Invasion of the Land Life on Earth

One of the most crucial steps in the story of Life On Earth

happened in a freshwater swamp about 350 million years ago.

The fish began to haul themselves out onto the land.

The land at the time was covered with the first plants.

Very different from these mangrove plants of today,

but nonetheless plants.

In order to get out among them, the fish had to solve two problems.

First, they had the mechanical problem

of hauling themselves onto land,

and second, they had to be able to breathe once they got there.

The way they solved the problem

of hauling themselves up onto the land,

we can see from a small fish

which lives in these mangrove swamps today.

It's in no way closely related to those early fish,

but it does give us an idea of what that scene must have been like.

The mudskipper.

They come up out of the water

to browse on small creatures swarming on the mud.

Their front fins have jointed bones

so the fish can use them as legs to lever itself along.

The mudskipper is not the only fish

to have developed muscular fins like these.

Fossils of one of the first have been found in rocks

laid down just before the time

that backboned animals ventured onto land.

The coelacanth.

Did this extremely ancient fish also use its fins as legs?

Unfortunately, no fossils of them younger than 70 million years

have ever been found, and up to 40 years ago,

scientists concluded that they wouldn't be able to answer

that question for certain as the fish was obviously extinct.

And then, in 1938, a living coelacanth was caught

off the coast of South Africa.

It was the scientific sensation of the century.

Before scientists could get to examine its entrails

and see how they confirmed or denied the deductions they'd made

from the very ancient fossil coelacanths,

the fish was already rotting.

Its guts were thrown away unexamined.

So a huge search was mounted to find another.

Leaflets were printed with pictures of the fish, offering a reward,

and were distributed among the countless fishing villages

off the African coast.

But nothing... until, 14 years later,

a second coelacanth was caught.

It came from a place over 1,000 miles away

from where the first one was landed.

Here in the tiny Comoro Islands,

a small group lying midway between

Madagascar and the coast of East Africa.

The first one, it seems, was a stray.

These waters are the true and only home

of this extraordinary rare fish,

and the people who live in that tiny village

are the world's experts in catching coelacanths.

A villager still had a dried coelacanth which he let me see.

From what we know of the habits of the living coelacanth,

which is not much,

it seems that these rear fins are used for swimming

but the front ones are used for manoeuvring

and for helping the fish to clamber about

along the rocky bottom where it lives.

All the fins have fleshy bases to them.

The fishermen catch them at night from depths of 300 metres or so.

Once hooked, the fish fight valiantly,

and it may take all night to haul one to the surface.

So it's usually dead on arrival.

Scientists have still not been able to observe one alive.

Then, while we were in the Comoros, one was caught.

Although it was weak, it was still alive when the cameras arrived.

350 million years ago, fish with fins like these

were cruising the seas of the world.

Some living in shallow waters produced descendants

which eventually clambered onto the land,

while this creature's ancestors moved down to the unchanging depths,

there to remain unchanged themselves.

The Comorians catch one or two coelacanths a year.

They used not to value them much, for their flesh isn't good to eat.

Now, however, big rewards are offered by scientific institutions,

so the old man who caught this one will soon be rich.

Some researcher in a few weeks' time

will be absorbed in examining the structure of this fin

which scientists agree must resemble closely those limbs

that first took backboned animals onto the land.

But how about that second problem? The problem of breathing up on land.

The gills, which had served them well while swimming in water,

extracting dissolved oxygen, wouldn't work in the air.

How did the fish solve that problem?

Well, this is East Africa and it's the height of the dry season.

There is not a drop of water to be found in this parched landscape.

And yet, here, close by me,

there are fish that are living and breathing in air.

If only I can find them.

Six months ago, this was a pond several feet deep in water.

But as the dry season progressed, the water evaporated

and the fish in it burrowed down into this,

which was soft liquid mud and is now brick-hard.

And there, somewhere, they cocoon.

And that...

That looks like...the nose of one.

Poking out from the mud, there.

Now, if I take this and drop it in a tank of water,

it should seem as though the rains have come early,

and the fish should come to life.

