All language subtitles for Earth_Story_S1E07_720p_The_Living_Earth_WEB_AAC2_0_x264_NOGRP

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MANNING: It's in tropical forests,
as here in Indonesia,

that you find the richest diversity of life
anywhere on Earth.

The origins of all this diversity was a mystery
that obsessed Victorian naturalists

when they returned from their collecting trips
with exotic animals and plants.

The answer finally came from two great scientists.

One was Charles Darwin.

The other, a lesser-known Englishman,
working in these forests in the 1850s,

Alfred Russel Wallace.

I've always felt a particular affinity for Wallace.

He was a professional collector

but he so obviously took a delight
in the natural world

and he loved its amazing variety.

He writes in his journals
of how he trembled with excitement

as a particularly beautiful butterfly
got into his net.

And he rhapsodises several times
about the beauties of the birds of paradise.

It was here he developed his theory,
identical to Darwin's,

about the origin of species
through natural selection.

The idea that as individuals compete
for space and resources,

selection acting over many generations
picks out those

that are best adapted to live in their environment.

Although it was Darwin
who developed these ideas most completely,

Wallace added a significant extra twist
to the theory.

He noticed that this narrow strait
between the islands of Bali and Lombok

marked a dramatic changeover

in the types of animals and plants
he was finding in Indonesia.

Lombok was inhabited by a species
which were common in Australia.

But in Bali, there was an unexpected changeover

to plants and animals much closer
to those found throughout Southeast Asia.

Wallace was puzzled
because the two islands were only 15 miles apart.

Then he made a bold intellectual leap.

In a letter which he wrote to Darwin in 1858,
he said,

"Facts like these can only be explained

"by the bold acceptance of enormous changes
in the Earth's surface."

And we know he was absolutely right.

He can't have had any idea of the mechanism,

but we know now that Lombok
and the points to the east

and Bali and points to the west

were on separate plates, thousands of miles apart.

And only by continental drift
have they been brought this close together.

80 million years ago,

the Australian region
was much further to the south.

But the plate carrying Australia
and nearby islands with their distinctive wildlife

was moving gradually northwards.

Eventually they were brought
closer and closer to Southeast Asia

to form the Indonesian Archipelago.

Perhaps we biologists have tended
to regard the evolution of life

as very much our own preserve.

But now that geologists are rethinking
the evolution of the planet itself,

we're forced to recognise
that this has been one of the major factors

shaping the history of life.

So I'm going on a journey
through time and around the world

to learn how the evolution of the Earth
has moulded the evolution of life.

And how life in turn has helped shape the Earth.

My journey starts in
the Barberton Mountain Land in Southern Africa.

This is where the rocks reveal,
better than any others in the world,

the secrets of the origin of life.

The rocks here
are three and a half billion years old.

And this is where geologist Maarten de Wit

has recently made a very exciting discovery.

What is so very special about this place?

Well, they're not the oldest rocks in the world

but they are without doubt
the best well-preserved rocks around.

What things are you looking for in particular?

We're interested in learning
how the planet operated at that time.

So we're interested in
reconstructing the processes,

reconstructing what the environment was like.

One of the things, for example, we're looking at

is to see if we can detect any life forms.

MANNING: Maarten suspected
that hidden in these rocks

could be the very earliest life forms,

fossils of primitive bacteria-like organisms.

He sent some rocks to Frances Westall.

I want to try and find...

MANNING: She's a microbiologist
with a particular ambition,

to find the oldest fossil on Earth.

So, in this particular image
we have this beautifully dividing bacteria

caught in the act of division by being frozen.

Instantaneously frozen and preserved.

Everybody wants to find
the oldest bacteria on Earth.

And I think we may actually be lucky here.

MANNING: So these could be
the oldest fossils in the world.

This is the actual rock
that we got the bacteria from.

MANNING: Despite the excitement
of their discovery,

Maarten is actually more interested
in the environment

where life on Earth first originated.

What do we know about the conditions
under which the bacteria formed?

Well, these rocks allow us
to reconstruct an environment

that is telling us
hot springs were prolific in this area

at the time that these bacteria were living.

When we look at the details of this rock,
we can reconstruct an environment

a bit like Iceland today,
where hot springs are prolific,

both just above sea level and below sea level.

