Earth: The Power of the Planet (2007) s01e05 Episode Script
Rare Earth
This is our planet, the Earth.
It's an amazing world, full of natural wonders, but there's far more to Earth than its natural beauty.
My name is Iain Stewart and in this series I'm exploring how our planet works.
How four great forces have combined to shape our remarkable planet.
Volcanoes, the atmosphere, ice and the oceans.
But now it's time to look at the bigger picture.
our planet is unique in the solar system, perhaps even in the universe, and that's no surprise when you consider the outrageous good fortune it's taken to create our extraordinary world.
What makes the Earth so special is the way that everything seems to come together to create the perfect conditions for life.
It's taken four and a half billion years to turn the Earth from a barren rock into the world we know today.
It's been an incredible journey of catastrophe and renewal.
But now this rare and remarkable planet is facing its greatest challenge, humankind.
The question is, will it survive? Ever since we humans have been able to scan the heavens, we've been looking for signs of intelligent life like us, up there in space.
This array of radio telescopes in America has been part of that search.
These telescopes can see deep into space.
But, so far, we haven't found anything.
Now it could be that we've not been looking for long enough or in the right part of space, but there is another possibility.
Perhaps we haven't found any signs of life because there's simply nothing out there.
It's hard to believe that we might be on our own.
After all, there are billions of stars in our own galaxy, the Milky Way.
And the Milky Way is just one of billions of galaxies.
You'd think there must be other Earths out there,just like ours.
Capable of supporting life just like us.
It's only a question of finding them.
But now, there's a new theory.
It suggests that planets like ours might be very rare.
It's called the Rare Earth Theory.
It says that simple life, like bacteria, the type of slime I've seen in hot volcanic pools, may be common in the universe.
But for complex life, like plants and animals to evolve, requires an extraordinary sequence of events.
And having travelled the world to make this series, what strikes me most is the incredible good fortune it's taken to make our planet a home for such rich and diverse life.
To get an idea of just how lucky Earth has been, let me take you back to what I reckon is the most important day in Earth's history.
This one day was responsible for much of what makes Earth so special.
And yet, it was just a chance event.
This is our planet just a few million years after it formed.
It's thought that back then Earth had a twin, Theia.
The two planets were born in similar orbits.
The result, a catastrophic collision.
Theia was obliterated, leaving a ring of debris orbiting the Earth.
This was a vital first step in creating the conditions for complex life.
That's because Earth consumed a part of its twin, and became a significantly larger planet.
The result was to increase the pull of Earth's gravity.
Without this, Earth's atmosphere might have gradually leaked into space.
Even an atmosphere needs gravity to hold it down.
on Mars, you can see what happens if a planet's gravity is weak.
Billions of years ago, Mars had an atmosphere similar to Earth's, so it had clouds, rain and rivers.
But Mars is smaller than Earth, and gradually its weak gravity allowed most of its atmosphere to leak away.
Today, it is a dead planet.
But because Earth consumed part of its twin, our planet had enough gravity to hold on to its atmosphere.
And it's this unique atmosphere that helps make Earth so special.
It keeps our planet warm.
It drives the climate.
And provides us with the oxygen we breathe.
But the atmosphere also gives us something else that's vital for life on Earth, protection.
I am in Arizona, heading for a place that shows just why it's so important for any planet to have an atmosphere to protect it.
This is a giant meteorite crater.
It was blasted out of the Earth's crust 50,000 years ago.
Well, there it is, Meteor Crater.
This place really gives you a sense of the destructive power of incoming meteorites.
The blast here would have vaporised a city larger than London, but the lump of rock that did it measured barely 50 metres across.
Down on the ground, the scale of the impact is even more breath-taking.
The crater is more than a kilometre across and nearly 200 metres deep.
The forces here were enormous.
The impact on this solid rock into this pulverised mush, it just bursts out in your hand.
I mean, look at that, they started out as the same kind of rock.
The impact exploded with a force a hundred times greater than the bomb that was dropped on Hiroshima.
Being hit by meteorites like this on a regular basis would be devastating.
But what makes this massive crater so significant isn't how big it is, but how rare it is.
To me, the really surprising thing is that events like this haven't happened more often.
It may not seem like it, but space is a hazardous place.
Up there, it's a shooting gallery where millions of objects are crossing Earth's path.
Cosmic shrapnel whizzing around at thousands of kilometres an hour.
other craters do exist.
This one is in Namibia.
Sometimes it takes specialist mapping techniques to reveal the presence of a crater, like this one in Brazil.
But even with this sophisticated equipment, less than 200 impact craters have been found across the entire planet.
Partly, this is because erosion continually rubs out the evidence of past impacts, but it's also because of the atmosphere's protective role.
To understand how something as intangible as thin air protects us from meteors, you need to find a meteorite that has survived its fall to Earth.
Meteorites are rare, so you need to go to great lengths to track one down.
Dr Phil Bland is a scientist on just such a mission.
I joined him on an expedition out onto the Nullarbor Plain in Western Australia.
The Nullarbor is vast, about the size of Great Britain.
But it's the ideal hunting ground for meteorites because it's as flat as a pancake.
And nothing has changed here for millions of years, allowing meteorites to accumulate undisturbed.
This is where the hard work begins.
The team are preparing to search the ground, scouring the desert like a police search team.
Phil, how easy is it to spot a meteorite? The whole plateau's limestone, so dark meteorites stand out pretty good against the pale limestone, and there's not too much vegetation, so you can see the ground pretty well.
So what am I looking for? Something metal and dark? BLAND: Yeah.
When they've been out here for a while, they rust a little bit.
They have chunks of metal in them, so you get a kind of a dark, rusty texture.
But after three hours of searching in temperatures above 45 degrees, we've not had much luck.
WOMAN: Hey, guys, I think I've got one here.
-That's a beauty.
Is that a meteorite? -Yeah, it's a lovely one.
This is incredible that it's something from outer space.
That's amazing.
What is amazing is that it actually makes it to the surface at all.
Something that size might have been part of an object the size of a washing machine or a small car at the top of the atmosphere, and as it punches into the atmosphere at high speed, the surface gets melted and stripped off and a new surface exposed and melted and stripped off, until you whittle down to something about that big.
But it's heavy, so that small car is a small car of rock weighing tons? -Yes.
-So, thin air does this to solid rock? -Yes.
-That's remarkable.
When meteors hit the atmosphere, they compress the air in front of them which generates intense heat.
Layer upon layer gets stripped away until most are completely burnt up.
We see this process in action when particles no larger than grains of sand burn up as they streak across the night sky.
