Nightfall…
reveals our planet's connection to the universe.
A shooting star.
Ancient rock from the depths of space…
falling to Earth.
Inside it are the chemical elements that built our universe.
For countless generations,
sea turtles have roamed the oceans,
gathering cosmic elements in the food they eat.
And as they've done for hundreds of millions of years…
every new generation
recycles this same stardust…
to produce the next.
Born in The Big Bang…
…and forged in the hearts of stars…
…90 chemical elements have created everything.
Endlessly recycled and reconfigured
throughout the entire history of the universe.
And now…
using the same elements,
each new turtle begins a life cycle…
…that mirrors the lives of the stars.
On Australia's Great Barrier Reef…
…something magical has happened.
After 50 days beneath the sand,
chemical elements from across the cosmos have created life.
Combining to form the next generation
of green sea turtles.
The eggs have now exhausted their supply of life-giving elements.
And today, the baby turtles will take their first taste of air.
And as her brothers and sisters scramble to make it to the water…
a little female…
is last to be born.
In order to grow to adulthood,
she will need to harvest chemical elements from across the ocean.
It's a dangerous journey…
…with treachery at every turn.
But for the baby turtles,
the call of the ocean is irresistible.
From a nest of 100,
sometimes only one will survive.
Sometimes none.
But despite the risk,
the little female knows
she must make it to the water.
She spots an opening…
and takes her chance.
Today…
she's a lucky one.
The hatchling won't stop swimming until she has left the reef far behind.
Safely past the predators that gather in the shallow coastal waters,
she'll disappear into the great void of the open ocean…
to spend her childhood feeding on atoms.
Atoms whose origins go back through the entire life of the cosmos.
To the time our universe
was in its infancy.
Expanding from nothing to everything in an instant.
Our universe was born in The Big Bang.
At first,
it was nothing but a hot, dense fog.
But as it continued to expand…
the fog began to cool.
And on the tiniest scale imaginable…
…something astounding began to occur.
The first atoms condensed out of the fog.
Atoms of hydrogen.
The very first element in the universe.
And in the ocean,
juvenile turtles are hunting it down.
As they emerge from the blue
to forage in the shallow waters of the reef.
This is Stella.
One of the lucky ones that made it off the beach
and into the ocean.
She's already more than 2,000 times her hatching weight
and growing fast.
Feeding on creatures she's found drifting in the open ocean,
she's harvested one element more than any other.
Hydrogen.
It makes up more than half of all the atoms in the food she eats.
And is the most abundant element in all living things.
From fish
to invertebrates.
Anemones and coral.
Hydrogen atoms formed at the dawn of the universe
have been passed on from star to planet,
planet to animal…
and from prey to predator.
Which means, for Stella,
the waters around the reef are full of danger.
Tiger sharks, hungry for the same atoms,
hunt sea turtles throughout their adult lives.
Stella's main defense is her trusty armor.
Its bone toughened with calcium and phosphorus.
Elements that date back to another pivotal moment…
in the life of our universe.
For the first 300 million years after The Big Bang…
there are no planets and no stars.
The dark ages of the universe.
Concealed in the gloom,
vast tendrils of hydrogen gas crisscross the cosmos.
And then, incredibly,
as they were drawn together by gravity…
…the first stars were born.
This is the cosmic dawn.
The moment the lights came on.
And deep within those first stars…
a remarkable process began.
Temperatures and pressures were so extreme…
…that new elements were created.
Atoms of hydrogen were forced together to make helium.
As they fused, energy was released.
Creating a chain reaction that produced increasingly heavier elements.
Including those essential for life.
Three helium atoms combined to make carbon.
The element on which all life is based.
Carbon and helium made oxygen.
The air we breathe.
Oxygen atoms combined to make phosphorus.
Then silicon and calcium.
The elements that make up every living thing
were created in the hearts of the very first stars.
For Stella,
some elements are so important…
…she'll travel great distances to find them.
The warm, shallow waters between Australia and Papua New Guinea
are a paradise of vibrant seagrass.
The prairies of the ocean.
Stella has traveled all the way here to access their magic ingredients.
As it grows,
seagrass absorbs elements from outer space
that have found their way to the ocean floor.
Burgeoning fronds rich in calcium and phosphorus
provide Stella with precisely the minerals she needs
to build and sustain her protective shell.
Hardening her armor for the trials ahead.
Stella will spend her adolescent years
searching the ocean
for atoms born in the stars.
Her journey toward maturity…
paralleled in the story of our universe.
Because three billion years after The Big Bang…
…the universe was also coming of age.
It was entering an incredibly active phase of star formation.
As they grew,
stars were pulled together into vast galaxies.
Like our Milky Way.
And within those galaxies,
the chemical elements each star produced…
…were transformed into planets and moons.
After billions of years,
chemical elements have matured into the Earth we know today.
Stella is now fully grown herself.
Over a meter long,
she has a thick shell covered in green algae.
Ideal camouflage for cruising coral reefs.
She is now at one with the ocean.
Atoms from the stars
perfectly adapted to life at sea.
Just one example of life on Earth…
exquisitely shaped from the cosmos.
