0:00:07 > 0:00:09I've spent most of my life
0:00:09 > 0:00:12trying to understand the forces that shaped our planet.
0:00:12 > 0:00:14As a geologist, it always seemed to me
0:00:14 > 0:00:17that rocks were right at the heart of things.
0:00:20 > 0:00:26But now, I'm discovering it's not only volcanoes and colliding continents
0:00:26 > 0:00:29that have driven the Earth's greatest changes,
0:00:29 > 0:00:33because at crucial moments in its history,
0:00:33 > 0:00:36another force has helped create the planet we live on...
0:00:37 > 0:00:39..plants.
0:00:39 > 0:00:41Just look at this seed.
0:00:41 > 0:00:44It's small, it's brown. It weighs hardly anything.
0:00:44 > 0:00:46Looks pretty ordinary,
0:00:46 > 0:00:49but actually nothing can be further from the truth
0:00:49 > 0:00:52because what it will become is truly extraordinary.
0:00:59 > 0:01:02These are giant sequoias.
0:01:02 > 0:01:05Some are over 3,000 years old.
0:01:07 > 0:01:12And sequoias are the largest single life form on Earth.
0:01:13 > 0:01:15All from a tiny seed.
0:01:17 > 0:01:19Yet even that pales into insignificance
0:01:19 > 0:01:22when compared to what the whole of the plant kingdom has done
0:01:22 > 0:01:24throughout the history of our planet.
0:01:25 > 0:01:27They harness light from a star,
0:01:27 > 0:01:30bringing energy to our world.
0:01:30 > 0:01:34They and their ancestors created our life-giving atmosphere.
0:01:35 > 0:01:39I'm breathing oxygen that was made two and a half billion years ago.
0:01:43 > 0:01:47They sculpted the very surface of the Earth
0:01:47 > 0:01:50and they drove the evolution of all animals...
0:01:51 > 0:01:53..including our own ancestors.
0:01:56 > 0:02:00It's a whole new story about our Earth...
0:02:02 > 0:02:05..told through remarkable images,
0:02:05 > 0:02:08captured for the very first time,
0:02:08 > 0:02:12and the latest scientific discoveries.
0:02:12 > 0:02:14Wish me luck.
0:02:21 > 0:02:23This is the start of that story.
0:02:23 > 0:02:27How plants took a barren alien rock, our planet,
0:02:27 > 0:02:30and transformed it into the home we know today.
0:02:44 > 0:02:47Ooh! It's a long way down.
0:02:50 > 0:02:52I'm in Central Vietnam
0:02:52 > 0:02:56and I'm descending into one of the largest caves in the world.
0:02:56 > 0:02:59The structure's absolutely fantastic.
0:02:59 > 0:03:00Ugh!
0:03:04 > 0:03:08At 7km long, this is known as Hang Son Doong.
0:03:12 > 0:03:14It's a dark, alien world.
0:03:14 > 0:03:18Down here, very little is alive.
0:03:22 > 0:03:25But I'm not here for the cave.
0:03:26 > 0:03:29Oh! Look at that!
0:03:31 > 0:03:34For goodness' sake!
0:03:34 > 0:03:36It looks like the roof has collapsed
0:03:36 > 0:03:39and the rainforest has just invaded.
0:03:39 > 0:03:43It's a rainforest inside a cave.
0:03:43 > 0:03:45After being in the darkness and the black for ages, look at that.
0:03:45 > 0:03:48You just suddenly see brilliant green.
0:03:49 > 0:03:51This isn't the entrance.
0:03:52 > 0:03:55We're three kilometres into the hear of the cave system.
0:03:59 > 0:04:04It's a thriving lost world with towering Polyalthia trees.
0:04:06 > 0:04:11And home to strange creatures like this Vietnamese flat-backed millipede.
0:04:13 > 0:04:16Isn't that incredible! It's got antlers.
0:04:19 > 0:04:22You really feel as if you've left the confines of that cave
0:04:22 > 0:04:26and just escaped really into this fantastic forest.
0:04:26 > 0:04:27It's a wonderland, really.
0:04:28 > 0:04:33This rainforest exists because of one thing above all.
0:04:33 > 0:04:36Something which has enabled plants to colonise
0:04:36 > 0:04:38almost everywhere on Earth...
0:04:39 > 0:04:41..light.
0:04:43 > 0:04:48Light which has travelled 150 million kilometres from the sun.
0:04:51 > 0:04:54Plants have this truly remarkable ability
0:04:54 > 0:04:58to harness energy from outer space to produce food.
0:04:58 > 0:05:03It's this ability to eat the sun, to manufacture life from light,
0:05:03 > 0:05:06that's allowed plants to dominate our planet.
0:05:09 > 0:05:14This is the most important natural process on Earth.
0:05:15 > 0:05:19It's how the plant kingdom has transformed a lifeless planet
0:05:19 > 0:05:21into a living world.
0:05:28 > 0:05:30But it wasn't always like this.
0:05:33 > 0:05:38And to see how it started, we need to go back three billion years.
0:05:43 > 0:05:47To begin with, our planet was like an alien world.
0:05:49 > 0:05:52There was very little oxygen.
0:05:53 > 0:05:57The atmosphere was a cocktail of toxic gases,
0:05:57 > 0:06:01like methane and sulphur dioxide.
0:06:01 > 0:06:04The land was lifeless.
0:06:08 > 0:06:11This barren saltpan in southern Kenya
0:06:11 > 0:06:13is about as close as you can get in the modern day Earth
0:06:13 > 0:06:16to that ancient world three billion years ago.
0:06:16 > 0:06:17But the one crucial difference
0:06:17 > 0:06:19between the planet then and the planet now
0:06:19 > 0:06:23is that back then I'd have been burnt to a crisp.
0:06:26 > 0:06:30That's because the primitive atmosphere couldn't screen out
0:06:30 > 0:06:33the sun's powerful ultraviolet rays.
0:06:35 > 0:06:39Back then, these UV rays were hundreds of times stronger
0:06:39 > 0:06:42than they are now.
0:06:44 > 0:06:48Nothing could survive on land.
0:06:51 > 0:06:55Yet all this was about to change.
0:06:56 > 0:06:58A momentous event
0:06:58 > 0:07:02that would create the planet's first life-supporting atmosphere.
0:07:04 > 0:07:09This event, between three and two and a half billion years ago,
0:07:09 > 0:07:14was the single greatest turning poin in the history of life on Earth.
0:07:14 > 0:07:19And it was all brought about by the earliest ancestors of plants.
0:07:27 > 0:07:31Here at the Sishen iron mine in South Africa,
0:07:31 > 0:07:35evidence of that epic event can still be unearthed today.
0:07:38 > 0:07:41But to get to it, you need a bit of help.
0:07:41 > 0:07:43SIREN
0:07:43 > 0:07:45MUFFLED VOICE
0:07:45 > 0:07:46Thirty seconds.
0:07:46 > 0:07:48Ten.
0:07:50 > 0:07:51LOUD EXPLOSION
0:08:01 > 0:08:07That is 200,000 tonnes of iron ore just been blasted apart.
