populations

and he's uncovered sections of DNA that can't be found

in other modern humans.

We were able to identify sequences

that we're confident are archaic in origin,

but when we try to match them to the Neanderthal or the Denisovan

genomes, they didn't match at all.

So this is clearly from an unknown group of humans

that existed in Africa. We don't have any fossil data for them,

we don't know anything about them, except for the trace amount of DNA

that they've left in the present-day individuals from Africa.

So, think about the human family tree.

We know that Neanderthals and Denisovans split off

from modern humans around 700,000 years ago.

And this archaic sequence split off from modern humans

probably 700,000 to 800,000 years ago.

And there's gene flow from this mystery group of archaic individuals

from Africa into the ancestors of present-day Africans,

50,000 to 80,000 years ago.

So they were survivors,

and we don't know much else about them.

Except that their fossil remains may be out there in Africa somewhere,

waiting to be discovered.

And what's fascinating to me is that we can make these inferences

not by discovering fossils or ancient DNA extracted from a fossil,

but simply by looking at the DNA sequence variation that is present

in modern African individuals.

And it provides this amazing window into human history and allows us to

find these missing twigs of the human family tree.

DNA evidence of genetic mixture between different forms of human

might also help solve the mystery of certain fossils

that don't fit neatly into our standard categories.

This skull, for instance, has a mixture of characteristics.

Some of which look over 100,000 years old,

others only 14,000 years old.

So, how do you explain that? Well, theoretically at least,

we can put forward the idea that

maybe something like Homo heidelbergensis

or a very primitive form of Homo sapiens survived for much

longer in central West Africa,

then interbred with the later forms of Homo sapiens.

And here we've got a fossil that reflects that interbreeding.

The possibility of ancient interbreeding with the lost tribes

is causing experts to re-evaluate many other fossils, too.

You know, there was a lot of exchange of interactions between

these different groups,

so we really have to look with new eyes to the old fossil records.

There are many of these fossil samples that don't fit the story,

because indeed, the story was built without them.

Could fossil evidence of ancient interbreeding

have been staring us in the face?

There are many fossils being found that I don't quite know

what they are. I think there will be surprises coming there.

Those could be further evidence of this interbreeding,

not just Neanderthals and Denisovans,

other examples of interbreeding, too.

And the new species don't stop here.

Experts are now much more open to the idea that there probably are

other archaic human tribes yet to be properly identified.

I think as we look in more geographically diverse individuals,

that there's a real potential that we'll discover

other branches of the human family tree.

We still have a lot to discover and to understand,

so whether it is new groups or new species,

or whether we have to start recognising those that were

the result of an interchange between different hominid groups,

I don't know.

But it's going to be something much more complicated than we thought.

So, when modern humans spread out from Africa,

they encountered a very different world to the one we know today.

The picture that is emerging

from fossils,

from scraps of ancient DNA

and from modern DNA

is a world of overlapping human groups.

It's now known that there were Neanderthals living in Europe,

the Middle East, and even nudging into Asia.

Denisovans in western Asia, but in all likelihood,

they lived right across Asia and South East Asia.

Floresian hobbits, minding their own business,

cut off on a remote island in Indonesia.

As yet nameless archaic Africans, the survivors.

And us.

Modern humans.

Homo sapiens.

The ultimate wanderers.

Originally from Africa,

but then restlessly, repeatedly exploring the globe.

And it turns out that our story is not so much about

where we came from, but who we slept with on the way.

The recent revelations emerging largely from a genetic research

are helping us to update our family tree.

So we know that there is at least...

..one other African archaic population, perhaps even species...

..going back about 800,000 years ago, to the branch point there.

And we've got DNA from that coming into modern humans.

And, of course, that wasn't the first glimpse that we've had

of genes coming in from other species.

The first indication we had was that there was some Neanderthal genes

coming this way into modern humans.

Now we've got evidence that there are modern human genes,

not surprisingly, going back in the opposite direction.

We've also got genes coming in from the Denisovans.

And there are other archaics coming into Denisovans

and also into Neanderthals at some point, too.

I think it's been quite difficult to come to this

concept of the family tree of humanity.

We seem to have been so wedded to that idea of a linear progression.

And what we can see now is that it is a family tree, but not just that.

It's a tangled web of connections as well.

Our DNA is a mosaic.

It contains traces of these other species.

We contain echoes of the lost tribes of humanity within us.

So I suppose you could say we are genetic mongrels.

But there's one final twist in our story.

A secret that each of us carries inside.

The archaic DNA within our modern genomes is not defunct.

It continues to have a profound effects on our lives today.

In Seattle, Josh Akey is taking advantage of modern data techniques

to look at our health.

There's been amazing technological innovations that have happened over

the last decade. Computing power to be able to crunch all the numbers,

that allows us to access genomic data on a scale that wasn't

really even imaginable a few years ago.

Josh has analysed the genetic data and detailed health records

of 28,000 people.

28,000 people fill half of this stadium.

The idea was to track down the parts of their genomes that were inherited

from their Neanderthal ancestors and ask

what is the spectrum of traits or diseases that the remaining

Neanderthal variation influences in present-day individuals?

We are still figuring out the full spectrum of consequences,

but we do know we've picked up some Neanderthal genes

that were important components of the immune system that provided

our ancestors a benefit to survive and reproduce

as they encountered pathogens that they hadn't seen before.

Increased genetic diversity related to our immune system could have been

a major advantage gained unwittingly through interbreeding.

There are certain places in our genome where we are actually more

Neanderthal-like than we are modern human.

And the frequency of the Neanderthal copy of the gene is 70 or 80%.

And those mountain-dwelling Denisovans

have also left their legacy.

Researchers have found a highly unusual gene variant found only in

Tibetans and a very few Han Chinese individuals.

But it is present in the Denisovan genome.

Denisovans contributed a version of the EPAS1 gene,

or EPAS1, to Tibetans

that allows them to exist and survive and thrive

at high altitudes.

So it's sort of interesting to think that we would perhaps

not have such big populations in the high plateau in Tibet today

if it wasn't for this contribution from Denisovans.

But it wasn't all good news.

New research suggests that our ancestors' fertility was negatively

affected by having too much Neanderthal or Denisovan DNA.

It seems that the merging of genomes that ultimately helped us to adapt

and proliferate was a tainted gift.

It appears that there was a trade off.

In present-day individuals,

many of those Neanderthal variants that were, at one time, protective

seem to also predispose to various autoimmune diseases.

In general, Neanderthal ancestry

explains about 2% of the disease risk to a wide number of traits.

As an anatomist,

I'm naturally much more comfortable with bones than with molecules.

But I find it remarkable how much DNA is now telling us

about our human origins.

I find it utterly extraordinary that we are now able to extract

DNA from bones that are not just tens of thousands of years old,

but hundreds of thousands of years old.

What we're currently working on is making the techniques we use

even more sensitive, so that we could go even further back in time.

I think these techniques will be like carbon dating or something like

that, which revolutionised archaeology and became

a standard tool in the tool box of archaeology.

So, in the future, we can expect genetics to shed even more light

on who we are and where we came from.

Over the next few years,

we will be able to convincingly show that there's multiple forms

of humans that we didn't know about before,

that we have no fossil data for

and the only evidence of their existence is in our DNA.

The last 20 years, particularly the last ten years,

has been a really exciting.

There is so much going on.

You don't know what's round the corner.

I've learnt in the last few years to expect the unexpected.

And I'm sure that will go on.