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00:00:08I spent most of my life trying to understand the forces that shaped our planet.
00:00:12And as a geologist it always seemed to me that rocks were right at the heart of things.
00:00:22But now I'm discovering it's not only volcanoes and colliding continents
00:00:27that have driven the Earth's greatest changes.
00:00:31Because at crucial moments in its history another force has helped create the planet we live on.
00:00:39Plants.
00:00:41Just look at this seed.
00:00:43It's small, it's brown, it weighs hardly anything.
00:00:46Looks pretty ordinary.
00:00:48But actually nothing could be further from the truth because what it will become
00:00:51is truly extraordinary.
00:01:02These are giant sequoias.
00:01:04Some are over 3,000 years old.
00:01:10And sequoias are the largest single life form on Earth.
00:01:16All from a tiny seed.
00:01:20Yet even that pales into insignificance when compared to what the whole of the plant kingdom's done
00:01:25throughout the history of our planet.
00:01:29They harvest light from a star bringing energy to our world.
00:01:34They and their ancestors created a life-giving atmosphere.
00:01:40And breathe an oxygen that was made two and a half billion years ago.
00:01:47They sculpted the very surface of the Earth.
00:01:51And they drove the evolution of all animals.
00:01:55We're here.
00:01:56Including our own ancestors.
00:02:01It's a whole new story about our Earth.
00:02:08Told through remarkable images.
00:02:11Captured for the very first time.
00:02:15And the latest scientific discoveries.
00:02:18Wish me luck.
00:02:27This is the start of that story.
00:02:29How plants took a barren alien rock.
00:02:32Our planet.
00:02:33And transformed it into the home we know today.
00:02:53It's a long way down.
00:02:57I'm in central Vietnam.
00:03:00And I'm descending into one of the largest caves in the world.
00:03:03The structure's absolutely fantastic.
00:03:07Ah!
00:03:11At seven kilometres long, this is known as Hansong Dung.
00:03:20It's a dark alien world.
00:03:23Down here, very little is alive.
00:03:30But I'm not here for the cave.
00:03:36Oh, look at that.
00:03:41For goodness sake.
00:03:43It looks like the roof has collapsed.
00:03:46And the rainforest has just invaded.
00:03:48It's a rainforest inside a cave.
00:03:52It has to have been in the darkness and the black for ages.
00:03:54Look at that.
00:03:55You just suddenly see brilliant green.
00:03:59This isn't the entrance.
00:04:02We're three kilometres into the heart of the cave system.
00:04:09It's a thriving lost world with towering polyalthea trees.
00:04:16And home to strange creatures like this Vietnamese flat-backed millipede.
00:04:24Isn't that incredible?
00:04:26It's got antlers.
00:04:30You really feel as if you've left the confines of that cave and just escaped really into this fantastic forest.
00:04:38It's a wonderland really.
00:04:40This rainforest exists because of one thing above all.
00:04:45Something which has enabled plants to colonise almost everywhere on Earth.
00:04:51Earth.
00:04:52Light.
00:04:55Light.
00:04:56Light which has travelled 150 million kilometres from the sun.
00:05:04Plants of this truly remarkable ability to harness energy from outer space to produce food.
00:05:11It's this ability to eat the sun.
00:05:13To manufacture life from light.
00:05:16It's allowed plants to dominate our planet.
00:05:22This is the most important natural process on Earth.
00:05:28It's how the plant kingdom has transformed a lifeless planet into a living world.
00:05:41But it wasn't always like this.
00:05:47And to see how it started, we need to go back three billion years.
00:05:57To begin with, our planet was like an alien world.
00:06:04There was very little oxygen.
00:06:08The atmosphere was a cocktail of toxic gases.
00:06:13Like methane and sulphur dioxide.
00:06:17The land was lifeless.
00:06:24This barn salt pan in southern Kenya is a bit as close as you can get in the modern day
00:06:28Earth
00:06:29to that ancient world three billion years ago.
00:06:32But the one crucial difference between the planet then and the planet now
00:06:35is that back then, it had been burnt to a crisp.
00:06:43That's because the primitive atmosphere couldn't screen out the sun's powerful ultraviolet rays.
00:06:52Back then, these UV rays were hundreds of times stronger than they are now.
00:07:01Nothing could survive on land.
00:07:08Yet all this was about to change.
00:07:14A momentous event that would create the planet's first life-supporting atmosphere.
00:07:23This event, between three and two and a half billion years ago,
00:07:28was the single greatest turning point in the history of life on Earth.
00:07:34And it was all brought about by the earliest ancestors of plants.
00:07:47Here at the Sishin Iron Mine in South Africa,
00:07:51evidence of that epic event can still be unearthed today.
00:07:58But to get to it, you need a bit of help.
00:08:0430 seconds.
00:08:06In.
00:08:22That has 200,000 tonnes of iron ore just been blasted apart.
00:08:30These explosions open a cross-section back in time to the distant origins of the plant kingdom.