0:08:09 > 0:08:13These explosions open a cross section back in time
0:08:13 > 0:08:16to the distant origins of the plant kingdom.
0:08:21 > 0:08:23This is iron ore.
0:08:23 > 0:08:24It's so heavy.
0:08:24 > 0:08:28Pure iron's got this metallic glint, it's shiny,
0:08:28 > 0:08:32but you can see that this has got loads of red in it.
0:08:32 > 0:08:35And it's red for a really simple reason. It's rusted.
0:08:35 > 0:08:38It's rusted because it's come into contact with oxygen.
0:08:38 > 0:08:42Oxygen produced by the very first burst of life.
0:08:46 > 0:08:49The miners want the ore for its iron content.
0:08:49 > 0:08:53But I'm going to use this iron oxide for a very different reason.
0:08:53 > 0:08:56Something I don't think has ever been done before...
0:08:56 > 0:08:58which is why I'm a wee bit excited.
0:08:59 > 0:09:02I've taken a chunk of the iron oxide rock
0:09:02 > 0:09:05and had it ground up into fine powder.
0:09:07 > 0:09:11It's then been turned into a solution.
0:09:11 > 0:09:14One I'm hoping will allow me
0:09:14 > 0:09:17to take a breath from the planet's earliest oxygen.
0:09:17 > 0:09:20Oxygen made by the ancient ancestors of plants.
0:09:21 > 0:09:24And now what I'm going to do is kind of jump start it, really,
0:09:24 > 0:09:27with this battery. I'm going to attach a lead
0:09:27 > 0:09:30and pass an electric current through it.
0:09:30 > 0:09:33And we should see a simple reaction.
0:09:34 > 0:09:37Oh yeah, yeah. There's some bubbles coming off.
0:09:40 > 0:09:43These bubbles are the gas oxygen.
0:09:43 > 0:09:48It's being released for the first time in over two and a half billion years,
0:09:48 > 0:09:51when it was locked away in the rock.
0:09:51 > 0:09:58There's a lovely little train of them just rising to the top and forming a little pocket of gas.
0:09:59 > 0:10:03You're never sure with these experiments whether you're really going to get it or not,
0:10:03 > 0:10:06but that's exactly what I was hoping to see.
0:10:06 > 0:10:11In just one hour, I've collected enough to fill the whole test tube.
0:10:16 > 0:10:18The thing is, this isn't any old oxygen.
0:10:18 > 0:10:22This is oxygen that's come from those iron bands.
0:10:22 > 0:10:26The very oxygen that changed our planet.
0:10:26 > 0:10:29In fact, I can't resist it. I'm going to have to...
0:10:29 > 0:10:31INHALES
0:10:32 > 0:10:34Ah!
0:10:34 > 0:10:35I can't believe it.
0:10:35 > 0:10:40I'm breathing oxygen that was made two and a half billion years ago.
0:10:40 > 0:10:43It's all gone. Liberated from the rocks now.
0:10:43 > 0:10:45It's up there somewhere.
0:10:47 > 0:10:51These iron bands tell a remarkable story.
0:10:51 > 0:10:55Oxygen was now flooding the Earth's atmosphere.
0:11:00 > 0:11:04It cleaned out the planet's toxic gases,
0:11:04 > 0:11:08leaving the sky a clear blue for the first time.
0:11:13 > 0:11:16Geologists call it the Great Oxidation Event.
0:11:16 > 0:11:18And it certainly was an event.
0:11:18 > 0:11:22This was an irreversible change between two very different worlds -
0:11:22 > 0:11:27a planet with virtually no free oxygen and a planet full of oxygen.
0:11:27 > 0:11:31This was the greatest change in the history of life on Earth.
0:11:33 > 0:11:36So how did this great event happen?
0:11:36 > 0:11:40The answer lies with the first burst of life,
0:11:40 > 0:11:45which emerged not on the hostile land but under water.
0:11:46 > 0:11:52Back then, water acted as a liquid sunscreen to the dangerous UV rays.
0:11:52 > 0:11:55Under the protection of water,
0:11:55 > 0:11:57the earliest organisms on Earth evolved
0:11:57 > 0:12:00in the form of tiny bacteria.
0:12:00 > 0:12:04And here in East Africa is a rare chance
0:12:04 > 0:12:07to see what it would have been like.
0:12:07 > 0:12:09This is Lake Magadi.
0:12:09 > 0:12:11The waters here are just super salty.
0:12:11 > 0:12:14Agh! Can feel it nipping away at my feet.
0:12:16 > 0:12:19But the bacteria I'm wading through are close descendants
0:12:19 > 0:12:24of the very first microorganisms that lived three billion years ago.
0:12:24 > 0:12:28It's fantastic to think that swimmin in the top layer here
0:12:28 > 0:12:31are some of the most primitive life forms on Earth.
0:12:31 > 0:12:35And those bacteria, just like the ones all that time ago,
0:12:35 > 0:12:37have got something surprising about them.
0:12:37 > 0:12:40They're purple.
0:12:44 > 0:12:50These are hallow bacteria and they didn't just occupy the occasional lake.
0:12:50 > 0:12:54Much of the world's oceans were purple, too.
0:12:56 > 0:12:59Imagine that from outer space.
0:13:00 > 0:13:03A purple Earth.
0:13:08 > 0:13:12The purple bacteria live by harnessing energy from the sun.
0:13:13 > 0:13:16But they only use part of the light.
0:13:16 > 0:13:20Some rays pass deeper into the water
0:13:21 > 0:13:26And over time, down there, a different type of bacteria evolved
0:13:26 > 0:13:29They had to live off the colours of light left over.
0:13:29 > 0:13:32This made them appear green.
0:13:32 > 0:13:35These were the green bacteria.
0:13:36 > 0:13:39This seemingly arbitrary event,
0:13:39 > 0:13:42bacteria absorbing one colour of light rather than another,
0:13:42 > 0:13:45would have colossal repercussions for the planet.
0:13:49 > 0:13:53Over time these green bacteria, a type of Cyanobacteria,
0:13:53 > 0:13:57came to dominate the waters of the world.
0:13:57 > 0:13:59Eventually, as we'll see,
0:13:59 > 0:14:03these green microorganisms became the ancestor of all plants on Earth.
0:14:03 > 0:14:07Because right from the start they were reflecting green light,
0:14:07 > 0:14:11the stalks of the plants became gree and the leaves were green.
0:14:11 > 0:14:14In fact, that's why all plants on Earth became green,
0:14:14 > 0:14:16from the grasses to the forests,
0:14:16 > 0:14:20and it's also why today, instead of living on a purple planet,
0:14:20 > 0:14:23we've got a green one.
0:14:30 > 0:14:33But it wasn't just about colour.
0:14:33 > 0:14:36Because the green bacteria did something
0:14:36 > 0:14:39their purple cousins couldn't.
0:14:39 > 0:14:41They produced oxygen.
0:14:44 > 0:14:48They would breathe life into the lifeless land.
0:14:48 > 0:14:52Without them, the story of our plane would be more like that of Mars.
0:14:55 > 0:14:58How the green bacteria did this is so complex
0:14:58 > 0:15:01that scientists still grapple with the details.
