NOVA

NARRATOR: Earth is a living planet.

(animals chirping, hissing, trumpeting) NARRATOR: But it wasn't always that way.

Life had a beginning.

KARYN ROGERS: When and how did life emerge on this planet?

What environments did it live on throughout Earth's history?

NARRATOR: These are some of our planet's greatest mysteries.

For a long time, scientists thought life could not have appeared very early in Earth's history, when the planet was under heavy bombardment by asteroids.

SIMONE MARCHI: A tremendous number of impacts, even large one.

Imagine, an object the size of the moon that could have collided with the Earth.

NARRATOR: But now, scientists are finding new clues in ancient rocks, on the surface of the moon, even on space rocks hundreds of millions of miles away, and inside craters made by massive asteroid impacts.

And they're wondering, instead of preventing life from starting, could violent impacts like these actually be essential?

DANIEL GLAVIN: Asteroids could have delivered the basic chemical building blocks of life to Earth.

NARRATOR: Leading some scientists to ask, "Could asteroids be the spark of life?"

Right now, on "NOVA."

NARRATOR: The Barberton Makhonjwa Mountains in South Africa.

Here lie some of the oldest and rarest rocks visible on the surface of our planet.

And here, geologists Nadja Drabon and Phumelele Mashele are searching for evidence of the conditions on the early Earth to help solve the mystery of how life got started.

So we'll be going right here, right where you see the purple meet the orange.

Right near the river.

Yeah.

DRABON: When someone comes into these mountains, they think, "Oh, wow, that's a really beautiful scenery and really gorgeous."

However, when I come here, I really start seeing Earth history unfolding layer by layer.

NARRATOR: Some of the Barberton Makhonjwa rocks are as old as 3.6 billion years.

They've only survived this long because the mountain range sits on a relatively stable part of the Earth's crust.

They date back to a geological eon called the Archean.

Earth itself had only formed about 900 million years before.

The Archean world was alien.

There was no breathable oxygen.

Erupting volcanoes poured vast quantities of greenhouse gases into the atmosphere.

The sun was a lot weaker than it is today, but these gases kept the planet warm-- warm enough for liquid water on its surface.

In fact, some of the minerals found in Archean rocks suggest the planet was already an ocean world.

So there was water.

Was there also life?

These Archean rocks are some of the best preserved in the world.

Could they contain signs of ancient life-forms?

In this vast landscape, Nadja thinks she may have found some traces.

DRABON: When you look at these rocks here, some of these layers look just really black.

But what I'm seeing here is really the remains of life back then.

People have taken a really close look through the microscope, and what they are finding is remains of single-cellular organisms preserved within the rock.

NARRATOR: What did these microbes look like?

Was this the first life on Earth?

Far from the mountains of Barberton, in the city of Orlans, France, geologist Frances Westall examines ancient rock samples, hunting for signs of life.

WESTALL: We have here in front of me a collection of rocks from Barbertons in South Africa and also from the Pilbara in Australia.

These rocks are more than three billion years old.

In these rocks, I have found traces of fossil microbial life.

NARRATOR: These rocky outcrops in the arid regions of the Pilbara in the northwest of Australia are as ancient as those in the Barberton Makhonjwa Mountains.

Rocks from both locations have given Frances and her colleagues some of the best evidence yet of what life may have been like in the Archean eon over three billion years ago.

(Westall speaking French) NARRATOR: To detect ancient life forms, Frances uses a scanning electron microscope.

A concentrated beam of electrons scans the sample and interacts with atoms on the surface, creating signals that can be translated into highly magnified images.

But this rock is not a good conductor of electrons.

So it's coated with a thin layer of a material that is-- gold.

In a 3.3 billion-year-old sample from Barberton, Frances identifies what many believe are fossilized life-forms.

WESTALL: Here you can see an individual filament.

Here, as well.

NARRATOR: Frances thinks each of these thread-like structures is a cell about 70 times thinner than a human hair.

How do we know that they're, they're microbial fossils and not something else that's got nothing to do with microbes-- minerals, for instance?

NARRATOR: One method is to compare them to microfossils of modern bacteria that Frances actually made in her lab.

She entombed living microbes in silica.

In nature, silica can fossilize an ancient organism by penetrating and coating its internal structure.

