Building Bodies Life on Earth

The creatures that live among the coral heads

of the Great Barrier Reef in Australia

must surely be among the most beautiful

and the most bewildering organisms

you can find anywhere in the world.

Sorting out these creatures into their various groups

is baffling work.

Often things are not what they seem.

These are the tentacles of a worm.

This is the cousin of a starfish.

This is a flatworm, and the creature advancing on it

is a snail that has lost its shell.

One thing is clear.

They're all animals without backbones, invertebrates.

But how are they related to one another?

Which is descended from what?

One way to find out is to trace the various groups, as fossils,

back through the rocks to their origins.

These limestones here in Morocco are so old,

getting on for 600 million years old,

that they date long before the time of any backboned animals.

There are no fish fossils here, for example.

But there are invertebrate fossils.

Not as many or as varied, it's true, as the invertebrates that live today

on the Barrier Reef, but invertebrates nonetheless.

And they fall roughly into three groups.

There are little shells, like this.

And a creature that looks like a flower

but was covered in stony plates.

And this, which is rather like a shrimp, with a shell,

and its body divided into segments.

What are the relationships between these three very, very early groups?

If we can understand that,

we will be close to understanding the origin of animal life.

The obvious place to look

is a few feet farther down in these limestones.

A million or so years earlier.

But suddenly, we come to a mystery.

Although these limestones look exactly the same as those above,

and must have been laid down in similar seas,

there are no fossil shells to be found here at all.

What's more, there are no fossil shells to be found

in any rocks in the world of an age of these.

And these extend for thousands of feet more,

representing hundreds of millions of years of deposit.

And not a fossil shell among them.

The explanation is precisely in that word "shell".

Shells fossilise easily.

Soft animal tissues rot, and hardly leave any trace behind.

There was life in the seas in which these limestones were deposited,

but without shells.

But why did it take so long for animals to develop shells?

After all, if you condense the whole history of life

from its beginnings until today into a year,

it wasn't until early November that the first shelled animals appeared.

Well, there's been a lot of debate on that question,

and a lot of suggestions.

One is that the chemistry of the seas wasn't suitable.

They were either too cold or too acid

to allow for the deposition of lime as shells.

Whatever the answer, the fact remains that for that immense period of time,

we have no fossil shells to help us chart the progress

of the very early stages of animal life.

But that doesn't mean we can't make some informed speculations.

For example, take this group of creatures,

the one like little shells.

What could their early ancestors have been like?

These microscopic creatures are among the simplest animals in the sea.

They're the larvae of corals and jellyfish.

We know that they appeared very early indeed.

But suppose some of them didn't grow up either to float

or to build skeletons, but took to a creeping life.

They might easily have become something like this.

This is a juvenile flatworm.

It has a cluster of spots on top at one end,

which are sensitive to light and to gravity,

and it swims with the aid of cilia that cover its surface.

When that settles on the sea bed, it becomes this.

Not a drifter like a jellyfish,

but an animal that moves in a purposeful way

with a definite front end and back end.

Flatworms are very flat,

and with such a great body surface in relation to their small bulk,

they absorb all the oxygen they need

through their beautifully patterned skin.

Many of them move by rippling their bodies

instead of relying entirely on the cilia.

And some are so good at it that they can swim.

A flat shape, however, is not so suited to burrowing.

And as mud and sand began to spread over the sea floor

about 1,000 million years ago,

burrowing became a desirable thing to do.

There were bits of food to be sifted from the mud,

and hidden beneath it, there was safety.

So some worms changed from being flat to being round and long,

and buried themselves in the mud.

Others were less active and remained with their front ends sticking out,

ringed by tentacles.

The beating of the cilia created currents

that enabled the tentacles to absorb oxygen,

and also swept food particles down to the mouth at their centre.

About 600 million years ago,

some of these worms secreted a pair of shields on the top

to protect the delicate tentacles

and channel the feeding currents over them.

This was such a success that variations appeared.

Shells were strengthened with lime and grew bigger

to allow more efficient breathing tentacles.

So eventually the original worm-like shape was lost.

We know from fossils that these creatures, the brachiopods,

were enormously abundant in the ancient seas.

They grew in many shapes and to a considerable size.

Some developed delicate coils of lime inside their shells

to support their feeding apparatus.

But some 70 million years ago, their fortunes waned,

and today only a few species survive.