As the water soaks in, the mud softens and falls away,

exposing a papery cocoon of dried mucus.

And there is the throat of this

extraordinary creature that can breathe in air and water.

It's a lungfish.

While its water-breathing apparatus,

the gills, are getting working again,

it snatches another gulp of air.

It's able to breathe air because it has, opening from its gut,

a long pouch lined with blood vessels,

and they can absorb gaseous oxygen through its moist lining.

The coelacanth has no lung, but it has got a simple leg,

that fin with a fleshy base to it, supported by bones.

Neither it nor the lungfish, therefore,

can be close to the ancestral creature that first moved to land.

But if those two crucial elements were to occur in one animal,

then such a creature would be a strong candidate.

And indeed, they do.

This fossil fish, from rocks 450 million years old, has them both.

It's called Eusthenopteron.

When the rock and scales around its fin are removed,

you can see the bones - one close to the body,

then two, then a group of small ones.

Exactly the pattern found in the limb of all land vertebrates.

And that adventurous ancestor may have been very like this.

But why should it have climbed onto the land?

Perhaps it was forced out by droughts.

Maybe it was tempted by food, the creatures that swarmed on the mud.

Whatever the reason,

its descendants came to spend more of their time out of water.

And over millions of years they evolved bodies

that were more suited to life on land

and became the first amphibians.

The vegetation of the time was different from that of today.

There were no flowering plants or conifers,

and one of the commonest was a kind of horsetail,

rather like these growing in the north of England,

except the horsetails then,

300 million years ago, grew to about 50 or 60 feet tall

and formed dense forests growing in swamps.

When they died, the horsetail trunks

fell into the water and formed a kind of peat.

Over the years, there were variations in the sea level

which flooded these swamps

and buried the peat beneath deposits of sand.

Under the accumulating weight of these sediments,

the peat then turned to coal.

And in the mine, you can see the sand that's been turned to stone

and beneath it, the compressed remains of the plants.

And in this particular seam have been found the bones

of some of the animals that crawled in those ancient swamps.

This is one of the most dramatic of them.

It's a skull. Here are its huge teeth,

which are simple teeth, rather like the peg-like teeth of the fish then.

We know that this creature had a paddle-shaped tail

and also four very good limbs.

So it really was a true amphibian.

It must have been a very formidable creature, too.

It grew to a length of about 12 feet.

There were many kinds of them,

and they dominated the land for 100 million years.

The largest amphibian alive today, the giant salamander from Japan,

grows to over 1.5 metres, four feet or so.

Even that is only a quarter as big as its ancestors.

Most of its living relations, the rest of the salamanders and newts,

are very much smaller, a few centimetres only from nose to tail.

Though newts spend much of their time out of water,

they don't go far from it.

In early spring, after hibernating, they must move back into it.

Their skin is permeable. It doesn't retain liquid very well.

If they dry out, they die.

They need to keep their skin moist,

for, like most amphibians, they breathe through it,

supplementing oxygen from their lungs

with more absorbed from the air.

And one final shackle keeps them tied to water.

They have to return to it to breed.

Once in water,

it sheds the thin outer skin used to protect it on land

and takes up an existence that is much more like that of a fish.

It often seems the newt is more at home here than on land,

and indeed, it retains many

characteristics of its fish ancestors.

The males become brightly coloured

and develop flamboyant crests along their backs.

Their courtship is reminiscent of that of fish.

They flex the frills along their backs

just as so many fish flex their fins,

and they beat the water with their tails,

sending powerful currents towards the female,

which she detects with a line of sensors

that resemble the lateral line system of the fish.

Two males are courting one female. She's in the middle,

without a crest.

The female lays several hundred eggs, each stuck to a leaf.

Development is swift. The tiny white sphere elongates.

Pigment appears.

And soon the young emerge,

and they're even more fish-like than their parents.

They have no legs, and breathe not with lungs but with feathery gills.

But slowly, their legs and lungs do develop,

and the newt tadpole for a short period can breathe both ways.

