Nova (1974–…): Season 36, Episode 13 - Becoming Human: First Steps - full transcript

NARRATOR:
Humans...

Without a doubt, the smartest
animal on earth.

Yet we're unmistakably tied
to our ape origins.

Millions of years ago,

we were apes,
living ape lives in Africa.

So how did we get from that...

to this?

What happened?

What set us on the path
to humanity?

The questions are huge.

But at last, there are answers.



More than six million years ago
we took that first step

to separate from the apes.

DON JOHANSON:
We see the launching
of the career

that ultimately led
to Homo sapiens.

NARRATOR:
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.

NARRATOR:
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.

MARK MASLIN:
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."

NARRATOR:
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.

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NARRATOR:
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.

(translated):
We are writing the first chapter
of human evolution.

We are very close
to the beginning.

Very close.

NARRATOR:
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.

ALEMSEGED:
On that afternoon, we decided
to survey this hillside

and the first thing that was
spotted was a cheekbone

of the face.

NARRATOR:
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.

ALEMSEGED:
Everything was squashed

against the base of the skull

and completely covered
by the sandstone block.

NARRATOR:
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.

NARRATOR:
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.

JOHANSON:
Lucy was terribly important
because she was really

an amalgam of different
characteristics

of ape and human.

ALEMSEGED:
I think specimens like Selam
and Lucy are

extraordinary simply because
you can look at them and see

evolution in the making.

NARRATOR:
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.

ALEMSEGED:
We spent hours, hours and hours
and days and years and years

and I removed the sand grains,
grain by grain,

working every day.

NARRATOR:
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.

ALEMSEGED:
An almost complete foot.

This is the kneecap.

The tibia here.

NARRATOR:
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.

JOHANSON:
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.

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

JOHANSON:
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.

NARRATOR:
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.

YANNICK GARDIN:
The Suguta Valley was entirely
covered in water,

up to an elevation of about
580 meters.

So you can imagine
that all this valley

was filled by a huge lake.

NARRATOR:
A huge lake that's deeper
than any of the Great Lakes.

In fact, the entire African
continent used to be

a lot wetter than it is today.

Many millions of years ago,
long before Selam and Lucy,

Africa was a wet,
tropical environment

covered with rainforest.

This is where the ancestors
of Selam and Lucy lived.

They probably looked a lot
like chimps.

But then, rica started
to gradually dry out.

The rainforest began to shrink.

By Selam's time, three to four
million years ago,

the Great Rift Valley
was a mosaic

of different environments.

We know that from the fossils
of the animals that lived here.

Their bones litter the ground.

This is a canine
of a hippopotamus,

so this is probably the skeleton
of a hippopotamus.

How can one find a hippo
in this type of environment?

Nice antelope here.

NARRATOR:
The fossils tell the story
of a vanished landscape.

ALEMSEGED:
This is the lower jaw
of an antelope.

NARRATOR:
Three million years ago,
the Rift Valley was a patchwork

of grassy plains, scattered
woodlands, lakes and rivers.

Definitely very different
from what we see here today.

Wow, a nice pig here.

NARRATOR:
As their environment changed,

scientists believe our ancestors
changed, too.

They had been creatures

who spent most of their time
in trees,

like chimps and orangutans
today.

But as their forests shrank,
some of them developed the trait

that we take for granted:

bipedalism--
walking on two legs.

This is one of the defining
characteristics of humans.

But how did bipedalism develop,
and why?

BRIAN RICHMOND:
Bipedalism is such
an unusual trait.

There's no other mammal

that habitually walks on two
legs like we do.

NARRATOR:
Because it's unique, it's hard
to figure out why it happened.

There are a lot of theories.

RICHMOND:
One of them is
that they stood up

to be able to see
over tall grass.

Another theory-- they stood up
to be able to pick fruits

off of the low branches of trees
the way chimpanzees do today.

Another theory states

that they stood up to cool
more efficiently,

so that we don't have as much
sun beating on

so much of our body.

DANIEL LIEBERMAN:
I think the most
compelling idea,

the most compelling hypothesis,
is that it saved us energy.

NARRATOR:
And energy is crucial
to survival.

Let's go back to
the dense forest,

home to our ancient ape
ancestors ten million years ago.

