Life on Earth

There are four million different kinds of animals and plants in the world.

Four million different solutions to the problems of staying alive.

This is the story of how a few of them came to be as they are.

The South American rainforest.

The richest and most varied assemblage of life in the world.

Those are howler monkeys up there.

There are around 50 different kinds of monkeys in these forests.

Some of the most beautiful creatures here are hummingbirds.

54 different kinds have been found within a few miles of here,

and over 300 have been found in South America as a whole.

Nobody knows how many kinds of animals there are here.

Wherever you look, there's life.

There are several hundred thousand insects that have been named,

and, without doubt, many more that haven't.

All these creatures and plants form one complex mosaic.

The orchid needs the bee to pollinate it.

The anteater couldn't have existed before the ants.

So unless the whole complex came about in a flash of instant creation,

different organisms must have appeared at different times.

But which came first, and why should there be such an immense variety?

Such questions obsessed a young 24-year-old Englishman

who came here in 1832.

His name was Charles Darwin

and he was enthralled to the point of ecstasy by the richness of life he found here.

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."

The conventional view of the time

was that every species of animal and plant had been individually created by God.

And Darwin was no atheist.

During the next three years, the Beagle sailed round South America

and up into the Pacific.

600 miles west of Ecuador,

they came to the lonely Galapagos islands.

It was here, on these volcanic islands,

that Darwin's doubts about the creation of species were reawakened.

Everywhere, Darwin found creatures that bore a general resemblance

to those he had seen on the mainland.

But nearly all were slightly different.

These, for example, were cormorants

similar to those he had seen flying along Brazilian rivers.

But in the Galapagos, their wings were so small, with such stunted feathers,

that the birds had lost their powers of flight.

And these were clearly iguanas.

He'd seen them climbing trees in the South American forests,

but on the Galapagos, with its sparse vegetation,

these iguanas fed on seaweed, and they were not the same.

Smaller, darker and with unusually long claws to help them keep a foothold

among the crashing breakers.

They also had extraordinary habits,

swimming fearlessly out to sea and diving deep to graze on the sea bed.

The Galapagos Islands got their name from the herds of tortoises that live on them,

which sailors for centuries had slaughtered for food.

But these too were obviously different from mainland tortoises.

They were many times bigger.

The English vice-governor of the islands told Darwin

that he could tell which island a tortoise came from by its shape.

This one, for example, with its deep, rounded shell,

comes from a well-watered island where it can feed on vegetation on the ground.

This one has a peak to the front of its shell that enables it to stretch its long neck upwards.

It comes from an arid island,

where the tortoises have to crane up to reach the only food there,

the branches of trees and cactus.

The suspicion grew in Darwin's mind that species were not fixed for ever.

Perhaps the tortoises were all descended from common ancestors

and had changed to suit their particular islands.

The differences Darwin noticed among these Galapagos animals

were, of course, tiny.

But if they could develop them, 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?

Was it not possible, perhaps,

that in the past amphibians had developed watertight skins and so turned into reptiles?

Or that a lizard-like reptile had developed a feathery kind of scale

and become a bird?

And even that man himself might be descended from a group of tree-swinging apes?

In truth, the idea was not a new one.

Others before Darwin suggested that all life on earth might have a common ancestry,

but Darwin went further.

He gave the idea irresistible force

by suggesting a mechanism which might have brought that about.

He called the mechanism natural selection.

Put briefly, his argument was this.

Individuals of the same species are not absolutely identical.

Some of these giant tortoise hatchlings may have, from birth,

slightly longer necks than others.

In times of drought, they will reach leaves and live,

while the shorter-necked ones die.

So those best fitted for the environment

will transmit their characteristics to their offspring.

After many generations, tortoises on arid islands

will have longer necks than those on well-watered ones.

And so one species will have given rise to another.

In these programmes, we're going to survey

the unmeasurable variety of animals produced by natural selection,

and look at them not as isolated oddities,

but as elements in a long and continuing story that began 1,000 million years ago,

and is still continuing today.

Some creatures, the mammals,

such as these sea lions and myself, are relatively recent arrivals on the scene.

Others: Birds, reptiles, amphibians, fish, have been here much longer than we have.

In places where conditions have remained unchanged over immense periods of time,

there are still creatures living which resemble very closely

their early ancestors.

They can tell us a lot.

But to disentangle the story, we shall also have to look for evidence in the rocks.

The bodies of animals fall into the bottom of ancient seas and swamps,

sometimes get entombed in the accumulating sediment.