Shooting stars.
Earth's early collision with its twin was also vital for our planet in many other ways.
For a start, Earth's core became larger.
And because it's made of iron, the churning of this huge metal core generates a powerful magnetic field.
It's possible to see this magnetic field at work.
This is the aurora.
You're watching the magnetic field deflecting the Sun's dangerous solar wind.
But the huge molten core contributes something else essential for life on Earth, a way of regulating the planet's temperature.
It's a remarkable system.
Hot rock heated by the core rises towards the Earth's surface.
As it spreads sideways, the Earth's crust is very slowly dragged apart.
This moves the continents, creating the Earth's restless and ever-changing surface.
But crucially for life, the core does more than just shape the planet.
Where the continents collide, it creates massive volcanic eruptions, which release carbon dioxide into the atmosphere.
Today, we think of carbon dioxide as a dangerous greenhouse gas that leads to global warming.
But throughout Earth's long history, carbon dioxide has played a vital role in keeping our planet the right temperature for complex life to survive.
And there was one final, rather special gift that the collision with Earth's twin bestowed upon our planet.
As the debris from the explosion orbited the Earth, it began to coalesce, until it formed a new planetary body.
The Moon.
It sits there just above the horizon like a vision of beauty.
Though, for the less romantically inclined, in reality it's a giant lump of rock devoid of air, water and life.
But whichever way you look at it, without it, we wouldn't even be here.
That is a precision instrument which we depend on for our entire existence.
Every other moon in our solar system is tiny in comparison to its parent planet.
But because of the way it formed, ours is unusually large and unusually close, and so it's made an unusually big difference to Earth's development.
As the Moon orbits Earth, its gravity pulls the water and the oceans towards it.
The constantly changing tidal zone this creates provided an evolutionary testing ground for Earth's early creatures.
But the tides are not the Moon's greatest contribution to life on Earth.
More importantly, it's the Moon that we have to thank for Earth's stable climate and regular seasons.
The Moon's gravitational pull prevents Earth from wobbling in space, which would cause climate chaos.
Without the Moon, Earth's temperature would regularly switch from baking hot to way below freezing.
Such wild climatic swings would have made the planet uninhabitable.
Yet there's something rather strange happening to the Moon that means we shouldn't take it for granted.
Meet Jerry Wiant.
Every morning, he rides up to the McDonald observatory in West Texas, and it's thanks to his work that we know something extraordinary about the Moon.
So, how many times do you make this journey? Every day for 37 years.
-37 years! -Yeah, on this bike.
-You're joking? -No, this one bike.
Jerry's dedication means that the work begun by the Apollo missions to the Moon over 30 years ago still continues to this day.
As a matter of fact, we're the last living Apollo project.
STEWART: And that's it up there, is it? WIANT: Yeah.
Jerry's daily routine is to fire a laser at the Moon in order to see how far it is from Earth.
So, how do you measure the distance to the Moon? Well, it really sounds complicated, but it really isn't.
We send light through our telescope and we aim it at the Moon and when it hits the Moon, it bounces back, and we time when the light left here and when the light comes back, that's the round trip time, and that's our data.
So how long does it take the light to get there and back? Two and a half seconds average time.
Two and a half seconds? That's really fast! It's very fast, yeah.
So what happens when that laser beam fires out your telescope? Well, most of the beam hits the surface of the Moon, but then a very small portion of our beam hits something like this.
This is a corner reflector, and this turns the light around and sends it back to us.
Jerry's target on the Moon is a panel of 1 00 of these corner reflectors put in position for his experiment by the Apollo astronauts.
STEWART: So, what does this work tell us about the Moon? Well, we've learned a lot of things about the Moon.
How the Moon wiggles around, top to bottom, left to right, and the fact that it's moving away from us.
-It's moving away? -It is, yeah.
-At about 3.
8 centimetres per year.
-So, nearly four centimetres a year? -That's the rate your fingernails grow.
-Yes.
-A few centimetres a year.
-It is.
So the Moon will not be with us forever.
Eventually, it will drift away into space and Earth will lose its climatic stability.
But there's no need to panic.
It'll take billions of years.
You know, you see the Moon in a different light when you realise just how essential it is for life on Earth.
Without it, this would be a very different planet, and yet if it wasn't for the most extraordinary cosmic fluke, we wouldn't have a moon at all.
Earth's chance collision with its twin was perhaps the critical moment in its history.
It ensured that the planet held on to its protective atmosphere.
It gave Earth a magnetic shield against the dangerous solar wind.
And it left us the Moon that provides climatic stability.
If the collision had never happened, Earth may never have been a home to complex life.
We've seen the extraordinary consequences of just one chance event on the Earth's destiny.
But there's another chance factor that was just as vital, its location.
It's only when you consider our planet's position in the solar system that you really begin to understand how fortunate it's been.
Too far from the Sun, like Mars, and it's simply too cold for life.
A little closer towards the Sun, and things improve.
This is where you find the Earth.
We are lucky enough to be in the only place in the solar system that's the right temperature for water, the most essential ingredient for life to exist on the surface.
Any closer to the Sun, like Venus, and it's far too hot.
Any water would boil away.
Look at the green zone.
This is the only place in the entire solar system that can support complex life, and Earth is the only planet there.
But it's not just Earth's proximity to the Sun that's important.
We're also lucky enough to orbit the right type of sun.
This is the actual surface of the Sun.
Every second, four million tons of the Sun is converted into pure energy.
It's been doing this for more than four and a half billion years.
And it's this long lifespan that makes our Sun so special.
Many other stars are bigger, but at a cost.
The larger the star is, the quicker it burns up and dies.
It took billions of years for complex life to get going on Earth, and it's our Sun's long life that made that possible.
And there's one final piece of good fortune that life on Earth depends on.
Jupiter.
of all the other planets in the solar system, Jupiter is by far the most important to Earth.
The reason is its enormous size.
Jupiter is more than three hundred times bigger than Earth, so it has a massive gravitational field.
This attracts dangerous flying objects that might otherwise hit Earth.
Jupiter is Earth's protector.
Scientists have even seen it in action.
In the early 1 990s, a comet was captured by Jupiter's powerful gravity.
It broke up into many different pieces, which smashed into the planet.
Some of the blasts left scars as big as Earth itself.
Without Jupiter, it's likely Earth would have been pummelled by huge impacts like this every 1 0,000 years or so.
Under that kind of bombardment, it's hard to imagine how complex life could ever have got going.
Even in a huge universe, you can begin to see why our planet might indeed be a rare Earth.