00:08:43This is iron ore.
00:08:44It's so heavy.
00:08:46Now, pure iron's got this metallic glint.
00:08:49It's shiny, but you can see that this, it's got loads of red in it.
00:08:54And it's red for a really simple reason.
00:08:56It's rusted.
00:08:58It's rusted because it's come into contact with oxygen.
00:09:01Oxygen produced by the very first burst of life.
00:09:09The miners want the ore for its iron content.
00:09:13But I'm going to use this iron oxide for a very different reason.
00:09:16Something I don't think's ever been done before.
00:09:19It's just why I'm a wee bit excited.
00:09:22I've taken a chunk of the iron oxide rock and had it ground up into a fine powder.
00:09:31It's then been turned into a solution.
00:09:35One I'm hoping will allow me to take a breath from the planet's earliest oxygen.
00:09:41Oxygen made by the ancient ancestors of plants.
00:09:45And now what I'm going to do is kind of jump start it, really, with this battery.
00:09:50I'm going to attach a lead and pass an electric current through it.
00:09:54And we should see a simple reaction.
00:09:59Oh, yeah, yeah.
00:10:00Yeah, there's some bubbles coming off.
00:10:05These bubbles are the gas oxygen.
00:10:08It's being released for the first time in over two and a half billion years, when it was locked away
00:10:14in the rock.
00:10:17There's a lovely little train of them just rising up to the top of the test tube and falling in
00:10:21a little pocket of gas.
00:10:24You're never sure with these experiments whether you're really going to get it or not, but that's exactly what I
00:10:28was hoping to see.
00:10:32In just one hour, I've collected enough to fill the whole test tube.
00:10:43The thing is, this isn't any old oxygen. This is oxygen that's come from those iron bands.
00:10:49The very oxygen that changed our planet.
00:10:53In fact, I can't resist it.
00:10:54I'm going to have to.
00:11:01I can't believe it.
00:11:03I'm breathing oxygen that was made two and a half billion years ago.
00:11:08It's all gone. Liberated from the rocks now.
00:11:11It's up there somewhere.
00:11:15These iron bands tell a remarkable story.
00:11:19Oxygen was now flooding the Earth's atmosphere.
00:11:28It cleaned out the planet's toxic gases.
00:11:33Leaving the sky a clear blue for the first time.
00:11:42Geologists call it the great oxidation event.
00:11:45And it certainly was an event.
00:11:47This was an irreversible change between two very different worlds.
00:11:51A planet with virtually no free oxygen.
00:11:54And a planet full of oxygen.
00:11:57This was the greatest change in the history of life on Earth.
00:12:03So how did this great event happen?
00:12:07The answer lies with the first burst of life,
00:12:10which emerged not on the hostile land, but underwater.
00:12:16Back then, water acted as a liquid sunscreen to the dangerous UV rays.
00:12:23Under the protection of water,
00:12:25the earliest organisms on Earth evolved in the form of tiny bacteria.
00:12:32And here in East Africa is a rare chance to see what it would have been like.
00:12:38This is Lake Magadi.
00:12:40The waters here are just super salty.
00:12:43I can feel it nipping away at my feet.
00:12:47But the bacteria I'm wading through are close descendants of the very first microorganisms that lived 3 billion years ago.
00:12:56It's fantastic to think that swimming in the top layer here are some of the most primitive life forms on
00:13:02Earth.
00:13:03And those bacteria, just not the ones all that time ago,
00:13:07I've got something surprising about them.
00:13:10They're purple.
00:13:17These are hollow bacteria, and they didn't just occupy the occasional lake.
00:13:23Much of the world's oceans were purple too.
00:13:29Imagine that from outer space.
00:13:34A purple Earth.
00:13:41The purple bacteria lived by harnessing energy from the sun.
00:13:47But they only used part of the light.
00:13:50Some rays passed deeper into the water.
00:13:55And over time, down there, a different type of bacteria evolved.
00:14:01They had to live off the colours of light left over.
00:14:05This made them appear green.
00:14:08These were the green bacteria.
00:14:11This seemingly arbitrary event, a bacteria absorbing one colour of light rather than another,
00:14:17would have colossal repercussions for the planet.
00:14:24Over time, these green bacteria, a type of cyanobacteria, came to dominate the waters of the world.
00:14:34Eventually, as we'll see, these green microorganisms became the ancestor of all plants on Earth.
00:14:40And because right from the start they were reflecting green light,
00:14:43the stalks of the plants became green, and the leaves were green.
00:14:47In fact, that's why all plants on Earth became green, from the grasses to the forests.
00:14:52And it's also why, today, instead of living on a purple planet, we've got a green one.
00:15:07But it wasn't just about colour.
00:15:10Because the green bacteria did something their purple cousins couldn't.
00:15:16They produced oxygen.
00:15:22They would breathe life into the lifeless land.
00:15:26Without them, the story of our planet would be more like that of Mars.
