NARRATOR: From a fiery hellscape to a thriving oasis filled with life, our planet has played host to a vast array of creatures.

STEVE BRUSATTE: It is one unfolding story with so many twists and turns and new characters coming in, and old characters going extinct.

It's like the longest-running television show of all time.

♪ ♪ NARRATOR: Throughout Earth's history... (eruption roars) ...powerful geological forces shape the course of evolution.

And 66 million years ago... (explosion roars) ...one catastrophe sparks the beginning of a new era.

AISHA MORRIS: One major event can have these ripple effects throughout the rest of history, and this event is almost unmatched.

NARRATOR: An era in which a species emerges that changes the planet faster than any before it.

AMELIA VILLASEÑOR: Humans have modified the planet in a geological blink of an eye.

We have basically altered every part that there is to alter.

♪ ♪ NARRATOR: What extraordinary series of events gave rise to us?

MÓNICA CARVALHO: This is one of those scenarios in which we see geology driving the evolution of life.

NARRATOR: And can lessons from our planet's past help secure our future?

ZERAY ALEMSEGED: Our survival as a species is intricately linked to the future of the planet.

NARRATOR: "Ancient Earth: Humans."

♪ ♪ Right now, on "NOVA."

♪ ♪ ♪ ♪ ♪ ♪ NARRATOR: Over the course of Earth's four-and-a-half-billion-year history, countless species have come and gone.

But for over 160 million years, it was dominated by creatures amongst the largest to have ever evolved.

This is the age of dinosaurs.

MORRIS: I think it would interesting, humbling, and a bit terrifying to be walking amongst some of these massive creatures that were roaming around and munching on these huge plants that were growing at the time.

(dinosaurs bellowing in distance) JANE FRANCIS: The sounds, I think, would have been really interesting.

Bird-like chattering.

Definitely a lot of loud and hoarse noises.

(dinosaurs lowing) BRUSATTE: The whole fantastic variety of meat-eating dinosaurs, plant-eating dinosaurs, long-necked dinosaurs, dinosaurs with horns and spikes, and dinosaurs with feathers and wings.

(dinosaur screeching) NARRATOR: But the reign of the dinosaurs nears its end, as a looming disaster will set the stage for our own evolution.

♪ ♪ ("Never Close Enough" by SIPHO.

playing) ♪ Oh, we won't ever hear the silence ♪ ♪ Or ever see the colors ♪ (exploding) ♪ That never lived in our minds ♪ ♪ ♪ ♪ Just a moment ♪ ♪ Never too far out ♪ ♪ Never close enough ♪ ♪ ♪ NARRATOR: An asteroid the size of Mount Everest is on a direct collision course with Earth.

♪ ♪ (dinosaurs calling) (explosion rumbling) NARRATOR: The blast from the impact annihilates everything in its path.

(shock wave rushing) MORRIS: The energy released when this asteroid struck was the equivalent of ten billion nuclear bombs.

MARK MASLIN: It caused earthquakes 100 times more powerful than any earthquake that humans have ever encountered.

GARETH COLLINS: It would have looked like a second sun on the horizon, a huge ball of fire.

It certainly wouldn't have been possible to bathe in its beauty, because if you were unfortunate enough to be able to see it, you were toast.

(shock wave rushing) JESSICA WATKINS: It is difficult to imagine what it would have been like to be a dinosaur on the surface that day.

Um, not a great day to be a dinosaur.

(laughs) This was probably the biggest asteroid that's hit the Earth in at least the last half a billion years.

And the dinosaurs had no idea what was coming.

♪ ♪ (thunder rumbling) NARRATOR: After billions upon billions of tons of super-heated debris are thrown up into the atmosphere, it begins to rain.

Not water, but bullets of rock known as spherules.

♪ ♪ In an asteroid impact, the molten rock vaporizes to form a gas.

That gas expands to form a plume, and inside the plume, that gas condenses, solidifies, and cools, and becomes these rounded droplets that then rain down on the surrounding environment.

COLLINS: These particular spherules were found about 300 miles away from the impact site.

