[0:04]This city, the people in it, in fact, all life on Earth, were only here by chance. We survived 99% of all the species that ever existed, didn't. They were wiped out in a series of global catastrophes, disasters that brought life to the verge of extinction. Four and a half billion years ago, the Earth collided with another planet. The impact nearly destroyed our world, but instead, it made it a home. This is the story of our planet's difficult birth.
[0:55]When I look around at all these people, it's hard to believe quite how lucky we are because we're only all here by chance. Evolution wasn't an orderly progression from single cells to us. It was an imperfect, often violent process and it's a miracle that we're here at all. In this series, we'll look at the theories about catastrophes that shaped our world and very nearly stopped evolution in its tracks. Like the time the planet was encased in ice for 25 million years, freezing the land and the oceans. Life survived, but only just.
[1:42]When huge volcanic eruptions poisoned the planet's atmosphere and pushed all life to the edge of extinction.
[1:56]Or the moment a six-mile-wide asteroid smashed into the planet.
[2:04]Killing 70% of life on Earth, including the dinosaurs.
[2:14]Even humans didn't escape unscathed. We might only have been around for a short while, but we've faced and survived supervolcanoes, ice ages, and even cosmic impacts. These catastrophes nearly wiped out life, but from their ashes, new species evolved. Without them, we might not be here at all. Tonight, we examine the first great catastrophe. The day our planet collided with another.
[2:53]The Earth's around 4.5 billion years old, so it's tough getting a handle on such an enormous amount of time. To put it in perspective, imagine the whole of Earth's history compressed into the 24 hours of a single day. Each minute on our clock represents around 3 million years. It starts ticking at midnight, and the first catastrophe was just minutes away. Our solar system hadn't even finished forming. 20 infant planets circled a new star, our sun. One of them was Earth.
[3:37]Its surface was a vision of hell, a barren, lifeless place shrouded in toxic volcanic gases. There was no water, no oxygen, and no moon. But this is all about to change. As our clock reaches 9 minutes past 12, that's 4.5 billion years ago, the Earth faces its first and greatest disaster, an impact of biblical proportions.
[4:14]Reconstructing Earth's early history is a major challenge. It's been obliterated and obscured by billions of years of erosion and volcanic activity. Astronomer Bill Hartman has spent his life studying the events of the early solar system. The record of the very first part of the Earth's history, all of that is wiped away on the Earth itself because we have erosion and rain and continental drift and continents colliding and mountains coming up and so on. But all is not lost. There is a record of Earth's earliest days, just not on Earth. It turns out the best place to look is on nearby Mercury and Mars. Their surfaces have barely changed in over 4 billion years, providing us with a unique record of events in the very earliest days of our solar system. They're dotted with ancient impact craters.
[5:18]You start looking at those craters and you discover that there are not only 100-mile craters and 200-mile craters, there's 600-mile features, I mean, some very large objects. These craters paint a picture of an intensely violent period. Of a solar system littered with cosmic debris where millions of asteroids and comets smashed into the young planets.
[5:40]In the thick of this all-out assault, the Earth too must have been struck. It was a window into the early history of the Earth, and it made us realize that the Earth itself has had this tremendous history of impacts. Enormous impacts that could have really damaged the whole planet.
[6:04]It got people thinking about what would be the effects of giant impacts on the Earth. Hartman realized that it wasn't just small asteroids hitting the planets, there were much larger objects too. And the larger the object, the more dramatic the consequences. So, the impact process, it's, it's wonderful kind of paradox, on the one hand, the small impacts tend to make everything the same. Millions and millions of impacts averaged out, but the big impacts give individual personalities to the planets. Take our planet, tilted on its axis at 23 degrees, with a nearby orbiting satellite, the Moon. We used to think that they'd been born together, until Hartman proposed a radical theory. The Earth had been hit by something the size of another planet, creating that tilt, and the Moon.
[7:07]The key to our idea was that as the planets grew, you had the finished planets, but you still had leftover bodies. If one of those crashes into the Earth, just as the Earth has finished forming, that can blow out material from which the Moon could form in orbit around the Earth.
[9:05]When Hartman suggested the idea in the 1960s, people found it hard to accept. Scientists were thinking of everything in terms of slow geologic processes, one grain of sand at a time, you know, wearing down mountains. To think of something as colossal as the Moon forming as a result of a single event was hard for people to swallow.
[9:39]But then Hartman got the first real clue that his theory might be true.
[9:47]The Apollo project.
[9:51]One small step for man, one giant leap for mankind.
[10:02]American astronauts made six visits to the Moon.
[10:08]They explored its surface, drove around its craters, and brought back 840 pounds of Moon rock.
[10:22]For the first time, scientists could find out what the Moon was actually made of. The lunar samples had a remarkably similar chemistry to the outermost layer of the Earth's crust. To most researchers, it was an interesting discovery, but to Hartman, it was vital new evidence. So you have crustal rocks, you have rocks on the surface, and a big impact comes in and blows all those crustal rocks away, and that material goes into space and forms the Moon. But many scientists were still skeptical. They just couldn't see how a massive impact could create the Moon and the Earth as they are today. Now, I actually had people telling me, we should exhaust every other theory first because this was such an outlandish idea. It was a chance meeting at a conference with astrophysicist Robin Canup that gave Hartman the breakthrough he was looking for. She was using computer models to study Saturn's moons.
