Original subtitles

I'm on a fantastic journey to look for the origins of life.

I shall be travelling, not only around the world, but back in time,

to try and build a picture

of what life was like in that very early period.

Last time I saw how, 600 million years ago,

simple cells evolved into the first multi-cellular animals.

In this programme, I investigate what happened next.

I will look for evidence in both

fossils and living creatures of what happened in that

far, distant past,

when the fundamental features of modern animals

were being established for the first time.

One group, the arthropods, were the great pioneers.

They were the first big predators.

They had eyes.

Legs.

And hard external skeletons,

They were the first to crawl out of water

to conquer the land and the air.

600 million years ago, the world was very different

from the planet we know today.

The land was entirely without animals or plants.

But the oceans were teeming with life.

The first proto-animals were immobile organisms

that lived on the sea floor and extracted their nourishment

from the water flowing around them.

But once animals developed mouths

and the ability move, evolution took off.

Canada's Rocky Mountains.

Here we can find evidence of a sudden explosion of life

when animals started to evolve with astonishing rapidity.

It happened during a period called the Cambrian.

And it began 542 million years ago.

During the next 10-20 million years,

animals increased in numbers, diversity and size as never before.

And as they got bigger, so they became more complex.

And they're preserved to an extraordinary degree of perfection

in the rocks right below me.

The Burgess Shales, where a rich seam of fossils

documents this Cambrian explosion in astonishing detail.

All this area was once the floor of a shallow sea, teeming with life.

As sediment settled down onto the floor, so it became compressed

and turned into mudstones and shales that you can see around me here.

About a century ago,

an American geologist from the Smithsonian Institution

was making a survey of this part of the Rockies.

And he came walking along this particular path.

And when he got to precisely this spot,

he noticed a tiny fossil of a kind he had never seen before.

He bent down and picked it up and it looked like this.

What sort of a creature could this be?

It was only the first of the enigmatic creatures

to come from the Burgess Shales.

Since then over 65,000 different specimens of now extinct

Cambrian animals have been from this one small quarry.

Many species have never been found elsewhere.

It seems that the Burgess Shales were deposited in a place

where conditions for fossilisation were uniquely perfect.

As a consequence, even bodies of animals that were soft

and lacking any hard parts were, nonetheless, preserved.

They survive as thin, almost imperceptible layers,

that you only see if you get the light just right.

It's these fossils that have transformed our understanding

of how animals we know today have come to be the way they are.

In some of these specimens we can glimpse shapes and forms

that look faintly familiar.

But many of these bizarre creatures seem like nothing we know of today.

This is one of the more mysterious animals from the Shales.

There are two clues as to how this creature might have lived.

It has flaps along the side of its body,

but no legs, and also a broad, flat tail.

So it's reasonable to assume that they helped it swim

and that it lived not crawling along the floor,

but up higher in the water.

But the really, truly mysterious thing about it is that here

on its head it had five eyes,

each of them like a kind of little mushroom.

And beneath that it had a long proboscis

with which it grabbed things.

It's a truly primitive animal

and one that, still, we don't fully understand.

It's been named opabinia.

And it seems to have been a kind of evolutionary experiment.

It's almost as if an assortment of different body parts

had been put together in something of a hurry.

What other animal has five eyes?

And opabinia wasn't the only oddball.

Wiwaxia was once thought to be an ancestor of earthworms,

but now is considered to be an early snail.

Most of the Burgess Shale creatures

are unlike anything ever discovered before.

There were countless bizarre creatures

living in the Cambrian Seas,

This unprecedented surge of diversity was something

that had never happened before and would never happen again.

For many years, scientists excavated and scrutinised the Shales

looking for the causes of the Cambrian explosion.

Their first task was to try and reconstruct

what these strange animals must have looked like when they were alive

and that was not at all easy.

This is one of the oddest of the fossils from Burgess Shales.

It seems to have five legs along the bottom,

and curious kind of lobes along the top,

which presumably were some devices, which help it to feed.

But what kind of animal is that with five walking legs

and feeding lobes along the top of its back?

It was such an extraordinary thought that the scientist

who described it thought it was a kind of hallucination,

and he called it "hallucigenia".

