Original subtitles

For centuries,

we've fantasized

about alien life on other planets.

And because life as we know it needs water,

we imagine seas on the Moon,

rivers on Mars,

and lakes beneath Europa's icy surface.

So far, we've discovered only dust

except on this watery blue marble

where inner space hides creatures

as weird as anything extraterrestrial.

There are oceans of oddballs

with strange shapes...

Strange rituals...

and strange bedfellows

right here in Earth's own alien abyss.

Our oceans are constantly changing.

For billions of years,

volcanoes, asteroids, and ice ages

have kept marine life on its toes.

If you want to live, you've got to adapt.

No solution is too strange if you survive.

That's the origin story of every watery weirdo in the sea,

beginning with the very earliest.

We've had life in our seas

for 3 1/2 billion years.

But intelligent life...

Not so much.

If martians had visited the Earth way back then,

they might not even have recognized it as life.

Lower landing leg.

Turn on your magnascope to third power.

How primitive.

In the beginning,

all life was too small to see,

except for these living rocks.

Hello. Take us to your leader.

These are stromatolites,

the oldest surviving life-forms on our planet.

Colonies of bacteria that have been around

for three and a half billion years.

They don't do much except survive.

Stromatolites ruled the oceans for most of Earth's history.

Now they survive in only a few spots

like Western Australia's Shark Bay.

The bacteria cells stick to each other

and the sea floor with mucus.

Sediment get stuck in the mucus too and starts to smother them.

The bacteria live by photosynthesis.

They need sunlight.

So they migrate upwards out of the muck and into the Sun,

building rocks at half a millimetre per year.

Stromatolites and other single-celled creatures

had the sea all to themselves

for about 3 billion years.

But eventually, bigger things came along--

things that like to eat them.

Thus began the familiar food chain.

Stromatolites, plants,

and anything else that can photosynthesize

get energy from the Sun.

Herbivores like this dugong come along and eat them.

Then carnivores come along, and...

yeah, you don't wanna see that.

Back to the stromatolites.

Sadly, most of them got eaten

by things like sea urchins.

But a few survived thanks to their one superpower:

they can live in extra-salty water.

That's why, today,

you'll only find stromatolites in tidal pools,

which, thanks to evaporation, are twice as salty as the sea.

Too salty for their enemies.

The march of evolution was tough

on single-celled creatures,

but it made life on Earth a lot more interesting.

It gave us weird beasties

like the red-lipped batfish...

and the tasselled wobbegong.

To make animals like this,

you need over 100 different kinds of cells,

with each kind of cell doing a special job...

that is, you don't make the tassels

with the same cells you use for the gills.

The very first animal

to crack the multi-cell formula was this.

No, not the frog fish.

That's hundreds of millions of years in the future.

The thing he's pretending to be...

The sponge.

Sponges were the weirdest thing

to happen to life on Earth since, well,

the beginning of life.

An animal with no organs, no tissues, no nervous system.

But a big leap forward all the same.

The first creatures with different cells

for different jobs.

Sponges make egg cells and sperm cells for reproduction.

Eggs, they keep to themselves.

Sperm, they release in a cloud to share with other sponges.

They also have an outer layer of cells similar to our skin,

and, hidden inside,

a layer of different cells

to filter bacteria from the water.

They are herbivores,

specializing in microscopic food.

Jellyfish are some of the first multicellular creatures

to turn their attention to hunting.

The challenge is how to hunt with a body that's 95% water.

The solution? Keep it simple.

Their bells are a jelly held together

by thin membranes.

A ring of muscles around the bell

contracts to force out water--

the first jet-propulsion system.

But their biggest innovation:

microscopic neurotoxic darts.

Nomura's jellyfish is king of them all.

arguably the largest jellyfish alive.

It's the size of a fridge.

Nomura's jellyfish plies the Yellow Sea

between China and Japan,

always searching for food.

This giant beast is stranger than most.

It has hundreds of mouths

all over the big feeding arms below the bell.

Its little fishy friends are safe, though,

as long as they avoid the many long, thin stinging tentacles.

Nomura's mouths are only one millimetre wide.

Only zooplankton--

teensy-tiny floating animals-- fit through.

But every one of them gets stung first.

It's mass murder on a microscopic scale.

If gelatinous stinging aliens freak you out,

get ready for some bad news.

Humans are doing everything we can to make more of them.

We pour nutrients into the sea, creating plankton blooms.

We catch the fish that would eat the plankton,

leaving more food for the Nomura.

We've given them an opportunity to launch an alien invasion

numbering in the hundreds of millions.

How can they take over so fast?