In an extremely old rock sample from the Pilbara in Australia, Frances finds a shape that looks like one of her modern silica-coated microbes.

Chemical analysis reveals signatures of what could be organic molecules, which means this might have been an ancient life-form.

WESTALL: I've been able to reveal traces of single cells.

We can see cell division.

We can see, also here, cell division.

These are cells that are preserved in a rock nearly three-and-a-half billion years old, and they are exquisitely preserved.

NARRATOR: They could be some of the oldest microfossils so far found on Earth.

Traces of a variety of single-celled life-forms that lived in different environments over three billion years ago have been found in Barberton and the Pilbara.

Even though they were single-celled, they came in a variety of shapes.

And so, this could not have been the first life.

Life was extremely diversified already by three-and-a-half billion years ago, which tells me that it must have emerged a lot earlier than we originally thought, possibly between 4.3 and 4.2 billion years ago.

NARRATOR: That would place the origins of life within the most mysterious and inhospitable eon in Earth's history-- the Hadean.

This was a time even before the Archean.

Archean rocks may be extremely rare, but Earth's Hadean rocks are nearly unheard of, because most rocks on Earth eventually get destroyed, eroded away, or melted down.

For most of our planet's history, Earth's crust has been broken into plates.

Sometimes, when two plates collide, one will slide under the other, pushing the surface rocks down into the mantle.

It's as though Earth is swallowing its past.

Direct evidence of the Hadean may be long gone, but what we do know is that over four-and-a-half billion years ago, our planet had just formed from dust and rock particles that circled our young sun, so its surface was unstable.

Scientists named the eon after Hades because they believed it must have been a hellish place, covered with molten lava from erupting volcanoes.

On top of that, giant asteroids left over from the formation of the solar system pummeled the planet.

Could life have emerged, and survived, in such hellish conditions?

To find the answer, scientists must first know what life actually is.

Karyn Rogers is an astrobiologist and geochemist.

Karyn and her team study the origins of life.

ROGERS: At some point in Earth's history, there wasn't life.

And there had to, from that entire planet that was abiotic, that had no life on it, there was a chemistry, or probably a series of chemistries and reactions that were intertwined, that eventually came into life.

NARRATOR: Scientists don't yet know the exact chemistry that created life, but they do know its building blocks: molecules containing elements like carbon, hydrogen, nitrogen, and oxygen, which are found all over the solar system, can join to form organic molecules, including sugars and amino acids.

These bond to make even bigger molecules: amino acids form proteins, vital for the functions of a cell.

ROGERS: How do we make the amino acids that turn into proteins?

How are the sugars that form the backbone of DNA and RNA originally synthesized?

So life has all of these ingredients, and they need to come together just right to eventually get to life as we know it.

NARRATOR: The recipe required a source of energy and one of life's most essential ingredients, liquid water.

(birds chirping) So if life did emerge 4.3 billion years ago, then this ancient, extremely hot planet had to also be a wet planet.

The rocks that could prove that might be long gone, but for years, scientists have been gathering clues from tiny ancient mineral crystals.

When rocks erode, some minerals can survive and get incorporated into new rocks.

In 2018, here in the Barberton Makhonjwa mountain range, Nadja Drabon and her team found grains of a type of mineral known to be the Earth's oldest surviving material, zircon.

Zircon is an extremely tough little crystal.

Once it forms, it's very hard to break down.

Zircon can withstand billions of years of weathering, so it retains evidence about the rock in which it originally formed.

(thunder rumbling) And that's why the zircons Nadja and her team discovered on this mountain are so special.

So this here is actually my favorite rock in the entire Barberton Greenstone Belt.

That is because we find zircons about 200 million years older than the oldest rock on Earth.

NARRATOR: Which means Nadja had discovered some of the rarest zircons on the planet, up to 4.2 billion years old-- from the Hadean.

Chemical analysis of these, along with even older Hadean zircons found in Australia in the 1980s, revealed that they had formed in the presence of a very special ingredient.

(rain falling) DRABON: By about 4.3 billion years ago, we've got evidence for liquid water preserved within these zircons.

(thunder clapping) NARRATOR: The presence of water in the Hadean 4.3 billion years ago is crucial, because water is one of the key ingredients for life.

And life as we know it could not have emerged without it.