But since then, more specimens have shown that in fact,

this is probably the wrong way up and that it was really like that.

The projections at the bottom are, in fact, legs.

And those along the top are tipped with sharp spines

that were presumably, defensive.

Perhaps these animals evolved these strange shapes

because they needed to protect themselves?

But if so, from what?

Where were the predators?

No-one could find a likely candidate.

And then the answer came from a couple of fossil species

that they had known almost from the very beginning.

One of the strangest fossils found here is this.

It's also one of the commonest.

But what is it?

Well, it has what looks like legs, so you might think

it was some kind of caterpillar, or shrimp maybe.

But the most mysterious thing about it was that

they never found one with a head.

Then there was another mystery,

not as common as the headless shrimp,

but one that looked like a sort of jellyfish,

with radiating lines out, and this strange hole in the middle.

And about twenty years ago,

it was discovered that actually, there is a link between

this and this.

This bit is not a separate shrimp, it's actually a claw.

And this bit is not a jellyfish, it's a mouth.

And in the mouth you can see something

that looks very significant.

Could these be teeth?

And were these not legs but spikes, used to stab and grab prey?

The two were, in fact, connected.

But now we have a most perfect fossil,

which really demonstrates that that is indeed the case.

This, you might say, is the Mona Lisa of the Burgess Shales.

This specimen, at last, gave scientists a picture

of the complete animal.

It had plates along its back, and a tail at the rear end.

It was a swimmer. And between those two spiked claws

at the front there was a mouth...

with teeth.

This was the hunter they had been looking for.

The scientist who discovered the claws called them anomalocaris,

meaning strange shrimp.

That name is now used for the whole animal.

With its large tail and flexible plates along its flanks,

anomalocaris could propel itself through the water at speed.

Other specimens show that it could grow to a length of nearly a metre,

two feet or so.

It was, as far as we know, the first big predator on Earth.

We can get clues as to what it was like

from an animal that is alive today.

It's much smaller than anomalocaris, though remarkably similar.

And it lives in Australia, here on the Great Barrier Reef.

Professor Justin Marshall has been studying these ferocious

and powerful hunters for over 20 years.

You have to very cautious about the way you handle them.

If you pick them up they can knock the ends off your fingers.

Fishermen call them thumb splitters because

as they handle them they get thumbs and fingers split open.

The other, slightly more technical name for them is mantis shrimp.

They have a very ancient ancestry.

Fossils of almost identical creatures have been found

that date back 400 million years.

This animal is almost as ancient as anomalocaris itself.

It lurks in burrows, waiting for its victims

to swim within range of its claws.

Looking at the fossils of anomalocaris

and comparing them to mantis shrimps,

one could imagine that these animals are similar.

They both have big raptorial appendages

that are shot out at the front to grasp prey.

You could imagine them lurking behind a rock

waiting for unwitting prey to come past.

And bang! Suddenly that's dinner.

The mantis shrimp illustrates the essential characteristics

of this brand new predator class of animals.

Superb vision, great speed and superior size.

Like anomalocaris, it's considerably larger than its victims.

It also has extremely acute vision,

with 12 different types of colour receptors in its eyes.

We have just three.

And it's one of the fastest animals alive,

some species striking with the speed of a pellet from a gun.

It's unlikely anomalocaris was as fast, or that it saw its prey

so clearly, but nonetheless, it was a formidable predator,

just as the mantis shrimp is today.

Even a glimpse of a finger through glass is enough

to make this animal strike,

and with alarming force.

So why did the mantis shrimp evolve in this way?

Well, obviously...

in order that it could

outfox and outmanoeuvre, and eventually catch its prey.

It's become very fast, very powerful,

and capable of great patience.

And those are characteristics of predators everywhere.

So the fossilised remains of anomalocaris

are evidence that hunting had begun in the Cambrian.

And as predators became bigger, faster and stronger,

so their prey had to develop increasingly elaborate defences.

Opabinia's five eyes helped it spot trouble.

And Hallucigenia protected itself with those spines along its back.

