Charles Darwin and the Tree of Life

Our Earth is the only known planet that sustains life,

and it does so in abundance.

I have been fortunate enough, over the years,

to travel to some of the most extraordinary and remote places on Earth

to find and film animals.

This is the biggest flower in the world.

The blue whale!

It's the biggest creature that exists on the planet.

The sheer number and variety of animals and plants is astonishing.

Estimates of the number of different species vary from six million to 100 million.

Nobody knows how many different kinds of animals there are here.

Wherever you look, there's life.

There are often a multitude of variations on a single pattern -

nearly 200 different kinds of monkeys, for example.

And 315 hummingbirds.

Nearly a thousand bats.

And beetles... at least 350,000 species of them.

Not to mention a quarter of a million different kinds of flowering plants.

The variety is astounding.

HE CHUCKLES

Even in this one small English woodland,

you might see four or five different kinds of finches.

Why should there be such a dazzling variety?

And how can we make sense of such a huge range of living organisms?

200 years ago,

a man was born who was to explain this astonishing diversity of life.

In doing so, he revolutionised the way in which we see the world

and our place in it.

His name was Charles Darwin.

This book, the Holy Bible,

explains how this wonderful diversity came about.

On the third day after the creation of the world, God created plants.

On the fifth day, fish and birds,

and then on the sixth day, mammals,

and finally, man.

That explanation was believed, literally,

by pretty well the whole of Western Europe for the best part of 2,000 years,

and generations of painters pictured it for the faithful.

This version was painted in Italy in the 16th century.

Here is God in the Garden of Eden,

which is now filled with all kinds of animals.

Here he is pulling Adam out of the Earth.

And here, creating the first woman by putting Adam to sleep,

and then taking one of his ribs and extracting Eve from his side.

She comes out, assisted by two angels.

And when God had finished, he said to Adam and Eve,

"Be fruitful and multiply and replenish the earth and subdue it,

"and have dominion over the fish of the sea and over the fowl of the air,

"and over every living thing that moveth upon the Earth."

That made it clear that, according to the Bible,

humanity could exploit the natural world as they wished.

This view of mankind's superiority still stood when, in 1831,

a British surveying ship, the Beagle, set off on a voyage around the world.

On board, as a companion to the captain,

was the 22-year-old Charles Darwin.

They crossed the Atlantic and made landfall on the coast of Brazil.

There, the sheer abundance of tropical nature astonishes the newcomer,

as I discovered when I retraced Darwin's steps 30 years ago

for a television series about the diversity of nature.

Darwin, as a boy, had been a fanatical collector of insects,

and here he was enthralled, almost to the point of ecstasy.

In one day, in a small area,

he discovered 69 different species of beetle.

As he wrote in his journal,

"It's enough to disturb the composure of the entomologist's mind

"to contemplate the future dimension of a complete catalogue."

They went south, rounded Cape Horn and so reached the Pacific.

And then, in September 1835,

after they had been away for almost four years,

they landed on the little-known islands of the Galapagos.

Here they found creatures that existed nowhere else in the world.

Cormorants that had lost the power of flight.

Lizards that swam out through the surf

to graze on the bottom of the sea.

Darwin, who had studied botany and geology at Cambridge University,

collected specimens of the animals and plants,

and as usual, when he went ashore to investigate,

described what he found in his journal.

"My servant and self were landed a few miles to the northeast,

"in order that I might examine the district mentioned above

"as resembling chimneys."

Volcanic chimneys, presumably.

"The comparison would have been more exact

"if I had said, 'the iron furnaces near Wolverhampton.' "

HE CHUCKLES

The British resident in the Galapagos

claimed that he knew from the shape of a giant tortoise's shell,

which island it had come from.

If it had a rounded front,

it came from a well-watered island, where it fed on lush ground plants.

Whereas one from a drier island had a peak at the front,

which enabled it to reach up to higher vegetation.

Were these tortoises, each on their separate islands, different species?

And if so, was each one a separate act of divine creation?

The differences that Darwin had noticed

amongst these Galapagos animals were, of course, all tiny,

but if they could develop,

wasn't it possible that over the thousands or millions of years

a whole series of such differences might add up

to one revolutionary change?

On his voyage home, Darwin had time to ponder on these things.