That began to paint a picture of what the Hadean landscape was like.

NARRATOR: It was not exactly the hellish place scientists once thought it was.

ROGERS: We had emergent land, but maybe not a lot of it.

And we had an ocean covering most of the planet.

NARRATOR: For oceans to exist, the planet's crust must have cooled much faster than scientists once thought.

But where did the water come from?

Some scientists believe it was delivered by asteroids and comets, but Earth may also have been born with water trapped deep inside the mantle, and volcanic activity delivered it to the surface as steam.

ROGERS: And, in addition, we were regularly getting bombarded with meteorites and asteroids.

And so they came in one after another, after another.

NARRATOR: Did life have to wait for a lull in the bombardments before it could spark?

Or was it hardy enough to emerge despite the chaos, snuffed out by an impact in one place, only to reemerge in another?

So far, no direct evidence of these early asteroid impacts has been found on Earth.

There may be traces of impacts that happened around three-and-a-half billion years ago back in the Barberton Makhonjwa Mountains.

While any craters would have been eroded away, much tinier clues can survive.

I just found one!

Woo-hoo!

(chuckles) These things are so difficult to find.

Like, in this whole package of rocks, these are the few that are well-preserved.

NARRATOR: This tiny circle is a spherule.

Spherules can form as a direct result of massive asteroid impacts.

DRABON: When you have a giant impactor, so think about ten kilometers in diameter or bigger, when that smashes into Earth, you actually have so much energy that it's going to form a rock vapor cloud that is going to be ejected out of the crater at speeds of up to 40,000 miles per hour.

This rock vapor cloud's going to start condensing to form these spherules that are going to rain out and blanket the entire globe.

NARRATOR: Without visible craters, these scattered spherules may be the only remaining evidence of an impact, so it's impossible to know exactly where on Earth the asteroid hit.

As the spherules rain down, they form layers.

The thicker the layer, the larger the original impact.

And the spherule layer that Nadja and Phumelele locate is very thick-- almost eight inches deep-- so the asteroid that created it was probably more than 20 miles across.

DRABON: For the impact energy, what we actually think about is the mass.

And that would have been 50 to 200 times bigger than that of the impactor that killed the dinosaurs.

These rocks tell the whole story from before the impact happened to the actual impact event, and then to how the environment and life responded.

Before the impact happened, about 3.26 billion years ago, here at this location, we would have been standing on the seafloor, on the shallow seafloor.

And there was a little bit of life present, but not too much.

And then all of a sudden, this is changing really dramatically.

NARRATOR: The asteroid hit the ocean, triggering an enormous tsunami that swept across the globe.

Evidence of the big wave is in the rocks.

DRABON: And that's what we see here, these big chunks that were ripped up from the seafloor right below.

NARRATOR: What effects did the tsunami have on the simple life that may have lived at the time?

Nadja finds clues in sediments that formed not long after the impact.

The rocks are red-- an indication of iron, an essential nutrient for life.

DRABON: What we think is that the tsunami that was sweeping by was bringing iron-rich waters from the deep oceans to the surface.

(saw whirring) NARRATOR: A closer look at the iron-bearing rocks reveals another surprise... ...signs that life bounced back very quickly after the impact.

How could this happen?

DRABON: So what we think we see in these rocks is that these microbes are starting to respond to that increase in nutrients and iron, and are actually starting to, to bloom.

When people think about giant meteorite impacts, they first think about the extinction of the dinosaurs.

What we think what we are seeing here is that life was not only able to survive these really disastrous consequences for the environment, life was actually able to thrive.

NARRATOR: So far, evidence of several giant impacts that happened between 3.5 and 3.2 billion years ago has been found in these mountains.

But what was happening earlier, during the Hadean, 4.3 billion years ago, when some scientists think the very first life emerged?

Traces of that time on Earth are long gone, but there is another place that can tell the story-- the moon, whose surface has retained the kinds of scars that once covered Earth.

DAVID KRING: We don't have any surviving remnants of that Hadean Earth.

And so it's easy, I think, to imagine that that surface was not cratered.

But the moon actually tells us otherwise.

MAN (on radio): Zero, zero, five, seven, two.

NARRATOR: David Kring is an astrobiologist at the Lunar and Planetary Institute in Houston.

