PBS NOVA: Becoming Human

More than six million years ago we took that first step

to separate from the apes.

We see the launching of the career

that ultimately led to Homo sapiens.

And three million years ago, we see the roots of our big brain

begin to take hold in a tiny creature

more like a chimp than a human.

PETER deMENOCAL: The frontier of human evolution

is really being brought to this razor sharp edge.

And we now know that for millions of years,

many different humanlike species lived together

on the planet until one day there was only us,

Homo sapiens,

the most complex, adaptable animal on earth.

So how did we get this way, and why?

A radical new theory reveals how episodes of cataclysmic change

forced our ancestors to adapt or die.

I think we

should actually look to our proud ancestry

and how we evolved in East Africa and say,

"That's how we survived that.

We can survive the future."

So get ready for a ride through millions of years

of our history.

It's the story of becoming human-- our story--

right now, on NOVA.

Millions of years ago, on the plains of Africa,

a momentous event took place.

Apes that had walked on four legs stood up and walked on two.

Eventually, this change in posture

would be followed by a change in their brains.

Somehow, over time, they would become us.

We know it happened, but we've never known when or why.

Until now.

In the Sahara desert, a six-million-year-old fossil

called Toumai may hold the secret

of how we first walked upright.

: We are writing the first chapter of human evolution.

We are very close to the beginning.

Very close.

And the fossilized bones of a child

from three-and-a-half million years ago,

hint to us about the beginnings of human thought.

We're discovering how many different human species

lived on earth at the same time and why all but one died out.

We, Homo sapiens, are the first ever to be alone.

So what powered our evolution?

Why did we become human?

Scientists are scouring the most remote parts of Africa

for clues.

The search for answers begins here, in the Afar--

northeastern Ethiopia.

It's part of the Great Rift Valley, a deep cut in the earth

where geologic forces are ripping Africa apart.

Millions of years of history are brought to the surface

in layers of exposed rock.

It's hot and desolate.

Dangerous, too.

Ancient rivalries and modern weapons have turned the Afar

into a no-man's-land of simmering conflict.

But Zeray Alemseged has made this forbidding place

his life's work.

He's searching for the fossilized traces

of our earliest human ancestors.

The fossil bones of animals like antelopes, elephants

and pigs are abundant.

But the fossils of our ancestors are extremely rare.

Then, in a stroke of luck, Zeray makes the find of a lifetime,

a find that illuminates our origins in a unique way.

On that afternoon, we decided to survey this hillside

and the first thing that was spotted was a cheekbone

of the face.

It was a face so tiny it had to be a baby.

But not a baby chimp; he could tell that from its shape.

The skull was embedded in sandstone,

but as Zeray turned it over, he could see more bones inside.

Everything was squashed

against the base of the skull

and completely covered by the sandstone block.

Clues to the age of the fossil

came from a distinctive feature in the landscape--

white bands of volcanic ash.

That is volcanic ash, dated to 3.4 million years ago.

If the volcanic ash is 3.4 million years old,

Zeray's fossil, which was lying just above it,

must be younger.

It was a child from the dawn of human evolution,

about 3.3 million years ago.

Zeray called the baby Selam, the Ethiopian word for peace.

Then he set off on a quest to unravel her many mysteries.

Her journey began a very, very long time ago.

Imagine the entire span of recorded human history

taking us back to the Egyptian pyramids,

about 5,000 years.

Double it-- 10,000 years ago--

when plants were domesticated and agriculture begins.

Double it again, to the time when Ice Age hunters

paint stunning images on cave walls.

And keep doubling six more times,

taking us back 1.3 million years,

when the first creature who really looked like us

hunted on the plains of Africa and then keep traveling back

another two million years

and only then do we arrive in the time when Selam lived

in Ethiopia nearly three- and-a-half million years ago.

What were Selam and her family like?

What kind of world did they live in?

The answers are hidden in their fossil bones.

Addis Ababa, Ethiopia.

Zeray's home.

He is one of a whole new generation of African scientists

trying to unravel the mysteries of human origins.

Zeray has brought his precious fossil here

to the National Museum.

His challenge is to release her from the tomb of sandstone

in which her bones are encased.

He quickly identifies her.

She is from a species considered by most scientists

to be an ancient ancestor.

Australopithecus afarensis.

A small, chimplike creature who walked on two legs.

This is the same species as the famous Lucy

discovered in the 1970s by Don Johanson.

Lucy was terribly important because she was really

an amalgam of different characteristics

of ape and human.

I think specimens like Selam and Lucy are

extraordinary simply because you can look at them and see

evolution in the making.

But seeing "Evolution in the making" Will take some work.

Selam's fossilized bones are solid rock

held together by a mesh of soft sandstone.

It has to be painstakingly removed.

We spent hours, hours and hours and days and years and years

and I removed the sand grains, grain by grain,

working every day.

He's been at it for eight long years,

but the payoff has been amazing.

As the work progressed,

Zeray revealed an almost complete skull...

and tucked beneath it was nearly her entire spine,

along with both shoulder blades.

Other bones were found nearby.

An almost complete foot.

This is the kneecap.

The tibia here.

Never before had a child's skeleton been found

so ancient and so complete.

Her bones would fit in a shoebox,

but they speak volumes about her life.

For example, to find out how old she was when she died,

Zeray looked at her teeth.

But not the baby teeth visible in her jaw...

the adult teeth growing inside the bone as seen in a CT scan.

From that, we know Selam died at age three.

Like Lucy, she testifies to a crucial step in our evolution.

Unlike apes, these creatures walked upright,

as the first fossil Don Johanson found clearly revealed.

It was sticking out of the ground like that.

And I gently tapped it with my sneaker

and this is what fell out of the ground.

And it is the... this is your... the top end of your shinbone.

So the kneecap would sit right in here.

And very close by in two pieces I found this bone.

And when you put them together

and you see how they move and articulate,

it has all the hallmarks of an upright person.

Other bones confirm that Lucy walked on two legs like us.

This is Lucy's pelvis.

And you can see how different a chimpanzee is.

And the reorientation of these hip bones--

in a chimp they're facing straight forward.

So here is... this is what everybody is sitting on

in their living room right now.

So they're not identical,

but clearly these two resemble each other

much more closely, right, than either one of these

resembles the pelvis of an ape.

From the waist down, Lucy was like us.

From the waist up, she and her kind were all ape.

Selam's skeleton is the same, with chimplike shoulder blades

giving her the range of motion

needed for climbing and swinging.

These ancient creatures must have spent time in the trees,

possibly sleeping there at night to keep away from predators,

but walking upright on the ground during the day.

They were at home in two worlds.

What was their environment like?

It must have been very different

from the Great Rift Valley of today.

Across the border in Kenya

is one of the hottest and most barren places on earth,

a vast expanse of volcanic rock and burning desert.

That's how it is now.

But there's good evidence

that for most of its history it was very different.

Researchers braving temperatures over 100 degrees

are seeing signs of a dramatic transformation

here in the Suguta Valley.

The Suguta Valley was entirely covered in water,