Like many apes today,
they were perfectly suited

to a life in the trees.

LIEBERMAN:
They're very good at climbing
in trees.

They're phenomenal at climbing
in trees.

NARRATOR:
On the ground,

these ancestral apes
could probably walk on two legs

for short distances if they had
to carry something.

LIEBERMAN:
Fantastic climbers, but also
able to walk and run

rapidly and effectively
but not economically.

NARRATOR:
Walking was tiring, but they
didn't have to walk far.

LIEBERMAN:
But if you're a chimp
and you only walk

two to three kilometers a day,

it doesn't really mean much,

it's not going to have that much
of an effect

on your energy budget.

NARRATOR:
But energy demands would change

as the forest started
to disappear.

Our ape ancestors had
to walk more.

LIEBERMAN:
They have to go farther

to get from one fruit patch

to another fruit patch,
for example.

NARRATOR:
Dan Lieberman is an expert
on bipedalism.

He believes that walking
on two legs evolved

because it saved energy.

When you compare the energy
consumption of humans to chimps,

there's no contest.

A chimp is an energy glutton.

It spends an enormous amount
of energy--

about four times as much energy
as a human walking.

NARRATOR:
Whether it walks on four legs...

or two...

a chimp can't compete
with the human gait.

LIEBERMAN:
It's poorly designed to
withstand the forces of gravity.

It has to spend a lot of
muscular effort to keep itself

from collapsing into a little
pile of "chimpness" or whatever

with each step.

NARRATOR:
According to Lieberman,
small anatomical differences

created large energy savings,

setting our ancestors on
the path to bipedalism,

a path that would eventually
lead to us.

But how long did it take?

When Lucy's kind were
first discovered,

many people thought they were
the so-called "missing link"

between apes and humans.

But the science of genetics has
transformed our understanding

with a technique called
"the molecular clock."

Today, scientists can compare
DNA from closely related species

to find out how long ago they
split from a common ancestor.

MARK STONEKING:
It's just a very simple idea

that the rate of change
in DNA sequences

is more or less constant
over time.

And that's an extraordinarily
powerful concept

because it means that you have
a way of determining

when two species last shared
a common ancestor.

NARRATOR:
Living forms evolve because DNA
sometimes spontaneously changes

as it copies itself.

These changes happen at
a surprisingly regular rate.

By counting the differences
between the genetic code

of chimps and humans,
we can calculate

how long they've been evolving
away from each other.

The dates that one
almost always gets are around

five to seven million years ago

for when humans and chimpanzees
last shared a common ancestor.

NARRATOR:
Here was proof that humans
diverged from the apes

much earlier than we thought,
about six million years ago.

It shows Lucy and Selam weren't
one step removed from chimps

but many.

They may even be
closer to us

than to the first human
ancestor.

So what came before
Lucy and Selam?

Who was our earliest ancestor?

Until the 1990s,
the fossil record was blank.

Fossil hunters combed East
Africa's Great Rift Valley

but could only find
small fragments

older than four million years.

Then, in 1997,
a French anthropologist

called Michel Brunet decided
to look somewhere else.

(Brunet speaking French)

(translated):
We decided to go to Africa,
but to the west,

to the west of the Great Rift.

NARRATOR:
1,600 miles to the west at
the edge of the Sahara desert

in northern Chad.

BRUNET (translated):
Obviously, if you only go
to the field in East Africa,

then you are going to find
fossils only in East Africa.

This was the situation.

(gusty winds blow)

NARRATOR:
Michel was looking in a place
where the few animal fossils

he turned up were all around six
million years old.

No one expected any humanlike
fossils to be found

in a layer that ancient.

(translated):
And everyone said,

"No. There just aren't any
fossils there."

NARRATOR:
Michel was not to be deterred.

He was stubborn, many thought
to the point of madness.

He and his team spent years
searching the desert

for signs of our ancestors.

And year after year,
they came up empty.

Then, on their 26th expedition,
in 2001,

they found a smashed,
misshapen skull...

around six million years old.

They called it
Sahelanthropus tchadensis.

There were no bones apart
from the skull.

Could it be a human ancestor?
Or just another ape?

The skull was so deformed
it was difficult to tell.

