Life from Light How to Grow a Planet

I've spent most of my life

trying to understand the forces that shaped our planet.

As a geologist, it always seemed to me

that rocks were right at the heart of things.

But now, I'm discovering it's not only volcanoes and colliding continents

that have driven the Earth's greatest changes,

because at crucial moments in its history,

another force has helped create the planet we live on...

..plants.

Just look at this seed.

It's small, it's brown. It weighs hardly anything.

Looks pretty ordinary,

but actually nothing can be further from the truth

because what it will become is truly extraordinary.

These are giant sequoias.

Some are over 3,000 years old.

And sequoias are the largest single life form on Earth.

All from a tiny seed.

Yet even that pales into insignificance

when compared to what the whole of the plant kingdom has done

throughout the history of our planet.

They harness light from a star,

bringing energy to our world.

They and their ancestors created our life-giving atmosphere.

I'm breathing oxygen that was made two and a half billion years ago.

They sculpted the very surface of the Earth

and they drove the evolution of all animals...

..including our own ancestors.

It's a whole new story about our Earth...

..told through remarkable images,

captured for the very first time,

and the latest scientific discoveries.

Wish me luck.

This is the start of that story.

How plants took a barren alien rock, our planet,

and transformed it into the home we know today.

Ooh! It's a long way down.

I'm in Central Vietnam

and I'm descending into one of the largest caves in the world.

The structure's absolutely fantastic.

Ugh!

At 7km long, this is known as Hang Son Doong.

It's a dark, alien world.

Down here, very little is alive.

But I'm not here for the cave.

Oh! Look at that!

For goodness' sake!

It looks like the roof has collapsed

and the rainforest has just invaded.

It's a rainforest inside a cave.

After being in the darkness and the black for ages, look at that.

You just suddenly see brilliant green.

This isn't the entrance.

We're three kilometres into the hear of the cave system.

It's a thriving lost world with towering Polyalthia trees.

And home to strange creatures like this Vietnamese flat-backed millipede.

Isn't that incredible! It's got antlers.

You really feel as if you've left the confines of that cave

and just escaped really into this fantastic forest.

It's a wonderland, really.

This rainforest exists because of one thing above all.

Something which has enabled plants to colonise

almost everywhere on Earth...

..light.

Light which has travelled 150 million kilometres from the sun.

Plants have this truly remarkable ability

to harness energy from outer space to produce food.

It's this ability to eat the sun, to manufacture life from light,

that's allowed plants to dominate our planet.

This is the most important natural process on Earth.

It's how the plant kingdom has transformed a lifeless planet

into a living world.

But it wasn't always like this.

And to see how it started, we need to go back three billion years.

To begin with, our planet was like an alien world.

There was very little oxygen.

The atmosphere was a cocktail of toxic gases,

like methane and sulphur dioxide.

The land was lifeless.

This barren saltpan in southern Kenya

is about as close as you can get in the modern day Earth

to that ancient world three billion years ago.

But the one crucial difference

between the planet then and the planet now

is that back then I'd have been burnt to a crisp.

That's because the primitive atmosphere couldn't screen out

the sun's powerful ultraviolet rays.

Back then, these UV rays were hundreds of times stronger

than they are now.

Nothing could survive on land.

Yet all this was about to change.

A momentous event

that would create the planet's first life-supporting atmosphere.

This event, between three and two and a half billion years ago,

was the single greatest turning poin in the history of life on Earth.

And it was all brought about by the earliest ancestors of plants.

Here at the Sishen iron mine in South Africa,

evidence of that epic event can still be unearthed today.

But to get to it, you need a bit of help.

SIREN

MUFFLED VOICE

Thirty seconds.

Ten.

LOUD EXPLOSION

That is 200,000 tonnes of iron ore just been blasted apart.

These explosions open a cross section back in time

to the distant origins of the plant kingdom.

This is iron ore.

It's so heavy.

Pure iron's got this metallic glint, it's shiny,

but you can see that this has got loads of red in it.

And it's red for a really simple reason. It's rusted.

It's rusted because it's come into contact with oxygen.

Oxygen produced by the very first burst of life.

The miners want the ore for its iron content.

But I'm going to use this iron oxide for a very different reason.

Something I don't think has ever been done before...

which is why I'm a wee bit excited.

I've taken a chunk of the iron oxide rock

and had it ground up into fine powder.

It's then been turned into a solution.