He studies the surface of the moon to learn about the early Earth.

MAN (on radio): Cherokee, one, zero, zero, zero.

KRING: In the early and mid-20th century, scientists debated the origin of the circular structures on the lunar surface.

Were they huge volcanoes or were they impact craters?

LAUNCH ANNOUNCER: Ignition sequence start.

KRING: Almost immediately, the Apollo 11 mission answered this question with the samples that were collected and brought back to Earth.

It became evident that nearly all of those circular features must have been generated by impacting asteroids and comets.

NARRATOR: It was a turning point, when scientists realized that violent asteroid impacts could reshape the surface of a planet, including our own.

The craters on the moon were beautifully preserved, undisturbed by erosion or plate tectonics, like we have here on Earth.

Scientists started to count them.

KRING: The older a planetary surface, the more time there has been for it to be hit by these impacting asteroids, and therefore, the greater the number of craters.

NARRATOR: Thousands of craters larger than a mile wide have been counted so far.

When the moon rocks from some of the craters were dated, scientists were in for a surprise.

KRING: In most cases, the samples that were returned by the Apollo astronauts had very, very old ages, in what we now call the Hadean of Earth history, indicating that there was, early in solar system history, a intense period of bombardment.

NARRATOR: If the moon suffered this many violent impacts, how many and how large were the asteroids that hit the early Earth?

How frequently did they impact?

And how could they affect the emergence of life?

Planetary scientist Simone Marchi has been piecing the story together.

MARCHI: There's one single process that's very important to me that I find it very fascinating, and that is the process of collisions.

Everywhere we look, everywhere in the solar system there is a solid surface, you'll find that there are craters.

NARRATOR: The craters are a reminder of our solar system's formation around 4.56 billion years ago.

It began as a disk of dust and gas in orbit around the young sun.

The solid materials collided and clumped together to gradually form the rocky planets.

MARCHI: As a result of the formation of the Earth, there were still lots of debris, and asteroids, and, and other, smaller objects flying around the sun.

Those object kept colliding with the surface of the Earth.

NARRATOR: About four-and-a-half billion years ago, a single object the size of Mars, or even bigger, may have crashed into the young Earth.

This high-resolution simulation reveals how the collision flung enough molten and vaporized debris into space to create the moon.

MARCHI: I'm trying to understand the early evolution of the Earth and the effects of all those impacts that were taking place during the Hadean Earth.

So we do this by building models.

NARRATOR: One of the most important sources of data comes from NASA's lunar reconnaissance mission.

This robotic spacecraft has made a 3D map of the moon's surface at extremely high resolution.

(camera shutter clicking) MARCHI: The first thing that we do is to look at the surface of the moon.

It is much older than the surface of the Earth.

The surface of the moon is full of impact craters, all the impact craters, and so we can use that information by mapping how many there are, and their sizes and their ages, and that will provide us the primary information that we need to build our models.

NARRATOR: It took an international team decades to collect the data, but they finally created a computer model that took what happened on the moon during the Hadean and simulated the asteroids that would have hit the Earth during the same stretch of time.

MARCHI: And the outcome of that first modeling was staggering.

We are seeing the entire surface of the Earth that is strongly affected by impacts.

Every single circle that you see here is, is an impact, is a collision.

The prediction was that there were a tremendous number of impacts, even large one.

Imagine, an object the size of the moon that could have collided with the Earth.

That would have basically wiped out almost entirely, perhaps, the oceans, vaporized the oceans and, and melted a large portion of the crust of the Earth.

NARRATOR: This series of apocalyptic bombardments might look like it created a chaotic hellscape on Earth.

But fossil evidence suggests that life did emerge during the Hadean.

So even during asteroid impacts, there must have been enough habitable conditions somewhere on the planet for life to get a foothold.

MARCHI: If life started on Earth around perhaps 4.2 billion years ago or thereabouts, then the question is how that was connected to the impacts that were taking place at the same time.

ROGERS: The origins of life community rarely thought about impacts as part of the origin story.

It's really hard to not think about them.

We really had to change our, our sort of frame of mind, and I certainly did.

NARRATOR: To investigate what effects the asteroid impacts had on the emergence of life, Karyn Rogers is recreating the conditions of the Hadean Earth in her lab.

