MANNING: It's in tropical forests, as here in Indonesia,
that you find the richest diversity of life anywhere on Earth.
The origins of all this diversity was a mystery that obsessed Victorian naturalists
when they returned from their collecting trips with exotic animals and plants.
The answer finally came from two great scientists.
One was Charles Darwin.
The other, a lesser-known Englishman, working in these forests in the 1850s,
Alfred Russel Wallace.
I've always felt a particular affinity for Wallace.
He was a professional collector
but he so obviously took a delight in the natural world
and he loved its amazing variety.
He writes in his journals of how he trembled with excitement
as a particularly beautiful butterfly got into his net.
And he rhapsodises several times about the beauties of the birds of paradise.
It was here he developed his theory, identical to Darwin's,
about the origin of species through natural selection.
The idea that as individuals compete for space and resources,
selection acting over many generations picks out those
that are best adapted to live in their environment.
Although it was Darwin who developed these ideas most completely,
Wallace added a significant extra twist to the theory.
He noticed that this narrow strait between the islands of Bali and Lombok
marked a dramatic changeover
in the types of animals and plants he was finding in Indonesia.
Lombok was inhabited by a species which were common in Australia.
But in Bali, there was an unexpected changeover
to plants and animals much closer to those found throughout Southeast Asia.
Wallace was puzzled because the two islands were only 15 miles apart.
Then he made a bold intellectual leap.
In a letter which he wrote to Darwin in 1858, he said,
"Facts like these can only be explained
"by the bold acceptance of enormous changes in the Earth's surface."
And we know he was absolutely right.
He can't have had any idea of the mechanism,
but we know now that Lombok and the points to the east
and Bali and points to the west
were on separate plates, thousands of miles apart.
And only by continental drift have they been brought this close together.
80 million years ago,
the Australian region was much further to the south.
But the plate carrying Australia and nearby islands with their distinctive wildlife
was moving gradually northwards.
Eventually they were brought closer and closer to Southeast Asia
to form the Indonesian Archipelago.
Perhaps we biologists have tended to regard the evolution of life
as very much our own preserve.
But now that geologists are rethinking the evolution of the planet itself,
we're forced to recognise that this has been one of the major factors
shaping the history of life.
So I'm going on a journey through time and around the world
to learn how the evolution of the Earth has moulded the evolution of life.
And how life in turn has helped shape the Earth.
My journey starts in the Barberton Mountain Land in Southern Africa.
This is where the rocks reveal, better than any others in the world,
the secrets of the origin of life.
The rocks here are three and a half billion years old.
And this is where geologist Maarten de Wit
has recently made a very exciting discovery.
What is so very special about this place?
Well, they're not the oldest rocks in the world
but they are without doubt the best well-preserved rocks around.
What things are you looking for in particular?
We're interested in learning how the planet operated at that time.
So we're interested in reconstructing the processes,
reconstructing what the environment was like.
One of the things, for example, we're looking at
is to see if we can detect any life forms.
MANNING: Maarten suspected that hidden in these rocks
could be the very earliest life forms,
fossils of primitive bacteria-like organisms.
He sent some rocks to Frances Westall.
I want to try and find...
MANNING: She's a microbiologist with a particular ambition,
to find the oldest fossil on Earth.
So, in this particular image we have this beautifully dividing bacteria
caught in the act of division by being frozen.
Instantaneously frozen and preserved.
Everybody wants to find the oldest bacteria on Earth.
And I think we may actually be lucky here.
MANNING: So these could be the oldest fossils in the world.
This is the actual rock that we got the bacteria from.
MANNING: Despite the excitement of their discovery,
Maarten is actually more interested in the environment
where life on Earth first originated.
What do we know about the conditions under which the bacteria formed?
Well, these rocks allow us to reconstruct an environment
that is telling us hot springs were prolific in this area
at the time that these bacteria were living.
When we look at the details of this rock, we can reconstruct an environment
a bit like Iceland today, where hot springs are prolific,
both just above sea level and below sea level.
A lot of volcanic activity driving a lot of sea water
and fresh water through these hydrothermal springs.
