Ancient Earth

Dragonflies the size of hawks.

Centipedes larger than humans.

A strange menagerie of giant insects and amphibians

reigned over the Earth 300 million years ago.

Over time, these huge creatures shrunk in size

or disappeared.

The reasons for their progressive extinction

remain controversial.

358 million years ago, the continents came together

to form the supercontinent, Pangea.

This was the beginning of the Carboniferous period.

Oxygen levels in the air were much higher back then.

35% compared to today's 21%.

For the first time on Earth,

giant trees stored carbon dioxide

and released oxygen in abundance.

Human beings would not have survived

in this high oxygen atmosphere.

But for some swamp dwellers, it was ideal.

Like the Arthropleura, measuring up to 10 feet.

This long lost cousin of the centipedes was a herbivore.

Or Meganeura, with a wingspan up to 25 inches,

this member of the dragonfly family

is the largest known flying insect ever discovered.

A tireless predator.

It had no airborne competitors at the time

since birds and flying reptiles didn't exist yet.

The high oxygen levels in the atmosphere

give the characteristic sepia color

to the sky during the Carboniferous period.

Oxygen also makes the air extremely flammable.

Such a hostile world is hard for us to imagine.

Lightning storms could set aflame the immense forests,

and their inhabitants.

During this period not a day went by

without huge forest fires,

and yet giant insects thrived.

Later when the first became less frequent,

these astonishing creatures simply disappeared.

Scientists are trying to determine

what caused their extinction.

There are several possible culprits.

In fact, it's a bit like an Agatha Christie novel,

where there's not one, but several murderers.

It's our job to take the clues we have

and reconstruct the investigations

in order to come up with the most likely scenario.

While we've known about giant insects

since the 19th century,

paleontologists do not understand why they have disappeared.

For a long time, a change in the composition

of the atmosphere was the only explanation.

But at the beginning of the 21st century,

the discovery of fantastic fossil insects

and their predators opened up new possibilities.

While that's a wonderful hypothesis,

and assuredly something was preying

upon these giant insects,

we don't have great evidence for it.

Around the world,

American, European, and Chinese scientists

confront the old theories using new fossil discoveries

unearthed by groundbreaking technology

to try and explain why these giants became extinct.

The earliest giant insect fossils

were found in the French region of Allier in 1880.

Under the surface of this pond

were the remains of animals that had died

350 million years ago during the Carboniferous period.

Meganeuras.

Now extinct, these tireless predators

are the largest flying insects that ever existed.

This abandoned industrial site

was an important coal field in the 19th century,

and as the coal was dug out,

fossils were discovered close to the town of Commentry.

The owner of the coal mine, Mr. Monyi,

gave his name to the specimen that is preserved

at the Natural History Museum in Paris.

Meganeura Monyi.

Andre Nel, a paleontologist

whose specialty is early insects,

watches over this valuable piece.

Miners would look for fossils

to make a little extra money,

and one day when they were opening slabs,

they cam across this animal.

Unfortunately, when they were digging it out

they hit it four times with a pick and we lost its head.

They were the super predators of the time.

Predators were other insects that were also very big.

These large size fossils are quite exceptional.

While thousands of insects were found

on the site of Commentry,

only five Meganeuras were ever discovered.

Meganeura, like all other insects, had four wings,

two on each side attached to the thorax in the center.

In front you had a head with big eyes

because it was a predator, so its eyes,

just like modern dragonflies,

were used to see its environment in 360 degrees.

So possibly even behind the animal.

To better understand

how this extinct animal once lived,

we must step back 300 million years in time.

This is what the French region of Allier

would've looked like then,

a giant swamp scattered with cypruses.

Humidity at nearly 100% made the atmosphere dense

and allowed Meganeura to easily carry

its heavy exoskeleton into the air.

It is part of a genus that is extinct today,

but it looks much like modern dragonflies,

and it's part of the same Odonatoptera superorder.

With wings that functioned independently of each other,

Meganeura was agile in flight,

but unlike its contemporary cousins,

it couldn't fold its wings.

Faced with this efficient airborne predator,

vegetarian insects such as Palaeodictyoptera

had to keep themselves out of sight.

By comparing its anatomy to modern dragonflies,

we can guess at Meganeura's main physical characteristics.

One, it could fly over 40 miles per hour.

Two, it was a sight predator.

Its head was independent from the rest of its exoskeleton

so it could keep it still while flying

and focus on its prey.

Three, it had a huge appetite.

It could eat its own weight in food every 30 minutes.

To catch all this food, Meganeura had an array

of attributes identified in fossils.

But what might explain its giant size?

Away from the public is the museum's library of species,

where they keep the specimens that scientists study.

Here we find Meganeuras and their prey,

both reaching impressive sizes.

So here you have an example of a Meganaeura,

on which we see the base of its wings,

the thorax, but what is most spectacular

are the forelegs equipped with strong spines

that were used to stab prey.

But the Meganeura's prey were also large sized insects,

like the Palaeodictyoptera.

You just have one wing from here to here,

so you can imagine the whole thing.

These were Meganeura's prey.

They were big guys too.

Big insects to escape big predators.

So in this case we have an arms race

between predators and prey.

But this battle to be the biggest

between Meganeura and its prey

seems to have had its limits,

otherwise paleontologists would certainly have found

even bigger and more terrifying flying insect fossils.

Most of Meganeura's day was spent looking for food

since its metabolism required a lot of energy.

According to scientists, the huge size of insects

during the Carboniferous period was possible

because of the high levels of oxygen in the air.

Insects don't have lungs, but instead use a unique system

of tubes, trachea, and tracheos

to bring air directly to their organs,

including their digestive system.

The downside to this system is it lacks efficiency.

Air travels through the tissues in the form of gas.

The bigger an insect is, the more oxygen it needs.

It is very strange that these animals

reached these sizes, because nowadays

we do not have such big insects,

and at the time of the dinosaurs,

when we had large vertebrates,

insects were much smaller.

It turns out that in the Carboniferous period,

for reasons linked to geochemistry,

the oxygen rate in the atmosphere

was higher than it is today,

which encouraged the development of animals

such as the large insects.

Meganeuras could not survive

in today's atmosphere,

because not enough oxygen would reach their organs,

including their brains, and they would faint.

Since the beginning of the 20th century,

scientists have proposed a link between the size of insects

and the concentration of oxygen.

But it wasn't until 2007

that an experiment finally proved it.

In the Chicago suburbs, the Argon National Laboratory

houses the United States' most powerful synchrotron,

a scanner that generates the brightest X-ray beams

in the northern hemisphere.

The distance around the particle accelerator

is more than half a mile,

so Jake Socha, the scientist in charge of the study,

uses a trike to get around.

Today live insects are being put under the scanner.

We use the idea that you can take living insects

and make inferences about insects that existed in the past.