Could it be that species were not fixed for all time, but could, in fact, slowly change?

On his return, he sorted out his specimens and sent them off

to relevant experts so that each could be identified and classified.

Most of the mammal bones and fossils he sent to Richard Owen.

Owen was one of the most brilliant zoologists of his time.

He was the first to recognise dinosaurs,

and indeed had invented their very name,

and he would later become the creator and first director of the Natural History Museum in London.

Many of the specimens that Darwin collected are still preserved and treasured

here among the 70 million other specimens housed in the museum that Owen founded.

And here is one of them.

It's obviously the lower jaw of some great animal, and when Darwin discovered it,

it had bits of skin and hair attached to it,

so that at first it was thought to be the remains of some unknown living species.

But now we know that it is a species that was extinct for some 10,000 years, a giant ground sloth.

Owen examined it in great detail

and eventually described it and gave it the name of Mylodon darwinii,

in honour of its discoverer.

But that mutual respect between two great men of science was not to last.

Soon after his return from his voyage, Darwin made his home here, in Down House, in Kent.

Here he wrote an account of his travels

and worked on detailed scientific treatises about corals

and barnacles and the geology and fossils of South America.

But he also pondered deeply on what he had seen in the Galapagos and elsewhere.

Maybe species were not fixed.

Every day, he took a walk in this small spinney

that he had planted at the end of his garden.

And it was here that he came to ponder on the problems of natural history,

including that mystery of mysteries -

how could one species turn into another.

He noted that most, if not all, animals

produce many more young than live to breed themselves.

This female blue tit, for example, may well lay a dozen eggs a year -

perhaps 50 or so in her lifetime.

Yet only two of her chicks need to survive and breed themselves

to maintain the numbers of the blue tit population.

Those survivors, of course, are likely to be the healthiest

and best suited to their particular environment.

Their characteristics are then inherited.

So perhaps, over many generations,

and particularly if there are environmental changes, species may well change.

Only the fittest survive, and that was the key.

He called the process "natural selection".

That would explain the differences that he had noted in the finches

that he had brought back from the Galapagos.

They were very similar, except for their beaks.

This one has a very thin, delicate beak which it uses to catch insects.

This one, on the other hand,

which came from an environment where there were a lot of nuts,

has a big, heavy beak which enables it to crack them.

So maybe,

over the vastness of geological time,

and particularly if species were invading new environments,

those changes would amount to very radical changes indeed.

Darwin drew a sketch in one of his notebooks to illustrate his idea,

showing how a single ancestral species might give rise to several different ones,

and then wrote above it a tentative "I think".

Now he had to prove his theory,

and he spent years gathering abundant and convincing evidence.

He was an extraordinary letter writer.

He wrote as many as a dozen letters a day to scientists and naturalists all over the world.

He also realised,

that when people had first started domesticating animals,

they had been doing experiments for him - for centuries.

All domestic dogs are descended from a single ancestral species - the wolf.

Dog breeders select those pups that have the characteristics that happen to please them.

Nature, of course, selects those young animals that are best suited to a particular environment,

but the process is essentially the same,

and in both cases it has produced astonishing variety.

In effect, many of these different breeds

could be considered different species because they do not, indeed they cannot, inter-breed.

For purely mechanical reasons, there's no way in which a Pekingese can mate with a Great Dane.

Of course, it's true that if you used artificial insemination,

you could get crosses between almost any of these breeds,

but that's because human beings have been selecting between dogs

for only a few centuries.

Nature has been selecting between animals for millions of years -

tens of millions, even hundreds of millions of years,

so what might have started out as we would consider to be breeds

have now become so different they are species.

Darwin, sitting in Down House, wrote to pigeon fanciers and rabbit breeders,

asking all kinds of detailed questions about their methods and results.

He himself, being a country gentleman and running an estate,

knew about breeding horses and sheep and cattle.

And he also conducted careful experiments with plants in his greenhouse.

But Darwin knew that the idea that species could appear without divine intervention

would appal society in general,

and it was also contrary to the beliefs of his wife, Emma,

who was a devout Christian.

Perhaps for that reason, he was keen to keep the focus of his work scientific.

He made a point of not being drawn in public about his religious beliefs,

but in the latter part of his life, he withdrew from attending church.