One I'm hoping will allow me

to take a breath from the planet's earliest oxygen.

Oxygen made by the ancient ancestors of plants.

And now what I'm going to do is kind of jump start it, really,

with this battery. I'm going to attach a lead

and pass an electric current through it.

And we should see a simple reaction.

Oh yeah, yeah. There's some bubbles coming off.

These bubbles are the gas oxygen.

It's being released for the first time in over two and a half billion years,

when it was locked away in the rock.

There's a lovely little train of them just rising to the top and forming a little pocket of gas.

You're never sure with these experiments whether you're really going to get it or not,

but that's exactly what I was hoping to see.

In just one hour, I've collected enough to fill the whole test tube.

The thing is, this isn't any old oxygen.

This is oxygen that's come from those iron bands.

The very oxygen that changed our planet.

In fact, I can't resist it. I'm going to have to...

INHALES

Ah!

I can't believe it.

I'm breathing oxygen that was made two and a half billion years ago.

It's all gone. Liberated from the rocks now.

It's up there somewhere.

These iron bands tell a remarkable story.

Oxygen was now flooding the Earth's atmosphere.

It cleaned out the planet's toxic gases,

leaving the sky a clear blue for the first time.

Geologists call it the Great Oxidation Event.

And it certainly was an event.

This was an irreversible change between two very different worlds -

a planet with virtually no free oxygen and a planet full of oxygen.

This was the greatest change in the history of life on Earth.

So how did this great event happen?

The answer lies with the first burst of life,

which emerged not on the hostile land but under water.

Back then, water acted as a liquid sunscreen to the dangerous UV rays.

Under the protection of water,

the earliest organisms on Earth evolved

in the form of tiny bacteria.

And here in East Africa is a rare chance

to see what it would have been like.

This is Lake Magadi.

The waters here are just super salty.

Agh! Can feel it nipping away at my feet.

But the bacteria I'm wading through are close descendants

of the very first microorganisms that lived three billion years ago.

It's fantastic to think that swimmin in the top layer here

are some of the most primitive life forms on Earth.

And those bacteria, just like the ones all that time ago,

have got something surprising about them.

They're purple.

These are hallow bacteria and they didn't just occupy the occasional lake.

Much of the world's oceans were purple, too.

Imagine that from outer space.

A purple Earth.

The purple bacteria live by harnessing energy from the sun.

But they only use part of the light.

Some rays pass deeper into the water

And over time, down there, a different type of bacteria evolved

They had to live off the colours of light left over.

This made them appear green.

These were the green bacteria.

This seemingly arbitrary event,

bacteria absorbing one colour of light rather than another,

would have colossal repercussions for the planet.

Over time these green bacteria, a type of Cyanobacteria,

came to dominate the waters of the world.

Eventually, as we'll see,

these green microorganisms became the ancestor of all plants on Earth.

Because right from the start they were reflecting green light,

the stalks of the plants became gree and the leaves were green.

In fact, that's why all plants on Earth became green,

from the grasses to the forests,

and it's also why today, instead of living on a purple planet,

we've got a green one.

But it wasn't just about colour.

Because the green bacteria did something

their purple cousins couldn't.

They produced oxygen.

They would breathe life into the lifeless land.

Without them, the story of our plane would be more like that of Mars.

How the green bacteria did this is so complex

that scientists still grapple with the details.

I've come to the Eden Project in Cornwall to try to understand it.

I'm to be the subject of an experiment

that's never been attempted before.

-Hi!

-Hello there.

I'm the guinea pig. Doctor, I presume?

Indeed. Dan Martin.

-Hi there.

-Hi. Katrina Hope. Nice to meet you.

Look at this! This is fantastic.

Incredible, isn't it?

I'm about to be locked inside this airtight chamber.

I hope to experience first-hand my very own Great Oxidation Event.

OK, everyone.

I'm going to start reducing the oxygen concentration in here now.

BEEPING

The first step is to lower oxygen levels closer to those of the early Earth.

So first of all,

this is going to monitor your heart rate and your oxygen levels

so if we pop that on we can just have a look here.

It's a lack of oxygen that complex life like us can't operate at for long.

So at the top is your heart rate.

How's that? Is that really high?

I think you might be a little bit anxious about going in there.

I am a little bit.

I'm sure your resting heart rate's not normally 95.

No, I have been thinking a lot about it.

My vital signs are being monitored, along with the oxygen levels in my blood.