00:15:33How the green bacteria did this is so complex that scientists still grapple with the details.
00:15:43I've come to the Eden Project in Cornwall to try to understand it.
00:15:49I'm to be the subject of an experiment that's never been attempted before.
00:15:55Hi.
00:15:56Hello there.
00:15:57I'm the guinea pig.
00:15:58Doctor, I presume.
00:15:59Indeed.
00:16:00Dan Martin.
00:16:00Hi there.
00:16:01Hi, Katrina.
00:16:02Nice to meet you.
00:16:03This is fantastic.
00:16:05Incredible, isn't it?
00:16:06I'm about to be locked inside this airtight chamber.
00:16:10I hope to experience first-hand my very own great oxidation event.
00:16:16OK, everyone, I'm going to start reducing the oxygen concentration in here now.
00:16:23The first step is to lower oxygen levels closer to those of the early Earth.
00:16:29So, first of all, this is going to monitor your heart rate and your oxygen levels.
00:16:34OK.
00:16:34So, if we pop that in, we can just have a look here.
00:16:37It's a lack of oxygen that complex life, like us, can't operate at for long.
00:16:43So, at the top is your heart rate.
00:16:46How's that? Is that really high?
00:16:48I think you might be a little bit anxious about going in there.
00:16:51I am, a little bit.
00:16:52I'm sure your resting heart rate's not normally 95.
00:16:55No, I have been thinking a lot about it.
00:16:58My vital signs have been monitored, along with the oxygen levels in my blood.
00:17:03Now it's time to be sealed inside the chamber for the next 48 hours.
00:17:10I'm as ready as I'll ever be, guys, so can we open this door?
00:17:15Wish me luck.
00:17:21Oh, it's small, isn't it?
00:17:28Oxygen levels in the air are normally 21%.
00:17:33Inside the chamber, they're far lower.
00:17:36Just over 12%.
00:17:40At these concentrations, the cellular activity in my body and brain is starting to slow down.
00:17:46Go.
00:17:48Green, yellow, red, green, yeah, it's kind of orange, purple, blue.
00:17:57You'll find that thinking becomes a little bit slower.
00:18:02My hand-to-eye coordination is being impaired.
00:18:07You can put them in any order you like.
00:18:09That's the way.
00:18:11Can you just tell us how exactly are you feeling?
00:18:14Yeah, it's funny.
00:18:14I felt very slow.
00:18:16Yeah, that slow, slowness is there, definitely.
00:18:22The doctors calculate that at the rate I use up oxygen, if it carried on like this, I'd be unconscious
00:18:29in just 24 hours.
00:18:31Your oxygen saturation, sort of 88%.
00:18:35If that was your level in hospital, we'd be pretty worried about you right now.
00:18:42The next crucial step is to see if the 300 plants in here with me can produce enough oxygen to
00:18:48keep me alive.
00:18:51It's all to do with a wondrous ability they inherited from those green bacteria.
00:18:57It's photosynthesis, of course.
00:18:59I think we can have the lights on, please.
00:19:04To kickstart it, you need light.
00:19:08Wow, suddenly the lights hit.
00:19:17Plants use photosynthesis to live and grow, and most importantly for me, to make oxygen.
00:19:28Photosynthesis is an intricate process that science is still trying to unlock.
00:19:35But the production of oxygen is one of its key features.
00:19:41To understand what's happening, you need to enter a complex and microscopic world.
00:19:50Inside every leaf, of every plant on the planet, are the direct descendants of those first green bacteria.
00:19:59Magnify a leaf a thousand times and you can see them.
00:20:02They're known as chloroplasts.
00:20:05Packed into every cell.
00:20:10They still behave a bit like bacteria.
00:20:13This is real footage of them moving towards a flash of light.
00:20:23They're just five thousandths of a millimetre across.
00:20:27And it's inside chloroplasts that photosynthesis happens.
00:20:35Light rays from the sun are made of photons.
00:20:40They're tiny, fast-moving particles of electromagnetic energy.
00:20:46When they hit the surface, the energy of the photons is captured by a ring, called the light harvesting complex.
00:20:58Inside this structure, the energy of two photons is used to split a water molecule.
00:21:10It's ripped into its two elements, hydrogen and oxygen.
00:21:18The plant uses the hydrogen to live and grow.
00:21:24But right now I'm interested in the other part of the water.
00:21:28The part plants pump out as a waste product.
00:21:31The oxygen.
00:21:47Scientists have calculated that the 300 plants in here with me should raise oxygen levels in this chamber from 12
00:21:55to 21% within 48 hours.
00:22:01I'm finding out how reliable the process of photosynthesis really is.
00:22:10It is quite concerning.
00:22:12You've been very busy this afternoon.
00:22:13A lot of activity.
00:22:15So we need to restrict the amount that you're talking.
00:22:18And really get you resting as much as possible.
00:22:21Not dashing around the chamber anymore.