We speculate back in time that this is the sort of place
where life might have started.
MANNING: It seems likely that these hot springs
are where the Earth gave birth to life.
It's here that the Earth's own energy came bubbling to the surface
and fed those primitive life forms.
So from the very beginning, the Earth and life were closely bound together.
Three and a half billion years ago, the Earth was a very different place.
It was probably covered with water,
the oceans were very shallow,
and it seems the continents were just starting to form.
Today, there's only one place in the world
to get a glimpse of what life must have been like on the ancient Earth.
This is Shark Bay in Northwest Australia.
These columns contain vast numbers of microscopic single-celled organisms.
So all the ancient Earth, all the shallow seas,
would have been filled with these things.
DE WIT: That's the amazing thing.
It's just like thinking back two to three billion years ago.
This is what all the shallow seas would have been like.
- They were just covered by this kind of life. - Yes.
And it was the only life essentially around at that time.
This is just a tiny piece that is preserved
- of what the Earth looked like then. - Yes.
And that, to me, after all these years of working in these old rocks, is amazing.
Here I'm walking on it. Live.
And these things are growing right now.
- It's quite a romantic thought, isn't it? - It's really amazing.
- Ancient landscape... - It blows my mind, actually. I must tell you that.
MANNING: The mounds are called stromatolites.
And today they're extremely rare.
But they represent a very important stage
in the evolution of early life.
These were the very first organisms to use energy from the sunlight to grow
and emitting oxygen as a by-product.
That's right. Of course, we know that today, that's photosynthesis.
Fixing carbon dioxide, giving off oxygen.
And of course, in the early stages of Earth evolution,
this was a major event.
This was the first time that we see oxygen production on planet Earth.
A very unexpected event.
And that event moves the evolution of planet Earth
into a totally different mode.
MANNING: As it spread around the planet,
life in turn began to influence the evolution and the geology of the Earth.
The dramatic effect of the oxygen produced by life on the Earth's geology
can be seen in the Hamersley Range in Northwestern Australia.
What an absolutely marvellous colour.
I think of iron, I think of rust. It looks as if it's covered in rust.
That's exactly what it is. It's rust.
There's a very thin coating of rust on these rocks.
And that tells you these rocks are just very rich in iron.
MANNING: And the way these iron-rich rocks were deposited
show what was happening to the ancient Earth
during the early stages of the evolution of life.
The clue is in what geologists call "banded iron formations".
Layers of dark red rock, rich in iron
alternating with pale layers with no iron.
MANNING: Where does the iron come from?
DE WIT: Well, the origin of this iron that comes out in these dark layers
is volcanic activity in the deep oceans.
And yet, the geology tells us these rocks must have been deposited
in very quiet, shallow seas.
So there is this paradox.
Why did this iron travel so far before it deposits itself?
Well, the key to that is that iron is very soluble in water that has no oxygen.
As soon as it comes out of the volcano,
it stays in solution and it circulates around the planet,
until it reaches an area where there is oxygen being produced.
And that's, of course, around these shallow oceans
where the stromatolites are producing that oxygen.
- Right. - As soon as it hits that,
the iron comes out of solution
and forms the start of these banded iron formations.
There must be periodic processes going on here
to produce these bands of iron then bands of the white material and so on.
Some of this banding, maybe, is sort of growth on an annual basis.
It's a bit like rings in trees, growth rings in trees.
Wintertime, very little oxygen produced by these stromatolites,
- and so the iron wouldn't come out. - Yes.
But in summertime, when the stromatolites are really going,
they just produce all this oxygen.
Iron sucks it up and gets deposited.
Rocks are polished by the water here.
MANNING: So, while these banded iron formations were being laid down,
any oxygen produced by life was being used up by the iron.
As a result, there was virtually no free oxygen
in the early oceans and atmosphere.
This went on for an incredibly long period of time.
Vast deposits of banded iron formations continued to be formed
for over a billion years.
And then, about two billion years ago,
the huge banded iron formations just stopped forming.
By now, life was producing so much oxygen
that the iron was immediately deposited where it was produced
and never reached the shallow waters.
