Original subtitles

An eye - from another world.

A smell-detector - investigating the path ahead.

We don't often see a snail that way.

And that's because we've only recently had the tiny lenses

and electronic cameras that we need to explore this miniature world.

But when we meet its inhabitants face to face,

we suddenly realize that their behavior can be just as meaningful

to us as the behavior of many animals. More our own size.

Look at this, for example.

It's an earwig.

Yes...

But it's also a female and a mother -

and, like so many mothers, she's guarding her young.

These two ants are not quite sure whether they like one another.

Stroking antennae is the equivalent of a cautious chat over the garden fence.

When big animals go courting,

they show off- and so do damselflies.

Courtship signals for a male wolf spider are rather more frantic

because if his female doesn't understand why he's approaching her,

she'll eat him.

This ant is a farmer - and these aphids the cows which it milks

for a drink of Honey-dew every day.

Other ants are eternally on the march.

Powerfully armed soldiers guard the flanks of their column as they travel,

protecting the workers who are carrying their helpless Young.

When it comes to craftsmanship,

few can beat this wasp using mud to construct an elegant jar

in which to store her eggs.

Mud is also used by termites.

They build tower blocks that, in proportion to their size,

are taller than New York sky-scrapers.

These two worlds - ours and theirs -

influence one another to an extraordinary degree.

If we and the rest of the back-boned animals were to disappear overnight,

the rest of the world would get on pretty well.

But if they were to disappear, the land's ecosystems would collapse.

For the fact is, they were the pioneers,

the first animals of any kind to colonize the Lands of the earth.

To tell their story,

we must go back to a time when the world was a very different place.

Some four hundred million years ago,

the lands of Planet Earth were totally without life.

They were bare, naked rock, roasted by sun during the day,

freezing cold at night and swept by terrible storms.

But in the waters of the world, conditions were much more stable.

Life had begun there some two thousand million years earlier still.

For a long time it remained microscopic.

But eventually larger animals appeared

- jellyfish, corals, starfish and snails,

and animals with segmented bodies.

All needed food.

Many would have eaten unguarded eggs, given the chance.

And then, around four hundred million years ago,

some enterprising creatures found it safer to lay their eggs out of the sea,

up on a beach.

They still do.

Every spring, on a few special nights along

the Atlantic coast of North America,

thousands of horseshoe crabs emerge from the sea

And here, in the wet sand, they spawn.

They may only stay for a few minutes or hours,

but animals like these may well have been the first

of any kind to leave the sea and venture on to land.

Although these creatures spend virtually all their lives at sea,

they can survive surprisingly well on land.

It's almost as if they were pre-adapted.

They have shells - external skeletons -

and that means that their legs are rigid and jointed.

And at the back they have a series of plates called book-lungs

which extract oxygen from sea water, but can also do the same thing

if they're kept reasonably moist, from the air.

So creatures like this can in fact, spend about a week on land.

And it only requires minimal modifications

to enable them to live up there permanently.

It was difficult to abandon the sea altogether until the land became green,

but eventually, it did.

Simple plants - algae and then mosses and liverworts -

began to advance over the mud and rock to clothe the earth.

And into these first green tangles came animals looking for food.

Some had armor,

for that in the sea had protected them from their enemies.

Now it would help them conserve moisture.

They were the ancestors of today's millipedes.

Small holes had developed along the underside of their bodies

that led to internal tubes with which they could absorb oxygen from the air.

Their rigid jointed legs, however,

were largely unchanged and worked very well on land -

even without the support of water.

Battering-ram heads enabled them

to bulldoze their way through the vegetation

to collect the rotting plants on which they fed.

They grew big, increasing the number of segments in their bodies.

Some had over three hundred -

each with two pairs of legs.

Some that didn't curl up

reinforced their armor with plates along their backs.

Crustaceans like shrimps came too.

They were the ancestors of woodlice.

So today, there is a huge and varied population of animals,

living on the land with bodies that are little different

from those of their ancestors who lived in the sea so long ago.

