The Great Dying Survivors: Nature's Indestructible Creatures

Since its emergence more than three billion years ago,

life on our planet has suffered

a series of devastating mass extinction events.

These have killed off uncountable species

and almost threatened to end life on Earth.

I'm Professor Richard Fortey of London's Natural History Museum

and all my life I've studied the remains of animals long extinct.

'But now, I'm setting off to discover why some animals and plants have survived.'

Hello, snaky.

'I'm going in search of living fossils...'

My goodness. '..Old-timers...'

Look at this little face.

'..That somehow managed to survive when so many others perished.'

Wow.

Do you know what, I'm used to seeing these things as fossils.

'In the process,

'I hope to find an answer to one of the most profound questions of all.'

Is being a survivor a question of having some very special features?

Or nothing more than pure chance?

'From living fossils that are our most ancient relations...

'..to gigantic relics from the age of dinosaurs.'

Life as we see it today is not just the product

of the processes of evolution,

it is also a consequence of mass extinction.

We are all the sons and daughters of catastrophe.

It's an early evening in late May. The tide is high.

Tonight, there will be no moon.

'Here, together with local naturalist Glenn Gauvry,

'I'm about to witness an annual event

'unlike any other in the animal kingdom.

'And one I've been waiting to see all my life.'

You know, Glenn, for a palaeontologist to come here

is rather like a Muslim going to Mecca,

or a Catholic going to see the Pope.

It is a special place.

Actually, probably the most productive place in the world

to see what we're getting ready to see.

Can I, dare I, ask how many are going to be there?

-Oh, a lot more than you can imagine.

-Well, I can't wait to see them.

It's a perfect night and it's calm. It's going to be quite spectacular.

Richard, wait till you see this.

-Look over there.

-Heavens above.

-Isn't it amazing?

'In the darkness,

'low, shelly mounds, the size of inverted colanders, loom into view.

'The horseshoe crab, an ancient survivor

'whose ancestors were swimming in shallow seas

'even before the first life struggled onto land.'

Well, for once, you can use the overused word "primeval".

-And it's absolutely spot on.

-Yeah, it is here.

-Well, of course, they're here to do business, aren't they?

-Yup.

Mating.

Look how many males are around this female.

There's one female right down here.

So, she's bearing her eggs, the males are trying to fertilise them.

Yup, and they're probably all doing it at the same time,

probably all releasing sperm at the same time.

-So, we're really looking at an orgy, here.

-Yeah.

'This orgy is being repeated all along America's east coast,

'from here in Delaware to Florida.

'An ancient ritual that has been going on every year

'since long before the dinosaurs.

'The poor, exhausted females are amply over-provided with mates.'

We've got one female here and one female here

and the rest of those are all males around those two females.

'The moist sand stops their gills drying out.

'And they may, eventually, struggle back to the sea

'when the laying is done.

'Although many do not.'

The fossil record shows the horseshoe crab is a survivor

from the greatest extinction event in the history of our planet.

250 million years ago,

during the Permian Age,

the continents were combined into a single landmass.

Terrestrial climate was greatly affected

and tough, arid conditions became widespread.

'Animals that might have prospered separately

'now came into direct competition.

'As a result, many died out.

'But it was much worse than that.'

At the same time, there was a volcanic event

of unimaginable scale.

It was one of the greatest outpourings of lava

in Earth's history.

For half a million years, more than a million square kilometres

of the united supercontinent were buried under flow after flow.

These released vast quantities of ash and gases,

poisoning the seas and triggering massive climate change.

Incredibly, the horseshoe crab was a survivor from this -

the most lethal of all extinction events.

'The next day, we return to a scene of devastation for some

'and abundance for others.

'While dead crab carcasses litter the sandy beach,

'million upon millions of eggs

'provide a feast for the migrating birds.

'Tens of thousands of birds can't make much of an impression

'on millions of eggs.

'But ensuring the continuation of the species

'takes a heavy toll on individuals.'

-Well, Glenn, this looks like the day after the night before.

-Sure does.

Or a scene after a major battle, doesn't it?

With these carcasses lying here.

It's amazing how many are on the beach right now.

'The night has resulted in many casualties.

'The carcasses of those that failed to make it back to sea

'lie scattered all around.

'Others have hunkered down in wet sand.

'So long as they can keep their gills even moderately damp,

'they can survive until the next tide.

'This gives Glenn an opportunity to show me

'how horseshoe crab reproduction has been key to its survival

'for nearly half a billion years.'

-Is that a female there?

-It is. She's been buried down since the last tide.

-Would she mind being dug up, do you think?

