Original subtitles

National Geographic Earth: Making of a Planet Original Air Date on March 6, 2011

== sync, corrected by elderman ==

The Earth, as far as we know it's the

only home to life in the universe.

So what makes our planet so special?

To find answers, we must travel back in time.

See the first humans walk the Earth.

Ride each colliding continent.

Face killer dinosaurs.

Dive into oceans full of bizarre life forms.

Feel the bitter chill of global ice ages.

And experience the fury of cosmic missile attacks.

By travelling back in time,

we can piece together our planet's incredible

story.

And discover why all of this, and all of us,

are here.

The journey starts almost five billion

years ago.

There's no sign of Earth just yet.

Only the Sun, a newborn star surrounded by dust.

The Earth hasn't formed yet.

But through time, gravity pulls the dust

into tiny rocks.

To our surprise, something as complex as

a planet is made from nothing more than dust

and rocks.

Over millions of years, gravity pulls these

rocks together to form the Earth,

one of at least a hundred planets

circling the sun.

But four and a half billion years ago,

Earth looks more like hell than home.

Up close the temperature is over

2,000 degrees Fahrenheit.

There's no air, just carbon dioxide,

nitrogen and water vapor.

It's so hot and toxic that if we got any

closer we'd be incinerated and

suffocated in seconds.

This newborn planet is a boiling ball of

liquid rock;

an endless ocean of lava.

A young planet called Theia is heading

straight for Earth.

The size of Mars, it travels nearly 10 miles

per second, twenty times faster than a

bullet.

The intruder's gravity distorts the Earth's surface.

The blast wave races around the planet.

It's as though both young planets turn to liquid,

trillions of tons of debris blast

out into space.

But over the course of a thousand years,

gravity turns the rubble into a ring of

red hot dust and rock that circles the Earth.

From this ring, a ball forms,

over two thousand miles wide.

It's the birth of our moon.

But it's much closer than the moon today,

just 14,000 miles away instead of a quarter of

a million miles.

The sun rises over a cooling Earth.

And sets just three hours later.

The Earth is spinning so fast that an entire

day lasts six hours.

The days may pass quickly but the Earth

changes slowly.

To understand the making of our planet we

need to fast foard through millions of years.

3.9 billion years ago,

a hail of meteors attack the Earth.

It's debris left over from the solar system's

formation.

Inside the meteors are crystals that look like

grains of salt.

And inside the crystals are minute droplets of

water.

These deadly missiles could contain the vital

ingredient for life on Earth.

Only a small amount of water exists inside

each meteorite, but as they bombard the Earth

for over 20 million years,

pools of water grow.

The Earth's core remains molten,

but its surface has cooled enough to form a

crust.

The air is around 170 degrees.

In the future, there's a chance we could

swallow this very water.

Every sip, every puddle,

every drop of water in every ocean,

is billions of years old.

And it may have travelled millions of

miles to reach us, carried inside a

meteor.

The Earth looks more familiar,

but it's still a dangerous place.

Winds are faster than the most destructive

hurricanes today.

It's a mega storm, whipped up by the

planet's rapid rotation.

The moon is so close to Earth that its gravity

is overwhelmed, creating huge tides

that race across the planet's surface.

But over time, the moon moves away,

the waves calm down and the planet spins slower.

700 million years after the planet's birth,

life-giving water covers its surface.

And scattered throughout are tiny

islands.

They seem to have appeared from nowhere.

So how did they get here?

Molten rock bursts through the Earth's

crust and rises up through the ocean.

Over time the lava cools,

forming a volcanic island.

In the future, these islands will join

together to form the first continents.

Earth now has water and land.

It's beginning to look like the planet we call home.

But the atmosphere is still toxic and the

temperature is scorching.

Life here is impossible.

Since the planet's formation,

meteors have been pummeling Earth.

But now, 3.8 billion years ago,

the assault enters a violent new phase.

Something's disturbed the orbits of these

meteorites.

In addition to water, the meteors carry

something else.

As they dissolve, the meteorites release

minerals.

Carbon and primitive proteins made of amino

acids are transported from outer space to the

bottom of the ocean.

