Meet the original Jaws,
the 400 million year old nightmare,
one of the first jawed fish.
Basically, one of the first jawed anything.
(dramatic music)
Dunkleosteus, it's one of the greatest fishes of all time.
It's about 40 feet long, it's head is bigger than I am
and it's jaws look like paper cutters,
just great big sharpened blades.
Dunkleosteus was just one of dozens
of doomed sea monsters that arose
during the Devonian period also known as the fishes.
And like all the rest of them from this time,
it was slated for death
by a barely believable series of disasters.
(dramatic music)
The Devonian spans about 50 million years,
some 180 million years before the dinosaurs appeared.
It's magnificent predators all descended
from boneless fish that squeaked
through the Earth's first mass extinction
along with a handful of trilobites,
conodonts, and other lifeforms.
There were crinoids, these stemmed sea lilies.
There were coral reefs.
There were of course lots of fish.
There were nautiloids that come back, some of them.
In the sea we call it the age of fishes
because now the fishes have evolved,
they're diversifying and you get some whoppers.
The massive supercontinent
of Gondwana made up of South America, Africa,
Antarctica, India, and Australia is drifting away
from the South Pole toward warmer latitudes.
What is now North America and what is now
Europe had collided and raised up the land.
So for the first time you had this big land area
with mountains and rivers running off
of the mountains into the sea.
Today we know this mountain chain
as the relatively diminutive Appalachians.
Back then they'd have risen higher than the Himalayas.
And when they came thrusting up out of the earth,
life on the planet would never be the same.
The uplift of the Appalachians during this
time period is a very big deal geologically.
You have a major mountain chain closing off a seaway,
so they're changing your climate globally
because you're cutting off ocean circulation pathways.
But there's another factor and that is all during
this period of early life algae were producing oxygen
as a waste product of photosynthesis.
If you make enough oxygen, it goes into the atmosphere,
that oxygen is hit by ultraviolet radiation
and it changes into ozone.
So the planet really sort of makes itself habitable
by first letting life produce oxygen
then the oxygen becoming ozone
and cutting out the ultraviolet radiation
and allowing the plants and the animals
to come out of the water for the first time.
Plants, once confined to the seas,
begin to spread inland.
Ferns and the first trees start covering the land
in vast often flooded forests.
The Devonian's when you get a land environment
that starts to look like something you might recognize.
You go from nothing to forests.
It was lush, it was lush and green.
There weren't flowers yet but it was very green
and very swampy and very alive.
In the oceans, new kinds of fish dominate reefs
larger than the world had ever seen.
Placoderms like Dunkleosteus are snacking on each other.
But other smaller fish are experimenting with lifestyles
that will change everything.
You have a variety of fish that actually are maneuvering
their way around in shallow water
and they have these globe-like fins that actually,
if you look at the fossils of them,
actually you can start to see the rays of fins
actually have distinct bones and it's this lineage
of fish that eventually evolves into amphibians.
With four sturdy fins to prop themselves up on,
these fish are now in prime position to snap up prey.
And some of them, like Dipterus,
develop not just rudimentary limbs
but little air sacs off their tracheas.
If the water was becoming stagnant,
they could go to the top of the water
and gulp air and get their oxygen that way.
The first lung fish are born.
Limbs and lungs are about to come in handy
because the Devonian climate is getting erratic.
Those mountains thrust up by the collision
of Europe and America catch water evaporating
off the seas creating the first massive rains on Earth
and the first fresh water habitats
for the world's fish to explore.
You had wet periods when it was very cloudy
and lots of rain and you had dry periods
when the rain would fail and so it became a very good
adaptation if you could get out of the water from one pond,
drying up pond, and get over into another drying up pond.
But Dipterus is still very much a fish.
It's limbs can't support its weight completely out of water.
For a long time, this was a sticking point
in our evidence of evolution.
There was just no transitional fossil
of a fish that could walk.
But in 2004 that all changed with the discovery
of the famous Tiktaalik, our first tetrapod,
or four-footed fossil.
The way they found Tiktaalik was they said,
"At this time period we should find a fossil
"of an animal that's got pretty well-developed arm bones
"and who looks like a fish but it has limbs
"that are on their way to evolving into arms.
"Let's see if we can go find it."
And they did.
It was one of those great point to the left field fence
and hit the home run kind of moments in paleontology.
Tiktaalik is basically a fish with feet.
They gave rise to amphibians.
And ultimately, to us.
They had to go back in the water to stay wet
and they had to go back in the water to lay their eggs
but they were partially freed from life in the water
and that's how life on the land for the vertebrates began.
But as Tiktaalik was venturing onto land,
planet Earth was about to experience
its second period of very bad days.
(suspenseful music)
And the end of Devonian around 375 million years ago,
there's another mass extinction.
This one looks like it might be a multiple mass extinction
meaning it was a longer term event
that was made up of separate individual rapid events.
Something is happening at very specific intervals
which is making the coral reefs disappear,
which is changing fish evolution,
which is changing some of the things with shells
that are living in the coral reefs.
The jury is still out
on what caused the extinction.
But from the latest research it looks
as though some of that new rather
unsuspecting Devonian life is a primary culprit.
It turns out all those trees covering the continents
were pumping out oxygen at rates the world had never known
while simultaneously soaking up carbon dioxide,
cooling the world in a sort of reverse greenhouse effect.
The byproducts of a forest and the byproducts
of soil is that you have river discharge,
you have submarine brown water discharge
that's full of nutrients.
They're gonna make a balloon
and that's gonna chose the oxygen off for everybody else.
What we think of as a coral reef is nothing
compared to what they used to be.
And to add insult to injury,
recent evidence indicates the earth took it on the chin
from an asteroid or other space rock large enough
to seal the fate of many species.
If you look at those layers,
one finds evidence of impact.
There are these little objects called microtektites.
They are little drops of molten rock
that were spewed out of the crater
and they're transported around the world.
And if you find a layer with microtektites,
that means there was impact somewhere
on the Earth at that time.
Whatever the cause,
the fossil record from the period reveals
that over the course of millions of years,
the mysterious catastrophe spelled bad news
for 3/4 of the species on earth.
Yes, it's a sad thing for the things that died
but there are winners and losers in every mass extinction.
Though the vast majority of fish
in the sea were wiped out in the great Devonian extinction,
the fish with feet planted them firmly on terra firma.
The tetrapods were set on world domination.
The great ages of amphibians,
reptiles, and mammals lay ahead.
(dramatic orchestral music)