0:15:04 > 0:15:09I've come to the Eden Project in Cornwall to try to understand it.
0:15:10 > 0:15:13I'm to be the subject of an experiment
0:15:13 > 0:15:16that's never been attempted before.
0:15:16 > 0:15:17- Hi!- Hello there.
0:15:17 > 0:15:20I'm the guinea pig. Doctor, I presume?
0:15:20 > 0:15:21Indeed. Dan Martin.
0:15:21 > 0:15:23- Hi there.- Hi. Katrina Hope. Nice to meet you.
0:15:23 > 0:15:25Look at this! This is fantastic.
0:15:25 > 0:15:27Incredible, isn't it?
0:15:27 > 0:15:30I'm about to be locked inside this airtight chamber.
0:15:30 > 0:15:36I hope to experience first-hand my very own Great Oxidation Event.
0:15:36 > 0:15:37OK, everyone.
0:15:37 > 0:15:40I'm going to start reducing the oxygen concentration in here now.
0:15:40 > 0:15:42BEEPING
0:15:42 > 0:15:47The first step is to lower oxygen levels closer to those of the early Earth.
0:15:49 > 0:15:50So first of all,
0:15:50 > 0:15:53this is going to monitor your heart rate and your oxygen levels
0:15:53 > 0:15:56so if we pop that on we can just have a look here.
0:15:56 > 0:16:01It's a lack of oxygen that complex life like us can't operate at for long.
0:16:01 > 0:16:05So at the top is your heart rate.
0:16:05 > 0:16:06How's that? Is that really high?
0:16:06 > 0:16:10I think you might be a little bit anxious about going in there.
0:16:10 > 0:16:11I am a little bit.
0:16:11 > 0:16:13I'm sure your resting heart rate's not normally 95.
0:16:13 > 0:16:17No, I have been thinking a lot about it.
0:16:17 > 0:16:21My vital signs are being monitored, along with the oxygen levels in my blood.
0:16:21 > 0:16:26Now it's time to be sealed inside the chamber for the next 48 hours.
0:16:28 > 0:16:32I'm as ready as I'll ever be, guys, so can we open this door?
0:16:32 > 0:16:34Wish me luck.
0:16:38 > 0:16:40It's small, isn't it?
0:16:45 > 0:16:49Oxygen levels in the air are normally 21%.
0:16:49 > 0:16:50BEEPING
0:16:50 > 0:16:53Inside the chamber, they're far lower.
0:16:53 > 0:16:55Just over 12%.
0:16:56 > 0:16:58At these concentrations,
0:16:58 > 0:17:01the cellular activity in my body and brain
0:17:01 > 0:17:03- is starting to slow down. - Three, two, one...
0:17:03 > 0:17:06- Go!- Green, yellow...
0:17:06 > 0:17:10red, green, ye... Kind of orange...
0:17:10 > 0:17:13Er... purple, blue.
0:17:13 > 0:17:16You'll find that thinking becomes a little bit slower.
0:17:16 > 0:17:21My hand-to-eye coordination is being impaired.
0:17:22 > 0:17:26You can put them in any order you like. That's the way.
0:17:26 > 0:17:28Can you just tell us how exactly are you feeling?
0:17:28 > 0:17:34It's funny. I felt very slow. That slowness is there, definitely.
0:17:37 > 0:17:40The doctors calculate that at the rate I use up oxygen,
0:17:40 > 0:17:46if it carried on like this, I'd be unconscious in just 24 hours.
0:17:46 > 0:17:50Your oxygen saturation, sort of 88%.
0:17:50 > 0:17:52If that was your level in hospital,
0:17:52 > 0:17:54we'd be pretty worried about you right now.
0:17:55 > 0:18:00The next crucial step is to see if the 300 plants in here with me
0:18:00 > 0:18:03can produce enough oxygen to keep me alive.
0:18:05 > 0:18:08It's all to do with the wondrous ability they inherited
0:18:08 > 0:18:10from those green bacteria.
0:18:10 > 0:18:13It's photosynthesis, of course.
0:18:13 > 0:18:15I think we can have the lights on, please.
0:18:17 > 0:18:20To kick start it, you need light.
0:18:21 > 0:18:24Wow! Suddenly the light's hit.
0:18:30 > 0:18:33Plants use photosynthesis to live and grow,
0:18:33 > 0:18:35and most importantly for me...
0:18:36 > 0:18:38..to make oxygen.
0:18:39 > 0:18:45Photosynthesis is an intricate process that science is still trying to unlock.
0:18:47 > 0:18:52But the production of oxygen is one of its key features.
0:18:52 > 0:18:54To understand what's happening,
0:18:54 > 0:18:59you need to enter a complex and microscopic world.
0:19:01 > 0:19:05Inside every leaf of every plant on the planet
0:19:05 > 0:19:09are the direct descendants of those first green bacteria.
0:19:09 > 0:19:13Magnify a leaf 1,000 times and you can see them.
0:19:13 > 0:19:15They're known as chloroplasts.
0:19:15 > 0:19:18Packed into every cell.
0:19:21 > 0:19:23They still behave a bit like bacteria.
0:19:23 > 0:19:28This is real footage of them moving towards a flash of light.
0:19:33 > 0:19:37They're just 5,000ths of a millimetre across.
0:19:37 > 0:19:41And it's inside chloroplasts that photosynthesis happens.
0:19:45 > 0:19:48Light rays from the sun are made of photons.
0:19:49 > 0:19:53They're tiny, fast-moving particles of electromagnetic energy.
0:19:55 > 0:20:00When they hit the surface, the energ of the photons is captured by a ring
0:20:00 > 0:20:03called the light-harvesting complex.
0:20:07 > 0:20:08Inside this structure,
0:20:08 > 0:20:12the energy of two photons is used to split a water molecule.
0:20:17 > 0:20:23It's ripped into its two elements - hydrogen and oxygen.
0:20:26 > 0:20:29The plant uses the hydrogen to live and grow.
0:20:31 > 0:20:35But right now, I'm interested in the other part of the water.
0:20:35 > 0:20:38The part plants pump out as a waste product -
0:20:38 > 0:20:40the oxygen.
0:20:53 > 0:20:57Scientists have calculated that the 300 plants in here with me
0:20:57 > 0:21:00should raise oxygen levels in this chamber
0:21:00 > 0:21:04from 12 to 21% within 48 hours.
0:21:07 > 0:21:12I'm finding out how reliable the process of photosynthesis really is.
0:21:15 > 0:21:17It is quite concerning.
0:21:17 > 0:21:21You've been very busy this afternoon, a lot of activity,
0:21:21 > 0:21:23so we need to restrict the amount that you're talking
0:21:23 > 0:21:26and really get you resting as much as possible,
0:21:26 > 0:21:28not dashing around the chamber any more.
0:21:28 > 0:21:31This is doctor's orders - bed rest.
0:21:31 > 0:21:32Night-night!
0:21:38 > 0:21:40As I drift off to sleep,
0:21:40 > 0:21:43the 11,000 leaves go to work.
0:21:46 > 0:21:49They have 30 cubic metres of the box to fill.