Jellyfish can clone themselves.

Their larvae attach themselves upside down to the sea floor.

But they don't just grow up to become one adult.

Each larva creates many identical copies of itself.

In just one generation,

they can launch a clone army.

If conditions are favorable-- and they are--

the jellyfish population explodes.

If this sounds terrifying,

this type of thing has happened many times in Earth's history.

500 million years ago,

there was such a massive burst of life

that we named a time period after it.

The Cambrian explosion...

A time when evolution itself blew up in all directions.

New animals tried out wacky new ways to live.

Some of their experiments really stood the test of time.

The most successful invention was a basic body plan.

Suddenly, alien animals start looking eerily familiar.

Meet the bilaterians.

A big word for animals with a top...

a bottom...

a left... and a right.

Essentially, anything that's not a sponge or a jellyfish.

Bilaterians include dugongs, remoras, scuba divers, and...

sharks.

Sharks have been around for 350 million years

in a form we'd recognized today as sharky.

You know the drill:

nasty teeth,

slicing fins,

dead eyes,

and that streamline torpedo shape.

But one design is just plain weird.

Why, hammerhead? Why?

Separating the eyes at its wingtips gives the hammerhead

extraordinary stereoscopic vision,

but it does nothing for the hydrodynamics.

All the spoiler does is increase drag

and spoil the sleek torpedo shape.

Hammerheads are ten times less energy efficient

than the classic shark's silhouette.

So why has the weird head survived?

It acts like a forward rudder.

The slightest twist creates a very sharp turn.

Perfect to chase down rays,

its favorite...

but very tricky...

prey.

But the strangest shark of all has almost given up

on swimming.

This is the epaulette shark.

Its other name is the walking shark.

The ones living on Australia's Great Barrier Reef

have developed extraordinary abilities to dice with death.

Their home is shallow and highly tidal.

It's easy to get stuck in a small pool

at extreme low tides.

The water temperature can skyrocket.

Oxygen levels drop by 80%.

The death zone.

Just walk away.

Fins with extra muscles and a greater range of motion

are almost as good as feet.

But even this low-tide specialist

can run out of pools, out of air, and out of luck.

Now it shuts down much of its brain.

Brains use a lot of oxygen.

Heart rate slows,

blood pressure crashes.

Essential systems only until tide comes in.

Engage brain...

and walk away, unscathed,

back to the alien abyss.

Feet or fins are not the only way to walk.

The sea floor abounds with many peculiar pedestrians.

Animals have adapted all sorts of appendages to do the job.

Sea stars have hundreds of tube feet,

which are for breathing,

smelling, and tasting, as well as walking.

Sea stars don't have brains,

so every tube foot makes its own decision

on where to go.

It's a miracle sea stars move at all.

And bristles can be legs too.

Bristle worms are highly successful

with little more than a head

and a whole lot of identical segments.

The biggest bristle is the bobbit,

a bizarre brute up to three meters long.

You heard right.

If you're swimming in Indonesia,

beware of three-meter-long walking worms.

They're killers.

The bobbit is too slow to run prey down.

And it's oblivious to the ones that hide.

But it has big ambitions.

It has its eyes set on a fish.

Wait, it has no eyes.

How does a blind, slow-moving worm catch a fish?

Watch.

The bobbit burrows tail first into the sand

and deploys its alien apparatus:

five surveillance antennae, tuned to any movement.

Razor-sharp, spring-loaded mandibles primed.

It's barely visible.

The slightest touch sets off the trap.

The muscle jaws can slice big fish in half.

The worm retracts into its burrow,

dragging its prize to a grizzly end.

Compared to the ruthless bobbit...

its cousins, the tube worms,

are, well, feather dusters.

They're bristle worms that have become builders.

They hide in hard, protective tubes

attached to any suitable support.

Instead of spring-loaded death traps,

they extend crowns of otherworldly feathers...

a marine multitool that they use to breathe,

find food, and gather building material.

The fine feathers sort the stuff

that floats in the water.

Large bits, they reject.

Small bits, they funnel to their mouths

to assess as food.

And middle-sized sand particles are just right

for sticking together with mucus

to build their tubes.

The Mediterranean fanworm is the most successful

of them all-- or the most invasive,

depending on your point of view.

You'd th

living in this idyllic place would stay put.

But it's the pretty fanworm,

not the vicious bobbit,

that has conquered our planet.

They're incredibly fast growing

and their larvae can settle on anything...

including passing ships.

They've hitchhiked from the Mediterranean

all the way to New Zealand on the other side of the world

and taken over...

at 1,000 per square meter.