One lives in some numbers on the muddy shores of a bay in Japan,

and at low tide, they are collected for food.

They call them shamisen-gai

because their shape is like that of the Japanese guitar, the shamisen.

These are the simplest type of brachiopod,

that have outlasted all the more ambitious kinds

that once were so abundant.

In fact, they're virtually identical to those earliest fossil shells.

It's an astounding example of survival

which occurs several times in the history of life.

An early species finds itself in surroundings

which suit it to perfection.

No other animal comes along later

which exploits the surroundings any better.

Its cousins may move away to colonise different environments,

or their environments might change,

and so they develop into different creatures.

But this creature, encountering no change, sees no cause for change.

So it plods doggedly on, an ultra-conservative.

This formula of a simple, worm-like body enclosed in a protective shell

had obviously a lot of potential.

Several groups of creatures in early periods were based on it,

and one group in particular, the molluscs,

exploited it very well indeed.

Today, there are around 80,000 different species of them.

The flatworm ancestors of the molluscs developed their shells

not over one end around the mouth, but in the middle of the back,

originally like a small tent under which the animal could hide,

as the limpet does today.

The shell is deposited by a part of the back, the mantle,

and the animal enlarges it by adding to the margins.

Some species, though, don't do so at an equal rate all round,

and that produces twists and coils in the shell.

They have a well-developed head, with eyes,

and sensory tentacles

for feeling the way and tasting the water.

And underneath it, a very efficient feeding organ.

It's a long, tongue-like ribbon.

The muscles around it press it down and pull it forward,

rasping it over the surface on which the animal is crawling.

Many species use it for eating algae.

Looked at under the electron microscope,

the reason for its efficiency is clear.

It carries rows and rows of minute teeth.

Each species, for some reason, with a different pattern.

Cowries secrete their shell in a way all their own.

They extend their mantle right round the shell

and deposit material on the top,

giving it that beautifully polished surface.

The spider shell has its ribbon tongue on a stalk

so it can scrape surfaces its shell would prevent it from reaching.

It also has a stalked eye to help it prospect for hidden pastures.

Its foot has become very muscular to help it get around.

Molluscs with paired shells, bivalves, don't often move far.

Their foot is used to pull them down into the sand

where they can sit and filter food safely and unobtrusively.

Scallops are also filter feeders.

They live on the surface,

and not only have good eyes, but a surprising way of moving.

Biggest of all is another filter feeder, the metre-long giant clam.

So huge, it can't move.

Its fleshy mantle joins its two shells,

forming a chamber through which water is sucked.

Every so often, it gives a convulsive shudder

and gets rid of a little waste.

A few molluscs have gone to the other extreme and become free-swimming

by reducing their shells to scales concealed within their bodies,

or doing without them altogether.

Unprotected by a shell,

these creatures defend themselves with a nasty-tasting slime.

And their brilliant colours may serve to warn off anything

that might contemplate eating them.

If that's so, they must be among the loveliest warning notices

in all nature.

These creatures are more complex and usually larger than flatworms,

and they need special breathing apparatus, the gills.

In some species,

they're exposed as a kind of trembling bouquet at the back.

Several kinds have developed feathery outgrowths

that enable them to float close to the sea's surface.

There, extraordinary though it may sound, they hunt for jellyfish.

This one is called glaucus, and it has found its prey.

The stinging cells of the jellyfish are no defence.

Indeed, some of these floating molluscs welcome them,

swallowing the stinging cells

and storing them in their own tentacles

to use as second-hand weapons.

This is another creature eaten by glaucus.

One of the most deadly of all jellyfish, a Portuguese man-of-war.

Beneath it trail its tentacles, loaded with stings.

Another mollusc also preys on this creature,

and this time, one with a shell.

It has a most ingenious solution to the problem of keeping afloat.

It produces bubbles by trapping air in mucus

with special movements of its spoon-like foot,

and builds them into a raft, from which it hangs.

When it drifts into a Portuguese man-of-war, it attacks immediately.

The stinging cells of the jellyfish,

lethal to other creatures, have no effect on the snail.

It munches them with the rest of the tentacles.

A raft of bubbles solves this snail's weight problems,

but that won't work for bigger creatures.

500 million years ago, however,

a group of molluscs evolved another method.

This fossil shell may look perhaps quite an ordinary sort of shell,

albeit rather large,

but inside it has got quite a complicated structure.

Here's one in a boulder where the outside has been worn away

so that we can see what's inside.