It's amazing to think that these tiny spherules were produced in this intense fireball.

♪ ♪ NARRATOR: The impact formed a vast crater over 110 miles wide.

(wind blowing, birds chirping) But, over time, this was buried under more than 1,600 feet of rock.

Today, at ground zero of the impact, the jungles burst with biodiversity.

It's hard to imagine the devastation inflicted 66 million years ago.

♪ ♪ To uncover the true extent of the catastrophe... ...scientists study geological clues it left behind.

♪ ♪ CHRIS LOWERY: I'm standing in this beautiful geological formation called a cenote.

Cenote is a Mayan word which means "a hole filled with water."

These cenotes form as rainwater trickles through cracks in the rock, slowly eroding those cracks, and they get wider and wider until they collapse, and a big sinkhole forms.

NARRATOR: Taking a satellite image of the Yucatán Peninsula and overlaying a map of the cenotes on top of it reveals a subtle pattern.

LOWERY: Each of these yellow dots is a cenote.

You can see thousands of cenotes.

There's as many as 10,000.

And if we look at the northwestern part of the Yucatán over here, we see about 400 of these cenotes that form this cluster.

NARRATOR: Scientists call this cluster the Ring of Cenotes, because if the arc it forms is extended into the ocean, it creates a circle.

LOWERY: This cluster of cenotes actually corresponds very closely to the inner rim of the crater itself.

Almost like a bull's eye of the impact crater.

NARRATOR: The asteroid impact weakened the rock around the crater's rim.

(birds chirping) So, over millions of years, rainwater eroded the weakened layers, creating caves, which collapsed to form the Ring of Cenotes.

LOWERY: So, these amazing natural features are some of the only visual reminders we have left of the impact.

♪ ♪ NARRATOR: But how could an impact on one side of the planet wipe out species on the other side?

To solve this mystery, scientists need to understand what happened in the days and months that followed.

In the hours after the chaos of the initial impact, debris and dust thrown up by the collision combine with soot and ash from wildfires, forming a vast, gray cloud, which engulfs the entire planet and causes death and destruction on a global scale.

One of the big mysteries is that the heavier particles rained out of the atmosphere within a few months, maybe a year, tops, but that doesn't really explain the extent of the extinction that followed.

So, there has been this global detective chase to understand how the extinction unfolded.

♪ ♪ NARRATOR: But it wasn't in the atmosphere that scientists found the smoking gun.

It was deep underground.

LOWERY: So, beneath our feet is the Earth's crust.

The Earth's crust is layers of rock, 20 miles thick in most places, and these layers of rock, uh, can be read like a story of the Earth's history.

We can drill into these layers, and we can recover samples, and we can use that to understand how things have changed through the past.

NARRATOR: In the 1950s and '60s, oil drilling in the Yucatán Peninsula unearthed samples that were rich in a particular type of rock.

LOWERY: So, this is a rock called anhydrite.

It might look very boring, but it's actually very rich in an element called sulfur.

And the sulfur is what's really important about this story.

♪ ♪ NARRATOR: In 2016, Chris Lowery and a team of scientists drilled through the seafloor into the impact crater.

LOWERY: When we drilled into the crater, we got the cores back from the layers of rock where the asteroid actually hit.

We found that there was no anhydrite.

What we think this means is all this anhydrite was vaporized by the force of the impact.

This would have put, we think, about 325 billion tons of sulfur into the upper atmosphere, and this is where this impact really had its devastating effect.

MORRIS: So, we end up with a lot of sulfur in the upper atmosphere, and the atmospheric circulation moves this material around the planet.

CARVALHO: Unlike carbon dioxide, that traps the heat from the sun, sulfur does the opposite, and it actually reflects a lot of the radiation that's coming in from the sun.

(wind blowing) NARRATOR: With less sunlight reaching the surface, it becomes dark and cold, plunging the planet into a global impact winter.

CARVALHO: With very little light, there's barely no photosynthesis.

And photosynthesis is the main process by which plants are able to produce the food that's feeding all the animals that live in ecosystems.