One of the world's leading experts on the Burgess Shales,

Dr Jean-Bernard Caron, believes that it was the arrival

of predators like anomalocaris that stimulated the great

Cambrian explosion of diversity.

It is during the Cambrian

that we can start seeing animals with legs, eyes, swimming.

This didn't exist before and this evolved very, very quickly

at the beginning of the Cambrian.

But once you have a big predator, presumably the rest of life,

which it was feeding on,

had to evolve quite fast to develop some sort of defences.

Would that be true?

Well, we think that this evolution occurred relatively quickly because,

in a place like the Burgess Shale you find organisms

that may have had some kind of defensive mechanism,

which is thought to be a response to higher predatory levels.

Arms race, if you want, between predators and prey.

One result of this duel between predators and prey

was the development of armour.

Animals everywhere were absorbing calcium carbonate

and other inorganic substances from the seawater

and mineralising their bodies.

Many of them, like wiwaxia, that early mollusc,

and ancestors of the squid, ammonites,

developed protective shells.

But one group, the arthropods, which had jointed legs,

encased their entire bodies with hard armour plating.

And what began as defensive armour, necessary for survival,

brought with it another great advantage.

Hard parts can be used not only to give protection,

but to provide support for a body.

Ha-ha!

This spider crab is a crustacean.

And it secretes chitin from its body,

which it then strengthens with calcium carbonate.

And a whole range of creatures

have skeletons like this, based on chitin.

Arthropods today include shrimps, lobsters and crabs,

as well as land-living creatures,

such as millipedes, scorpions and insects.

But the ancestors of all of them first appeared in the Cambrian Seas.

Over 50% of fossils in the Burgess Shales

are arthropods of one kind or another.

But one family was particularly abundant and varied.

Just across the valley from the quarry,

near the summit of Mount Stephen,

almost every rock you turn over contains their remains.

Here, they are found all over the place.

They're called trilobites.

Trilobites because their bodies were in three sections.

Here on this slab there are several of them.

That's the head.

There's the middle bit. And there's the tail.

One, two, three trilobites.

Trilobites, at this particular time,

right at the beginning of the Cambrian,

began to proliferate into all sorts of forms.

These creatures, for the next 250 million years,

were probably the most advanced forms of life on this planet.

To see how advanced the trilobites eventually became,

I'm going to North Africa.

In Morocco, on the southern flanks of the Atlas Mountains,

the hills contain an amazing variety of them.

They were only discovered a few years ago,

but now the demand for them is so great

that digging them out has become a major industry.

These rocks, which were laid down about 150 million years after

the Burgess Shale, also contain trilobites.

The trouble is, the rock is very hard

and the trilobites are quite rare.

But when these people find them,

their specimens are absolutely extraordinary.

Some species have features that are so delicate

that it can take days, sometimes weeks,

to fully prepare a specimen.

Skilled technicians use dentists' drills

to get down to the finest detail.

Every particle of rock must be carefully removed,

with enormous patience and absolute precision.

The end results reveal that trilobites

moulded their external skeletons

into an almost unbelievable variety of shapes.

And that enabled them to colonise a great variety of habitats,

just as modern arthropods still do today.

There were about 50,000 different trilobite species that we know of,

and doubtless there are still many more to be discovered.

Their hard exoskeletons

not only ensured their abundance in the fossil record,

they also tell us a lot about their owners' lives.

Many of the trilobites that are found in these cliffs

are curled up like this one.

Sometimes even more tightly than this is,

with their tail tucked underneath their heads.

And it's clear that this was some kind of protective posture,

just as it is for some kinds of woodlice

that you find in the garden today.

That protected them against their enemies.

But there are so many that are curled in these deposits,

together with others that have their backs arched upwards

and others in other strange postures,

that it seems that they are the victim of some kind of catastrophe.

The sea floor, it seems, was quite steep.

And every now and again,

the mud that accumulated on the bottom slipped down

in a submarine avalanche,

carrying the animals that lived in it and on it,

higgledy-piggeldy, and burying them alive.

Moroccan trilobites are big business these days.

Particularly rare species sell for thousands of pounds.

The world's leading trilobite experts,

such as Professor Richard Fortey,

come here to study these extraordinary animals.

