Chapter Content
Okay, so turning into a fossil, wow, it's harder than you think. I mean, almost every single living thing, like over 99.9% of them, just… vanishes. Poof! Your little spark of life goes out, and every single molecule you ever had, gets, like, eaten or washed away, and then used to build something else. That's just how it is, you know? And even if you manage to be part of that tiny fraction that turns into, you know, a little blob of microbes and doesn't get chomped on, the chances of actually becoming a fossil are still super slim.
To become a fossil, you need, like, a whole bunch of things to line up perfectly. First, you gotta die in the right spot. Only, like, maybe 15% of rocks can even preserve fossils, so, you know, kicking the bucket in what's gonna be a granite quarry isn't gonna cut it. You basically gotta get buried in sediment, leaving an imprint, kind of like a leaf in mud. Or, you know, decompose without any oxygen so that dissolved minerals can replace the molecules in your bones and your hard bits, and sometimes, even your soft bits, creating, like, a stone version of the original. And then, the fossil needs to, like, survive the Earth's movements, all that squeezing and folding and pushing, you know? It has to stay recognizable. And finally, and this is a big one, after hiding for millions, or even billions of years, somebody has to find it and think it's worth keeping.
They reckon that only about one in a billion bones actually turns into a fossil. If that's true, it means that all of the living Americans today, all 270 million of us, with our 206 bones each, will only leave, like, 50 fossils between us. That's like, a quarter of a full skeleton! Of course, even that doesn't mean that any of those fossils will ever be found. Remember, they could be buried, anywhere! And only a tiny, tiny bit of that land is ever gonna be dug up, and even less of it is gonna be looked at carefully. So, yeah, if even a few of those bones get found, it's basically a miracle. Fossils are, like, super rare in every sense of the word. Most of the creatures that have ever lived on Earth are just, gone. It's estimated that less than one in 10,000 species even has a fossil record. Which is a crazy small fraction. But if you go with the generally accepted idea that Earth has produced, like, 30 billion species, and that, you know, roughly 250,000 of them have fossils, then that ratio drops to something like one in 120,000. Either way, what we have is just the tiniest sample of all the life that's ever been.
And, the record we *do* have is, like, super unbalanced. Most land animals, obviously, aren't gonna die in sediment. They're gonna fall out in the open, and get eaten, or rot away, or get blown away by the wind and rain, you know? As a result, the fossil record is, like, massively skewed towards sea creatures, to the point of being, almost comical. About 95% of the fossils we've found are from animals that lived underwater, mostly in shallow seas.
I bring all of this up because, I went to the Natural History Museum in London one gloomy day to meet a really nice, kind of scruffy, paleontologist named Richard Fortey.
Fortey is a smart guy, seriously. He wrote this really funny and cool book called, “Life: An Unauthorized Biography” which covers the whole story of how life began. But what he's really into is a type of sea creature called a trilobite. They used to fill the oceans, but now they're gone, except as fossils. They basically had the same body structure, divided into three sections, or lobes. That's where their name comes from. Fortey found his first trilobite fossil as a kid while climbing around on the rocks. And he's been obsessed with them ever since.
He took me to this room filled with tall metal cabinets. Each cabinet had these shallow drawers, and each drawer was crammed with trilobite fossils. He had like, 20,000 specimens in total.
“It looks like a lot,” he admitted, “but you have to remember that trillions of trilobites lived in the ancient oceans for hundreds of millions of years, so 20,000 is not that much. And most of them are just, bits and pieces. Finding a complete trilobite fossil is still a big deal for paleontologists.”
Trilobites first showed up around 540 million years ago, right at the start of what's called the Cambrian explosion, when complex life just, appeared, seemingly from nowhere. Then, about 300 million years later, they vanished, along with lots of other creatures, during the Permian extinction, which is still a bit of a mystery. You know, like other extinct creatures, you might think of them as failures. But they were actually one of the most successful animals ever. They lasted for 300 million years. That’s twice as long as dinosaurs. Humans have only been around for, like, a tiny fraction of that.
Because they were around for so long, there were tons of them. Most were pretty small, like the size of a beetle, but some were as big as dinner plates! There were at least 5,000 genera, 60,000 species. And new ones are still being found. Fortey mentioned he was at a conference in South America recently, and this researcher from some university in Argentina approached him with a box full of stuff, trilobites, and other things, that had never been seen before. She didn’t have the resources to study them properly or to find more. He said that a lot of the world hasn't even been explored.
“You mean for trilobites?” I guess someone asked.
“No, for everything.” he clarified.