On Sundays, he would escort Emma and the children here to the parish church in Down,

but while they went into the service,

he remained outside and went for a walk in the country lanes.

Perhaps because he feared that his theory would cause outrage in some quarters,

he delayed publishing it year after year after year.

But he wrote a long abstract of it,

and then on July 5th 1844, he wrote this letter to his wife.

"My dear Emma.

"I have just finished this sketch of my species theory..."

Some sketch - it was 240 pages long.

"I therefore write this in case of my sudden death,

"that you will devote £400 to its publication."

He then goes on to list his various naturalist friends

who would be asked to edit it and check it,

and he ends the letter charmingly,

"My dear wife...

"yours affectionately, CR Darwin."

He continued to accumulate evidence and refine his theory for the next 14 years.

But then, his hand was forced.

In June 1858, 22 years after he got back from the Galapagos,

here in his study in Down, he received a package

from a naturalist who was working in what is now Indonesia.

His name was Alfred Russell Wallace.

He had been corresponding with Darwin for some years.

But this package was different.

It contained an essay that set out exactly the same idea as Darwin's...

of evolution by natural selection.

The idea had come to Wallace as he lay in his hut, semi-delirious in a malarial fever.

But although his idea of natural selection was the same as Darwin's,

he had not spent 20 years gathering the mountain of evidence to support it, as Darwin had done.

But whose idea was it?

In the end, the senior members of the Linnean Society decided that the fairest thing

was for a brief outline of the theory from each of them to be read out, one after the other,

at a meeting of the society, here in Burlington House in London.

The Linnean, then as now,

was the place where scientists studying the natural world held regular meetings

to present and discuss papers about their observations and thoughts.

The one held on July 1st 1858 was attended by only about 30 people.

Neither of the authors were present.

Wallace was 10,000 miles away in the East Indies,

and Darwin was ill and devastated by the death a few days earlier of his infant son,

so he was still at his home in Kent.

As a consequence, the two papers had to be read by the secretary,

and as far as we can tell, they made very little impression on anyone.

Darwin spent the next year writing out his theory in detail.

Then he sent the manuscript to his publisher, John Murray,

whose firm - then as now - had offices in Albemarle Street, just off Piccadilly in London.

Murray was the great publisher of his day

and dealt with the works of Jane Austen and Lord Byron,

whose first editions still line these office walls.

Darwin regarded his work as simply a summary,

but even so, it is 400 pages.

It was published on November 24th 1859.

This is not a first edition, more's the pity.

First editions are worth literally hundreds of thousands of pounds.

This is a 6th edition - my copy,

which I bought as a boy, when I was 18, I notice.

And it cost me the princely sum of one shilling.

The first edition of 1,250 copies sold out immediately,

and it went for a reprint.

And then another reprint and another reprint.

It's a book that contains very few technical terms - it's easily understood by anybody.

And predictably, it caused an outrage,

not only throughout this country, but indeed all the civilised world.

What scandalised people most, it seems, was the implication

that human beings were not specially created by God,

as the Book of Genesis stated,

but were descended from ape-like ancestors -

a notion that provided a lot of scope for cartoonists.

The leaders of the Church, headed by Samuel Wilberforce, the Bishop of Oxford,

attacked it on the grounds that it demoted God

and contradicted the story of Creation as told by the Bible.

"That Mr Darwin should have wandered from this broad highway

"of nature's works into the jungle of fanciful assumption is no small evil."

"I have read your book with more pain than pleasure..."

"It is the frenzied inspiration of the inhaler of mephitic gas."

"Fails utterly."

Darwin's theory implied that life had originated in simple forms,

and had then become more and more complex.

He knew perfectly well that the whole idea of evolution raised a lot of questions.

In fact, some of those questions would not be answered until comparatively recently.

But in his own time, many distinguished scientists

raised what seemed to be insuperable difficulties.

And foremost among them was Richard Owen,

the man who 20 years earlier had named the extinct ground sloth in honour of Darwin.

Over the years, the two men had developed a deep personal dislike of one another

and had quarrelled frequently.

It wasn't that Owen thought that the story of the Garden of Eden was literally correct,

but nonetheless, he was a deeply religious man.

He had, after all, ensured that his museum, which would display the wonders of Creation,

echoed in its design the great Christian cathedrals of medieval Europe.

And Owen knew about the diversity of life.