He believes that their external skeleton

was the key to their success.

The trilobites did almost everything

you possibly can do with an exoskeleton.

I think that skeleton was what gave them an advantage.

They were protected. They could do all kinds of interesting things.

They could grow spines.

They could get flat, like pancakes.

They could protect themselves by getting thick exoskeleton

with pobbles all over it.

It was a great advantage to them, just as it is to crabs and lobsters

living today, which of course weren't around

at the time of the trilobites.

So they utilised the virtues of having a tough exoskeleton,

to radiate into all kinds of ecological niches.

You can see one of the most comprehensive collections

of trilobite fossils

just a few miles from where they're quarried, at Erfoud Museum.

The collection here reveals just how varied

the trilobite skeleton could be.

There is no question that an exoskeleton

gave the trilobites protection.

But it also gave them something else of great value.

There must have been many reasons why trilobites were so successful.

But one of them, unquestionably, was their power of sight.

They had eyes.

not just eyespots that could tell the difference

between light and dark,

but complex eyes that could form detailed pictures

of their surroundings, for the first time in the history of life.

Eyes like these.

Most animals on Earth today have eyes of one kind of another.

Most are made of soft tissue, as ours our.

But trilobite eyes are unique.

Their lenses are derived from their mineralised external skeleton.

They're made of rock.

Each one of these little dots is a lens.

And each is made from calcite,

a crystalline form of chalk.

Trilobites were the only organisms

ever really to use this stuff as their lens material.

And in doing so they evolved very sophisticated vision indeed.

For example, these sorts of trilobites had very large lenses.

And each lens is readily visible with the naked eye

and each one is biconvex.

And it's been proven that individual lenses have little bowls inside them

to help them focus more precisely.

These creatures were among the first,

certainly, to actually focus a picture, weren't they?

It wasn't just a question of telling light from dark,

they could do better than that?

On no, these, these had really sophisticated vision.

The kind of trilobites that have these eyes were probably hunters.

Some people have claimed that they could form stereoscopic images,

using both eyes, so they could really home in on the prey.

May predators today, including ourselves,

have 3D, or stereoscopic vision.

It makes it possible for a hunter to accurately judge the distance

between itself and its prey.

But not all trilobites were predators.

Some were inoffensive creatures that lived by munching mud.

But sight must have been valuable

for them too, enabling them to spot enemies in time to escape.

There are trilobite eyes with more than 5,000 lenses.

5,000? More than 5,000 lenses.

Now each of those, does it have an image?

Each doesn't have an image, but if they go for lots of tiny lenses,

they're particularly sensitive to movement,

i.e. something changing between one lens and the next.

This trilobite's eyes are so big they extend right round its head

and meet in the middle.

And that suggests that the animal swam high above the sea floor

and had a 360-degree view of the scene below.

With each lens capable of detecting movement,

its owner must have been able to see an enemy coming from any direction.

But the shape of a trilobite's eyes can reveal more than the

kind of image they produced.

Eyes can tell us a surprising amount about how and where an animal lived.

This one with its eyes on turrets probably lived in the sea where it

was gloomy, but nonetheless there was enough light for the animal to

be able to see on either side of it.

This one, on the other hand, has eyes also on turrets, but at the top

it has flanges, like sun shades.

So it's, er, likely that it lived in the shallow, sunlit sea

and valued shades above its eyes so it didn't get dazzled.

This one, however, has very reduced eyes, and it may well be

that it skated along the mud along the bottom,

where it was gloomy anyway and there wasn't much to see,

so like an animal living in a cave, it slowly lost the use of its eyes.

And finally there's this creature,

and this is the one I think is particularly delightful.

This one has its eyes on stalks.

And probably lived under the mud,

gobbling up food there with its, just its eyes

peeking out of the top, to see whether there was danger around.

So trilobites were the first animals to see clearly.

But they had other senses as well, perhaps some

we don't even know about.

Take this species with this bizarre trident structure on its nose.

What was it for? Some kind of motion sensor?

Prehistoric radar, perhaps?

Trilobites were, without question,

the most successful animals of their time.

They flourished in all parts of the ocean.

Indeed, they could be counted as one

of the most successful kinds of animals in the entire history of life.

