It's morning in the forest.
And for this troop of chimpanzees,
it's time to get out of bed
and get on with the day.
But when you sleep in a nest 40 meters up in the air,
it can be a bit of a challenge.
This morning ritual has been playing out like clockwork
for countless generations.
A rhythm driven by one of the most mysterious properties of our universe.
Time.
And time is playing an extra important role
for one female chimp.
Celeste is in her mid-30s,
and she has a ticking clock inside her.
A clock that was set 13.8 billion years ago.
From The Big Bang came atoms…
…and energy…
…which have journeyed across vast reaches of space.
And these ingredients
are now assembling…
to create a new life.
In Celeste's womb, a tiny chimpanzee is forming.
It will be another 150 days before this baby meets the world.
That's the time it takes.
And without time,
nothing in our universe would ever be born.
From the youngest chimpanzee…
to the oldest black hole.
But how did time start in the first place?
Does everything experience time in the same way?
And will our universe's clock ever stop ticking?
The day begins for a chimp much as it does for us.
Breakfast.
A critical meal… when you're eating for two.
Celeste is already an experienced mom.
Her firstborn, Apollo, is the dominant male of the troop.
And he can be a bit slow to get going in the morning.
Each chimp is a different age.
Each has a distinct personality.
But all of them are creatures of routine.
Their lives governed by the passage of time.
We can't see it.
-Or hear it.
-We can't influence it in any way.
And yet, every heartbeat, every choice and action,
every inaction is a consequence of time's continuous flow.
It can whiz by or drag on.
But time ticks ever onward.
Stars and planets form, age, and die.
In their wake, new stars emerge.
And from a rare few,
the most mysterious objects in the universe are born.
Black holes.
Time is the cosmic conductor.
Nothing can escape its grasp.
Although we have devices that can track and measure time…
…chimps don't.
Celeste and her family use the sun as their guide.
Rising and arcing across the sky.
As it moves…
so do they.
Chimps rarely sleep in the same nest twice.
Each morning, they head off in search of fresh fruit.
Journeying from one part of their territory to another.
But right now, the ripest fruit…
is on the other side of an open plain.
Apollo and the strongest members of the troop go ahead
to make sure it's safe for the others.
Out in the open,
they're vulnerable to predators, rival chimps,
and the heat of the rising sun.
They need to be quick.
Time is ticking.
The sun traveling across the sky
forms the chimps' basic unit of time.
From sunrise to sunset and back to sunrise.
The 24-hour day.
But a day is not actually a measure of time at all.
It's a measure of motion.
As our planet rotates on its axis,
the forest moves through space at 1,600 kilometers per hour.
It takes 24 hours for Earth to do one full turn.
Giving the chimps 24 hours of movement,
12 in sunshine and 12 in darkness.
Again and again.
It's our planet's steady spin that sets Earth's master clock.
And is the chimps' trusted timekeeper.
But this essential cycle we take for granted…
is not universal.
As soon as you leave our home planet…
…the 24-hour day no longer applies.
Flying closest to the sun…
…is the planet Mercury.
Compared to us, it rotates at a snail's pace.
Our neighbor Venus spins even slower.
It takes longer to rotate on its axis
than it does to orbit the sun.
Making Venus's day longer than its year.
Each planet has its own unique day length.
Including planets far beyond our solar system.
Sixty-three light years away,
orbiting a young star,
is Beta Pictoris b.
It spins so fast…
…it bulges out at the sides.
It is the fastest-spinning world we know of.
Here, a day lasts just eight hours.
Every planet in our universe spins at its own speed.
Billions of worlds,
each with their own clock,
ticking away to different rhythms.
But all moving forward on a universal cosmic timeline.
Back on Earth,
with the sun now almost directly overhead,
the chimps know exactly what time it is.
It's lunchtime.
And their crossing of the savanna has been timed to perfection.
They've hit the jackpot.
These sweet berries of the African grape tree
are only around for a few weeks.
A rare seasonal gift from the forest.
Where the middle of the day is a very productive time.
Animals of all sizes are tending to their daily tasks.
And each experiences time differently.
For small creatures that live fast, short lives…
…lunchtime can feel like a lifetime.
For slower, longer living things,
an hour can go by in the blink of an eye.
But whether you're living in high speed
or in slow motion,
all life in the forest must adhere to the same 24-hour day.
And as the world keeps spinning…
…those days keep ticking by…
dictating the lives of every chimpanzee.
One in particular.
At just 15 weeks,
this baby's entire body could fit in its mother's hand.
For it to fully form,
a cascade of perfectly timed events must occur.
For the next 130 days,
every minute, every nanosecond is critical.
New cells divide and grow with remarkable speed and precision.
Right now,
brain cells are multiplying at a rate of around 15 million per hour.
All of us, from our very first moments,
have a deep biological relationship with time.
As the sunlight wanes,
the chimps instinctively switch gears.
To evening mode.
Nesting calls signal to the whole troop.
It's time to find a safe place to spend the night.
Their bedding needs to be soft but strong.
It doesn't hurt if it tastes good either.
The rhythm of every day in the forest
is determined by the sun's position in the sky.
But the chimp's innate sense of time
comes from something inside them.
Deep in their brains is a biological clock.
Bundles of neurons act as a pendulum…
…controlled by genes that switch on and off…
and a cycle that repeats every 24 hours.
These clock genes create a circadian rhythm in the body
that waxes and wanes in sync with our planet's rotation.
Controlling hormones…
…heart rates,
and energy levels.
And right now, the chimps' body clocks are sending the message,
"It's time to get sleepy."
Not everyone can though.
Celeste's baby's brain is still forming.
With no internal clock of its own,
it relies on its mother's hormones to keep it synchronized.
But baby doesn't always get the memo.