-Probably not.

Look, she's got some nice spawning scars, there.

Let's pick her up.

-Here we go.

-Sorry, old girl. If I...

-Shall I hold this down?

-You want to lift up the operculum?

-If I keep up that...

If she's got eggs, you can sometimes encourage some of them to come out.

-There we go.

-They're like caviar, really, about the same size.

-I haven't tasted them myself.

-I haven't either. Other people have.

'Each female lays up to 20 clutches of several thousand eggs.

'Although only a few will even get to hatch, such a vast quantity

'ensures the horseshoe crab is amongst our great survivors.'

-And while we've got her here, we can see the other limbs here.

-Yup.

The legs, which seem exceedingly strong

and they can crush a clam if they need to.

Yeah, if it's not too large. She's pretty feisty.

Pretty feisty, this one.

-Her mouth right in the centre, right there.

-That's where the mouth is.

And they can eat almost anything that's got some nourishment in it?

They're opportunistic feeders. They love little clams and mussels

but if there's a dead fish, they'll go over and they'll eat that as well.

They're just not an aggressive animal,

so they can't chase after anything.

So, that lack of choosiness might also be a factor

in their survivability.

Well, I think being a generalist is a good strategy.

I mean, it's just like going to a restaurant.

If you can eat anything on the menu, you're going to go out full.

That's right!

I think we should give her a chance to get a bit of oxygen back in her.

-I agree.

-And there she goes.

-She was rather accommodating.

-Thank you, girl.

-Thank you. Thank you very much.

'Unfussy diet and a scattergun approach

'to reproduction are two good strategies for a species to survive.

'But even better is being virtually indestructible.'

Look, he's missing his tail.

And he's got this huge damaged area, right there.

-This is why they're so incredibly tough.

-It is.

And this must be one of the reasons for their durability

-and capacity to survive.

-It is.

Look at this one, up here. Look at the concaved area on the front.

Yeah, that one's really been, that's really been through the wars.

-Look how deep this concaved area is.

-And they can carry on carrying on.

-Yeah.

-So, durability, really, is the name of the game.

But they do have something to help them.

-Which is special blood.

-Their blood.

Their copper-based blood, which is blue.

Actually, it's a very pretty blue.

-It's almost the colour of your shirt when it hits the light.

-Yeah.

-It's a really nice blue.

-And that has the capacity to coagulate.

-So, they can kind of wall-off a wound.

-Yes.

-And keep on trucking.

Yup, they'll pretty much say, "You can have this part of me

"but you can't have that part of me."

'As well as its amazing power to coagulate,

'copper-based blood is also more efficient

'than ordinary iron-based blood in oxygen-poor environments.

'This would have been a life-saving quality

'when the atmosphere and oceans turned toxic

'during what's usually known as the Great Dying.'

250 million years ago, life almost died.

90% of the animals living in the oceans,

the poisoned oceans, became extinct.

Yet, somehow, the ancestors of the horseshoe crab negotiated

this terrible time.

They aren't actually found at the critical interval.

They're known before, they're known after.

They reappear like Lazarus from the dead.

For that reason, they're sometimes called Lazarus taxon.

The horseshoe crab wasn't the only species

to have seemingly returned from the dead

all those millions of years ago.

During the Great Dying, many other species also vanished.

They either became extinct, or dwindled to such small numbers

they left no fossil record we have ever found.

They were casualties of the massive volcanism

that afflicted our planet,

creating a lethal brew of toxicity and climate change

that made it almost uninhabitable.

Rocks preserve the evidence of ancient landscapes

and vanished lives.

'In special places around the world, we can still find evidence

'for living things far more ancient even than horseshoe crabs.

'Living things whose ancestors are even older

'than these ancient mountains

'once intruded as liquid rock into the depths of the Earth.'

They look almost like Henry Moore sculptures,

these fretted pieces of rock -

all that's left of a vast mass of granite

that might have stretched for many, many miles.

And, of course, once buried deep beneath the earth.

So, everything above, everything above has been eroded away.

'The Remarkable Rocks at the southern tip of South Australia

'are 500 million-year-old granites.

'One of the toughest rocks.'

Some of our survivors are as durable as this granite.

They have lasted for hundreds of millions of years,

but yet they are not entirely unchanged -

nothing in nature is ever entirely unchanged.

These rocks have been sculpted by natural forces.

And our organisms have continued to evolve in subtle ways,

while still retaining ancient features

that tell us of deep time and vanished worlds.

'In search of these ancient organisms

'and how they have changed over the ages,

'I take a journey backwards in time.