ROGERS: In some ways, the early Earth was a big experimental laboratory doing prebiotic chemistry.

We can do experiments that were similar to what the early Earth was doing, and hopefully discover the chemistry that eventually led to life.

NARRATOR: Karyn's team can create tiny Hadean environments with the same temperatures, pressures, gases, water composition, and types of rocks that may have existed at the time... ...echoing the places which may have had the chemistry needed for simple molecules to join and eventually lead to the first cell.

ROGERS: So what we think about the origins of life, there are a few different ideas that have been around for a while that people have been studying for quite some time.

One of them is a hydrothermal system origin of life.

NARRATOR: These are hot water systems heated by volcanic activity.

ROGERS: One of the really special things about hydrothermal systems is that they can provide energy either for life or maybe the chemistry that allows life to emerge.

NARRATOR: Hydrothermal systems can appear on land, where magma pushes towards the surface, creating hot springs and geysers, like the ones at Yellowstone, and in the deep sea.

Here, the cold seawater descends into fractures in the rock and interacts with the minerals.

Heated by magma, it reemerges through chimney-like structures, now enriched with the types of organic molecules necessary for life.

ROGERS: We have really hot hydrothermal fluid coming out of a chimney.

It's full of metals, and it's mixing with seawater.

And so when those two fluids come together, they could also provide energy to do organic chemistry that might lead to life.

NARRATOR: So, did the very first primitive cells emerge and survive in hydrothermal systems?

And if so, where?

On land?

In the deep sea?

Or somewhere else?

Was that even possible under a steady barrage of asteroids?

ROGERS: Well, when we think about life surviving on the Earth, we think about things that allow it to thrive and things that might actually destroy it.

And probably one of the most prominent sort of events that destroyed life was the impact that killed the dinosaurs.

NARRATOR: 66 million years ago, long after the Hadean ended, a space rock bigger than Mount Everest hurtled toward our planet.

It was a moment that would change the evolution of life on Earth.

A vivid reminder of the havoc and devastation that an asteroid impact can wreak.

(asteroid approaching) That one impact alone wiped out 75% of Earth's species after it hit the Yucatn Peninsula.

(dinosaur growling) KRING: The Chicxulub impact crater was produced by an asteroid.

It hit with an energy equivalent to 100 million megatons of energy.

That's a tremendous blast.

NARRATOR: David Kring has studied rocks from beneath the Chicxulub impact crater.

Looking at tiny slices of the rock under a microscope... ...he found quartz crystals that had been shocked and deformed by the intense pressure generated by the impact.

But he also saw something much more surprising: minerals like anhydrite, which are produced hydrothermally in the presence of very hot water.

So this is evidence that the impact event heated the Earth's crust, heating the water within the Earth's crust, and then generated a vast, circulating hydrothermal system.

NARRATOR: This system would have been similar to volcanically driven hydrothermal systems, where some scientists believe life first emerged.

But this one was much larger.

As David probed further, he found something else locked inside the minerals.

Signs that ancient microbes were living in Chicxulub's hydrothermal system just after the impact.

KRING: We now have evidence that it hosted a microbial ecosystem.

They provided the habitat in which these organisms thrived and grew throughout the crust of the Earth beneath the floor of the Chicxulub impact crater.

NARRATOR: And it wasn't just Chicxulub.

As scientists surveyed the 200 known impact craters and structures on Earth, they discovered that about a third of them show signs of the same type of hydrothermal activity.

KRING: And so we began to realize that this was a common process that would have occurred in impact craters throughout the Hadean period.

NARRATOR: What did these ancient hydrothermal systems look like?

There could be clues inside one of Earth's best-preserved craters.

About 15 million years ago, an asteroid hit Southern Germany, making an almost 16-mile-wide crater known as Ries.

(bell ringing) The impactor was about the same size as a medieval town called Nrdlingen, built inside the crater, which is nearly invisible today.

(bell rings) Even from the highest tower, the rim is hard to make out.

But human-made quarries have exposed the secrets that lie beneath the crater.

All along the rock walls, excavation has exposed strange, vertical, pipe-like structures.

(wind blowing) Planetary geologist Livio Tornabene is at Ries Crater to learn more about these formations, which are visible as rust-colored rock.

