Conquest of the Waters Life on Earth

Life began in the sea.

The water carries oxygen so that creatures can breathe,

and microscopic organisms to provide them with food.

It's a rich world, it covers three quarters of the planet,

and the fish are masters of it.

The world of water is a varied one.

But the fish, by developing into thousands of different forms,

exploit almost every part of it.

Collecting different food requires different-shaped bodies.

And some are quite unexpected.

They've developed a multitude of different ways of propelling themselves through the water.

In size, they vary enormously.

There are giants.

A grouper like this can grow to be twice as long as a man.

Others are so tiny that they can slip inside a big fish's mouth and pick its teeth for it.

Fish have developed some surprising ways of finding their way about

in this varied underwater world.

The four-eyed fish has eyes divided horizontally

so that it can look above the surface and below it at the same time.

On the other hand, the cave fish,

which normally lives in eternal blackness, has no eyes at all.

How did this astounding variety come about?

What were the earliest fishes like,

whose descendents now exploit the resources

of the seas, lakes and rivers in such a multitude of different ways?

The answer may lie with one of the simplest organisms in the sea.

It's a tiny, insignificant little blob of jelly.

And amazing, indeed, fantastic though it is,

there are good reasons to suppose that it was a creature like this

that gave rise to a line which led not only to the fish,

but through them to the amphibians, reptiles, mammals and man.

It's called, not very attractively but quite accurately, a sea squirt.

And to know why, we have to look at it in water.

Its structure is very simple indeed.

Just a U-shaped tube enclosed in jelly.

It sucks water in at the top,

passes it through a grid inside the body

that filters out the food particles,

and then squirts it out at the side.

When it first hatches, however, it's rather different.

And here is the clue that links it to fish.

It has a tail with a thin, flexible rod in it.

Little bunches of muscle are attached to the rod

so that the animal can swim

by beating it from side to side.

In front, it has some sensory pits,

so it has some perception of its surroundings.

We know that this is a very ancient body pattern.

A fossil creature with both these characters has been found in rocks 530 million years old.

Here again, those bunches of muscles attached to a rod.

It's larger, but built on the same principles as the young sea squirt.

And a creature very like this still survives today - the lancelet.

This tiny sliver of flesh has no jaws,

just a mouth surrounded by tentacles.

The bunches of muscles attached to the rod in its back

enable it to swim with an S-shaped wriggle,

each bend pushing against the water so the creature moves forward.

Here it's filmed in slow motion.

It's an action that's going to appear again and again in what is to come.

Lancelets live half-buried in the bottom of the sea

with just their heads projecting above the gravel,

so that they can filter-feed.

Another creature has the same kind of lifestyle

and is built on similar lines, and it swims in the same way as the lancelet.

It's a lamprey.

And later, in some species, it will change from filter-feeding to a parasitic way of life,

using a rasping sucker at its head end.

It extracts oxygen from the water,

and continues to suck it in at the mouth and expel it through gill slits on the neck.

Its close relative, the hagfish, lives in the sea,

sometimes burying itself in the mud,

sometimes fastening itself to fish with its teeth and eating with a sucker-like mouth.

So, and judging from the design of their bodies and the way they move them,

there does seem to be a connection between the young sea squirt, the lancelet

and the hagfish and the lamprey.

But although the hagfish looks like a fish, it's not one.

It has no strengthening to that rod in the back,

no real backbone and no jaws.

Of course, it could be that the reason that the lamprey and the hagfish haven't got any jaws

is not that they are primitive creatures that never developed them,

but they are degenerate ones that lost them.

The way to find the answer to that is to look in the rocks.

The earliest fossils of shells and corals appear about 600 million years ago.

And then, for 200 million years, there's no sign whatever of any backboned animals.

But then, suddenly, they appear.

Some of the finest specimens have been found in these ancient rocks

at the mouth of the Severn, in the west of England.

And these creatures have no jaws either.