A lot of volcanic activity driving a lot of sea water

and fresh water
through these hydrothermal springs.

We speculate back in time
that this is the sort of place

where life might have started.

MANNING: It seems likely that these hot springs

are where the Earth gave birth to life.

It's here that the Earth's own energy
came bubbling to the surface

and fed those primitive life forms.

So from the very beginning,
the Earth and life were closely bound together.

Three and a half billion years ago,
the Earth was a very different place.

It was probably covered with water,

the oceans were very shallow,

and it seems the continents
were just starting to form.

Today, there's only one place in the world

to get a glimpse of what life
must have been like on the ancient Earth.

This is Shark Bay in Northwest Australia.

These columns contain vast numbers
of microscopic single-celled organisms.

So all the ancient Earth, all the shallow seas,

would have been filled with these things.

DE WIT: That's the amazing thing.

It's just like thinking back
two to three billion years ago.

This is what all the shallow seas
would have been like.

- They were just covered by this kind of life.
- Yes.

And it was the only life essentially around
at that time.

This is just a tiny piece that is preserved

- of what the Earth looked like then.
- Yes.

And that, to me, after all these years
of working in these old rocks, is amazing.

Here I'm walking on it. Live.

And these things are growing right now.

- It's quite a romantic thought, isn't it?
- It's really amazing.

- Ancient landscape...
- It blows my mind, actually. I must tell you that.

MANNING: The mounds are called stromatolites.

And today they're extremely rare.

But they represent a very important stage

in the evolution of early life.

These were the very first organisms
to use energy from the sunlight to grow

and emitting oxygen as a by-product.

That's right. Of course, we know that today,
that's photosynthesis.

Fixing carbon dioxide, giving off oxygen.

And of course, in the early stages
of Earth evolution,

this was a major event.

This was the first time that we see
oxygen production on planet Earth.

A very unexpected event.

And that event moves the evolution
of planet Earth

into a totally different mode.

MANNING: As it spread around the planet,

life in turn began to influence the evolution
and the geology of the Earth.

The dramatic effect of the oxygen produced by life
on the Earth's geology

can be seen in the Hamersley Range
in Northwestern Australia.

What an absolutely marvellous colour.

I think of iron, I think of rust.
It looks as if it's covered in rust.

That's exactly what it is. It's rust.

There's a very thin coating of rust on these rocks.

And that tells you these rocks
are just very rich in iron.

MANNING: And the way
these iron-rich rocks were deposited

show what was happening to the ancient Earth

during the early stages of the evolution of life.

The clue is in what geologists call
"banded iron formations".

Layers of dark red rock, rich in iron

alternating with pale layers with no iron.

MANNING: Where does the iron come from?

DE WIT: Well, the origin of this iron
that comes out in these dark layers

is volcanic activity in the deep oceans.

And yet, the geology tells us
these rocks must have been deposited

in very quiet, shallow seas.

So there is this paradox.

Why did this iron travel so far
before it deposits itself?

Well, the key to that is that iron
is very soluble in water that has no oxygen.

As soon as it comes out of the volcano,

it stays in solution
and it circulates around the planet,

until it reaches an area
where there is oxygen being produced.

And that's, of course, around these shallow oceans

where the stromatolites
are producing that oxygen.

- Right.
- As soon as it hits that,

the iron comes out of solution

and forms the start
of these banded iron formations.

There must be periodic processes going on here

to produce these bands of iron
then bands of the white material and so on.

Some of this banding, maybe,
is sort of growth on an annual basis.

It's a bit like rings in trees, growth rings in trees.

Wintertime, very little oxygen produced
by these stromatolites,

- and so the iron wouldn't come out.
- Yes.

But in summertime,
when the stromatolites are really going,

they just produce all this oxygen.

Iron sucks it up and gets deposited.

Rocks are polished by the water here.

MANNING: So, while these banded iron formations
were being laid down,

any oxygen produced by life
was being used up by the iron.

As a result, there was virtually no free oxygen

in the early oceans and atmosphere.

This went on for an incredibly long period of time.

Vast deposits of banded iron formations
continued to be formed

for over a billion years.