And they are extraordinarily successful -

some are the most numerous of all land-living species.

But we seldom see them.

This pin will give you an idea of why.

They are tiny.

This minute little creature is a springtail.

It's less than half a millimeter long - the size of a full stop.

In one square meter of soil, there maybe over ten thousand of them.

Drying out is a very real danger for them

and some waterproof themselves regularly with a droplet

of special grooming fluid.

You might even say they have turned bathing into an art form.

They even have two inflatable tubes that enable them

to get to those hard-to-reach laces.

To help them get around through the leaf litter,

these springtails - as their name suggests,

have a rather novel way of jumping.

They have a tiny two-pronged lever beneath their abdomen.

One small flick from it can catapult them six inches -

some fifteen centimeters - into the air.

It's the equivalent of a human being jumping over the Eiffel Tower.

And if they happen to land upside down - well,

they have a special way of righting themselves.

They use their grooming-fluid dispenser to stick onto the ground

so that they can pull themselves back onto their feet.

So the foundations were laid for

the ecosystems that now flourish on earth -

and on which we ourselves depend.

It has to be said, however,

that sometimes some of us regard a few of these pioneers

more as our enemies than our friends.

Many of the mollusks in the sea developed shells

to protect themselves from predators.

But on land, those shells served just

as well to keep the occupant nice and moist.

So without any major change to their anatomy,

mollusks were able to creep up out of the water

and graze in the forests of algae and mosses

that were then spreading over the land.

And given the right conditions they still do.

With rain and the coming of night, a secret army comes out of hiding.

These are the conditions they like best.

Dark - and - best of all - wet!

Gliding along a carpet of slime works

just as well on land as it does under water.

And a rasping tongue scrapes algae off rocks wherever they are.

In times of drought,

snails may be unable to move around for months on end,

so when conditions are right, they eagerly set off to find food.

Their upper pair of tentacles carry those eyes with which they look around.

The lower ones smell what's beneath.

They breathe by means of a small pouch on the right hand side of their body,

just within the shell which, because it's permanently moist,

is able to absorb oxygen.

This is what they're seeking - a succulent green leaf.

No time to be lost.

Dawn will bring a change in conditions.

So they have to return to their shelters

and clamp down their shells once more so that they retain their moisture.

The ancient forests were colonized by all kinds of plant-eaters long

before there were any frogs or lizards, birds or insect-eating mammals.

But there were, nonetheless,

hunters prowling through the vegetation.

This was one of the first.

Fossils very like it have been found in rocks

that are five hundred and forty million years old.

This is a velvet worm.

It too has scarcely changed since it lived in the sea,

and today it's only found in wet, humid forests.

It usually hunts at night,

but infra-red cameras can reveal it in action.

Soft stumpy legs enable it to move in total silence

and it finds its way with long sensitive feelers.

It's a master of stealth.

This cricket has huge eyes,

but its very difficult to see what's going on around it.

Although the velvet worm has fangs, it will attack its prey,

when it finds it, with a very special weapon.

Anxiously, the cricket probes around

in the darkness with its long antennae.

The velvet worm will only know if it's found its prey

when it touches it.

So when it does, it has to react immediately.

There!

And the cricket is trapped.

A slow motion camera shows the remarkable way

in which the velvet worm attacks.

Two nozzles beneath its feelers squirt twin streams of glue.

The more the cricket struggles, the more it becomes entangled.

With the prey immobilized,

the velvet worm reclaims its glue by eating it

and then it starts on the cricket.

There were other hunters too in the ancient forests -

relatives of the horseshoe crabs and they were even more formidable.

This is a whip-spider.

Like its ancestors, it has a hard external skeleton.

Two of its limbs have been turned into highly mobile sensitive feelers.

It uses them to probe around delicately both in front and behind.

Any prey within a foot of it will be immediately detected.

It's extremely territorial and it has no hesitation

in attacking one of its own kind.

The width of its claws is a good indicator of strength

and a smaller animal will quickly back down.

But these two are equally matched - and they will fight.

The loser retreats.

