Original subtitles

The Earth, home to millions of species.

But what might live... beyond?

Astronomers have discovered thousands of planets

outside our solar system.

They believe there are trillions more.

If life exists on only a fraction of them,

then the universe must be... alive.

All living things have the same needs.

To feed...

reproduce...

and evolve.

By applying the laws of life on Earth

to the rest of the universe...

it's possible to imagine

what could live...

on alien worlds.

Most planets we know of are so hellish,

it seems impossible that anything could live.

But it's amazing where life can take hold.

For as long as I can remember, my mom always told me I was an alien,

because I just had my mind on outer space from the beginning.

I just had always known that I wanted to get out there and explore the universe.

Oh, my God.

This is the most incredible place I think I've ever seen.

This is what I dream other planets looking like.

It's... it's incredible.

So, yeah.

The Danakil Depression is known locally as "The Gateway to Hell."

120 meters below sea level and volcanic...

it's the hottest place on Earth.

We're at 79C. 82C.

This one's topping out at 87.

If life can exist here...

it could exist on any number of worlds in the universe.

Life on other planets, they're gonna have, uh, chemistry.

Whether it looks exactly like ours, we don't know,

but they're gonna have chemistry that helps them live.

And that chemistry is going to be subject to limits.

For example, all life is made up of cells.

Inside these cells they have biomolecules

that allow the cells to function and live and work.

That's our machinery that runs and makes this chemistry happen,

that keeps life living.

And so we really wanna understand how that machinery works

and what's gonna make that machinery break down.

Until recently, it was assumed

that Danakil was too extreme to support life.

But astrobiologists come here to look for simple single-celled microbes...

known as extremophiles.

Microbes are too small to see with the naked eye,

so it's not going to be as simple as taking a little bit of water out of a lake

and putting it under a microscope and seeing lots of teeming bugs.

The only way to detect the presence of extremophiles

is to find traces of their DNA,

using a portable genetic sequencer.

This is a very small device and it's very sensitive to temperature,

and we're in one of the hottest places on the planet Earth.

So, here in the field right now, we do... a cool block,

to kind of keep it cool, and we have our channels running.

And we have...

We have sequencing!

We actually have sequencing happening.

We have DNA in here.

Excellent.

Sequencing of DNA is proof

that extremophiles can survive in the acidic waters of Danakil.

These guys are like the superheroes of microbes, and it's just amazing,

as we learn more and more about the different strategies and mechanisms

that they come up with to survive these kinds of environments.

Finding things alive here opens up an intriguing possibility.

But some life forms are tough enough to survive anywhere...

even in the deepest reaches of space.

A red dwarf star.

The most common type of star in our galaxy.

Imagine a planet in such a close orbit,

its rotation is locked by the star's gravity,

so it always shows the same face to its sun.

This is Janus.

How would life adapt on such an extreme world?

On one side of the planet, it's always daytime,

a searing desert.

On the other side, it's forever night,

a frozen shadowland.

Squeezed between the two, a sliver of perpetual twilight.

Freezing meltwater flows from the cold side,

carving canyons through the landscape.

Even here, where the planet's most hospitable,

there's little vegetation.

But deep in these canyons

lives an extraordinary five-legged creature...

a pentapod.

No bigger than a cat,

it's the dominant life form on Janus,

capable of adapting to any conditions.

On the cold side,

it's stocky and hairy.

On the hot side, it's shiny, slender and skittish.

Pentapods can become whatever they need to be

in order to survive.

On Earth, all animals adapt to fit their environment.

But some are more adaptable than others.

This trail that I just came across here,

it's a beautifully manicured trail,

and you would think it was made by some bigger animal or maybe even a human.

But it's actually made by ants.

The leaf-cutter ant, in particular.

All looks quiet right now,

but if we come back at night, this will actually turn into an ant super-highway,

with leaves just streaming up, like, towards their nest.

Oh, wow.

Look at that.

This is the ant nest.

You can really see into the heart of this colony.

There's a lot going on.