This part was where the animal lived,

and at the back of it, there were these chambers

which in life were filled with gas and acted as flotation chambers.

How can we be so sure?

Well, because this is another of those creatures

that have survived virtually unchanged

for hundreds of millions of years.

This is a nautilus, and there are nautilus swimming in the seas today.

They live in the South Pacific, but few people ever see them alive,

for they spend most of their time in depths of up to 500 metres.

They can swim at any depth,

by pumping fluid in and out of their chambers,

and so controlling their buoyancy.

Being so mobile, they need good sense organs,

and their eyes, although they have no lenses,

are the best of any creature we've seen so far.

Their bodies have become modified into dozens of tentacles.

Some carry sense organs to detect food, some are used in reproduction

and others to grapple with their prey,

which is usually carrion, or lobsters or crabs.

This proved to be an immensely successful design.

And from it came another great group of molluscs, the ammonites.

The ammonites were to dominate the seas of the world

for the next 200 million years.

They left behind in the rocks,

particularly here in Lyme Regis in southern England,

fossils that to my mind are some of the loveliest fossils of all.

Like the nautilus,

the ammonites added new flotation chambers as they grew,

while their bodies occupied only the outer one.

Because ammonites were so numerous and their shells fossilised so well,

we know a great deal about the way they developed

over a period of 200 million years.

But their history is full of puzzles.

Why, for example, did some groups develop uncoiled species

and then, over generations, slowly coil up again?

And why did the junctions between the flotation chambers,

which originally had been simple curves,

become increasingly elaborate and intricate, and eventually florid?

Small ones may have lived in shallow water near the bottom,

but others grew to an immense size

and probably sailed the upper waters of the prehistoric seas

like galleons.

And there is one final mystery.

Why, 50 million years ago, did they all die out?

There is not one surviving ammonite today.

But these paper-thin shells look remarkably like them.

On very rare occasions,

they are washed up on lonely beaches in New Zealand.

They belong neither to an ammonite nor a nautilus but a relative,

a kind of octopus called the argonaut,

which is sometimes stranded with them.

The animal doesn't live in the shell.

It secretes it from one of its arms and then lays its eggs in it.

Few people have ever seen that happen.

Just once in a while, a storm catches the breeding shoals

and drives these delicate cradles ashore,

some of them still holding their eggs.

For most of its life, the argonaut, like all other octopus,

is totally without a shell.

Only on this one occasion does it demonstrate its relationship

with the nautilus so vividly.

It's difficult to remember at times that the octopus is a mollusc

and that most of its relations are weighed down with shells

and a very long way from being quick-moving or intelligent.

Its molluscan tentacles have become heavily armoured with suckers.

The siphon, used by the clams for filter feeding,

serves as a nozzle for jet propulsion.

Its eyesight is excellent, and it has a lively brain and quick reactions.

The squid is very similar.

It has two more arms than the octopus

and is a very much more active swimmer.

Squids still keep within their bodies

a last relic of their ancestral shell.

A horny, sword-shaped structure that helps to support their long body.

As they swim, they hold their tentacles out horizontally.

They use jet propulsion for speed,

but they can also idle along in either direction

by waving fin-like extensions of their mantle.

The squids and octopuses are the most active and intelligent of molluscs,

able to solve complicated problems.

They're also the largest.

This giant squid that ran aground in Norway was nine metres long,

and there are reports of others twice the size.

They all developed from ancestors like flatworms

that lived in the seas of 600 million years ago.

But what about the second group of creatures from these ancient rocks?

The ones represented by this flower-like fossil

with a radial symmetry.

Well, within the next few million years,

these developed into a multitude of most beautiful forms

that we call sea lilies or crinoids.

It's a reasonable guess that these too evolved from worm-like creatures

that developed limey plates to strengthen and protect themselves.

These are about 300 million years old,

and a very few species like them still survive in the ocean depths.

But on the Barrier Reef,

some close relatives still flourish in great numbers:

feather stars.

These are just like crinoids,

but without stems, except when they're very young.

These adults swim freely around,

mostly at night, in search of feeding places

where they can cling to the rocks

and collect floating particles with their arms.

Their relatives, the starfish,

show clearly another characteristic of this group.

Their bodies have a five-fold symmetry.

The mouth is underneath, at the centre.

They move on tube feet, another unique feature.