Thousands of feet below the waves, it is dark,

and close to freezing.

But a dramatic illusion is taking shape.

Underwater chimneys, spewing hot liquid.

Seawater seeps down into the Earth through

cracks in the crust, collecting minerals and

gases along the way.

This potent mixture is released back out into

the ocean, building these towers.

Combined with the minerals and chemicals

from the meteorites, the water becom a

chemical soup.

It's impossible to know how or when,

but somehow these chemicals come together

to create life.

The water is now full of microscopic

organisms.

These single-celled bacteria are the

earliest forms of life on Earth.

This is a defining moment in the making of

the planet.

Microscopic life is underway.

For hundreds of millions of years

nothing changes.

The only life is single-celled bacteria.

To find more complex life,

we need to fast forward to 3.5 billion years ago,

and a shallow ocean.

These look like rocks, or even plants,

growing out of the sea bed.

Each mound is a colony of living bacteria

called a stromatolite.

These bacteria turn sunlight into food,

a process called photosynthesis.

It uses the power of sunlight to traform

carbon dioxide and water into glucose,

a simple form of sugar.

And this magical transformation releases

a by-product, a gas called oxygen.

Underwater, the stromatolites slowly

fill the oceans with oxygen.

The oxygen then turns traces of iron into

rust, which fall to the ocean floor,

to form deposits of iron rich rock.

It's this mineral that will one day be used to

build bridges, ships, and skyscrapers.

Above the waves, the oxygen transforms the

atmosphere.

These stromatolites are creating the most

important element for life on earth.

Without them, virtually every living thing

wouldn't exist.

Over the next two billion years oxygen

levels continue to rise,

and as the planet's spin slows,

the days get longer, lasting at least 16 hours.

3 billion years after the planet's birth,

there's still no complex life,

no plants, no dinosaurs, no humans.

But the Earth has something unlike any

other planet in this solar system,

a force with the power to change everything.

One and a half billion

years ago, the Earth is a beautiful blue ball,

dotted with volcanic islands.

Over millions of years, a mysterious force

rearranges these islands.

Hidden beneath the ocean,

the Earth's crust breaks into vast

plates.

But even deeper, the Earth's core is so hot

that it generates movement in the rock

beneath the crust.

These movements push and pull the plates

around the globe, carrying the oceans and

islands with them.

Millions of years race by,

our planet is alive, and changing.

Over 400 million years a vast new super

continent takes shape.

It's called Rodinia.

In the shallow waters surrounding Rodinia,

stromatolites work their magic for more

than two billion years, pumping oxygen into the

atmosphere.

The temperature is 85 degrees.

And the days are 18 hours long.

But this planet looks more like Mars than Earth.

To find life, we must fast forward through time.

Washington State: 750 million years ago.

A force deep inside the planet rips the crust

to pieces.

It's as though the world is breaking

apart.

And there's only one force powerful enough

to do this, heat.

It escapes from the Earth's molten core,

stretching and weakening the crust.

Inch by inch, year by year,

the great super-continent is

splitting in two.

The intense geological activity spawns a mass

of volcanoes.

They pump carbon dioxide into the

atmosphere.

The carbon dioxide mixes with water,

and creates acid rain.

When the continent tore apart,

many rocks were exposed.

These rocks absorb the acid rain,

including its carbon dioxide.

Vast quantities of CO 2 are absorbed out of the

atmosphere.

Now, there's not enough carbon dioxide in the

air to trap the sun's heat around the planet.

In just a few thousand years,

the temperature plummets to around

minus 60 degrees.

This frozen wasteland is Southern Australia,

650 million years ago.

It's the start of what some scientists call

Snowball Earth, a period they believe to

be the longest, coldest ice age ever to grip

the planet.

A vast wall of ice looms thousands of feet

high.

And it's unstoppable.

The more ice there is the more sunlight it

reflects away from the planet,

and the faster the ice spreads.

Nearby is a second ice sheet just as high.

The two sheets spread away from the poles,

toward each other to meet at the Equator.

Now an ice sheet up to 10,000

feet thick covers the entire planet.

Earth first began as a molten ball of fire,

now it's a frozen ball of ice.