00:22:23This is doctor's orders. Bed rest.
00:22:26Night-night.
00:22:33As I drift off to sleep, the 11,000 leaves go to work.
00:22:42They have 30 cubic metres of the box to fill.
00:22:48Some plants, like this maize and the banana plant, are particularly efficient at pumping out oxygen.
00:23:01So you can see the increase every hour here.
00:23:03That's incredible really.
00:23:04They're really pushing out a lot of oxygen as you can see.
00:23:12Every hour, my plants are producing over 40 litres of oxygen.
00:23:18Hi Ian, it's Katrina.
00:23:19We're now 41 hours in.
00:23:21Really? 41 hours in?
00:23:23Yeah.
00:23:23So the oxygen levels are still climbing.
00:23:25Gradually every hour, so it's going really well.
00:23:29With my vital signs returning to normal, I'm now a top attraction at the Eden Project.
00:23:35Hello. Hello. Can you see him?
00:23:38What are you doing in that box?
00:23:39He looks very happy in there, doesn't he?
00:23:41He's measuring his oxygen levels.
00:23:44Eat your heart out, David Blaine.
00:23:50Finally, after 48 long hours, oxygen levels are almost back to normal.
00:23:56The plants have triumphed.
00:24:04Wee!
00:24:06Oh, I'm out!
00:24:09I survived it. Fantastic.
00:24:12It's amazing.
00:24:13I was just thinking that I've survived, but actually, I guess I've really survived because of them.
00:24:19Because of the plants.
00:24:20I leave here thinking that I needed those plants way more than they needed me.
00:24:26It's easy to think of this as just an experiment, but to me, when you're lying in there, you realise
00:24:31that this place is a metaphor for something much bigger.
00:24:33For the planet, really, and for our relationship with plants through photosynthesis to keep life going.
00:24:43The early Earth was like my chamber.
00:24:46It was transformed from a world with very little oxygen to a world rich in oxygen.
00:24:53And all that oxygen began to do something else.
00:24:57High in the stratosphere, it created ozone.
00:25:04This was a protective blanket, which enveloped the Earth and blocked most of the sun's dangerous UV rays.
00:25:14It meant that for the first time in the planet's history, plants could move on to the land.
00:25:20But it was no small step.
00:25:34The thing is, if you'd been protected by water for billions of years, then the move to the land was
00:25:40going to be a rude shock.
00:25:43Yet over 400 million years ago, plants finally made that leap.
00:25:51Surprisingly, the best evidence for these pioneers doesn't come from some exotic corner of our planet, but from Britain.
00:26:04I've come to just outside the village of Rhiney in North-East Scotland to see this.
00:26:09A stone wall.
00:26:10But not just any stone wall, of course.
00:26:12For me, this is the most important stone wall in the history of science.
00:26:17A stone wall.
00:26:17A stone wall.
00:26:25A stone wall.
00:26:26Back 410 million years ago, Scotland was located well south of the equator and looked like another world.
00:26:39Hot springs and geysers boiled out across a rocky and barren landscape.
00:26:50But something else was happening.
00:26:52A scientists discover when they came across some curious markings on this wall.
00:26:58And this is one of them.
00:26:59Look at this.
00:27:00You see these really strange, elongated kind of shapes here.
00:27:05At first people just didn't really know what they were.
00:27:07They thought maybe at first it was some kind of lava, but when they looked really closely, especially when they
00:27:12got it cut and polished, this rock literally came alive.
00:27:23Because you can see these dark features here.
00:27:26They realised that this was something that was once living.
00:27:33And when they were alive, this is what they looked like.
00:27:40Just a few centimetres tall, they're called aglophyton.
00:27:47Bulbous shapes on the end of naked stems.
00:27:53A time before leaves or roots, yet somehow these bizarre life forms survived along the water's edge.
00:28:01What geologists had found right here in Scotland were some of the earliest pioneering plants to make that giant leap.
00:28:09To colonise the land.
00:28:15And around this time, all along the margins of lakes and rivers, primitive plants were coming ashore.
00:28:27For the first time when viewed from space, the land began to look alive.
00:28:33The beginning of a transformation from hostile world to fertile earth.
00:28:42Yet this wasn't a full scale invasion.
00:28:46Just a toehold.
00:28:49Plants were still tied to the water's edge.
00:28:52Unable to head inland and penetrate the harsh, rocky surface.
00:28:57But all this was about to change.
00:29:03Plants evolved an inspired solution to the problem.
00:29:07A brilliant device for collecting water and nutrients.
00:29:10And something that they never really had before.
00:29:13Roots.
00:29:24Cambodia.
00:29:24The 12th century temple here at Taprom is a wonder of civilisation.
00:29:31But it's also a wonder of the natural world.
00:29:35Although the roots of these strangler things are very different from the first ones to evolve.
00:29:40It's a superb place to reveal how roots allowed plants to invade inland.
00:29:50Roots are hugely powerful. I mean, I love this one.