Each foot has a tiny suction pad at the end,

and the many thousands of them are worked by hydraulics,

for they are all connected to water-filled vessels

that run through the body.

Their cousins, the brittle stars,

are much the speediest creatures in the group.

Sea urchins are more typical.

They too have tube feet,

but they move largely with the help of their spines.

Some of the tube feet are specialised for jobs

such as moving bits of debris from around the mouth,

which, like that of the starfish, is on the underside of the animal.

Urchins feed by grazing slowly on algae.

The food is gnawed by hard jaws, taken into the gut

and then, in most species, excreted from a pore at the top.

The spines are attached to the plates of the urchin's shell

by ball-and-socket joints,

so they can move in any direction.

Those on the top are for defence. If a shadow falls on the urchin,

it swivels its spines quickly to point towards a possible attacker.

These creatures may seem different from the original crinoids,

but they all have a radial symmetry and tube feet.

Although we can't be sure of evolutionary pathways,

relationships can be plainly seen.

If the head of a crinoid drops on its face, it becomes a starfish.

This, thinned down, turns into a brittle star,

but if it thickens, curls its arms back on itself and grows spines,

it becomes a sea urchin.

One group became elongated and lay down on its side to feed.

It's obvious why it's called a sea cucumber.

Most of these creatures work their way over the sea floor,

feeding on detritus.

A pretty nondescript animal, you might think.

But their tube feet give the clue to their true relationship.

This whole group of hydraulically driven creatures

hasn't produced any swift-moving highly intelligent forms,

but in their own terms, they've been successful.

There are about 5,000 species of them,

and wherever there's a suitable opportunity,

they miraculously appear, often in great numbers.

The crown of thorns starfish is normally uncommon.

But periodically, thousands appear on a reef and start to eat the coral.

The secret of the group's success lies in their larvae.

Too small to be noticed by the naked eye,

these larvae swim in millions in the sea.

This will eventually become a starfish.

And this, similar in many ways, turns into a sea cucumber.

Nearly all marine invertebrates -

molluscs, sea urchins, worms, corals, jellyfish -

all reproduce by larval forms like these

which are swept by the currents into every part of the oceans.

The vast majority will be eaten by fish.

Great numbers fail to find a suitable home,

and simply die and dissolve into nothing.

But their presence everywhere

ensures that no suitable corner goes unoccupied.

The larval sea snails have to support the weight of their developing shells

with lobes covered by beating cilia.

The similarities between larval forms

are just as valid evidence of relationship as those between adults.

And the fact that this mollusc larva looks like this,

the larva of a segmented worm,

is a strong indication the two groups are descended from a common ancestor.

Eventually, this larva becomes a worm such as this,

the simplest member of our third group of animals, the segmented ones.

They probably developed segments in their bodies,

each with its own pair of movable bristles,

because it made sustained burrowing easier.

Soft-bodied animals hardly ever fossilise,

but in one site in south Australia, in rocks 650 million years old,

older than those limestones in Morocco,

have been found what appear to be segmented worms.

This is one of the earliest records of a soft-bodied animal

that has ever been found.

There is one other highly exceptional fossil site

where soft bodies have left their impressions in the rocks.

It lies in the heart of the Rocky Mountains in British Columbia.

In the rocks here, you can get a unique glimpse

of the animals that crawled around the bottom of the seas

100 million years after those early Australian ones.

In fact, at about the same time as those in Morocco.

These rocks are shales.

Mudstones, and of the very finest texture.

From a detailed examination of them,

we can be pretty sure they were laid down

at the bottom of the sea about 500 feet deep.

But this particular patch was a very special one.

There were virtually no currents, and in consequence no oxygen.

That meant that no creatures could actually live

in this little part of the sea bottom.

There were no scavenging animals, for example,

and equally, there was no oxygen to fuel the processes of decay.

So that meant that if any dead creatures

drifted down to settle on these muds,

their bodies would remain intact for a very long time.

And come they did.

Fine mud settled on top of them and so they were entombed.

Over millions of years, the mud consolidated to form these shales.

And here as fossils they have remained,

miraculously escaping the distortions and crushings

that happened when these rocks were rucked up by earth movements

to form the Rocky Mountains.

And these freak conditions

have preserved the most delicate of creatures.

Here, for example, is a little worm.

Several species of segmented worms have been found,

and their preservation is so remarkable

that you can almost count their bristles.