'Just a few miles away is one of the best fossil sites in the world.

'Emu Bay is owned by farmer Paul Buck.'

Here, just...

'My old friend Jim Jago has been coming here ever since the time

'of Paul's father, who discovered the fossils way back in the 1950s.'

It's places like this that provide us

with the geological evidence for our survivors.

Some of the animals living here have been extinct

for tens, if not hundreds, of millions of years.

But some of them still have living relatives.

Those are the ones we're going to investigate.

'Buried here are fossils of many of the organisms

'I hope to search out.

'As well as evidence of one that didn't make it.

'Discovered during his last dig just a few weeks earlier,

'is a fossil specimen of a creature

'I have spent most of my professional life studying.'

-Carefully pull this back.

-Wow! 'The trilobite.'

-That's enormous!

-We've measured it.

-It's about 24, 25 centimetres long.

-That's huge.

You know, we tend to think of trilobites

as little, tiny things in the middle of our hands,

but they grew to really quite a substantial size, didn't they?

Yes. This is about as big as they get here.

I've never seen one any bigger than this.

'Trilobites survived for hundreds of millions of years,

'evolved into all sorts of elaborate species.

'But then, were laid low before the Great Dying.'

'But although trilobites didn't make it,

'swimming alongside them were the aquatic ancestors

'of the scorpion.

'The largest species of sea scorpions grew as big as a man.

'Then, around 300 million years ago, some of their smaller cousins

'were among the earliest creatures to scurry onto land.

'By the time of the Great Dying, scorpions had adapted to deserts,

'making them perfectly suited to the arid conditions

'that spread over the newly-formed supercontinent

'250 million years ago.

'They've been adapting ever since.

'They can now make their homes almost anywhere.

'Including hotel rooms.'

The scorpion is another survivor

that passed through major extinction events,

probably because it lived in a habitat

where it didn't worry about arid conditions and changes in climate

and habitat that extinguished so many of its contemporaries.

They have their sting, of course, in the tail which they can arch over

to administer venom to unfortunate prey.

'Today, there are scorpions that can tolerate not just heat,

'but freezing temperatures as cold as -30, live to be 30 years old

'and go for 12 months without feeding.'

The scorpion is a real survivor and I intend to survive this encounter.

So, we're going to bring this take to a speedy conclusion.

Exit stage left! Ha-ha.

Whoops.

Even before relatives of the scorpions crawled onto land,

it was already clothed with plants.

It was not long before lush forests appeared -

the source of what became coal.

They endured for millions of years,

though the mighty trees that flourished

'in early, damp forests could not survive the increasing aridity

of the Earth's landmasses coming together into one supercontinent.

But, amazingly, some humble survivors

are relatives of these extinct giants,

and still lurk in secret corners of the world.

The Daintree Rainforest of northern Queensland is probably

the oldest tropical rainforest in the world.

A habitat that has remained largely unchanged

for tens of millions of years.

We're in Daintree, a very special tropical rainforest,

in which are hiding some plants that have been with us on the planet

for more than 250 million years.

It's a place of survivors, and we're going to try

and find some, living in their natural habitat.

'Daintree still hides relatives of some of the first plants

'to colonise the land.

'Today, the vast majority of all plants are vascular.

'That is, they use veins to transport nutrients

'towards the growing shoot.

'Among the first vascular plants were lycopods.

'Normally, they're ground-dwelling,

'but to find one, I venture under the forest canopy

'to meet a botanist with the rather appropriate name

'of Dr Ashley Field.'

-Ah, Ashley, how's it going?

-Good, you've made it in.

-I have.

Well, I've been walking through the bush looking for lycopods.

-For lycopods?

-And I can't find any.

Well, that's because your eyes are glued to the ground.

The lycopods here are way up above us in the canopy.

If you follow this tree up, you can see a clump of a plant

that's hanging down with branches dividing.

It's a soft, fern-like plant. That is a tropical lycopod.

That's an epiphytic Huperzia.

What's the right thing about the habitat up there that suits them?

They're high up in the trees because they need a lot of light

and there's three or four good host trees just in this small cluster.

-So, they don't occur everywhere.

-I'm looking forward to seeing one.

Well, what I'll do is I'll go up to this one, just up here,

and see if I can get a small sample to show you,

-because I work on these guys.

-That would be great. Good luck.

'Epiphytic plants live perched upon other plants.

'But the tiny plant Ashley pursues at such dizzying height

'once grew to great height itself.

'350 million years ago, lycopods were trees,

'the equal of any hereabouts.