TORNABENE: It's really bounding this pipe structure.

I mean, it looks like it disappears, but it probably goes into the rock.

Probably have to see this in three dimensions, and it would be sort of branching out and trying to find the quickest route up to the surface.

NARRATOR: After decades of research, scientists believe they know how these pipes were formed.

Here, we're, we're actually below the surface of the, of the deposit as it would have been 15 million years ago.

It's really well-preserved, and for a crater this size, it would have produced a lot of melt that would have been superheated.

NARRATOR: The heat from the melt released water from the rock and turned it into gas.

The energy of the escaping steam forged pathways up though the hardening rock, creating what scientists call degassing pipes.

The darker color of the pipes is evidence that liquids and gases once flowed through them.

TORNABENE: We know that there was fluid here running through these rocks, there was heat, there were available nutrients, and that is definitely the combination that you want to look for when looking for life here on Earth, or on Mars, or elsewhere in the solar system.

NARRATOR: These degassing pipes were like the plumbing of a vast hydrothermal system.

With every excavation, more are exposed.

So how large were the hydrothermal systems that formed during the Hadean?

The Chicxulub crater may help scientists in their estimates.

So, Chicxulub is a good model for some of the smaller impact events that occurred during the Hadean.

NARRATOR: At the Southwest Research Institute, geologist Amanda Alexander runs one of the latest Chicxulub models.

The blue-green color shows the areas where the impact would have fractured the rock, allowing water to flow through, creating a hydrothermal system.

It was much bigger than scientists thought.

About ten times larger than was previously expected, and about 100 times larger than we think is the current Yellowstone hydrothermal system.

NARRATOR: And this astonishing estimate is for only one crater.

ALEXANDER: So the Chicxulub impactor was about 14 kilometers in size, but the impacts that were happening on the Hadean were much larger and much more frequent.

Impacts like these would have been occurring over the half-billion-year duration of the Hadean.

NARRATOR: So at some point, vast hydrothermal systems might have covered much of the planet.

All of this research is leading to a remarkable idea.

It's looking more and more like asteroid impacts were double-edged swords.

While they were certainly bringers of chaos and destruction, they might have created ideal conditions for life.

But what about the raw ingredients?

How can we know if they were present at asteroid impact sites?

(asteroid falling) Danny Glavin is an astrobiologist at NASA's Goddard Space Flight Center near Washington, D.C.

He's been looking for the ingredients of life in space rocks.

GLAVIN: Meteorites are really fascinating objects.

These are fragments of asteroid material that are constantly bombarding the Earth.

Something like 5,000 metric tons of material is falling to the Earth each year.

NARRATOR: In 1969, what would become one of the world's most studied meteorites fell to Earth in Southeast Australia.

It was named Murchison, after a nearby town.

This meteorite was a treasure trove, containing hundreds of amino acids and other fundamental building blocks of life.

GLAVIN: The way we extract these meteorite samples to look for the chemical building blocks of life is, we start with a small chunk, maybe the size of a sugar cube, start chopping it up, grinding it up.

We make kind of a meteorite flour, and then we take that powder and we put it in a test tube, with water, to extract it.

So we're making kind of a, a meteorite tea, if you will.

We take the liquid water, we purify it through several steps-- we want to remove the salts from the extract, so that we can really focus on the amino acids and detecting them.

And then the final step is, we inject that liquid into a mass spectrometer to separate out the individual amino acid peaks and identify them by name.

NARRATOR: But there's a problem with studying meteorites that have made their way to Earth's surface.

(explosions echo) GLAVIN: One of the challenges with meteorites, that, as soon as they hit the atmosphere and hit the ground, they, they immediately become contaminated.

LAUNCH ANNOUNCER: Lift off of OSIRIS-REx.

To boldly go to the asteroid Bennu and back.

GLAVIN: We really do need to go to space, to go to asteroids and bring back pristine samples that have never seen the Earth's biosphere.

NARRATOR: In 2016, NASA's OSIRIS-REx mission headed to the asteroid Bennu.

Bennu is slightly taller than the Empire State Building, and, from a distance, it looked like it would have had the same kind of rocky makeup as meteorites like Murchison.

In October 2020, OSIRIS-REx bounced off Bennu and grabbed a sample of its surface material.