At last, the amount of oxygen in the oceans and the atmosphere
started to rise.
One might say, then, that this is the first time
that we have evidence of life affecting the structure of the planet.
Yes, you're right.
And I find that an intriguing and wonderful idea.
For the first time in Earth history
life is influencing geological processes.
MANNING: The oxygen in the atmosphere
also changed the course of the evolution of life forever.
Oxygen molecules in the upper atmosphere
was transformed by the sun's rays into ozone.
This created a shield protecting the Earth and life
from the sun's harmful radiation
and allowed more complex life forms to survive.
The Earth itself was also changing.
By one billion years ago, large continents existed.
And life had evolved into a variety of different single-celled organisms
which thrived in the shallow waters on the edges of these continents.
Life began on Earth really very early in its history.
By about 3.6 billion years ago, it was established.
What I find so remarkable is that for the next three billion years,
it stayed as single-celled, very microscopic organisms.
And then about 600 or 700 million years ago,
something amazing happened.
And it's here on the rocky coast of Newfoundland in eastern Canada
that scientists like Ed Landing have found evidence
of a major change in the evolution of life on Earth.
LANDING: We're on a surface here that is just covered
with all sorts of fossils.
Hundreds and hundreds of soft-bodied organisms,
some of them look like feathers, almost like little ferns.
And some of these guys will be up to a half metre in size.
A holdfast and something that almost looks like a feather that comes up.
Now, what are these things? Are they plants or animals?
Well, if you look at this rock section,
you can interpret the ancient environments.
The ancient environment was one that was very deep.
It was on a continental slope or rise setting.
We're dealing with something that is probably thousands of metres of water.
And that's important here because light penetrates ocean water
only a couple of 100 metres.
And below that it's completely dark.
These things are living in darkness.
So these are not plants, these are animals.
These are the oldest soft-bodied multicellular animals that are known on Earth.
MANNING: These fossils are nearly 600 million years old.
So why did single-celled life evolve into multicellular animals?
Evidence along the coast suggests that, yet again,
the Earth may have played a major role.
This is a rock unit we can trace around this part of Newfoundland.
It's very distinctive.
It consists of pebbles, boulders up to great sizes,
of volcanic rocks and sedimentary rocks
surrounded by mud and sand.
This can't be laid down by flowing water.
And the interpretation is that this is an ice deposit.
MANNING: The rocks and pebbles were carried here by glaciers.
This was one of the coldest periods in Earth's history.
The planet practically froze up.
And some glaciers had nearly reached the equator.
This massive ice age probably occurred because of a combination of events.
About 600 million years ago,
the continents formed a vast region in the Southern Hemisphere,
near the South Pole.
This reduced the circulation of oceanic waters
and stopped warm air and warm water reaching the poles.
Huge sheets of ice developed across the planet.
The effect of all this on life was devastating.
70 percent was killed.
The single-celled organisms were the victims of the first mass extinction of life on Earth.
The Earth which had given birth to life had nearly destroyed it.
And then the massive continents broke apart.
The increased circulation in the oceans warmed the Earth.
The big freeze was over.
This worldwide event which actually brought glaciers
down to sea level in the tropics is thought to be...
It's a change in Earth history.
And what we end up having at this time is, well, two things.
One is cold, glacial waters sink
and the deep oceans became oxygenated.
When the ice melted, sea level rose
and you had shallow seas covering the continents.
And this was a time of evolutionary radiation
of soft-bodied animals.
MANNING: The end of the Ice Age created the right conditions
for more complex, multicellular life forms to flourish and evolve.
LANDING: Before the glaciation,
we're starting to see very enigmatic,
multicellular things that might be animals.
But after the glaciation, we're seeing
quite complex soft-bodied organisms living attached
and perhaps crawling around on the bottom.
MANNING: It seems that once multicellular life had evolved,
there was an explosion of new life forms.
And the best place to see this
is in a remote area of the Canadian Rocky Mountains.
I'm coming very close now
to a place which is holy ground for zoologists.