Now it's time to be sealed inside the chamber for the next 48 hours.

I'm as ready as I'll ever be, guys, so can we open this door?

Wish me luck.

It's small, isn't it?

Oxygen levels in the air are normally 21%.

BEEPING

Inside the chamber, they're far lower.

Just over 12%.

At these concentrations,

the cellular activity in my body and brain

-is starting to slow down.

-Three, two, one...

-Go!

-Green, yellow...

red, green, ye... Kind of orange...

Er... purple, blue.

You'll find that thinking becomes a little bit slower.

My hand-to-eye coordination is being impaired.

You can put them in any order you like. That's the way.

Can you just tell us how exactly are you feeling?

It's funny. I felt very slow. That slowness is there, definitely.

The doctors calculate that at the rate I use up oxygen,

if it carried on like this, I'd be unconscious in just 24 hours.

Your oxygen saturation, sort of 88%.

If that was your level in hospital,

we'd be pretty worried about you right now.

The next crucial step is to see if the 300 plants in here with me

can produce enough oxygen to keep me alive.

It's all to do with the wondrous ability they inherited

from those green bacteria.

It's photosynthesis, of course.

I think we can have the lights on, please.

To kick start it, you need light.

Wow! Suddenly the light's hit.

Plants use photosynthesis to live and grow,

and most importantly for me...

..to make oxygen.

Photosynthesis is an intricate process that science is still trying to unlock.

But the production of oxygen is one of its key features.

To understand what's happening,

you need to enter a complex and microscopic world.

Inside every leaf of every plant on the planet

are the direct descendants of those first green bacteria.

Magnify a leaf 1,000 times and you can see them.

They're known as chloroplasts.

Packed into every cell.

They still behave a bit like bacteria.

This is real footage of them moving towards a flash of light.

They're just 5,000ths of a millimetre across.

And it's inside chloroplasts that photosynthesis happens.

Light rays from the sun are made of photons.

They're tiny, fast-moving particles of electromagnetic energy.

When they hit the surface, the energ of the photons is captured by a ring

called the light-harvesting complex.

Inside this structure,

the energy of two photons is used to split a water molecule.

It's ripped into its two elements - hydrogen and oxygen.

The plant uses the hydrogen to live and grow.

But right now, I'm interested in the other part of the water.

The part plants pump out as a waste product -

the oxygen.

Scientists have calculated that the 300 plants in here with me

should raise oxygen levels in this chamber

from 12 to 21% within 48 hours.

I'm finding out how reliable the process of photosynthesis really is.

It is quite concerning.

You've been very busy this afternoon, a lot of activity,

so we need to restrict the amount that you're talking

and really get you resting as much as possible,

not dashing around the chamber any more.

This is doctor's orders - bed rest.

Night-night!

As I drift off to sleep,

the 11,000 leaves go to work.

They have 30 cubic metres of the box to fill.

Some plants, like this maize, and the banana plant

are particularly efficient at pumping out oxygen.

So we can see the increase every hour here.

That's incredible really.

They're really pushing out a lot of oxygen, as you can see.

Every hour, my plants are producing over 40 litres of oxygen.

Hi, Iain. It's Katrina. We're now 41 hours in.

Really? 41 hours in?

Yeah. The oxygen levels are still climbing gradually every hour

so it's going really well.

With my vital signs returning to normal,

I'm now a top attraction at the Eden Project.

-Hello!

-Hello!

-Can you see him?

-What's he doing in that box?

He looks very happy in there.

-What's that on his finger?

-It's measuring his oxygen levels.

Eat your heart out, David Blaine!

Finally after 48 long hours,

oxygen levels are almost back to normal.

The plants have triumphed.

Wa-hay!

Oh, I'm out!

Ah! Survived it. Fantastic!

It's amazing just thinking that I've survived, but actually...

I guess I've really survived because of them, cos of the plants.

I leave here thinking that I needed those plants

way more than they needed me.

It's easy to think of this as just an experiment

but to me, when you're lying in there,

you realise this place is a metaphor for something much bigger,

for the planet, really,

and for our relationship with plants

through photosynthesis to keep life going.

The early Earth was like my chamber.

It was transformed from a world with very little oxygen

to a world rich in oxygen.

And all that oxygen began to do something else.

High in the stratosphere, it created ozone.

This was a protective blanket which enveloped the Earth

and blocked most of the sun's dangerous UV rays.

It meant that for the first time in the planet's history,

plants could move on to the land.