When, after millions of years, those sediments turn to rock,

those remains of animals and plants survive as fossils.

Since the discovery of radioactivity,

scientists have developed techniques of measuring the age of rocks

based on the rates at which some chemical elements decay.

So fossils can be dated to within a few million years.

But there are much simpler ways of establishing the ages of rocks that anyone can use,

and there is no more dramatic place to do so than in the Grand Canyon in the American west.

The Colorado river, aided by wind and rain,

has cut a gigantic section through the sandstones and limestones of Arizona.

The layers still lie largely undisturbed,

so obviously the lower ones were deposited before the upper ones.

So if we want to trace the ancestry of life back to its beginnings here,

we have to go deeper and deeper into the canyon.

This is the greatest gash that exists in the surface of the earth.

From the rim to the river at the bottom is a vertical mile.

There are a number of trails down. The usual way is on the back of a mule.

Here, we're about 500 feet below the lip of the canyon.

Already the rocks are about 200 million years old.

There are no mammal fossils here, but there are some four-legged land animals.

Small reptiles: A little lizard-like creature

that has left its tracks along here, which was once the face of a sand dune.

Farther down, there are no signs of any reptiles,

but in limestones 400 million years old, the bones of strange armoured fish have been found.

The trail winds on through rocks formed on the bottom of ancient seas.

With every 20 feet we descend, we go back a further million years.

The Grand Canyon is really two canyons, one inside the other.

For a while, the trail flattens out as it approaches the rim of the inner canyon.

Here, I'm about two thirds of the way down, 3,500 feet below the rim.

The rocks here are about 500 million years old.

These rocks have no back boned animals in them at all, no fish.

The only creatures are those without backbones, including a whole lot of worms

which have left this delicate tracery of trails in what was mud on the bottom of a shallow sea.

At last, the bottom and the Colorado river.

It's taken nearly a day, going fairly easily, to get this far.

We've ridden seven miles of trail and have descended a mile into the earth's crust.

The rocks here are getting on for 2,000 million years old.

For the past 700 or 800 feet of our descent,

they've had no signs of any fossils in them.

For years, it was thought that all rocks of this age had no fossils.

Why was this? Was it because they were so unimaginably old

that they'd had all life crushed from them?

Or did life begin with creatures as big as a worm?

For many years, this was a great puzzle.

And then, 20 or 30 years ago,

people realised they'd been looking in the wrong rocks and in the wrong way.

These are the right rocks.

They're a kind of flint called churt,

and they're on the shores of Lake Superior, in Canada,

about 1,000 miles east and north of the Grand Canyon.

They were well-known during the last century because the pioneers

used them in their flintlock guns.

And scientists have recognised for a long time

that they were ancient rocks.

We now know they are about the same age as the rocks at the bottom of the Grand Canyon,

about 2,000 million years old.

But these strange rings in them...

For a long time, these were a subject of great controversy.

Some scientists maintained they were signs of very early life.

Others, that they were no more than the result

of the ordinary chemical processes during the rocks' formation.

But in the 1950s, scientists started looking at them in the right way.

First of all, you have to cut a wafer-thin slice of the gunflint rock.

This is then ground down further for several hours

until the slice is translucent.

When scientists first prepared churt to look at under the microscope,

many doubted that primitive life forms, even if they existed 2,000 million years ago,

could be preserved as tiny fossils.

And then scientists saw this.

Marks in rocks can be deceptive.

They may just be the result of mineral action.

But these filaments were almost identical to primitive algae growing today.

The search continued.

Soon, the fossilised remains of other kinds of primitive life were found

that had once lived in those early seas.

Since those discoveries, other micro-fossils have been found elsewhere

in rocks that are even more ancient, some over 3,000 million years old.

These immense periods of time baffle the imagination.

But perhaps we can get an idea of the relative lengths of the stages

if we condense the history of life on earth into one year.

Then 10 million years become one day.

On that calendar, I'm talking in the last moment of December 31st,

and primitive man will have appeared only a few hours ago, in the early afternoon.

The first back boned animal will have crawled up onto land during the last week of November,

and these churts will have been formed on June 15th.

Now let's go back way, way, to the beginning of January.

To the beginning of life.

Over 3,500 million years ago,

our planet was radically different in almost every way from the one we live on now.

Erupting volcanoes built up islands of lava and ash in the global seas.

The atmosphere was filled with gases such as ammonia, methane, hydrogen and steam.

There was virtually no oxygen. In consequence, there was no ozone layer.