Most trilobites are quite small, rather like beetles are today.

But the biggest living beetle is about that big, the goliath beetle.

Trilobites, on the other hand, grew very big indeed. Like this one.

And this is by no means the biggest.

The biggest known is nearly a metre, nearly three feet long.

And it's thought that these really big ones grew to this size

because they lived in cold waters, and that's a tendency of animals

in cold, to grow large.

And at the time that these rocks were laid down, Africa,

where we are now, and where these are found,

was down by the South Pole.

Spectacular though these are,

they were by no means the largest arthropods in the ocean at the time.

The trilobites had remote cousins, also arthropods, that had grown

into monsters.

Their remains are much rarer, and often fragmentary,

but some of the most complete have been found in Scotland.

ALARM SOUNDS

One of the best is held in the vaults

of Edinburgh's National Museum.

Gosh!

Well, this is a magnificent example of just how big an animal can grow

if it has an external skeleton.

This is a creature called the Eurypterid, or a sea scorpion.

And it was a hunter.

It had a pair of powerful pincers at the top, just behind its head.

It was obviously a monster, a terror of the seas.

And this is by no means the biggest of the eurypterids.

Sea scorpions were the top predators of their day.

As far as we know, they were the biggest arthropod

that has ever existed.

The discovery of a large fossilised claw suggests

that they could grow up to two and a half metres, eight feet in length.

So arthropods of one kind or another

were certainly dominant 420 million years ago.

The seas were full of life.

From huge complex animals like this sea scorpion

creeping along the bottom,

to simple creatures, like jellyfish, floating on the surface waters.

But the land was barren and without animals of any kind.

But there was food up there, simple plants,

and that tempted some animals to venture out of the water.

Surviving on land, however, was a problem for them.

Coming from the sea, they had to evolve ways

of preventing their bodies from drying out.

And even more difficult, they had to develop a method of breathing air.

The very first animals had simply absorbed

dissolved oxygen from the water

through the skins of their soft bodies.

As they began to move and grow bigger, they needed more energy,

more quickly.

And that meant

they had to improve their method of collecting dissolved oxygen.

Bigger, more complex animals,

like for example, this lobster,

have to have specialised devices, which are called gills.

Here in the lobster they are these flaps underneath its abdomen,

which is flaps forwards

and backwards to increase the flow of oxygenated water over them.

But the trouble with gills is that they only work when they're wet.

In the dry, they do not absorb oxygen.

So if animals are to live on land,

they had have to have a new way of breathing.

The Burgess Shales,

that astonishingly rich treasury of Cambrian fossils,

contain the remains of just one

particularly rare species that may well have been the very first animal

to make that move onto land.

It was not, as you might think, an amphibian, it was not even

a true arthropod, but one of their far distant cousins.

This little creature,

from the Burgess Shale seas, is thought to be the ancestor

of the very first creature that went on to land. It's called Aysheaia.

And we don't have to imagine what it was like in life,

because there's a creature, that seems to be almost identical,

that is alive today.

It lives in many parts of the tropics, including the rainforest,

here in Queensland, Australia.

It's nocturnal and seldom seen.

It spends most of its time hidden away inside rotten logs.

Ah, it's nice and wet!

Certainly, er, perfect for what we're looking for.

You need local expertise to find one.

I generally find that it's just from the outside

of the, er, core of the tree.

All nice and... Oh! What is that? Ooh, look at that.

And this enchanting little creature

is what we were looking for.

Sometimes called a velvet worm,

or to give it its scientific name, Peripatus.

If there is such a thing as a living fossil,

this surely must be one of them.

Because it seems to be almost identical

with that fossil, Aysheaia, which we saw in the Burgess Shales.

It looks at first sight like a worm.

But of course no worm has legs. In fact,

it seems to be halfway

between a worm

and an insect.

Aysheaia, of course, lived in the sea.

But this little creature lives on land.

And it has one further attribute,

which Aysheaia could not have had.

It has tiny little holes all along its flanks,

which enable it to breathe air.

So this is one of the first creatures

that moved on to land,

540 million years ago.