'Now, they're a pale shadow of their former selves

'and live here, in the branches of other trees.

'Not as parasites, but as unobtrusive lodgers.

'Amazingly, they still maintain some of their unique features,

'even millions of years later and in their miniature version.'

-Well, welcome back.

-Oh, I hope it's in here somewhere.

Now, this is all I was after, it's a very, very small piece

for DNA work of an epiphytic Huperzia.

That one just up there.

What's the structure at the end, there?

The structure on the end is a strobilus and in this species

the strobilus is distinct from the other shape.

That's the spore body, isn't it?

The spores are produced in sporangia, which are little capsules

in under those leaves, there, and many hundreds of them,

thousands of them, are dispersed by the wind,

they blow around and, hopefully, land on the right position

on one of these trees so that they can grow.

-It's a sort of scattergun approach, really.

-It is.

I mean, spores,

it's an old solution to a problem of spreading yourself around.

-It may be old but it works.

-It certainly does.

So, we're talking about something that's pushing on beyond 400 million years.

-Yeah. Some of the oldest plants around.

-Isn't that incredible?

-It is.

Have you a theory why this sort of habitat should suit them?

The reason this particular place is good is the high rainfall,

high humidity, and it's been here for a long time.

That environment's been here long enough for species to establish and stay here.

And, presumably, it's a case of, "if it ain't broke, don't fix it."

These things have got a kind of morphology, life habit,

that can always find a niche somewhere

-and this is one of those special places.

-It certainly is.

'Their survival may be tied up with their reproduction.

'Releasing spores onto the wind is one of the oldest

'and most successful of all methods of reproducing.

'It has been repeated numerous times with numerous plant species

'ever since.

'But enduring by being smuggled through time

'on the back of another species is usually a survival strategy

'adopted by different types of organisms.

'Parasites.

'And it is one of the oldest

'and most persistent parasites that is my next survivor.

'Its home is the English countryside.

'But not on land. We're heading back in time

'and into water, where life began.

'It's a species currently being sought by Adam Hilliard

'and his team from the UK Environmental Agency.'

We come on a beautiful spring day to this beautiful,

clear, chalk stream in search of a very special old-timer.

A fish.

A fish that has survived for more than 400 million years.

'It's a creature that resembles an eel

'but is, in fact, a jawless fish.'

Well, this is rather an inelegant

but no doubt very effective way of fishing.

How does it work? What are you doing?

Basically, we're doing an electric fishing survey.

On our boat, there, we've got a generator which provides power

to the control box at the front of the boat, there.

And then the control box feeds the power out to the electrodes,

the anodes we've got here and the cathode at the back,

the positive and the negative.

The fish are attracted towards the anodes here.

We then net the fish out, put them into an aerated bin

and do that over a 100-metre stretch

and we can work out numbers of fish and weight of fish within that area.

-So, you're doing a kind of census?

-Effectively, yeah.

We look at the numbers of fish, the diversity of the fish.

We've caught five different species today.

Brown trout, bullhead, stickleback, grayling and, of course,

the brook lamprey as well.

This is one of the very few habitats in which it successfully lives.

-Yeah.

-Why do you think that is?

Well, the River Lambourn is a very clean water source.

It's got very good habitats for the lamprey's sort of

various life stages. The sort of larval stage where they live in silt,

and the adult stages where they spawn and produce the eggs

which the larval form come from.

So, this place, as well as being beautiful

is rather a special conservation area.

It is, yes. It's a special area of conservation.

-OK, well, thank you, Adam. I'd better let you get on with it.

-OK.

'Regrettably, although the purity of the water is as good

'as it ever was,

'brook lamprey numbers have declined in recent years

'as foreign invaders, like these North American crayfish,

'have colonised their habitat.

'But the lamprey burrow down here,

'their young living on microscopic algae.'

We're going to try and catch one of these slippery customers.

But it's not as easy as you might think. How are we doing, Adam?

Um, not so good at the moment.

-Ah, well done.

-There we go.

-Now, is it a grown-up?

-This is an adult brook lamprey, yes.

-Oh, excellent. Can I have a look?

-An adult brook lamprey.

-Oh, my goodness, it's about...

Can you sort of...?

Perhaps we'll have better luck next time. Let's have another go.

-There you go.

-There, oh, ah, yes. There we are.

It's a charming, little creature.

And I think this one's a little bit calmer. You can see its, kind of,

eel-like body, but can you see its little head is sucking onto my hand?

With a special adaptation at the mouth end.

It hasn't got a proper jaw at all but it has got little teeth.