There's actually, for leaf-cutter ants, there's different castes.

There is the soldiers, there are different workers.

There's also the queen.

She's the only one who's laying eggs, and there's only a single queen.

Without her, none of this would be here.

An ant colony depends on the division of labor.

Different castes have different jobs, and they come in all shapes and sizes.

I just grabbed one of the soldiers,

and I also grabbed the really, really small worker.

And it's not the smallest worker in this colony,

but it's... it's pretty close.

And I mean, the soldiers, I barely can hold her.

It's really hard. I can feel her power.

I can feel she's trying to get out. Her jaws are wide open.

But, I mean, the difference is incredible.

And to think that they all started from the same... the same larva.

All ants in a colony are closely related.

They share most of the same genes.

But as they grow up, some genes are switched on, others switched off,

depending on the food they are fed.

Any of them can become workers, foragers or soldiers.

So, this ability to create such different shapes and forms from the same blueprint,

this is actually called polyphenism.

And "poly" just means "many," and "phenism" would be "shapes."

Oh, the spider just did exactly what I was trying to avoid all night.

She walked exactly into one of these trails of these leaf-cutters here.

And that's not good news.

So here, you really see the power of these soldiers and their jaws.

If something disturbs the colony,

it's their job to come in and get it, and that's exactly what they did.

But wow.

Aaah!

All right, I'm already being attacked.

Ooh, that's gonna hurt.

They are now coming for me.

So, any creature that has this ability to morph

and react to changes in its environment as quickly as ants can do,

has an amazing advantage to not just survive,

but actually dominate its world.

On Janus, pentapods are masters of polyphenism.

Like ants, they're able to develop into different forms.

But they all begin life in the same place...

the Twilight Zone.

A pair of adults is ready to breed.

They're both male and female.

Hermaphrodites.

Each can produce offspring,

doubling their chances of success.

Having mated, a pentapod leaves the shelter of the canyon.

It climbs to high ground,

ready to spawn.

At the top, a constant wind blows between the cold and hot sides of the planet.

But the wind provides an opportunity to scatter their larvae.

This way, they can colonize the planet.

On the hot side, water is scarce...

condensing only in shaded areas.

If there's nothing to eat, they move on.

With five legs, they can skitter in any direction.

And with ten eyes,

not much escapes their attention.

If they sense a meal,

they use feelers to root it out.

They need to make the most of every opportunity.

Life in any desert is an ordeal.

Only the tough survive.

And in the natural world,

there's nothing much tougher...

than a scorpion.

- Oh. Anything? - Here?

I don't think so.

My first childhood memory of the scorpion,

I was, uh, ten years old and I see this big scorpion.

I screamed "Mom!"

Uh, so she came to my aid

and she was like, "What? What happened?"

And I point to the scorpion, uh, on the ceiling,

I said, "That's gonna eat me."

And she was just like, "No, come on. Be cool. This is just a scorpion."

So, as a cute Mexican mother,

she took, uh... her sandal

and just smashed the, uh, little bastard on the ceiling.

Carlos Santibáñez-López is now an expert on scorpions,

specializing in the study of their venom.

I go at night and I use the UV light,

because UV light makes scorpions glow.

They have a special protein in their shell.

Yeah, let's... Let's take a look at this one.

- Oh, look at this! - There's a lot of them.

Yeah, this is great.

Scorpions have barely changed in 400 million years.

Perfectly adapted for life in the desert.

Scorpions have a very slow metabolism.

They can spend long periods without food or water.

We have records of some species lasting one year

without eating anything.

And they have just enough energy

to sustain essential functions to keep them alive.

They need to ensure that the next time food is available,

they catch it.

So, scorpions have developed a lot of weapons that they can use to hunt prey,

including the most important weapon...

venom.

To study scorpion venom,

Carlos needs to extract it from glands in the tail.

Venom is a powerful cocktail that contains minerals, salts

and hundreds of proteins with a different function.

Try to be as fast as you can.

There you go.