Back in the 1800s, trilobites were almost the only early complex life forms that we knew about. So, they were collected and studied a lot. One of the big mysteries with trilobites is how suddenly they appeared. Fortey said that even today, if you go to the right kind of rock formations and dig down, you know, through all those layers of history, you don't find any visible life, and then, all of a sudden, boom! A fully formed *Profallotaspis* or *Elenellus*, just jumps into your waiting hand. And these were animals with limbs, gills, a nervous system, antennae, a brain, and the strangest eyes. Eyes that were made of the same stuff that makes limestone, calcite. The earliest known vision system. And it wasn't just one adventurous species. There were dozens, all over the place. Back in the 1800s a lot of people used this as an argument for God, you know, against Darwin's theory of evolution. They asked, how could these complex animals just appear so suddenly if evolution is so slow? And the truth is, Darwin couldn’t answer that.
So the question remained unanswered until a paleontologist named Charles Doolittle Walcott made a big discovery in the Canadian Rockies.
Walcott was born in 1850. He was really good at finding fossils, especially trilobites. He built up a pretty impressive collection. Louis Agassiz, a famous scientist, bought the collection for his museum, and that money paid for Walcott's education. He became a major authority on trilobites, even though he didn't have much formal schooling.
In 1879, Walcott started working for the newly formed U.S. Geological Survey. He did so well that he became the director within 15 years. Then, he became the secretary of the Smithsonian Institution. But he still did fieldwork. He wrote a lot. He was also a founding board member of the National Advisory Committee for Aeronautics, which later became NASA. So, he could totally be considered one of the fathers of the space age.
But, we mainly remember him for a lucky find he made in the summer of 1909 in British Columbia. According to the story, Walcott was riding on a mountain path with his wife when her horse slipped and fell. Walcott helped her up and noticed that the horse had dislodged a piece of shale. Inside the shale was a fossil of a very old, very rare, crustacean. It was snowing, so they didn't stick around. But, the next year, Walcott went back. He climbed up to where the rock must have slid. There, he found a shale outcrop that was just packed with fossils. Fossils from right after the Cambrian explosion. He had found the paleontological equivalent of the Holy Grail. The outcrop became known as the Burgess Shale.
By reading Walcott's diaries, someone noticed that the story about the horse slipping might be a little embellished. But, it was still a huge discovery, no doubt about it.
It's hard to imagine just how long ago the Cambrian explosion was. If you could fly back in time at one year per second, it would take you about half an hour to reach the time of Jesus. It would take you over three weeks to get back to the beginning of humans. But, it would take you 20 years to get to the start of the Cambrian period. It was a completely different world back then.
First of all, when the Burgess Shale formed, it wasn't on a mountaintop. It was at the bottom of the ocean, at the foot of a steep cliff. The ocean was full of life, but normally, animals didn't leave a record because they were soft-bodied and decomposed. But, at the Burgess Shale, the cliff collapsed and buried the creatures in a mudslide, preserving them in incredible detail, like flowers pressed in a book.
From 1910 to 1925, Walcott spent every summer digging there. He collected thousands of specimens and brought them back to Washington. His collection was amazing. Some of the Burgess fossils had shells, but many didn't. There were over 140 different species.
Unfortunately, Walcott didn't seem to realize how important his discovery was. He classified the fossils using modern categories, seeing them as ancestors of today's worms, jellyfish, and other creatures. He didn't recognize how different they were.
Walcott died in 1927. For almost half a century, the Burgess Shale fossils were locked away in drawers at the Smithsonian. Nobody looked at them. In 1973, a graduate student named Simon Conway Morris went to see the collection. He was blown away by the fossils. They were so much more spectacular than Walcott had described. They were also much more diverse. Conway Morris realized that there were so many unique designs in those drawers that it was unbelievable that the original discoverer had missed it.
Over the next few years, Conway Morris and his colleagues systematically reclassified the entire collection. They kept noticing new things, making amazing discoveries. Many of the creatures had body plans that had never been seen before or since. For example, *Opabinia* had five eyes and a trunk with a claw at the end. *Peytoia* was shaped like a disk and looked kind of like a pineapple ring. And *Hallucigenia* had rows of stilt-like legs, and it was so bizarre that they named it after hallucinations. There were so many new and unrecognized things that one time someone heard Morris say, "Oh damn, there's not a new phylum in here!"
The reclassification showed that the Cambrian period was a time of amazing innovation and experimentation when it came to animal body plans. For almost 4 billion years, life had been slow and uneventful. Then, in just 5 to 10 million years, it created all the basic body plans that we use today. Every animal you can name, from worms to humans, uses an architecture that was invented at the Cambrian party.