Indeed, he had spent his whole career cataloguing it.

But even so, he refused to believe that a species could change over time.

He, and other pioneer Victorian geologists,

as they established their comparatively new science,

recognised that the outlines of the history of life could be deduced

by examining the land around them.

Look at these rocks in northern Scotland.

We know from fossils that are associated with them that they are very ancient.

And they are sand stones.

Compacted sand that was laid down at the bottom of the sea,

layer upon layer upon layer.

But look how many layers there are!

Clearly, those at the top must have been laid down

after those beneath them.

So as you descend from layer to layer, you are in effect going back in time.

So a fossil species, if it comes from a particular layer,

is of a particular age.

And if you can recognise each one, then you can begin to piece together the outlines of life's history.

Ah, Micraster...

The ability to identify fossils and place them in their geological time zone

was still an essential skill when I was at university a century later.

We worked our way through drawers like these,

which are full of fossils of one sort or another -

but none of them have labels, only numbers.

So you were expected to be able to pick up one...

..and say, "Yes, that's a belemnite."

Now, actually which belemnite it is, I can't remember now.

And when you came to your practical exam,

your examiners would produce one of these and say,

"OK, what's that?"

And you either knew or you didn't,

and the way you knew was because of all the work you did in drawers like these, hour after hour.

Owen did not deny the sequence in which all these different species appeared.

But he believed that each was separate, each divinely created.

Darwin's theory, however, required that there should be connections not just between similar species,

but between the great animal groups.

If fishes and reptiles and birds and mammals had all evolved from one another,

then surely there must be intermediate forms between those great groups.

And they were missing.

And then, just two years after the publication of The Origin Of Species,

Richard Owen himself purchased the most astonishing fossil for his museum.

It had been found in this limestone quarry in Bavaria.

The stone here splits into flat, smooth leaves

that have been used as roofing tiles since Roman times.

Most are blank.

But occasionally, when you split them apart,

they reveal a shrimp or a fish.

It's almost impossible to resist the temptation

of pulling down almost every boulder you see,

and then opening it like a book, to look at each unopened page

to see whether maybe it contains yet another fossil.

But this fossil was something unprecedented.

It is still one of the greatest of the treasures that are stored in the Natural History Museum.

And this is it - it's called archaeopteryx.

It has unmistakable feathers on its wings

and down its tail.

So Owen had no hesitation in calling it a bird.

But it was unlike any other bird that anyone knew of,

because it had claws on the front of its wings,

and, as was later discovered, it didn't have a beak, but jaws with teeth in it.

And a line of bones supporting its tail.

So it was part reptile, part bird.

Here was a link between those two great groups

that was no longer missing.

Gosh, you really can see the filaments there.

Other examples of the same creature show its feathers even more clearly.

We know from the bones of archaeopteryx that it was, at best, a very poor flyer.

So it's not surprising that eventually it was superseded

by more modern, more efficient birds.

And that's the fate of these links between great groups.

Eventually they become extinct,

and the only way we know they existed is from their fossilised remains.

Even so, there is a bird alive today

that illustrates the link between modern birds and reptiles.

The hoatzin nests in the swamps of tropical South America.

There are cayman in the water beneath, ready to snap up any chick that might fall from its nest,

so an ability to hold on tight is very valuable.

And the nestlings have a very interesting way of doing that.

The young still have claws on the front of their wings, as archaeopteryx did.

Here is vivid evidence that the wings of birds are modified forelegs

and once had toes with claws on them.

There's another creature alive today that represents a link between the great animal groups,

a descendant of a group of reptiles that took a different evolutionary course

and evolved not feathers, but fur -

the platypus.

When specimens of this creature first reached Europe from Australia

at the very end of the 18th century,

people refused to believe their eyes.

They said it was a hoax -

bits and pieces of different creatures rather crudely sewn together.

And yet in a way those early sceptics were right.

The platypus is the most extraordinary mixture of different animals.

It's part mammal and part reptile.

And so it can give us some idea of how the first mammals developed.

When it comes to breed, it does something that separates it from all other mammals except one.

In its nest, deep in a burrow, it lays eggs.

It's this that links the platypus with the reptiles,

and this that entitles it to be regarded

as the most primitive living mammal.

So the links between the great animal groups

are not, in fact, missing,

but exist both as fossils and as living animals.