0:21:51 > 0:21:55Some plants, like this maize, and the banana plant
0:21:55 > 0:21:59are particularly efficient at pumping out oxygen.
0:22:04 > 0:22:07So we can see the increase every hour here.
0:22:07 > 0:22:08That's incredible really.
0:22:08 > 0:22:11They're really pushing out a lot of oxygen, as you can see.
0:22:15 > 0:22:21Every hour, my plants are producing over 40 litres of oxygen.
0:22:21 > 0:22:24Hi, Iain. It's Katrina. We're now 41 hours in.
0:22:24 > 0:22:26Really? 41 hours in?
0:22:26 > 0:22:29Yeah. The oxygen levels are still climbing gradually every hour
0:22:29 > 0:22:31so it's going really well.
0:22:31 > 0:22:34With my vital signs returning to normal,
0:22:34 > 0:22:37I'm now a top attraction at the Eden Project.
0:22:37 > 0:22:39- Hello!- Hello!
0:22:39 > 0:22:41- Can you see him? - What's he doing in that box?
0:22:41 > 0:22:43He looks very happy in there.
0:22:43 > 0:22:46- What's that on his finger? - It's measuring his oxygen levels.
0:22:46 > 0:22:48Eat your heart out, David Blaine!
0:22:51 > 0:22:54Finally after 48 long hours,
0:22:54 > 0:22:57oxygen levels are almost back to normal.
0:22:57 > 0:23:00The plants have triumphed.
0:23:05 > 0:23:07Wa-hay!
0:23:07 > 0:23:09Oh, I'm out!
0:23:09 > 0:23:12Ah! Survived it. Fantastic!
0:23:12 > 0:23:17It's amazing just thinking that I've survived, but actually...
0:23:17 > 0:23:20I guess I've really survived because of them, cos of the plants.
0:23:20 > 0:23:23I leave here thinking that I needed those plants
0:23:23 > 0:23:25way more than they needed me.
0:23:25 > 0:23:28It's easy to think of this as just an experiment
0:23:28 > 0:23:30but to me, when you're lying in there,
0:23:30 > 0:23:33you realise this place is a metaphor for something much bigger,
0:23:33 > 0:23:34for the planet, really,
0:23:34 > 0:23:37and for our relationship with plants
0:23:37 > 0:23:40through photosynthesis to keep life going.
0:23:42 > 0:23:45The early Earth was like my chamber.
0:23:45 > 0:23:49It was transformed from a world with very little oxygen
0:23:49 > 0:23:51to a world rich in oxygen.
0:23:51 > 0:23:55And all that oxygen began to do something else.
0:23:56 > 0:24:00High in the stratosphere, it created ozone.
0:24:03 > 0:24:06This was a protective blanket which enveloped the Earth
0:24:06 > 0:24:10and blocked most of the sun's dangerous UV rays.
0:24:11 > 0:24:15It meant that for the first time in the planet's history,
0:24:15 > 0:24:18plants could move on to the land.
0:24:18 > 0:24:21But it was no small step.
0:24:32 > 0:24:35If you'd been protected by water for billions of years
0:24:35 > 0:24:39then the move to the land was going to be a rude shock.
0:24:39 > 0:24:44Yet over 400 million years ago, plants finally made that leap.
0:24:47 > 0:24:50Surprisingly, the best evidence for these pioneers
0:24:50 > 0:24:54doesn't come from some exotic corner of our planet,
0:24:54 > 0:24:56but from Britain.
0:24:59 > 0:25:03I've come to just outside the villag of Rhynie in northeast Scotland
0:25:03 > 0:25:06to see this - a stone wall.
0:25:06 > 0:25:08But not just any stone wall, of course.
0:25:08 > 0:25:13For me, this is the most important stone wall in the history of science
0:25:20 > 0:25:27Back 410 million years ago, Scotland was located well south of the equato
0:25:27 > 0:25:30and looked like another world.
0:25:33 > 0:25:38Hot springs and geysers boiled out across a rocky and barren landscape.
0:25:43 > 0:25:48But something else was happening, as scientists discovered
0:25:48 > 0:25:51when they came across some curious markings in this wall.
0:25:51 > 0:25:53This is one of them. Look at this.
0:25:53 > 0:25:57You see these really strange elongated shapes here.
0:25:57 > 0:26:00The first people just didn't really know what they were.
0:26:00 > 0:26:03They thought maybe, at first, it was some kind of lava
0:26:03 > 0:26:05but when they looked really closely,
0:26:05 > 0:26:09especially when they got it cut and polished, this rock literally came alive.
0:26:15 > 0:26:19Because you can see these dark features here,
0:26:19 > 0:26:23they realised that this was something that was once living.
0:26:25 > 0:26:29And when they were alive, this is what they looked like.
0:26:32 > 0:26:36Just a few centimetres tall, they're called Aglaophyton.
0:26:38 > 0:26:41Bulbous shapes on the end of naked stems.
0:26:44 > 0:26:46A time before leaves or roots,
0:26:46 > 0:26:52yet somehow these bizarre life forms survived along the water's edge.
0:26:53 > 0:26:56What geologists had found right here in Scotland
0:26:56 > 0:26:58were some of the earliest pioneering plants
0:26:58 > 0:27:02to make that giant leap, to colonise the land.
0:27:06 > 0:27:11And around this time, all along the margins of lakes and rivers,
0:27:11 > 0:27:13primitive plants were coming ashore.
0:27:17 > 0:27:20For the first time when viewed from space,
0:27:20 > 0:27:23the land began to look alive.
0:27:23 > 0:27:28The beginning of a transformation from hostile world to fertile Earth.
0:27:32 > 0:27:34Yet this wasn't a full-scale invasion.
0:27:34 > 0:27:37Just a toehold.
0:27:37 > 0:27:40Plants were still tied to the water's edge,
0:27:40 > 0:27:46unable to head inland and penetrate the harsh, rocky surface.
0:27:47 > 0:27:50But all this was about to change.
0:27:52 > 0:27:55Plants evolved an inspired solution to the problem,
0:27:55 > 0:27:58a brilliant device for collecting water and nutrients
0:27:58 > 0:28:01and something that they never really had before.
0:28:01 > 0:28:03Roots.
0:28:11 > 0:28:13Cambodia.
0:28:13 > 0:28:15The 12th-century temple here at Ta Prohm
0:28:15 > 0:28:18is a wonder of civilisation,
0:28:18 > 0:28:21but it's also a wonder of the natural world.
0:28:21 > 0:28:25Although the roots of these strangler figs are very different
0:28:25 > 0:28:27from the first ones to evolve,
0:28:27 > 0:28:33it's a superb place to reveal how roots allowed plants to invade inland.
0:28:36 > 0:28:40Roots are hugely powerful. I love this one.
0:28:40 > 0:28:42Look at it prising its way into that roof,
0:28:42 > 0:28:45just lifting that whole structure up.
0:28:45 > 0:28:46And then boring down here
0:28:46 > 0:28:49through these stone blocks and then disappearing.
0:28:49 > 0:28:55Just tiny pressures exerted over decades of centuries.