But there's one tadpole that remains like this all its life.

Its external gills are large and feathery and permanent.

It lives in one lake in Mexico and the Aztecs

called it the water monster, axolotl.

But the most surprising thing about this overgrown, eternal tadpole

is that it breeds in this condition.

The eggs start developing immediately.

The black part is the beginning of a body

which will grow round and enclose the cream-coloured yolk.

Food supply for further development.

Though the axolotl never changes

into a land-living salamander in the wild,

it has a close relative in Mexico which retains its options.

Sometimes it breeds like the axolotl, but if its lake dries,

it can turn into a normal land-living salamander.

The tadpoles, still with their feathery gills,

wriggle in the tepid, shallowing pools.

But as time passes, the gills disappear.

For now, the animal has developed lungs.

And eventually the little creature hauls itself up onto the mud.

But many salamanders aren't enthusiastic walkers

and show signs of abandoning the habit.

This one, from California,

has tiny legs and spends its time burrowing under stones.

One entire group of amphibians

has opted totally for this way of life

and lost their legs altogether - the Sicilians.

You might well confuse these with large earthworms.

This one comes from South-East Asia.

Its eyes are covered in skin, and to replace them,

it has small white feelers below its eye.

Their bodies have become elongated

and they've lost all traces of limbs.

Most of them don't come up to the surface until night.

But then you really see that they're not earthworms

solidly champing through soil.

Blind though they are, they're hunters.

This one comes from South America.

Sicilians constitute the smallest amphibian group.

160 species are known, compared with over twice that

for salamanders and newts.

But they're so unobtrusive

and so easily mistaken for worms and therefore ignored,

that there may well be many more kinds

still to be discovered in the soft, damp soils of the Tropics.

But most of the amphibians living in the world today

belong to a third group.

A group that doesn't live below ground like the Sicilians,

but above it, and far from having lost their legs,

they have developed their legs to a spectacular degree -

the frogs and toads.

And this is the king of them all,

the largest frog in the world, the Goliath frog.

It's a very rare animal that lives in a small part of West Africa.

In captivity, it lives on small birds and rats or mice,

and even fish. But in the wild, its diet is not quite so ambitious.

It takes dragonflies and other insects

as well as crabs from the bottom of the river.

It's a very good swimmer,

with very large webs at the bottom of its feet.

But these huge legs...these huge legs

also enable it to jump very well.

This particular one can jump nine or ten feet, ten times its body length.

But in the kingdom of frogs, that's not much.

Some of the smaller frogs can do very much better than that

and are dazzling athletes.

When each foot is webbed to form a parachute,

your leaps are spectacular indeed.

This is the famous flying frog,

though it'd be more accurate to call it a glider.

Even so, in one leap and glide, it can cover 15 metres or so,

say, 100 times its body length.

Several species have developed this talent.

There's one in Japan, another in Malaya,

and this one lives in Costa Rica.

Its feet are not only webbed but each toe ends in a sucker

so it can also cling to vertical leaves, if it has a mind to.

But its unique splendour is only revealed

when it leaps and opens its four extraordinary parachutes.

The flying frog seeks safety by launching itself into the air.

This frog takes refuge underground.

And its pointed nose gives it a very good start.

And then its legs provide a pile-driving thrust.

The holy cross toad of Australia also buries itself,

but it goes rear end first, with a different kind of leg action.

It's easy to understand why they hide.

Frogs to a hungry hunter appear appetising

and vulnerable with their soft bodies.

And indeed, many of them are.

But some have developed defences, and very surprising ones, too.

This grass snake is about to tackle an ordinary European toad.

The combination of standing on tiptoe

and inflating its body makes it look much bigger than it really is.

Whether this frightens the snake or baffles it, who can say?

Whatever its effect, it works.

The fire-bellied toad. Watch.

This extraordinary posture deters predators

by revealing the pattern on its stomach,

a combination of colours that's widely recognised

by animals as a warning. It's not all bluff, either.