And then, about two billion years ago,

the huge banded iron formations
just stopped forming.

By now, life was producing so much oxygen

that the iron was immediately deposited
where it was produced

and never reached the shallow waters.

At last, the amount of oxygen
in the oceans and the atmosphere

started to rise.

One might say, then, that this is the first time

that we have evidence of life affecting
the structure of the planet.

Yes, you're right.

And I find that an intriguing and wonderful idea.

For the first time in Earth history

life is influencing geological processes.

MANNING: The oxygen in the atmosphere

also changed the course
of the evolution of life forever.

Oxygen molecules in the upper atmosphere

was transformed by the sun's rays into ozone.

This created a shield protecting the Earth and life

from the sun's harmful radiation

and allowed more complex life forms to survive.

The Earth itself was also changing.

By one billion years ago, large continents existed.

And life had evolved into a variety
of different single-celled organisms

which thrived in the shallow waters
on the edges of these continents.

Life began on Earth really very early in its history.

By about 3.6 billion years ago, it was established.

What I find so remarkable
is that for the next three billion years,

it stayed as single-celled,
very microscopic organisms.

And then about 600 or 700 million years ago,

something amazing happened.

And it's here on the rocky coast
of Newfoundland in eastern Canada

that scientists like Ed Landing
have found evidence

of a major change in the evolution of life on Earth.

LANDING: We're on a surface here
that is just covered

with all sorts of fossils.

Hundreds and hundreds of soft-bodied organisms,

some of them look like feathers,
almost like little ferns.

And some of these guys will be
up to a half metre in size.

A holdfast and something
that almost looks like a feather that comes up.

Now, what are these things?
Are they plants or animals?

Well, if you look at this rock section,

you can interpret the ancient environments.

The ancient environment was one
that was very deep.

It was on a continental slope or rise setting.

We're dealing with something
that is probably thousands of metres of water.

And that's important here because
light penetrates ocean water

only a couple of 100 metres.

And below that it's completely dark.

These things are living in darkness.

So these are not plants, these are animals.

These are the oldest soft-bodied
multicellular animals that are known on Earth.

MANNING: These fossils
are nearly 600 million years old.

So why did single-celled life evolve
into multicellular animals?

Evidence along the coast suggests that, yet again,

the Earth may have played a major role.

This is a rock unit we can trace around
this part of Newfoundland.

It's very distinctive.

It consists of pebbles, boulders up to great sizes,

of volcanic rocks and sedimentary rocks

surrounded by mud and sand.

This can't be laid down by flowing water.

And the interpretation
is that this is an ice deposit.

MANNING: The rocks and pebbles
were carried here by glaciers.

This was one of the coldest periods
in Earth's history.

The planet practically froze up.

And some glaciers had nearly reached the equator.

This massive ice age probably occurred
because of a combination of events.

About 600 million years ago,

the continents formed a vast region
in the Southern Hemisphere,

near the South Pole.

This reduced the circulation of oceanic waters

and stopped warm air and warm water
reaching the poles.

Huge sheets of ice developed across the planet.

The effect of all this on life was devastating.

70 percent was killed.

The single-celled organisms were the victims
of the first mass extinction of life on Earth.

The Earth which had given birth to life
had nearly destroyed it.

And then the massive continents broke apart.

The increased circulation in the oceans
warmed the Earth.

The big freeze was over.

This worldwide event
which actually brought glaciers

down to sea level in the tropics is thought to be...

It's a change in Earth history.

And what we end up having at this time
is, well, two things.

One is cold, glacial waters sink

and the deep oceans became oxygenated.

When the ice melted, sea level rose

and you had shallow seas covering the continents.

And this was a time of evolutionary radiation

of soft-bodied animals.

MANNING: The end of the Ice Age
created the right conditions

for more complex, multicellular life forms
to flourish and evolve.

LANDING: Before the glaciation,

we're starting to see very enigmatic,

multicellular things that might be animals.

But after the glaciation, we're seeing

quite complex
soft-bodied organisms living attached

and perhaps crawling around on the bottom.

MANNING: It seems that
once multicellular life had evolved,

there was an explosion of new life forms.

And the best place to see this

is in a remote area
of the Canadian Rocky Mountains.