Any planet that's to be a home to complex life needs liquid water, a sun that doesn't burn out too quickly and a giant neighbour to protect it from meteorites.
Earth has benefited from all of these.
It's incredible when you think about it.
Our Earth formed in the right bit of space.
We've got the right kind of sun and a moon that gives us stability.
An environment that remains relatively stable is essential for complex life to survive.
But there's a curious twist.
Too much stability and evolution can get stuck in a bit of a rut.
Every now and then life needs a catastrophe to shake things up.
In the jungles of Mexico lies the evidence of one such catastrophe, one that turned out to be very important for us humans.
You know, everywhere you go around here, you find these holes.
They're called cenotes.
There's thousands of them, and most of them haven't even been explored.
But these are the right people to be with because these guys are cave explorers and they're going to show me a new world, one beneath our feet.
-It's quite deep, isn't it? -It's actually very deep.
Team leader Bernadette is the first to descend.
Okay, I'm down! Wow! Look at the size of this.
This is magnificent.
That's beautiful.
This cave may be stunning, but it provides the evidence for one of the greatest catastrophes in the Earth's history.
And that water, it's so clear.
So, is this what all the cenotes are like? No, some of them are completely flooded with water all the way to the ceiling, and there's some others, like this one, and the water level is slightly lower.
You see there on the back, that's the gateway to the underground river systems.
So, lower the gear, please.
There's actually much more to this amazing cavern than first meets the eye.
But to understand the scale of what happened here, you have to go deeper still, underwater.
Cave diving is notoriously risky.
It's all too easy to get trapped.
But Bernadette's a professional cave diver, so I'm in good hands.
Okay.
(SIGHING) I'm not sure if I'm ready for this.
I've got all the equipment, but there's something about going down into the water when you're not quite sure where your exit is.
But I trust Bernadette completely here! She knows what she's doing, so I'm as ready as I'll ever be.
-Ready? -All right.
Descending into the depths of the cenote is like entering a new world.
Fewer people have visited some of these drowned caverns than the surface of the Moon.
As divers have explored further, they've discovered the cenotes are actually part of a huge complex of tunnels and caves.
In fact, when you look from above, you can see there are cenotes scattered across hundreds of kilometres.
And when they're mapped, it becomes clear that they follow a distinctive circular course through the jungle.
They mark out the rim of a giant crater.
Scientific instruments show the structure of the underlying rock has been deformed, revealing the boundaries of a colossal meteorite impact crater.
This amazing cavern is part of a bigger story, a much bigger story.
Sixty-five million years ago, this was the site of one of the most catastrophic impacts in Earth's history.
What became known as the Chicxulub meteorite landed here and that triggered the extinction of the dinosaurs.
The meteorite was 1 5 kilometres across, enough to cause utter devastation across the whole planet.
It exploded with the force of one hundred million million tons of TNT.
The blast sent a giant plume of vaporised rock out into space.
A crater was punched 30 kilometres into the Earth's crust.
It was above this rim of weakened rock that the cenotes formed, millions of years later.
The blast would have been ferocious.
But it was what happened next that made the impact a global catastrophe.
The blast plume that shot into space fell back to Earth.
Billions of molten particles superheated the air to a temperature of hundreds of degrees.
Fires swept the planet, choking the atmosphere with soot and dust.
The dinosaurs and most other creatures were doomed.
But fortunately, underground there were some survivors.
It took a few million years, but eventually conditions on Earth returned to normal.
By wiping out the dinosaurs, the impact cleared the way for the rise of a new type of animal, the mammals and, ultimately, for us.
So from our point of view, this global disaster turned out to be no bad thing.
Catastrophes like the Chicxulub meteorite are disasters for the creatures that happen to be around at the time.
But because they stimulate evolution, they can actually be a blessing in disguise.
As I've seen in this series, the extinction of the dinosaurs wasn't the only time that a catastrophe kick-started a major evolutionary change.
In the Dolomites, I saw how the dinosaurs themselves benefited from a previous catastrophe.
This layer of black rock marks a time when over 90% of all life perished.
A disaster thought to be triggered by the collapse of the Earth's system of ocean currents.
When it was over, it was the dinosaurs that inherited the Earth.
In Australia, I also saw how an earlier catastrophe, a colossal ice age, ended the long era of the stromatolites.
The demise of these colonies of bacteria cleared the way for the first steps towards complex life.
Without this catastrophe to kick-start evolution, Earth might still be a home to nothing more than bacteria.
It wasn't until I made this series that I fully appreciated just how many chance events are needed to make a planet fit for complex life.
Earth has been blessed with just the right balance of stability and the occasional catastrophe.
Too many catastrophes and life never gets a chance to develop.
Too few, and life gets stuck in a rut.
I've always enjoyed the idea that, up in space, there must be lots of planets like Earth, many of them with complex intelligent life.
But when you consider what it's taken to make this place, you realise that planets like Earth could be very rare.
Perhaps we are alone in space on our own special planet.
It's taken four and a half billion years to turn the Earth from a barren rock into the world we see today.
But now the planet's facing perhaps its greatest challenge.
There's now a new force that's threatening this unique planet.
It's a force that's been steadily gathering strength for thousands of years, and that force is us.
Just look at Earth from space.
At night, our lights mark out a domination of the world.
By burning fossil fuels like coal and gas, we've changed the very composition of the atmosphere, and as a result, warmed the climate.
The human race now moves more rock and soil on the surface of the Earth than all of nature's processes put together.
In fact, our influence is now so great that scientists have declared that a new geological era has begun.
The Anthropocene, the human era.
The question is, are we now beginning to threaten the very conditions that have made our planet so special? A home to complex life.
A good place to find out is here on the island of Madagascar, off the coast of Africa.
It wasn't until just 2,000 years ago that humans first set foot on Madagascar.
Europe had been settled for 40,000 years by then, but this is a remote location stuck out in the Indian Ocean, isolated from Well, everywhere.
When those first settlers arrived, they must have been amazed by what they saw.
They'd landed in a kind of Garden of Eden.
85% of all life here is unique to this island.
Most of it is found in the forests Madagascar is famous for.
To me, the forest here symbolises so much about our planet.
It's incredibly complex, and yet everything seems to work together to maintain the health of the whole system.
And just like the whole planet, we're changing the forests here without fully understanding the consequences.
I've joined up with a science expedition to see for myself what's happening to Madagascar's forests.
FISHER: All right, guys, here we are! Dr Brian Fisher is cataloguing the life found in the forests to help understand how the delicate ecosystem is changing.
Well, this is it.