Although the fossil record provides an answer to the problem

of missing links, it also posed a major problem.

It started very abruptly.

The earliest known fossils in Darwin's time

came from a formation called the Cambrian,

and there were two main kinds -

these, which look like fretsaw blades and are called graptolite,

and these, like giant woodlice, which are called trilobites.

Could it really be that life on Earth started

with creatures as complex as these?

As a boy, I was a passionate collector of fossils.

I grew up in the city of Leicester,

and I knew that in this area, not far from the city, called Charnwood Forest,

there were the oldest rocks in the world, older even than the Cambrian.

So therefore, by definition, they would be without fossils.

There was no point in me looking for fossils in these ancient rocks.

There were, it's true, very rarely,

some rather odd shapes in these rocks,

like this one here.

But they were dismissed as being some kind of mechanical aberration.

I mean, after all,

how could there be anything living in these extremely ancient rocks?

And then in 1957,

a schoolboy with rather more patience and perspicacity than I had

found something really remarkable -

and undeniably, the remains of a living creature.

And here it is in Leicester Museum,

where it's been brought for safekeeping.

It's called Charnia.

Who could doubt that this is the impression of a living organism?

It has a central stem, and branches on either side.

In fact, it seems to have been something like the sea pens

that today grow on coral reefs.

Since its discovery, a whole range of organisms have been found

in rocks of this extreme age,

not only here in the Charnwood Forest, but in many other different parts of the world.

Fossil hunters searching these rocks in the Ediacra Hills of Australia

had also been discovering other strange shapes.

At first, many scientists refused to believe

that these faint impressions were the remains of jellyfish.

But by now, enough specimens had been discovered to make quite sure

that that indeed is what they are.

So now we know that life did not begin suddenly with those complex animals of the Cambrian.

It started much, much earlier,

first with simple microscopic forms which eventually became bigger,

but which were still so soft and delicate that they only very rarely left any mark in the rocks.

The question of the age of the Earth posed another problem for Darwin's theory.

In the 17th century, an Irish bishop had used the genealogies recorded in the Bible

that lead back to Adam to work out that the week of Creation

must have taken place in the year 4004 BC.

That may seem to us to be a very naive way of doing things,

but what other method was there anyway?

The Victorian geologists had already concluded that the Earth must be millions of years old.

But how many millions, no-one could say.

Then, less than 50 years after the publication of The Origin,

a discovery was made in what seemed a totally disconnected branch of science

that would ultimately provide the answer.

A Polish woman working in Paris, Marie Curie,

discovered that some rocks contained an element called uranium

that decays over time at a steady rate through a process called radiation.

Today, a century after she made her extraordinary discovery,

the method of dating by measuring

changes in radioactivity has become greatly refined.

This is a sample taken from those very ancient rocks in Charnwood Forest.

And these tiny crystals are revealed to be 562 million years old.

That provides more than enough time

for natural selection to produce the procession of fossils

that eventually leads to the living animals and plants we know today.

But there was another objection.

If all animals within a group have a common origin,

how is it that some kinds of animals are distributed

throughout the continents of the world, except for Antarctica?

How is it that, for example, frogs in Europe and Africa

are also found here in South America,

on the other side of the Atlantic Ocean,

bearing in mind that frogs have permeable skins and can't survive in sea water?

Darwin himself had a couple of suggestions.

One was that they might have floated across accidentally on rafts of vegetation,

and the other is that maybe there were land bridges between the continents,

but even he was not convinced by either explanation.

Even as late as 1947, when I was a geology student here at Cambridge,

there was no convincing explanation.

It's true that back in 1912, a German geologist had suggested

that at one time in the very remote, distant past,

all the continents of the Earth that we know today were grouped together

to form one huge super-continent,

and that over time this broke up and the pieces drifted apart.

That would have provided an answer.

But when I asked the professor of geology here who was lecturing to us

why he didn't tell us about that in his lectures,

he replied, rather loftily I must say,

"When you can demonstrate to me that there is a force on Earth that can move the continents by a millimetre,

"I will consider it, But until then, the idea is sheer moonshine, dear boy!"

But then, in the 1960s,

it became possible to map the sea floor in detail,

and it was discovered not only that the continents have shifted

in just the way that the German geologist had suggested,

but that they were still moving.