Michel would have
to reconstruct it.

His first step was
to take the skull,

now nicknamed Toumai,
to a particle accelerator

in Grenoble, France, to use
its powerful X-ray scanner.

Over a thousand pictures
of the fossil were taken

to build a 3-D image
of the crushed skull.

Using the virtual image,
the skull could be restored

to its original shape.

It was then reproduced
by a type of 3-D printer

equipped with lasers
which harden plastic.

When it finally rose
from its bath,

the cast of Toumai's skull was
ready for detailed study.

The cast allows Michel to answer
an important question.

Did this ancient creature
walk on two legs

millions of years before
Lucy or Selam?

It's how the skull connects
to the spine

that provides the vital clue.

And Michel could infer that from
the shape of Toumai's skull.

If Toumai's skull is set
on the neck of an ape

that walks on all fours,
his eyes point downward.

That can't be right.

Set on the upright spine
of a biped,

his eyes point straight ahead.

For Michel, this proved
Toumai walked upright.

(translated):
Anatomically speaking,

he had the receding back skull
of a biped.

The back of his skull is not
that of a gorilla,

like some people are trying
to say.

No, not at all.

All you have to do is look.

NARRATOR:
Some scientists still question

whether Toumai was
really a biped.

But if Michel is right, his
six-million-year-old fossil

is a good candidate for
the first human ancestor.

Discoveries like this
are changing the way

we see human evolution.

Scientists used to have
a simple idea:

the growth of open grasslands
forced our ancestors

out of the trees.

They became bipeds, and in short
order, brain size increased.

Human evolution took off.

We were on our way
to becoming human.

That simple idea prevailed
for more than a century.

JOHANSON:
Darwin thought that
we left the trees,

walked on the ground upright,

freed our hands, made tools,
got big brains,

reduced our canines and so on

all at the same time.

NARRATOR:
But walking upright may not have
automatically led

to big brains at all.

From Toumai to Selam, both
bipeds, brains stayed small.

And they weren't the only ones.

Over millions of years there was
a profusion of upright walkers

with complicated names
and chimp-sized brains...

Like Orrorin tugenensis.

LIEBERMAN:
What we're seeing is a
florescence of species,

multiple species.

They're probably subtly
different from each other.

NARRATOR:
Ardipithecus ramidus.

LIEBERMAN:
But it's important to recognize

that there are not major
differences among these species.

NARRATOR:
Australopithecus africanus.

LIEBERMAN:
They were all bipeds,
big snouts,

more or less
chimp-sized brains.

NARRATOR:
Kenyanthropus platyops.

This way of life, this suite
of adaptations,

lasted for millions of years.

NARRATOR:
Small-brained bipedal apes
were extremely successful.

Debates rage among scientists

about which one
eventually led to us.

But as a group, they flourished
for about 25 times longer

than we've been around.

They survived and thrived

as brain size flat-lined
for almost four million years.

But that doesn't mean
nothing changed.

There's evidence that the seeds
of our humanity were growing

in these apelike creatures.

One key difference
between humans and apes

is the length of childhood.

But what do we know
about the childhood

of our early ancestors?

ALEMSEGED:
We knew all about

the adult individuals,

but we didn't know much
about the children.

NARRATOR:
The brains of baby chimps have
an early growth spurt.

They're almost
fully formed by age three.

In humans that growth spurt
is slower,

and it takes nearly two decades
for our brains to fully mature.

But what about Selam's brain,
3.3 million years ago?

Her skull tells us
all we need to know.

We have her milk teeth
and her adult teeth,

which give us her age--
three years old.

And we have a cast of the inside
of her skull,

which tells us about her brain.

When you have this
you can directly measure

how much of the brain was
formed at age three.

NARRATOR:
From other fossils,
we know how large

Selam's brain would have been
as an adult.

So Zeray could calculate how
much of her brain

was already formed by age three,
when she died.

He knows what the answer
would be for a chimp.

ALEMSEGED:
By age three,
a chimpanzee would have

over 90% of the brain formed.

NARRATOR:
But Selam's brain was only
around 75% of its adult size,

suggesting it was
growing up slower.

(squeaking)

NARRATOR:
Childhood would have been
her time to learn,

to learn the survival strategies
her family group needed

to live in a dangerous world.