Virtually all the sun's light, and warmth,

reflects back into space.

And it seems as though nothing,

not even the Sun can rescue the planet now.

650 million years ago,

ice entombs the planet.

For 15 million years, the Earth is a vast

frozen snowball.

But it can't last forever;

something must release the Earth from this

frozen prison.

And when it does, who knows whether life has

survived beneath the ice.

The surface is frozen but the core is still

hotter than the sun's surface.

Volcanoes have been erupting since the

world began to freeze.

But up until now even their heat and power

made no impact on the ice.

Volcanoes pump out billions of tons of

carbon dioxide.

Before the big freeze, the Earth's rocks

absorbed most of the CO2.

But now, with the rocks smothered in ice,

there's nothing to absorb the gas.

So it fills the atmosphere.

Like a blanket, it traps the sun's warmth

around the planet, allowing the

temperatures to rise.

Now, after 15 million years,

the ice begins to melt.

It's thought that during Snowball Earth

the ice pushed the crust down.

But now, as it melts, the crust bounces up,

creating fissures and weak spots,

and more volcanoes.

The volcanoes release more carbon dioxide and

push the temperature up even higher.

The melt gathers momentum.

And oxygen levels rise.

Through a series of chemical reactions the

ice has created oxygen.

While the planet was frozen,

the Sun's ultraviolet rays reacted with water

molecules in the ice, to produce hydrogen

peroxide, a chemical rich in oxygen.

Now, as the ice melts, the hydrogen peroxide

breaks down and releases massive

amounts of oxygen.

The Earth is waking up, and it's a very

different place.

600 million years ago, the atmosphere is

warmer, like a summer day.

And the days are about 22 hours long.

Add all this water and it's the perfect recipe

for life.

Before Snowball Earth, primitive bacteria

emerged in the oceans.

But surely they couldn't have survived

an ice age 75 times longer than the entire

span of human history.

If something has survived,

then life may be found where it was last seen,

in the oceans.

Now, 540 million years ago,

in an ocean full of oxygen,

those primitive bacteria have evolved.

A handful must have clung on through the

big freeze.

There are plants scattered on the sea

floor.

And something else.

What looks like an armored slug takes up

residence.

It's called Wiwaxia, a new generation of

complex multi-celled organisms.

The Earth enters one of the most dynamic

periods in its history, the Cambrian explosion.

Increased oxygen levels allow creatures to grow

larger and develop bony skeletons.

There are worms, sponges,

and these trilobites: distant relatives of

insects, lobsters, even scorpions.

Life in the oceans blossoms.

From microscopic bacteria, to monsters.

This is Anomalocaris.

Nearly two feet long, it has large eyes,

razor sharp teeth and grasping limbs.

All it has to do, is take its pick.

The trilobite can't right itself,

its soft belly is exposed.

Also found in the sea is Pikaia.

Only an inch and a half long,

they may be the first organism with a spine.

Over millions of years, this simple structure

will evolve into the spine that keeps us

standing.

Expecting to find life in ruins,

the oceans are in fact full of life.

Where trilobites scavenge and monsters

prowl.

And where creatures begin to take on

familiar forms.

Beneath the waves there are already tens of

thousands of plant and animal species.

The advance of life seems unstoppable.

460 million years ago,

the plates have been moving, again.

A new continent now exists, Gondwana.

It's 90 degrees and oxygen levels are close

to what they are today.

The land should be covered with plants and

crawling with creatures.

But there's not much here,

besides a few patches of algae.

There's only one explanation, the Sun.

It blasts the surface with deadly radiation,

so the complex life in the ocean doesn't stand

a chance on land.

But 30 miles up, where the rays enter the

Earth's atmosphere, something is happening.

When oxygen meets the sun's radiation,

it turns into another kind of gas,

call ozone.

This gas forms a blanket around the

planet and absorbs the lethal radiation.

Over 120 million years, the ozone layer gets

thicker and prevents more and more radiation

from reaching the Earth's surface.

Without this layer, life on land simply

wouldn't exist.

Now, shielded from radiation,

life takes off.

Small mossy lumps are the first land plants.

They pump out even more oxygen.

Levels soar.

