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 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

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 up.

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 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 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 on land.

And over millions of years they evolved bodies 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, 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 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 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.

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 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 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 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.