This is a very harmless relative of a rather more sinister animal

that lives in bigger rivers and seas

that lives by being a parasite on other fish.

It rasps with its suckers into the flesh of the fish.

'The ancient ancestors of the brook lamprey possibly survived

'the toxic waters of the Great Dying by using the same survival tactic

'as modern sea lamprey.

'They use their jawless mouths to rasp onto other fish,

'burrowing into their flesh.

'As long as there were other fish to prey on,

'there would surely be lampreys.'

I can see it gasping for air, can you? Poor little thing.

I think I maybe ought to put it back into its natural medium, don't you?

And leave it to live another 400 million years, let's hope.

The animals and plants we've seen so far

prove the importance of one thing.

Habitat.

But as well as pure streams and rainforests,

you can find survivors, sometimes, in the most unexpected places.

'Hong Kong is one of the most densely-populated areas

'in the world.

'There are more skyscrapers here than any other place on Earth.

'And much of the city is built on land reclaimed from the sea.

'Yet, some truly ancient survivors are still to be found

'lurking in the waters off this cramped island.

'First is a sea creature related to the lamprey.

'It's commonly called the lancelet.

'And, believe it or not, it's one of our most distant ancestors.

'To find it, I join an old friend,

'Dr Paul Shin of Hong Kong University.

'Together, we must dare the seaworthiness

'of a local fishing boat.

'At the rear of the boat, which doubles as our host's home,

'the family has a Daoist shrine.

'They light traditional incense sticks to bless our journey.

'The South China Seas can be choppy and unpredictable.'

OK, lower.

'Finally, when we reach our destination,

'it's time to lower the steel jaws of the dredger

'to the sandy seafloor in the hope it will trap our quarry,

'the lancelet.'

So, the lancelet is another animal

that's survived for 500 million years.

-Exactly.

-In this special place.

What's special about this habitat, do you think?

So, the animal literally

gathers food from the surrounding waters and lives on that?

Yes.

And they grow and reproduce in this very special habitat.

'The sandy waters turn up a variety of different,

'as well as ancient, sealife.

'Here are tiny sea urchins,

'whose relatives survived from the age of dinosaurs.

'But after much frantic dredging and dedicated shifting

'and sieving, at last, one of us finds the elusive lancelet.

'We've struck survivor gold.'

It's an extraordinary thought that a creature so delicate,

so fragile, could last for such an inconceivably long period of time.

And yet, that's exactly what has happened with the lancelet.

Particularly when you compare it

with the big, Sherman tank-like horseshoe crab.

But, even more extraordinary, this little, tiny, fragile animal

is probably the nearest thing we have to our own, distant ancestor.

Here, Richard, you can see now, a very nice part of the lancelet.

Can you see?

Oh, yeah. That's very, very nice indeed.

'In search of what makes this tiny ancestor of ours

'such a determined survivor, we return to Paul's lab.'

This is the lancelet,

and it's been stained to reveal some of its internal structure.

And the head end is this end

and the most important feature is probably the notochord,

which is this little, tubular structure at the front end.

This is the feature that connects us

with all other chordates including vertebrates,

-like fish, mammals and other organisms.

-That's right.

And above the notochord is the nerve cord.

-And that runs all the way along the back.

-That's correct.

This is the same nerve cord that runs up our spines

and the spines of fish and other vertebrates.

And moving along the body of the animal... Ah, well, now.

-Here we've got the gonads.

-Yes.

Lots of them. I don't know how many pairs.

-20 pairs.

-20 pairs.

So no shortage of genetic material here, then.

-And both sides, too.

-And both sides, good Lord.

There we are, lots of gonads.

And then, at the back end, well, you can call it a tail, can't you?

-You can call it, yes, tail.

-There's the fin. The caudal fin.

-Yes.

And in the centre, some very obvious, rather strong muscles.

So, this is why the animal can swim and move about quite strongly

and propel itself and wiggle around.

-That's right.

-Very good.

'The lancelet has neither a skeleton nor much of a brain.

'But it does have the beginnings of a backbone.

'In 2008, its mitochondrial DNA was sequenced

'which confirmed, for once, what my biology master told me

'when I was still a youth.

'It's related to the vertebrates.

'It is like a vertebrate with almost everything subtracted.

'A half-sketched blueprint.

'Yet, without the ancestors of the lancelet,

'bony fish, reptiles, amphibians, birds, mammals

'and even us might never have come to exist.'

So, why did the lancelets survive?

As a filter feeder, its needs were simple,

but then many other simple organisms did not survive the Great Dying.

It must be something else.

And that something else was the survival of its habitat

in shallow seas.

