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(dramatic orchestral music)

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It's been some 450 million years

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since these hardy creatures,

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Horseshoe crabs, first appeared on Earth.

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And though they have barely
changed in that time,

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the world around them sure has.

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Since they started scurrying onto beaches

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the vast majority of life around them

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was nearly obliterated.

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It's a story of life,

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death,

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and life reborn.

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(gentle dramatic music)

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Horseshoe crabs aren't actually crabs.

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Turn them over and you'll see

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they're much more closely
related to spiders.

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{\an8}These things are called living fossils.

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{\an8}And they've been around

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{\an8}for hundreds of millions of years.

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I think they did so because
they were generalists.

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They didn't have one specific
way of making a living.

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They were very opportunistic.

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By coming ashore to mate,

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they're reenacting a brilliant

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but doomed moment in deep history,

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when their spidery ancestors

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during what we now call
the Ordovician Period

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became the first life forms

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to try to wriggle their way onto land.

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For the almost four billion
years life was evolving

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on Planet Earth up to this point,

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land had been nothing more
than a barren wasteland.

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All the continents of Earth
just blank lifeless rock.

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There were effectively no land plants,

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which means no land animals.

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{\an8}So if you were walking around
on land in the Ordovician,

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{\an8}there might be some lichen or some fungus,

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{\an8}some bacteria, but no plants.

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No real terrestrial ecosystem.

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But dip below the waves
during the Ordovician,

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and you'll find a very different world.

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Life's been on the planet

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probably as much as four billion years ago.

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And in the Ordovician almost
all live was still in the sea.

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Life started in the sea,

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and it's still very abundant in the sea.

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Five hundred millions years ago,

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at the dawn of the Ordovician,
most of the planet was sea.

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Chunks that will eventually become Africa,

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South America, Australia, and Antarctica

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are beginning a slow drift south.

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Parts of the future
North America and Europe

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peek out at the equator.

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But the vast majority

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of these familiar places are submerged,

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with no ice caps.

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Sea levels are 2,000 feet
higher than we're used to.

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In the Ordovician sea,

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the things that were swimming

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would be things that you would recognize

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as relatives of squids and octopuses.

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It's like life is booming,

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it's blowing and going in the oceans.

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Coral appears for the first time,

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creating reef communities that
teem with ancient critters,

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and completely new ones.

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You had animals like trilobites,

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like little pill bugs.

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They have eyes, they have legs,

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they have segmented body parts,

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and they're cruising
around on the sea floor

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eating organic matter or
other smaller animals.

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Feathery crinoids wave microscopic food

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into their mouths.

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They look like plants but
they are in fact animals.

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It's simply feeding in the sea water.

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It's plankton eating, it's almost like

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if you want to think about
it, a little attached whale,

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it's a filter feeder.

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Snails and clams make their debut.

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And the first bony fish,
jawless but armored,

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are sucking up garbage from the seabed.

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And then there are these
mysterious eel-like conodonts,

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basically tubes with teeth
and fins roaming the seas.

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We didn't know what the animals ate

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for a long time.

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We found the actual teeth parts.

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Then one day someone found an
imprint of the whole animal

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with its teeth in place.

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Now these things are a
little more familiar.

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Scary new hunters at the
top of the food chain.

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Cephalopods,
relatives of squids and octopuses,

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the greatest predators of the Ordovician.

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These are nasty marine predators.

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If you are out swimming
with one of those things,

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it's not a pleasant concept
to be eaten by a squid.

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It's reverse calamari,
right, the calamari eats you.

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This busy seascape is
ideal for many species.

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But after 50 million
years in the Ordovician

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the natives are getting
restless, and daring.

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The spores of water plants drift inland

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and take root for the first time.

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Creatures like our
Horseshoe crab's ancestors

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creep up on shore to reproduce.

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First the plants have to colonize the land

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so there's something for animals to eat.

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And the first animals to
colonize the land were insects.

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They already had legs to
walk on the ocean bottom.

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They breathed air through their spiracle,

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through their skin,

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and so they could very
easily or readily adapt

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from being marine organisms
to being non-marine organisms.

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The Ordovician is set

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to host the most amazing revolution,

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the rise of life on land.

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But it's too late.

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Something was about to
stop life in its tracks.

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A leading theory blames the
supercontinent of Gondwana,

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which has continued its move south

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and has finally reached the South Pole.

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Rocks from the time indicate glaciers

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are starting to form at its center.

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When there's glaciation a lot of water

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gets drawn out of the
ocean, into the atmosphere,

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precipitates as snow, forms as ice,

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and as the ice grows
the sea level goes down.

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If you drop sea level off the
edge of the continental shelf,

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you decrease the area
where animals can live

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and you can get extinctions.

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That's one possible scenario.

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Another scenario has the whole dying

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beginning even earlier.

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Somewhere in our galaxy,

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a super massive star went hypernova,

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spewing out the universe's most
deadly weapon, a gamma ray.

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If such a thing hit the earth's atmosphere,

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it would have toasted the ozone
layer that protects all life

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from the deadly ultraviolet
power of the sun.

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Yet another theory ends with a whimper

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instead of a cosmic bang.

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Recent research shows that
the seas started running out

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of trace elements, like
zinc, magnesium, and selenium

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long before the end.

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Whatever the cause, or causes,

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our trusty Horseshoe crab
somehow managed to survive.

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But the combined catastrophes snuffed out

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50 to 85% of all life in the oceans.

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The Ordovician, extinction on
land is kind of a moot point

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because there was nothing there to kill.

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But in the sea, something happened

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that snuffed out a lot of it.

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It was the Earth's first mass extinction

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that we can see in the fossil record.

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And the second worst,
according to most scientists.

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But these terrible days on Earth,

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like all the extinctions to come,

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were blessings in disguise.

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Life fights back.

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There's always winners in
the lottery of extinction.

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And whatever those survivors are,

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that's where the next radiation happens,

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that's where the next
evolutionary burst happens.

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So if you want a simple way

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to think about extinctions
on Planet Earth is,

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the survivors are the winners,

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and their descendants are the ones

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that populate the next time period.

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Within five million years

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of the Ordovician collapse,

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the Earth would warm again.

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Sea levels would rise.

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And new life forms would
explode to fill the voids.

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With a fresh, clean slate,
life is just getting started.

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(dramatic orchestral music)

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{\an8}(bright music)