But even whip spiders were not

the most formidable hunters in these forests.

There were others with an even more venomous weaponry.

This centipede has powerful jaws, poison fangs and is very, very fast.

It's a very good hunter,

but it's only half as long as my little finger.

There are centipedes in the world, however, that are as big as my fore-arm.

This is one of these alarming giants.

It's over thirteen inches - 35 centimeters -

long and with the muscular strength of a small snake.

And the poison in its black-tipped fangs is lethal.

It hunts in the dark, bat-haunted caves of Venezuela.

Like the whip spider and the velvet worm,

it uses its antennae to feel for its victims.

The beetles that swarm on the rocky floor

of the cave are of no interest to it.

It's after bigger prey.

And it knows it can find that by climbing.

Its many legs give it a secure hold on the vertical rocks.

It's heading for the ceiling.

Now, in the darkness, it can sense bats flying past it.

Holding on with its hind legs, it reaches out into their flight path

and almost immediately it has one.

An injection of venom from its fangs kills

the bat almost instantaneously.

It will take it an hour or so, but it will eat all the bat's flesh.

So all these animals, having left the sea,

solved the problems of moving around and breathing air -

in their own differing ways.

But there was another difficulty. Mating.

In the sea, animals need only release their eggs and sperm

and the water mixed the two together.

On dry land, that couldn't happen -

even for the most moisture-loving of creatures.

An individual slug carries both male and female organs,

but even then, that was of no help.

Each had to both give and receive.

Somehow or other, pairs of individuals had to get together.

And the ways they have evolved in which to do so are quite extraordinary.

Indeed, some of them are almost beyond imagining.

The leopard slug, you might think, has the simplest of habits.

Maybe. But not when it comes to mating.

When an individual is looking for a partner,

it gives its trail of slime a special taste that advertises the fact.

Another, if it feels the same way,

will detect the invitation and start to follow.

The pursuer, to confirm that it's there and ready to mate,

gives the pursued a nibble.

The leader heads upwards. An overhang is what's needed.

The underside of a branch will do very nicely.

The two start to circle one another more and more closely until they entwine.

For an hour or so they continue to wind themselves around one another.

Then suddenly the pair release their hold on the branch

and start to slide downwards on a rope of mucus.

Now in midair, they move to the next stage in their pairing.

Each exerts its male organ from just behind its head.

These grow longer and longer.

Then they too begin to entwine.

They fan out to form a translucent flower-like globe.

And now at last, sperm passes from one slug to the other.

The transfer is complete. Each has been fertilized.

Finally, their strange balletic relationship comes to an end

- with a bump.

A millipede, unlike a slug, is either a male or a female.

In southern Africa, where there are many different species,

both sexes spend the inter in hibernation,

curled up in the leaf litter or beneath the bark.

As the temperatures rise with the coming of spring,

they all unwind themselves and set off to look for a mate.

Finding one in the tangled undergrowth is not easy.

But this male forest millipede knows

that he can increase his chances if he heads upwards into the trees.

Leaving the safety of the undergrowth may seem a risky thing to do,

but these millipedes secrete a poison from pores in their armor

and their conspicuous red

and black colors warn predators to leave them alone.

They emerge in thousands.

Surprisingly perhaps, a male, when he does find a female,

is not met with a friendly greeting. Quite the reverse. She coils up.

This is her way of sorting out the men from the boys.

Only the strongest and fittest male will have

the strength to force her coils apart.

To help him do so, he has white suction pads

on the bottom of his feet which give him a good grip.

Eventually, she relaxes and he lifts her up

so that he can extend two specially modified legs

with which he inseminates her.

Once inserted, these legs swell so that the partners

become fastened together.

And that's important,

because it will take him a couple of hours to transfer his sperm.

But there are lots of males around,

and before long, another one turns up.

The new arrival checks out the pair with his antennae.

If they're not tightly bound together he may have a chance of taking over.

He pushes between them, levering them apart.

Gradually he manages to unzip their legs.

The first male's white mating legs are dragged out.