One day in August 1909,
the famous American palaeontologist Charles Walcott
was riding along here on horseback.
He knew this area quite well.
There was a slab lying across the trail.
And he was afraid that his horse would stumble.
So he dismounted and tried to shift it.
But it was too heavy.
So he took his hammer and struck it.
And it split open to reveal a miraculously preserved little fossil.
And Walcott saw immediately that it was an animal
of a type that he'd never seen before.
So he knew he simply had to find
the place from which that rock had slid.
And that's where I'm going now.
Every year, Des Collins and his team spend the summer collecting fossils
from the most famous fossil quarry in the world,
the Burgess Shale.
There's a little bit of sponge right here on this side,
an ostracod there.
MANNING: I suppose what strikes me, coming to this very famous place,
is how small it is.
It's a really very small area.
Sure. Sure. Well, the fossils are really mostly concentrated in front of the cliff area.
And we've collected something like,
in total, something like 100,000 specimens have been collected here.
- Over the years. Yeah. - Quite amazing.
MANNING: The sheer quantity and quality of fossils from the Burgess Shale
is quite staggering.
But even more surprising is that all the major categories of modern animals
had already evolved.
I mean, you can see the finest details of the hairs on... Are these the gills?
The gills there, right.
Well, this is one of the best specimens of this that we've ever collected.
Well, what you can see is all the beautiful swimming flaps
with the gills running on the end.
And you can see the gut running right through the body.
And at the front, you've got these very large great appendages.
A beautiful arthropod.
Looks very much like the arthropods and the shrimps that you see in the sea today.
It's the soft body parts that you get so well here. I mean...
- Right. - What about... I mean, you get jellyfish?
Well, this is a ctenophore, that's a comb jelly...
MANNING: And scientists could see that for the very first time,
these animals were part of a complex community of plants and animals
and some of the animals lived by eating others.
This is the claw of Anomalocaris that belongs to an extinct class of arthropods.
And this is one of the claws that comes out of the front of the animal?
Right, right, the front of the animal. And the other extraordinary thing are the jaws.
One guy thought that was a jellyfish. That's a very strange-looking jellyfish.
We now know it's got 32 teeth, all going in, they've got points at the end of them.
I've got a model here of the Anomalocaris.
And you can see the jaws.
MANNING: Right.
And if you compare the size of this to the size of this in this proportion to the body,
then these jaws came from an animal that was probably about a metre in length.
- That's a really big animal. - Yeah.
MANNING: The biggest animal that ever lived at that time.
So 520 million years ago, both predator and prey had evolved.
They were inextricably bound together as part of a food chain.
From now on, the evolution of life became
even more vulnerable to any changes on the planet
which could affect this food chain.
But life was evolving only in the oceans.
The water teemed with different animals and plants.
The land was barren and lifeless.
Then, about 450 million years ago,
as the numbers and complexity of species in the sea multiplied,
plants at last made the evolutionary step which allowed them to leave the water.
Once plants had made the move, everything changed.
Soil started to build up, trapping water, transforming the surface of the continents.
But the plants didn't have the land to themselves for long.
Soon after they invaded, the animals followed.
I'm here on the Scottish island of Arran with geologist Chris Nicholas.
And this is Carboniferous period.
NICHOLAS: That's right. These rocks here are Carboniferous in age.
And what we find here is that all of these plants,
when they die, they're being buried and compressed together
- to form coal. - And that's coal, all right.
MANNING: This coal is graphic evidence that the land was covered with dense forests,
forests where strange creatures lurked.
NICHOLAS: So this is a fossil track way.
You can see we've got these two parallel...
Yeah.
...lines of footprints that go round and they curve
and head off under that slab.
MANNING: This, I think, is the first track of a terrestrial animal.
Yes, we think that we can work out how many legs it had
from the repetition of these footprints.
- So we reckon this thing had about 23... - Twenty-three pairs of legs.
Pairs of legs. That's right.
So it really was a relative of the modern centipedes and millipedes.
Well, we think so, yeah.
But much bigger. It must have been over a metre long.
That's right...