I'm coming very close now

to a place which is holy ground for zoologists.

One day in August 1909,

the famous American palaeontologist
Charles Walcott

was riding along here on horseback.

He knew this area quite well.

There was a slab lying across the trail.

And he was afraid that his horse would stumble.

So he dismounted and tried to shift it.

But it was too heavy.

So he took his hammer and struck it.

And it split open to reveal
a miraculously preserved little fossil.

And Walcott saw immediately
that it was an animal

of a type that he'd never seen before.

So he knew he simply had to find

the place from which that rock had slid.

And that's where I'm going now.

Every year, Des Collins and his team
spend the summer collecting fossils

from the most famous fossil quarry in the world,

the Burgess Shale.

There's a little bit of sponge right here
on this side,

an ostracod there.

MANNING: I suppose what strikes me,
coming to this very famous place,

is how small it is.

It's a really very small area.

Sure. Sure. Well, the fossils are really mostly
concentrated in front of the cliff area.

And we've collected something like,

in total, something like 100,000 specimens
have been collected here.

- Over the years. Yeah.
- Quite amazing.

MANNING: The sheer quantity and quality
of fossils from the Burgess Shale

is quite staggering.

But even more surprising is that
all the major categories of modern animals

had already evolved.

I mean, you can see the finest details
of the hairs on... Are these the gills?

The gills there, right.

Well, this is one of the best specimens
of this that we've ever collected.

Well, what you can see
is all the beautiful swimming flaps

with the gills running on the end.

And you can see the gut
running right through the body.

And at the front, you've got these very large
great appendages.

A beautiful arthropod.

Looks very much like the arthropods
and the shrimps that you see in the sea today.

It's the soft body parts that you get so well here.
I mean...

- Right.
- What about... I mean, you get jellyfish?

Well, this is a ctenophore, that's a comb jelly...

MANNING: And scientists could see
that for the very first time,

these animals were part
of a complex community of plants and animals

and some of the animals lived by eating others.

This is the claw of Anomalocaris
that belongs to an extinct class of arthropods.

And this is one of the claws that comes out of
the front of the animal?

Right, right, the front of the animal.
And the other extraordinary thing are the jaws.

One guy thought that was a jellyfish.
That's a very strange-looking jellyfish.

We now know it's got 32 teeth, all going in,
they've got points at the end of them.

I've got a model here of the Anomalocaris.

And you can see the jaws.

MANNING: Right.

And if you compare the size of this
to the size of this in this proportion to the body,

then these jaws came from an animal
that was probably about a metre in length.

- That's a really big animal.
- Yeah.

MANNING: The biggest animal
that ever lived at that time.

So 520 million years ago,
both predator and prey had evolved.

They were inextricably bound together
as part of a food chain.

From now on, the evolution of life became

even more vulnerable
to any changes on the planet

which could affect this food chain.

But life was evolving only in the oceans.

The water teemed
with different animals and plants.

The land was barren and lifeless.

Then, about 450 million years ago,

as the numbers and complexity of species
in the sea multiplied,

plants at last made the evolutionary step
which allowed them to leave the water.

Once plants had made the move,
everything changed.

Soil started to build up, trapping water,
transforming the surface of the continents.

But the plants didn't have the land
to themselves for long.

Soon after they invaded, the animals followed.

I'm here on the Scottish island of Arran
with geologist Chris Nicholas.

And this is Carboniferous period.

NICHOLAS: That's right.
These rocks here are Carboniferous in age.

And what we find here is that all of these plants,

when they die, they're being buried
and compressed together

- to form coal.
- And that's coal, all right.

MANNING: This coal is graphic evidence
that the land was covered with dense forests,

forests where strange creatures lurked.

NICHOLAS: So this is a fossil track way.

You can see we've got these two parallel...

Yeah.

...lines of footprints that go round and they curve

and head off under that slab.

MANNING: This, I think, is the first track
of a terrestrial animal.

Yes, we think that we can work out
how many legs it had

from the repetition of these footprints.

- So we reckon this thing had about 23...
- Twenty-three pairs of legs.

Pairs of legs. That's right.

So it really was a relative
of the modern centipedes and millipedes.

Well, we think so, yeah.

But much bigger.
It must have been over a metre long.