He's been defeated.

It will be the second, stronger male who fertilizes her eggs.

So mating on land isn't as random as it had been for so many in the sea.

Now it's selective.

But brute force isn't the only basis on which to select.

A female springtail is bigger than a male and she prefers a partner

who can give her a sustained head-to-head push.

Other males are eager to try their luck -

but butting her sides won't get them anywhere.

She seems unimpressed by any of them,

But one is determined to stay as her dance partner.

She simply can't get rid of him.

He confidently signals victory with a couple of fancy twirls.

Then he deposits a droplet of sperm on to the leaf -

and she graciously takes it on board.

One group of colonists were of particular importance,

for they changed the nature of the soil and thus made it possible

for new kinds of plants and animals to evolve.

They outweigh all other animals in any given area of the forest.

A single hectare may be home to eight million of them.

They spend nearly all their time below ground.

Worms. They eat their way through the earth,

extracting edible vegetable material and making it suitable for plants.

And at night,

they come to the surface and collect dead leaves.

They also take the opportunity to call on their neighbors,

poking their heads into next door burrows.

They're looking for partners.

Like slugs, they're hermaphrodite,

each individual both male and female.

They mate by lying alongside one another.

Two narrow grooves form between their two bodies.

These are the conduits that carries sperm from one partner to the other.

Their bodies slowly pulse as sperm travels along the space between them.

But the process is a long one and it may be three hours

or so before they separate, each carrying the other's sperm.

Like so many of the inhabitants of the undergrowth,

earthworms can only live in a Moist environment.

But they are found in soils of every continent except Antarctica.

This small valley in southern Australia is home to one of the rarest

and the most extraordinary of all earthworms.

And I know they're around because I can hear them.

Those gurgling noises, believe it or not,

are being made by giant earthworms Megascolides australis

as they squelch along their water-filled burrows.

The vibrations of my footsteps are enough to stir them into activity.

They never come to the surface.

But in places where there's been a small landslip,

you can sometimes find their burrows.

They are over an inch in diameter.

And in them, if you're very lucky, you may occasionally find one of these.

This is one of their egg cocoons - and it's enormous.

If I hold it up against the light,

I can see the young worm inside, wriggling.

It will take a year for this to develop

and when the young one finally does break free,

it's already twenty centimeters long.

Huge!

It will take a further five years to reach full size -

and become this remarkable creature.

So the question is, how long is a giant earthworm?

Well, it's not an easy question to answer.

The fact of the matter is

they are rather delicate creatures and they break.

If I were so unfeeling as to try and stretch it,

well I guess it might stretch to a couple of meters -

almost six feet long.

How long they live?

Well, some say up to twenty years, but we really don't know.

And we certainly don't know how they manage to mate deep underground

as they squelch their way through their lonely tunnels.

The land may have been a safe place for eggs

when horseshoe crabs first laid theirs up on the beach,

but as new kinds of animals appeared so it

became increasingly important for animals to protect their eggs.

Most creatures just hid them, but a few now actively defend them.

The builder of this circular mud wall,

in the Central American rain forest is one.

During the day, he conceals himself.

But when night comes he emerges to inspect his collection of eggs.

His body is smaller than a grain of wheat.

He's a relative of the spiders - a harvestman.

His eggs - up to a hundred of them -

are half buried in the floor of his nest

and he regularly inspects each one of them.

If it has a fungus on it, he carefully cleans it

before putting it back into its moist bed.

He also continually repairs and improves his nest,

for females will only call on those who have well built and well-kept ones.

Some males, however, follow a different policy.

They don't bother to build a nest for themselves.

They try to take over an existing one.

A nest-holder has to leave sometimes,

to feed and that gives an intruder his chance.

But the owner is back almost immediately.

And they fight -

trying to bite one another in the weakest point of their armor -

the joints of the legs.

The intruder retreats - and the nest owner checks his eggs.

No damage done.

And then another, more welcome, visitor arrives.

This is a female.

She is bigger than he is and she's touring all the nests

in the neighborhood to choose the one where her eggs

will be best cared for.