There's also a group of creatures that,

while they seem to be related to the segmented worms

and are rather more complex than they are,

are nonetheless quite unlike any creatures alive today

or any other later fossils we know of.

You might call them experiments in animal design,

experiments that didn't quite come off.

They weren't efficient enough

to survive in the battle for living

that was becoming increasingly intense.

Look at this one, for example.

It appears to have seven pairs of supports,

and above each, a tentacle with its own mouth.

Even compared with some of today's strange creatures,

it seems grotesque and outlandish.

This five-eyed creature has a long trunk, here bent back along its body.

It was probably used for detecting and manipulating food.

This one is of particular interest,

for it has stumpy little legs down each side.

In this case, there does seem to be a close living parallel.

It's not a sea creature but one that lives in moist jungles.

Peripatus.

Clearly, segmentation was a great evolutionary success.

The appendages on each segment becoming more and more specialised

as legs and gills and mouth parts.

Some of the commonest fossils here are trilobites,

like the one we saw in Morocco.

These had hard shells, part calcium carbonate and part chitin,

and they fossilised well all over the world,

for they swarmed everywhere in the seas of 400 to 500 million years ago,

during November in our "life on Earth" year.

Because their body armour was not expandable,

the trilobites had to shed their shells regularly in order to grow.

Indeed, many trilobite fossils are of these discarded shells.

Sometimes they occur in great drifts.

Here, almost entirely the tail ends,

presumably sorted out by the sea currents as shells are today.

When the complete animal has been fossilised,

we can see from various positions

that some trilobites could roll up for protection, like woodlice today.

More information can be discovered by X-raying some perfect fossils.

They even reveal details of the gut

and muscle fibres inside the animal's body.

But perhaps the most astounding thing about trilobite fossils

is the preservation of their eyes.

Although our knowledge of the internal structure is limited,

the hard part, the outer lens system, is often fossilised in superb detail.

Even the earliest trilobites had compound eyes,

each element providing a part of a mosaic picture,

which in this species gave the animal an almost spherical field of view.

If the fossil eye is sliced,

we can discover how each lens was constructed.

It was a single crystal of calcite,

lined up in such a way as to give the clearest image.

There could be several thousand in each eye.

Later in their history,

some trilobites evolved even more sophisticated eyes.

Here, the lenses are less numerous but larger,

and it's thought that each provided a separate image instead of a mosaic.

By slicing one of these fossilised lenses,

a remarkable discovery has been made.

The lens is really a doublet. It has an upper and a lower element.

This is the line of their contact.

It's almost identical with the design recommended

by mathematicians in the 17th century

for correcting spherical aberration in thick lenses.

Evolution solved the problem for the trilobites

400 million years before man.

The doughnut shapes of the lower lens elements

have been preserved alone in these fossil eyes.

In most cases, it's the upper lenses that can be seen.

Although trilobites possessed

the first sophisticated optical system on earth,

some species were blind.

They must have inhabited dark, muddy waters

where there was no light and no need for eyes.

The great variety of shape and size in trilobites

suggests that they had a wide range of habits.

It's probable that some scavenged their living on the muddy bottom,

whilst others were quite active swimmer-hunters.

Finally, some 250 million years ago, their great dynasty came to an end.

Though one relative managed somehow to hang on.

This is it. The horseshoe crab.

It's sufficiently different from a trilobite,

with this very big head shield,

for us to put it in a group on its own.

It's also sufficiently similar for us to be pretty sure

that the two groups are closely related.

It's got a pair of these eyes on the front,

which are mosaic eyes, very like those of a trilobite,

and underneath it's got a segmented body

with a pair of legs on each segment.

And at the front, a fist with a hook on it.

That is the sign that this is a fully mature male,

because it uses that in breeding.

On a few nights in the spring,

when the moon and the tides are just right, and this is one of them,

these antique animals crawl up out of the sea to nest here on the beach.

This male is one of the advance guard,

but as the night wears on,

there should be hundreds and thousands of them.

Horseshoe crabs are found along the eastern seaboard of North America.

This beach in Delaware Bay

is the best place to see them in large numbers.

Here, at least, we can get some idea of what things may have been like

when their distant relatives, the trilobites,

swarmed in the seas of long ago.

At the centre of each mass is a large female,

and directly behind her, attached by his claws,

is a male who will fertilise the eggs.

Other unsuccessful males crowd around.

The egg mass is laid several inches down in the sand

and remains there while the tiny larvae develop inside.