Perhaps this set the stage for
our longer human childhood,

when culture is handed down.

But is there any other evidence
Selam's brain was becoming

more human and less ape?

To find out, compare a human
brain to a chimp's.

TODD PREUSS:
This is the brain
of our closest relative,

the chimpanzee brain.

It's slightly larger
than you would expect

of a typical primate
for their body size.

Not greatly so.

NARRATOR:
Scientists look for clues
to the evolution of the brain

in the folds and furrows
on its surface.

One important structure is
called the lunate sulcus.

PREUSS:
In chimpanzees
as in many primates,

there is this big, deep sulcus
here, the lunate sulcus.

NARRATOR:
The lunate sulcus is a deep
furrow in a primate's brain.

It divides parts of the brain
related to vision

from the rest of the neocortex,

which is where
more complex thought happens.

The human brain doesn't have
this deep furrow,

and the neocortex is bigger
than the vision structures,

which have moved
far to the back.

So did Selam have the deep
furrow and small neocortex

of a chimp?

Or had something changed?

Brains don't fossilize, but
her remarkably complete skull

provides a way to see some
of the different structures

of her brain.

A cast of the brain case,
called an endocast,

preserves the impression
of the brain's surface.

Ralph Holloway has a collection
of 300 brain endocasts

from many of our ancestors.

HOLLOWAY:
What a paleoneurologist
like myself will be looking for

are those indications on the
endocast that might suggest

reorganization taking place.

And that's why things

like the so-called infamous
lunate sulcus becomes important.

NARRATOR:
Ralph claims that as chimplike
ancestors evolved

into creatures
like Selam and Lucy,

the lunate sulcus, the furrow
marking the vision structures,

moved back, making room
for a larger neocortex,

the thinking part of the brain.

HOLLOWAY:
If you look carefully,

what you've got here
is a depression

that could very likely be
the lunate sulcus.

And so that suggests, then,
by australopithecine times,

that, you know, you're having
a beast that is simply smarter

than present-day chimpanzees.

NARRATOR:
If that's the case,

although still the size
of a chimp's,

Selam's brain had been rewired.

But there was a long way to go.

She and her kind were
still very apelike.

It would take
another million years

for the seeds of humanity
contained in Selam's tiny frame

to bear fruit.

It's a time still shrouded
in mystery.

For almost half a million years,

the fossil record is
virtually silent.

But in this blank period,
something is happening.

In 2.5 million-year-old layers,

scientists begin to find
something new.

We might be tempted
to call them rocks,

but someone was shaping them.

They are the first stone tools.

RICHMOND:
The way we know this is a tool
instead of just a broken rock

is that it's broken
in a very particular way,

breaking a flake off this way,
that way, this way,

back and forth.

So there is a method behind
how this rock was broken

in order to make it into a tool,

and it's not a random method.

NARRATOR:
It's considered unlikely

they were made by
Australopithecus, Lucy's kind.

RICHMOND:
Australopithecus was around

for a couple of million years
and did not make stone tools.

NARRATOR:
But if not Lucy's kind,
then who?

The gap in the fossil record
makes it difficult to say,

but that's not surprising.

Tools preserve easily,
bones much less so.

Finally, the skulls of a new
creature begin to turn up.

Is this the toolmaker?

The skulls are different
from what came before.

They represent the dawn
of a new era

beginning around two million
years ago.

This is our era, the era
of the genus Homo, humans.

The mysterious toolmaker,
Homo habilis,

is the first
of these new creatures.

RICHMOND:
We definitely have evidence

that the stone tools were being
used to break the long bones

in order to get to the marrow
inside the long bones.

There were clear cut marks
on the bones of turtles,

crocodiles, big antelopes,
little antelopes, even hippos--

really big animals
like hippos.

So we know that meat had become
a new important part

of the diet of Homo habilis.

NARRATOR:
The first fossil to be called
Homo habilis included

21 bones of the hand and was
nicknamed "Handy Man."

RICHMOND:
This little bone is the bone
at the end of the thumb,

and that little bone in
Homo habilis, like in humans,

is very broad.

And the broad bone reflects
having a broad pad on the thumb,

with a lot of surface area
for fine, precision grip.