Once established,

they're almost impossible to get rid of

thanks to a few secret weapons in their quiver.

They absorb heavy metals, like arsenic, from the seawater

and concentrate it in their crowns

to deter predators.

Stranger yet,

they have simple eyespots

all over their feathery mouthparts.

Any sign of danger,

like the shadow of an approaching camera operator,

and they retreat.

Eyes.

Now that was a useful development.

All animal eyes have a common origin

going all the way back to the Cambrian.

Starting as simple eye spots,

they developed in many bizarre ways,

from the relatively simple eyes of the scallop

to the sophisticated compound eyes of the mantis shrimp.

But the eyes of octopus, squid, and cuttlefish

are out of this world...

maybe.

Meet the head-foots...

cephalopods, if you want to get all Greek about it.

The basic blueprint:

tentacles...

and a head with big eyes.

Eyes with freaky features

like the ability to detect polarized light

or to focus independently of each other.

Some squid eyes even have built-in lights

to see in the dark.

Basically, it's the recipe for a real-life alien.

And some say that how cephalopods got to be so weird

is a genuine deep-space mystery.

In 2018,

a few scientists suggested that octopus eggs

might have come from outer space,

frozen on a comet.

Sadly, all the other scientists said they were tripping.

The conventional, less-exciting wisdom

is that cephalopods and their eyes

started out much simpler.

More like the nautilus.

These are the most primitive surviving cephalopods.

They're the only ones that still have an external shell

and the only ones without amazing visual abilities.

Half a billion years ago,

2 1/2 thousand nautiloids ruled the ocean.

Today, just six remain.

The South Pacific's pearly nautilus is the largest.

Like Captain Nemo's submarine,

the Nautilus sinks by filling chambers

in its shell with seawater.

It can sink an impressive 800 meters

before its shell implodes.

But its eyes are much less impressive.

They are open to the sea

with no lens or protective cornea.

Unique in the animal kingdom,

the nautilus eye is basically a pinhole camera.

The image that pinhole pupil creates

is 400 times less bright than that produced by octopuses,

but at least it was a start.

Later cephalopods, like octopuses,

added lenses and other features

to make their eyes state-of-the-art.

They're a very different design to those of vertebrates,

but just as successful.

They don't have different color receptors on their retinas.

Instead, they seem to see color

by splitting light into its components

with their strange, horizontal pupils.

And instead of changing the shape of their lenses

to focus like we do,

they move them back and forth like a camera.

It means their ability to focus never deteriorates.

But great eyesight is only one reason

that they're brilliant hunters.

The other reason is their phenomenal brain,

which is unlike any other on Earth.

This is a run-of-the-mill common octopus.

Its central brain has decided

it wants to check out some other rock pools.

It's instructed the eight auxiliary brains

to make that happen.

They're all working on the problem independently,

one brain in each arm.

2/3 of the octopus's brain cells

are in its tentacles.

They may even be helping to process visual information

they could be seeing with their arms.

One thing the octopus still can't do is breathe air.

It's breathing from water stored in its mantle.

It has two hearts dedicated to pumping blood to its gills.

The third heart supplies the organs.

Maybe they really are aliens...

Or just an incredibly over-engineered solution

to the problem...

of how to kill a crab.

In the oceans,

problems are just opportunities waiting to be cracked.

For some of the most alien life-forms,

no problem is too extreme.

Earth is a planet of extremes.

There are places where no sensible life-form

should survive...

like hot acid lakes,

or the depths of the abyss.

But even here, life has found a way.

Quick recap.

If you live on Earth,

you're probably getting your energy from the Sun,

directly through photosynthesis or indirectly.

If you're not, then chances are,

you're an extremophile

on a hydrothermal vent...

places we've only known about since 1977...

when we started exploring the deepest parts of the ocean.

Everything down here is more alien,

even the octopuses.

The water temperature is near freezing;

the pressure, immense.

Volcanic vents pump out superheated water

and noxious chemicals.

And vent shrimp wouldn't have it any other way.

They thrive in the Goldilocks zone

between boiling and freezing.

In fact, they're unusually active for deep sea creatures.

The question is, where do they get their energy?

They get it from extremophile bacteria

that use chemicals

from the vents as their energy source.

It's chemosynthesis instead of photosynthesis.

The shrimp don't eat the bacteria.

Instead, they receive nutrients directly from bacteria

actually living inside their guts and on their gills.

This is life completely independent of the Sun.

Many scientists now think life started in this dark,

hot, chemical soup.

It may be that learning to tolerate the Sun

was the real alien innovation.