00:29:53Look at it, prizing its way into that roof.
00:29:55Just lifting that whole structure up.
00:29:58And then boring down here through these stone blocks and then disappearing.
00:30:04Just tiny pressures exerted over decades and centuries.
00:30:11Add these up and you get phenomenal strength.
00:30:14A pressure of up to 10 kilograms per square centimetre.
00:30:19Around 400 million years ago, the first roots appeared.
00:30:25And gave plants the ability to smash up the rocky planet.
00:30:31And this created a vital ingredient for life on land.
00:30:38When the tiny broken up fragments of rock get mixed up with kind of dead plant material.
00:30:44It ends up as this ideal environment for storing water.
00:30:49An environment that we call...
00:30:52Soil.
00:30:55Today, soil covers 40% of the planet's land.
00:31:00It takes a long time to form.
00:31:02A thousand years to make just two centimetres of soil.
00:31:06But it's essential for plant life.
00:31:09Just as it was back then.
00:31:14Because the primitive, leafless plants could now break free from the water's edge.
00:31:22Roots and the soil they created made plants unstoppable.
00:31:28Allowing them to colonise inland for the first time.
00:31:33An invasion that would have a dramatic influence on all life on Earth.
00:31:45For millions of years, animals had been confined to the rivers and oceans.
00:31:52Now they could finally emerge from the water.
00:31:57And get an idea of those first tentative steps by travelling back in time with a creature that's barely changed
00:32:05for 500 million years.
00:32:18I've come to the east coast of America, where these ancient creatures still come ashore at dusk to mate.
00:32:28Here they are.
00:32:30Horseshoe crabs.
00:32:33Looks like something from another planet.
00:32:37You can see the two main eyes here, but they've got something like...
00:32:40Like ten eyes scattered across their body.
00:32:43And the really weird bit is if you lift them up.
00:32:48I mean, look at that.
00:32:50I mean, for a start they've got five pairs of legs.
00:32:52Look, one, two, three, four, five.
00:32:54Whereas normal crabs just have four.
00:32:56They're actually more related to the scorpion than to normal crabs.
00:33:00Look at that.
00:33:00But the really interesting bit is, tucked under here, these things called bookgills.
00:33:07Look at that there.
00:33:08It's like sheaves of a book.
00:33:10And that allows them to extract oxygen, not just from the water, but also from the air.
00:33:16It's an amazing breathing apparatus.
00:33:20Got to put her back now.
00:33:21Come on, dear.
00:33:22There you go.
00:33:23Ah!
00:33:29As long as they're kept moist, these lung-like gills enable the crabs to stay out of water for days
00:33:35at a time.
00:33:38Fossils show that horseshoe crabs appeared on land at least 400 million years ago.
00:33:44They're some of the first animals ever to come ashore.
00:33:55Amphibians and insects soon followed.
00:33:59Oxygen allowed them to move onto land.
00:34:02But something else was also enticing them.
00:34:08It's funny, plants create oxygen as a waste product.
00:34:11And it's that waste product that has transformed our atmosphere.
00:34:15But, of course, the main reason that plants photosynthesise is to create sugars.
00:34:20Sugars that are vital for plants to live and to grow.
00:34:23And also provide a source of food for all animals.
00:34:33Plants make this sugar from water.
00:34:37Carbon dioxide from the air.
00:34:41And energy from the sun.
00:34:46And again, it all happens in those tiny chloroplasts.
00:34:53We've seen how light splits water into oxygen and hydrogen.
00:35:00Well, the plant takes that hydrogen and combines it with carbon dioxide to make sugar.
00:35:09By exposing a plant to carbon dioxide tagged with a radioactive marker, you can see the sugar being created.
00:35:18For the first time, scientists have imaged its creation and movement through a plant.
00:35:23In this case, maize.
00:35:28As soon as carbon dioxide is sucked into the plant cells, they begin to glow.
00:35:35This is the actual moment that photosynthesis turns the carbon dioxide into sugar.
00:35:44In just 15 minutes, the newly formed sugar is sent to the roots for storage.
00:35:52The plant can then use this sugar to grow and thrive.
00:35:59That's why photosynthesis is nature's most astonishing achievement.
00:36:04The ability of plants to be powered by light from beyond our planet sets them apart from all other life.
00:36:11And that connection with that star, our sun, makes plants a foundation stone for all living things.
00:36:19It's just such a wonderful thought.
00:36:27400 million years ago, leafless plants were flourishing like never before.
00:36:36But a dramatic transformation of the atmosphere was about to throw plants into a global crisis.
00:36:44Not only would it change their shape, it would change all life on our planet.
00:37:09This is Lake Tarawera in New Zealand.
00:37:16This ancient landscape is home to a plant that 360 million years ago
00:37:22confronted that crisis and came up with an inspired solution.
00:37:29Looks like the land that time forgot, doesn't it?
00:37:32Just that strange mixture of different shapes of plants and trees.