[11:32]Bill Hartman came up to me after my talk and asked me, have you ever thought about applying your models about how moons form within and near Saturn's rings to the origin of the Moon? And I said, no.
[11:47]So she tried it. Canup used modeling software to recreate the early solar system. Then she plotted a planetary collision of the kind Hartman was suggesting.
[12:00]So we're four and a half billion years ago at the end of the Earth's formation. And we're in space and we're watching as a small planet, the planet on the right is about to hit the young Earth, represented by the larger planet on the left. The collision takes place and as we see it hit, it hits in a glancing blow. And you can see the impacter is completely destroyed by the collision. Thea, the impacter, is annihilated. Earth survives, but only just. Now, this collision is incredibly violent, so violent that there's enough energy to completely melt the Earth. And in fact, at the end of this impact, the Earth is surrounded by an atmosphere of vaporized rock. Trillions of tons of debris blast out into space. Here we see part of the impacting planet sheared out into this long arm of material that produces a disk that we're seeing almost edge-on in this view. And it's from that disk that the Moon later coalesces. Canup's model demonstrates that the Moon was probably made of debris from both Thea and Earth. It explains why those Moon rocks were so similar to rocks from the Earth's crust. So I actually called my colleague and said, you're not going to believe this, but I tried, I tried this Mars-sized impactor case with about a 45° impact angle, and everything worked. And he said, you better check it again. And so I did check it again and did many more of these simulations. And sure enough, that type of impact is the one that, uh, gives us the Earth-Moon system today. Canup's work was further evidence that Hartman's radical theory might just be right. So, it was very exciting as Robin did her models and they started to say, yes, there can be moon-forming debris left in orbit around the Earth and the Moon would form from that debris.
[14:03]Hartman and Canup's work showed that our Moon was created by a violent, cataclysmic collision between Earth and its twin Thea. The Earth narrowly survived complete destruction, but the collision triggered a series of events that transformed our planet. It became a climate hellhole with extreme weather conditions and giant tides. But bizarrely, those deadly conditions created the building blocks for life itself to evolve.
[17:10]If you actually look closely at these modern corals, you can see tiny lines that they build while they're growing. They're kind of similar like growth rings on trees. We know nowadays that one of these lines represents a day.
[17:32]If you count these daily growth rings, you can actually, in modern corals, count 365 growth lines per year. But the 400 million year old fossilized corals don't have 365 growth lines per year. They have 410. When these corals were alive in the ancient oceans 400 million years ago, the world was very different. A year didn't last 365 days, it lasted 410. But whether you measure it in days or hours, the Earth's orbit around the sun always takes the same amount of time. A year is always constant. The only explanation for more days in a year has to be that millions of years ago, each day was shorter. That means that back in the day when this animal was actually living in the in the ocean, that the days had less hours than they have today. 400 million years ago, a day lasted just 21 hours. And if days were shorter, then the Earth must have been spinning faster. Calculate back from 400 million years ago to 4.5 billion years ago, just after the huge collision, and each day would have lasted just 6 hours. And that means the Earth must have been spinning much faster than it does today.
[19:13]It seems the massive impact that created our Moon also set the Earth spinning like a top. It was the first step towards the habitable Earth we live on today, but you'd never know it. The rapid rotation unleashed the worst weather the planet has ever seen. It looked as though the collision with Thea had left the Earth an uninhabitable world. At 12 minutes past midnight on our clock, 40 million years after the Earth first formed, frozen comets started smashing in from space.
[19:51]It was a savage bombardment, but it wasn't a disaster. In fact, it was a blessing. The icy comets melted, helping to create the first oceans. But the young Earth was still a violent, hostile place. Its rapid rotation whipped up 500-mile-an-hour hurricanes, rain and storm-force winds scoured the planet's surface. And the atmosphere was a lethal cocktail of carbon dioxide and acid rain. It seemed that there was no way life could ever have got started. Were it not for one thing. When the Moon rose 4 billion years ago, it wasn't the familiar Moon we see today. It was 10 times closer to the Earth and dominated the horizon. Its proximity was another consequence of that huge collision, one that led directly to the emergence of life on Earth.
[21:01]Because the Moon was so much closer than today, its gravitational pull on the Earth was much stronger. It pulled hard on the Earth's newly formed oceans. The result, huge tides that ripped across the planet at hundreds of miles an hour. You might think these made it tough for life to get going, but you'd be wrong.
[21:50]Because what the tides stripped from the land, they gave to the ocean, creating the perfect environment for life to emerge.
[25:23]To get life started, you need a huge amount of minerals in the oceans that are free to mix and interact. The only way that the Earth could have gotten that was from the huge tides that the Moon gave when it was much, much closer. The tides ripped minerals and nutrients from the land and mixed them into the oceans, creating a primordial soup. Scientists think that chemical interactions in that soup created the very first amino acids and basic proteins, the building blocks of life.
[26:04]From these ingredients, the first primitive cells would eventually emerge. Life that might never have developed, were it not for the tides, tides created by the Moon, a Moon born out of catastrophe. A disaster that nearly destroyed the Earth, but without which, life might never have evolved.