This is the end of the road for the vehicles.
We're heading off on foot now down there into the jungle.
Oh, looks like the porters are arriving.
Right, that's that one done.
This is fresh, is it? I mean, you can drink? FISHER: Yeah, you can drink this water, it comes right from the mountain.
Be careful of the rocks.
They're slippery.
Brian, it's humid, it's messy.
There's creepy-crawlies, bloodsuckers.
-Why here? -Look at this place.
You come in, it's like a cathedral of beauty because you see the trees, you see all the green things, but what you don't see To appreciate the real ecosystem, you've got to be on your hands and knees three millimetres above the ground, looking at life.
The thump, the power, the beauty of the forest is really the creepy-crawly things.
Yeah.
Brian's team are searching for new species that he's convinced lurk in the undergrowth.
He believes it's a particularly good area for ants.
FISHER: And they live in rotting wood.
They live in the ground, they live under stones.
We're just trying to peek into their homes, you know, get onto their level.
Check this out.
There are cocoons there, look at that.
Yellow cocoons, look at that.
Yes.
-I'm sure this is a new species.
-New species of ant? Yep, new species in genus Cerapachys.
Oh, look at them all! The whole nest structure is right here.
STEWART: Oh, yeah! Look at that.
FISHER: Oh! Look at them all.
So, you only find these ants in a place that's got a lot of biodiversity, it's unique.
Right.
You can't support this on a marginal habitat because they require, underneath it, a whole system of interactions to be happening.
Look at that one stinging me right now.
Look at that.
It's an encouraging find.
The fact that this part of the forest is teeming with insect life means that here, at least, it's still pretty healthy.
And by convention, whoever discovers a new species gets to give it a name.
In a very generous gesture, Brian chose to name the new ant after me.
Say hello to Cerapachys iainstewarti.
Bit of a mean-looking critter if you ask me.
The more we find out about these forests, the more we're discovering how vital they are to the well-being of the whole planet.
The jungle is a wonderful place.
You can really get a sense of how the trees are part of an intricate web and how the other plants and animals depend on them.
And that's not only true for Madagascar.
Trees like these are crucial for life right across the planet, and that's because jungles play a critical role in removing carbon dioxide from the atmosphere.
They're like the lungs of the Earth.
As they grow, trees absorb huge amounts of carbon dioxide from the atmosphere through their leaves.
The remarkable thing is that even as carbon dioxide levels in the atmosphere rise, trees respond by growing faster, so absorbing more carbon dioxide.
In fact, since humans started pumping greenhouse gases into the atmosphere, the world's jungles have absorbed around 25% of all the carbon dioxide we've produced, helping to reduce the effects of global warming.
So, healthy forests like those I've experienced in Madagascar play a vital role in regulating the planet's climate.
But the pristine natural beauty of this island isn't all that it seems.
This is one of the most famous places in Madagascar.
These giants are the baobab trees, the island's national symbol.
They're actually a symbol of something else as well.
The only reason you can see them is that all the other trees have been cut down.
The baobabs remain because to the local people, they're sacred.
Right across Madagascar, the forest is under attack.
Wherever you go, you catch the smell of wood smoke, the smell of farmers clearing the jungle because the local practice, known as tavi, uses fire to open up new land.
These days, there's only about a fifth of the island's original jungle left.
of course, deforestation is not a problem that's confined to Madagascar.
In Bolivia, this area of rainforest covering 50,000 square kilometres was turned into farmland and towns in just 20 years.
Each year across the planet, an area of forest about the size of Scotland is destroyed.
So not only are we increasing the levels of greenhouse gases in the atmosphere, we're also reducing the ability of the planet to deal with the problem.
It's just one example of the way that humans are disrupting systems that have taken millions of years to build up.
In this series I've already witnessed some of the changes this is causing.
As temperatures rise, glaciers and ice sheets are shrinking.
The result will be sea level rise which may threaten many of our cities.
In the oceans, rising carbon dioxide levels are beginning to acidify sea water.
Ultimately, this is expected to damage coral reefs and, with them, the complex food chain they support.
And in Siberia, I saw how, as temperatures rise, the permafrost is beginning to melt, releasing yet more greenhouse gases into the atmosphere.
It's hard to deny that human civilisation is having a profound effect on our planet.
Very few places remain untouched.
But what of the future? Are we now so powerful that we're beginning to damage the planet's ability to support complex life? or is human influence nothing like as great as we sometimes imagine? You only have to go back to Mexico, the place where the meteorite that wiped out the dinosaurs landed, to see our influence on the planet from a very different perspective.
Just inland from the ocean, here in Mexico's Yucatan Peninsula, the jungle stretches unbroken for hundreds of miles.
But every now and then, you stumble upon a strange mound of stones like this.
Some of them are enormous, the pyramids of a lost civilisation.
This is the pyramid of Uxmal.
It was built more than a thousand years ago by the Mayans, and was at the heart of a city of 50,000 people.
Today, little remains of that once-thriving culture.
There are lots of theories for why the Mayan civilisation disappeared.
Some say it was war, others that drought finished them off, but there's one thing for sure and that's they're not here now.
It's a reminder that no civilisation will last forever.
In the human imagination, the 1,500-year rise and fall of the Mayan Empire seems incredible.
But it's nothing compared to what's happened here over Earth's lifetime.
This area hasn't always been jungle.
At times it's been beneath the sea.
At other times, covered in ice.
And 65 million years ago, this is where the Chicxulub meteorite landed.
But even that hasn't left much trace.
Earth and life recovered, sometimes even benefited, from this and from every other major catastrophe.
It's this ability to deal with catastrophe that's a truly special thing about Earth.
Our planet is really tough, and there's nothing to suggest that that's going to change anytime soon.
In the long run, Earth can cope with anything we can throw at it.
We could clear all the jungles, but a jungle can re-grow over a few thousand years.
We could burn all Earth's fossil fuels, flooding the atmosphere with carbon dioxide, but even then it would take the planet only a million years or so for the atmosphere to recover.
Even the animals we're wiping out will eventually be replaced by others equally rich in diversity, as the relentless work of evolution continues.
It's only a question of time.
The Earth will be just fine.
That's not to say that the rapid changes we're forcing on Earth don't matter.
That's because humans operate on a different timescale.
We've evolved to live in the world as it is now.
So, in changing this world, we're altering the very environment that has allowed the human race to thrive.
We could be creating conditions that threaten the long-term survival of our civilisation.
So, all this stuff about saving planet Earth, well, that's not the problem.
Planet Earth doesn't need saving.
Earth is a great survivor.
