Season 1 Recap: The Hadean
Season 1 Recap: The Hadean (Ep. 4-23)
Welcome back! I hope you’ve had a nice break. I certainly have. I’ve been studying Michigan’s oldest fossils with two undergrads, fossil pond scum around 2 billion years old, half the age of the Earth. The podcast won’t cover these rocks until Season 6, but I might interview the students so you can hear what they’re up to. I’m also writing a book chapter on fossil pond scum, specifically fossils we’ll meet in Seasons 4 and 5. I’ll let you know when that’s coming out. So things are busy, but good, and I’ve finally been able to relax a bit as well.
But time marches forward, and it’s time to pick up where we left off.
Maybe you’ve binged Season 1 and want a quick summary. Maybe you’ve been waiting patiently for the past few months and want to dust the cobwebs off.
In either case, here’s a quick recap to get caught up.
For the past 20 episodes, Season 1 covered Earth’s earliest days, a time called the Hadean. On our imaginary Earth Calendar, the Hadean covers January 1st to February 14th. In real time, that’s 4.6 to 4 billion years ago. In the Hadean, we saw the origins of the Earth, the Moon, the first oceans and islands, the oldest crystals on the planet, and maybe the first life. Here are a few major arcs:
Episodes 4-7: The Solar System and Earth
Our Solar System formed 4.6 billion years ago, long, long after the Big Bang. On the outer edge of the Milky Way Galaxy, in the darkness between stars, a huge dusty cloud collapsed into itself. Most material fell into the cloud’s center, fusing into the Sun. The leftovers clumped into smaller asteroids, which then clumped into larger planets. The early Solar System was a violent, chaotic realm and many early planets were destroyed. Some were pulled into the Sun’s furnace, and many more crashed into each other. Dozens of worlds were winnowed down to just eight.
One of these lucky survivors was Earth. As the Earth grew, different materials settled into different layers- a core of dense iron, coated by a mantle of green olivine crystals. On the surface, an ocean of magma cooled into a crust of black basalt. Basalt is still made today by volcanos in Iceland, Hawaii, and many other places, but these modern rocks are much younger than the Hadean. In fact, there are virtually no Hadean rocks left on planet Earth- they’ve all been recycled away.
So how do we know this all happened? Most Hadean history is gathered from objects in space: asteroids and comets. Almost every asteroid that falls to Earth has the same age: 4.6 billion years old. Asteroids also have the same ingredients as Earth- iron, olivine, water, and carbon. Asteroids are one thing, but there’s a much larger and much closer relic of the Hadean world hovering just above us.
Episodes 8-9: The Moon
After 100 million years, the first week of the Earth Calendar, Earth was hit by a rogue planet the size of Mars, a leftover from the early Solar System. The titanic collision sprayed tons of rock into space, which clumped together into our Moon. The early Moon was much closer and much hotter than today, a red ball of molten rock the size of a fist held up to the sky. Over time, the Moon slowly cooled down and drifted away. By the end of Season 1, the Moon looks like a golf ball in the sky- smooth, pale, and closer than today. There are a few craters, but the Man in the Moon won’t show up until next season.
The story of a planetary collision might sound like science fiction, but we have evidence from Moon rocks collected by the Apollo astronauts, and by meteorite hunters down below. The Moon preserves far more Hadean history than the Earth because there’s no plate tectonics or flowing water to recycle rocks away. Speaking of which, let’s return to the Hadean Earth.
Episodes 10-15: Crystals, Oceans, and Islands
After the collision that formed the Moon, the Earth looked more like a metal album cover than the world we know today. The heat and force of the impact melted the crust back into an ocean of magma. The Earth was also spinning much faster. For reference, the modern Earth’s spin gives us a 24-hour day. Back in the Hadean, a single day was only 10 hours longs- 5 hours of daylight, 5 hours of night.
The short nights were filled with constant meteor showers, the last debris from the solar system’s birth. These shooting stars delivered cold ice from outer space, which melted into liquid water. Ponds grew into lakes, and lakes grew into seas, turning Earth’s surface from black to blue. The Hadean Earth became a waterworld, one vast ocean peppered with small volcanic islands. The oceans have survived until today, but the islands have not. In fact, nearly all of Earth’s Hadean crust has been recycled into new, younger rocks. So how do we know about these ancient oceans and islands?
