Original subtitles

Viewed from above, Planet Earth is a riot of colours.

But there's one particular colour that marks Earth out as special.

The colour that shows it's a living, breathing planet.

Green.

Here we go.

Take a look at this little beauty. This is Lysimachia glutinosa.

And I know it's not the most

glamorous plant in the world,

but its claim to fame is that it grows here

and only here,

on this one side of this one small island,

which makes it sound very fragile, very vulnerable.

That couldn't be further from the truth.

Because in the story of this plant, indeed all plants,

lies the story of our Earth.

It's a story that begins billions of years ago...

..in the chaos of Earth's early years...

..before plant life transformed it...

..into a world of opportunity...

..as plants rose from the oceans...

..to conquer a hostile and alien land,

fighting and evolving through triumph and disaster.

And just as they finally built the perfect garden world,

their global domination almost wiped out all life on the planet.

The story of plants begins deep in Earth's ancient history.

Four billion years ago.

When the planet was an inhospitable world...

..shrouded in a noxious atmosphere of methane clouds...

..and covered by an endless ocean...

..broken only by a few remote volcanic islands...

..with no sign of life.

But to find plants' ancestors,

you'd have to go about as far from the surface as you can get.

In the depths of the oceans...

..sheltered inside geothermal vents...

..are something miraculous.

Extremophiles.

An extraordinary form of single-celled life.

The ancestors of every living organism on Earth,

including plants.

But they are stuck here.

At this point, their chances of making the leap onto dry land...

..are virtually nil.

CHRIS SIGHS

Four billion years ago,

any dry land on Earth would have looked like this -

black, barren, volcanic islands peeping out of a vast ocean.

If plants had any aspirations to leap out onto land,

it was going to be very rapidly disappointed,

because this land was very short-lived.

The Earth's earliest islands were made up of basalt.

Solidified lava...

..that was easily devastated by explosive eruptions...

..smashed by extreme tides.

This was no place for life.

So the question is,

and it's a big question,

how did plants forge a permanent base on the land?

Because, if the Earth's only trick

when it came to land-building was volcanism,

it's very likely that that life

would've never made it out of the ocean.

What was needed was another land-creating force,

and it came in the form of a celestial intervention.

The culprits were giant asteroids.

Some nearly 60km in diameter.

More than four times the size of the one

believed to have caused the extinction of the dinosaurs.

The consequences were earth-shattering.

Fracturing our planet's crust...

..and triggering a process that would re-write Earth's story.

Plate tectonics.

Vast subterranean plates were formed,

and where they meet and collide,

rocks like basalt,

along with sea water and sediment,

are pulled into Earth's fiery mantle...

..where they're transformed into a new type of rock.

A rock with a superpower.

Granite.

So what is it about this hard, heavy,

unforgiving rock that sets it apart from the crowd?

Well, rather counterintuitively, it's its buoyancy.

Now, we know that ice is less dense than water,

therefore ice floats in water,

therefore we have icebergs.

But it turns out that granite, here, is 10% less dense than basalt.

So, when it's formed deep down inside the Earth,

it naturally rises to the surface.

So you could say that the continents on which we are walking

are vast floating granite icebergs.

So, when the tectonic plates collide

and the basaltic crust is forced down,

granite isn't.

Granite is pushed up,

where it's crumpled into these giant mountain ranges that we see today.

And over the course of geological time,

more and more granite accumulates on the surface,

perched on those tectonic plates,

which are gyrating around the planet

in a grand continental dance.

Now, you may be wondering how we know this.

- Ten, nine...

Ignition sequence started.

- Part of the answer is that, since the 1960s...

- Zero, all engines running.

- ..the space programme has provided a unique insight

into the workings of our planet.

For the last 42 years,

we've been able observe the movement of the Earth's tectonic plates

from orbit.

In 1976, Nasa launched Lageos -

the Laser Geodynamic Satellite -

which used a high precision laser measuring system

and thousands of reflectors...