She seems to approve of the standard of his house-keeping.

So now, face-to-face through the tangle of legs,

she mates with him.

He has a rod with which he injects his sperm.

He withdraws and she's been fertilized.

Half an hour later she lowers her white tubular ovipositor,

feeling for a suitable place for her egg.

She thrusts the egg into the floor of the nest

and then covers it with a thin blanket of mud.

She leaves. He will now tend and guard the egg

with the rest of his collection for the month that it will take to hatch.

His nest is clearly one of the best in the neighborhood,

for throughout the night, a succession of females call on him.

But not all have come to lay.

Life is not that simple.

This one starts, as usual, with a routine inspection and then,

without more ado, she mates.

He waits for her to produce a new egg.

But nothing appears.

He constantly checks the nest floor with his feelers -

but there are no signs of a new egg.

And then she grabs one from his collection -

she wants to eat one.

She grabs again.

He bites her leg joints and tries to pull her away.

She's had enough. And he has rescued his egg.

He checks it over,

cleans it with great care and then takes it away to rebury it.

A month after the eggs were laid, his young begin to emerge.

The skins from which they hatched provide them with their first meal.

He will now guard his young for a couple of days

until they leave the nest.

Excellently adapted though harvestmen are to a life on land,

they cannot survive for very long away from this damp undergrowth.

In fact, most of the direct descendants of those early colonists

that came from the sea,

are still trapped in a world of moisture.

Those with no external skeletons are always

in imminent danger of death by drought.

Even those with exo-skeletons are not safe,

for most have armor that is not totally water-tight

and will eventually dry out

and die if they leave the dank shelter of the undergrowth.

But beyond the reach of the forests,

in the center of continents where little or no rain falls,

there is a very different territory - empty and hostile.

Here there is little shelter from the scorching sun.

Temperatures rise above seventy degrees centigrade. -

and there may be no rain whatsoever for years on end.

Deserts, like this one in the south-west of the United States,

represented the ultimate challenge for those ancient creatures,

whose ancestors first left the sea.

Here there's virtually no water at all.

And yet those early creatures,

the very first to walk on land, reached even here.

And they're still around.

In order to survive the ferocious heat of the day,

they take refuge in little burrows like this,

which go quite a long way down into the ground.

But I can use this special optical probe

to see whether anyone is at home.

And there it is. It's a scorpion.

They won't come out for the rest of the day, but at night,

when it gets cool scorpions all over the desert will be emerging.

And then we have a very special way of finding them.

In ultra-violet light, scorpions are magically transformed.

They glow with fluorescence.

So with an ultra-violet torch,

you can get a better idea of just how abundant scorpions actually are -

even in this arid wilderness.

That's because they have managed to develop external

skeletons that are virtually watertight.

They also have powerful stings

and pincers so getting together to mate could be dangerous.

A male looking for a female must be careful.

She is powerful enough to kill and eat him.

So he begins to dance.

Is she impressed?

Apparently so - and his solo becomes a pas de deux.

But stings are still held high, ready to strike.

She tries to sting him.

His response is to give her a dose of her own medicine with a quick jab.

But it's so slight it merely makes her a little drowsy.

At last, she seems more amenable.

He pulls her to a part of the dance ground

that is smooth and level.

He has extruded a small packet of sperm on a stalk glued to the ground.

He maneuvers her so that as she dances she goes over the stalk -

and takes the sperm-packet up into her body.

The nuptial dance is over.

Her fertilized eggs stay within a special chamber

in her body for more than a year while they slowly develop.

And then in her burrow deep underground, she gives birth.

She has produced up to fifty young ones.

They cling tightly to her back for a few weeks after birth,

each sustained by a small blob of yolk in its stomach.

And then at last they're all ready to venture

into the open desert for themselves.

By colonizing this, the most hostile of environments,

the first animals to walk

on land finally broke their link with open water.

And they did that about three hundred million years ago

at a time when the animals with backbones -

including our own ancestors - were still swimming in the seas.