All amphibians have mucus glands in their skin

which help keep them moist,

and some of these glands in the

fire-bellied toad produce a bitter-tasting poison.

Skin has become versatile in the amphibians for breathing, defence,

and it comes in all sizes, shapes and colours.

In South America, some frogs have developed defence

so far that they've become real killers.

The poison in their skins is so powerful,

it can paralyse a monkey or a bird immediately.

There are at least 20 different kinds of poison frogs

in Central and South America,

and conspicuousness is an important part of their defence strategy.

They don't want to be eaten by mistake,

for it's of no value to them if their attacker dies

soon after they've been eaten.

So they're all dressed in spectacular colours.

Colour is of no use at night when it can't be seen,

so unusually for frogs,

these little creatures are active in the daytime,

moving boldly around the forest,

confident and secure in their brilliant livery.

This particular species has good reason to be confident.

It has the most poisonous skin secretion of all.

It's only recently been discovered by science.

And just a tiny smear from its skin could kill a man.

The local Indians in Colombia use its poison on blowgun darts

by rubbing the tips on the backs of the living frogs.

One frog in Argentina has developed

a unique way of safeguarding against that,

and, at the same time, keeping itself watertight

when the weather's dry.

It gives itself a varnish.

There are many wax glands in its skin,

and when it feels its body is drying out,

it gives itself a good going-over to produce a thin,

waterproof covering.

But there is, of course, another opposite strategy.

If you're without defences of any kind,

then it may be much more effective to spend the day concealed,

camouflaged as part of a leaf.

Some conceal themselves not to escape, but to lurk in ambush.

This big toad will pounce on mice and fledglings as well as worms.

Cleaning earth and twigs from the worm is important,

for the toad has no teeth and swallows whole

whatever gets into its mouth.

It doesn't want any hard or spiky, inedible bits.

A tongue that can be stuck out is an amphibian invention.

No fish ever had one, and very effective it is, too.

The blink is an essential part of swallowing.

Frogs and toads have no bony base to their eye sockets,

so their eyeballs bulge down into their mouths.

When they blink, the underside of the eyes

helps to squeeze the food in its mouth back towards its throat.

Their tongue is not attached to the back of the mouth but to the front,

so they can stick it out much farther than we can,

which is very useful for an ungainly hunter without a neck like a toad.

Its end is sticky and muscular

and it grabs the worm with the underside.

And then the tongue has one final function. It lubricates the food

so it can be swallowed easily

without scratching the delicate membranes of the throat.

They eyes of the amphibians are fundamentally the same

as those of fish. There was no need to change them,

for they work as well in air as water.

But they have to be kept moist and clean,

so the amphibians have developed an ability to blink

and a membrane to wipe the surface.

Protection from strong light by closing the iris.

Or by using a membrane which still lets light in.

In air, however, you do need a different hearing apparatus

than in water. Eardrums. And with them came a voice.

Some frogs call during the day, like these edible frogs.

RASPING CALL

But most sing at night. Before the amphibians had crawled

out of the water 300 million years ago,

the only animal sounds on earth had been chirps of insects.

So the first animal chorus to break the silence of the land

may well have been like this.

WAILING CALL

The frog's lungs which blow air through its tiny vocal cords

are feeble. But resonating sacs bulging from the

angle of the jaws or the throat amplify it many times,

so that some calls can be heard for over a mile away.

HIGH-PITCHED CALL

SHORT CHIRPS

TWANGING CALL

HAMMERING CALL

VARIOUS CALLS

CHORUS OF CALLS

The cue for these choruses is usually a change in the weather,

for these songs are the prelude to mating.

In the Tropics, the trigger is usually

the onset of the rainy season.

THUNDER CLAPS

As the forest is drenched,

so the moisture-loving amphibians can get out,

seeking mates and laying eggs.

In this very rare species from Costa Rica,

the male is yellow, the female red and brown.

They abandon their eggs after they've laid them to return to land.

Streams and ponds and other such spawning sites

often swarm with fish that will eat any eggs or young they can find.

So hundreds must be laid if just one or two are to survive.