Off the coast of Taiwan,

there are very shallow hydrothermal vents

just below the surface.

They belch out superheated water

and a lot of gas,

mostly carbon dioxide.

Warm acidic seas, high CO2 levels.

Sound familiar?

Scientists love places like this.

It's like studying the end result of global warming.

Life, on the other hand, hates it.

Super sulfur vent crabs are the only creatures

that can hack it here.

Among other things,

they've learned to metabolize sulfuric acid.

The crabs have evolved special biochemistry

to cope with CO2 levels that kill all lesser creatures.

Anything that asphyxiates becomes crab food.

Most of their meals are much smaller

than a lost puffer fish.

Plankton dies the moment

it drifts into this watery hell.

All the crabs have to do is pick it up.

They thrive in conditions where nothing else survives.

But that's hardly surprising.

Crabs do well almost everywhere...

So much so that many imitators

have tried to follow their recipe for success.

Crabs may look a bit like alien robots made from spare parts,

but their sturdy design has stood them in good stead

for 200 million years.

With eight legs,

massive claws,

body armor,

360-degree vision

and amphibious capabilities,

they are the perfect planetary rovers.

Maybe that's why nature

has thrown up the crab body plan

many times over.

This is a crab.

Crab. Crab.

Not a crab.

Crab. Crab.

Not a crab.

Yep, that's a crab.

And... not a crab.

This porcelain crab is an extreme example

of carcinisation.

That would be the tendency for crustaceans

to become crab-like.

Porcelain crabs are family of squat lobsters,

squashed up and flattened.

King crabs are relatives of hermit crabs,

which, by the way, are also not crabs.

There are many advantages to being crab-like.

It's easier to hide under rocks or in crevices

and you expose less of your vulnerable side to predators.

The crab-eyed goby is obviously convinced.

It's done some carcinisation of its own.

But no defense is perfect.

For extra protection,

many crabs augment their armor

with elaborate camouflage.

Others carry defensive weapons.

And the king of all crab-like things

has become so big, no one bothers it.

Coconut crabs are the world's largest hermit crab.

Again, not really a crab.

There isn't a shell big enough for an adult female to hide in.

Instead, she has hardened plates for protection.

She can breathe air through primitive lungs...

so she has no need to return to the sea

except to release the eggs that she carries under her tail.

When they're ready to hatch, the ocean calls her back.

She may be a landlubber at heart,

but her offspring start life as beach babes.

They'll have to take their chances with the tides.

The point where the land meets the sea is a great place

for extraterrestrial sightings.

In a sense, sea creatures that come ashore really are aliens.

The action of waves

and the coming and going of the tides

catch many a careless creature unaware.

One tidal terrestrial

has not only adapted to these hardships,

it's based its entire life around them.

The finger plough snails of Southern Africa

are confirmed beach bums.

As the incoming tide flows over the sand,

plough snails emerge... by the hundreds.

They've avoided the low tide hidden under the sand.

Emerging on the incoming tide

means they don't run the risk of drying out.

But they have an unerring nose for any unfortunate

left stranded at the high-tide mark.

But how to get all the way up there

traveling at, well, a snail's pace.

That requires genius.

These snails have sails.

They unfurl their freakishly big feet

to catch a wave all the way to dinner.

Time for a bit of alien probing.

The long maneuverable proboscis searches for an opening.

Once it's in,

the snail hoovers up soft tissue and liquid

up to a third of its own body weight.

Fully sated, they wait for the retreating tide

to take them back

to where they started.

The sneaky snails successfully straddle two worlds.

Why choose when you can have the best of both?

Mudskippers have raised this dual-citizenship approach

to an art form.

From Africa to Australia,

these enigmatic little fish are waiting for low tide.

They all have joints of a sort in their fins

that function a bit like elbows.

It's what puts the skip in their step

and helps them keep their heads up high.

Some of these proud land dwellers

spend 3/4 of their time out of the water.

And they're not lying around, gasping for air.

These little oddballs love to throw their weight around.

Some fight for mates and territories.

Some turn into formidable land-based predators.

Others graze on the nutritious slime the tide leaves behind.

Lots of highly active stuff that uses up a lot of oxygen.

The weird thing about mudskippers

is that they can't breathe air, at least not directly.

Like all fish, they have gills.

But they can also breathe through their skin,

like amphibians do.

To do that, they have to keep it nice and moist.

The inside lining of those giant mouths...

is especially good at absorbing oxygen.

Some mudskippers love air so much

that when the tide comes in,

they take another big gulp...

to fill up their burrows with a pocket of air.