MANNING: These first fossil footsteps on land are 350 million years old.
But at the same time, descendants of fishes,
the land vertebrates, had also emerged from the sea.
There was an explosion of new life on land.
And just 50 million years later,
the land was alive with amphibians and reptiles.
There are more fossils of these early reptiles here in South Africa
than anywhere else in the world.
Amongst these reptiles was one special group.
They're called the mammal-like reptiles.
Gideon Groenwald has found hundreds of mammal-like reptile fossils.
If we look at, for instance, the skull of this animal,
we will find a very reptile-like skull.
But if we turn it over and you look at the inside, you will find a palate in the mouth.
Now, the palate indicates to us
that this animal could chew and breathe at the same time,
which is very important.
If you look at reptiles today, they throw the head back
and they chomp off big chunks of food and they swallow all this food in,
a very inefficient way of eating.
Whereas, if you look at a mammal, it will breathe while it's chewing.
It actually grinds the food into much smaller pieces. It's a very efficient feeder.
And for that reason, the mammal-like reptiles
had the ability to survive much worse conditions than the reptiles of that age.
MANNING: These mammal-like reptiles, our own very distant relatives,
were the first reptiles to dominate the world.
But they didn't evolve into true mammals
until the planet itself had gone through some extraordinary upheavals.
250 million years ago,
a series of unrelated changes to the Earth
resulted in a dramatic change to life.
The continents had been moving together
and had formed one huge supercontinent called Pangaea.
This huge landmass reduced the coastline,
reducing the habitats for marine life.
At the same time, sea levels dropped dramatically,
exposing and killing all those species living on the continental shelf.
90 percent of marine life was destroyed.
On land, this coincided with a catastrophic event
in what is now Siberia.
COURTILLOT: It's very hard to imagine what may have happened when this lava erupted.
The volumes, the size, the speeds involved.
This is not something that has been seen since the human race exists,
since the human species exists.
You should try to imagine a fissure, a crack in the Earth's crust
possibly 400 kilometres long,
spewing lava,
throwing material, dust but most importantly gases,
carbon dioxide, sulphur dioxide, that will eventually lead to acid rain,
darkness, cooling, altering vegetation,
destroying animals that would need vegetation for support
and eventually killing animals that would eat animals that ate vegetation.
So the whole life chain being completely changed.
Many teams around the world have collected samples from this lava
and dated it very accurately
and found that it was precisely 250 million years old.
MANNING: Massive volcanic outpourings like these which ruin ecosystems, destroying food chains,
are regular events in the history of the Earth.
COURTILLOT: These volcanic eruptions are part of a very important rhythm of the Earth.
The planet is essentially trying to cool down
and that heat leaves with that big bubble of rock which, when coming to the surface,
melts and produces, through cracks induced in the Earth's crust,
the gigantic lava outpourings.
It shows that the Earth's geology has a direct influence
in changing the course of evolution at certain times but in a gigantic way.
Was it not for these catastrophes,
well, life on Earth today would be completely different,
and most likely, we would not be here.
That combination of events on Earth 250 million years ago
led to the biggest and most catastrophic mass extinction of life
that's ever occurred.
It probably lasted for several million years.
But whilst it was going on, the carnage was enormous.
It's hard to comprehend now, but during that time,
80 to 90 percent of life, on land and especially in the sea, disappeared.
So our modern understanding of how the Earth works, how it operates,
has given us a new view on the processes of evolution.
Yes, because it does seem that there have been these marked events,
these punctuations in the history of life caused by whatever reasons,
which have caused massive extinctions
and therefore provided opportunities for other groups to evolve.
So rather than being just a steady
and almost ineluctable change from simple to more complex,
life has these punctuation marks across it.
And each punctuation mark
provides a period of opportunity for new and perhaps slightly unpredictable groups
to evolve and diversify.
And therefore the whole of life on Earth has been affected by these singularly chance events.
It's difficult to know who will be the beneficiaries of these events and who will actually lose out.
I think that's the way you've got to look at life.
There are always winners and losers in any situation like that.
But it adds that sort of...
spicy sort of unpredictability to the history of life on Earth.