Because of their shape at this early stage,

they're known as trilobite larvae.

At the next high tide, a month after the eggs were laid,

the sea reaches them again.

The eggs rupture and the larvae swim free.

Thousands will get eaten within hours.

But a few will survive to continue this very ancient line.

Swimming with them are creatures related to those segmented animals

in the British Columbian shales.

These survived unobtrusively

throughout the reign of the trilobites

but have since come into their own - the crustaceans.

This is one of them. A copepod with its remarkable simple eye.

But there are about 3,500 species of crustacean today.

Most of them have adopted a floating way of life

and are the staple food of many kinds of fish, as well as of whales.

They have many lifestyles. Some of them are completely unknown.

This creature has, on a number of times,

been seen holding a tiny jellyfish.

Is it using the jellyfish's stinging cells as protection?

Or is there some other relationship between the two?

Whatever their way of life,

all crustaceans have one problem in common:

the same one the trilobites had.

Their external skeleton won't expand.

So if the animal is to grow, it must be shed.

First, it extracts some of the important salts from its skeleton

and reabsorbs them into its bloodstream.

Then it begins to moult.

Its new skeleton is soft and crumpled, but it quickly expands.

For a while, the animal is vulnerable,

but as the salts are slowly fed back into it,

the shell hardens.

In spite of its problems,

an external skeleton as developed by the crustaceans

is clearly a very effective and efficient way of building a body.

And nothing could demonstrate its potential better

than creatures that live in this bay off the coast of Japan.

Because down on the sea bottom, 600 metres down,

there live the largest crabs in the world.

And this boat is fishing for them right now.

Without the support of water, its long legs flop.

The muscles are not strong enough to hold them rigid in air.

Each leg is a tube down which a strand of muscle runs.

The muscle is attached to a projection from the next joint

so that when the muscle contracts, the joint moves.

It's rather like the arm of an industrial crane,

which has an outer network of steel girders

down which a wire hawser runs.

Of course, the one sort of joint you can't put on such a system

is a ball-and-socket joint which gives a sort of universal movement

that I can have in my shoulder or my thigh.

But the crab deals with that

by having each joint working in different planes.

So one way or another,

it can reach almost anything within its immediate neighbourhood

and convey it with its pincers to the mouth, where it's chewed up.

Its body is protected by this heavy armour of shell

and the crab can tell what's going on around it

because through the armour there project tiny little sensory bristles.

This creature is indeed spectacular,

but every now and again from the waters of this bay,

the fishermen bring up a real giant.

And creatures like this are over 11 feet across.

Most crustaceans, however, are of a more modest size.

Apart from the myriads of tiny ones in the ocean,

there are vast numbers of small crabs and prawns and shrimps,

all with specialised ways of life.

All these, for example,

come from just one small patch of the Great Barrier Reef.

Crustaceans use pigments for camouflage in the most elegant way.

Some, in fact, are very difficult to see at all

unless photographed in close-up.

The crustaceans show clearly what advantages can come

from having a body divided into segments.

Each can bear appendages, and the crustaceans have modified them

into many different tools.

Sometimes they're used for respiration,

sometimes for reproduction,

some as antennae, mouth parts, food manipulators, pincers

and, of course, legs.

An external jointed skeleton has one quality I've not yet mentioned.

Mechanically, it works just as well on land as it does in water.

So from that point of view you might say

the crustaceans are pre-adapted to life on land,

and indeed, one group has made the move.

Quite formidable animals they are too.

This is a rubber crab.

I must handle him with some care

because you can get quite a nip from these pincers.

He uses them to cut down young coconuts, on which it feeds.

It's said that he can even hammer a hole into a mature coconut,

though no-one's actually seen him do it.

He breathes through a chamber at the back of the shell here.

It doesn't contain gills,

but the oxygen is absorbed through the puckered lining of the chamber.

So here's a creature that can breathe on land, move on land, eat on land.

It's true, it has to go back into the sea in order to breed,

but otherwise, it's a fully operational land-living animal.

Other descendants of sea-living invertebrates

have also made the move onto land at various times.

Snails, for example.

Though robbed of the support of water,

they're never able to grow their shells on land

as big as they do in the sea.

It's the segmented animals that have adapted best to land.

And of all those, it's the ones that did it first

who have been most spectacularly successful.

The insects. They emerged some 400 million years ago

and they wrote the next great chapter in the history of life on Earth.