GLAVIN: We fired the nitrogen to collect the sample.

There was a huge plume of material.

When we imaged the sample collector, we saw there was a bounty of material from asteroid Bennu in the collector, so we had done our job.

NARRATOR: It took just under three years for the sample to be returned to Earth.

Altogether, the mission brought back about 122 grams, the largest sample ever collected from an asteroid.

The precious space dust was divided up and sent to labs around the world for analysis.

Danny's lab got about five grams.

GLAVIN: So you're looking at a very tiny speck of Bennu.

Even in a particle as small as a half a millimeter, we can extract these samples and look for amino acids and other chemical building blocks of life.

NARRATOR: Bennu was rich in carbon, the element of life.

14 of the 20 amino acids found in life on Earth were also detected, along with all of the chemical bases of our genetic code.

(rumbling) Which begs the question, what would happen if an asteroid that created a vast hydrothermal system on impact also delivered the building blocks of life directly to that site?

(groaning) (rumbling) Could those building blocks survive such a violent, destructive event?

ROGERS: Nobody really knew what happened to these organic compounds once they got delivered.

And we do know that when impactors hit the Earth, they create hydrothermal systems.

And nobody asked, "Gee, what happens to those organic compounds in those hydrothermal systems?"

So we did lots and lots of experiments.

NARRATOR: Meri Herrero Perez conducts the experiments under early Earth conditions.

Meri takes a mineral thought to be on the Hadean Earth and found in impact craters today.

To this she adds a mixture of simulated Hadean water and soluble organic compounds, including amino acids, like those present in some meteorites.

She then places these ingredients into a chamber, where they're exposed to the conditions typical of a hydrothermal system made by an impact.

It requires liquid water, energy, and heat.

Hydrothermal systems made by impacts cool over a long period of time.

So the experiments are conducted using a range of temperatures.

Meri conducted hundreds of these trials, each lasting seven days.

I remember the first time that I looked at all the experiments that needed to be done.

It was 180, and I thought that that was a lot.

But the actual thing is, I ended up doing probably more than double or triple that.

NARRATOR: After the tests, Meri used a nuclear magnetic resonance spectrometer to help identify the structures of any molecules that may have formed during the experiment.

The results were beyond surprising.

HERRERO: The first time I saw the results of a successful experiment, I did not believe what I was seeing.

(laughs) I thought I had done it wrong.

And then I spoke with Karyn, and we, we could understand that there was more complex chemistry happening that we envisioned.

ROGERS: Our question was, is, did these molecules start to react with each other as they went through this impact-generated hydrothermal system?

There was always a possibility that they just broke down and never led to life, but what we found is, instead, these molecules actually got together and made new and bigger molecules.

And we're still trying to figure out what those new and bigger molecules are.

But as you make bigger molecules, you really are pushing in the right direction to build the complex chemistry that could eventually lead to life.

NARRATOR: Exactly where and when that complex chemistry made the leap to biology, and life first emerged, remains a mystery.

Many scientists still believe that hydrothermal systems created by volcanic activity deep in the sea or on the surface are the best candidates.

But some scientists are rethinking what role asteroid impacts might have played in the origins of life.

MARCHI: We always think about an asteroid's colliding with the surface of the Earth as a very negative event.

Maybe that's what we needed in order to get the chemistry necessary for life to form.

KRING: Those very same impact events were perfect crucibles for the origin and early evolution of life.

DRABON: They really generated an environment to life that allowed it to evolve to what it is today.

NARRATOR: Many lines of evidence have led to a remarkable hypothesis, that life might have begun as a result of those huge impacts, rather than in spite of them.

Perhaps over four billion years ago, a space rock laden with the building blocks of life hit the Earth, creating a vast hydrothermal system, one of hundreds of thousands that covered our planet, each one with the water, the energy, and the ingredients to brew the chemistry of life.

And these asteroid impacts were happening all over the solar system.

On Mars, the remains of hydrothermal systems have been discovered beneath many asteroid impact craters.

And inside one four billion-year-old crater, NASA's Perseverance rover has collected the most tantalizing evidence yet of potential microbial life on the Red Planet.

Who knows what discoveries may await on distant, rocky surfaces elsewhere in our own solar system, and beyond, that might finally reveal, once and for all, that we are not alone?