We use electricity to stimulate the muscles.

We're only gonna get small... very small amounts,

a teeny, tiny drop of it.

But this drop is so valuable that we don't wanna lose it.

Give him a nice massage.

Yeah. There's a tiny droplet.

- There. - Oh, yes, it's... Okay, so it's working.

Uh, we have venom.

It is right there, but it's a teeny, tiny drop.

- Uh... do you see it? - That's right. Yeah.

I mean, it's... it's like a... a drop of water.

Venom is such an effective weapon,

it's used by reptiles, insects and fish.

It has evolved, separately, at least 30 times.

The more often a trait like venom has evolved on Earth,

the more likely it is to exist on other planets.

On the hot side of Janus,

pentapods use venom to catch their prey.

But hunting alone is risky.

Even a venomous predator is powerless

against a swarm.

On the cold side of the planet, things are just as tough.

In perpetual darkness and with no energy from the sun...

how can anything survive?

On Earth, nearly all life depends on sunlight.

Plants convert solar energy into food,

which feeds other life forms.

But there are some extraordinary places on Earth

where life has evolved without any light.

When I contemplate going in a cave like this,

one of the things I know is it's an alien environment.

- It's fogging up a little bit. - Yeah.

It is full of good ways to die.

One of the big risks in this cave is there is a lot of hydrogen sulfide.

Hydrogen sulfide can kill you.

- Okay. You ready? - Yeah.

And so my husband comes along to act as a safety officer.

He's not afraid to pull us out and say, "Time to go, you've had enough."

Cueva de Villa Luz is a maze of limestone chambers.

Hydrogen sulfide bubbles up from the Earth's crust.

Mixing with oxygen, it forms sulfuric acid and turns the water milky white.

And yet, some life forms survive,

and even thrive in these conditions.

Snottites...

dripping colonies of bacteria.

There are a lot of organisms in one snottite.

Instead of relying on photosynthesis,

they are eating hydrogen sulfide

and are using it as their energy source.

- Okay, ready to flame? - Yep.

So, make it nice and sterile.

Okay, cap that, please.

Okay, let's go for this one.

Okay.

Okay. One snottite.

These are some of the best snottites in the world.

So acidic.

More acidic than car battery acid.

An entire ecosystem has evolved

to feed off the snottites.

Creatures that have never seen daylight.

Can you see those?

Yes, I do.

So, there's some really big ones over here to the right.

So, whenever you see one of those white streamers of bacteria,

you'll see a fish go after it.

Okay, here comes some more over there. See? Here comes a big one, see?

It's coming. There, it got it!

Studying life in total darkness

has expanded how we look for life in other places in the universe.

It's made us think about the fact

that we could find life in places we never considered.

- Yep, there we go. - 25.

25. It's getting pretty high.

All right.

You think we ought to get out of here?

Yeah.

All life needs some form of energy.

On the hot face of Janus, it comes from starlight.

On the cold face, it comes from deep within.

Geothermal activity.

When volcanic pressure forces water to the surface,

there's a flurry of new life...

which stirs pentapods into action.

They're after grubs that emerge around geothermal pools.

But it's not so easy.

The grubs have evolved a defense.

They jump and flash a warning signal to others...

acting as one to stay alive.

On Earth, some animals use light in the same way...

to send signals.

Every single point of light that you see out there is a firefly.

Right as the sun sets, you'll start to see them come up,

like little sparks rising from a fire.

It's just spectacular, and it only gets better as the night gets darker.

Fireflies flash as a form of communication.

They are among the only land animals

that communicate with light of their own making.

Bioluminescence evolved among fireflies

as a warning to predators

that they're packed with toxins and should be given a wide berth.

Now the same mechanism

is used for finding a mate.

So, the firefly produces light from an organ on its abdomen called the lantern.

And this organ has an enzyme

that produces one photon of light.

This chemical reaction involves oxygen.

So, by controlling the flow of oxygen to their lantern,

they can produce, uh, flashes.

They can turn the chemical reaction on and off,

and use these flashes to communicate with each other.