New rock wells up from deep below the Earth's crust,

and flows away on either side of the mid-ocean ridges, carrying the continents with it.

Amphibians had originally evolved on this super-continent

and had then travelled on each of its various fragments as they drifted apart.

Problem solved!

Perhaps the biggest problem of all for most people

was the argument put forward for the existence of God

at the beginning of the 19th century by an Anglican clergyman called William Paley.

He said, supposing you were walking in the countryside and you picked up something like this.

You would know from looking at it

that it had been designed to tell the time.

There must, therefore, be a designer.

And the same argument would apply if you looked at one of the intricate structures found in nature,

such as the human eye.

And the only designer of the human eye could be God.

Anti-evolutionists maintain that the eye would only work

if it was complete in all its details.

Darwin, on the other hand, argued that the eye had developed,

becoming increasingly complex over a long period of time.

That would only work if each stage of development

was an improvement on the previous one,

and today we know enough about the animal kingdom to know that that is indeed the case.

Some very simple animals have nothing more than light-sensitive spots

that enable them to tell the difference between light and dark.

But if a patch of such spots formed even the shallowest of pits,

one edge of the pit would throw a shadow,

and so reveal the direction of light.

If the pit got deeper and started to close, then light would form a blurred image.

Mucus secreted by the cells would bend the light and focus it.

If this mucus hardened, it would form a proper lens

and transmit a brighter and clearer image.

All these different fully-functional stages

at different levels of complexity are found in living animals today.

This single-celled creature has one of those light-sensitive spots.

Flatworms have a small pit containing light spots,

so they can detect the shadow of a predator.

A snail's blurry vision is good enough to enable it to find its way to food.

And the octopus has an eye with a proper lens and can see as much detail as we can.

So the structure of the human eye does not demand the assistance of a supernatural designer.

It can have evolved gradually,

with each stage bringing a real advantage, as Darwin's theory demands.

Natural selection, of course,

requires that an animal's characteristics are handed from one generation to the next.

It's obvious that children resemble their parents.

Anyone knows that.

But when you come to think of it, how does that come about?

In Darwin's time, nobody had the faintest idea

about the mechanism or the rules that governed that process,

except perhaps for one man who was working in the city of Brno,

in what is now the Czech Republic, at exactly the same time that Darwin was writing his book in Kent.

That man's name was Gregor Mendel.

He discovered the laws of inheritance

by breeding thousands of pea plants

and observing how they changed from one generation to the next.

He found that while many characteristics

were passed down directly from one generation to another,

others could actually skip a generation. How could that happen?

Mendel explained this by suggesting that each plant, each organism,

contained within it factors which were responsible

for creating those particular characteristics.

Today, we call those things genes,

but nobody had any idea how they worked until 100 years after Mendel's time.

And then the answer was discovered in Cambridge.

In 1953, here in the Cavendish laboratories, two young researchers,

Francis Crick and James Watson, were building models like this.

It was their way of thinking about and investigating the structure

of a complex molecule that's found in the genes of all animals - DNA.

The crucial bit are these chains,

which encircle the rod -

and here is a second - and entwine.

This is a double helix.

The workings of the DNA molecule are now understood in such detail

that we can demonstrate something that is truly astounding.

A gene taken from one animal can function in another.

The gene that causes a jellyfish to be luminous, for example,

transplanted into a mouse, will make that mouse luminous.

The genetic code can also reveal relationships.

Even our law courts accept that DNA fingerprinting

can establish whether a man is the father of a particular child.

And it can also reveal whether one kind of animal is related to another.

It proves, for example, that kangaroos -

ground-living animals that run with great leaps -

are closely related to koalas that have taken to climbing trees.

That insect-eating shrews have cousins that took to the air

in search of insects - bats.

And that one branch of the elephant family, way back in geological history,

took to the water and became sea cows.

So, 150 years after the publication of Darwin's revolutionary book,

modern genetics has confirmed its fundamental truth -

all life is related.

And it enables us to construct with confidence

the complex tree that represents the history of life.

It began in the sea,

some 3,000 million years ago.

Complex chemical molecules began to clump together

to form microscopic blobs - cells.

These were the seeds from which the tree of life developed.

They were able to split, replicating themselves as bacteria do.