0:28:57 > 0:29:00Add these up and you get phenomenal strength.
0:29:00 > 0:29:03A pressure of up to 10kg per square cm.
0:29:05 > 0:29:09Around 400 million years ago, the first roots appeared...
0:29:10 > 0:29:16...and gave plants the ability to smash up the rocky planet.
0:29:17 > 0:29:21And this created a vital ingredient for life on land.
0:29:23 > 0:29:26When the tiny, broken-up fragments of rock
0:29:26 > 0:29:29get mixed up with dead plant material,
0:29:29 > 0:29:33it ends up as this ideal environment for storing water.
0:29:33 > 0:29:37An environment that we call soil.
0:29:39 > 0:29:43Today, soil covers 40% of the planet's land.
0:29:44 > 0:29:49It takes a long time to form, 1,000 years to make just 2cm of soil
0:29:49 > 0:29:54But it's essential for plant life, just as it was back then.
0:29:57 > 0:29:59Because the primitive leafless plant
0:29:59 > 0:30:02could now break free from the water's edge.
0:30:04 > 0:30:09Roots, and the soil they created, made plants unstoppable...
0:30:10 > 0:30:15...allowing them to colonise inland for the first time.
0:30:15 > 0:30:20An invasion that would have a dramatic influence on all life on Earth.
0:30:27 > 0:30:29For millions of years,
0:30:29 > 0:30:32animals had been confined to the rivers and oceans.
0:30:33 > 0:30:37Now they could finally emerge from the water.
0:30:39 > 0:30:42We get an idea of those first tentative steps
0:30:42 > 0:30:45by travelling back in time
0:30:45 > 0:30:49with a creature that's barely changed for 500 million years.
0:30:58 > 0:31:01I've come to the east coast of America,
0:31:01 > 0:31:05where these ancient creatures still come ashore at dusk to mate.
0:31:08 > 0:31:09Here they are.
0:31:09 > 0:31:11Horseshoe crabs.
0:31:13 > 0:31:16Looks like something from another planet.
0:31:17 > 0:31:19He sees with two main eyes here,
0:31:19 > 0:31:23but they've got something like ten eyes scattered across their body
0:31:23 > 0:31:27and the really weird bit is if you lift them up.
0:31:27 > 0:31:29Look at that.
0:31:29 > 0:31:31For a start, they've got five pairs of legs.
0:31:31 > 0:31:33Look - one, two, three, four, five,
0:31:33 > 0:31:35whereas normal crabs just have four.
0:31:35 > 0:31:38They're actually more related to the scorpion than to normal crabs.
0:31:38 > 0:31:39Look at that.
0:31:39 > 0:31:43But the really interesting bit is tucked under here.
0:31:43 > 0:31:45You get these things called book gills.
0:31:45 > 0:31:48Look at that there. It's like sheaves of a book.
0:31:48 > 0:31:51And that allows them to extract oxygen,
0:31:51 > 0:31:54not just from the water but also from the air.
0:31:54 > 0:31:56It's an amazing breathing apparatus.
0:31:58 > 0:31:59Better put her back now.
0:31:59 > 0:32:01Come on, dear. There you go.
0:32:07 > 0:32:09As long as they're kept moist,
0:32:09 > 0:32:11these lung-like gills enable the crabs
0:32:11 > 0:32:14to stay out of water for days at a time.
0:32:15 > 0:32:19Fossils show that horseshoe crabs appeared on land
0:32:19 > 0:32:20at least 400 million years ago.
0:32:21 > 0:32:24They are some of the first animals ever to come ashore.
0:32:31 > 0:32:34Amphibians and insects soon followed.
0:32:35 > 0:32:39Oxygen allowed them to move onto land.
0:32:39 > 0:32:42But something else was also enticing them.
0:32:43 > 0:32:47It's funny. Plants create oxygen as a waste product
0:32:47 > 0:32:51and it's that waste product that has transformed our atmosphere.
0:32:51 > 0:32:54But of course, the main reason that plants photosynthesise
0:32:54 > 0:32:55is to create sugars.
0:32:55 > 0:32:59Sugars that are vital for plants to live and to grow
0:32:59 > 0:33:03and also provide a source of food for all animals.
0:33:08 > 0:33:10Plants make this sugar from water...
0:33:12 > 0:33:14..carbon dioxide from the air...
0:33:16 > 0:33:17..and energy from the sun.
0:33:21 > 0:33:24And again, it all happens in those tiny chloroplasts.
0:33:27 > 0:33:32We've seen how light splits water into oxygen and hydrogen.
0:33:34 > 0:33:36The plant takes that hydrogen
0:33:36 > 0:33:40and combines it with carbon dioxide to make sugar.
0:33:43 > 0:33:48By exposing a plant to carbon dioxid tagged with a radioactive marker,
0:33:48 > 0:33:51you can see the sugar being created.
0:33:51 > 0:33:52For the first time,
0:33:52 > 0:33:56scientists have imaged its creation and movement through a plant.
0:33:56 > 0:33:58In this case - maize.
0:34:01 > 0:34:05As soon as carbon dioxide is sucked into the plant's cells,
0:34:05 > 0:34:07they begin to glow.
0:34:07 > 0:34:09This is the actual moment
0:34:09 > 0:34:14that photosynthesis turns the carbon dioxide into sugar.
0:34:16 > 0:34:20In just 15 minutes, the newly-formed sugar
0:34:20 > 0:34:22is sent to the roots for storage.
0:34:22 > 0:34:28The plant can then use this sugar to grow and thrive.
0:34:31 > 0:34:35That's why photosynthesis is nature' most astonishing achievement.
0:34:35 > 0:34:40The ability of plants to be powered by light from beyond our planet
0:34:40 > 0:34:42sets them apart from all other life.
0:34:42 > 0:34:46And that connection with that star, our sun,
0:34:46 > 0:34:50makes plants a foundation stone for all living things.
0:34:50 > 0:34:53It's just such a wonderful thought.
0:34:57 > 0:34:59400 million years ago,
0:34:59 > 0:35:03leafless plants were flourishing like never before.
0:35:06 > 0:35:10But a dramatic transformation of the atmosphere
0:35:10 > 0:35:13was about to throw plants into a global crisis.
0:35:14 > 0:35:16Not only would it change their shape
0:35:16 > 0:35:19it would change all life on our planet.
0:35:21 > 0:35:25MAORI CHANTING
0:35:38 > 0:35:41This is Lake Tarawera in New Zealand.
0:35:44 > 0:35:47This ancient landscape is home to a plant
0:35:47 > 0:35:52that 360 million years ago confronted that crisis.
0:35:52 > 0:35:54It came up with an inspired solution
0:35:57 > 0:36:00Looks like the land that time forgot, doesn't it?
0:36:00 > 0:36:01Just that strange mixture
0:36:01 > 0:36:04of different shapes of plants and trees.
0:36:04 > 0:36:06Really unfamiliar and alien.
0:36:06 > 0:36:08It's almost primeval.
0:36:12 > 0:36:15The early plants had become victims of their own success.
0:36:15 > 0:36:19They were gorging on so much carbon dioxide in the atmosphere
0:36:19 > 0:36:21that they were using it up.