..to confirm the theory that the continents

are constantly moving.

Nothing on Earth is staying still.

And by combining this data with other measurements,

we've been able to rewind the clock,

to see how the continents have evolved

over hundreds of millions of years.

And what this tells us

is that one billion years ago...

..the surface of the Earth was a place

full of possibility and promise.

Vast granite landmasses covered the planet.

All a potential home for life.

If only it could find its way there.

And, fortunately,

one life form was waiting in the wings,

ready to seize its opportunity.

Plants.

Life had migrated from the depths to the shallows...

..in the form of marine algae.

The first instantly recognisable plant-like organism on Earth.

They had mastered a revolutionary new art.

Harvesting energy from the sun...

..using photosynthesis.

But before these plants could escape the ocean,

they needed to overcome a hurdle...

..greater than anything they'd faced before.

It's quite difficult to get your head around

just what a challenge getting onto dry land was for plant life.

The world that it would have to overcome was harsh, hostile,

gravity bound, constantly battered by storms, wind, rain,

UV light, pounded by hot sunlight.

So, just like these contemporary relatives,

green algae rapidly chose the easy pickings,

living in those freshwater rivers and lakes that formed

on the early landmasses.

Cocooned in the safety of the water,

where nutrients and minerals were abundant.

Life stayed in the water for 500 million years...

..until a moment about half a billion years ago,

when, for reasons we don't entirely understand,

plants' ancestors set off into the unknown.

Making base camp on rocky sediments at the water's edge.

Having evolved a thick waxy coating

to stop themselves drying out in their harsh new environment.

But this brilliant adaptation proved to be a double-edged sword...

..making it much harder to absorb the nutrients they needed.

So, despite their best efforts,

they slowly dried out,

dying on the rocks.

But plants aren't the type to give up easily.

They just needed to find something to help them.

And they did,

because, as it turned out...

..they were not alone.

Half a billion years before plants

successfully made it onto dry land,

it's believed that another group of organisms were surviving

on these hostile early landmasses.

In this small rock are the fossilised remains

of Tortotubus protuberans,

a 440-million-year-old species.

Now, you can't see it.

It's 200 micrometres in length.

But its earlier ancestors were those that were surviving on that land.

Their ongoing success was down to their ability

to chemically degrade that substrate to get nutrients.

They were feasting on the bare rock itself.

Now, if that sounds otherworldly, I've got to tell you,

you probably know these organisms very well.

You might have even had some on toast

for breakfast this morning.

Because they're fungi.

The next waves of plant life making their way onto the land

developed a new trick.

They evolved specialised cells that could connect with fungi,

allowing them to trade resources

like nutrients and food between each other.

And this new, mutually beneficial partnership

turned out to be a match made in heaven.

This was truly a pivotal moment.

Fungi and plants had come together to produce

the first complex terrestrial ecosystem on Earth.

Now, the plants got from the fungi

the nutrients they could extract from the rocks -

and they repaid their fungal partners with glucose,

the sugar product of photosynthesis,

using energy from the sun and CO2 from the atmosphere.

And this symbiosis meant that plants could survive

permanently on these new landmasses.

They'd finally made it out of the water,

and they were ready to start conquering the world.

It's incredible to think that that first collaboration

between fungi and plants would lead to such

an extraordinary relationship,

and one which would endure till today.

Look at this bracket fungus here, growing on this log.

This species is all about decay and decomposition,

but we mustn't think about fungi being about death.

Here in the forest, they're very much about life.

There's an extensive network of their hyphae,

their roots if you like, stretching out into the woodland here,

intrinsically linking with the roots of all of the trees.

And they are allowing them to share nutrients,

even communicate with one another.

And the key thing is that none of these plants could survive

without the fungi

and the fungi couldn't survive without the plants.

And yet we always think of them being down here in the damp,

in the undergrowth, very much subservient to the plants

which are up here, towering above them.

But in the earliest days of terrestrial plants,

the situation couldn't have been more different.

