NOVA

Deep underground,

in a flooded Mexican cave system,

divers make an amazing discovery.

The feeling is like one of those outer space black holes

that sucks all your light.

I see no reflection of my light.

The heart starts to beat very hard.

At the bottom of a vast sunken pit...

A forest of prehistoric bones.

In their midst, the skull of a girl.

It's just incredible to see

another human in this environment... it's amazing.

He said you can't tell anyone

because we've never really seen anything quite like this before.

Who was she?

How did she die?

It's kind of a missing link right?

And suddenly you are sitting with that.

The mystery of the girl in the cave

leads scientists on a journey

into the world of the very first humans

to arrive in the Americas.

Who were they?

How did they get here?

Finally, there are answers.

It's the most remarkable project

I'll probably ever work on.

They discover the way of life of an Ice Age people.

Rapidly moving big game hunters.

But it all begins with the story of a girl

who lived 13,000 years ago.

Between 15 and 16 years of age.

We know a lot more about the early lives of the Americans

than we would ever have known without her.

And what happened on the day she died.

Fracture at death.

Astonishing new finds

and a glimpse of the "First Face of America."

Right now on "NOVA."

Major funding for "NOVA" is provided by the following.

In a dark cave, deep underwater,

a cache of prehistoric bones,

Ice Age animals that walked the earth thousands of years ago.

Among the ancient bones, the skeleton of a girl,

one of the very first Americans.

Her skeleton is so complete,

it will allow scientists

to reconstruct her life and death in amazing detail,

providing answers to questions that have long puzzled them

about the peopling of the Americas.

In her death she left us this incredible record

of the life of these earliest people.

13,000 years after she died, a young girl launches

an exciting archaeological adventure,

finally unlocking a great mystery:

how and when did humans first enter the new world?

In the remote jungles of Yucatan in Mexico...

a team of cave divers,

intrigued by reports of prehistoric bones,

is on its way to explore a system of underground cenotes.

It's a dangerous undertaking.

Beneath the surface of the Yucatan Peninsula,

the cenotes are a vast network

of underground caves and tunnels.

They stretch for hundreds of miles

through the limestone bedrock of the peninsula.

They were once dry.

But flooded at the end of the last ice age

about 10,000 years ago.

Only a fraction of the network has been explored.

The possibilities of getting trapped,

lost, or running out of air...

are ever-present.

Over the years, dozens of experienced divers

have drowned in these flooded sinkholes.

Because they don't know how big the cenote system is,

they've prepared carefully.

Their tanks have a special mix of oxygen, nitrogen, and helium

that will allow them to dive deep.

They also have rebreathers that recycle their air,

extending their dive time.

Propelled by underwater scooters,

they travel along submerged tunnels for almost an hour.

In the first passages of the cenote

they notice that guide wires had been laid,

a sign that someone has explored these tunnels before them.

But before long, the lines end.

They are now in a part of the cenote system

that is completely unknown.

With no idea of what lies ahead, they press on.

Suddenly the floor and walls of the tunnel drop away.

At the end of this tunnel we can see darkness.

And then everything was black.

I was in front.

When I see no reflection of my light,

it wasn't... the heart starts to beat very hard.

They find themselves suspended in a vast watery pit.

The feeling is like

we were faced with one of those outer space black holes

that suck all your light.

And then I am floating in the dark.

You can't even see the floor,

you can't even see the next wall.

The black hole, Hoyo Negro, is so big,

the beams of their flashlights cannot find its floor.

Breathless with excitement, they begin their descent.

Finally, at a depth of over a hundred feet,

their flashlights detect the bottom

and reveal a treasure trove of ancient bones

beyond their wildest dreams.

All of a sudden, we start finding bones.

We see this huge pelvis.

And there was this beautiful broken femur on top of a rock.

Big, big bones,

we knew could be something similar to an elephant.

Here is the massive thigh bone of an extinct elephant,

the skull of a cave bear, a giant sloth.

The floor of the Hoyo is littered

with over 20 skeletons of long-extinct ice age species.

But the most amazing find comes last.

Just as we thought it couldn't get any better,

all of a sudden, we go a little bit up.

There's this human skull.

It's amazing.

I mean this is the discovery of our lifetime,

it's not going to get any better than this.

The rest of the skeleton is not far away.

A whole human skeleton surrounded by the bones

of ice age megafauna has never been found before.

Who is it?

How did this person get here?

And when?

For many months the divers explore the Hoyo,

taking bone samples and photos.

Finally, they decide to send them

to the director of subaquatic archaeology

at Mexico's National Institute of Anthropology and History,

Pilar Luna.

In 2009, Alejandro Alvarez got in touch with me

about an extremely important find

in a place called Hoyo Negro.

The news was very exciting

because I had always believed

that there is much to be discovered

in this area of the Yucatan Peninsula

about the past and the first people

to populate the Americas.

Pilar decided to send a CD of the divers' photos

to the scientist who has studied

more ancient human remains from the Americas

than any other... Jim Chatters.

I said, "I've been looking at your CD,

do you want to know what you have?"

And she said, "Yes, no one's been able to tell us."

And I said, "Well, you have an adolescent female."

And she said, "Would you like to take over the study

of the human skeleton?"

To work with ancient humans and extinct animals

at the same time, nobody's been able to do that before.

Knowing that the skeleton they have found is a young girl,

the divers give her the name of a mythological water nymph...

Naia.

You get a connection,

and you get more respect if we have a name.

And Naia was a kind of water nymph.

It's a little bit related to the spirits of the cave.

Jim must wait to examine Naia's skeleton

until the divers retrieve her.

But from a few bone and tooth fragments they bring him,

he can at least try to find out how long ago she lived.

He hopes a radiocarbon dating lab

will give him the dates he needs.

It proves to be difficult.

To get radiocarbon dates,

it's necessary to extract proteins such as collagen,

which contain carbon.

But her long sojourn under water

has destroyed all the collagen in Naia's bones.

The problem we have is in tropical environments

bone does not produce good radio carbon dates.

The protein part of the bone is dissolved away

by bacteria and warm-warm weather.

And so what we're trying is sort of second best,

which is the tooth enamel.

And tooth enamel, being a very tight crystalline matrix,

has the best chance of having

a non-contaminated material we can work with.

The key is to find the radioactive isotope

carbon-14, present along with other elements,

in the cells of all living things.

The molecules of this form of carbon are unstable,

slowly losing protons to become nitrogen.

They do this at a steady rate measured as a half-life,

the time it takes half the carbon-14 to decay.

Once an organism dies, the carbon-14 in its tissues

stops being replaced.

So its density in the tooth

relative to other more stable elements

will give Jim a reading on Naia's age.

First of all, they dissolve the tooth fragment...