0:36:22 > 0:36:25Levels plummeted by 90%.
0:36:27 > 0:36:31Without enough of this vital gas, plants began struggling.
0:36:31 > 0:36:36If they couldn't find a way to breathe in more carbon dioxide,
0:36:36 > 0:36:38they'd suffocate.
0:36:40 > 0:36:44The early plants, plants like these gorgeous ferns here,
0:36:44 > 0:36:47came up with a remarkable new structure.
0:36:47 > 0:36:52Large, flat surfaces that house within them a complex breathing apparatus.
0:36:52 > 0:36:55We call them leaves.
0:36:59 > 0:37:03Leaves were the answer to all plants' breathing problems.
0:37:04 > 0:37:09They massively increased their surface area by over a hundredfold,
0:37:09 > 0:37:12allowing them to absorb far more carbon dioxide.
0:37:15 > 0:37:17Now, for the first time,
0:37:17 > 0:37:23shade was cast by a beautiful and delicate canopy, like these Dicksonia.
0:37:24 > 0:37:26These ferns are incredible.
0:37:26 > 0:37:28They're like giant umbrellas.
0:37:31 > 0:37:34The key to this advanced breathing apparatus
0:37:34 > 0:37:37is on the underside of each fern leaf.
0:37:37 > 0:37:41They're microscopic holes called stomata.
0:37:43 > 0:37:46Filmed and actioned with an electron microscope,
0:37:46 > 0:37:48this is them opening and closing.
0:37:48 > 0:37:52Speeded up 140 times.
0:38:00 > 0:38:04There are thousands of stomata on every leaf on Earth.
0:38:06 > 0:38:08They allow a single fern
0:38:08 > 0:38:12to breathe in five litres of carbon dioxide a day.
0:38:21 > 0:38:24The evolution of leaves, rich in stomata,
0:38:24 > 0:38:27saved plants from suffocation.
0:38:28 > 0:38:31But leaves also allow plants to capture more light.
0:38:32 > 0:38:36This in turn fuelled fierce competition,
0:38:36 > 0:38:39each plant desperate for the sun's rays.
0:38:42 > 0:38:47This family squabble would lead to a new type of plant.
0:38:47 > 0:38:50One that would have surprising repercussions for the planet.
0:38:52 > 0:38:55How do we know?
0:38:55 > 0:38:58Well, it's all thanks to some rare evidence here
0:38:58 > 0:39:01in Nova Scotia in Canada.
0:39:03 > 0:39:07To reach it, you have to abseil to the bottom of this 30m cliff.
0:39:09 > 0:39:14Here, scientists discovered the remains of a mysterious world.
0:39:15 > 0:39:17You know, this is just the best way to see rocks.
0:39:17 > 0:39:20You really feel as if you're a time traveller,
0:39:20 > 0:39:23peeling back the layers of history one by one as you go down.
0:39:23 > 0:39:28These rocks are over 300 million years old.
0:39:29 > 0:39:33But it's what's locked inside the rocks at the base of this cliff
0:39:33 > 0:39:35that took scientists' breath away.
0:39:40 > 0:39:43These fossilised remains are just spectacular!
0:39:43 > 0:39:45I mean, look at the texture.
0:39:45 > 0:39:47You can tell it's a plant
0:39:47 > 0:39:50but this isn't some big shrub or overgrown fern.
0:39:50 > 0:39:54You can see here, look, you can get traces of bark.
0:39:54 > 0:39:57And down here, you can see there are some roots coming off.
0:39:57 > 0:40:00This is completely extinct, you don't get this any more,
0:40:00 > 0:40:02but what I'm actually crouching beside
0:40:02 > 0:40:05is one of the planet's very early tree trunks.
0:40:05 > 0:40:07And not just one tree, cos look here
0:40:07 > 0:40:08There's another one here.
0:40:10 > 0:40:11There's another there.
0:40:13 > 0:40:16This is a fossil forest.
0:40:18 > 0:40:23These Lepidodendron trees had strange diamond-shaped bark.
0:40:24 > 0:40:27Each diamond sprouting a needle-like leaf.
0:40:29 > 0:40:32Over 300 million years ago,
0:40:32 > 0:40:36they made up the planet's first tropical forests.
0:40:46 > 0:40:49Found in swamps throughout the Earth's tropics,
0:40:49 > 0:40:51these first forests were so extensiv
0:40:51 > 0:40:55you'd have seen a band of dark green from space.
0:40:57 > 0:41:01And all those new leaves were pumping out oxygen,
0:41:01 > 0:41:06so much that levels of oxygen increased
0:41:06 > 0:41:09to not far off double what they are today.
0:41:12 > 0:41:14It was having a very odd effect on animals...
0:41:16 > 0:41:19..in particular, insects and their cousins.
0:41:21 > 0:41:25Instead of lungs, invertebrates have simple breathing tubes
0:41:25 > 0:41:31that rely on diffusion for oxygen to reach their internal organs.
0:41:31 > 0:41:33The size of these animals is therefore limited
0:41:33 > 0:41:36by the concentration of oxygen in the air.
0:41:38 > 0:41:41Increase the oxygen, just as the first forest did,
0:41:41 > 0:41:44and things get interesting.
0:41:47 > 0:41:49Do you see these markings on the rock here?
0:41:49 > 0:41:53There's two lines of little dents, one here and one here.
0:41:53 > 0:41:55They are fossilised footprints
0:41:55 > 0:41:58that date back to the very early forests.
0:41:58 > 0:42:00When scientists first studied them
0:42:00 > 0:42:03they realised they weren't made by some reptile or amphibian.
0:42:03 > 0:42:05They were made by a millipede.
0:42:05 > 0:42:09Now, here is one of the biggest millipede species
0:42:09 > 0:42:12alive on Earth today.
0:42:12 > 0:42:13That's pretty big.
0:42:18 > 0:42:20Using the tracks for scale,
0:42:20 > 0:42:25it's clear the ancestors of this little fellow were massive.
0:42:32 > 0:42:37Called Arthropleuridea, it was over 2m long.
0:42:39 > 0:42:41The forests would have been terrifying.
0:42:42 > 0:42:46With giant scorpions and giant spiders.
0:42:50 > 0:42:53And not just on the land.
0:42:53 > 0:42:57I think the most impressive of all were the dragonflies.
0:42:57 > 0:43:01Most modern dragonflies have wing spans up to 10cm across,
0:43:01 > 0:43:04but back then they were way larger.
0:43:04 > 0:43:07Some were up to a metre across.
0:43:14 > 0:43:19These Meganeura were the largest insects ever to take to the skies.
0:43:22 > 0:43:26But in this oversize world pumped with oxygen,
0:43:26 > 0:43:29the plant kingdom still reigned supreme.
0:43:31 > 0:43:34Then, 230 million years ago,
0:43:34 > 0:43:38a new group of animals emerged from the shadows of the swampy forests.
0:43:38 > 0:43:42They would become the largest creatures to roam the Earth
0:43:42 > 0:43:46and they were ready to do battle with the kingdom of the plants.