It's not the planet we should be worrying about, it's us.
It's an amazing world, full of natural wonders, but there's far more to Earth than its natural beauty.
My name is Iain Stewart and in this series I'm exploring how our planet works.
How four great forces have combined to shape our remarkable planet.
Volcanoes, the atmosphere, ice and the oceans.
But now it's time to look at the bigger picture.
our planet is unique in the solar system, perhaps even in the universe, and that's no surprise when you consider the outrageous good fortune it's taken to create our extraordinary world.
What makes the Earth so special is the way that everything seems to come together to create the perfect conditions for life.
It's taken four and a half billion years to turn the Earth from a barren rock into the world we know today.
It's been an incredible journey of catastrophe and renewal.
But now this rare and remarkable planet is facing its greatest challenge, humankind.
The question is, will it survive? Ever since we humans have been able to scan the heavens, we've been looking for signs of intelligent life like us, up there in space.
This array of radio telescopes in America has been part of that search.
These telescopes can see deep into space.
But, so far, we haven't found anything.
Now it could be that we've not been looking for long enough or in the right part of space, but there is another possibility.
Perhaps we haven't found any signs of life because there's simply nothing out there.
It's hard to believe that we might be on our own.
After all, there are billions of stars in our own galaxy, the Milky Way.
And the Milky Way is just one of billions of galaxies.
You'd think there must be other Earths out there,just like ours.
Capable of supporting life just like us.
It's only a question of finding them.
But now, there's a new theory.
It suggests that planets like ours might be very rare.
It's called the Rare Earth Theory.
It says that simple life, like bacteria, the type of slime I've seen in hot volcanic pools, may be common in the universe.
But for complex life, like plants and animals to evolve, requires an extraordinary sequence of events.
And having travelled the world to make this series, what strikes me most is the incredible good fortune it's taken to make our planet a home for such rich and diverse life.
To get an idea of just how lucky Earth has been, let me take you back to what I reckon is the most important day in Earth's history.
This one day was responsible for much of what makes Earth so special.
And yet, it was just a chance event.
This is our planet just a few million years after it formed.
It's thought that back then Earth had a twin, Theia.
The two planets were born in similar orbits.
The result, a catastrophic collision.
Theia was obliterated, leaving a ring of debris orbiting the Earth.
This was a vital first step in creating the conditions for complex life.
That's because Earth consumed a part of its twin, and became a significantly larger planet.
The result was to increase the pull of Earth's gravity.
Without this, Earth's atmosphere might have gradually leaked into space.
Even an atmosphere needs gravity to hold it down.
on Mars, you can see what happens if a planet's gravity is weak.
Billions of years ago, Mars had an atmosphere similar to Earth's, so it had clouds, rain and rivers.
But Mars is smaller than Earth, and gradually its weak gravity allowed most of its atmosphere to leak away.
Today, it is a dead planet.
But because Earth consumed part of its twin, our planet had enough gravity to hold on to its atmosphere.
And it's this unique atmosphere that helps make Earth so special.
It keeps our planet warm.
It drives the climate.
And provides us with the oxygen we breathe.
But the atmosphere also gives us something else that's vital for life on Earth, protection.
I am in Arizona, heading for a place that shows just why it's so important for any planet to have an atmosphere to protect it.
This is a giant meteorite crater.
It was blasted out of the Earth's crust 50,000 years ago.
Well, there it is, Meteor Crater.
This place really gives you a sense of the destructive power of incoming meteorites.
The blast here would have vaporised a city larger than London, but the lump of rock that did it measured barely 50 metres across.
Down on the ground, the scale of the impact is even more breath-taking.
The crater is more than a kilometre across and nearly 200 metres deep.
The forces here were enormous.
The impact on this solid rock into this pulverised mush, it just bursts out in your hand.
I mean, look at that, they started out as the same kind of rock.
The impact exploded with a force a hundred times greater than the bomb that was dropped on Hiroshima.
Being hit by meteorites like this on a regular basis would be devastating.
But what makes this massive crater so significant isn't how big it is, but how rare it is.
To me, the really surprising thing is that events like this haven't happened more often.
It may not seem like it, but space is a hazardous place.
Up there, it's a shooting gallery where millions of objects are crossing Earth's path.
Cosmic shrapnel whizzing around at thousands of kilometres an hour.
other craters do exist.
This one is in Namibia.
Sometimes it takes specialist mapping techniques to reveal the presence of a crater, like this one in Brazil.
But even with this sophisticated equipment, less than 200 impact craters have been found across the entire planet.
Partly, this is because erosion continually rubs out the evidence of past impacts, but it's also because of the atmosphere's protective role.
To understand how something as intangible as thin air protects us from meteors, you need to find a meteorite that has survived its fall to Earth.
Meteorites are rare, so you need to go to great lengths to track one down.
Dr Phil Bland is a scientist on just such a mission.
I joined him on an expedition out onto the Nullarbor Plain in Western Australia.
The Nullarbor is vast, about the size of Great Britain.
But it's the ideal hunting ground for meteorites because it's as flat as a pancake.
And nothing has changed here for millions of years, allowing meteorites to accumulate undisturbed.
This is where the hard work begins.
The team are preparing to search the ground, scouring the desert like a police search team.
Phil, how easy is it to spot a meteorite? The whole plateau's limestone, so dark meteorites stand out pretty good against the pale limestone, and there's not too much vegetation, so you can see the ground pretty well.
So what am I looking for? Something metal and dark? BLAND: Yeah.
When they've been out here for a while, they rust a little bit.
They have chunks of metal in them, so you get a kind of a dark, rusty texture.
But after three hours of searching in temperatures above 45 degrees, we've not had much luck.
WOMAN: Hey, guys, I think I've got one here.
-That's a beauty.
Is that a meteorite? -Yeah, it's a lovely one.
This is incredible that it's something from outer space.
That's amazing.
What is amazing is that it actually makes it to the surface at all.
Something that size might have been part of an object the size of a washing machine or a small car at the top of the atmosphere, and as it punches into the atmosphere at high speed, the surface gets melted and stripped off and a new surface exposed and melted and stripped off, until you whittle down to something about that big.
But it's heavy, so that small car is a small car of rock weighing tons? -Yes.
-So, thin air does this to solid rock? -Yes.
-That's remarkable.
When meteors hit the atmosphere, they compress the air in front of them which generates intense heat.
Layer upon layer gets stripped away until most are completely burnt up.
We see this process in action when particles no larger than grains of sand burn up as they streak across the night sky.
Shooting stars.