BRUSATTE: What I have here is a replica of a skull.

It's a type of dinosaur called an ornithomimid, and it was thriving during those last glory days before the asteroid hit.

And it probably ate a lot of plants.

The dinosaurs and other animals that ate plants, they didn't have any food, so they died.

And the meat-eaters then died, and so on.

Ecosystems collapsed like houses of cards.

So it was really that global impact winter that sealed the fate of most of the dinosaurs.

NARRATOR: But it isn't just the dinosaurs that are wiped out.

AMMIE KALAN: We have an estimate that about 75% of all living plants and animals at that time went extinct as a result of the asteroid's impact.

This was one of the worst mass extinctions in Earth history.

♪ ♪ NARRATOR: The asteroid impact wipes out all of the dinosaurs except for some smaller ones that are the ancestors of birds.

And, crucially, some mammals also survive.

FRANCIS: Extinctions are really important because they change things.

They give us a change in the direction of evolution, and it gives an opportunity for new species of animal and plants to evolve into the landscape.

(birds calling) NARRATOR: You don't have to look into the past to find examples of the types of animals that survived that long winter, because areas of ecological destruction on Earth today demonstrate that with a depleted habitat, but just enough opportunity, there is always a chance that some species will find a way to exploit the devastation that is left.

BRUSATTE: There are some types of organisms, because they're adaptable, because they can grow fast, because they can eat lots of different things, they are well suited for living in conditions that other animals and plants just can't handle.

This was a huge catastrophe for mammals, but just enough survived that they were able to inherit a planet that was barren of dinosaurs.

So, these are replica fossils of a tiny early mammal called Purgatorius.

Purgatorius is one of these early mammals that thrived once the dinosaurs went extinct.

So, here is Purgatorius's jaw, and you can even see the teeth there, and the tiny little heel bone, which is minute, and its ankle bone here.

And it's really on the backs of these really tiny small mammals that the evolution of the rest of mammals really lies on.

♪ ♪ NARRATOR: It is thought that the cold, dark conditions last for over a decade.

But as the skies clear and the sunlight returns to full strength, temperatures rise again, creating a climate warmer than the one we have today.

♪ ♪ And mammals, which have lived in the shadows of the dinosaurs for around 140 million years, find a way to gain a foothold.

BRUSATTE: And this new world was a world of prime opportunity for the mammals that survived.

It was their springboard to an entirely new future.

NARRATOR: A new chapter for life is beginning: the age of mammals.

But it's not an asteroid from space that will spur the next big change in the course of evolution.

It's powerful forces within Earth itself.

♪ ♪ Deep beneath the North Atlantic Ocean, volcanic activity starts to bake organic matter within the seafloor.

As this carbon-rich material is heated, bubbles of the greenhouse gases carbon dioxide and methane stream out and warm the atmosphere.

FRANCIS: Methane acts much faster and much more strongly than carbon dioxide.

So, if we're releasing methane from the seafloor, it would have caused rapid warming.

NARRATOR: As this warming releases other reserves of methane, it nudges the climate past a tipping point.

We had what we think was a runaway effect.

LOWERY: The rate of things really matters when you're talking about warming, and this global temperature spike 56 million years ago is a example of that sort of runaway effect and the devastating consequences of that in terms of climate.

(thunder clapping) NARRATOR: On an already warm planet, global temperatures rise by at least nine degrees Fahrenheit, a dramatic spike... (thunder claps) ...that triggers chaos in Earth's climate.

Violent storms batter the planet with flash floods... ...prolonged droughts, and destructive hurricanes.

JAMES ZACHOS: The warming event had impacts on virtually every environment on Earth.

With the combination of warming and ocean acidification, there was one of the largest deep sea mass extinctions in recent Earth history.

NARRATOR: Alongside catastrophic impacts in the deep oceans are surprising changes in life near the poles, where temperatures now rise above 70 degrees Fahrenheit.

BRUSATTE: There were alligators above the Arctic Circle, trying to take shade underneath palm trees.