The only solid pieces of the Hadean Earth are a few tiny zircon crystals, purple grains of sand scattered around the world. The best treasure trove is in the Jack Hills of Western Australia, miles from any city. The Jack Hills Zircons are the oldest stuff on Earth, 4.4 billion years old. Using the Jack Hills Zircons as time capsules, we can paint a hazy picture of our planet just after the Moon formed. Oxygen isotopes tell stories of the first oceans, hafnium isotopes of the first islands. But there are some stories that the Jack Hills just can’t tell- most importantly, the story of life.
Episodes 16-23: The Origins of Life
Just like the stories of stone and water, the story of life begins in space. The explosions of distant stars shoved carbon and hydrogen atoms together- the first organic molecules. These molecules were captured by asteroids and delivered to Earth by meteor showers. But the molecules weren’t alive just yet. Across Earth’s oceans and hot springs, organic molecules were mixed into a “primordial soup”. It took a little heat and a lot of time to build more complex materials: carbs, proteins, and fats, the building blocks of life and your diet. But they weren’t alive just yet.
Years of trial and error slowly formed even more complex molecules- DNA and RNA. RNA came first- it’s special because it can manipulate other molecules and make copies of itself. Eventually, RNA developed a “backup drive”, a library of chemical recipes that we call DNA. But DNA by itself isn’t alive.
Back in the primordial soup, fat molecules formed bubbles, just like olive oil or bacon grease in your kitchen. These fatty bubbles captured tiny pockets of primordial soup, mixtures of RNA, DNA, and proteins. Most bubbles were duds, popping back into oblivion. In some lucky bubbles, RNA and proteins stabilized the walls to build a tiny enclosure- the first cell. RNA did not do this consciously- but these stable cells survived longer than other bubble experiments. Eventually, RNA manipulated cells to dividing over and over, copying itself into future generations. Again, RNA did not plan this, but a stable cell making copies of itself is more likely to survive and change than a regular bubble in your kitchen. Finally, we can call these cells alive.
Some cells did very well, multiplying across the Hadean Earth. They looked like tiny bacteria, microscopic rods huddled around hot volcanic waters. Scientists call these lucky cells LUCA- our first ancestor. Over 4 billion years, evolution has copied and edited LUCA’s DNA into every living thing on Earth today. In short, LUCA is our greatest grandmother. We don’t have any fossils of LUCA, but we know these stories by looking carefully at the DNA and RNA it left behind inside us and other life on Earth- our distant cousins.
Summary
Let’s stand on the Earth at the end of the Hadean, 4 billion years ago. We’re on the shore of a small volcanic island just before dawn, watching ocean waves lapping on a dark sandy beach. If we turn around, the land behind us would have no plants, no animals- just black volcanic rock with steaming hot springs. If there’s any life in these springs or the oceans, it’s too small for us to see yet.
In the sky above, there is a constant backdrop of shooting stars, day and night. The sun and moon hurtle around the sky at twice their modern speed. The moon is noticeably larger and smoother. The sun is the same size, but less bright, as if someone turned down a dimmer switch. It is a stark but beautiful ocean world, a little closer to the modern day than the hellscape of the early Hadean. As the sun rapidly rises again, we pass from the Hadean into the next chapter of Earth history- the Eoarchean.
So what changed? What sets this next chapter apart? To answer that question, you’ll have to stick around for the next episode. For now, here’s one final thought for everyday life.
Everything on Earth is a gift from outer space. As Carl Sagan said- we are star stuff. The three greatest gifts were the minerals that built the planet, the ice that melted into oceans, and the carbon that grew into life. Every time you drink a glass of water, you’re drinking meteorite juice. Every time you look at the moon, you’re seeing the aftermath of Earth’s largest catastrophe. And every living thing you see is related, the great grandchildren of a tiny microbe huddled in hot water more than 4 billion years ago.
If you want to learn more, check out Season 1. Episodes 1-3 give you some Geology 101, but if you want to dive straight in, head to Episode 4. Whether you’ve been listening this whole time, or have just joined the party, you have my deepest thanks. For listeners old and new, stick around as we start Season 2 next episode.