Earth's early collision with its twin was also vital for our planet in many other ways.
For a start, Earth's core became larger.
And because it's made of iron, the churning of this huge metal core generates a powerful magnetic field.
It's possible to see this magnetic field at work.
This is the aurora.
You're watching the magnetic field deflecting the Sun's dangerous solar wind.
But the huge molten core contributes something else essential for life on Earth, a way of regulating the planet's temperature.
It's a remarkable system.
Hot rock heated by the core rises towards the Earth's surface.
As it spreads sideways, the Earth's crust is very slowly dragged apart.
This moves the continents, creating the Earth's restless and ever-changing surface.
But crucially for life, the core does more than just shape the planet.
Where the continents collide, it creates massive volcanic eruptions, which release carbon dioxide into the atmosphere.
Today, we think of carbon dioxide as a dangerous greenhouse gas that leads to global warming.
But throughout Earth's long history, carbon dioxide has played a vital role in keeping our planet the right temperature for complex life to survive.
And there was one final, rather special gift that the collision with Earth's twin bestowed upon our planet.
As the debris from the explosion orbited the Earth, it began to coalesce, until it formed a new planetary body.
The Moon.
It sits there just above the horizon like a vision of beauty.
Though, for the less romantically inclined, in reality it's a giant lump of rock devoid of air, water and life.
But whichever way you look at it, without it, we wouldn't even be here.
That is a precision instrument which we depend on for our entire existence.
Every other moon in our solar system is tiny in comparison to its parent planet.
But because of the way it formed, ours is unusually large and unusually close, and so it's made an unusually big difference to Earth's development.
As the Moon orbits Earth, its gravity pulls the water and the oceans towards it.
The constantly changing tidal zone this creates provided an evolutionary testing ground for Earth's early creatures.
But the tides are not the Moon's greatest contribution to life on Earth.
More importantly, it's the Moon that we have to thank for Earth's stable climate and regular seasons.
The Moon's gravitational pull prevents Earth from wobbling in space, which would cause climate chaos.
Without the Moon, Earth's temperature would regularly switch from baking hot to way below freezing.
Such wild climatic swings would have made the planet uninhabitable.
Yet there's something rather strange happening to the Moon that means we shouldn't take it for granted.
Meet Jerry Wiant.
Every morning, he rides up to the McDonald observatory in West Texas, and it's thanks to his work that we know something extraordinary about the Moon.
So, how many times do you make this journey? Every day for 37 years.
-37 years! -Yeah, on this bike.
-You're joking? -No, this one bike.
Jerry's dedication means that the work begun by the Apollo missions to the Moon over 30 years ago still continues to this day.
As a matter of fact, we're the last living Apollo project.
STEWART: And that's it up there, is it? WIANT: Yeah.
Jerry's daily routine is to fire a laser at the Moon in order to see how far it is from Earth.
So, how do you measure the distance to the Moon? Well, it really sounds complicated, but it really isn't.
We send light through our telescope and we aim it at the Moon and when it hits the Moon, it bounces back, and we time when the light left here and when the light comes back, that's the round trip time, and that's our data.
So how long does it take the light to get there and back? Two and a half seconds average time.
Two and a half seconds? That's really fast! It's very fast, yeah.
So what happens when that laser beam fires out your telescope? Well, most of the beam hits the surface of the Moon, but then a very small portion of our beam hits something like this.
This is a corner reflector, and this turns the light around and sends it back to us.
Jerry's target on the Moon is a panel of 1 00 of these corner reflectors put in position for his experiment by the Apollo astronauts.
STEWART: So, what does this work tell us about the Moon? Well, we've learned a lot of things about the Moon.
How the Moon wiggles around, top to bottom, left to right, and the fact that it's moving away from us.
-It's moving away? -It is, yeah.
-At about 3.
8 centimetres per year.
-So, nearly four centimetres a year? -That's the rate your fingernails grow.
-Yes.
-A few centimetres a year.
-It is.
So the Moon will not be with us forever.
Eventually, it will drift away into space and Earth will lose its climatic stability.
But there's no need to panic.
It'll take billions of years.
You know, you see the Moon in a different light when you realise just how essential it is for life on Earth.
Without it, this would be a very different planet, and yet if it wasn't for the most extraordinary cosmic fluke, we wouldn't have a moon at all.
Earth's chance collision with its twin was perhaps the critical moment in its history.
It ensured that the planet held on to its protective atmosphere.
It gave Earth a magnetic shield against the dangerous solar wind.
And it left us the Moon that provides climatic stability.
If the collision had never happened, Earth may never have been a home to complex life.
We've seen the extraordinary consequences of just one chance event on the Earth's destiny.
But there's another chance factor that was just as vital, its location.
It's only when you consider our planet's position in the solar system that you really begin to understand how fortunate it's been.
Too far from the Sun, like Mars, and it's simply too cold for life.
A little closer towards the Sun, and things improve.
This is where you find the Earth.
We are lucky enough to be in the only place in the solar system that's the right temperature for water, the most essential ingredient for life to exist on the surface.
Any closer to the Sun, like Venus, and it's far too hot.
Any water would boil away.
Look at the green zone.
This is the only place in the entire solar system that can support complex life, and Earth is the only planet there.
But it's not just Earth's proximity to the Sun that's important.
We're also lucky enough to orbit the right type of sun.
This is the actual surface of the Sun.
Every second, four million tons of the Sun is converted into pure energy.
It's been doing this for more than four and a half billion years.
And it's this long lifespan that makes our Sun so special.
Many other stars are bigger, but at a cost.
The larger the star is, the quicker it burns up and dies.
It took billions of years for complex life to get going on Earth, and it's our Sun's long life that made that possible.
And there's one final piece of good fortune that life on Earth depends on.
Jupiter.
of all the other planets in the solar system, Jupiter is by far the most important to Earth.
The reason is its enormous size.
Jupiter is more than three hundred times bigger than Earth, so it has a massive gravitational field.
This attracts dangerous flying objects that might otherwise hit Earth.
Jupiter is Earth's protector.
Scientists have even seen it in action.
In the early 1 990s, a comet was captured by Jupiter's powerful gravity.
It broke up into many different pieces, which smashed into the planet.
Some of the blasts left scars as big as Earth itself.
Without Jupiter, it's likely Earth would have been pummelled by huge impacts like this every 1 0,000 years or so.
Under that kind of bombardment, it's hard to imagine how complex life could ever have got going.
Even in a huge universe, you can begin to see why our planet might indeed be a rare Earth.