That's right...

MANNING: These first fossil footsteps on land
are 350 million years old.

But at the same time, descendants of fishes,

the land vertebrates, had also emerged
from the sea.

There was an explosion of new life on land.

And just 50 million years later,

the land was alive with amphibians and reptiles.

There are more fossils of these early reptiles
here in South Africa

than anywhere else in the world.

Amongst these reptiles was one special group.

They're called the mammal-like reptiles.

Gideon Groenwald has found
hundreds of mammal-like reptile fossils.

If we look at, for instance, the skull of this animal,

we will find a very reptile-like skull.

But if we turn it over and you look at the inside,
you will find a palate in the mouth.

Now, the palate indicates to us

that this animal could chew
and breathe at the same time,

which is very important.

If you look at reptiles today,
they throw the head back

and they chomp off big chunks of food
and they swallow all this food in,

a very inefficient way of eating.

Whereas, if you look at a mammal,
it will breathe while it's chewing.

It actually grinds the food into
much smaller pieces. It's a very efficient feeder.

And for that reason, the mammal-like reptiles

had the ability to survive much worse conditions
than the reptiles of that age.

MANNING: These mammal-like reptiles,
our own very distant relatives,

were the first reptiles to dominate the world.

But they didn't evolve into true mammals

until the planet itself had gone through
some extraordinary upheavals.

250 million years ago,

a series of unrelated changes to the Earth

resulted in a dramatic change to life.

The continents had been moving together

and had formed one huge supercontinent
called Pangaea.

This huge landmass reduced the coastline,

reducing the habitats for marine life.

At the same time, sea levels dropped dramatically,

exposing and killing all those species
living on the continental shelf.

90 percent of marine life was destroyed.

On land, this coincided with a catastrophic event

in what is now Siberia.

COURTILLOT: It's very hard to imagine
what may have happened when this lava erupted.

The volumes, the size, the speeds involved.

This is not something that has been seen
since the human race exists,

since the human species exists.

You should try to imagine a fissure,
a crack in the Earth's crust

possibly 400 kilometres long,

spewing lava,

throwing material, dust
but most importantly gases,

carbon dioxide, sulphur dioxide,
that will eventually lead to acid rain,

darkness, cooling, altering vegetation,

destroying animals
that would need vegetation for support

and eventually killing animals
that would eat animals that ate vegetation.

So the whole life chain being completely changed.

Many teams around the world
have collected samples from this lava

and dated it very accurately

and found that
it was precisely 250 million years old.

MANNING: Massive volcanic outpourings like these
which ruin ecosystems, destroying food chains,

are regular events in the history of the Earth.

COURTILLOT: These volcanic eruptions are part
of a very important rhythm of the Earth.

The planet is essentially trying to cool down

and that heat leaves with that big bubble of rock
which, when coming to the surface,

melts and produces, through cracks
induced in the Earth's crust,

the gigantic lava outpourings.

It shows that the Earth's geology
has a direct influence

in changing the course of evolution
at certain times but in a gigantic way.

Was it not for these catastrophes,

well, life on Earth today
would be completely different,

and most likely, we would not be here.

That combination of events on Earth
250 million years ago

led to the biggest
and most catastrophic mass extinction of life

that's ever occurred.

It probably lasted for several million years.

But whilst it was going on,
the carnage was enormous.

It's hard to comprehend now,
but during that time,

80 to 90 percent of life, on land
and especially in the sea, disappeared.

So our modern understanding
of how the Earth works, how it operates,

has given us a new view
on the processes of evolution.

Yes, because it does seem that there have been
these marked events,

these punctuations in the history of life
caused by whatever reasons,

which have caused massive extinctions

and therefore provided opportunities
for other groups to evolve.

So rather than being just a steady

and almost ineluctable change
from simple to more complex,

life has these punctuation marks across it.

And each punctuation mark

provides a period of opportunity for new
and perhaps slightly unpredictable groups

to evolve and diversify.

And therefore the whole of life on Earth has been
affected by these singularly chance events.

It's difficult to know who will be the beneficiaries
of these events and who will actually lose out.

I think that's the way you've got to look at life.

There are always winners and losers
in any situation like that.

