Hank: Welcome back to our mini-series on the history of life on Earth! So far, we’ve covered the Archean and Proterozoic eons -- the first 3 billion years of life, which was mostly single-celled that whole time. Next comes the Phanerozoic eon, and here, we’re going to zoom in a bit: to the first chunk of the Phanerozoic eon, the Paleozoic era, which lasted from 542 to 252 million years ago.

Right at the beginning of the Paleozoic, there was a huge explosion of more complex life. And that’s when things started to get really interesting. The Paleozoic era is divided up into 6 periods.

And by the first one, the Cambrian, multicellular life -- including animals -- already existed. But it had not... done much. Animals were simple things like sponges. They didn’t have complex organs, and really, they weren’t much more than lumps, eating bacteria they strained out of the water.

All that changed about 542 million years ago, with the Cambrian Explosion. There was an increase in oxygen levels just before the beginning of the Cambrian, caused by a boom in life that produced oxygen using photosynthesis.

Scientists still debate how big this oxygen event was and when exactly it happened, but it might have made predation -- the predator-prey relationship -- possible for the first time. And the filter-feeding lumps got gobbled up.

Predation involves chasing after the things you want to eat, and that takes more energy than sitting on the ocean floor waiting for food to come to you. To maintain that high-energy lifestyle, predators needed a whole lot of oxygen. Once predators evolved, prey started to evolve better defenses and ways to run away -- which led to predators getting faster and better at capturing their prey. It was basically an evolutionary explosion.

Hard body tissues like shells and skeletons begin to show up in the fossil record just before the Cambrian. It took a lot of energy to produce that tissue, but it was worth it since these animals were less likely to be eaten. Officially, the beginning of the Cambrian was when animals started to burrow under the thick mat of bacteria on the ocean floor to escape predators.

These predator-prey relationships combined with other factors, like changes in the minerals in the oceans and flooding that opened up shallow habitats. This led to such an enormous boom in diversity that almost every major animal group that exists today evolved during the Cambrian -- including arthropods, molluscs, and the chordates that eventually gave rise to vertebrates.

The second period of the Paleozoic era was the Ordovician, which started 485 million years ago. The name comes from a Celtic tribe, since many of the best-studied rocks from the Paleozoic come from Britain. The Ordovician was when vertebrates first appeared. They were fish without jaws.

Then came the Silurian period, named for another Celtic tribe, which started 443 million years ago. Sometime during the Ordovician and into the Silurian, life made the jump to land. Moving to land wasn’t as easy as washing up onshore and going about your business. Life began in water, and living in water has some advantages over living in air. Water holds your body up, it helps you with gas exchange, if you release your sperm or eggs into the Big Blue, it’s much more likely that they’ll meet up with other gametes to reproduce.

None of that is true for air. You have to lift your body up, and in order to make the next generation you have to get physically close to each other. So the earliest land organisms had to evolve support structures, new respiratory systems, ways to avoid drying out, and methods of reproduction that were a little more controlled. Extremely simple plants might go back as far as the early Ordovician or even the Cambrian. They were spore-forming and had very little in the way of internal support.

The oldest fossil we have that isn’t a plant spore or a single-celled organism is a fungus called Tortotubus. It’s about 440 million years old, from around the Ordovician-Silurian cutoff. This fungus was a total game-changer. Tortotubus probably helped pave the way for more complex plants, and for animals too. It lived as an underground network of filaments, much like modern fungi. It might have formed mushrooms to disperse its spores, but we don’t know for sure. It fed by rotting the few other organisms on land, like early plants and microbes. By breaking down nutrients, it helped develop the soil on Earth’s surface. That helped complex plants grow and develop soil even further.

Now, there are Dr. Who creatures called Silurians. But they’re pretty badly named, because there were no land vertebrates during the Silurian period, let alone intelligent humanoid reptiles.

But! There were insects and other arthropods, earthworms, and other terrestrial invertebrates colonizing the land. They formed the first simple land ecosystems, along with plants and fungi.

The fourth period of the Paleozoic, called the Devonian, started 419 million years ago and ended 359 million years ago. It’s sometimes called the Age of Fishes, because it’s when the first fish with jaws appeared, and it was followed by a LOT of fish with jaws.

These fish, called placoderms, had tough, bony armor surrounding their skulls. They were the earliest vertebrates with jaws, and jaws were pretty useful for eating stuff, so they were a big success, in an evolutionary sense. Placoderms showed up in the middle Devonian, before the Devonian was over, the first tetrapods, or four-footed creatures with backbones, had already evolved. In fact, there’s evidence that tetrapods may go back 395 million years or more -- smack in the middle of the Devonian.

Which means that those fish -- the ancestors of birds, mammals, reptiles, and amphibians -- got around to having legs and crawling out of the ocean almost instantly, on an evolutionary timescale.