They have scales down their flanks and a head covered by a heavy bony shield.

They must have swum by wriggling this body

and pushing their head along the bottom.

And at the front, between two small eyes, there is a nostril.

In fact, it's a kind of lamprey in armour.

At the time of which we are talking, about 400 million years ago,

the face of the Earth was not at all like what it is today.

The relationships of the continents, the ocean basins, the coastlines, all were very different.

Only in a few places can you today get a clear picture of what those ancient shores were like.

And here, in Western Australia, in the Kimberly Ranges,

there's one of them.

And the best place to see it is from the air.

Rising above the parched and sandy scrub,

there are strangely shaped outcrops of rock.

Those bluffs owe their curious shape not to the erosion of wind and rain

but to the labours, millions of years ago, of coral polyps.

We are flying over an ancient seabed,

with the original coast and the land behind it, now a rocky plateau,

stretching away in the distance.

Once, this plain was covered by a shallow blue lagoon

in which corals built their great constructions of limestone.

Over the millennia, rivers eroded the continent nearby,

washed down the sand and mud

and deposited it over the sea floor.

So the lagoons slowly silted up and the sea retreated.

Then the continent rose, rain and sun eroded the mudstones

and eventually the coral reefs were exposed once more as cliffs on a sun-baked plain.

And here I am walking on the ancient seabed.

The surface of the sea would have been near the top of those reefs.

So here I would have been about 200 feet down.

And the sediments that lay on the bottom of that ancient sea

are still here, turned into sandstones and mudstones.

And in them are the remains of the creatures

that lived in those seas.

Here is one that I picked up only a few minutes ago.

It's the scale of a huge fish.

And this is the flank of a smaller fish with many scales on it.

And this, which is perhaps the least impressive of all,

is actually the most interesting, because this is a fossil skull.

There is the line of its lower jaw.

And if this nodule is treated with acids, the matrix will be eroded away

and expose the perfectly preserved bones of the skull.

These creatures, 400 million years old,

were a considerable advance on the lancelets and lampreys

because they had true jaws.

And on their edges, the scales grew particularly long and sharp

so that the fish could bite and cut.

Jaws armed with teeth

enabled the fish to become very effective food-gatherers,

and so grow into large and powerful creatures.

And some of them became monsters.

Judging from the size of these gigantic teeth,

the shark was about 45 feet long.

It's extinct, but its relatives are very much alive.

Sensitive pits in the front of the head, nostrils,

enable them to detect their prey from great distances.

The hammerhead shark is said to be particularly sensitive.

And this may explain the grotesque shape of its head.

There's a nostril at the end of each side of the hammer.

And the fish habitually swings its head from side to side.

So when the scent is equally strong in both nostrils,

then it must know that its prey lies straight ahead.

That rod in the back has now been strengthened with cartilage.

And the entire skeleton of sharks is built from this soft, light material.

They still swim like the lancelets, with sideways beats of their body

which are restricted mostly to the back half and to the tail.

The thrust created tends to drive the nose downwards,

and to compensate for that, sharks have a pair of horizontal fins on either side at the front,

like the vanes of a submarine.

But these fins are stiff and inflexible.

The shark can't twist them vertically to act as brakes.

Indeed, a charging shark can't stop, only swerve to one side.

Nor can it swim backwards.

Furthermore, since its body is heavier than water, if it stopped swimming, a shark would sink.

The wobbegong, a shark from Australian waters,

has a tendency to do just that.

It's largely abandoned the effort of perpetually swimming

to keep in mid-water,

and has settled on the sea floor where it leads a more restful life.

The transition from continuous swimming in the open sea

to a life more or less permanently on the bottom

can be seen in a series of fishes.

The dogfish is very shark-like.

The angel shark is rather more flattened,

with wide side fins and a rather smaller tail.

The ray has flattened its body to an extreme degree,

dispensing with that rear engine, the powerful thrashing tail,

and expanding the lateral fins

so their ripples can take over the job of propelling the fish through the water.