In temperate areas, breeding begins when the weather warms in spring.

European toads migrate from miles around to a single favoured pond

and assemble there in great numbers, all within a few days.

The breeding period may only last a week or so,

and towards the end, females with eggs still to lay become rare.

And the males, in their frenzy to couple with them,

clasp anything in the neighbourhood that moves, male or female,

and so form tangled, writhing groups.

These toads rely for breeding success on numbers.

A single female may deposit 6,000 eggs.

Mass production, be it in temperate or tropical places,

seems to be a very effective strategy.

These tadpoles, for example, developed from eggs

that were laid in enormous numbers

in pools beside this South American river.

But amphibians have another option.

They, after all, can climb up onto land,

so they can lay their eggs in places no fish could possibly reach.

When they had the land to themselves,

that must have been a particularly effective strategy.

Even today, there are many frogs

that go to quite extraordinary lengths

in order to lay their eggs away from ponds and rivers.

Of course, they have to keep their eggs moist, or they would dry out.

But around this river, at any rate, that's not too difficult.

These are the Kaieteur Falls on the Potaro River in Guyana.

The forest round here must be paradise for frogs,

for here, in effect, there is permanent rain.

And quite warm rain, at that.

The Potaro, above the falls, is 100 or so metres across,

and its waters fall sheer for over 200 metres.

Much of these great masses of falling water

turn to spray and drenching mists.

And as the mist comes swirling up, it condenses into drops

which fill the centre of such plants as this

and turn them into miniature ponds, idea for the frogs' purposes.

Inside this particular one lives a tiny, beautiful, golden frog.

It shares its minute pool with a few larvae,

but nothing that does it or its eggs any harm.

There are many plants with water-filled chalices in their

centres in the South American rainforest,

and many of them grow high up on branches,

their roots dangling in the moist air.

So they are, in effect, ponds up trees.

They provide the frogs with little oases

where they can live and spawn away from predators for generations

without ever coming down to the ground.

In the African savannahs, with much less rain,

there are no such plants, but there is a frog

that manages to breed in the trees.

Instead of water, it uses foam. The trick is done at night.

The female excretes a liquid

which she beats into a lather with her hind legs.

The male joins her and fertilises

the 150 or so eggs which she deposits in the foam.

The sun will bake the outside into a hard crust.

But inside, it remains liquid, and there the eggs develop.

The nests are always made above water,

so in due time, when the crust cracks and the young ooze out,

they drop straight down into a river or a swamp.

This frog from South America also

has a way of keeping its eggs away from the river.

Here, however, where the air is more humid,

it doesn't need foam,

because the jelly surrounding its eggs doesn't dry out.

The young tadpoles develop inside the jelly, like many other species,

but these stay there while they go through

most of their larval development.

They even develop gills inside the egg.

And their hearts begin to beat.

Eventually, they too will emerge and drop down into water.

The tadpoles of one Caribbean frog have managed, astonishingly,

to dispense with water altogether for their development.

The whistling frog lays a cluster of eggs on the ground.

They're only small, but inside each there is liquid.

And in it, the young develop not only to the tadpole stage,

but beyond.

Their tiny stomachs are full of yolk

that must fuel their entire development.

The front legs are formed.

And so are the back legs.

And at last, it becomes virtually a tiny,

fully-developed replica of its parent.

On its nose, it has a tiny spike,

and with that, it punctures the egg membrane.

And after about 18 days, it hatches, having eliminated altogether

the tadpole's normal need for open water.

Laying eggs away from water and its dangers

is a successful breeding strategy.

But yet other frogs have taken a different line.

Instead of abandoning their eggs in a safe place,

they stay with them and look after them.

The midwife toad lives in Europe. Its name is not accurate

because it's the male that carries the eggs entangled round his legs.

There may be 60 or so of them,

and he carries them for six or seven weeks.

At hatching time, he takes them down to water and the tadpoles swim away.

Some toads spend all their time in water.

This pipa from Brazil,

instead of laying and abandoning 6,000 eggs like the European toad,

lays a mere hundred or so.