The clues were strange circular fossils.

At first, scientists thought they were ancient trees.

But, looking closer, they found microscopic filaments,

revealing them to be fungi,

but on a scale bigger than anything we know today.

70 million years after plants and fungi first teamed up,

something utterly astonishing has happened to fungi.

They are now giants.

In the staggering form of Prototaxites.

Gargantuan, leathery, spore-bearing fungal monsters

standing an incredible eight metres tall.

They towered over the tiny plants

still clinging to the water's edge.

Before plants could challenge the dominance of fungi,

they needed to come up with yet another cunning plan.

The problem was that most of the planet's surface

was just rock.

So, away from the water's edge,

where plants had ready access to that water,

they really had no hope.

As soon as any moisture appeared on those impervious surfaces,

it drained away into the streams, into the rivers, into the lakes,

leaving the rock too exposed, too dry for plants to survive.

So, at this point, it did appear as if the Earth would be

a fungal paradise for all eternity.

If the plants wanted to compete,

if they wanted to stake their claim on the land,

they would have to change their equation.

They needed some magic.

Today, plants are everywhere.

In every niche and every environment.

But almost all plants have one thing that allows them to thrive.

Soil.

In the modern world, there are many different types of soil.

Most, like this crumbling brown wonder stuff here,

are made through the activities of invertebrates, fungi,

bacteria, enzymes, all breaking down organic matter -

like leaf litter or animal excrement -

and then mixing it with minerals that have eroded

from the bedrock below.

And the result is this magical substance,

packed full of nutrients and, essentially, able to hold moisture,

meaning that plants can get what they want from it all year round,

those nutrients and that water.

But 450 million years ago,

there was no soil.

No soil because there were few or no animals living on land

and precious little organic matter for anything to work with.

The very idea of making soil was seemingly impossible.

But plants weren't going to let a little thing like that stop them.

So they began scratching the rock with tiny root-like hairs...

..turning it to dust...

..which they mixed with enzymes secreted by fungi.

But the truly transformative ingredient...

..was the plants themselves.

Generation after generation...

..laying themselves down to form the magical substance

that would set their descendants free.

Fast forward 30 million years and, thanks to soil,

plants have transformed.

They are much bigger

and in possession of brand-new evolutionary tricks.

A vascular system that allows them to move water

through their tissues.

And the first true roots that draw nutrients from the soil

and support taller stems.

The colossal Prototaxites still tower over them.

But now the soil offers fertile ground

for countless wind-borne, seed-like spores released by plants.

Thanks to this, plants can finally break free

and move away from the water...

..riding the wind far and wide,

spreading across plains and hillsides.

For the first time in history,

Planet Earth was turning green.

Ironically, there was a very real danger

that this new-found success could have instigated

the beginning of the end.

You see, no matter how big we think it is,

the Earth is essentially a closed system.

So any massive increase or decrease in the amount of plants

wouldn't occur in isolation,

it would have a profound effect both then and now.

I mean, just imagine, if we were monumentally stupid enough

to cut down all of the trees and poison all of the plants,

then the amount of CO2 in the atmosphere

would rocket up, along with the global temperature,

and the amount of oxygen would decrease.

So ultimately we wouldn't be able to breathe.

But of course, we wouldn't be silly enough to do that.

400 million years ago, however,

the situation was the polar opposite,

there was a massive increase in the amount

of terrestrial plants.

And as a consequence,

the amount of carbon dioxide in the atmosphere

began to plummet.

In fact, in the first 30 million years of the Devonian period,

it went down by 25%.

Now, given that CO2 is one of the most important

resources for terrestrial plants,

this had the potential to develop into a very real problem.

You see, if plants wanted to continue to grow bigger,

they were going to need some new, well, inspiration.

The funny thing about plants

is that we generally think of them as fairly inanimate.

But look closer and there's an awful lot going on.

Such as the photosynthetic dance of green chloroplast,

excited by the sunlight they capture.