So, I've been to the Arctic and collected fossil plants, and I have a leaf about 45 million years old from the forest that once grew close to the North Pole.

Here we are, standing among banks of snow, and my hands are freezing cold, and yet here is a leaf that's telling me that millions of years ago, there was warmth and lush life in the polar regions.

It's incredibly exciting to find fossils from this time period, because it's an unimaginable thing.

It's the warmest the planet has been in 180 million years.

And for the first time, you are looking at these plants that were actually thriving in these ecosystems.

NARRATOR: This warmer world brings new opportunities for mammals on land, because now, for the first time, across vast areas of North America, Europe, and Asia, one habitat starts to flourish and spread out from the Equator.

CARVALHO: Tropical rain forests, as we know them today, started spreading north and south.

And from the pollen record, we see many new types of pollen showing up during this time period, which is reflecting many new species of plants.

♪ ♪ NARRATOR: This rich environment would play a vital role in the emergence of a new type of mammal.

One that is more like us.

(birds chirping) Tropical forests evolved into some of the most biodiverse habitats on Earth.

(birds chirping) Home to countless species of animals and plants.

♪ ♪ And 56 million years ago, many of these plants were providing a crucial ingredient.

In my hand, I have a fossilized flower, and it's pretty incredible.

It's really tiny.

(chuckling): It's only about the size of my fingernail.

And it was found in Utah, in North America, and it's been dated to around 51 million years old.

So, flowering plants have been on Earth for over 130 million years.

But during this period of time on Earth, which was really hot and humid, the tropical forests were spreading, and at that time, we think the flowering plants also continued to evolve and diversify.

♪ ♪ NARRATOR: Flowering plants are one of the great drivers of biodiversity, and, following the spike in global temperature, Earth's blossoming forests were full of them.

♪ ♪ KALAN: As flowering plants flourished, so did the species that relied on them, such as insects, but also birds and mammals.

And that's really significant, because flowering plants also produce fruit.

And we think fruit was one of the factors that drove the evolution of our mammal ancestors.

NARRATOR: Against a thriving backdrop of opportunity and reward, a new branch of mammals is evolving, which takes full advantage of this rich food source.

They are known as the first true primates.

♪ ♪ KALAN: So, I have in my bag an illustration of one of the first true primates, called Teilhardina.

And it would have evolved around 56 million years ago, during this period of climate change, where there was a lot of warming.

And we know from the fossils that this primate would have been very small, only around maybe two ounces or so.

And you can see here they had very big, forward-facing eyes and these grasping hands and feet, very much in line with modern-day primate characteristics.

NARRATOR: It took millions of years of evolution, but this was the first time a species had evolved that truly resembled the primates we see around the world today.

KALAN: They're climbing and leaping through the canopy.

And right now they're watching us.

I have one looking right at me right now.

(chuckles) Showing their curiosity.

And their acrobatic skills.

(chuckles) (animals calling in background) These are the black howler monkeys, and they are basically a tree canopy-adapted species.

They use their grasping hands and feet and this amazing prehensile tail to make their way through the canopy.

(animals calling in background) So, the first true primates, like Teilhardina, would have had traits very similar to the howler monkeys that we see here today.

♪ ♪ NARRATOR: Abundant fruit may have helped these true primates evolve some of their distinctive characteristics.

KALAN: Fruit is essentially a wonderful source of energy for them that's packed with calories, and it can be really hard to find these small fruits in this large, dense, green canopy, and that's where their big eyes and their hands help them to be able to find and pick out ripe fruits.

And this would have been essential for them to be able to adapt to a life in the trees.

♪ ♪ ♪ ♪ So, with this global temperature spike, forests started to spread further and further, even up into the northern hemispheres, and, with that, primates were then able to expand into Europe, North America, Asia, and Africa.

MASLIN: We enter the golden age of primates.

We have them evolving into lots of different species.

This was perhaps, at that moment in time, the pinnacle of primate diversity.

At this time, it really was not really "Planet of the Apes," but it definitely was "Planet of the Primates."

♪ ♪ NARRATOR: But the global spread of primates is about to come to an end.

