Life on Earth

The creatures on the coral heads of the Great Barrier Reef in Australia

must surely be among the most beautiful and the most bewildering organisms

you can find anywhere in the world.

Sorting out these creatures into their various groups is baffling work.

Often things are not what they seem.

These are the tentacles of a worm.

This is the cousin of a starfish.

This is a flatworm, and the creature advancing on it is a snail that has lost its shell.

One thing is clear. They're all animals without backbones, invertebrates.

But how are they related to one another? Which is descended from what?

One way to find out is to trace the various groups, as fossils,

back through the rocks to their origins.

These limestones here in Morocco are so old, getting on for 600 million years old,

that they date long before the time of any backboned animals.

There are no fish fossils here, for example.

But there are invertebrate fossils. Not as many or as varied, it's true,

as the invertebrates that live today on the Barrier Reef, but invertebrates nonetheless.

And they fall roughly into three groups.

There are little shells, like this.

And a creature that looks like a flower but was covered in stony plates.

And this, which is rather like a shrimp, with a shell, and its body divided into segments.

What are the relationships between these three very, very early groups?

If we can understand that, we will be close to understanding the origin of animal life.

The obvious place to look is a few feet farther down in these limestones.

A million or so years earlier.

But suddenly, we come to a mystery.

Although these limestones look exactly the same as those above,

and must have been laid down in similar seas, there are no fossil shells to be found here at all.

What's more, there are no fossil shells to be found in any rocks in the world

of an age of these.

And these extend for thousands of feet more,

representing hundreds of millions of years of deposit.

And not a fossil shell among them.

The explanation is precisely in that word "shell".

Shells fossilise easily. Soft animal tissues rot, and hardly leave any trace behind.

There was life in the seas in which these limestones were deposited, but without shells.

Why did it take so long for animals to develop shells?

After all, if you condense the whole history of life from its beginnings until today into a year,

it wasn't until early November that the first shelled animals appeared.

Well, there's been a lot of debate on that question, and a lot of suggestions.

One is that the chemistry of the seas wasn't suitable.

They were either too cold or too acid to allow for the deposition of lime as shells.

Whatever the answer, the fact remains that for that immense period of time,

we have no fossil shells to help us chart the progress in the very early stages of animal life.

But that doesn't mean we can't make some informed speculations.

For example, take this group of creatures, the one like little shells.

What could their early ancestors have been like?

These microscopic creatures are among the simplest animals in the sea.

They're the larvae of corals and jellyfish.

We know that they appeared very early indeed.

But suppose some of them didn't grow up either to float or to build skeletons,

but took to a creeping life.

They might easily have become something like this.

This is a juvenile flatworm.

It has a cluster of spots on top at one end, which are sensitive to light and to gravity,

and it swims with the aid of cilia that cover its surface.

When that settles on the sea bed, it becomes this.

Not a drifter like a jellyfish, but an animal that moves in a purposeful way

with a definite front end and back end.

Flatworms are very flat, and with such a great body surface in relation to their small bulk,

they absorb all the oxygen they need through their beautifully patterned skin.

Many of them move by rippling their bodies instead of relying entirely on the cilia.

And some are so good at it that they can swim.

A flat shape, however, is not so suited to burrowing.

And as mud and sand began to spread over the sea floor 1,000 million years ago,

burrowing became a desirable thing to do.

There were bits of food to be sifted from the mud, and hidden beneath it there was safety.

So some worms changed from being flat to being round and long,

and buried themselves in the mud.

Others were less active and remained with their front ends sticking out,

ringed by tentacles.

The beating of the cilia created currents that enabled the tentacles to absorb oxygen,

and also swept food particles down to the mouth at their centre.

About 600 million years ago, some of these worms secreted shields on the top

to protect the delicate tentacles and channel the feeding currents over them.

This was such a success that variations appeared.

Shells were strengthened with lime and grew bigger

to allow more efficient breathing tentacles.

So eventually the original worm-like shape was lost.

We know from fossils that these brachiopods were enormously abundant in the ancient seas.

They grew in many shapes and to a considerable size.

Some developed delicate coils of lime inside their shells

to support their feeding apparatus.

But some 70 million years ago, their fortunes waned,

and today only a few species survive.

One lives in some numbers on the muddy shores of a bay in Japan,

and at low tide, they are collected for food.

They call them shamisen-gai because their shape is like that of the Japanese guitar, the shamisen.

These are the simplest kind of brachiopod,

that have outlasted all the more ambitious kinds that once were so abundant.

In fact, they're virtually identical to those earliest fossil shells.

It's an astounding example of survival

which occurs several times in the history of life.

An early species finds itself in surroundings which suit it to perfection.

No other animal comes along later which exploits the surroundings any better.

Its cousins may move away to colonise different environments,

or their environments might change and so they develop into different creatures.

But this creature, encountering no change, sees no cause for change.

