24: A New Dawn

Part 1: The Eoarchean

Welcome to Season 2 of Bedrock! If you’re new to the show, I’d recommend pausing here, and backing up an episode for a healthy recap of Season 1. If you’re on a binge, more power to you.

Last season, we covered Earth’s earliest chapter, a time called the Hadean. In the Hadean, we saw the dusty origins of the Solar System and the Earth. We saw the Moon born from a titanic planetary collision. We saw legions of asteroids and comets bring water to make the first oceans. Last but not least, the same asteroids brought organic molecules, starting the recipe for primordial soup and eventually, the first life on Earth. All this happened between 4.6 and 4 billion years ago. On our imaginary Earth Calendar, that’s January 1st to February 14th.

Now let’s look forward to Season 2, a brand new chapter in Earth history. Today, we’ll learn the chapter’s name, what separates it from the Hadean, and how geologists divide prehistory into these different chapters. Next episode, we’ll dive back into the past.

So, what came after the Hadean? The next big chapter of Earth history is called… the Archean. The word Archean comes from ancient Greek, meaning origin or beginning. It’s where we get the words archaic and archetype. In other words, Archean means really old.

The Archean covers the first half of Earth history, all the way to June on the Earth Calendar. In real time, that’s 4 to 2.5 billion years ago. The Archean is a much longer chapter than the Hadean, so geologists carve it into 4 smaller slices. For your sanity and mine, this show will do the same: each slice of the Archean will be its’ own season.

Now that we have the big picture, we can finally introduce Season 2. Drumroll, please!

Season 2 will last from February to March on the Earth Calendar, 4 to 3.6 billion years ago. This time is called the Eoarchean (ee-oh-ark-ee-an). I know that’s a mouthful, but we’ll get used to it. Say it a few times just to get familiar. Hopefully there’s no one else around, or that might look awkward.

Scientists didn’t name the Eoarchean just to be obtuse- the name means something.

We know that Archean means “beginning”, but what about that Eo- part?

Eo comes from Eos, the Greek goddess of the dawn. That’s a fitting name for this time. The Eoarchean is not a dawn of the sun, but a dawn of new information.

So all together, Eoarchean means the Archean’s dawn, or the dawn of the beginning. That might sound strange, since we’ve already seen Earth’s beginning. In fact, we’ve spent 600 million years and 20 episodes just to get here, the so-called “dawn”. So, what gives?

To answer that question, we need to learn how geologists divide Earth’s past into different chapters.

Part 2: A Wrinkle in Time

Let’s start with human history. Humans have only been on Earth for 300,000 years, the last half-hour of the Earth Calendar. You could simply lump all that history into a giant “Age of Humans”, but it makes more sense to split it into different chapters. For example, the Stone Age, the Bronze Age, and the Iron Age are based on human tools. These chapters help scientists study the past, but there are some problems when carving up time. The first is… timing. Most major cultural shifts don’t happen overnight, and they don’t happen everywhere at once. For example, the Iron Age began in Asia centuries before arriving in northern Europe, and several groups of modern people still don’t use iron tools.

Another issue is choosing the right name for a slice of time. The names Dark Ages and Renaissance are loaded terms, describing a world oppressed by plagues and backward thinking before a new interest in art and science. But there were plenty of philosophers and beautiful art before the Renaissance, especially outside Europe. The Dark Ages weren’t entirely dark. In short, dividing history into chapters helps track change over time, but we need to beware the pitfalls.

Let’s take these ideas to Earth’s truly ancient past before humans. Just as the Dark Ages weren’t always dark, the Hadean of Season 1 wasn’t always hellish, and the Eoarchean of Season 2 certainly wasn’t the beginning. For now, the names have stuck.

Names are one thing, but let’s return to the main question of this episode: how do geologists carve time into separate chapters? How do they know where to make the cut?

Early geologists noticed changes in fossils over time. One layer of rock could have a very different fossil menagerie than layers above or below. In the early 1800s, scientists across the world started to compare notes and began to see broader patterns. For example, there was clearly a time when giant reptiles roamed the Earth, which then suddenly, mysteriously, disappeared, replaced by more familiar creatures like dogs, cats, and horses. Without getting too technical, fossils were placed into the Age of Fish, the Age of Dinosaurs, and the Age of Mammals. These chapters were carved into even smaller slices, such as the famous Jurassic or Cambrian Periods. Fossils come, fossils go, and when they do, scientists mark a different chapter in Earth history.

Things were going great, and geologists kept digging deeper down, eager to find even older fossils. And then, they ran out of road. Around the world, scientists found a clear division: younger rocks full of fossil bugs, shells, and fish, sitting above older, deeper rocks with no fossils at all, not ones they could recognize. The fossil-rich layer was called the Cambrian, and this apparently sudden appearance of life was eventually called the Cambrian Explosion or Radiation. The older rocks were simply called the Precambrian. As we discussed in Episode 20, the Precambrian was one of Darwin’s big head-scratchers- if evolution was real and gradual, how could he explain this chapter break? As we’ll see in future seasons, this question has been answered to a degree. Turns out, there are plenty of Precambrian fossils, you just need to know where to look. However, these bacterial fossils aren’t as useful of clocks as shells or bones.