At the beginning of the Devonian, jawless vertebrates were the most complex life around. By the end of the Devonian: there were early, amphibian-like land dwellers walking around. That is a gigantic leap. Arthropods and land plants had a huge boom too, meaning those simple land-based ecosystems from the end of the Silurian were a lot more complex by the end of the Devonian.

Then, 359 million years ago, the 5th period of the Paleozoic began:, the Carboniferous period. You might’ve heard that fossil fuels are made of dinosaurs. But they’re actually much, much older than that. The Carboniferous was when land plants really started to establish themselves. The climate was mild enough for plants to grow year-round, and huge forests grew.

The word “carboniferous” means coal-bearing, and for good reason: hundreds of millions of years later, we’re digging up the remains of those forests as coal. The forests pumped oxygen into the atmosphere like crazy -- much more oxygen than there is today -- which led to the development of the first big land animals: arthropods.  Bugs, basically. They grew huge in the oxygen-rich atmosphere. That’s right. 350 million years ago, Earth was full of giant bugs.

Land vertebrates were still fairly small in the Carboniferous, but they did develop one major evolutionary innovation: the amniotic egg, which is the reason you can store a chicken egg without it drying out. Amniotic eggs don’t need to be kept in water, because they have a tough shell and membranes to manage gas exchange without letting the embryo dry out.

The reptiles that laid these eggs were less dependent on water than the first tetrapods, who still had to return to the water to lay their eggs. But the amniotes could spend their entire life cycle on land, and they got better and better at it. And they got bigger.

The Permian, the last period of the Paleozoic, began 299 million years ago, arthropods, It was the first age that was dominated by land vertebrates -- including the first big vertebrate land predators, like the fin-backed Dimetrodon. If you had a dinosaur-themed coloring book or toy set that featured Dimetrodon as a kid, you should know two things: First, dinosaurs didn’t evolve until after the Paleozoic era, during the Mesozoic era. Dimetrodon is way older than those guys! Second, Dimetrodon was on the same evolutionary branch as today’s mammals, not today’s reptiles and birds -- so it’s more closely related to you than to any dinosaur.

It was a member of the group of so-called “mammal-like reptiles” that came before the dinosaurs. Even though they weren’t technically reptiles, it can be a helpful way to think of them. Not mammals yet, but getting there. Dimetrodon was a carnivore, but there were synapsids that ate plants, as well. Like the similar-looking Edaphosaurus, which Dimetrodon probably ate.

Plant-eating was its own kind of evolutionary innovation, because herbivores couldn’t really survive until there were enough plants to sustain the animals that ate them. Plus, herbivorous animals had to evolve digestive systems that could extract nutrients from leaves, which is much harder and less energy-efficient than getting all your calories from meat.

So plant-eating was another major evolutionary development that happened during the Permian. At the end of the Permian, 251 million years ago, the Paleozoic era ended. And everything else nearly ended along with it. There was a mass extinction event so unimaginably widespread that it’s sometimes called the Great Dying. Something like 90% -- or more -- of Earth’s marine species went extinct. Most of those big synapsids died out, marine species were hit even harder, something so awful happened that life nearly met its match.

There were ice ages and smaller extinctions throughout the Paleozoic, but this one was the big one. So what was it? What caused the Great Dying? We don’t know for sure. The prime suspect is a plume of lava in present-day Siberia that was deposited 250 million years ago, just when the Permian extinction took place. This was a volcanic eruption of sorts, but if you’re imagining a Vesuvius or Krakatoa, think bigger.

A huge plume of heat welled up under Earth’s crust and melted it for hundreds of square kilometers. The region was flooded by enough lava to cover two thirds of the United States.

The reason this volcanic plume is such a likely suspect is because it could have done all sorts of life-ending things. It could have caused rapid cooling by blocking out the sun. It could have also set fire to buried coal, releasing carbon dioxide and causing runaway global warming -- there’s evidence for both kinds of temperature extremes. It could have released chemicals into the atmosphere that led to large-scale acid rain, or changed the chemistry of the oceans. We don’t know exactly what those eruptions did, but we know they did something and it probably wasn’t pretty.

Other suspects include methane-producing bacteria warming the planet, a catastrophe that somehow got rid of all the oxygen in the oceans, an asteroid impact. The Great Dying could also have been caused by the formation of the supercontinent Pangaea -- continents crashing into each other would have destroyed a lot of continental shelf habitat, killing some of the richest parts of the oceans. Having one big continent in one place would also have rearranged ocean currents and altered the climate.

But Pangaea formed a little too early to account for such a widespread die-out, and all the other hypotheses have their strengths and weaknesses, too -- none of them can explain everything. So, some scientists have suggested what’s known as the Murder on the Orient Express Hypothesis: #spoiler like in Agatha Christie’s classic novel, there are multiple culprits. It’s like an exam question where the answer might be “some of the above” or “all of the above.”

Whatever the cause, nearly everything died. But a few lucky life forms hung on, clearing the way for the archosaurs, the group that includes the dinosaurs. They were the dominant vertebrates during the Mesozoic era, which we’ll talk about next time.

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