Incredible as they are,

mudskippers have never really conquered

anything more than mud.

Leaving the sea for good would take fundamental changes

to the way animals walked and breathed.

The first animals to take up permanent residence on the land

were fish.

Not the modern fish that we have today,

but ancient animals called lobe-finned fish.

The most famous is the coelacanth,

extinct for 15 million years

until someone fished up a live one in 1938.

The coelacanth is a fish with arms

and may prove to be the link between man

and his sea ancestors.

We now know that the closest living relative of land animals

is the only other type of lobe-finned fish:

the air-breathing freshwater lungfish.

Even today, you can see how its prehistoric relatives

might have crawled up a mudflat.

Paired fins eventually became

the arms and legs of land animals.

Lungfish do have gills,

but their environment is often low in oxygen.

Gills alone won't cut it.

Lungfish must surface regularly to gulp fresh air

at the surface.

These strange fish have a pair of lungs

very similar to our own.

Of course, things were different

when their ancient ancestors slithered out of the water.

Today's lungfish face a problem their forebears never did...

Land-based predators.

The shoebill is just another creature

that evolved from that pioneering fish.

Once animals had conquered the land,

there was only one thing left to do:

head back to the sea.

What a waste of time that was.

Yes, otters are the descendants of homesick land fish.

5 million years in the sea have given them webbed feet,

paddle-like tails, and waterproof fur,

but they wouldn't look out of place on land.

The reason is that, in evolutionary terms,

they haven't been back in the water all that long.

Given more time,

the transitions get a lot stranger

than these oversized water weasels.

Whales are most closely related to hippos.

A penguin is a fat albatross with tiny wings.

And the dugong is a cousin of the hyrax.

But the animal voted least likely to succeed at sea

lives here.

The Galapagos Islands

are famous for their bizarre animals.

Blue-footed boobies,

giant tortoises,

tropical penguins.

Charles Darwin, famous scientist, made his name

studying the least-interesting inhabitant:

the finch.

The coolest, most alien creatures,

he dismissed as, and I quote,

"most disgusting, clumsy lizards."

The only thing he liked about these imps of darkness

was that they tasted good.

Harsh words for the only lizard to have reinvaded the sea.

Marine iguanas are so completely bound to the sea

that seaweed is the only thing they eat.

The bizarre thing is they're not really much different

to the neighboring land iguanas.

They're all iguana and very little marine.

So how does a cold-blooded lizard

that moves slowly in full sun

turn into this alien action hero?

The big males feed exclusively beyond the surf zone.

The water is cold.

Every stroke saps his energy.

Now he fills his lungs and, battling his own buoyancy,

dives ten meters or more.

Finally, food.

Anchored with his sharp claws,

he grazes quickly before his body freezes up.

Under water, they're not as stupid and sluggish

as Darwin thought.

The clever iguana times his return to the surface

so that he has just enough energy in reserve

to make it back to shore.

When he reaches the warm basalt,

it's all he can do to lie, spent, in the sun.

Now his black body comes into its own.

It's important to get warm as fast as possible

because there's one last thing

that makes these iguanas weirder than any alien.

They can't digest the only food they eat--

at least, not on their own.

They rely on bacteria in their guts

to break down the tough seaweed,

and they only work when they're warm.

He just musters up the will to sneeze out some salt

extracted from his blood by a special gland.

Meanwhile, his land-based cousins

haven't even left their cactus.

The two species are so similar,

they can breed to produce hybrids.

Marine iguanas really are land iguanas

with strange habits rather than strange features.

Swimming.

Compared to land iguanas,

their toes are minimally webbed;

their tails, only slightly flattened into paddles.

Holding their breath.

No special adaptations here.

You can hold a land iguana under water for an hour

without killing it.

Please take our word for it.

Both types switch to anaerobic respiration

when the air runs out,

even though land iguanas seldom need to.

And this trick...

Most lizards have salt-removing glands.

In fairness, marine iguanas do have the best salt glands

of any reptile.

The amount of salt they eat

would pickle any other lizard's kidneys.

All in all, their adaptations are mostly behavioral.

They time their excursions with the midday sun.

They leave hot, come home cold,

and spend the rest of the day recovering and digesting.

Sometimes an alien is just a normal creature

exploring a strange environment.

It just needs one...

to take the plunge.

The ocean was the cradle of all life on Earth,

the place where fundamental new designs were first tried out.

And it's still the place where some of the wackiest

evolutionary experiments get tested.

Animals will do anything to exploit an opportunity.

You may end up as one of the ocean's oddballs.

But when you win,

no one cares what you look like

in the alien abyss.