Any planet that's to be a home to complex life needs liquid water, a sun that doesn't burn out too quickly and a giant neighbour to protect it from meteorites.
Earth has benefited from all of these.
It's incredible when you think about it.
Our Earth formed in the right bit of space.
We've got the right kind of sun and a moon that gives us stability.
An environment that remains relatively stable is essential for complex life to survive.
But there's a curious twist.
Too much stability and evolution can get stuck in a bit of a rut.
Every now and then life needs a catastrophe to shake things up.
In the jungles of Mexico lies the evidence of one such catastrophe, one that turned out to be very important for us humans.
You know, everywhere you go around here, you find these holes.
They're called cenotes.
There's thousands of them, and most of them haven't even been explored.
But these are the right people to be with because these guys are cave explorers and they're going to show me a new world, one beneath our feet.
-It's quite deep, isn't it? -It's actually very deep.
Team leader Bernadette is the first to descend.
Okay, I'm down! Wow! Look at the size of this.
This is magnificent.
That's beautiful.
This cave may be stunning, but it provides the evidence for one of the greatest catastrophes in the Earth's history.
And that water, it's so clear.
So, is this what all the cenotes are like? No, some of them are completely flooded with water all the way to the ceiling, and there's some others, like this one, and the water level is slightly lower.
You see there on the back, that's the gateway to the underground river systems.
So, lower the gear, please.
There's actually much more to this amazing cavern than first meets the eye.
But to understand the scale of what happened here, you have to go deeper still, underwater.
Cave diving is notoriously risky.
It's all too easy to get trapped.
But Bernadette's a professional cave diver, so I'm in good hands.
Okay.
(SIGHING) I'm not sure if I'm ready for this.
I've got all the equipment, but there's something about going down into the water when you're not quite sure where your exit is.
But I trust Bernadette completely here! She knows what she's doing, so I'm as ready as I'll ever be.
-Ready? -All right.
Descending into the depths of the cenote is like entering a new world.
Fewer people have visited some of these drowned caverns than the surface of the Moon.
As divers have explored further, they've discovered the cenotes are actually part of a huge complex of tunnels and caves.
In fact, when you look from above, you can see there are cenotes scattered across hundreds of kilometres.
And when they're mapped, it becomes clear that they follow a distinctive circular course through the jungle.
They mark out the rim of a giant crater.
Scientific instruments show the structure of the underlying rock has been deformed, revealing the boundaries of a colossal meteorite impact crater.
This amazing cavern is part of a bigger story, a much bigger story.
Sixty-five million years ago, this was the site of one of the most catastrophic impacts in Earth's history.
What became known as the Chicxulub meteorite landed here and that triggered the extinction of the dinosaurs.
The meteorite was 1 5 kilometres across, enough to cause utter devastation across the whole planet.
It exploded with the force of one hundred million million tons of TNT.
The blast sent a giant plume of vaporised rock out into space.
A crater was punched 30 kilometres into the Earth's crust.
It was above this rim of weakened rock that the cenotes formed, millions of years later.
The blast would have been ferocious.
But it was what happened next that made the impact a global catastrophe.
The blast plume that shot into space fell back to Earth.
Billions of molten particles superheated the air to a temperature of hundreds of degrees.
Fires swept the planet, choking the atmosphere with soot and dust.
The dinosaurs and most other creatures were doomed.
But fortunately, underground there were some survivors.
It took a few million years, but eventually conditions on Earth returned to normal.
By wiping out the dinosaurs, the impact cleared the way for the rise of a new type of animal, the mammals and, ultimately, for us.
So from our point of view, this global disaster turned out to be no bad thing.
Catastrophes like the Chicxulub meteorite are disasters for the creatures that happen to be around at the time.
But because they stimulate evolution, they can actually be a blessing in disguise.
As I've seen in this series, the extinction of the dinosaurs wasn't the only time that a catastrophe kick-started a major evolutionary change.
In the Dolomites, I saw how the dinosaurs themselves benefited from a previous catastrophe.
This layer of black rock marks a time when over 90% of all life perished.
A disaster thought to be triggered by the collapse of the Earth's system of ocean currents.
When it was over, it was the dinosaurs that inherited the Earth.
In Australia, I also saw how an earlier catastrophe, a colossal ice age, ended the long era of the stromatolites.
The demise of these colonies of bacteria cleared the way for the first steps towards complex life.
Without this catastrophe to kick-start evolution, Earth might still be a home to nothing more than bacteria.
It wasn't until I made this series that I fully appreciated just how many chance events are needed to make a planet fit for complex life.
Earth has been blessed with just the right balance of stability and the occasional catastrophe.
Too many catastrophes and life never gets a chance to develop.
Too few, and life gets stuck in a rut.
I've always enjoyed the idea that, up in space, there must be lots of planets like Earth, many of them with complex intelligent life.
But when you consider what it's taken to make this place, you realise that planets like Earth could be very rare.
Perhaps we are alone in space on our own special planet.
It's taken four and a half billion years to turn the Earth from a barren rock into the world we see today.
But now the planet's facing perhaps its greatest challenge.
There's now a new force that's threatening this unique planet.
It's a force that's been steadily gathering strength for thousands of years, and that force is us.
Just look at Earth from space.
At night, our lights mark out a domination of the world.
By burning fossil fuels like coal and gas, we've changed the very composition of the atmosphere, and as a result, warmed the climate.
The human race now moves more rock and soil on the surface of the Earth than all of nature's processes put together.
In fact, our influence is now so great that scientists have declared that a new geological era has begun.
The Anthropocene, the human era.
The question is, are we now beginning to threaten the very conditions that have made our planet so special? A home to complex life.
A good place to find out is here on the island of Madagascar, off the coast of Africa.
It wasn't until just 2,000 years ago that humans first set foot on Madagascar.
Europe had been settled for 40,000 years by then, but this is a remote location stuck out in the Indian Ocean, isolated from Well, everywhere.
When those first settlers arrived, they must have been amazed by what they saw.
They'd landed in a kind of Garden of Eden.
85% of all life here is unique to this island.
Most of it is found in the forests Madagascar is famous for.
To me, the forest here symbolises so much about our planet.
It's incredibly complex, and yet everything seems to work together to maintain the health of the whole system.
And just like the whole planet, we're changing the forests here without fully understanding the consequences.
I've joined up with a science expedition to see for myself what's happening to Madagascar's forests.
FISHER: All right, guys, here we are! Dr Brian Fisher is cataloguing the life found in the forests to help understand how the delicate ecosystem is changing.
Well, this is it.
This is the end of the road for the vehicles.
