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Meet the original Jaws,

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the 400 million year old nightmare,

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one of the first jawed fish.

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Basically, one of the first jawed anything.

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(dramatic music)

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Dunkleosteus, it's one of the
greatest fishes of all time.

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It's about 40 feet long,
it's head is bigger than I am

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{\an8}and it's jaws look like paper cutters,

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{\an8}just great big sharpened blades.

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Dunkleosteus was just one of dozens

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of doomed sea monsters that arose

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during the Devonian period
also known as the fishes.

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And like all the rest
of them from this time,

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it was slated for death

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by a barely believable series of disasters.

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(dramatic music)

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The Devonian spans about 50 million years,

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some 180 million years before
the dinosaurs appeared.

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It's magnificent predators all descended

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from boneless fish that squeaked

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through the Earth's first mass extinction

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along with a handful of trilobites,

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conodonts, and other lifeforms.

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{\an8}There were crinoids,
these stemmed sea lilies.

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{\an8}There were coral reefs.

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There were of course lots of fish.

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There were nautiloids that come
back, some of them.

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In the sea we call it the age of fishes

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because now the fishes have evolved,

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they're diversifying and
you get some whoppers.

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The massive supercontinent

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of Gondwana made up of South
America, Africa,

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Antarctica, India,
and Australia is drifting away

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from the South Pole
toward warmer latitudes.

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What is now North America and what is now

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Europe had collided and raised up the land.

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So for the first time you
had this big land area

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with mountains and rivers running off

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of the mountains into the sea.

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Today we know this mountain chain

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as the relatively diminutive Appalachians.

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Back then they'd have risen
higher than the Himalayas.

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And when they came thrusting
up out of the earth,

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life on the planet would never be the same.

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{\an8}The uplift of the Appalachians during this

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{\an8}time period is a very
big deal geologically.

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You have a major mountain
chain closing off a seaway,

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so they're changing your climate globally

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because you're cutting off
ocean circulation pathways.

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But there's another factor
and that is all during

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this period of early life
algae were producing oxygen

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as a waste product of photosynthesis.

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If you make enough oxygen,
it goes into the atmosphere,

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that oxygen is hit by ultraviolet radiation

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and it changes into ozone.

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So the planet really sort
of makes itself habitable

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by first letting life produce oxygen

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then the oxygen becoming ozone

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and cutting out the ultraviolet radiation

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and allowing the plants and the animals

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to come out of the water
for the first time.

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Plants, once confined to the seas,

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begin to spread inland.

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Ferns and the first trees
start covering the land

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in vast often flooded forests.

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The Devonian's when you
get a land environment

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that starts to look like
something you might recognize.

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You go from nothing to forests.

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It was lush, it was lush and green.

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There weren't flowers
yet but it was very green

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and very swampy and very alive.

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In the oceans,
new kinds of fish dominate reefs

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larger than the world had ever seen.

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Placoderms like Dunkleosteus
are snacking on each other.

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But other smaller fish are
experimenting with lifestyles

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that will change everything.

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You have a variety of fish
that actually are maneuvering

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their way around in shallow water

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and they have these
globe-like fins that actually,

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if you look at the fossils of them,

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actually you can start
to see the rays of fins

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actually have distinct
bones and it's this lineage

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of fish that eventually
evolves into amphibians.

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With four sturdy fins to
prop themselves up on,

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these fish are now in prime
position to snap up prey.

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And some of them, like Dipterus,

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develop not just rudimentary limbs

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but little air sacs off their tracheas.

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If the water was becoming stagnant,

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they could go to the top of the water

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and gulp air and get their oxygen that way.

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The first lung fish are born.

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Limbs and lungs are about to come in handy

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because the Devonian
climate is getting erratic.

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Those mountains thrust up by the collision

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of Europe and America
catch water evaporating

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off the seas creating the
first massive rains on Earth

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and the first fresh water habitats

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for the world's fish to explore.

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You had wet periods when it was very cloudy

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and lots of rain and you had dry periods

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when the rain would fail
and so it became a very good

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adaptation if you could get
out of the water from one pond,

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drying up pond,
and get over into another drying up pond.

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But Dipterus is still very much a fish.

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It's limbs can't support its
weight completely out of water.

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For a long time, this was a sticking point

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in our evidence of evolution.

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There was just no transitional fossil

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of a fish that could walk.

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But in 2004 that all
changed with the discovery

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of the famous Tiktaalik,
our first tetrapod,

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or four-footed fossil.

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The way they found Tiktaalik was they said,

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"At this time period
we should find a fossil

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"of an animal that's got
pretty well-developed arm bones

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"and who looks like a fish but it has limbs

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"that are on their way
to evolving into arms.

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"Let's see if we can go find it."

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And they did.

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It was one of those great
point to the left field fence

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and hit the home run kind
of moments in paleontology.

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Tiktaalik is basically a fish with feet.

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They gave rise to amphibians.

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And ultimately, to us.

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They had to go back in
the water to stay wet

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and they had to go back in
the water to lay their eggs

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but they were partially
freed from life in the water

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and that's how life on the
land for the vertebrates began.

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But as Tiktaalik was venturing onto land,

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planet Earth was about to experience

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its second period of very bad days.

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(suspenseful music)

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And the end of Devonian
around 375 million years ago,

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there's another mass extinction.

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This one looks like it might
be a multiple mass extinction

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meaning it was a longer term event

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that was made up of separate
individual rapid events.

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Something is happening at
very specific intervals

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which is making the coral reefs disappear,

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which is changing fish evolution,

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which is changing some
of the things with shells

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that are living in the coral reefs.

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The jury is still out

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on what caused the extinction.

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But from the latest research it looks

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as though some of that new rather

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unsuspecting Devonian
life is a primary culprit.

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It turns out all those trees
covering the continents

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were pumping out oxygen at
rates the world had never known

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while simultaneously
soaking up carbon dioxide,

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cooling the world in a sort
of reverse greenhouse effect.

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The byproducts of a
forest and the byproducts

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of soil is that you have river discharge,

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you have submarine brown water discharge

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that's full of nutrients.

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They're gonna make a balloon

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and that's gonna chose the
oxygen off for everybody else.

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What we think of as a coral reef is nothing

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compared to what they used to be.

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And to add insult to injury,

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recent evidence indicates
the earth took it on the chin

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from an asteroid or other
space rock large enough

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to seal the fate of many species.

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If you look at those layers,

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one finds evidence of impact.

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There are these little
objects called microtektites.

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They are little drops of molten rock

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that were spewed out of the crater

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and they're transported around the world.

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And if you find a layer with microtektites,

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that means there was impact somewhere

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on the Earth at that time.

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Whatever the cause,

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the fossil record from the period reveals

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that over the course of millions of years,

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the mysterious catastrophe spelled bad news

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for 3/4 of the species on earth.

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Yes, it's a sad thing
for the things that died

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but there are winners and
losers in every mass extinction.

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Though the vast majority of fish

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in the sea were wiped out in
the great Devonian extinction,

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the fish with feet planted
them firmly on terra firma.

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The tetrapods were set on world domination.

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The great ages of amphibians,

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reptiles, and mammals lay ahead.

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(dramatic orchestral music)