The males are flying around, and they're doing little advertisements,

little flashes that say, "Here I am. This is me, check me out."

And then the females are waiting on the ground below and they're looking up.

And when they see a guy that they like, they flash back.

And then the male will come over, he'll flash again.

She flashes back and they come together and mate.

So, this is actually one big flirtatious dance

that we're seeing right here behind us.

Each species creates its own little language

that they can use to find each other in the dark.

But using light to communicate is inherently risky.

Some species have evolved to emit fake signals.

What we have here is a predatory Photuris female,

also called femmes fatales, because they have a reputation

for luring in males of other species to eat them.

They can hack in to the communication between a male and a female

and use that to her advantage.

So, you can tell this is a predatory firefly

because her lantern has a different shape to it.

Only two... two little lines light up, almost like an equals sign.

So, basically, a male will be flying around looking for a mate

and then when the males flash, they flash back,

just as if they were a female of that species.

So, they're imitating the females of that other species

to get the male to come over,

but they don't wanna mate with that male...

she wants to eat that male.

And she proceeds to do so in an extremely violent way.

This is a classic example of a predator using mimicry.

One species copying the behavior of another...

in order to kill it.

Lying and deceit is...

is pretty widespread in the animal kingdom, I would say.

And I think it would be on any planet.

On Janus, the grubs use light to stay alive.

But their system is about to be hacked.

By catching a few grubs, the pentapods absorb their bioluminescence...

and mimic their warning signals.

The grubs instinctively jump away from flashing lights.

But this time, it's a trap.

The pentapods have them where they want them.

Again, adaptability is the secret to their survival.

But for all creatures on Janus,

existence depends on one key ingredient...

whether in the hot, cold or twilight worlds.

Water.

Everybody knows the story of Goldilocks and the Three Bears.

She sneaks into their house and tries to eat their porridge.

The first bowl is too hot.

The second bowl is freezing cold.

It's only when she tries the third bowl that she's happy.

"This one," Goldilocks says, "is just right."

When we look around the universe in the search for life,

we are looking for conditions that are "just right."

And "just right," in this case,

is defined by the presence of this stuff.

Water.

Water transports the essentials for life around the planet

and around every living body.

It's the most important substance in the universe.

Because water is so familiar to us,

it's not until you really stop and think about it that you realize how weird it is.

Water is made up by one oxygen atom...

and two hydrogen atoms.

And because of the nature of the elements and the way they're bound,

we have a net negative charge at the oxygen side

and positive on the two hydrogens.

This means that every hydrogen in a molecule

can interact with the oxygen on the other molecules neighboring.

Replicate this attraction for billions of neighboring molecules

and you get a substance that is very sticky.

And water molecules don't just attract each other,

they attract almost any other molecule they come in contact with.

They pull other substances apart,

break them down, and dissolve them.

Water is the universal solvent

because of its unique ability to dissolve nutrients and chemicals.

It seems certain the first spark of life on Earth happened not on land...

but in water.

Deep underwater, in the primordial oceans,

the chemical ingredients for life were swirling around.

What was needed was a place to concentrate these ingredients...

to allow the chemical reaction that would eventually lead to life.

That place could have been a hydrothermal vent.

Water interacts with hot rocks deep underground.

It comes gushing back up into the freezing seawater

carrying nutrients and chemicals...

little bubbles of life.

The link between water and life is so strong,

it's hard to imagine any life anywhere in the universe

existing without it.

It's mind-boggling to think,

you know, how life can find a way in very extreme places.

Very hot, very cold.

And at the end, it's all about the water.

If there's liquid water, you'll find life.

Water on Janus is scarce.

It either freezes as ice...

or evaporates into the wind.

But there's just enough liquid water

for life to take hold...

and for the toughest of creatures

to adapt and survive.

At NASA, they have a saying.

"If you want to find life...

follow the water."

How might life adapt on a different world?

One with two stars,

where there's more energy than on Earth.

A planet perfect for life.