But they look after them in the most extraordinary manner.

The male, with these elegant movements of its hind feet,

takes care that as many eggs as possible

are gathered on the female's back.

And they stick.

Then the skin on the female's back begins to swell.

The eggs rapidly become embedded in it.

Soon, a membrane grows over them to enclose them completely.

After only 30 hours, almost all the eggs have disappeared

and the skin is complete again.

After nearly three weeks, it's moving.

And then, after another three weeks or so, the young begin to emerge.

Now the parent leaves the young to fend for themselves.

But at least they're now independent swimmers,

able to find hiding places.

So a higher proportion is likely to

survive than if they'd been abandoned as eggs.

This little South American frog

also keeps her eggs and young on her back

in a pouch with an opening just above the base of her spine.

Her developing young remain inside it for three months or more,

until at last she releases them into a pool as tadpoles.

They're on their way towards their final change into adults,

for they have back legs.

These many differing ways of carrying their developing young

may seem extraordinary enough, but other frogs in South America

actually retain their tadpoles inside their bodies,

in the most unlikely parts.

There is one which calls in the beech forests of southern Chile.

It's Darwin's frog.

These creatures, only a few centimetres long,

are all males, even though they vary in colour.

They're still calling but the breeding is over.

The females have laid their eggs in groups of 20 or 30 on the forest floor.

As soon as the males see a movement in the eggs, they will, apparently, eat them.

Each male may take a dozen or so but he doesn't swallow them.

Instead, they go into his vocal sac down the front of his throat

and there they develop...and wriggle.

The males sit about, struck dumb by their own offspring.

After some weeks, their extraordinary

vocal pregnancy comes to an end.

And here is that amazing birth once again, in slow motion.

The male's vocal sac is now ready again for singing,

before it's turned next season once more into a nursery.

The prize for the care of the young must surely go to this frog,

that lives only on a remote mountain in West Africa.

The female, only about a centimetre long,

keeps the eggs inside her distended oviducts

and holds them there throughout the nine months of the dry season.

As they grow, she secretes internally some white flakes.

The tadpoles, moving around freely inside the oviduct,

eat the flakes and digest them in

their gut just as they would do in the pond.

When the rain comes, she gives birth.

Her stomach and oviduct don't have muscles which can expel the young,

so she does it by bracing her body against the ground with her forelegs

and inflating her lungs so they bulge into her abdomen

and squeeze the young out.

And they're born fully-formed froglets,

a triumph of parental care.

By providing their young with moisture of some kind

and using all these varying and astonishing techniques,

frogs and toads have managed to colonise almost all the world.

Even so, you would think that with their thin, permeable skins,

they would never be able to survive in the Australian desert.

But one or two species manage to live even here.

They spend nearly all their lives in the ground, away from the sun.

They may lie here for years,

waiting, but eventually the rains do come.

When the frogs burrowed down here during the last rains,

they were bloated with water,

and they've conserved it in their chambers

by sealing themselves inside a membrane secreted from their skins.

But when the rain arrives again, they must get rid of their packaging

to be ready for breeding.

In the brief period when the desert is wet,

these frogs will dig themselves out and mate,

and their tadpoles will develop

in the few days there's water in the pools.

Then, as the desert dries out, the young frogs will bury themselves

and remain underground for perhaps five years or more.

In times of drought, the desert aborigines

search eagerly for frogs like this, and this is why.

If you squeeze one, you can get a reasonable drink of water from it.

It's tasteless and really quite drinkable.

And now I'm going to have to find a pond for this little creature,

where it will survive until the next rains come.

For the fact of the matter is

that its success as a desert liver is limited.

It can only be active and breed

during that short period when there's rain.

In order to survive in a desert and breed there,

if necessary with no rain at all,

you need a device that no frog or amphibian has got.

This.

An egg with a waterproof shell.

That was the next great evolutionary breakthrough.

And it was achieved by the reptiles.

The astonishment is that without it,

amphibians managed to colonise so much of the world.

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