But most incredibly,

looked at the right way,

you might almost swear

you can see them breathing.

Stomata - like tiny green mouths...

..taking in carbon dioxide

and exhaling oxygen and water vapour.

Back in the Devonian period,

most stomata existed only in plant stems.

But falling atmospheric carbon dioxide meant they needed

more stomata to absorb the same quantity to survive.

The problem was where to put them.

The answer was as elegant as it was revolutionary.

Leaves.

Just look at these beauties.

These are called elephant's ear.

Now, of course, the earliest leaves were nowhere near as big,

but they were a similar triumph of botanical form and function.

Waxy on top, keeping them waterproof.

And underneath, these strong ribs to keep them flat.

Also, a greater surface area,

allowing many more stomata in here for better gas exchange.

And on the top, that surface area provides more room

for more chlorophyll to harvest more sunlight from the sun.

But there is one problem with leaves -

they generate shade,

which promotes competition.

And this fired the starting gun on a race for light

that once again would completely transform

the surface of the Earth.

Leaves did far more than just allow plants to harvest

more carbon dioxide.

They made photosynthesis more efficient,

which boosted energy...

..leading to the birth of a new magic ingredient

in the form of wood.

This wonder material led to the creation

of biological machines,

unlike anything Earth had ever seen.

With strong, durable trunks that could push past

the competition towards the sunlight.

Trees.

For terrestrial plants,

trees represented a quantum leap forwards.

I suppose we could say

they were the epitome of everything that plants had learned

up until this point.

Deep-rooted, long-lived

photosynthetic powerhouses,

perfectly adapted to exploiting all of the resources

that they required.

And perhaps most ahead of its time was Archaeopteris,

considered by many to be the first true tree -

enormously successful,

fossils found all over the world.

And just like this contemporary Sitka spruce,

it had a timber trunk, thick bark and lateral branches

covered with masses of green photosynthetic leaves

competing for light.

And with that competition came the need to grow ever taller.

And they did.

Until they towered above everything else,

reaching heights of 30 metres.

Earth was now on its way to becoming a forest world.

A home for countless new species of plants

and insects at every level,

from the canopy to the forest floor.

The former masters of the land, Prototaxites,

were gone, never to return.

Fungi were reduced to life in the shadows,

where they've remained,

working their quiet magic, ever since.

The meteoric rise of plant life,

from uncertain pioneers

to undisputed masters of the land,

was complete.

It was a new chapter in Earth's story.

But this triumph brought with it

the threat of global catastrophe.

If you've never stood and gazed up into the high canopy of a forest,

then it's something that I can thoroughly recommend.

Because if you're in the right place at the right time,

with the right species, you might see something special.

So stand, stare and blink,

and look for a unique pattern.

You see, all of the branches and leaves

from neighbouring trees don't quite meet,

leaving a silvery line between them.

It's almost as if they're being kind to their neighbours.

It's a phenomenon called crown shyness,

part of a peaceful process of evolution

which has allowed all of the species in this ecosystem

to come together and live harmoniously.

And it works.

It's beautiful.

And when they're living,

these magnificent trees are providing homes,

shelter and food for a whole range of different animals,

other species of plants and fungi.

And you know, even when they're dead,

even when they are dead,

they just keep giving.

Through this process of decomposition,

again, they're feeding animals, other species of plants and fungi.

But it hasn't always been like this.

There was a time when it was different,

when intense competition was driving an arms race

that produced a very dangerous substance,

a substance which could have led to the end of all life on Earth.

The rapid spread of terrestrial plants has changed the Earth.

Atmospheric carbon dioxide has fallen even further,

causing global cooling.

In the southern hemisphere,

ice sheets have formed for the first time

in more than a quarter of a billion years.

But near the equator,

the climate is still extremely hot and very wet.

Fluctuating sea levels have caused huge deltas to form,

where vast carbon-hungry swamp forests have sprung up...

..covering as much as 20 million square kilometres.

A sweltering jungle paradise

teeming with life...