But it adds that sort of...

spicy sort of unpredictability
to the history of life on Earth.

MANNING: And the winners are the survivors,

those species with the ability
to live through the changes.

And after the mass extinction
of 250 billion years ago,

a few of the mammal-like reptiles
managed to survive.

The mammal-like reptiles
survived these very difficult times

because they were burrowing.

And this is a very good example
of a cast of a burrow that you find in the rocks.

And this is really a unique find.

Where we found the animal inside a burrow.

MANNING: Burrowing allowed these animals
to survive in an increasingly harsh world.

The whole of Pangaea
was gradually moving northwards,

taking this part of Southern Africa
closer to the Equator.

The climate became hotter and hotter,
drier and drier.

And by 200 million years ago,
some of the mammal-like reptiles

had evolved into a new group
of much smaller animals.

Now for the first time

we have the remains of true mammals.

And this here is the skull
of one of these tiny mice-like creatures

that managed to survive
these very difficult living conditions.

By burrowing, they lived in burrows
and they fed only during the night.

They're small, furtive, highly sensitive,
really quite intelligent mammals.

And you'd think they would just,
in an evolutionary sense, explode,

dominate everything around us,
because we're so familiar with mammals today.

But the mammals don't take over the Earth,
do they?

Curiously enough, those hot, dry desert conditions
don't suit mammals.

Reptiles such as dinosaurs
were perfect for those conditions.

Deserts usher in dinosaurs and really from then on

mammals are curiously confined
just to a nocturnal insectivore sort of niche

and scurry around at night,
perhaps around the sleeping bodies of dinosaurs,

while dinosaurs dominate the daytime.

MANNING: The dinosaurs were
the most successful of all the vertebrates

that have ever lived on land.

There were the dominant group
for over 170 million years.

And then, as we all know, they disappeared.

65 million years ago,
there was another mass extinction.

The dinosaurs went, the pterodactyls,

the ichthyosaurs, those marine reptiles,

many other species,

even the beautiful ammonites
were never seen again.

60 to 70 percent of life became extinct.

It disappeared from the fossil record.

Many scientists now believe that
a meteorite from outer space struck the Earth.

They believe that this created a gigantic fireball.

Dense clouds of material and dust
were thrown into the sky,

obliterating the sun for months or even years.

The whole ecostructure would have collapsed.

But some scientists have doubts.
Did a meteorite really destroy the dinosaurs?

A crater caused by the impact of a meteorite
has been found.

The centre is right here,
in the village of Chicxulub in Mexico.

The crater is the right date, 65 million years old.

But is it the right size?

Some scientists think the meteorite
was too small to have wiped out the dinosaurs.

Trying to settle the argument are seismologists
Mike Warner and Jo Morgan.

They're working out the size of the crater.

Using over 100 seismometers
buried in the ground and out at sea,

they can build up a seismograph,

a picture of the crater which is hidden
under 65 million years' worth of sediment.

And their results were surprising.

WARNER: The size of the object that hit the Earth
was about 12 kilometres in diameter.

But it was, relatively speaking, rather small
on the size that people had been guessing.

MANNING: It seemed to be too small
to have killed the dinosaurs.

But their seismograph shows the meteorite
couldn't have landed in a worse place.

A small impact shouldn't be devastating.
It shouldn't mess up the environment.

And maybe the dinosaurs were lucky.

If we look at these rocks here,

they're formed in shallow lagoons
much like we see around Chicxulub.

And they contain a lot of sulphur.

WARNER: Rocks with sulphur in,
if you hit them really hard with an impact,

then they generate sulphur dioxide.

And if you put sulphur dioxide in the atmosphere

it combines with water
and it makes sulphuric acid.

Little droplets of sulphuric acid
way up in the stratosphere.

And that's particularly deadly,

it stops sunlight getting to the ground
for 10 years, perhaps 100 years.

So, maybe the dinosaurs were killed
just by these rocks that we're seeing here

because those were hit very, very hard.

MANNING: But maybe the dinosaurs
were doubly unlucky.

Because at the same time
on the other side of the world in India,

another catastrophic event was taking place.

Volcanoes were again pouring out
massive amounts of lava.

It was an unimaginable amount,

the like of which had not been seen
for 200 million years.