Velvet worms may have been the first animals to set foot on land,

but they have hardly changed during the following half-billion years.

Why?

Well, unlike true arthropods, their bodies are covered,

not by an exoskeleton, but by soft, permeable skin.

That lack of an external skeleton means that their bodies,

unsupported by water, can't grow any bigger.

It also means that in order to prevent themselves from drying out,

they have to stay in damp environments.

True arthropods, like this scorpion,

a descendent of those giant sea scorpions, were not so restricted.

They had external skeletons.

That meant that not only were their bodies protected from drying out,

but they were strong and rigid enough to allow them to grow bigger

and get around without the support of water.

So how and when did true arthropods with exoskeletons

draw their first breath of air?

The answer can be found in this.

It is perhaps the smallest and most fragmentary fossil I've seen so far,

but don't be fooled by appearances.

It's almost certainly one of the most significant.

This specimen was collected in Cowie Harbour, here in Scotland, in 2004.

Even though it's so small, under the microscope you can see

extraordinary detail.

This is the main body of the animal with its segments.

And here are its legs.

But above each there is a tiny hole.

That is a spiracle, through which the animal was able to breathe air

just as insects do today.

And since it breathed air, if it had gone into the water

it would have drowned.

So this is a truly land-living animal and what is more,

it's the first and oldest that we know.

It's 428 million years old.

But what kind of creatures were these early land-dwelling arthropods

Animals very like them are still quite common

in many parts of the world.

There are certainly plenty of them in those Australian rainforests.

One sort are millipedes,

which today grow as long as that and live on vegetation

and rotting wood, harmless vegetarians.

But there's also another multi-leg creature, which is a much more

difficult customer.

This is one of them.

A centipede. A very formidable hunter, with a powerful bite,

and some centipedes have bites that are lethal to human beings.

What kind of a bite this one has,

I don't know.

But when I let him out I shall do so

very carefully, because I don't propose to find out.

Come on.

So multi-legged arthropods invaded the land and became

more successful than ever.

Back in Scotland,

there is impressive evidence of just how successful they became.

This is a small fishing village

on the East Coast of Scotland called Crail.

Nothing particularly strange about it, you might think...

until, that is, you go down to the shore.

And then you can see something that is really extraordinary.

Standing here and there on the beach are fossils,

not of animals, but of plants.

This huge circular stump looks just like the base of a tree.

And indeed that is what it is, or rather,

what it was, 335 million years ago.

But it wasn't a tree like trees we know today.

It was related

to the small plants that are alive today called horsetails.

But this tree grew to 90 feet.

It was immense.

When they were alive, during a period called the Carboniferous,

long after the Cambrian,

this whole area was very different

from the windswept coastline of today.

This was a time when the continents of the world were grouped together

and forests were widespread.

So much plant life was pumping out oxygen

that the composition of the atmosphere began to change.

This had a profound effect on animal life.

In the forest that was growing near Crail, the ancient trees

were rooted in a sandy swamp.

And on the expanses of sand that stretched between those huge trees,

sand that's now turned to this sandstone,

there are tracks.

Tracks that come in pairs, there's one pair that goes up there.

There's another pair that goes up here.

And when you look at them in detail, you can see,

particularly on this pair, that each track has a number of dimples in it.

And those are the imprints of individual feet.

So this animal had a lot of feet.

It's thought to have been a giant millipede.

It was about...

four and a half feet long, one and a half metres.

And it had 26 or 28 segments.

A magnificent beast.

Arthropleura.

A giant millipede,

probably the biggest terrestrial arthropod that has ever existed.

The largest specimen discovered so far was nearly as long as a car...

two and a half metres.

The Carboniferous was the golden age for the arthropods,

for the air was now particularly rich in oxygen.

Today the atmosphere contains around 21% oxygen.

Back in the Carboniferous,

it was around 35% and that enabled animals to grow very big indeed.

But growing large was not their only success.

Some other arthropods in these carboniferous rainforests

were evolving in a different way.

Instead of becoming huge and ponderous,

they became agile and speedy.

To do that it's better to be short rather than long, and some

reduced their segments and ran around on just three pairs of legs,

as silverfish and bristletails do today.