The most shocking thing was that so many of those body plans just, disappeared. They didn't leave any descendants. Out of all the Burgess animals, at least 15 or 20 didn't belong to any recognized phylum. It looked like success in evolution was a matter of pure luck.
However, one animal did make it through. It was a little worm-like creature called *Pikaia gracilens*. It had a primitive spinal cord, which made it the earliest known ancestor of all later vertebrates, including us. There weren't many *Pikaia* fossils in the Burgess Shale, so who knows how close they came to going extinct.
There was a famous quote that made it clear that the success of our lineage was a huge fluke: "If you could rewind the tape of life back to the early Burgess Shale and replay it, the chances of anything like human intelligence re-evolving are negligible."
The book came out and started a lot of debate. A lot of scientists didn't agree with the conclusions, and things got kind of ugly. It soon became clear that the "explosion" of the Cambrian period had more to do with modern tempers than with ancient biology.
It turns out, that complex life was around way before the Cambrian period. We should've known it a long time ago. Nearly 40 years after Walcott's discovery in Canada, a geologist named Reginald Sprigg found something even older and more amazing in Australia.
In 1946, Sprigg was investigating abandoned mine sites in the Ediacara Hills. One day while he was eating lunch, Sprigg flipped over a piece of sandstone and was surprised to find that the surface was covered in delicate fossils that looked like imprints of leaves in mud. These rocks were older than the Cambrian explosion. He was looking at the beginning stages of visible life.
Sprigg wrote a paper about his discovery, but it was rejected. Then, he presented his findings at a conference, but the head of the conference wasn't impressed. The head said that the Ediacara imprints were just "fortuitous markings of inorganic origin." He left his government job and started exploring for oil.
Nine years later, a schoolboy named John Mason found a strange fossil in England. The fossil looked like a modern sea pen, similar to some of the specimens that Sprigg had found. The schoolboy gave the fossil to a paleontologist, who recognized it as something from before the Cambrian period. The schoolboy was in the newspapers.
Today, Sprigg's original Ediacara specimens are displayed in a glass case at the South Australian Museum. They don't attract much attention. The delicate patterns aren't very clear. They're mostly small and disc-shaped, with faint stripes.
What were these things? How did they live? There's no real consensus. They didn't seem to have mouths for eating or anuses for eliminating waste.
They were mostly simple, like jellyfish. Ediacaran animals were diploblastic, meaning that they were made of two layers of tissue. All animals today, except for jellyfish, are triploblastic.
Some experts think that they weren't animals at all. They could have been plants or fungi. Even today, the line between plants and animals isn't always clear. Sponges spend their entire lives in one place, without eyes, brains, or hearts. But they're animals.
There's a lot of disagreement about whether the Ediacaran animals were ancestors of any animals alive today. Many people see them as a failed experiment. They might have been outcompeted by the more mobile and complex Cambrian animals.
The animals that are alive today don’t seem very similar to them.
Some people believe that there was a mass extinction at the end of the Precambrian period, and that none of the Ediacaran animals made it to the next stage. In other words, complex life started with the Cambrian explosion. At least that's how some see it.
Almost immediately, people started questioning the interpretations, especially the interpretations of the Ediacaran animals.
I think, the best way to put it, is that the interpretations are now much more nuanced.
They argue that the Burgess Shale animals weren't as strange or as diverse as they seemed at first.
It's a difficult task to interpret ancient animals from incomplete and distorted evidence.
It turns out that the Burgess fossils weren't really all that different. The "Hallucigenia" fossil had been upside down. It's stilt-like legs were actually spines on its back. The pineapple-like thing was actually part of a larger animal called *Peytoia*.
So the Burgess Shale specimens weren't that unbelievable after all.
However, where did those animals come from? How did they appear so suddenly? That's still a difficult question.
Maybe the Cambrian explosion wasn't as explosive as we thought. It's now believed that the Cambrian animals were around a long time ago, but they were too small to see. The trilobites give a clue. It's because different species of trilobites seem to have mysteriously spread across the globe.
It suggests that they must have an ancestor, something that pioneered that lineage in the distant past.
Maybe we didn't find those earlier species because they were too small to be preserved. Even today, the ocean is full of tiny arthropods that don't leave a fossil record. There are tons of tiny copepods in the modern oceans. We only know about their ancestors from a single specimen found in the belly of an ancient fossilized fish.
The Cambrian explosion was more likely an increase in size, rather than the sudden appearance of new body plans.
Reginald Sprigg did get recognition. An early genus was named after him. The whole find was later called the Ediacaran fauna, named after the hills where he found the fossils. By that time, Sprigg had long since stopped looking for fossils. He built a successful oil company and retired to a ranch in the Flinders Ranges where he created a wildlife reserve. He died a wealthy man.