00:37:36Really unfamiliar and alien.
00:37:38It's almost primeval.
00:37:44The early plants had become victims of their own success.
00:37:49They were gorging on so much carbon dioxide in the atmosphere that they were using it up.
00:37:55Levels plummeted by 90%.
00:38:00Without enough of this vital gas, plants began struggling.
00:38:05If they couldn't find a way to breathe in more carbon dioxide, they'd suffocate.
00:38:14With the early plants, plants like these gorgeous ferns here came up with a remarkable new structure.
00:38:22Large flat surfaces that house within them a complex breathing apparatus.
00:38:27We call them leaves.
00:38:30We call them leaves.
00:38:34Leaves were the answer to all plants' breathing problems.
00:38:39They massively increased their surface area by over a hundred fold,
00:38:44allowing them to absorb far more carbon dioxide.
00:38:51Now, for the first time, shade was cast by a beautiful and delicate canopy, like these Dixonia.
00:38:59These ferns are incredible.
00:39:03They're like giant umbrellas.
00:39:07The key to this advanced breathing apparatus is on the underside of each fern leaf.
00:39:13They're microscopic holes called stomata.
00:39:20Filmed in action with an electron microscope,
00:39:23this is them opening and closing.
00:39:26Speeded up 140 times.
00:39:38There are thousands of stomata on every leaf on Earth.
00:39:43They allow a single fern to breathe in five litres of carbon dioxide a day.
00:39:59The evolution of leaves, rich in stomata, save plants from suffocation.
00:40:06But leaves also allowed plants to capture more light.
00:40:12This in turn fueled fierce competition.
00:40:15Each plant desperate for the sun's rays.
00:40:21This family squabble would lead to a new type of plant.
00:40:26One that would have surprising repercussions for the planet.
00:40:33How do we know?
00:40:35Well, it's all thanks to some rare evidence here in Nova Scotia in Canada.
00:40:43To reach it, you have to abseil to the bottom of this 30 metre cliff.
00:40:49Here, scientists discovered the remains of a mysterious world.
00:40:55You know, this is just the best way to see rocks.
00:40:58You really feel as if you're a time traveller, peeling back the layers of history one by one as you
00:41:03go down.
00:41:04I mean, these rocks are over 300 million years old.
00:41:10But it's what's locked inside the rocks at the base of this cliff that took scientists' breath away.
00:41:22These fossilised remains are just spectacular.
00:41:25I mean, look at the texture. You can tell it's a plant, but this isn't some big shrub or overgrown
00:41:31fern.
00:41:32You can see here, look, you can get traces of bark.
00:41:36And down here, you can see there's some roots coming off.
00:41:39This is completely extinct. You don't get this anymore.
00:41:42But what I'm actually crouching beside is one of the planet's very early tree trunks.
00:41:47And not just one tree, because look here.
00:41:50There's another one here.
00:41:53There's another one there.
00:41:56This is a fossil forest.
00:42:01These lepidodendron trees had strange diamond-shaped bark.
00:42:08Each diamond sprouting a needle-like leaf.
00:42:13Over 300 million years ago, they made up the planet's first tropical forests.
00:42:31These forests were so extensive, you'd have seen a band of dark green from space.
00:42:42And all those new leaves were pumping out oxygen.
00:42:47So much that levels of oxygen increased to not far off double what they are today.
00:42:57It was having a very odd effect on animals.
00:43:01In particular, insects and their cousins.
00:43:08Instead of lungs, invertebrates have simple breathing tubes that rely on diffusion for oxygen to reach their internal organs.
00:43:17The size of these animals is therefore limited by the concentration of oxygen in the air.
00:43:25Increase the oxygen, just as the first forest did, and things get interesting.
00:43:34Do you see these markings on the rock here?
00:43:37There's two lines of little dents, one here and one here.
00:43:41They are fossilised footprints that date back to the very early forests.
00:43:45When scientists first studied them, they realised they weren't made by some reptile or amphibian.
00:43:50They were made by a millipede.
00:43:53Now, here is one of the biggest millipede species alive in the air today.
00:44:00That's pretty big.
00:44:07Using the tracks for scale, it's clear the ancestors of this little fellow were massive.
00:44:21Called Arthropluridae, it was over two metres long.
00:44:28The forests would have been terrifying.
00:44:32With giant scorpions and giant spiders.
00:44:40And not just on the land.
00:44:42I think the most impressive of all were the dragonflies.
00:44:47I mean, most modern dragonflies have wingspans up to 10 centimetres across,
00:44:51but back then they were way larger.
00:44:54Some were up to a metre across.
00:45:05These mega-neura were the largest insects ever to take to the skies.
00:45:13But in this oversized world, pumped with oxygen, the plant kingdom still reigns supreme.
00:45:23Then, 230 million years ago, a new group of animals emerged from the shadows of the swampy forests.
00:45:30They would become the largest creatures to roam the earth.