They have thrived for over 20 million years on a warm Earth.

But now, the planet's climate is cooling dramatically.

♪ ♪ FRANCIS: The mechanism is still debated, but we think natural cycles reduced carbon dioxide levels in the atmosphere.

And there were other impacts, as well, such as the movement of the tectonic plates that changed ocean circulation, that had an impact on the atmosphere, and gradually, the polar regions began to, to cool, particularly Antarctica.

NARRATOR: Across the northern continents, cooler, drier conditions decimate the lush forest ecosystems, and the habitats where the first true primates emerged begin to disappear.

KALAN: This was really bad for primates, because it meant that their habitat was shrinking.

Along with that comes the fact, then, that they cannot get access to all the food resources that they need.

NARRATOR: Primate populations plummet in Europe and disappear completely from North America.

But around Earth's warm Equator, their habitat continues to thrive.

The critical thing about those primates in Africa is that we can trace our evolutionary lineage all the way back to them.

♪ ♪ NARRATOR: As primates prosper in the warmth of East Africa... ♪ ♪ ...tectonic forces deep within the Earth's crust begin to tear the continent apart.

♪ ♪ Hot magma wells up, driving the creation of a new environment.

♪ ♪ Over millions of years, deep valleys are forged and mountain ranges rise.

♪ ♪ Until, running thousands of miles through present-day Ethiopia in the north to Mozambique in the south, the East African Rift Valley is formed.

♪ ♪ APRIL NOWELL: The Rift Valley system created both these deep valleys and, of course, these really high mountain ranges, and that would have created this rain shadow, so it blocked the monsoon rains from coming across, and that created a whole new landscape.

CARVALHO: So, this is one of the perfect scenarios in which we see geology and tectonics actually driving the evolution of life.

With this drier climate, all the vegetation changes.

You go from a complete cover of forest to having a mosaic of environments.

Patches of forests connected by grasslands.

NARRATOR: This shifting landscape presents these primates with new challenges.

KALAN: This was really an evolutionary fork in the road for primates.

Food resources became more dispersed, which meant that primates had to travel further in order to find enough food to survive.

And what we see is that that likely led to evolving more efficient ways of moving through the landscape.

ALEMSEGED: Africa is fundamental to our origin story because most of our development happened in Africa.

The conditions were unique not only for the flourishment of our species and our lineage in general, but for the preservation of their remains.

(talking indistinctly) NARRATOR: The fossils discovered here help paleontologists like Zeray Alemseged retrace the complex story of our evolution.

VILLASEÑOR: There is evidence in the fossil record that there's a mix of walking and climbing traits, and so our ancestors were experimenting with walking on two legs.

NARRATOR: They were still spending time in the trees, but a specimen almost three-and-a-half-million years old, which Zeray named Selam, adds to evidence that our ancestors were regularly walking upright.

What I'm holding here is a replica of a skull of Selam, which is earliest child ever discovered.

This hole here, which is where the spine would insert and connect to the brain, is more centralized, and that is what happens when you have a upright, walking individual.

By studying the skull in general, we were able to comprehend that the species to which she belonged was at the cusp of being human.

The ability to walk upright changed everything for our ancestors.

It allowed us to run, it allowed us to have shoulders that we could actually throw things, so we could hunt.

Over time, tool use becomes much more complex.

So, for example, I have here something called a hand axe.

This one here was used to butcher a horse, and we know that because we were able to extract blood residue from the edge.

These stone tools tell us that our ancient ancestors were much more cognitively, socially, technologically sophisticated than we ever thought before.

Every time we find a new fossil or a new artifact, it's like adding a new page to that human story.

♪ ♪ (animal trumpeting) NARRATOR: The fossil record reveals that around 300,000 years ago, a number of human-like species are thriving.

And it's in Africa that our own ancestors eventually emerge-- Homo sapiens.

But the world humans encounter as they leave Africa is vastly different from the one their predecessors inhabited.

♪ ♪ Over millions of years, Earth has continued to cool... ...and is now in the thick of the Ice Age.

