Cosmos

Yes, this is home.

This is Earth.

Having trouble finding a familiar continent?

The past is another planet.

Actually, many.

I'm standing on the great expanse of time that has elapsed

since the Big Bang. In order to think about it,

we've compressed it all into a single year.

It's the early morning of December 23

on this Cosmic Calendar of ours,

or about 350 million years ago,

when our world was a mere four billion years old.

Earth looks so different.

You might not even know the place.

The stars wouldn't help you.

Even the constellations would have been different back then.

The dinosaurs were still

more than 100 million years in the future.

There were no birds, no flowers.

And the air was different, too.

The atmosphere had more oxygen than at any other time

in Earth's history, before or since.

This allowed insects to grow much larger than they do today.

How?

Insects don't have lungs.

Life-giving oxygen is taken in through openings

in the outside of their bodies

and transported through a network of tubes.

If an insect were too large,

the outer reaches of these tubes would absorb all the oxygen

before it could ever get to its internal organs.

But during the Carboniferous Period,

the atmosphere had almost twice the oxygen as today.

Insects could then grow much bigger

and still get enough oxygen in their bodies.

That's why the dragonflies here are as big as eagles

and the millipedes the size of alligators.

So why was there so much oxygen back then?

It was produced by a new kind of life.

What kind of life could've changed

the Earth's atmosphere so dramatically?

Plants that could reach for the sky--

trees.

In their competition for sunlight,

trees evolved a way to defy gravity.

Before trees, the tallest vegetation was

only about waist-high.

And then something wonderful happened.

A plant molecule evolved that was both strong and flexible,

a material that could support a lot of weight,

yet bend in the wind without breaking.

Lignin made trees possible.

Now life could build upward.

And this opened a whole new territory,

a three-dimensional matrix

for communities far above the ground.

Earth became the Planet of the Trees.

But lignin had a downside:

it was hard to swallow.

When nature's demolition crew, the fungi and bacteria,

tried to eat anything with lignin in it,

they got a really bad case of indigestion.

And termites wouldn't evolve

for at least another 100 million years.

What to do with all those dead trees?

It took the fungi and bacteria millions of years

to evolve the biochemical means to consume them.

Meanwhile, the trees just kept springing up,

dying, falling over

and getting buried by the mud that built up over eons.

Eventually, there were hundreds of billions of trees

entombed in the Earth,

buried forests all over the Earth.

What possible harm could come from that?

This cliff in Nova Scotia is another kind of calendar.

It tells the story of that other world

that once flourished right here.

And this is the death mask

of that 300 million-year-old tree.

It was cast by minerals that replaced the original wood

cell by cell-- in other words, a fossil.

The tree surrendered its organic molecules

to the environment long ago,

its carbon and water. Only its shape remains.

When this tree was alive, it took in carbon dioxide

and water and used sunlight

to turn them into energy-rich organic matter.

The tree gave off oxygen as a waste product.

That's what trees and other plants still do.

When plants die, they decay,

and this reverses the transaction.

Their organic matter combines with oxygen

and decomposes,

putting carbon dioxide back into the air.

This balances the books

for the chemistry of Earth's atmosphere.

But if the trees are buried before they can decay,

two things happen... they take the carbon

and stored solar energy with them

and leave the oxygen behind

to build up in the atmosphere.

That's what happened around 300 million years ago.

There was an oxygen surplus.

That's how the bugs got so big.

And what became of all that buried carbon?

It lay there for eons before dealing life on Earth

its most devastating blow of all time.

There are places on this planet where you can walk through time

and read the history written in the rocks.

This beach in Nova Scotia is one of them.

Every layer is a page.

Each one tells the story of a flood,

one after another, over millions of years.

The layer cake of flood deposits was slowly buried

and turned into rock by heat and pressure.

The same forces that built mountains

then tilted and uplifted them,

along with the entombed fossil forest.

The newer layers were always

deposited on top of the older ones.

All the pages are in the correct order,

bearing witness to what happened here

over millions of years.

Back that way

lies the more distant past.

And with every step I take,

I move about 1,000 years closer to the present

and away from the world of 300 million years ago.

50 million years later lies that way.

This was the beginning of the end of the Permian world,

an event of unequalled carnage.

The Permian is the darkest corridor

in this memorial to the broken branches on the Tree of Life--

the Halls of Extinction.

Death has never come so close

to reigning supreme on this world

in the quarter billion years since.

The eruptions, in what is now Siberia,

lasted for hundreds of thousands of years.

The lava flooded and buried more than a million square miles.

This event dwarfs any volcanic eruption

in historical times.

_

Huge quantities of carbon dioxide came pouring

out of the volcanic fissures.

This greenhouse gas warmed the climate.

And this is where the long-buried forests

of the earlier Carboniferous Period reenter the story.

During the intervening 50 million years,

those trees had turned into immense deposits of coal,

and as it happened,

one of the world's largest

accumulations of coal

was buried right there in Siberia.

The heat from the lava baked the coal,

driving methane and sulfur-rich gases out of the ground.

They were laden

with toxic and radioactive ash particles--

coal smoke.

This witch's brew polluted the atmosphere

and radically destabilized Earth's climate.

A sulfuric acid haze blocked incoming sunlight

and darkened the planet.

Global temperatures plummeted to subfreezing.

During lulls in the eruptions,

the acid haze fell back to the surface.

But the carbon dioxide remained

and built up in the atmosphere to cause global warming.

Years of frigid cold

alternating with millennia of stifling heat

battered a dwindling population of plants and animals.

They had no chance

to adapt to the drastic swings in climate.

As the global warming

continued, the surface and the bottom waters

slowly mixed, raising the temperature

of the once-frigid depths of the sea floor.

Methane-rich ices

that had been frozen in the sediments began to melt.

Newly liberated methane gas made its way to the surface

and into the atmosphere.

Methane traps heat far more efficiently

than carbon dioxide, so the climate

got even hotter.

And the methane also destroyed

the ozone layer in the stratosphere.

The natural sunscreen that protects life

from deadly ultraviolet rays was eaten away.

The circulatory system of the world ocean shut down.

These stagnant waters became oxygen-starved,

killing almost all the fish in the sea.

But one kind of life flourished in this brutal environment...

bacteria that produced deadly hydrogen sulfide gas

as a waste product.

That was the last straw.

The poison gas killed almost all the remaining plants

and animals on the land.

This was the Great Dying.

The closest life on Earth has ever come to annihilation.

Nine in ten of all species perished.