And it spends most of its time lying on the bottom.

A light dusting of gravel does wonders for camouflage.

The sawfish shark is another bottom-liver.

It uses its extraordinary blade like a double-edged scythe,

excavating in the sand and gravel for shells and crabs

and sometimes flailing through a shoal of fish,

slashing them so that they fall injured and can be eaten.

So, bodies with cartilaginous skeletons

developed into two main shapes.

Long ones, like sharks,

and wide ones, like rays and skates.

But having learned, as it were, to live on the bottom,

some rays took off again.

Undulating side fins are effective motors for mid-water swimming,

provided that speed is not needed.

So they are suitable for fish like the manta ray

that drifts through these surface waters, filter-feeding on plankton.

The blades on either side of the manta's head help to channel the food-bearing water

into the slot-like mouth.

The manta cannot swim much faster than this,

but it wouldn't help its feeding even if it did.

For the water can't flow through the sieve in the gill slits

any faster than it's doing now.

Filter-feeding in the surface of the ocean is clearly a very effective way of life.

It doesn't require much energy, there's an unlimited supply of food,

and some of the fish that have taken to it have become very large indeed.

The basking shark grows to a length of 15 metres - 45 feet.

Only one fish today is any bigger - the whale shark.

And that too is a filter-feeder.

And there, clinging under its tail, is a primitive jawless lamprey,

sucking at its flesh, a reminder of the fish's remote past.

A close relative of the earliest swimmers.

Another filter-feeder - the paddlefish.

But this is only very distantly related to the sharks and rays.

400 million years ago, right at the beginning of fish history,

a group started constructing their skeletons not of cartilage but of solid bone.

And the ancestors of the paddlefish were among them.

And another of these primitive bony fish, the sturgeon.

Not only does it have bone in its internal skeleton,

it also has heavy bony scales in its skin.

It's the eggs of this fish that are made into caviar.

It still swims very like a shark,

with sweeps of its hind body and tail.

And the tail looks shark-like too.

Soon after the bony fish first appeared,

they spread from the seas up the rivers to colonise the fresh waters of the world.

It was an invasion that was to have revolutionary consequences.

The waters of rivers and lakes are shallow compared to the sea,

and often, as a consequence, they get quite warm.

And the warmer water becomes,

the less oxygen it can hold dissolved in it.

That presents a serious problem to any fish living there.

How are they to breathe?

This is one of them, the polypterus. And this is its solution.

It gulps air and then absorbs the gaseous oxygen from a pouch that leads off its gut.

In other words, it has developed a very simple lung.

But an air-filled pouch within the body brings another incidental advantage.

It gives buoyancy. So the bony fish acquired a swim bladder.

A controllable bag of air inside the body.

Now the elements of the modern fish have been assembled.

A swim bladder for buoyancy,

a backbone with muscles attached for strength,

and gills for breathing.

And for further precision and control, there is the lateral line,

a row of tiny pits that are sensitive to pressures and currents in the water.

And so the modern bony fish, like this trout,

is very finely tuned to its world.

This perfection of senses and control of movement

is critical when a pike is on the hunt for roach.

With buoyancy provided by the swim bladder,

the fins can be used entirely for fine adjustments of its position as it hovers.

At normal speed, it's almost impossible to see what happens, it's so fast.

Slowed down, it's possible to see

the enormous acceleration and accuracy.

The actual bite only lasts a split second.

And the prey goes straight in.

I'm standing on the brink of one of the most densely populated parts of the sea.

I'm on the edge of a coral reef at a low tide.

A few feet out there, the bottom sinks dramatically,

and there you will find an abundance of life of all kinds.

Microscopic plants, invertebrates, corals, and, of course, a multitude of fish

that come there to harvest this rich source of food.

Each kind of fish has its own particular place in this mosaic, its own particular food,

and each has, in consequence, developed its own way of swimming,

its own way of using its fins.