NARRATOR:
With newly dexterous hands,

this creature could make
better tools.

But what was it like?

The few skeletal bones
that have been found

indicate a creature
much smaller than us,

about the same size as Lucy
and Selam's kind--

Australopithecus--
three to four feet tall.

Homo habilis was still apelike
in many ways,

but with a critical difference.

RICHMOND:
What we see in the evolution
of Homo habilis

is an expansion
in the brain size

compared to Australopithecus.

So here is the skull
of Australopithecus,

and it has no forehead,

it just has the straight slope
behind the orbits.

Whereas here in Homo habilis

you see a sloping,
elevated forehead.

And in Australopithecus,
the area behind the orbits

is pinched in, also reflecting
a small frontal region.

In contrast, in Homo habilis,

we see an expansion of that area
behind the orbits

that points to an expansion
in the cognitive capabilities

of higher functions,

of the higher reasoning
functions of the brain.

NARRATOR:
It was an expansion equivalent
to a doubling of brain volume.

HOLLOWAY:
Once you go from something

like 400 cc
in australopithecines

to, say, 700, 800 cc
in Homo habilis, yes,

you're getting a big increase
in cognitive capacity.

NARRATOR:
And along with his bigger brain,

Homo habilis was starting
to look a lot more human.

The contours of fossil skulls

allow reconstructionist
Viktor Deak

to reveal the faces
of early human beings.

Gone is the projecting snout
of an ape.

In Homo habilis, the face
of humanity is emerging.

This poses a great enigma.

Why, after millions of years
of flat-lining,

did brain size and mental
capacities suddenly take off?

Two million years ago, what
jump-started human evolution?

Scientists all over Africa
looked for clues.

Here in Kenya they found some,

at the southern end
of the Great Rift Valley.

It's a hotbed
of tectonic activity

where ancient layers are
forced to the surface.

Ten million years ago, Africa
was a much wetter place,

a tropical jungle which has been
slowly drying out ever since.

But these rocks in Kenya show

that Africa's gradual drying
trend was punctuated

by bursts of wild climate
fluctuation.

Rick Potts is an expert
in reading the rocks.

POTTS:
This layer right here

represents about 1,000 years
of environmental stability,

but then we had an abrupt
volcanic eruption,

and then the lake was around
for perhaps 500 years

before a drought,
then the lake came back.

So in some cases we saw this
through layer after layer

of environmental change.

NARRATOR:
With his trained eye,
Rick could see

some layers were once lake beds,
others desert sands.

Still others came from
volcanic eruptions--

a snapshot of a million years
of climate history.

This observation led him
to an amazing new idea--

rapid change as a catalyst
for our evolution.

POTTS:
And I began to think that, well,

maybe it's not the particular
environment of a savanna

that was important,

but the tendency of
the environment to change.

NARRATOR:
Could it be that the need

to survive violent swings
of climate

made our ancestors
more adaptable?

A group of scientists has come
here from Germany to find out

just how radical these swings
of climate really were.

It's hard to believe,
but these huge rock formations

are made of the shells of tiny
one-celled organisms

called diatoms.

There are many different kinds,
but they all live in water.

Their shells collect in layers
of rock that pile up

over millions of years,

proving that this whole valley
was once a giant lake.

Annett Junginger analyzes
these rock samples

under the microscope.

What I've discovered was

that those white layers consist

of a special kind of diatoms
which only live in deep lakes.

NARRATOR:
But between the white layers

she also finds other species
of diatoms

which only live
in shallow water.

It means that in this spot,
a massive lake appeared

and disappeared and reappeared
many times.

JOHN KINGSTON:
These lakes are really
significant;

these are not small ponds.

And what we've been able
to document now

is a series of lakes
that are cycling.

MASLIN:
We're talking freshwater lakes
the size of Lake Victoria,

filling the whole Rift Valley,
and then disappearing.

Enormous amount of water rushing
through this area.

This constant flux
of turnover, of change.

An awful time to live here.

It's not just
a unidirectional change.

It's going back and forth.

NARRATOR:
Against the backdrop
of a slow drying trend,

Africa was periodically pulsing
with climate change:

wet, dry, then wet again,

sometimes in the space
of 1,000 years.