But it was no small step.

If you'd been protected by water for billions of years

then the move to the land was going to be a rude shock.

Yet over 400 million years ago, plants finally made that leap.

Surprisingly, the best evidence for these pioneers

doesn't come from some exotic corner of our planet,

but from Britain.

I've come to just outside the villag of Rhynie in northeast Scotland

to see this - a stone wall.

But not just any stone wall, of course.

For me, this is the most important stone wall in the history of science

Back 410 million years ago, Scotland was located well south of the equato

and looked like another world.

Hot springs and geysers boiled out across a rocky and barren landscape.

But something else was happening, as scientists discovered

when they came across some curious markings in this wall.

This is one of them. Look at this.

You see these really strange elongated shapes here.

The first people just didn't really know what they were.

They thought maybe, at first, it was some kind of lava

but when they looked really closely,

especially when they got it cut and polished, this rock literally came alive.

Because you can see these dark features here,

they realised that this was something that was once living.

And when they were alive, this is what they looked like.

Just a few centimetres tall, they're called Aglaophyton.

Bulbous shapes on the end of naked stems.

A time before leaves or roots,

yet somehow these bizarre life forms survived along the water's edge.

What geologists had found right here in Scotland

were some of the earliest pioneering plants

to make that giant leap, to colonise the land.

And around this time, all along the margins of lakes and rivers,

primitive plants were coming ashore.

For the first time when viewed from space,

the land began to look alive.

The beginning of a transformation from hostile world to fertile Earth.

Yet this wasn't a full-scale invasion.

Just a toehold.

Plants were still tied to the water's edge,

unable to head inland and penetrate the harsh, rocky surface.

But all this was about to change.

Plants evolved an inspired solution to the problem,

a brilliant device for collecting water and nutrients

and something that they never really had before.

Roots.

Cambodia.

The 12th-century temple here at Ta Prohm

is a wonder of civilisation,

but it's also a wonder of the natural world.

Although the roots of these strangler figs are very different

from the first ones to evolve,

it's a superb place to reveal how roots allowed plants to invade inland.

Roots are hugely powerful. I love this one.

Look at it prising its way into that roof,

just lifting that whole structure up.

And then boring down here

through these stone blocks and then disappearing.

Just tiny pressures exerted over decades of centuries.

Add these up and you get phenomenal strength.

A pressure of up to 10kg per square cm.

Around 400 million years ago, the first roots appeared...

...and gave plants the ability to smash up the rocky planet.

And this created a vital ingredient for life on land.

When the tiny, broken-up fragments of rock

get mixed up with dead plant material,

it ends up as this ideal environment for storing water.

An environment that we call soil.

Today, soil covers 40% of the planet's land.

It takes a long time to form, 1,000 years to make just 2cm of soil

But it's essential for plant life, just as it was back then.

Because the primitive leafless plant

could now break free from the water's edge.

Roots, and the soil they created, made plants unstoppable...

...allowing them to colonise inland for the first time.

An invasion that would have a dramatic influence on all life on Earth.

For millions of years,

animals had been confined to the rivers and oceans.

Now they could finally emerge from the water.

We get an idea of those first tentative steps

by travelling back in time

with a creature that's barely changed for 500 million years.

I've come to the east coast of America,

where these ancient creatures still come ashore at dusk to mate.

Here they are.

Horseshoe crabs.

Looks like something from another planet.

He sees with two main eyes here,

but they've got something like ten eyes scattered across their body

and the really weird bit is if you lift them up.

Look at that.

For a start, they've got five pairs of legs.

Look - one, two, three, four, five,

whereas normal crabs just have four.

They're actually more related to the scorpion than to normal crabs.

Look at that.

But the really interesting bit is tucked under here.

You get these things called book gills.

Look at that there. It's like sheaves of a book.

And that allows them to extract oxygen,

not just from the water but also from the air.

It's an amazing breathing apparatus.

Better put her back now.

Come on, dear. There you go.

As long as they're kept moist,

these lung-like gills enable the crabs

to stay out of water for days at a time.

Fossils show that horseshoe crabs appeared on land

at least 400 million years ago.

They are some of the first animals ever to come ashore.

Amphibians and insects soon followed.

Oxygen allowed them to move onto land.

But something else was also enticing them.

It's funny. Plants create oxygen as a waste product

and it's that waste product that has transformed our atmosphere.

But of course, the main reason that plants photosynthesise

is to create sugars.