375 million years ago, a new species lives in

the water, swimming.

It's a strange fish called a Tiktaalik.

It uses its neck to raise itself up.

And its fins function as legs,

allowing it to move out of the water.

This is one of the most important moments in

the making of our planet.

Over 15 million years, creatures like these

evolve.

They grow stronger limbs and spend more

time out of the water, where plant life is

exploding.

Until, 360 million years ago,

their relatives, called tetrapods,

make the land their home.

From tetrapods, four legged vertebrates

evolve into dinosaurs, birds,

mammals and eventually humans.

There are mosses and ferns everywhere,

some at least a hundred feet tall.

A seed is carried by the wind.

Until now,

plants reproduced using spores,

single-celled particles that need plenty of

water to grow.

But this seed is miles from water.

This embryonic plant has its own food and

water supply.

Unlike a spore, this seed can survive far

from water for months, even years.

The humble seed spreads life across the planet.

And each tree and plant pumps out even more

oxygen.

It's an environment very much like today.

The planet has come a long way,

from a lump of burning rock and dust,

to a blue and green world bursting with

life.

There are now fish, plants, and this,

the dragonfly.

It's called Meganeura.

This insect is the size of an eagle.

What were once legs have evolved into

wings, extending the dragonfly's hunting

territory over a vast area.

There are also millipedes,

and spiders down there.

These creatures, called arthropod,

were among the first to set foot on land

hundreds of millions of years ago.

They look almost identical to the bugs

that invade our homes today.

Except for one big difference.

Like the Meganeura, they're monsters.

It's a world full of giants,

where millipedes are 6 feet long,

andcorpions are the size of wolves.

This is because the oxygen level is much

higher than it is today,

which allows their respiratory systems to

be more efficient, fueling thier bodies to

grow larger.

So far, animals have been laying their eggs

in the water.

But a lizard-like creature,

called the hylonomus, prefers land.

Its eggs contain all the water and nutrients

that the developing fetus needs.

The babies grow in their own

self-contained pond.

The egg is a major evolutionary

breakthrough.

Allowing animals to leave the water behind,

and conquer land.

This baby hylonomus leads the advance,

as a new creature, the reptile.

Inevitably, with life, comes death.

Dead plant matter builds up and decays

into dense soggy layers.

Over hundreds of millions of years,

rocks cover these layers.

And heat from the Earth's core and

pressure from the overlying rocks

transform these layers into seams of coal.

Each lump of coal burned today is made of

plants that died 300 million years ago.

Amidst the decay, hidden from sight,

life is stirring.

Soon seeds will germinate,

plants will grow, and this wasteland will

live again.

Life seems to have conquered the planet.

But will it last?

A herd of creatures graze

the Siberian plains, and they are big.

Evolution takes a huge leap forward,

small lizards are now giant reptiles.

These scutosaurs are distant relatives of

turtles.

If these plant eaters look this tough,

the carnivores must be seriously mean.

Like this gorgonopsid.

A perfectly engineered pre-historic killing

machine.

The gorgonopsid's sabre-teeth wound the scutosaur.

The predator watches,

as its prey grows weak from blood loss.

But before it can make its final move,

something strange happens.

The ground gets hot.

Enormous pressure builds beneath the surface,

and lava spews into the air.

But the lava isn't from one single volcano,

the entire landscape is erupting.

It's a flood basalt eruption.

A massive plume of mantle rises up from

deep inside the Earth, and pushes molten rock

out through fissures in the Earth's crust.

The lush paradise is now a lifeless hell.

The scutosaurs and the gorgonopsids are dead.

They're the first casualties in the

greatest mass extinction the world

has ever seen, the Permian Extinction.

On the other side of the continent Gondwana,

nothing's changed, yet.

It appears to be snowing,

but the temperature is about 70 degrees.

It's actually ash, fallout from the

eruptions some 10,000 miles away.

The ash burns and suffocates the animals,

killing them around the world.

Sulphur dioxide from the eruptions fills the

atmosphere.

As it rains, the gas turns to sulphuric

acid, and burns everything it falls on.

What was a local disaster has now turned

global.

The Siberian eruptions increase the Earth's

carbon dioxide levels.