So it plods doggedly on, an ultra-conservative.

This formula of a simple, worm-like body enclosed in a protective shell

had obviously a lot of potential.

Several groups of creatures in early periods were based on it,

and one group in particular, the molluscs, exploited it very well indeed.

Today, there are around 80,000 different species of them.

The flatworm ancestors of the molluscs developed their shells

not over one end, but in the middle of the back,

originally like a small tent under which the animal could hide, like the limpet does.

The shell is deposited by a part of the back, the mantle,

and the animal enlarges it by adding to the margins.

Some species, though, don't do so at an equal rate all round,

and that produces twists and coils in the shell.

They have a well-developed head, with eyes

and sensory tentacles for feeling the way and tasting the water.

And underneath it, a very efficient feeding organ.

It's a long, tongue-like ribbon.

The muscles around it press it down and pull it forward,

rasping it over the surface on which the animal is crawling.

Many species use it for eating algae.

Looked at under the electron microscope, the reason for its efficiency is clear.

It carries rows and rows of minute teeth.

Each species, for some reason, with a different pattern.

Cowries secrete their shell in a way all their own.

They extend their mantle right round the shell

and deposit material on the top, giving it a beautifully polished surface.

The spider shell has its ribbon tongue on a stalk

so it can scrape surfaces its shell would prevent it from reaching.

It also has a stalked eye to help it prospect for hidden pastures.

Its foot has become very muscular to help it get around.

Molluscs with paired shells, bivalves, don't often move far.

Their foot is used to pull them down into the sand

where they can sit and filter food safely and unobtrusively.

Scallops are also filter feeders.

They live on the surface and not only have good eyes but a surprising way of moving.

Biggest of all is another filter feeder, the metre-long giant clam.

So huge, it can't move.

Its fleshy mantle joins its two shells, forming a chamber through which water is sucked.

Every so often, it gives a convulsive shudder and gets rid of a little waste.

A few molluscs have gone to the other extreme and become free-swimming

by reducing their shells to scales concealed within their bodies,

or doing without them altogether.

Unprotected by a shell, these creatures defend themselves with a nasty-tasting slime.

And their brilliant colours may serve to warn off anything that might contemplate eating them.

If that's so, they must be among the loveliest warning notices in all nature.

These creatures are more complex and usually larger than flatworms,

and they need special breathing apparatus, the gills.

In some species, they're exposed as a kind of trembling bouquet at the back.

Several kinds have developed feathery outgrowths for floating near the sea's surface.

There, extraordinary though it may sound, they hunt for jellyfish.

This one is called glaucus, and it has found its prey.

The stinging cells of the jellyfish are no defence.

Indeed, some of these floating molluscs welcome them, swallowing the stinging cells

and storing them in their own tentacles to use as second-hand weapons.

This is another creature eaten by glaucus.

One of the most deadly of all jellyfish, a Portuguese man-of-war.

Beneath it trail its tentacles, loaded with stings.

Another mollusc also preys on this creature, and this time, one with a shell.

It has a most ingenious solution to the problem of keeping afloat.

It produces bubbles by trapping air in mucus with special movements of its spoon-like foot,

and builds them into a raft, from which it hangs.

When it drifts into a Portuguese man-of-war, it attacks immediately.

The stinging cells of the jellyfish, lethal to other creatures, have no effect on the snail.

It munches them with the rest of the tentacles.

A raft of bubbles solves this snail's weight problems, but that won't work for bigger creatures.

500 million years ago, however, a group of molluscs evolved another method.

This fossil shell may look perhaps quite an ordinary sort of shell, albeit rather large,

but inside it has got quite a complicated structure.

Here's one in a boulder where the outside has been worn away

so that we can see what's inside.

This part was where the animal lived, and at the back of it, there were these chambers

which in life were filled with gas and acted as flotation chambers.

How can we be so sure?

Well, because this is another of those creatures that have survived virtually unchanged

for hundreds of millions of years.

This is a nautilus, and there are nautilus swimming in the seas today.

They live in the South Pacific, but few people ever see them alive,

for they spend most of their time in depths of up to 500 metres.

They can swim at any depth, by pumping fluid in and out of their chambers,

and so controlling their buoyancy.

Being so mobile, they need good sense organs,

and their eyes, although they have no lenses, are the best of any creature we've seen so far.

Their bodies have become modified into dozens of tentacles.

Some carry sense organs to detect food, some are used in reproduction

and others to grapple with their prey, which is usually carrion, or lobsters or crabs.

This proved to be an immensely successful design.

And from it came another great group of molluscs, the ammonites.

The ammonites were to dominate the seas of the world for the next 200 million years.

They left behind in the rocks, particularly here in Lyme Regis in southern England,

fossils that to my mind are some of the loveliest fossils of all.

Like the nautilus, the ammonites added new flotation chambers as they grew,

while their bodies occupied only the outer one.