♪ ♪ FRANCIS: There were times when about 25% of the land surface would have been covered by ice.

Particularly in the Northern Hemisphere, glaciers and glacial landscapes extended as far as New York and London.

So much water was locked up as ice that the oceans dropped by hundreds of feet.

♪ ♪ ALEMSEGED: As Homo sapiens moved from one place to another, they would be facing many challenging conditions.

Remember, we are a tropical species.

But at the same time, there seem to have been some type of shift in terms of our behavior.

Humans were doing all sorts of things that you wouldn't have imagined to have happened during the Ice Age.

(birds chirping) ♪ ♪ NARRATOR: Today, we uncover hidden evidence of these behaviors.

♪ ♪ Clues that our ancestors had evolved impressive powers of creativity.

Wow!

The panel of the spotted horses is one of my absolute favorite in all of cave art, and I've seen it reproduced a thousand times in books and so on, but nothing compares to standing right in front of it.

You see the colors, you see the textures.

In some ways, it looks deceptively simple, but these lines are so carefully placed that you immediately know that this is a horse, just from looking at its contour.

It's a real combination of what they're seeing in their environment, as well as maybe some symbolic meaning through the placement of the dots around them.

And then with the hand prints around it.

I look at those hands, and you know that's us, you know that's a human.

That's what connects us to the people who made these 25,000 years ago.

♪ ♪ NARRATOR: Ancient art like this has been found all over the world.

Within the caves of Indonesia are paintings dated to around 45,000 years ago-- the world's oldest known images of animals.

Studying this expanding footprint of creativity helps scientists piece together the puzzle of what makes us human.

♪ ♪ NOWELL: Prehistoric people around the world chose to recreate nature through their art.

And, of course, the big question is, why?

For me, the most compelling explanation is that these images were probably part of an oral storytelling tradition.

That they were the illustrations that went along with their stories.

And they're not just for entertainment value, but they actually also communicate a lot of really important information.

In order to be able to survive in a particular environment, one person's knowledge isn't enough.

♪ ♪ But humans live in communities, and we share our knowledge.

It's not one mind, but many minds working together.

It's this grand total, this sum of all the knowledge that we have that we then pass on from generation to generation over time, and that's what archaeologists call cumulative culture.

For me, this is key.

This is what makes humans unique, and it's really what has allowed us to move out into all different kinds of environments and essentially live in basically every corner of this planet.

♪ ♪ NARRATOR: The most extreme cold and dry conditions of the Ice Age don't last.

Because subtle changes in Earth's orbit alter the amount of sunlight reaching its surface.

This, along with increasing carbon dioxide in the atmosphere, drives temperatures up and causes much of the ice to melt.

♪ ♪ And as it retreats, humans apply their skills in a revolutionary new way.

They begin to farm.

♪ ♪ VILLASEÑOR: Farming was a major turning point for humans.

We started to modify the landscape in a way that we'd never done before.

With farming, we transitioned from using the environment to owning the environment through domesticating animals, but also having a permanent landscape that we control.

NARRATOR: Within a few thousand years of those first seeds being sown, humans are farming across the planet on an ever-increasing scale.

♪ ♪ Today, about half of the habitable land on Earth is used for agriculture, and our takeover of the natural world has had an unprecedented impact.

♪ ♪ We have cut down three trillion trees-- that's half the trees on the planet-- to make way for agriculture and our cities.

♪ ♪ NARRATOR: Humans now have a greater effect on shaping Earth's surface than many of its natural processes, and human-made materials like concrete and plastic outweigh the combined biomass of all life on the planet.

♪ ♪ MORRIS: Humans have done a lot to create a place where they can thrive in relative comfort.

We have buildings, we have very tall buildings.

We have cars, buses, trains.

We've conquered the sky.

♪ ♪ NARRATOR: And the engine of this progress is powered by materials deposited over the course of Earth's history.

MASLIN: Modern human society is built on the use of fossil fuels.

Coal, oil, and natural gas, you can see them as fossilized sunlight.

♪ ♪ Plants and animals have trapped energy from the sun, stored it in their carbon, and then been laid down in geological strata.