The huge number of fish that swarm on the reef, harvesting the great variety of food it offers,

causes considerable social problems.

Each species has its own particular niche on the reef and is designed accordingly.

Many are slim for slipping through the tangle of coral.

Others, like the cowfish, have a rigid box of bony plates

and can stop dead with precise control from its fins.

The trigger sticks its fin-free front half between coral branches to feed.

The angelfish picks off small morsels from the surface of corals,

once again with perfect control.

And this butterfly fish has elongated jaws

that enable it to probe into narrow crevices

with the accuracy of forceps.

For turning sharply, banking steeply, or simply flapping along,

most coral fishes have been able to abandon the S-shaped wriggle.

They've deployed their fins and adjusted their bodies to live in this particular world.

No shark could do this, but then they are adapted to a different kind of life.

The puffer fish doesn't wriggle its body but it does flex its fins, and to great effect.

The S-shape action is now being used there.

And fins have another important role, as flags.

In such a mixed and dense crowd, it's very much to the advantage of every individual fish

to proclaim its presence and identity from among the throng.

So rivals will be aware that this particular food patch has got an owner.

The same markings will also serve, when the time comes,

to attract a mate of the right species.

The sharks and rays have eyes that, though they see shapes, are largely blind to colour.

It's hardly surprising that they are largely drab-coloured creatures.

But the bony fish have excellent colour vision.

And so they are able to signal to one another with stripes and spots and blotches,

and in the most wonderful variety of colours.

The coral fish can risk making themselves conspicuous

because the reef is full of crevices and corners

where they can dart to safety if danger threatens.

Away from the reef, however, the sea is a dangerous place.

For there, there is nowhere to hide, except among your fellows.

And these are designed for a very different way of life.

Fast swimming, fast feeding in the open sea with plankton at the base of the food chain.

And it's that wriggling body action that pushes them along.

Open-water fish often form huge shoals.

And this may be for safety's sake.

The drifting, darting multitudes of fish may tend to baffle and confuse predators.

And if you meet a shark on your own, it'll go for you.

But if you are with others, your chances are much better.

From the plankton to the small fish and on up the food chain to the big fish.

In the open ocean, speed is of great value.

And since water is very dense, 800 times more so than air,

streamlining is of the greatest importance to fish.

Both hunters and the fish they pursue have developed very similar shapes.

Pointed in front and tapering to a two-bladed symmetrical tail at the back.

Barracuda. Among the most voracious and swift of the bony fish.

And this is a hunter's-eye view

of a fish that escapes not by swimming fast,

but in a quite different way.

It's a flying fish. Its front pair of fins are greatly enlarged so that with a flick of its tail,

it launches itself into the air and out of the hunter's sight.

This is the flight in slow motion.

The fish is already swimming fast when it comes to the surface,

and it takes off, helped by the beating tail which has a specially enlarged lower lobe.

The front fins are then spread to assist the glide.

Occasionally they dip their tails into the surface to give themselves an additional boost.

And so they can sometimes fly for several hundred metres.

Some fish have sought safety by going not upwards but downwards.

These eggs, that float in astronomic numbers on the surface of the sea during the summer,

have come from one of the bottom dwellers.

Only one in 100,000 will survive.

But those that do will pass through a most extraordinary transformation

before they become adult.

After about a week, they hatch into what looks like a fairly normal kind of fish fry.

They hang near the surface where it's warm and there's a lot of oxygen,

feeding on micro-organisms.

Each is not much bigger than a pinhead.

Each one still contains a tiny bag of yolk that will sustain it for a day or two more.

The young fish has eyes on either side of its head,

but they won't remain that way for long.

Its body deepens as it begins to feed, and its stomach swells.

And it develops a swim bladder.

Its eyes are beginning to look a little lopsided.

One is higher than the other.

Now they've developed pigment, but only on one flank,

and they swim on their sides with that coloured flank upwards.