00:45:33And they were ready to do battle with the kingdom of the plants.
00:45:37I'm talking, of course, of dinosaurs.
00:45:45It's the meat-eaters that get all the press.
00:45:50But recent research has revealed that out of the 700 species discovered, over two-thirds were herbivores.
00:46:00Vegetarians ruled.
00:46:03Led by the biggest herbivores in history.
00:46:06The sauropods.
00:46:11To discover the impact of these huge dinosaurs on the plant kingdom,
00:46:16I've come to an animal sanctuary to see it in the flesh.
00:46:23Unfortunately, this place doesn't have a living sauropod, but what it does have is the biggest herbivore that the planet's
00:46:30got to offer.
00:46:31The African elephant.
00:46:34Come and meet Butch.
00:46:36This beautiful four-ton elephant can help me truly appreciate the staggering size of the dinosaurs.
00:46:50Can we go up?
00:46:55Now, Butch here is about as big as a big African bull gets.
00:46:59And, you know, that's already...
00:47:02What's that?
00:47:03Four metres high.
00:47:05But if we want to get to the height of a sauropod, we have to go much higher.
00:47:10Six metres, we've got to be higher than that.
00:47:13We're now at eight metres.
00:47:15Still got to go higher.
00:47:16Yeah.
00:47:17Where are we at?
00:47:18Ten metres now.
00:47:20A bit higher than that.
00:47:21We're still not at the height of a sauropod yet.
00:47:24Okay, we're getting there.
00:47:25Nearly.
00:47:27Okay.
00:47:28Fight.
00:47:31So my head's now about 12 metres, which is about the height of a four-storey building,
00:47:37and also the height of a sauropod.
00:47:40And the thing is, on the end of a nine metre neck, this is the skull of a sauropod.
00:47:48It seems quite small.
00:47:50But the point was that this had to be manoeuvrable and nimble to get right up at that high-level
00:47:56foliage.
00:48:00Sauropods were like nothing else a planet had ever seen.
00:48:04They weighed more than ten times an African elephant.
00:48:09Now, Butch here eats about 90 kilograms of foliage every day, which is roughly about that much hay.
00:48:16But scientists have estimated that sauropods eat about 1,500 kilograms of hay every day.
00:48:23In other words, about 20 times that daily diet.
00:48:27Or 50 bales of hay.
00:48:30Now, if you imagine, you've got herds of about 30 sauropods, much bigger than these beasts here.
00:48:37And you realise that the plant kingdom was up against the ultimate salad predator.
00:48:45150 million years ago, dinosaurs were stripping the land of vast swathes of foliage.
00:48:53For the first time, the plant kingdom was under serious attack from another dynasty.
00:49:01To fight back, plants began to evolve a whole arsenal of defences for their precious leaves.
00:49:11Here in California, we can see just how intense this arms race was in one of the world's most unusual
00:49:18gardens.
00:49:20It's full of a group of bizarre and extremely rare plants called cycads.
00:49:26That once, they made up a quarter of all plants on Earth.
00:49:35This is incredible.
00:49:39Exactly the kind of place you'd expect a dinosaur just to pop out.
00:49:46To stave off attack from those ravenous dinosaurs, cycads developed some clever lines of defence.
00:49:52And the most obvious being physical weapons like needles and spikes and acca.
00:49:58These are vicious.
00:49:59The main point was to make leaves as painful as possible to eat.
00:50:12These defences came in all shapes and sizes.
00:50:19And some plants also spice things up with chemical weapons.
00:50:29This is a trap's valley, cycad from South Africa.
00:50:32But it's pretty typical in that the leaves contain a nerve age and that if you ingest it,
00:50:36it causes vomiting, diarrhoea, paralysis of limbs and then, of course, death.
00:50:42Now, obviously, I'm not going to eat one of these, but I can eat a plant whose ancestors emerged around
00:50:48the time of the dinosaurs
00:50:49and who also have a chemical weapon.
00:50:51And that is...
00:50:53a chilli.
00:50:55In particular, a harberness chilli.
00:50:59Which is supposed to be one of the most powerful in the world.
00:51:04There's a chemical in here called capsicum.
00:51:08But...
00:51:09It's contained in the fruit.
00:51:13And that, essentially, is a toxin.
00:51:19Ah!
00:51:23Which is, at this precise moment, burning and inflaming all of my mouth.
00:51:31Oh, my gosh.
00:51:37The thing is, the toxins in the cycads, right?
00:51:40They were...
00:51:41They were far more powerful even than chillies.
00:51:44Oh, my gosh.
00:51:46So you can imagine what...
00:51:48the dinosaurs were had to endure.
00:51:50Oh, my.
00:51:52I could have to...
00:52:01I can't even see how sore that is.
00:52:08Mmm.
00:52:09Mmm.
00:52:11Eat a chilli, they said.
00:52:14It'll be funny, they said.
00:52:17Ooh.
00:52:18You know what? Forget about cycads.