The atmosphere gets hotter.

Water evaporates.

And vegetation dies.

Around the world, life on land is being wiped out.

And life in the oceans,

has also been compromised.

The waters turn pink.

Plants, trilobites, and predators, disappear.

The new, hotter atmosphere heated the

oceans and stripped them of oxygen.

Now, this pink algae is one of the few life

forms that can survive in the stagnant water.

The Siberian eruptions transform the entire

planet, nothing, not even the deepest ocean

floor, is beyond their reach.

Bubbles of methane gas escape from beneath the

sea bed.

Methane is a greenhouse gas,

at least twenty times deadlier than carbon dioxide.

Until now, the gas has been frozen.

But as the sea temperature rises,

it begins to melt.

Released into the atmosphere,

this powerful gas pushes up temperatures

even further.

It's now almost 105 degrees,

11 degrees hotter than before the Siberian

eruptions.

Creatures that survive the initial destruction

now face a new and deadly environment.

Few will live.

It's been five hundred thousand years since

the eruptions first began,

And all this time, the lava's been pouring out.

By now, it covers an area the size of the

United States with a layer of molten rock

nearly 4 miles deep.

95 per cent of the species are gone.

A few survive by eating anything they can find

and living in burrows underground.

But everything else, is dead.

250 million years ago, the Earth reverts back

to an almost lifeless planet.

But that's about to change, again.

It's been fifty million

years since virtually all life on Earth was

wiped out.

And the planet has been transformed.

It's now 200 million years ago,

and just one supercontinent, Pangea,

stretches from pole to pole.

After the trauma of the mass extinction,

the planet heals.

Temperatures stabilize; the acid rain

neutralizes and vegetation returns.

With 95 per cent of all life on Earth wiped

out, the field opens for a new species to

emerge.

One that will dominate the planet like no

other.

The dinosaurs.

These are called Ammosaurs.

Like all dinos, they evolved from the

reptiles that survived the Permian Extinction.

At 15 feet tall, their size makes them slow,

and vulnerable.

Nearby is the dilophosaurus.

It's small,

and fast.

The ammosaurus is too big a meal for one

dilophosaurus.

But not for two.

The dinosaurs have repopulated the Earth.

But no species can tame this restless,

volatile planet.

The earth's crust is thinning.

It's releasing lava and shaking with

earthquakes, as if being stretched by some

unseen force.

It's also happening near what will become

North America's eastern seaboard.

The Earth's plates are on the move again.

190 million years ago, the great

super-continent of Pangea tears apart.

A vast slab of land breaks away,

creating a chasm.

It fills with a new ocean called the Tethys

over what will one day be the Middle East.

Currents push nutrients up into

the coastal waters.

And the nutrients attract fish,

in the millions.

But again, with so much life, also comes death.

Dead fish and plankton carpet the ocean floor.

Over the next 10 million years,

layers of rock will bury and heat

the dead creatures.

Ancient fish and

plankton will become oil.

Every gallon of gas in our cars,

every piece of plastic on the planet,

the paint on our walls, the carpet under our

feet, even the soap we wash with,

almost all originated in this way.

180 million years ago, the North American

plate is still moving away from the European

and Asian plate.

It's shifting about one inch each year,

the same speed at which our fingernails grow.

But fast forward 35 million years and a new

ocean forms, as well as new continents.

The United States breaks away from

Africa.

The world as we know it takes shape.

The chasm between the two continents fills,

creating the Atlantic Ocean.

And in the middle, lies a volcano.

Like before, currents deep beneath the

Earth's crust move the plates again.

The entire sea floor tears in two and is

pushed up into a ridge of

mountains and volcanoes.

It grows higher than the Himalayas,

and longer than the Rockies.

The water here is hot, as molten lava forces

its way out from deep inside the earth.

As the lava cools, it creates a new range of

volcanic mountains, and a new ocean floor.

Pangea breaks apart, and our world

rearranges again.

It's this geological activity that makes the

Earth restless, creative and unique.

And every time the planet reinvents

itself, its inhabitants adapt and evolve.

Like the ichthyosaurs.

Their reptile ancestors lived on land,

but as the planet changed,

so did these animals.