These are going to be flatfish.

Turbot, plaice, sole and flounder also go through such a transformation.

And they finally settle on the bottom.

One eye is now on the edge of the fish.

Now the transformation is complete

and the fish has lost that swim bladder,

for buoyancy is a positive hindrance on the sea bottom.

A bony fish has joined the skates and the rays on the sea floor

by the simple if drastic expedient of lying on its side

and moving one eye right round its body.

Many other bony fish have abandoned the swim bladder and settled down.

Each has found its own way of adapting to life where skill in swimming is less important

than an ability to merge into the background of the sea floor.

So fins can be used for all kinds of other purposes.

This looks like a rock lying on the bottom,

but it has a gill, an eye and an upturned mouth.

It's a stonefish, a hunter that relies on invisibility to catch its prey unawares.

And its fins are coloured and shaped to help its camouflage.

The angler fish uses its fins not for swimming but for walking.

And the front spine of its dorsal fin is a fishing rod.

With that lure, it attracts unsuspecting creatures within range of its mouth.

The bearded ghoul uses its fins for defence.

Without a membrane between them,

they are no longer of any use as stabilisers when swimming,

and instead they are sharp and tipped with poison.

Very effective protection.

It's just what you need if you are lying on the bottom.

The gurnard uses some of the rays of its front pair of fins

as delicate legs for finding food in the gravel.

Fish like those live in comparatively shallow waters,

100 feet, 30 metres, something like that.

Their world is a heavily inhabited one and also quite a bright one

because the water is shallow enough to receive light from the sun.

It's also one that's comparatively familiar to us.

For one thing, all the sea fish that we eat come from it.

For another, hundreds of thousands of people regularly visit it wearing aqualungs.

But in fact it is only a tiny proportion of the seas of the world.

Most of the oceans are very, very much deeper than that.

And to visit those deep waters, you can't go down in an aqualung,

you have to use something like this - a submersible.

These craft work on the sea floor, helping in the drilling for oil.

They give a splendid view of what's happening at depth, both to oil engineers and to fish watchers.

There's a highly sensitive television camera on the outside of the hull,

with a monitor screen in the cockpit

and spotlights to illuminate places

that the sun's rays have never reached.

As we go down, it gets darker and darker.

And the pressure increases too, very quickly.

By the time we are 500 feet, the loading on this viewing dome here will be about 70 tons.

And it also gets colder and colder.

At one point, and the precise depth varies according to where we are in the world,

between, say, 20 metres, which is about 60 feet,

and 150 metres, 450 feet,

it suddenly gets very much colder indeed and drops to about five degrees above freezing.

That point is called the thermocline,

and it's a kind of frontier in the ocean,

separating two very different worlds between which there is very little traffic.

Above, there is the sunlit, warm waters near the surface which have their own circulation,

and below the thermocline, there's the black, near-freezing world of the ocean depths.

And there, there live very different fish indeed.

This is part of the world that man is only just beginning to explore.

Until a few years ago, most of our knowledge of these creatures

came from specimens hauled up in dredges.

But, as they came up, changes in pressure and temperature

usually distorted their bodies and they quickly died.

Only now, from such craft as the submersibles, are we beginning to get an accurate idea

of what life is really like in the deeper parts of the oceans.

A shark, built on the same pattern as its relatives above.

Like most of the inhabitants of these oxygen-poor waters,

it moves comparatively slowly.

Probably never meeting a boundary or a barrier in the endless deep sea.

A red prawn, doubtless food for some big fish.

An extraordinary relative of the prawn, another crustacean, called an ostracod.

Fossils of species very like these have been found in extremely ancient rocks,

so we know that they were here long before the fish arrived.

This fish still uses that antique way of swimming with S-shaped undulations of its body.

A fangtooth, one of the hunters of this lightless world.

Our knowledge of the fish at these depths is still very fragmentary.