And as time passed, they diversified into different groups.

Some remained attached to one another, so that they formed chains.

We know them today as algae.

Others formed hollow balls which collapsed upon themselves,

creating a body with an internal cavity.

They were the first multi-celled organisms -

sponges are their direct descendents.

As more variations appeared, the tree of life grew and became more diverse.

Some organisms became more mobile and developed a mouth that opened into a gut.

Others had bodies stiffened by an internal rod.

They understandably developed sense organs around their front end.

A related group had bodies that were divided into segments

with little projections on either side that helped them to move around on the sea floor.

Some of these segmented creatures developed hard protective skins

which gave their bodies some rigidity.

So now the seas were filled with a great variety of animals.

And then, around 450 million years ago,

some of these armoured creatures crawled up, out of the water and ventured onto land.

And here, the tree of life branched into a multitude of different species

that exploited this new environment in all kinds of ways.

One group of them developed elongated flaps on their backs,

which, over many generations, eventually developed into wings.

The insects had arrived.

Life moved into the air and diversified into myriad forms.

Meanwhile, back in the seas,

those creatures with the stiffening rod in their bodies had strengthened it by encasing it in bone.

A skull developed, with a hinged jaw that could grab and hold on to prey.

They grew bigger, and developed fins

equipped with muscles that enabled them to swim with speed and power.

So fish now dominated the waters of the world.

One group of them developed the ability to gulp air from the water surface.

Their fleshy fins became weight-supporting legs and 375 million years ago,

a few of these backboned creatures followed the insects onto the land.

They were amphibians with wet skins and they had to return to water to lay their eggs,

but some of their descendents evolved dry, scaly skins

and broke their link with water by laying eggs with watertight shells.

These creatures, the reptiles, were the ancestors of today's tortoises, snakes, lizards and crocodiles.

And of course they included the group that back then came to dominate the land - the dinosaurs.

But 65 million years ago, a great disaster overtook the Earth.

Whatever its cause, a great proportion of animals were exterminated.

All the dinosaurs disappeared, except for one branch

whose scales had become modified into feathers.

They were the birds.

While they spread through the skies,

a small, seemingly insignificant group of survivors began to increase in numbers on the ground beneath.

These creatures differed from their competitors

in that their bodies were warm and insulated with coats of fur -

they were the first mammals.

With much of the land left vacant after the great catastrophe, they now had their chance.

Their warm, insulated bodies enabled them to be active at all times,

at night as well as during the day.

And in all places, from the Arctic to the Tropics.

In water as well as on land.

On grassy plains and up in the trees.

HE CHUCKLES

There can be no doubt about our close relationship to these chimpanzees.

Our bodies are so similar,

the proportions of our limbs or our faces may differ,

but otherwise we are very, very similar.

The arrangement of our internal organs, the chemistry of our blood,

the way our bodies work... All these are almost identical.

And DNA confirms that.

Indeed, we are as closely related to chimpanzees

and the rest of the apes and monkeys as, say, lions are to tigers

and to the rest of the cat family.

Suddenly, an image from our remote past comes vividly to light -

the time when our distant ancestors,

in order to keep up with the changing environment,

had to wade and keep their heads above water

in order to find food.

That crucial moment when our far distant ancestors

took a step away from being apes and a step towards humanity.

The Natural History Museum is one of the most important museums of its kind in the world.

Richard Owen brought it into existence, but over a century later,

discoveries from many branches of science have shown that his belief

that species can never change, but always remain exactly the same, was mistaken.

It was Charles Darwin's profound insights that have proved to be true.

And now, to mark the 200th anniversary of his birth,

his statue is being taken from its out-of-the-way location

to be placed centre stage in the main hall.

Darwin's great insight revolutionised the way in which we see the world.

We now understand why there are so many different species,

why they are distributed in the way they are around the world.

And why their bodies and our bodies are shaped in the way that they are.

Because we understand that bacteria evolve,

we can devise methods of dealing with the diseases they cause.

And because we can disentangle the complex relationships

between animals and plants in a natural community,

we can foresee some of the consequences when we start to interfere with those communities.

But above all, Darwin has shown us that we are not apart from the natural world.

We do not have dominion over it.

We are subject to its laws and processes,

as are all other animals on Earth - to which indeed we are related.

HE CHUCKLES

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