..where intense competition for light

has given rise to a whole host of new plant species...

..who would go on to threaten the future

of terrestrial life on Earth.

The largest amongst them were Lepidodendrons,

known as "scale trees",

towering up to an incredible 50 metres tall.

In many ways, we can see this as the modern equivalent

of a carboniferous swamp forest.

It's certainly very swampy, soft underfoot.

And in spring and summer,

green, lush, very productive - as it was back then,

when large scale trees proliferated

because their roots had adapted to allow them to grow

on the land and beneath the surface of the water.

But unlike these modern day cypresses -

and Archaeopteris, which preceded them -

their trunks were very different.

They weren't made of wood.

The interior was a soft, corky material

and the exterior, a very robust, tough structural shell,

which allowed them to perhaps grow to 50 metres

in as little as 15 years.

But maybe that tough structural shell

was just a little too indestructible.

Because when they finally matured and died and toppled

into this oxygen depleted ooze,

they didn't decompose as modern trees do -

breaking down slowly, giving their carbon back to the system.

No, those scale trees hung on to it, they hoarded that carbon,

and the consequences for planet Earth

were astonishingly dire.

The floor of the swamp forests became log-jammed

with fallen trees and decaying plant matter.

When this carbon-rich mixture was then buried

under millions of tonnes of sediment,

all the elements were in place for a remarkable alchemy.

Under intense heat and pressure,

and consumed by the passage of time,

this vast swathe of plant material was transformed

by the Earth into a new type of rock,

a type of rock that would come back to haunt us.

Here we are.

It's coal.

Yes, coal.

And there's a seam of coal running through this cliff here -

that black line -

which is constantly being eroded by the wind, waves and rain.

Now, throughout the 60 million years of the Carboniferous,

plants fixed carbon in the form of coal

to the tune of 100,000 million tonnes every single year,

taking an enormous amount of free carbon out of the carbon cycle.

And what this added up to was a deadly downward spiral.

These carbon-hoarding swamp forests

had pushed the Earth to the brink.

In the frozen south,

the Archaeopteris forests are long dead.

And to make matters worse,

atmospheric carbon dioxide is plummeting fast.

Nearly a quarter of the world's land

is now buried beneath a blanket of ice.

Earth sits within a hair's breadth of descending

into a snowball event...

..where reflection of the sun's rays by the frozen surface

could lead to the total glaciation of the planet...

..threatening almost all life on Earth.

This could've been the end of plants' journey...

..but for another timely intervention.

Beneath the frozen surface,

the giant tectonic plates that set all these events in motion

in the first place

had been continuing their perpetual dance.

And over the 60 million years of the Carboniferous,

they'd slowly been shifting the landmasses

where the swamp forests thrived...

..raising huge granite mountains in their place,

which changed weather patterns,

denying water to the deltas below.

And with this intervention

about 280 million years ago,

most of the coal-producing swamps dried up for good.

Atmospheric carbon dioxide began to rebound.

Temperatures rose...

..melting the southern glaciers...

..which eventually disappeared...

..setting the scene for a plant renaissance.

Allowing plants to diversify,

developing flowers and fruit,

grasses and grains.

Transforming their signature green into a kaleidoscope of colour.

Evolving new species to exploit every niche on the planet.

Right down to one side of one small island.

Like our old friend, Lysimachia glutinosa.

Plants' long journey has been an astonishing

four-billion-year struggle from humble beginnings...

..through the deadly fight to escape the water...

..to the countless generations that have reshaped

the surface of our planet.

Transforming it from bare rock

to a lush and verdant home for life.

In the aftermath of all of these tumultuous events

came a new world order.

That partnership of forces which had been shaping the planet

found a harmony.

Even plants finally found a balance,

instinctively aligning the amount of biomass on Earth

with the carbon cycle

and the composition of the atmosphere.

And this equilibrium has lasted more than a quarter of a billion years.

You see, plants had taken up that role as guardians

of the Earth's climate,

breathing in and out as and when required

and paving the way for the world that we've inherited today.