MANNING: And the winners are the survivors,
those species with the ability to live through the changes.
And after the mass extinction of 250 billion years ago,
a few of the mammal-like reptiles managed to survive.
The mammal-like reptiles survived these very difficult times
because they were burrowing.
And this is a very good example of a cast of a burrow that you find in the rocks.
And this is really a unique find.
Where we found the animal inside a burrow.
MANNING: Burrowing allowed these animals to survive in an increasingly harsh world.
The whole of Pangaea was gradually moving northwards,
taking this part of Southern Africa closer to the Equator.
The climate became hotter and hotter, drier and drier.
And by 200 million years ago, some of the mammal-like reptiles
had evolved into a new group of much smaller animals.
Now for the first time
we have the remains of true mammals.
And this here is the skull of one of these tiny mice-like creatures
that managed to survive these very difficult living conditions.
By burrowing, they lived in burrows and they fed only during the night.
They're small, furtive, highly sensitive, really quite intelligent mammals.
And you'd think they would just, in an evolutionary sense, explode,
dominate everything around us, because we're so familiar with mammals today.
But the mammals don't take over the Earth, do they?
Curiously enough, those hot, dry desert conditions don't suit mammals.
Reptiles such as dinosaurs were perfect for those conditions.
Deserts usher in dinosaurs and really from then on
mammals are curiously confined just to a nocturnal insectivore sort of niche
and scurry around at night, perhaps around the sleeping bodies of dinosaurs,
while dinosaurs dominate the daytime.
MANNING: The dinosaurs were the most successful of all the vertebrates
that have ever lived on land.
There were the dominant group for over 170 million years.
And then, as we all know, they disappeared.
65 million years ago, there was another mass extinction.
The dinosaurs went, the pterodactyls,
the ichthyosaurs, those marine reptiles,
many other species,
even the beautiful ammonites were never seen again.
60 to 70 percent of life became extinct.
It disappeared from the fossil record.
Many scientists now believe that a meteorite from outer space struck the Earth.
They believe that this created a gigantic fireball.
Dense clouds of material and dust were thrown into the sky,
obliterating the sun for months or even years.
The whole ecostructure would have collapsed.
But some scientists have doubts. Did a meteorite really destroy the dinosaurs?
A crater caused by the impact of a meteorite has been found.
The centre is right here, in the village of Chicxulub in Mexico.
The crater is the right date, 65 million years old.
But is it the right size?
Some scientists think the meteorite was too small to have wiped out the dinosaurs.
Trying to settle the argument are seismologists Mike Warner and Jo Morgan.
They're working out the size of the crater.
Using over 100 seismometers buried in the ground and out at sea,
they can build up a seismograph,
a picture of the crater which is hidden under 65 million years' worth of sediment.
And their results were surprising.
WARNER: The size of the object that hit the Earth was about 12 kilometres in diameter.
But it was, relatively speaking, rather small on the size that people had been guessing.
MANNING: It seemed to be too small to have killed the dinosaurs.
But their seismograph shows the meteorite couldn't have landed in a worse place.
A small impact shouldn't be devastating. It shouldn't mess up the environment.
And maybe the dinosaurs were lucky.
If we look at these rocks here,
they're formed in shallow lagoons much like we see around Chicxulub.
And they contain a lot of sulphur.
WARNER: Rocks with sulphur in, if you hit them really hard with an impact,
then they generate sulphur dioxide.
And if you put sulphur dioxide in the atmosphere
it combines with water and it makes sulphuric acid.
Little droplets of sulphuric acid way up in the stratosphere.
And that's particularly deadly,
it stops sunlight getting to the ground for 10 years, perhaps 100 years.
So, maybe the dinosaurs were killed just by these rocks that we're seeing here
because those were hit very, very hard.
MANNING: But maybe the dinosaurs were doubly unlucky.
Because at the same time on the other side of the world in India,
another catastrophic event was taking place.
Volcanoes were again pouring out massive amounts of lava.
It was an unimaginable amount,
the like of which had not been seen for 200 million years.