0:43:46 > 0:43:49I'm talking, of course, of dinosaurs
0:43:49 > 0:43:50ROARING
0:43:52 > 0:43:56It's the meat-eaters that get all the press.
0:43:58 > 0:44:00But recent research has revealed
0:44:00 > 0:44:03that out of the 700 species discovered,
0:44:03 > 0:44:06over two-thirds were herbivores.
0:44:07 > 0:44:12Vegetarians ruled, led by the biggest herbivores in history...
0:44:13 > 0:44:15..the sauropods.
0:44:18 > 0:44:22To discover the impact of these huge dinosaurs on the plant kingdom,
0:44:22 > 0:44:26I've come to an animal sanctuary to see it in the flesh.
0:44:29 > 0:44:32Unfortunately this place doesn't have a living sauropod,
0:44:32 > 0:44:35but what it does have is the biggest herbivore
0:44:35 > 0:44:37that the planet's got to offer -
0:44:37 > 0:44:39the African elephant.
0:44:39 > 0:44:41Come and meet Butch.
0:44:42 > 0:44:46This beautiful four-tonne elephant can help me truly appreciate
0:44:46 > 0:44:48the staggering size of the dinosaurs
0:44:54 > 0:44:56Can we go up?
0:45:00 > 0:45:04Butch here is about as big as a big African bull gets
0:45:04 > 0:45:09and that's already four metres high,
0:45:09 > 0:45:13but if we want to get to the height of a sauropod, we have to go much higher.
0:45:13 > 0:45:17Six metres. We've got to be higher than that.
0:45:17 > 0:45:19We're now at eight metres.
0:45:19 > 0:45:20We've still got to go higher.
0:45:20 > 0:45:24Where are we at? Ten metres now? A bit higher than that.
0:45:24 > 0:45:27We're still not at the height of a sauropod yet.
0:45:27 > 0:45:29OK, we're getting there. Nearly.
0:45:30 > 0:45:33OK, fine.
0:45:33 > 0:45:34ELEPHANT ROARS
0:45:34 > 0:45:37So my head's now about 12 metres,
0:45:37 > 0:45:40which is about the height of a four-storey building
0:45:40 > 0:45:43and also the height of a sauropod.
0:45:43 > 0:45:47The thing is, on the end of a nine-metre neck,
0:45:47 > 0:45:50this is the skull of a sauropod.
0:45:50 > 0:45:53It seems quite small.
0:45:53 > 0:45:57But the point was that this had to b manoeuvrable and nimble
0:45:57 > 0:45:59to get right up at that high-level foliage.
0:46:01 > 0:46:06Sauropods were like nothing else the planet had ever seen.
0:46:06 > 0:46:09They weighed more than ten times an African elephant.
0:46:11 > 0:46:15Now, Butch here eats about 90kg of foliage every day,
0:46:15 > 0:46:17which is roughly about that much hay
0:46:17 > 0:46:19But scientists have estimated
0:46:19 > 0:46:24that sauropods ate about 1,500kg of hay every day.
0:46:24 > 0:46:27In other words, about 20 times that daily diet.
0:46:27 > 0:46:30Or 50 bales of hay.
0:46:30 > 0:46:34Now if you imagine you've got herds of about 30 sauropods,
0:46:34 > 0:46:37much bigger than these beasts here,
0:46:37 > 0:46:39and you realise that the plant kingdom
0:46:39 > 0:46:42was up against the ultimate salad predator.
0:46:44 > 0:46:48150 million years ago,
0:46:48 > 0:46:52dinosaurs were stripping the land of vast swathes of foliage.
0:46:52 > 0:46:57For the first time, the plant kingdo was under serious attack
0:46:57 > 0:46:59from another dynasty.
0:47:01 > 0:47:05To fight back, plants began to evolve a whole arsenal
0:47:05 > 0:47:08of defences for their precious leaves.
0:47:10 > 0:47:15Here in California, we can see just how intense this arms race was
0:47:15 > 0:47:18in one of the world's most unusual gardens.
0:47:18 > 0:47:24It's full of a group of bizarre and extremely rare plants called cycads
0:47:24 > 0:47:29but once, they made up a quarter of all plants on Earth.
0:47:34 > 0:47:35This is incredible.
0:47:36 > 0:47:40Exactly the kind of place you'd expect a dinosaur just to pop out.
0:47:43 > 0:47:46To stave off attack from those ravenous dinosaurs,
0:47:46 > 0:47:49cycads developed some clever lines of defence,
0:47:49 > 0:47:52the most obvious being physical weapons
0:47:52 > 0:47:57like needles and spikes and... Agh! These are vicious.
0:47:57 > 0:48:01The main point was to make leaves as painful as possible to eat.
0:48:08 > 0:48:12These defences came in all shapes and sizes.
0:48:15 > 0:48:20And some plants also spiced things up with chemical weapons.
0:48:25 > 0:48:27This is a Trapps Valley cycad from South Africa,
0:48:27 > 0:48:31but it's pretty typical in that the leaves contain a nerve agent
0:48:31 > 0:48:34that if you ingest it, it causes vomiting, diarrhoea,
0:48:34 > 0:48:35paralysis of the limbs
0:48:35 > 0:48:37and then, of course, death.
0:48:37 > 0:48:40Obviously I'm not going to eat one of the leaves,
0:48:40 > 0:48:42but I can eat a plant whose ancestors emerged
0:48:42 > 0:48:44around the time of the dinosaurs
0:48:44 > 0:48:47and who also have a chemical weapon and that is...
0:48:48 > 0:48:50..a chilli.
0:48:50 > 0:48:53n particular, a habaneros chilli...
0:48:54 > 0:48:57...which is supposed to be one of the most powerful in the world.
0:48:59 > 0:49:02There's a chemical in here called capsicum...
0:49:02 > 0:49:05that is contained in the fruit.
0:49:07 > 0:49:09And that essentially is a toxin.
0:49:10 > 0:49:12COUGHS AND LAUGHS
0:49:13 > 0:49:14Agh!
0:49:14 > 0:49:15EXHALES
0:49:17 > 0:49:20Which is, at this precise moment,
0:49:20 > 0:49:24burning and inflaming all of my mouth.
0:49:24 > 0:49:26Oh, my gosh!
0:49:30 > 0:49:33The thing is, the toxins in the cycads were...
0:49:33 > 0:49:37they were far more powerful even than the chillis.
0:49:37 > 0:49:38Oh, my gosh!
0:49:38 > 0:49:43So you can imagine what...the dinosaurs would have had to endure.
0:49:43 > 0:49:46Oh, my... I'm going to have to...
0:49:51 > 0:49:52Ah!
0:49:54 > 0:49:56Can't even say how sore that is!
0:49:59 > 0:50:01Mmmm.
0:50:03 > 0:50:05Eat a chilli, they said.
0:50:06 > 0:50:08It'll be funny, they said.
0:50:09 > 0:50:11You know what? Forget about cycads.
0:50:11 > 0:50:14That could have brought down a 70-tonne sauropod.
0:50:18 > 0:50:21And the arms race didn't stop there.
0:50:21 > 0:50:24Plants evolved a new tactic.
0:50:25 > 0:50:30Not so much a line of defence as a line of communication.