This bountiful, blooming miracle.

This blue-green jewel.

This Eden.

How do scientists piece together what was happening on our Earth

millions or even billions of years ago?

- Our planet has a 4.5-billion-year history of change.

And when I say change, I mean radical, dramatic,

just astonishing change.

- This episode featured the bizarre giant fungi, Prototaxites,

that dominated land over 400 million years ago.

- Prototaxites was a fossil fungal spike.

They could grow up to be about 26 feet tall.

So about the size of a two-storey house.

- Not only are these one of the strangest organisms

ever to grace the planet,

the only clue to their existence were a series

of mysterious fossils first discovered in 1843.

- So here you have this great big thing.

And when they started finding more of them, they were like,

"Well, this is kind of like a trunk,

"or it's shaped like a chunk of wood."

But there's no wood.

There's no trees.

So what is this?

- Until, in 2007,

when microscope technology was able to take a closer look,

revealing a cellular structure that was strangely familiar.

- So what this is, is a very thin slice of Prototaxites.

And we find that, unlike a log,

which would be full of woody cells,

instead we find a mass of these fungal filaments.

- Looking at it more closely,

they realised the structures were actually more similar to fungi.

- These were gigantic tree-like fungi.

- It creates, in my mind, one of the most bizarre

prehistoric landscapes of all.

And it's a great example of how ancient organisms

sometimes look completely different from anything that's alive today.

- Sometimes the challenge isn't identifying what a fossil is,

it's figuring out how the parts fit together.

As it was with Archaeopteris,

one of the earliest trees on Earth.

- Archaeopteris has a remarkable fossil history.

So, first, the stem was discovered,

and they recognised it because of its distinct type of wood.

And then, at the same time, they found lots of fern-like foliage.

However, we didn't think they were connected

because they looked so drastically different.

- Scientists initially thought they had two distinct plants.

- Eventually, someone found a specimen that showed

the two structures connected.

And that's how we discovered that this was all

part of the same plant.

- And so Archaeopteris was discovered...

..and another chapter of the story of plants came into focus.

- Archaeopteris fundamentally changed the Earth's landscape.

For the first time,

we had forests that we're so familiar with today.

- Now, finding ancient fossilised plant life is one thing.

How do we begin to learn about geological processes

billions of years in the past?

Like the origin of plate tectonics, an event still shrouded in mystery.

- Studying the onset of plate tectonics

is a hugely controversial area,

and that's because the evidence is just so scant.

- An eye-catching new idea leads some scientists to think

plate tectonics started with a bang.

- I would so love to have been there to watch

a 30-mile asteroid smash into Earth.

Now, I'd want to be out in space someplace,

maybe on the moon.

- But what's the evidence for such a cataclysmic event?

- One thing that geologists can do is they can pick out

small minerals from rocks and date them using isotopes.

- For example, there are now evidence for very large asteroid impacts -

in Australia, in South Africa -

and that seems to correlate with some of the starting

of plate tectonics.

- Inside ancient rocks, geologists have discovered spherules,

tiny droplets of melted material that form

under the intense heat and pressure of asteroid impacts.

Modelling of these impacts indicates that these massive bombardments

played a role in triggering plate tectonics.

- What happens if an asteroid hits that crust?

Well, it smashes it like a plate falling on the floor,

and those pieces get pushed down and moved around.

And that's how the mantle and the crust

could begin plate tectonics.

- By hunting down clues today,

scientists can unlock the secrets of the Earth's deep history,

allowing us to tell our planet's story like never before.

We journey back to where it all began...

..to tell the story of our atmosphere.

How it emerged from a toxic orange hell

and transformed a violent ball of rock

into a beautiful, life-sustaining blue bubble,

unique in the universe.

If the Earth could talk, what would it tell us?

Well, the Open University imagine how it might answer

some of our questions.

To experience this interactive presentation,

go to the website on the screen

and follow the links to the Open University.