Many species have never been filmed,

and we know them only from a few mangled specimens and still photographs.

This bait on a line is suspended from a rod dangling in front of an upper jaw

lined with needle teeth.

It's another kind of angler fish.

There are so few animals at these depths that when a meal arrives,

a hunter must make quite sure of catching it.

This angler normally looks like this,

but when it's had its meal, its stomach becomes hugely distended.

The gulper is little more than a swimming mouth,

also with a stomach capable of great extension.

The bigger the stomach can go, the wider the choice of prey,

and meals may be few and far between.

Many fish produce lights in this blackness, some as face patches or dots along the sides.

At night, many of them move up to shallower water where, of course, it's still dark.

With special light-sensitive cameras and a little illumination,

you can just make out the fish that are producing these lights.

They are called flashlight fish.

With no illumination at all, they become disembodied green spots again,

mysteriously circulating in the blackness.

The light is produced by bacteria which live in this one patch of skin

and glow as a normal by-product of the chemistry of their life processes.

The light may serve the flashlight fish in several ways.

It may be a sign to other members of the shoal. It may baffle predators.

After a fish switches off its light, it immediately darts away to a different position.

Or it may simply be a method of finding your way around in the blackness.

The problem of finding the way in the dark faces other fish too.

They live in turbid waters and under a thick carpet of floating vegetation.

Not an easy place to find them.

This somewhat surprising piece of apparatus...

is the latest device developed and designed by research workers interested in electric fishes.

This plastic tube has got two leads which come up through this cable

along here to this extraordinary hat.

There, they go into this amplifier,

and on the brim of the hat, there's a loudspeaker here

and, very thoughtfully, a counterweight here,

so that when I put the hat on my head, it doesn't flop over one ear.

And then, if I turn on the amplifier, the speaker is next to my ear,

I have one hand free and the wires will pick up the signals of those electric fishes

and I can hear them as a series of clicks.

LOW HUMMING

The fish produce their electricity

from stacks of plate-shaped shells embedded in jelly

that lie in a column along each flank, like batteries.

Each fish sends out a particular kind of discharge.

And each makes a different sound on the loudspeakers.

So these may be another kind of call sign,

like the Morse code of the flashlight fish,

a way of proclaiming identity.

They also certainly help the fish in navigation.

Each creates in the water around it a weak electric field.

Any other solid object in the water causes a change in that field,

and the fish have special pores, spaced out over their bodies, which detect such alterations.

As a result, they are able to find their away about in total darkness.

And they can swim just as accurately backwards as they can forwards.

HIGH-PITCHED HUMMING

PULSING

Electricity has evolved independently in many fishes.

From South America and Africa, in rivers, lakes and also in the sea.

In some, the tail muscles are used.

Others, the head area and even the eye muscles.

A straight, knife-shaped body is characteristic of all these fish,

and it may be important to keep the body stiff in this position

in order to maintain an accurate output of navigational signals.

And the fins do the manoeuvring.

Some frequently rest, wedged between plant stems.

HARSHER AMPLIFIED TONE

Most of the discharges produced by those electric fish

are extremely weak.

You couldn't possibly detect them without special amplifying equipment.

But that is very much not the case with all of them.

This is the most powerful electric fish of all,

the famous electric eel from South America.

This has two kinds of electricity.

Not only does it have batteries which produce the discharges used for navigation,

but it's also capable of delivering a massive electric shock,

which it stuns its prey with and which is quite sufficient to throw me on my back,

if I were not wearing rubber gloves.

And I can demonstrate that electric shock

by tapping him near his head and tail

with these electrodes, which will then, if he gives a shock,

light up these bulbs.

And the more powerful the shock, the more bulbs he will light.

There's a rapid output of volts,

peaking in this case at about 400. Four bulbs were lit.

Of all backboned animals, fish are the only ones to produce electricity in their bodies.