They grew fins and moved into the newly

formed Atlantic Ocean.

This one is 20 feet long,

and can travel at a quick 25 miles an hour.

It's the ocean's fastest creature and

most efficient predator.

It rules the oceans for 50 million years.

But now, there's a new contender.

The pliosaur.

Longer than a bus, and as heavy as a truck,

its jaws are eight times more powerful

than a great white sharks',

with teeth 12 inches long.

The Earth, and its inhabitants,

have changed beyond recognition.

This was once solid ground,

now it's the Atlantic Ocean.

The very spot where ammosaurs grazed and

dilophosaurs stalked their prey.

The dinosaurs' world may be different,

but they're as dominant as ever.

They appear invincible.

They're one of the planet's most

successful species.

They've survived volcanic eruptions,

earthquakes, and entire continents splitting

apart, The dinosaurs have thrived for 165

million years.

A number of mammals have also survived,

like this shrew-like animal.

They're prey to the dinosaurs.

So they live in trees or underground,

and venture out only at night.

Mammals pose no threat to the dinosaurs.

Nothing on Earth can challenge their

dominance.

Nothing, on Earth.

A large piece of space rock,

heads toward Earth.

This asteroid is at least 6 miles across,

and travelling over 40,000 miles an hour.

It's bigger than Mount Everest,

and heads straight for the Gulf of Mexico.

It travels so fast that its impact would be

missed in a blink of an eye.

A split second changes the world forever.

At impact, the asteroid's back edge

stands at 35,000 feet.

It strikes with such immense force that it

destroys everything it hits.

The asteroid itself instantly vaporizes,

unleashing the energy of millions

of nuclear weapons.

Nowhere is safe,

with debris flying everywhere.

Debris as big as entire city blocks

circle the Earth.

The blastwave races out from the impact zone

like shrapnel from an exploding bomb.

Minutes after impact, hundreds of miles

from where the asteroid struck,

the earth is under attack.

Boulders rain down.

Earthquakes shake the ground.

And tsunamis batter the coasts.

But the onslaught has only just begun.

The plume of molten rock and dust spreads

out, and engulfs the planet.

The entire sky acts like a giant lamp,

heating the Earth's surface to

over 500 degrees.

Vegetation spontaneously ignites.

Even months after the impact,

smoke and ash still block out

the sun's rays.

And with less sunlight, plants die and the

animals starve.

65 million years ago,

the impact has blasted,

stoned and burnt the dinosaurs.

Their 165 million year reign is over.

But the dinosaur's demise brings

opportunity for another species.

A mammal.

By living underground, mammals have avoided

the heat and fires.

And by eating anything, they thrive while more

selective eaters die.

They're the unlikely inheritors of the

dinosaurs' crown.

And as one story ends.

Another begins.

With the dinosaurs out of the way,

our ancestors' may now have a chance, at life.

The dinosaurs are long dead.

The planet is peaceful.

In this new world, our mammal

ancestors are evolving.

47 million years ago, this lake,

in what will one day be Germany,

is the perfect place to spot them.

Unlike earlier mammals, its eyes and

brain are bigger.

This is Darwinius Massilae, or Ida.

She looks nothing like humans,

but fossil evidence today indicates that

these creatures might evolve into monkeys,

apes, and eventually humans.

Looking back through 47 million years of

evolution might reveal one of our earliest

known ancestors.

The lake sits on a volcanic crater.

It belches out noxious gas,

that eventually kills Ida.

The lake preserves her in its

oxygen depleted depths.

One day, when the water is gone and Ida's

fossil is found, this primitive primate could

help piece together the beginning of our own

story, the story of human life.

We're closer to understanding how

everything we've seen, from ocean bacteria,

through walking fish and subterranean

rodents, leads to us.

And to understanding how our

planet was made.

47 million years ago, the atmosphere

is much like today.

The temperature is 75 degrees.

And a day lasts just under

twenty four hours.

The Earth is now almost identical to

the planet we call home.

Almost.

The Earth's plates are on the move again.

India moves north toward Asia.

The Indian and Asian plates are locked

in a titanic struggle,

and neither plate is winning.