Stella is now ready for the next stage of her life.
Because she's about to become a mother herself.
Inside her body…
she's carrying a clutch of 100 eggs.
The first of many she'll have over her lifetime.
Her own precious cargo of stardust.
All across the reef,
atoms are being assembled into new life.
Creating one of the richest ecosystems on Earth.
But our planet…
…is an outlier.
In the rest of the solar system,
the very same elements
have created worlds far less suited to life.
Closest to the sun,
Mercury is a planet rich in carbon.
But rather than creating life,
the carbon is baked into its crust.
On a world scorched by starlight.
While in the farthest reaches of the solar system,
Neptune's freezing temperatures
cause carbon and hydrogen to form clouds of methane.
The suffocating gas that gives it its brilliant blue.
And even on Mars,
a planet with all the same ingredients as Earth…
the lack of an oxygen-rich atmosphere
means complex life couldn't survive here.
Only on Earth are conditions just right.
But even here,
competition for elements is fierce.
And danger…
…is never far away.
Luckily, Stella has become an expert at hide-and-seek.
Safe for now…
Stella faces the biggest challenge of her life.
To return to the Great Barrier Reef
and the island of her birth…
to lay her own eggs in the sand.
But a lifetime spent wandering the ocean
means she's now 2,000 kilometers away.
How Stella navigates home
is one of the wonders of the natural world.
Deep within the Earth,
our planet's core generates a magnetic field…
which gives every place on its surface a unique magnetic signature.
When she hatched,
her island's signature was imprinted in Stella's brain.
And to find her way back,
she'll rely on yet another chemical element.
One that connects her to the planet's core.
An element that's only created when stars die.
In a nearby part of the galaxy,
two and a half million years ago,
a dramatic change took place in the core of a giant star.
For all its life, it had been powered by nuclear fusion.
The energy released…
…creating heavier and heavier elements.
Until the core became so hot,
it started creating…
…iron.
Which caused a fundamental change to take place.
Because making iron doesn't release energy.
It consumes it.
As more and more of the metal was produced,
it spread through the core like a disease.
Feeding off the star's heat.
Throttling it.
With no heat source to hold it up.
From this moment onward…
…the star was doomed.
The resulting explosion shone with the brightness of 10 billion suns.
A supernova which propelled iron-rich dust across the galaxy.
All the way to our planet.
And even now,
100 tons of stardust enters Earth's atmosphere every day.
Some left over from the formation of the solar system.
Some blown here from distant supernovas.
A direct link between our planet and the stars.
And it's the iron in this dust
that allows Stella to find her way.
Because deep in her brain…
…specialized cells are thought to contain crystals of magnetite.
An iron-rich mineral that aligns itself to the Earth's magnetic field.
A compass,
pointing her in the right direction.
Allowing Stella to find the unique magnetic fingerprint of home.
And so an element forged in a supernova…
…will now guide her across kilometers of featureless ocean.
Back to the island…
on which she was born.
But there are other creatures with magnetic homing abilities.
The tiger sharks have it too.
And it's leading them back to the same island.
Because for a few months every year,
thousands of female turtles come here to lay their eggs.
It's the largest gathering of green sea turtles
anywhere on the planet.
And as they come to shore,
the feeding begins.
Stella must wait for the right moment…
to find an opening…
and avoid the onslaught.
When the danger has passed…
she finally swims ashore.
After an epic journey…
Stella emerges onto the sand
she last touched as a hatchling.
The elements she spent a lifetime collecting…
are now safely delivered into the sand.
Stella's mission is complete,
and she returns to the ocean…
to begin gathering stardust once more.
The cycle of her life
mirroring the grandest cycle of all.
The lives and deaths of the stars.
Because far away across the galaxy,
the ashes of countless supernovas
have come together to form a giant nebula.
A cloud of stardust tens of light-years across.
And buried within it are places where the next generation of stars
are beginning to form.
Called proplyds.
They're like vast cosmic eggs.
Each one bigger than our solar system.
Inside their shells of ionized gas,
the force of gravity is pulling stardust together.
And the stardust inside Stella's eggs…
is also beginning to take shape.
As the elements she collected are being transformed…
into something that's starting to resemble a tiny creature.
And within the proplyd…
gases like hydrogen and helium…
…are feeding an embryonic star.
And what was once an embryo…
is now a tiny turtle…
with shell, flippers, and beak.
Elements reimagined…
for a life at sea.
In the proplyd,
the new star's development is also nearing completion.
Reborn from the ashes of the previous generation,
its light now reveals a family of growing planets in orbit.
New worlds.
But perhaps, new possibilities for life.
And beneath the sand,
the same stardust,
passed down through countless generations of stars and turtles…
will start yet another journey.
Each new life
directly connected to the origin of the universe.
Each new story beginning at the end of the last.
And so, the story of our universe continues…
…with Earth's most precious resource…
…water.
And a family of elephants on a dangerous mission to find it.
Their journey every bit as extraordinary…
…as water's journey across the vastness of space to our planet.
But why is Earth the only world in the solar system
whose surface is covered with liquid water?
And why is it so essential for life?