So the bony fish, one way or another, have managed to colonise all the waters of the world,

from the black depths of the sea to inland rivers and lakes, even lakes like this.

Lake Magadi in the Rift Valley of East Africa

is, I think, just about the most hostile environment that I know for land animals, let alone fish.

It's a lake not of water but of solid soda and potash,

solidified by the baking African sun

from solutions bubbling up from volcanic rocks far below.

And here, at last, is somewhere you might think you'll get a place to cool your feet.

You might get a nice refreshing drink of clear, cool water.

And yet in fact...

this water is so...hot that actually it's really quite difficult to bear.

And when you taste it, the water is sickeningly salty.

This is actually one of the hot volcanic springs

where water bubbles up from deep below the surface of the ground,

bringing up a solution of soda and salt to trickle down and crystallise out in the lakes.

You could hardly imagine a less likely place to find a fish.

And yet there it is. A species of tilapia.

The water here can be as high as 43 degrees centigrade,

110 degrees Fahrenheit.

Algae grow here and the fish survive by feeding on it.

Yet another niche, a most unlikely one,

has been filled by the incredibly adaptable fishes.

At the other extreme, there's one fish in the coldest waters on Earth.

Sea water freezes below the temperature of fresh water.

The ice fish from the seas of the Antarctic has developed a substance in its blood

which keeps it liquid even when the sea water above it freezes solid.

It has, in fact, a kind of antifreeze.

But if one wanted to pick out of the 30,000 or so species of fish alive today

the king of them all, my vote would go to this, the salmon.

In the acuteness of its senses, the skilfulness of its navigation,

the strength and athleticism of its body, this surely must be a paragon among fish.

In the Pacific, there are several different kinds.

They spend much of their time in the ocean, feeding on plankton and small fish.

But in the summer, they assemble off the North American coast

and then they begin to battle their way up the rivers.

Even falls don't stop them.

The flexible rod that first appeared in the young sea squirt

is here marvellously muscled and strengthened with a jointed column of bone.

So with a thrash of its hind end and fins,

the movement first developed by the earliest ancestors of the fish,

the salmon can swim and leap up the fastest torrents.

And its lateral line can sense the details of the surge.

The salmon's sense of smell is almost unbelievably acute.

This river is not just any river.

It is the precise one in which all these fish were hatched.

Each has retained a memory of the taste and smell of these waters.

And this has drawn them back across thousands of miles of ocean

so that they may complete their lives where they began them.

During the past few days,

their bodies have been changing with astonishing speed.

These pink salmon have developed a high humped back with thin bodies,

and their lower jaws have become hooked.

The front teeth of the males have developed

into long and powerful fangs.

Hopeless for feeding,

but then the time of feeding is long since over.

Their teeth are for battle.

The males fight for a scrape scooped in the gravel of the river bed.

When one takes possession, a female will join him and lie alongside.

Then, as she sheds her eggs into the gravel, his milt will fertilise them.

And now they're totally spent.

They don't even have enough energy to heal their battered, wounded bodies.

Their scales fall off and the once-powerful muscles, the flesh, dwindles and shrivels.

And they die. All of them.

Not a single one of the millions of fish

which fought their way up this river

ever goes back to the sea.

But their eggs remain, and will stay here throughout the winter, safe in the gravel,

until, in the spring, they'll hatch and the fry will be swept down the river to the ocean.

There they will feed and grow until, at the appointed time, two years hence exactly,

as far as these pink salmon are concerned, the fully-grown fish will once again

beat its way powerfully upriver to the place where it was born.

The salmon is the master both of salt water and fresh,

but one part of the world is closed even to it - the land.

And yet a few fish can survive even there for a short time.

The walking catfish travels over land

using bony fins and a sideways wriggle.

But it's not the first fish to do that.

One managed it some 350 million years ago, and that was a momentous move.

Because from that fish developed frogs and lizards, birds and mammals

and, ultimately, ourselves.

Subtitles by Red Bee Media Ltd