0:50:34 > 0:50:37We know that when some plants are attacked
0:50:37 > 0:50:42they activate a quick-acting toxin that deters herbivores.
0:50:43 > 0:50:47Now we're discovering that this defence goes even further,
0:50:47 > 0:50:50because plants can actually warn other plants
0:50:50 > 0:50:52that a herbivore is eating them.
0:50:56 > 0:51:00And at last, scientists here at Exeter University
0:51:00 > 0:51:04are beginning to listen in to this hidden conversation.
0:51:08 > 0:51:10They're finding that when plants are attacked,
0:51:10 > 0:51:14they also release an unseen gas from their leaves.
0:51:15 > 0:51:17What it does is extraordinary.
0:51:19 > 0:51:23And this will be the first time it's been captured on film
0:51:23 > 0:51:25using specialist imagery.
0:51:25 > 0:51:30These two Arabidopsis plants are being put inside a chamber.
0:51:32 > 0:51:35A third plant is then cut to mimic an attack.
0:51:41 > 0:51:44It's added to the undamaged plants.
0:51:48 > 0:51:50The chamber is sealed.
0:51:59 > 0:52:03The plant leaves are now releasing the gas.
0:52:05 > 0:52:09As they do so, their biological activity can be seen changing.
0:52:15 > 0:52:18Something remarkable happens.
0:52:18 > 0:52:22The gas triggers a change in the biological activity
0:52:22 > 0:52:23in the two neighbouring plants.
0:52:29 > 0:52:33They have detected the message warning them to protect themselves.
0:52:42 > 0:52:45Scientists don't know all the detail of this plant language,
0:52:45 > 0:52:50but increasingly they believe there's a chatter between plants all around us.
0:52:53 > 0:52:57I think most people assume that plants lead a rather passive life.
0:52:57 > 0:53:00That they're static and unresponsive
0:53:00 > 0:53:01That's just not true.
0:53:01 > 0:53:05In reality they move, they sense, they communicate.
0:53:05 > 0:53:08It's almost as if they show a kind of intelligence.
0:53:12 > 0:53:17For 200 million years, the dinosaurs and the plants
0:53:17 > 0:53:21were locked in a titanic evolutionary battle,
0:53:21 > 0:53:25each trying to gain the upper hand.
0:53:26 > 0:53:30But it was now that some plants played their trump card.
0:53:39 > 0:53:43They used wood to grow taller and taller.
0:53:45 > 0:53:50In California's Sierra Nevada, I'm about to find out just how tall.
0:53:53 > 0:53:57To do that, I need the help of biologist Jim Spickler.
0:54:00 > 0:54:02I'm as ready as I'll ever be.
0:54:02 > 0:54:05- Take your time. We've got time. - I was going to, absolutely.
0:54:12 > 0:54:16This is the grandest example of them all.
0:54:16 > 0:54:19The giant sequoia.
0:54:20 > 0:54:23What you see is just impossible for your mind to process.
0:54:23 > 0:54:26- The scale is so large. - It's extraordinary.
0:54:26 > 0:54:29I feel as if I'm in Lord of the Rings.
0:54:29 > 0:54:31GRUNTING
0:54:49 > 0:54:51What a great tree!
0:54:53 > 0:54:5670 million years ago,
0:54:56 > 0:55:01the ancestors of this type of tree, the conifers, got ever taller.
0:55:03 > 0:55:07This was the ultimate in plant construction.
0:55:08 > 0:55:10Conifers like the giant sequoias
0:55:10 > 0:55:13raised their precious leaves out of reach.
0:55:14 > 0:55:19The dinosaurs were no longer the biggest organisms on Earth.
0:55:19 > 0:55:22That title had been well and truly won back
0:55:22 > 0:55:25by these giants of the plant kingdom
0:55:28 > 0:55:30This is so tall, but I've still got...
0:55:30 > 0:55:33I don't know, another third to go.
0:55:33 > 0:55:36By using wood to grow really tall like this,
0:55:36 > 0:55:39it gave trees another advantage over plants
0:55:39 > 0:55:42because it allowed them first pick of the sun's strongest rays.
0:55:42 > 0:55:47The thing is, of course, for plants, light means success.
0:55:51 > 0:55:56If you had a satellite image of the dinosaur era 70 million years ago,
0:55:56 > 0:55:59you'd see the Earth like it had never been before
0:55:59 > 0:56:01and never would be again.
0:56:03 > 0:56:07The climate was so warm the poles had no ice.
0:56:07 > 0:56:10Instead they were covered with conifers -
0:56:10 > 0:56:12a vast polar forest.
0:56:14 > 0:56:18And the mighty sequoia trees were not just found
0:56:18 > 0:56:21in small areas of the Sierra Nevada, as they are today.
0:56:21 > 0:56:23They were global.
0:56:23 > 0:56:28Stretching along the Pacific coast and as far south as Australia.
0:56:30 > 0:56:34- How far are we from the top, then? - We're getting close.
0:56:34 > 0:56:36GRUNTING
0:56:37 > 0:56:39Ah! It's extraordinary.
0:56:44 > 0:56:47This is it. This is the top of the tree.
0:56:47 > 0:56:48Ooh!
0:56:49 > 0:56:51Unbelievable!
0:56:56 > 0:57:01It's staggering to think that using just a gas, carbon dioxide,
0:57:01 > 0:57:04and a liquid, water,
0:57:04 > 0:57:07together with light energy from beyond our world,
0:57:07 > 0:57:13you can construct a cathedral of wood 90 metres tall.
0:57:20 > 0:57:22Since they first appeared,
0:57:22 > 0:57:26plants and their ancestors have revolutionised our planet.
0:57:28 > 0:57:31They created oxygen for the atmosphere...
0:57:34 > 0:57:37..which would allow them to conquer the land
0:57:37 > 0:57:39and transform rock into soil,
0:57:39 > 0:57:44in turn fuelling the explosion of all life.
0:57:45 > 0:57:50From a barren alien planet, plants have made a living Earth.
0:57:50 > 0:57:55And left on its own, the world would have continued like this,
0:57:55 > 0:58:00dominated by large dinosaurs and endless forests.
0:58:00 > 0:58:02But 65 million years ago,
0:58:02 > 0:58:04something happened that would change everything.
0:58:04 > 0:58:07A chance event that would have dramatic consequences
0:58:07 > 0:58:09not just for plants but for all life
0:58:09 > 0:58:13And it would originate not on Earth, but in outer space.
0:58:13 > 0:58:15WHOOSH
0:58:15 > 0:58:17EXPLOSIONS
0:58:22 > 0:58:24The asteroid would kill off the dinosaurs.
0:58:24 > 0:58:27And the next chapter would see the triumph
0:58:27 > 0:58:31of a whole new group of plants...
0:58:32 > 0:58:36...flowers transformed the bond between animal and plant,
0:58:36 > 0:58:40even sculpting the very planet itself.
0:58:40 > 0:58:45Above all, plants would drive our human story.
0:58:45 > 0:58:49But all that was still to come.
0:59:09 > 0:59:13Subtitles by Red Bee Media Ltd