Sugars that are vital for plants to live and to grow

and also provide a source of food for all animals.

Plants make this sugar from water...

..carbon dioxide from the air...

..and energy from the sun.

And again, it all happens in those tiny chloroplasts.

We've seen how light splits water into oxygen and hydrogen.

The plant takes that hydrogen

and combines it with carbon dioxide to make sugar.

By exposing a plant to carbon dioxid tagged with a radioactive marker,

you can see the sugar being created.

For the first time,

scientists have imaged its creation and movement through a plant.

In this case - maize.

As soon as carbon dioxide is sucked into the plant's cells,

they begin to glow.

This is the actual moment

that photosynthesis turns the carbon dioxide into sugar.

In just 15 minutes, the newly-formed sugar

is sent to the roots for storage.

The plant can then use this sugar to grow and thrive.

That's why photosynthesis is nature' most astonishing achievement.

The ability of plants to be powered by light from beyond our planet

sets them apart from all other life.

And that connection with that star, our sun,

makes plants a foundation stone for all living things.

It's just such a wonderful thought.

400 million years ago,

leafless plants were flourishing like never before.

But a dramatic transformation of the atmosphere

was about to throw plants into a global crisis.

Not only would it change their shape

it would change all life on our planet.

MAORI CHANTING

This is Lake Tarawera in New Zealand.

This ancient landscape is home to a plant

that 360 million years ago confronted that crisis.

It came up with an inspired solution

Looks like the land that time forgot, doesn't it?

Just that strange mixture

of different shapes of plants and trees.

Really unfamiliar and alien.

It's almost primeval.

The early plants had become victims of their own success.

They were gorging on so much carbon dioxide in the atmosphere

that they were using it up.

Levels plummeted by 90%.

Without enough of this vital gas, plants began struggling.

If they couldn't find a way to breathe in more carbon dioxide,

they'd suffocate.

The early plants, plants like these gorgeous ferns here,

came up with a remarkable new structure.

Large, flat surfaces that house within them a complex breathing apparatus.

We call them leaves.

Leaves were the answer to all plants' breathing problems.

They massively increased their surface area by over a hundredfold,

allowing them to absorb far more carbon dioxide.

Now, for the first time,

shade was cast by a beautiful and delicate canopy, like these Dicksonia.

These ferns are incredible.

They're like giant umbrellas.

The key to this advanced breathing apparatus

is on the underside of each fern leaf.

They're microscopic holes called stomata.

Filmed and actioned with an electron microscope,

this is them opening and closing.

Speeded up 140 times.

There are thousands of stomata on every leaf on Earth.

They allow a single fern

to breathe in five litres of carbon dioxide a day.

The evolution of leaves, rich in stomata,

saved plants from suffocation.

But leaves also allow plants to capture more light.

This in turn fuelled fierce competition,

each plant desperate for the sun's rays.

This family squabble would lead to a new type of plant.

One that would have surprising repercussions for the planet.

How do we know?

Well, it's all thanks to some rare evidence here

in Nova Scotia in Canada.

To reach it, you have to abseil to the bottom of this 30m cliff.

Here, scientists discovered the remains of a mysterious world.

You know, this is just the best way to see rocks.

You really feel as if you're a time traveller,

peeling back the layers of history one by one as you go down.

These rocks are over 300 million years old.

But it's what's locked inside the rocks at the base of this cliff

that took scientists' breath away.

These fossilised remains are just spectacular!

I mean, look at the texture.

You can tell it's a plant

but this isn't some big shrub or overgrown fern.

You can see here, look, you can get traces of bark.

And down here, you can see there are some roots coming off.

This is completely extinct, you don't get this any more,

but what I'm actually crouching beside

is one of the planet's very early tree trunks.

And not just one tree, cos look here

There's another one here.

There's another there.

This is a fossil forest.

These Lepidodendron trees had strange diamond-shaped bark.

Each diamond sprouting a needle-like leaf.

Over 300 million years ago,

they made up the planet's first tropical forests.

Found in swamps throughout the Earth's tropics,

these first forests were so extensiv

you'd have seen a band of dark green from space.

And all those new leaves were pumping out oxygen,

so much that levels of oxygen increased

to not far off double what they are today.

It was having a very odd effect on animals...

..in particular, insects and their cousins.

Instead of lungs, invertebrates have simple breathing tubes

that rely on diffusion for oxygen to reach their internal organs.