Survival of habitat buoyed it through the time of crisis.

'The lancelet isn't the only survivor from deep time

'that still lives in Hong Kong.

'With Paul as my guide,

'we set off to hunt for another elusive sea creature.

'And our adventure begins with a monstrous warning.'

'The sandy flats surrounding Hong Kong's New Territories

'may be constantly shifting,

'but they're a remarkably persistent habitat.

'Many inter-tidal zones, like this one,

'have hardly changed in 250 million years,

'giving my next survivor, a shelled animal called lingula,

'a place to hide out for a very long time.

'Fortunately, Dr Shin's assistant Jessica is on hand

'with a sharp eye to help.'

Well, this is fabulous. This is, this is lingula.

This is a real, living fossil.

And let's see if I can get it out of the mud from its home.

Have I managed?

-Have I got its stalk?

-Yeah, long stalk.

-Well, that's great.

That really is great. OK, let's wash him off.

So we can have a look.

Do you know, I'm used to seeing these things

as fossils in rocks that are greater than 400 million years old.

You see, it's a simple kind of shellfish

although it's not a mollusc, not related to any kind of mollusc.

It's got a valve at the top and it's got this funny, fleshy stalk

hanging down below that anchors it in the mud

and allows it to pull itself down when the tide is out.

And when the tide comes in, it lifts itself up

and it opens its little valves at the top

and extracts edible particles and micro-organisms,

using a little contraption on the inside covered in tiny hairs.

This mode of life has allowed the animal

to continue happily onwards through many millions of years

while many other organisms have died out around it.

It's a very special thing to see, indeed.

'Many shell-bearing animals are molluscs,

'but, in fact, lingula belongs to a distinct group of animals

'called brachiopods,

'most of which did not make it through the Great Dying.

'On the outside, lingula doesn't offer us

'very many clues as to how it survived.

'But peering at the hinged shell, we can see the edible stalk

'which it uses to anchor itself in place in the sand.

'Also, the two valves that take seawater inside the animal,

'where tiny edible particles are removed by a kind of ribbon

'carrying cilia, called a lophophore.

'While little hairs around the valve edges

'prevent large, unwanted particles entering the feeding chamber,

'any edible material is eventually passed

'into a simple digestive system.

'Lingula almost certainly survived

'because it was a filter feeder whose habitat lay at the edge

'of the toxic world.

'It is also a simple but effective life form living in a habitat

'that has hardly changed in hundreds of millions of years.

'My last Hong Kong survivor is easy to find in the city's markets.

'It litters seafloors all over the world

'and market stalls all over China.'

My quest for survivors has brought me to some odd places,

perhaps none quite so strange as this fish stall in Hong Kong.

Well, I can see all sorts of interesting things

like dried puffer fish and I think they must be bladders,

swim bladders of some kind.

Oysters, perhaps smoked oysters, I'm not quite sure.

Bottled fish, dried shrimps - they look rather appetising -

but I can't quite see the thing I want.

Ah, that's more like it.

Sea cucumbers. Holothurians.

Refugees from the Ordovician.

Though, I must say, they don't look particularly appetising.

And I believe these have to be soaked for four days

before you can even begin to cook them.

So, maybe we should find a shortcut and go to a restaurant

and see what happens.

'Sea cucumbers survived the Great Dying.

'Their name suggests they are plants, but they are animals,

'distant relatives of sea urchins and starfish.

'An old Chinese proverb says,

'"In China, we eat everything with legs except the table."

'Sea cucumbers don't need legs.

'They use collagen to twist

'and contort their bodies into almost any shape

'and any sheltering crevice.

'A handy form of defence.

'Sightless, brainless, hermaphrodites.

'They're also scavengers, which may be the best strategy

'for getting through an extinction event.

'Especially one as severe as the Great Dying.

'But my current preoccupation is rather less academic.

'What do they taste like?'

I'm a little bit apprehensive about eating something

that, in the dried state, looks rather like a turd.

But here it is, properly prepared. So I'm going to give it a taste.

After 400 million years of existence.

And, um, well...

..rubbery hardly does it justice.

'It's a good job the chef knows what he's doing.

'Living sea cucumbers secrete a poison that can kill fish

'and cause blindness in humans.'

Yummy.

'Poison is a defence sea cucumbers share with creatures

'even further down the evolutionary tree of life.

'The sea sponges.

'But sea sponges don't use toxins for attack,

'they use them to make themselves inedible.

'But this is just one of an array of survival skills that make them

'some of the most ancient and toughest creatures on Earth.

