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.

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

How did this astounding variety come about?

What were early fish 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.

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 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 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 be 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 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?