The size of these animals is therefore limited

by the concentration of oxygen in the air.

Increase the oxygen, just as the first forest did,

and things get interesting.

Do you see these markings on the rock here?

There's two lines of little dents, one here and one here.

They are fossilised footprints

that date back to the very early forests.

When scientists first studied them

they realised they weren't made by some reptile or amphibian.

They were made by a millipede.

Now, here is one of the biggest millipede species

alive on Earth today.

That's pretty big.

Using the tracks for scale,

it's clear the ancestors of this little fellow were massive.

Called Arthropleuridea, it was over 2m long.

The forests would have been terrifying.

With giant scorpions and giant spiders.

And not just on the land.

I think the most impressive of all were the dragonflies.

Most modern dragonflies have wing spans up to 10cm across,

but back then they were way larger.

Some were up to a metre across.

These Meganeura were the largest insects ever to take to the skies.

But in this oversize world pumped with oxygen,

the plant kingdom still reigned supreme.

Then, 230 million years ago,

a new group of animals emerged from the shadows of the swampy forests.

They would become the largest creatures to roam the Earth

and they were ready to do battle with the kingdom of the plants.

I'm talking, of course, of dinosaurs

ROARING

It's the meat-eaters that get all the press.

But recent research has revealed

that out of the 700 species discovered,

over two-thirds were herbivores.

Vegetarians ruled, led by the biggest herbivores in history...

..the sauropods.

To discover the impact of these huge dinosaurs on the plant kingdom,

I've come to an animal sanctuary to see it in the flesh.

Unfortunately this place doesn't have a living sauropod,

but what it does have is the biggest herbivore

that the planet's got to offer -

the African elephant.

Come and meet Butch.

This beautiful four-tonne elephant can help me truly appreciate

the staggering size of the dinosaurs

Can we go up?

Butch here is about as big as a big African bull gets

and that's already four metres high,

but if we want to get to the height of a sauropod, we have to go much higher.

Six metres. We've got to be higher than that.

We're now at eight metres.

We've still got to go higher.

Where are we at? Ten metres now? A bit higher than that.

We're still not at the height of a sauropod yet.

OK, we're getting there. Nearly.

OK, fine.

ELEPHANT ROARS

So my head's now about 12 metres,

which is about the height of a four-storey building

and also the height of a sauropod.

The thing is, on the end of a nine-metre neck,

this is the skull of a sauropod.

It seems quite small.

But the point was that this had to b manoeuvrable and nimble

to get right up at that high-level foliage.

Sauropods were like nothing else the planet had ever seen.

They weighed more than ten times an African elephant.

Now, Butch here eats about 90kg of foliage every day,

which is roughly about that much hay

But scientists have estimated

that sauropods ate about 1,500kg of hay every day.

In other words, about 20 times that daily diet.

Or 50 bales of hay.

Now if you imagine you've got herds of about 30 sauropods,

much bigger than these beasts here,

and you realise that the plant kingdom

was up against the ultimate salad predator.

150 million years ago,

dinosaurs were stripping the land of vast swathes of foliage.

For the first time, the plant kingdo was under serious attack

from another dynasty.

To fight back, plants began to evolve a whole arsenal

of defences for their precious leaves.

Here in California, we can see just how intense this arms race was

in one of the world's most unusual gardens.

It's full of a group of bizarre and extremely rare plants called cycads

but once, they made up a quarter of all plants on Earth.

This is incredible.

Exactly the kind of place you'd expect a dinosaur just to pop out.

To stave off attack from those ravenous dinosaurs,

cycads developed some clever lines of defence,

the most obvious being physical weapons

like needles and spikes and... Agh! These are vicious.

The main point was to make leaves as painful as possible to eat.

These defences came in all shapes and sizes.

And some plants also spiced things up with chemical weapons.

This is a Trapps Valley cycad from South Africa,

but it's pretty typical in that the leaves contain a nerve agent

that if you ingest it, it causes vomiting, diarrhoea,

paralysis of the limbs

and then, of course, death.

Obviously I'm not going to eat one of the leaves,

but I can eat a plant whose ancestors emerged

around the time of the dinosaurs

and who also have a chemical weapon and that is...

..a chilli.

n particular, a habaneros chilli...

...which is supposed to be one of the most powerful in the world.

There's a chemical in here called capsicum...

that is contained in the fruit.

And that essentially is a toxin.

COUGHS AND LAUGHS

Agh!