'To learn from an expert, I join Dr John Hooper

'on the way to Magnetic Island on the Barrier Reef.

'Here, sea sponges are found in abundance,

'and some local specimens live to be hundreds of years old.

'John explains how their bewildering array of chemical powers

'make them among the most versatile animals in the world.'

I've got your favourite organism here, the sponge.

And also, of course, one of the great survivors

for our consideration because it's been around for,

well, the group has been around for 600 million years.

It's probably got all kinds of chemical defences

-hidden away in its tissues. Is that right?

-Yes, that's right.

There's two principal reasons why sponges survived.

One is they've got these cells called archaeocytes, they're totipotent,

which means they can change from one function to another, and then back again.

Very few other animals or any organisms can do that.

So, it gives them a plasticity of growth form.

So, if you put a sponge in a particular environment,

it'll adapt to that environment morphologically.

But probably the real survivorship is that, as you can see,

this thing doesn't have arms or legs or any spikes

or any way of removing predators or defending itself against parasites.

So, remarkably enough, they've evolved, sponges have evolved

an arsenal of biochemical compounds.

What, special poisons, toxins, defences?

You name it, they've done it. And they haven't just done it by themselves,

some chemicals are very much sponge-produced ones.

So, it's part of their metabolism. What do they eat?

They break down the waste products,

they re-use those waste products as a chemical arsenal.

Also, these things are called sponge hotels.

-So, they're full of bacteria.

-Yeah.

So, they have evolved defence mechanisms to harvest

those bacteria and re-use their chemicals and the waste products

in their own defences - it's called sequestering.

Or they pull in the toxins from the coral reefs above them

and they re-use those toxins and modify them.

-So, simple they may be, but complex also, they certainly are.

-Indeed.

'Sponges can filter five times their own body weight every hour.

'They can also grow in very oxygen-poor water,

'which has to be one of the reasons they survived

'the toxic seas of the Great Dying.'

'Another is also almost certainly

'the sponge's incredibly long life span.'

Now, how old is that specimen?

This specimen would be, probably, around about 100 years old.

It takes 100 years to make something the size of a large walnut?

One quarter of a millimetre per year growth rate.

This one here, which is a bit larger,

is probably a couple of hundred years old.

And we know of specimens from the Outer Great Barrier Reef approximately this big,

which would be thousands of years old.

Presumably they're capable of reproducing themselves

-during that long period?

-That's right.

So, a combination of a capacity to live an enormously long time

and to protect themselves from all sorts of attacks

probably adds up to a great survival strategy.

Well, the proof's in the evidence as you see in front of you.

'Sponges are even older than the eroded mountain tops

'of the 500 million-year-old Remarkable Rocks.

'But life itself is even older.

'Sponges may appear to confuse the dividing line between vegetable

'and animal, but my next survivor confuses the dividing line

'between animal and mineral.

'Tucked away on the remote western edge of Australia is Shark Bay.

'Here, in this natural time machine, endure colonies of species

'that have survived almost since life first began on our planet.

'These simple-looking objects are the magicians that transform

'the Earth into a place habitable by animals.'

They're stromatolites.

This one's just a few thousand years old.

That's its kind of lumpy, crusty appearance when fresh.

If you break it, you'll see it's full of fine layers, laminations,

rather like filo pastry, running parallel to the edge.

Each layer is built up by a thin film of living cells, bacteria.

'These particular bacteria utilise carbon dioxide

'to build up their laminations and exhale oxygen.

'Just as plants do today.

'This had the effect of releasing oxygen into the early atmosphere,

'transforming it into the air that we could all breathe.

'But it was a slow process.'

This one is no less than 3,500 million years old.

That's 3.5 billion years.

Inside, a section shows the same kind of fine layering.

It's probably the oldest organic structure anywhere on Earth.

If it hadn't been for these tiny cells, breathing out oxygen,

organisms like ourselves that need oxygen

would not be able to live today.

'Arguably, stromatolites are victims of their own success.

'By changing the composition of the atmosphere

'and making other forms of life possible,

'they eventually drove themselves to the margins.

'But persistent, marginal, amazingly inhospitable habitats

'are where I will find the very oldest of all survivors.'

This is a vision of a world 3.5 billion years ago.

Here, in hot water, live countless, tiny micro-organisms -

we call them archea and bacteria - that relish the heat.

Under these hot springs, delicate crusts and algal mats

and mats of bacteria combine together

to form ancient, living communities.

Still here, even after 3.5 billion years.

'Yellowstone is itself a unique habitat

'where survivors from a much later extinction event,

'the end of the Ice Age, still thrive.