00:52:20That could have brought down a 70-tonne sawpoint.
00:52:27And the arms race didn't stop there.
00:52:30Ah!
00:52:30Plants evolved a new tactic.
00:52:35Not so much a line of defence,
00:52:38as a line of communication.
00:52:45We know that when some plants are attacked,
00:52:47they activate a quick-acting toxin that deters herbivores.
00:52:53Now we're discovering that this defence goes even further.
00:52:58Because plants can actually warn other plants
00:53:00that a herbivore is eating them.
00:53:07And at last, scientists here at Exeter University
00:53:11are beginning to listen in to this hidden conversation.
00:53:19They're finding that when plants are attacked,
00:53:21they also release an unseen gas from the leaves.
00:53:26What it does is extraordinary.
00:53:31And this will be the first time it's been captured on film,
00:53:34using specialist imagery.
00:53:37These two Arabidopsis plants are being put inside a chamber.
00:53:44A third plant is then cut to mimic an attack.
00:53:54It's added to the undamaged plants.
00:54:01The chamber is sealed.
00:54:13The plant leaves are now releasing the gas.
00:54:18As they do so,
00:54:20their biological activity can be seen changing.
00:54:32The gas triggers a change in the biological activity
00:54:36in the two neighbouring plants.
00:54:43They have detected the message warning them to protect themselves.
00:54:47They have detected the message warning them to protect themselves.
00:54:57Scientists don't know all the details of this plant language,
00:55:01but increasingly they believe there's a chatter between plants all around us.
00:55:08I think most people assume that plants lead a rather passive life.
00:55:12That they're static and unresponsive.
00:55:16That's just not true.
00:55:17In reality, they move, they sense, they communicate.
00:55:21It's always as if they show a kind of intelligence.
00:55:29For 200 million years, the dinosaurs and the plants were locked in a titanic evolutionary battle.
00:55:38Each trying to gain the upper hand.
00:55:43But it was now that some plants played their trump card.
00:55:56They use wood to grow taller and taller.
00:56:03And in California's Sierra Nevada, I'm about to find out just how tall.
00:56:11To do that, I need the help of biologist Jim Spickler.
00:56:19Ready as I'll ever be.
00:56:21Take your time.
00:56:22I mean, we've got some time.
00:56:23What I was going to do. Absolutely.
00:56:32This is the grandest example of them all.
00:56:36The giant sequoia.
00:56:40What you see is just impossible for your mind to process.
00:56:42The scale is so large and...
00:56:44Ah, it's extraordinary.
00:56:46I feel like as if you're the Lord of the Rings.
00:56:52the world.
00:57:10Ah, what a great tree.
00:57:15Seventy million years ago, the ancestors of this type of tree, the conifers, got ever
00:57:22taller. This was the ultimate in plant construction. Conifers like the giant
00:57:30sequoias raised their precious leaves out of reach. The dinosaurs were no longer the
00:57:38biggest organisms on earth. That title had been well and truly won back by these giants
00:57:46of the plant kingdom. This is so tall, but I've still got, I don't know, another third to go.
00:57:55The thing is by using wood to grow really tall like this, it gave trees another advantage over
00:58:01plants because it allowed them first pick of the sun's strongest rays. And the thing
00:58:06is, of course, for plants, light means success.
00:58:14If you had a satellite image of the dinosaur era 70 million years ago, you'd see the earth
00:58:20like it had never been before and never would be again.
00:58:26The climate was so warm the poles had no ice. Instead they were covered with conifers, a vast
00:58:35polar forest. And the mighty sequoia trees were not just found in small areas of the
00:58:43Siena Nevada as they are today. They were global, stretching along the Pacific coast and as far
00:58:51south as Australia. How far are we from the, from the top then?
00:58:58We're getting close. Oh, that's extraordinary. This is it. This is the top of the tree.
00:59:22It's staggering to think that using just the gas, carbon dioxide and a liquid, water, together
00:59:31with light energy from beyond our world, you can construct a cathedral of wood 90 metres tall.
00:59:47Since they first appeared, plants and their ancestors have revolutionised our planet.
00:59:55They created oxygen for the atmosphere, which had allowed them to conquer the land and transform
01:00:05rock into soil. In turn, fueling the explosion of all life. From a barren, alien planet, plants
01:00:17had made a living earth. And left on its own, the world would have continued like this, dominated
01:00:24by large dinosaurs and endless forests. But 65 million years ago, something happened, it would
01:00:32change everything. A chance of end, it would have dramatic consequences, not just for plants,
01:00:37but for all life. And it would originate, not on earth, but in outer space. The asteroid would kill off
01:00:53the dinosaurs.
01:00:54And the next chapter would see the triumph of a whole new group of plants. Flowers transformed the bond
01:01:05between animal and plant. Even sculpting the very planet itself. Above all, plants would drive our human story.
01:01:15But all that was still to come.
01:01:18.
01:01:48.
01:01:49.
01:01:49.
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