Punishing drought alternated
with storms and monsoons.

Rivers and forests sprang up,
then turned to dry grassland

all in the evolutionary blink
of an eye.

So we have a complete change
of our ideas,

from this slow drying out,
to this incredible change

between wet and dry,
wet and dry.

NARRATOR:
What effect did that have
on our ancestors?

Could these periods of climate
instability be the key

to understanding
the evolutionary leap

from small bipedal apes

to the larger brained
toolmaker, Homo habilis?

To know that, scientists needed
a detailed record

that went back further
than the diatoms--

way back to the time when
Homo habilis was evolving

two million years ago.

That's only found in one place--
under the ocean.

Layers of deep-sea sediment
tell a story

that goes back millions
of years.

They have to be drilled
from the ocean floor.

At his laboratory
in Upstate New York,

Peter deMenocal keeps
thousands of columns

of sand, silt and rock--
a library of ocean cores.

deMENOCAL:
One of the really attractive
features about ocean sediments

is that they accumulate very
slowly but very gradually

and continuously over time.

NARRATOR:
Each three-foot long core holds
a continuous record

of dust carried on the wind
from Africa into the ocean,

where it now sits
on the bottom.

Oh, nice! Wow!
There we go!

Sweet-- okay.

NARRATOR:
An expert eye can detect
distinct layers--

thick in dry years

when the dust is easily picked
up by the wind;

thin in wet years.

By measuring the layers,

they can tell when
the climate was wet or dry.

deMENOCAL:
So we can read
these deep-sea sediments

almost like an earth history
book of past changes in climate.

NARRATOR:
To make sense of all this dirt,
they have to know

when it blew into the ocean.

They can do this by dating the
shells of tiny sea creatures

that sank to the bottom
at the same time.

deMENOCAL:
So this gives us an age,

the other analysis gives us
the climate.

Oh, nice!

NARRATOR:
Peter took this finely detailed
climate diary

and compared it to the grand arc
of our human evolution.

For the three million years
between Toumai and Selam,

when brain size was flat-lining,
African climate was stable--

dry... getting a little drier.

Then came 200,000 years
of wildly varying climate,

careening unpredictably
between wet and dry.

During that time,
stone tools appeared

along with the larger brained
creatures that made them.

Africa was also home to many
other humanlike species.

Climate instability put pressure
on all of them.

So there are these time periods

when African climate was
really unstable,

so anything that was living
there at the time

would have had to adapt

to really dramatically different
climate changes.

NARRATOR:
Those that couldn't adapt died
out, like Selam and Lucy's kind.

Better problem-solvers,
like Homo habilis, survived.

The new discoveries

about ancient climate upheavals
in Africa

have led Rick Potts to formulate
a bold theory

of human evolution.

POTTS:
The traditional idea we have had
about human evolution

is that it was the savanna--
the grassy plain

with some trees on it--
that was the driving force.

But instead,
what we've discovered

is that climate changed
all the time.

And so the idea that
we've come up with is

that variability itself

was the driving force
of human evolution

and that our ancestors were
adapted to change itself.

NARRATOR:
It's a simple
but revolutionary idea--

human evolution is nature's
experiment with versatility.

We're not adapted to any
one environment or climate,

but to many.

We are creatures
of climate change.

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."

Because we are that creature,
because we are that smart.

NARRATOR:
Today, climate change seems
to threaten our survival.

But it may have held the keys
to the astonishing story

of how we became who we are.

Because it didn't stop
two million years ago.

These dramatic upheavals
would continue

for another million
and a half years,

propelling our ancestors down
a road leading ultimately

to the smartest creature
the world has ever known.

NOVA's got a brand-new
evolution Web site

with lots to explore
about our ancestors.

We want to know
what you think.

Bookmark it today
and give us your feedback.

A mystery unearthed...

A nearly complete skeleton.

This is a big, strong creature.

The first species
that looks like us.

Looks like us... and,
surprisingly, is like us.

Nearly two million years ago.

Now meet him face to face.

A groundbreaking NOVA special

tells a new story
of human evolution,

our story.

Part Two of Becoming Human--

"The Birth of Humanity,"
next time on NOVA.

Major funding for NOVA
is provided by the following:

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