VILLASEÑOR: Humans have basically mined the geological record to fuel many of the technologies that we depend on today.

NARRATOR: Humanity is acting as a geological force, adding greenhouse gases to the atmosphere around ten times faster than the volcanic activity that caused global warming 56 million years ago.

We've driven 20,000 years' worth of climate change in only 170 years.

As someone who studies natural events, it's remarkable, the change that we have made.

We're changing every aspect of the planet.

The oceans, the land, the atmosphere, the ice.

♪ ♪ MORRIS: I think this would be an unrecognizable planet to our ancestors.

NARRATOR: The planet we have transformed now supports more than eight billion people.

A remarkable milestone for a species that was unlikely to have evolved at all.

MASLIN: Every single one of your ancestors must have survived and reproduced to produce you.

The chances that any one of us actually exists, the chances of our own species existing, are so, so small, it must make us realize how lucky we are.

♪ ♪ NARRATOR: Since Earth formed four-and-a-half billion years ago, the evolution of humanity has been far from inevitable.

Life has been threatened by asteroids.

(explosions roar) ♪ ♪ Catastrophic volcanic eruptions.

♪ ♪ And the almost complete glaciation of Earth's surface.

BRUSATTE: In the history of the Earth, the history of life, it is one unfolding story with so many twists and turns and plot lines and new characters coming in, and old characters going extinct.

It's like the longest-running television show of all time.

♪ ♪ NARRATOR: But from a barren environment once devoid of an atmosphere... ♪ ♪ ...to thriving ecosystems bursting with plants and animals, our planet's geology and climate shaped a world where Homo sapiens could evolve, the first species able to look not only into Earth's past... ...but also toward its future.

♪ ♪ MISSION CONTROL: T minus 15.

NARRATOR: In 2021, NASA launched a rocket... MISSION CONTROL: Ten... ANNOUNCER: Nine, eight, seven... NARRATOR: ...toward an asteroid seven million miles from Earth.

ANNOUNCER: Three, two, one... And lift-off of the Falcon 9 and DART, on NASA's first planetary defense test to intentionally crash into an asteroid.

♪ ♪ NARRATOR: Even though this asteroid was a fraction of the size of the one that wiped out the dinosaurs, humanity was about to make history.

Oh, my goodness!

(applauding) (applause continues) (cheering and applauding) We have impact!

(cheering and applauding) COLLINS: As someone that studies asteroid impacts and knows how disastrous their consequences can be, it was really exciting to watch the NASA DART spacecraft slam into the asteroid and successfully deflect it.

(cheering and applauding) ♪ ♪ NARRATOR: Our technology allows us to consider a future free from the threat of a cataclysmic asteroid impact.

But it also gives us a perspective never experienced by our ancestors.

One that brings into sharp focus threats far closer to home.

♪ ♪ WATKINS: It takes 90 minutes to orbit the Earth on the I.S.S.

So, we see 16 sunrises and sunsets every day.

It is hard to pull yourself away from watching the world go by.

♪ ♪ We're up in... ...the Canadian plains now.

(interview): Seeing the Earth for the first time from the cupola windows was just absolutely breathtaking.

It's difficult to describe.

Just really seeing the planet as one body.

Getting to see how all of the different climates and environments of the Earth are really connected.

And also, how fragile that ecosystem is.

It really drives home the importance of taking care of this planet and the responsibility we've been given to do so.

♪ ♪ I think oftentimes we as a species focus too much on the bombastic.

On the big, bold, brash things.

When it comes down to it, the risk of an asteroid hitting us is tiny, but the risk that climate and environmental change pose to us right now, every day, day in and day out, that risk is so much higher, it's so much more real.

We do have a responsibility to look in the past and use that information wisely to make decisions about our future and the future of this planet.

It is our responsibility.

We are the only species that understands the consequence of our actions.

WATKINS: I think, as human beings, we've been given a gift, and that is our intelligence and capability to investigate our impact on the environment around us, and I think using that gift to understand how we can affect our future is really imperative.

♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