These early insects then made another dramatic move...

they developed wings and became the first animals of any kind to fly.

Truly the invertebrates had colonised

not only the land, but the air.

And in an atmosphere so rich in oxygen,

they did so in a truly dramatic way.

This giant dragonfly,

the biggest flying insect that has ever existed, is called Meganeura.

Its wings were nearly three feet across.

But the golden age of the giant arthropods was not to last.

The rainforest died back, and oxygen in the atmosphere dropped.

Giant insects are no longer alive today and that may be

because the proportion of oxygen in the atmosphere is very much lower.

But nonetheless, insects have managed to find a way

of overcoming the problems of size.

They've become colonial.

Just as in the far distant, remote past,

individual cells clubbed together to form a larger organism,

such as a sponge,

so hundreds of thousands of individual insects, termites,

have cooperated to build this nest.

And a colony like this can crop as much vegetation

from the surroundings as a bigger animal like an antelope.

So by living in vast colonies like this,

arthropods can still dominate their surroundings.

They've become super-organisms...

hundreds of thousands of individuals all descended from the same female,

working and behaving as one.

So arthropods remain

one of the most successful groups of animals on the planet.

They've spread to all its corners.

Insects alone make up at least 80% of all animal species.

But arthropods weren't the only ones to make this move on to land.

The Burgess Shales -

the place where the beginnings of all this proliferation of life

in the Cambrian period are recorded in unparalleled detail.

Among the ancestors of all the insects,

spiders, the scorpions, the shellfish, the crustaceans,

the shrimps, the sponges,

there's just one tiny little creature, very insignificant,

which we human beings might think is perhaps the most important of all.

Because this...

is the first creature to have the sign of a backbone,

and thus, therefore, is probably the ancestor of us all.

It's a tiny, worm-like creature called Pikaia.

It was not a fearsome hunter.

It had no teeth for attack and no external skeleton for defence.

But Pikaia did have something new.

Instead of an external skeleton,

it had an internal one, a thin gristly rod...

the beginnings of a backbone.

It, or something very like it, was the ancestor of all vertebrates.

From such a creature as this, the first fish evolved.

Some of them, living in swamps, started to gulp air and wriggled up

onto the land. They gave rise to moist-skinned amphibians.

Some of them developed scaly, impermeable skins that enabled them

to colonise the driest places...

they were the reptiles.

And from them came the birds.

And the mammals.

Today mammals, like this rhinoceros,

are the biggest of all living animals.

Hello, old boy. How are you?

How are you?

'All mammals, including ourselves, extract oxygen from the air with

'the end of internal lungs, and distribute it through our bodies

'in our blood.'

There we are. There's a good lad.

'But we also owe our success, and our size,

'to the nature of our skeletons.'

Animals with an internal skeleton, like this rhinoceros,

have a huge advantage over animals whose skeleton is external.

A white rhinoceros, like this,

is one of the biggest land animals alive today.

Compare him

with him... a rhinoceros beetle.

Its skeleton is external.

It's very powerful.

It can carry 850 times its own weight.

But it can't grow much bigger. Because the only way it can grow is

by shedding its skeleton and growing a new one.

And while its skeleton is not there, its body is unsupported.

And after a certain size, the body will collapse under its own weight.

Here.

Here we are, come on boy. Come on boy.

Despite these differences, it's no coincidence that

backboned animals evolved many of the same features as the arthropods.

Teeth.

Legs.

Shells. Eyes.

And wings.

Any animal group needs such things if they are to colonise

all the Earth's varied habitats.

A journey that began for me near my boyhood home in Charnwood Forest

has taken me around the world and through 600 million years

of evolutionary history.

I've seen evidence of how single-celled life

dominated the planet for billions of years,

until a global ice age triggered the emergence of the first animals.

Many animal groups lasted millions of years.

But eventually their time ran out and they disappeared.

But others endured.

And between them they evolved

into the wondrous variety of life that inhabits this planet today.

Life originated in the oceans.

After an immense period of time, some creatures managed to crawl up

onto the land.

Those animals may seem to us to be very remote,

strange, even fantastic.

But all of us alive today owe our very existence to them.