The volcanoes were also belching out
clouds of dust, turning day into night.

The combination of the meteorite
and the Indian volcanoes was too much.

It turned the Earth's surface
into a dark, burning world

where all the rules
governing survival of the fittest changed.

The dinosaurs never had a chance.

Those events 65 million years ago

resulted in another dramatic turning point
in the evolution of life.

The dinosaurs had all gone,
most of the other reptiles had gone

and there was a void of large animals.

Into that void the mammals
and the birds jumped very quickly.

The mammals in particular
had been held down by the reptiles,

dominance of the reptiles,
for many millions of years.

And they radiated out extensively
all over the Earth.

The primates emerged
and eventually we emerged ourselves.

The last leg of my journey
through the story of life is here in Africa.

Not too far north of where I started
with the origin of life.

I'm in Kenya, in the Great Rift Valley of Africa.

And it's here that scientists are discovering

just how closely the evolution of the Earth

is linked to the evolution of our own ancestors.

Over the last million years,

the Earth has experienced a succession of
ten glaciations interspersed with warm periods.

This waxing and waning of the ice sheets

has had a profound effect
on the rest of the world's climate

and our own evolution.

Rick Potts spends every summer
searching for evidence

to show why modern humans evolved
from our ape-like ancestors.

We're going up through time,
up through time here

and then we reach a point
where the soil disappears

and this white sediment of the lake bed comes in,

which shows that the lake had expanded and
covered the whole area of the southern Kenya Rift,

just in this area.

And again these fluctuations continue

and then the soil comes back
just for a short period of time.

What do you mean by a short period of time here?

Probably, oh, a few hundred years at most.

And then the soil disappears
and it's replaced by the lake up here.

And then the soil comes back in again
for a short period of time.

Lake again and then a very sharp demarcation

where the soil again
goes as far as the eye can see.

And this occurred at a time when the Ice Age
fluctuations in Europe and North America

were just really getting going
and tremendous fluctuations going on.

MANNING: But how can rapid fluctuations
between dry and wet periods

be the driving force of the evolution
of modern humans?

POTTS: This is the species Homo habilis
and it had still a relatively small brain.

This was a clever creature, make no doubt
about it, but it wasn't quite us.

It reminds me very much of the chimpanzee.

I mean, why didn't we stay as upright apes
like chimpanzees?

In my view, we evolved
because of climate fluctuation,

of the tremendous fluctuations

and uncertainty of environments
that we can see here in the geologic record.

This is Homo sapiens, our own species.

And this one evolved during those fantastic
fluctuations of environment

that we see beginning about
600 to 700 thousand years ago.

And you can see the incredibly large size
of the brain case.

So the rate of change began to accelerate?

Yes. Tremendous acceleration
in the rate of brain growth.

And this had to do with I think, with flexibility,

with the ability to adapt
to those tremendous changes in environment.

For example, communication.

Through language we are able to say,

"You know, my grandfather told me that

"the time before him, you could find fruits
to eat on the other side of that mountain

"when there is a drought."

And that's something that no other animal can do.

Being able to refer to places and things
and abstract things that you can't even see.

And yet we can communicate about them.

We're used to the idea

that organisms are adapted
through natural selection

to match the specific environment
in which they live.

But I think there's another process of selection

that is... represents an adaptation to the
variability and the fluctuation of environments.

And this is a process that distances an organism

from any one specific environment.

And I think that we, Homo sapiens,

are the paramount example
of this kind of species.

(HERDSMAN CALLING OUT AND CRACKING WHIP)

MANNING: So we, too, like the rest of life,

are the product of constant physical changes
that have occurred here on Earth.

As a biologist,

I've always concentrated
on the evolution of life in biological terms.

But it adds a completely new dimension

to recognise how closely
the evolution of life is linked

with the inexorable changes of the Earth.

We now know that Earth isn't simply
the place where we happen to live.

We know it's a dynamic planet

and we know the Earth has been
one of the driving forces,

shaping evolution since
the very beginnings of life.

We also know that the way
life has evolved on Earth

makes our planet different
from our neighbouring planets.

But the Earth is also special in its geology.

And in the next programme,
we'll be exploring why we are so different.

What makes the Earth so special?

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