So how do geologists divide the 4 billion years of the Precambrian, without fossils to help? Let’s look at one specific case- the man who sliced time in half.

Part 3: Married to the Rocks

The year is 1848. Charles Darwin has begun to write The Origin of Species. The word “dinosaur” has just been invented. But our story takes place far away in the Canadian wilderness, on the northern shores of Lake Huron. It’s a summer night in the Northwoods. A few men are sleeping on the beach, covered in blankets to protect from mosquito clouds. All the men, except one. A middle-aged man is writing notes dangerously close to the fire. His red hair and beard are matted with sweat, his clothes are torn and patched after long days in the field, and he is quietly murmuring to himself. He looks like a madman at the edge of the world.

But if we peek over his shoulder, we see beautifully drawn maps and thoughtful notes. He is writing about a new discovery- a pair of very different, very ancient rocks. The man is William Edmond Logan, one of Canada’s greatest scientists, and he will help define the Archean chapter of Earth’s history.

William Logan was born in Montreal in 1798, but spent his youth in the UK, taking lessons in art and math. He was a good student but dropped out of college to work as an accountant for his uncle’s mining company. Logan was a hard worker, spending many evenings at the office instead of hitting the bars. He soon realized the mines lost valuable time and money searching for coal. The local maps just weren’t very good. Logan’s dedication and artist’s eye soon produced incredibly accurate geology maps. He could point to a spot, tell you how deep the coal was, and he would be right.

Word of Logan’s skills quickly spread around the UK. When Parliament needed someone to survey Canada’s geology, Logan jumped at the chance. It only paid half of his old accounting salary, but by this point he was hooked on geology.

I could spend a whole episode listing Logan’s achievements- he founded the Geological Survey of Canada, received a knighthood from Queen Victoria, and the French Cross from Napoleon III. After he died, Canada’s highest mountain was named Mount Logan – if the mountain had a name before, I couldn’t find one. Despite all these accolades, Logan never let them go to his head. Just like his younger years, Logan was addicted to work. He was single his whole life, claiming he was “married to the rocks”.

As Logan traveled across Canada, he noticed a pattern in the ancient stones he saw- two different types of rocks sitting next to each other, over and over again. Neither of these rocks held fossil shells or bones, but a simple look showed a few key differences. Logan discovered that the older rocks were squeezed, folded, and tilted like a broken accordion. In contrast,

the younger rocks had even, horizontal layers and more importantly, held a lot more iron, which would become a backbone of Canada’s industry.

Logan knew he had found two distinct chapters within the massive Precambrian Era, but he would never know just how old the division was. It would be more than a century before a proper date was found- roughly 2.5 billion years ago. By this point, scientists had found similar division around the world- older, altered rocks, separated from younger, iron-rich rocks around 2.5 billion years. It was finally time to give these chapters proper names, the names we met at the beginning of the episode.

For now, we’ll focus on Logan’s older, jankier rocks. This giant stretch of Earth history is our new friend, the Archean, from 4 to 2.5 billion years ago. Eventually, geologists would slice the Archean into smaller chunks, including the Eoarchean, the spotlight of Season 2.

Before we finish, there’s one final question we need to address: Logan and others helped define the Archean’s end, 2.5 billion years ago, but not the beginning, where the show currently is. What boundary separates the Hadean and Eoarchean 4 billion years ago? Was there a huge catastrophe, a mass extinction, or something weirder? The real answer is annoyingly simple: the Eoarchean’s start is defined by Earth’s oldest rock. As of this episode, the oldest rock is 4 billion years old. If we find a rock 4.1 billion years old, then the Eoarchean’s start will be pushed back in time. So it’s very possible that the dates I’m giving here will be outdated in the next few decades.

If that sounds un-scientific, you’re not alone. In future seasons, we’ll see that many chapter breaks are even more arbitrary, and there are many researchers who are crying out for updates. For now, these chapter breaks are the best thing we have, and they make our lives easier as this podcast moves forward. For now, Season 2 and the Eoarchean will start 4 billion years ago, and it’s high time to meet the rocks that mark this boundary.

Summary:

The Precambrian covers 90% of Earth’s history and is divided into many different chapters. Sometimes there’s a clear reason for a chapter break- a new fossil, a mass extinction, or a global change in the rocks. But many other breaks are simply placeholders while we look for better alternatives. Seasons 2 starts 4 billion years ago, but that could change if we find older rocks.

There’s been a lot of names and dates flying around this episode, so thanks for hanging in there.  For now, there are only three names to remember:

1)    The Hadean, Season 1 that we just finished.

2)    The Archean, Seasons 2-5, the next big chapter, which includes…

3)    The Eoarchean, Season 2 going forward.

Next episode, we’ll return to the Canada and finally start the Eoarchean, the oldest rocks on Earth.