EXHALES

Which is, at this precise moment,

burning and inflaming all of my mouth.

Oh, my gosh!

The thing is, the toxins in the cycads were...

they were far more powerful even than the chillis.

Oh, my gosh!

So you can imagine what...the dinosaurs would have had to endure.

Oh, my... I'm going to have to...

Ah!

Can't even say how sore that is!

Mmmm.

Eat a chilli, they said.

It'll be funny, they said.

You know what? Forget about cycads.

That could have brought down a 70-tonne sauropod.

And the arms race didn't stop there.

Plants evolved a new tactic.

Not so much a line of defence as a line of communication.

We know that when some plants are attacked

they activate a quick-acting toxin that deters herbivores.

Now we're discovering that this defence goes even further,

because plants can actually warn other plants

that a herbivore is eating them.

And at last, scientists here at Exeter University

are beginning to listen in to this hidden conversation.

They're finding that when plants are attacked,

they also release an unseen gas from their leaves.

What it does is extraordinary.

And this will be the first time it's been captured on film

using specialist imagery.

These two Arabidopsis plants are being put inside a chamber.

A third plant is then cut to mimic an attack.

It's added to the undamaged plants.

The chamber is sealed.

The plant leaves are now releasing the gas.

As they do so, their biological activity can be seen changing.

Something remarkable happens.

The gas triggers a change in the biological activity

in the two neighbouring plants.

They have detected the message warning them to protect themselves.

Scientists don't know all the detail of this plant language,

but increasingly they believe there's a chatter between plants all around us.

I think most people assume that plants lead a rather passive life.

That they're static and unresponsive

That's just not true.

In reality they move, they sense, they communicate.

It's almost as if they show a kind of intelligence.

For 200 million years, the dinosaurs and the plants

were locked in a titanic evolutionary battle,

each trying to gain the upper hand.

But it was now that some plants played their trump card.

They used wood to grow taller and taller.

In California's Sierra Nevada, I'm about to find out just how tall.

To do that, I need the help of biologist Jim Spickler.

I'm as ready as I'll ever be.

-Take your time. We've got time.

-I was going to, absolutely.

This is the grandest example of them all.

The giant sequoia.

What you see is just impossible for your mind to process.

-The scale is so large.

-It's extraordinary.

I feel as if I'm in Lord of the Rings.

GRUNTING

What a great tree!

70 million years ago,

the ancestors of this type of tree, the conifers, got ever taller.

This was the ultimate in plant construction.

Conifers like the giant sequoias

raised their precious leaves out of reach.

The dinosaurs were no longer the biggest organisms on Earth.

That title had been well and truly won back

by these giants of the plant kingdom

This is so tall, but I've still got...

I don't know, another third to go.

By using wood to grow really tall like this,

it gave trees another advantage over plants

because it allowed them first pick of the sun's strongest rays.

The thing is, of course, for plants, light means success.

If you had a satellite image of the dinosaur era 70 million years ago,

you'd see the Earth like it had never been before

and never would be again.

The climate was so warm the poles had no ice.

Instead they were covered with conifers -

a vast polar forest.

And the mighty sequoia trees were not just found

in small areas of the Sierra Nevada, as they are today.

They were global.

Stretching along the Pacific coast and as far south as Australia.

-How far are we from the top, then?

-We're getting close.

GRUNTING

Ah! It's extraordinary.

This is it. This is the top of the tree.

Ooh!

Unbelievable!

It's staggering to think that using just a gas, carbon dioxide,

and a liquid, water,

together with light energy from beyond our world,

you can construct a cathedral of wood 90 metres tall.

Since they first appeared,

plants and their ancestors have revolutionised our planet.

They created oxygen for the atmosphere...

..which would allow them to conquer the land

and transform rock into soil,

in turn fuelling the explosion of all life.

From a barren alien planet, plants have made a living Earth.

And left on its own, the world would have continued like this,

dominated by large dinosaurs and endless forests.

But 65 million years ago,

something happened that would change everything.

A chance event that would have dramatic consequences

not just for plants but for all life

And it would originate not on Earth, but in outer space.

WHOOSH

EXPLOSIONS

The asteroid would kill off the dinosaurs.

And the next chapter would see the triumph

of a whole new group of plants...

...flowers transformed the bond between animal and plant,

even sculpting the very planet itself.

Above all, plants would drive our human story.

But all that was still to come.

Subtitles by Red Bee Media Ltd