We're heading off on foot now down there into the jungle.
Oh, looks like the porters are arriving.
Right, that's that one done.
This is fresh, is it? I mean, you can drink? FISHER: Yeah, you can drink this water, it comes right from the mountain.
Be careful of the rocks.
They're slippery.
Brian, it's humid, it's messy.
There's creepy-crawlies, bloodsuckers.
-Why here? -Look at this place.
You come in, it's like a cathedral of beauty because you see the trees, you see all the green things, but what you don't see To appreciate the real ecosystem, you've got to be on your hands and knees three millimetres above the ground, looking at life.
The thump, the power, the beauty of the forest is really the creepy-crawly things.
Yeah.
Brian's team are searching for new species that he's convinced lurk in the undergrowth.
He believes it's a particularly good area for ants.
FISHER: And they live in rotting wood.
They live in the ground, they live under stones.
We're just trying to peek into their homes, you know, get onto their level.
Check this out.
There are cocoons there, look at that.
Yellow cocoons, look at that.
Yes.
-I'm sure this is a new species.
-New species of ant? Yep, new species in genus Cerapachys.
Oh, look at them all! The whole nest structure is right here.
STEWART: Oh, yeah! Look at that.
FISHER: Oh! Look at them all.
So, you only find these ants in a place that's got a lot of biodiversity, it's unique.
Right.
You can't support this on a marginal habitat because they require, underneath it, a whole system of interactions to be happening.
Look at that one stinging me right now.
Look at that.
It's an encouraging find.
The fact that this part of the forest is teeming with insect life means that here, at least, it's still pretty healthy.
And by convention, whoever discovers a new species gets to give it a name.
In a very generous gesture, Brian chose to name the new ant after me.
Say hello to Cerapachys iainstewarti.
Bit of a mean-looking critter if you ask me.
The more we find out about these forests, the more we're discovering how vital they are to the well-being of the whole planet.
The jungle is a wonderful place.
You can really get a sense of how the trees are part of an intricate web and how the other plants and animals depend on them.
And that's not only true for Madagascar.
Trees like these are crucial for life right across the planet, and that's because jungles play a critical role in removing carbon dioxide from the atmosphere.
They're like the lungs of the Earth.
As they grow, trees absorb huge amounts of carbon dioxide from the atmosphere through their leaves.
The remarkable thing is that even as carbon dioxide levels in the atmosphere rise, trees respond by growing faster, so absorbing more carbon dioxide.
In fact, since humans started pumping greenhouse gases into the atmosphere, the world's jungles have absorbed around 25% of all the carbon dioxide we've produced, helping to reduce the effects of global warming.
So, healthy forests like those I've experienced in Madagascar play a vital role in regulating the planet's climate.
But the pristine natural beauty of this island isn't all that it seems.
This is one of the most famous places in Madagascar.
These giants are the baobab trees, the island's national symbol.
They're actually a symbol of something else as well.
The only reason you can see them is that all the other trees have been cut down.
The baobabs remain because to the local people, they're sacred.
Right across Madagascar, the forest is under attack.
Wherever you go, you catch the smell of wood smoke, the smell of farmers clearing the jungle because the local practice, known as tavi, uses fire to open up new land.
These days, there's only about a fifth of the island's original jungle left.
of course, deforestation is not a problem that's confined to Madagascar.
In Bolivia, this area of rainforest covering 50,000 square kilometres was turned into farmland and towns in just 20 years.
Each year across the planet, an area of forest about the size of Scotland is destroyed.
So not only are we increasing the levels of greenhouse gases in the atmosphere, we're also reducing the ability of the planet to deal with the problem.
It's just one example of the way that humans are disrupting systems that have taken millions of years to build up.
In this series I've already witnessed some of the changes this is causing.
As temperatures rise, glaciers and ice sheets are shrinking.
The result will be sea level rise which may threaten many of our cities.
In the oceans, rising carbon dioxide levels are beginning to acidify sea water.
Ultimately, this is expected to damage coral reefs and, with them, the complex food chain they support.
And in Siberia, I saw how, as temperatures rise, the permafrost is beginning to melt, releasing yet more greenhouse gases into the atmosphere.
It's hard to deny that human civilisation is having a profound effect on our planet.
Very few places remain untouched.
But what of the future? Are we now so powerful that we're beginning to damage the planet's ability to support complex life? or is human influence nothing like as great as we sometimes imagine? You only have to go back to Mexico, the place where the meteorite that wiped out the dinosaurs landed, to see our influence on the planet from a very different perspective.
Just inland from the ocean, here in Mexico's Yucatan Peninsula, the jungle stretches unbroken for hundreds of miles.
But every now and then, you stumble upon a strange mound of stones like this.
Some of them are enormous, the pyramids of a lost civilisation.
This is the pyramid of Uxmal.
It was built more than a thousand years ago by the Mayans, and was at the heart of a city of 50,000 people.
Today, little remains of that once-thriving culture.
There are lots of theories for why the Mayan civilisation disappeared.
Some say it was war, others that drought finished them off, but there's one thing for sure and that's they're not here now.
It's a reminder that no civilisation will last forever.
In the human imagination, the 1,500-year rise and fall of the Mayan Empire seems incredible.
But it's nothing compared to what's happened here over Earth's lifetime.
This area hasn't always been jungle.
At times it's been beneath the sea.
At other times, covered in ice.
And 65 million years ago, this is where the Chicxulub meteorite landed.
But even that hasn't left much trace.
Earth and life recovered, sometimes even benefited, from this and from every other major catastrophe.
It's this ability to deal with catastrophe that's a truly special thing about Earth.
Our planet is really tough, and there's nothing to suggest that that's going to change anytime soon.
In the long run, Earth can cope with anything we can throw at it.
We could clear all the jungles, but a jungle can re-grow over a few thousand years.
We could burn all Earth's fossil fuels, flooding the atmosphere with carbon dioxide, but even then it would take the planet only a million years or so for the atmosphere to recover.
Even the animals we're wiping out will eventually be replaced by others equally rich in diversity, as the relentless work of evolution continues.
It's only a question of time.
The Earth will be just fine.
That's not to say that the rapid changes we're forcing on Earth don't matter.
That's because humans operate on a different timescale.
We've evolved to live in the world as it is now.
So, in changing this world, we're altering the very environment that has allowed the human race to thrive.
We could be creating conditions that threaten the long-term survival of our civilisation.
So, all this stuff about saving planet Earth, well, that's not the problem.
Planet Earth doesn't need saving.
Earth is a great survivor.
It's not the planet we should be worrying about, it's us.