The volcanoes were also belching out clouds of dust, turning day into night.
The combination of the meteorite and the Indian volcanoes was too much.
It turned the Earth's surface into a dark, burning world
where all the rules governing survival of the fittest changed.
The dinosaurs never had a chance.
Those events 65 million years ago
resulted in another dramatic turning point in the evolution of life.
The dinosaurs had all gone, most of the other reptiles had gone
and there was a void of large animals.
Into that void the mammals and the birds jumped very quickly.
The mammals in particular had been held down by the reptiles,
dominance of the reptiles, for many millions of years.
And they radiated out extensively all over the Earth.
The primates emerged and eventually we emerged ourselves.
The last leg of my journey through the story of life is here in Africa.
Not too far north of where I started with the origin of life.
I'm in Kenya, in the Great Rift Valley of Africa.
And it's here that scientists are discovering
just how closely the evolution of the Earth
is linked to the evolution of our own ancestors.
Over the last million years,
the Earth has experienced a succession of ten glaciations interspersed with warm periods.
This waxing and waning of the ice sheets
has had a profound effect on the rest of the world's climate
and our own evolution.
Rick Potts spends every summer searching for evidence
to show why modern humans evolved from our ape-like ancestors.
We're going up through time, up through time here
and then we reach a point where the soil disappears
and this white sediment of the lake bed comes in,
which shows that the lake had expanded and covered the whole area of the southern Kenya Rift,
just in this area.
And again these fluctuations continue
and then the soil comes back just for a short period of time.
What do you mean by a short period of time here?
Probably, oh, a few hundred years at most.
And then the soil disappears and it's replaced by the lake up here.
And then the soil comes back in again for a short period of time.
Lake again and then a very sharp demarcation
where the soil again goes as far as the eye can see.
And this occurred at a time when the Ice Age fluctuations in Europe and North America
were just really getting going and tremendous fluctuations going on.
MANNING: But how can rapid fluctuations between dry and wet periods
be the driving force of the evolution of modern humans?
POTTS: This is the species Homo habilis and it had still a relatively small brain.
This was a clever creature, make no doubt about it, but it wasn't quite us.
It reminds me very much of the chimpanzee.
I mean, why didn't we stay as upright apes like chimpanzees?
In my view, we evolved because of climate fluctuation,
of the tremendous fluctuations
and uncertainty of environments that we can see here in the geologic record.
This is Homo sapiens, our own species.
And this one evolved during those fantastic fluctuations of environment
that we see beginning about 600 to 700 thousand years ago.
And you can see the incredibly large size of the brain case.
So the rate of change began to accelerate?
Yes. Tremendous acceleration in the rate of brain growth.
And this had to do with I think, with flexibility,
with the ability to adapt to those tremendous changes in environment.
For example, communication.
Through language we are able to say,
"You know, my grandfather told me that
"the time before him, you could find fruits to eat on the other side of that mountain
"when there is a drought."
And that's something that no other animal can do.
Being able to refer to places and things and abstract things that you can't even see.
And yet we can communicate about them.
We're used to the idea
that organisms are adapted through natural selection
to match the specific environment in which they live.
But I think there's another process of selection
that is... represents an adaptation to the variability and the fluctuation of environments.
And this is a process that distances an organism
from any one specific environment.
And I think that we, Homo sapiens,
are the paramount example of this kind of species.
(HERDSMAN CALLING OUT AND CRACKING WHIP)
MANNING: So we, too, like the rest of life,
are the product of constant physical changes that have occurred here on Earth.
As a biologist,
I've always concentrated on the evolution of life in biological terms.
But it adds a completely new dimension
to recognise how closely the evolution of life is linked
with the inexorable changes of the Earth.
We now know that Earth isn't simply the place where we happen to live.
We know it's a dynamic planet
and we know the Earth has been one of the driving forces,
shaping evolution since the very beginnings of life.
We also know that the way life has evolved on Earth
makes our planet different from our neighbouring planets.
But the Earth is also special in its geology.
And in the next programme, we'll be exploring why we are so different.
What makes the Earth so special?