Lucky for Celeste,
with so much growing to do,
her baby does eventually drift off.
While the chimps go to sleep high in the treetops…
other forest dwellers are just waking up.
Body clocks tick away
in the cells of plants and animals throughout the forest.
Clock genes evolved two and a half billion years ago
in the first living cells.
Those that could anticipate the light and dark of a day had an advantage.
And as life evolved,
it synchronized to Earth's motion.
So that today,
these clocks exist in almost every creature on Earth.
Although some creatures prefer day and others night…
all life is dancing to the same beat.
Thanks to cellular clocks
that tick in time with the spin of our Earth.
As yesterday becomes a memory…
…another new day begins in the forest.
A fresh cycle,
and yet, no clock can ever truly be reset.
And there are subtle clues all around the forest that reveal why.
Nothing is the same today as it was yesterday.
Least of all Apollo,
who seems to have woken up on the wrong side of the bed.
When he's in a bad mood…
the rest of the group just need to keep their heads down.
He soon finds a target for his rage, though.
Some of the younger chimps have found honey for breakfast.
And they forgot to invite Apollo to join them.
You soon learn, never get on the wrong side of the dominant male.
He's got a nasty bite.
Thankfully, it's not too serious.
With a bit of care, this wound should heal.
But wounds leave behind scars,
etched into the chimps' bodies.
Indelible marks of the past that reveal the one universal truth about time.
It has a direction.
You can never go back in time…
…only forward into the future.
This is time's arrow,
and it leaves scars on everything
across Earth…
…and beyond.
Our moon is pockmarked with battle scars that tell stories of ancient events.
So too are the planets.
Their surfaces testament to a lifetime of bombardment by asteroids and comets.
But perhaps the most impressive impact crater in our solar system
belongs to a moon of Saturn.
Mimas.
The Herschel crater is 130 kilometers across.
Whatever created this enormous scar…
must have come close to shattering the moon to pieces.
Mimas lives to tell the tale.
But events like this are irreversible.
And they differentiate the past from the present.
Impact craters are timestamps
that mark a unique moment in history.
We can never revisit the moment they were created.
But we can piece together traces of past events
and map a course back through time.
Before craters.
Before planets.
Before stars.
Back to a time before time.
Our universe had a beginning.
A very big bang…
that created three-dimensional space
and time.
But space and time are not separate.
Time is woven into the fabric of the universe.
Bound to space as our universe's fourth dimension.
The Big Bang set the universe's clock ticking.
Propelling everything into the future.
It created the arrow of time.
From our universe's very first moments…
…it pushes us all into the future,
causing everything to grow,
age…
…and change.
From childhood…
…to adulthood,
we all travel through life in the same direction.
Getting older day by day.
And with age comes wisdom
as youthful innocence is replaced by experience.
Time's arrow leads us through these stages of life.
It shapes us.
Defines who we are.
From our first moments…
to our last.
Chimps rarely live past the age of 40.
Nothing is immortal.
Everything that exists in our universe has a finite lifespan.
Including, perhaps, the universe itself.
Today, the galaxies are filled with stars.
But 95% of stars that will ever exist have already been born.
So, as our universe journeys into the future…
…the stars…
-…one by one, will blink out…
…leaving the universe to face an ever-darkening future,
stretching out for trillions,
that's thousands of billions, of years.
The age of starlight
will be followed by a dark age of black holes.
These monsters feed on the remnants of dead stars and planets.
And as they do,
their ever-increasing gravity scars the very fabric of the universe.
Distorting both space and time.
The edge of a black hole, called the event horizon,
is a region where gravity has become so strong
it causes time to slow down practically to a standstill.
But even black holes can't escape time's arrow.
In the far, far future…
…when there are no stars or planets left to consume,
they will begin to starve.
And slowly…
very slowly,
they will shrink.
When they die,
no one truly knows what comes next.
But it could be the end of our universe.
Here on Earth, none of us can slow the passage of time.
And one of Celeste's aunts is reaching the end of her time.
Retreating from the group, she heads into the forest alone.
She has lived many years.
But this day will be her last.
While the world continues to spin,
her body lies still.
But time doesn't just carry us toward endings.
It also creates beginnings.
As days turn into weeks and weeks into months,
there are big changes for one special chimpanzee.
Say hello to Cosmo.
Her brain and limbs are now fully formed.
And her eyes are ready to see the world.
The exact timing of her birth is impossible to know.
But she could be born any day now.
She's been in the womb for eight months.
But her journey began long, long ago.
The elements that made her came from dying stars.
The water that flows through her body came from distant worlds.
The energy that powers her muscles came from the heart of our sun.
But it's that most mysterious ingredient,
the one that started ticking 13.8 billion years ago…
that has led all other ingredients to this moment.
In this place.
On this planet.
Another sunrise, another morning.
And this one is extra special.
Welcome to Earth, Cosmo.
This precious moment will never be repeated for Celeste.
But Cosmo's journey has just begun.
It will take around three months
for her body clock to synchronize to the rhythm of the planet,
one year before she can truly communicate,
and perhaps 15 years until she is fully grown.
So much lies ahead.
Like Cosmo,
our universe is still in its infancy
at the beginning of an unimaginably long lifespan.
We are living in the age of starlight.
A moment when time's arrow has filled the universe with beauty…
…and wonder.
But it's a brief window of time
that will never be repeated in the history of our universe.
No one can slow the passage of time,
nor should we try.
Because without time,
there would be no now.
No moment in the sun.
No moments, period.
Without time,
there would be no life.
Next in the story of our universe,
Earth's seasons.
Like a mother bear raising her cubs…
…we all experience our planet's yearly transformations.
But what created Earth's seasons?
How are they connected to the formation of our moon?
And why does only our planet have conditions just right for life?