'The bison are prepared for a sudden cold snap, but I'm not.'

Welcome to springtime in the Northern Rockies, Richard.

Well, you know, it's amazing to think that with all this snow

there are still little organisms out here flourishing in extreme heat.

Yes, it's not a question of surviving - they like it here,

they love it here.

You take them out of this environment,

they either stop metabolising or they just altogether die.

This is home sweet home.

'With my guide, the intrepid Dr Tim McDermott, we set off

'to seek out survivors that would not be out of place on an alien world.'

Look at the different colours in these pools.

Yeah, we have the yellows and the browns and the reds

and the turquoise and the blacks.

There's a bubbling hole, is that hydrogen sulphide coming out?

Hydrogen sulphide and carbon dioxide, both are coming up there.

And you can see the yellow and red round that hole there, the source,

that's elemental sulphur.

-So, pure sulphur.

-Yes.

-And then around that, a brown ring.

Yes, that's iron.

And what's happening is, hydrogen sulphide and the ferrous iron

are coming to the surface and the microbes are transforming

both of those chemicals to now form the same chemicals

that we can now see.

So, that's their food and the food leaves a coloured stain behind.

Precisely, this is how they make a living.

And then outside that, green, I see. That's from alga, yes?

Yes, it's a lone eukaryotic organism out here in Norris Geyser Basin.

Everything else is archea and bacteria.

So, just in this small area we can see five different organisms,

all leaving a different colour imprint on the ground.

-Precisely.

-Well, that's fantastic.

And snowy.

'These slippery walkways are certainly treacherous in the snow.

'One slip, and I'd be in for a scalding bath.

'But it's a curious fact

'that something that's effectively immortal can, at the same time,

'be so brittle and fragile.

'Like so many of our other survivors,

'these organisms still persist because their unique habitat does.

'A habitat that still replicates the conditions of our planet

'not so long after it was born.

'From the extremophiles of Yellowstone,

'with their unique, high-temperature home,

'to the strange plants of Daintree...

'and even the sea cucumbers of Hong Kong...' Hmm.

'..All animals and plants are adapted to their own habitat.

'If it persists,

'then they will usually survive, along with their home.'

We're used to thinking, perhaps,

of those really richly biodiverse places,

like rainforests and coral reefs, as needing special protection.

But other places, where species that have endured for millions

and millions of years still live on, are, perhaps, even more valuable

as windows seeing back deep into geological time,

telling us about our own history and the history of life.

I call these places time havens and I think they're worth protecting.

Well, Kevin, what survivors have we got here?

'Unglamorous these time havens and the species that live in them may be,

'but because of human interference, many are now under greater threat

'than at any time since the Great Dying.'

Well, that's a true, living fossil for you.

'Luckily, there are those amongst us

'determined to help our greatest survivors survive.'

-The second species that we have.

-Oh, what a giant.

This is the Chinese horseshoe crab. Be careful.

-This is also a lady.

-Fantastic.

And, oh, look, the spikes are almost getting me that time.

'Kevin Laurie is a retired Hong Kong policeman

'who shares my passion for horseshoe crabs.

'They were once common in Hong Kong's waters, but pollution,

'fishing and industrialisation have taken a heavy toll.'

My goodness. So, they take a long time to grow.

It's a very slow generation time.

'Kevin has joined Dr Paul Shin in trying to repopulate

'the dwindling numbers of horseshoe crabs

'found around the New Territories.

'Bred in captivity, when they're a year old,

'these tiny horseshoe crabs are released back into the wild.'

OK, these are the babies from the breeding programme.

I took them out this morning. These are yearlings.

Yearlings, sweet little creatures.

And they'll go and feed on the seagrass

and then mature further out to sea

and then come back and breed in 16 years' time.

16 years to become adults. 16 or 17 months all on this seagrass bit.

Well, let's put some more of these little creatures

back into the wild and help them to grow to adults.

Just lay them on the mudflats - and there's one moving off,

quite quickly.

They're beginning to bury themselves already.

Well, they may not do a great deal, these horseshoe crabs,

but what they do, they do extremely well.

Maybe that's one of the secrets of their survival.

'As I watched the next generation of crabs hide itself away in the mud

'of this time haven, I'm struck by a final poignant thought.

'Had the Great Dying never happened,

'life as we know it might be profoundly different,

'and human beings might never have evolved in the first place.

'So it is no exaggeration to say,

'we are all the sons and daughters of disaster.'

'In the next episode, I go in search of survivors

'from the most dramatic of all extinction events.

'The asteroid impact that wiped out the dinosaurs.'

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