As they buckle, the ocean floor contorts

upward along a 1,500 mile line.

A vast mountain range rises up

over 27,000 feet.

These are the Himalayas.

And the highest mountain of all,

Mount Everest.

Its summit reaches up into the

Earth's jet stream, the same altitude

where planes fly.

When the snow on the peaks melts,

it feeds rivers like the Ganges, Indus,

Yellow and Yangtze.

The Himalayas are like a vast water tower.

One day, their rivers will supply water for

almost half the world's population.

20 million years ago, this is our planet,

with every continent, and every ocean

as we know it.

Except one thing is still missing,

the human race.

And for humans to evolve,

something needs to change.

Along the African

coast, a great rift opens up.

It stretches nearly 4,000 miles,

with mountains growing along its edge.

This ape-like creature may stay in these trees

forever, but its world is changing.

The growing mountains act like a wall,

prohibiting moisture from the Indian Ocean

from passing over the land.

It's getting hotter, and drier.

The lush rainforest becomes an arid

savannah.

The new hotter climate changes the creatures'

habitat, forcing them to search further for

food, soon, they stop dragging their knuckles

like apes.

And begin to stand, and walk on two feet.

It's the most important step in

the human story.

This mountain range could be the reason

humans walk on two feet.

The random movement of two plates may have

kick-started a chain of events that leads to

the first humans.

A man and child.

A scene very much like today.

But it's one and a half million years ago.

They are an early species of human called

Homo Erectus.

And these are the first footprints like our

own; civilizations past and present,

everyone that's ever lived,

the greatest inventions,

the most brilliant ideas.

Human history, in all its complexity and

splendor, begins here and now.

The climate changes again.

70,000 years ago sea levels fall.

And the gap between Africa and Arabia

shrinks to just eight miles.

The Red Sea is now narrow and shallow

enough for a small group to cross,

out of Africa.

They're another later species of human,

called Homo Sapiens.

Scientists believe that the entire world beyond

Africa was populated from descendants of

these two hundred individuals.

Over time our ancestors multiply and spread

out; to India, onto Asia and into Europe.

But while humans head north,

a giant wall of ice travels south.

Europe, 40,000 years ago.

Homo sapiens arrive, only to find a world

that's quickly changing.

It should be the height of summer,

but it's getting colder.

The plants are frostbitten,

and the rivers are frozen.

Natural changes in the Earth's orbit,

CO2 levels, and the flow of warm water

around the planet conspire to lower the

Earth's temperature.

The Earth, and its inhabitants,

enter an ice age.

Glaciers as high as skyscrapers creep over

the northern hemisphere about a foot every day,

Slow, and powerful, they sculpt the

landscape, gouging out great depressions.

The planet will never look the same again.

Now, around twenty thousand years ago,

they grind to a halt.

Much of the northern hemisphere is covered

by ice sheets up to one and a half miles thick.

With trillions of gallons of water locked

up as ice, sea levels fall.

A strip of land emerges from the ocean between

Siberia and Alaska.

It's a bridge between two vast continents,

a gateway that takes humans from Asia to a

new world, the Americas.

They're the last great continents to be

colonized, the last great human migration,

and the first Americans.

Now, 14,000 years ago, the changes that

triggered the ice age reverse.

As the ice retreats, it reveals a very

different northern hemisphere.

The huge depressions that the glaciers

create fill with water to become North

America's Great Lakes.

6,000 years ago, the ice retreats back to

the poles, toward the Arctic and Antarctic.

After a four and a half billion year journey,

we're, finally home.

This is our world, our time.

The planet's incredible story helps us

understand how and why everything around us is

here today.

From the skies above.

To water, the essential ingredient for life.

To the ground beneath our feet.

And finally life, a chain of catastrophes

and coincidences, creating a

spectacular result.

Each triumph and disaster is a step

leading to here, each and every one of

us, right now.

But the Earth's story isn't over.

This journey is only half the story.

A lot more is yet to come.

The Earth will live for at least another 4.5

billion years.

More wonders, more terrors,

and more strange creatures lie ahead for

our restless and creative planet.

The next chapter of Earth's story is still

to be written.

-- sync, corrected by elderman --