(SciShow Intro)

Stefan: The Earth formed about 4.6 billion years ago, which is an almost unimaginable amount of time, but that's how long it took for the Earth to settle down and for life to develop in the forms we see today.  It was the longest miniseries we've ever done, but it was worth it.  It turns out, the history of our planet is kind of incredible, and now we've collected those videos in this compilation so you can watch the full series in one place.  Our first episode covers the first few billion years, which was just enough time for life to get going and for very simple, mostly single-celled organisms to form.

Michael: Where did life come from and how did it come to this? The path from bacteria to baseball wasn’t simple. In this miniseries, we’re going to explore the evolution of life on Earth. It’s been a wild journey, with plenty of twists and turns, and there were a lot of times we almost didn’t make it through.

This first episode is about two of the earliest geological eons: the Archean and the Proterozoic. Eons are the second-largest way to divide up Earth’s history, after supereons.  There was one geological eon before the Archean: the Hadean, which was 4.5 to 4 billion years ago, but because Earth’s rocks are constantly being destroyed and reshaped, literally all the rocks that old are gone. It’s hard to have geology without rocks, so we’re going to start with the Archean.

The Archean began about 4 billion years ago, and ended around 2.5 billion years ago when the chemistry of rocks began to change, and plate tectonics -- the way Earth’s crust moves around -- started to become more of a thing. That kicked off the Proterozoic Eon, which continued until 542 million years ago. These two eons are where life began and developed in complexity, though it took its sweet time about it. It’s also the time when life transformed the atmosphere, making it more suitable for some living things--but more dangerous for others.

Earth and the creatures that live on it have a complex relationship: we’ve influenced each other. The geology of the planet has guided the evolution of life, and life has shaped Earth just by existing. So Earth’s geology during the Archean shaped what were probably the first forms of life, but you wouldn’t recognize the planet. 

The atmosphere contained a lot of methane, ammonia, hydrogen, and carbon dioxide -- a type of chemical mixture that’s called a reducing environment, which means it can make electrons available for chemical reactions. That was probably good news for the molecules that would eventually turn into biological molecules -- they were able to react with each other and start becoming more complex. Carbon dioxide and methane are greenhouse gases, so the Earth was much hotter then, even though the sun was younger and fainter. In fact, there’s no evidence for polar ice caps or glaciers in the Archean. It was too hot for ice.

Meanwhile, the continents were solidifying. Plate tectonics have shuffled them around a whole bunch since then, but the innermost rocky centers of most of today’s continents date to the Archean. Volcanoes belched carbon compounds and water into the atmosphere. The oceans condensed pretty quickly from that water. And because there was almost no oxygen in the atmosphere, there was no ozone layer. UV radiation might have been pretty intense. If you hopped in the TARDIS and time-traveled to Archean Earth, you might think you were on Venus. That’s very different from the temperate climate and nitrogen-oxygen atmosphere we have now.

So what changed? Well, life happened. At some point in the warm oceans and carbon-rich atmosphere, biological molecules that contained the necessary information to copy themselves, and had the chemical ability to do so, formed from organic compounds. These became encapsulated in an oily membrane that kept them safe from the outside world -- the first things to resemble a living cell. And eventually, this early life transformed the atmosphere and climate.

Some scientists believe the earliest biological molecules were RNA, the molecule our DNA now uses to send messages outside a cell’s nucleus. It’s called the RNA world hypothesis. RNA is very similar to DNA, but it’s easier to form from simple components. It also stores genetic information the same way DNA does, but can tangle itself into shapes that make it easier for chemical reactions to happen, the way proteins do in our cells now. The RNA world hypothesis would explain why RNA is the go-between for DNA and protein in our cells.  But some scientists argue that it would have been too complicated for life to switch over from RNA to DNA and protein, and it’s more likely that all three evolved together. 

Whether life started as an RNA world or not, by the time the common ancestor of everything alive today came along, the system was based on DNA.

The earliest fossils show that life, in the form of bacteria, existed 3. 5 billion years ago, but a recent finding shows life could be even older than that. A team of researchers from California found evidence of life that’s 4. 1 billion years old. Now, I know I just said that there aren’t any rocks older than 4 billion years. And there aren’t, which means there aren’t any fossils either. But the team found resilient little crystals called zircons, which can be preserved when the rock surrounding them is destroyed. Then they get incorporated into new rocks. The zircons in question, found in Australia, contain traces of 4.1 billion year old carbon. There are lighter and heavier forms of carbon, called isotopes, and living things tend to have more of the lighter ones compared to the heavier ones. And so do these zircons.

For many scientists, it’s hard to believe life could possibly be that old. For one thing, Earth was pelted by asteroids 3.8 billion years ago in what was called the Late Heavy Bombardment. Next time you look up at the moon, check out the craters, a lot of them come from the Late Heavy Bombardment. And as you can probably guess from what happened to the dinosaurs, asteroid impacts aren’t great for the survival of living things. But some analyses suggest that the Late Heavy Bombardment would have only wiped out most life, not all of it, so if there were living things that old, some of them could have survived. Either way, we’re more confident about those fossils that are 3. 5 billion years old.

They’re called stromatolites, and they’re made up of layers laid down by films of bacteria. Stromatolites are pretty uncommon today, although you can find them living in Australia’s Shark Bay. But pretty much up until the time grazing animals evolved half a billion years ago and started eating all the bacteria lying around, they were an extremely widespread form of life. That means they basically ruled Earth for a solid three billion years. They may look like slimy lumps of rock, but stromatolites are actually kind of incredible.

Even if life only goes back 3. 5 billion years, that’s still pretty amazing, because it means that Earth was about a billion years old when the first life emerged. That’s not a whole lot of time, especially since the continents and oceans and stuff were still forming. 

So it might not have been that hard for life to get started. The hard part would have been staying alive once there was oxygen everywhere. A few kinds of microorganisms make stromatolites. One of them is cyanobacteria. And cyanobacteria are pretty special. Instead of acquiring energy from their environments, like the other first organisms, cyanobacteria could capture energy from the sun. They could photosynthesize.

Photosynthesis seems to have evolved in the Archean, but didn’t really kick into high gear until early in the Proterozoic. That’s because the water cycle needed time to work on the brand new continents. Water weathers continents. It carries sediment into the sea, and that creates a shallow region around the shore: the continental shelf. The continental shelf is great for photosynthesis because it’s shallow and gets plenty of sun. But photosynthesis has a nasty, chemically voracious, toxic byproduct: Oxygen, one of the most greedy, electron-stripping elements on the periodic table. It reacts with practically anything. It’s where we get the word oxidizing, for chemically stealing electrons.

Once continental shelves formed, the cyanobacteria started pumping out oxygen like there was no tomorrow. And for many of the anaerobic, or oxygen-intolerant, life forms on the planet, there was no tomorrow. They weren’t used to oxygen and couldn’t handle it. It was poison to them -- it reacted with and destroyed them. All that oxygen even changed the rocks, it reacted with iron that had been dissolved in the oceans, laying down bands of iron ore that we still find today. The Earth basically rusted.

But there was another problem. The cyanobacteria also pulled those carbon-containing greenhouse gases out of the atmosphere. Earth’s temperature plunged. In the early Proterozoic -- and again near the end, maybe twice -- the planet became a snowball. Life nearly poisoned and then froze itself to death. Cyanobacteria changed the atmosphere so much they destabilized the climate and changed the composition of the Earth itself. Eventually, the volcanoes coughed up more greenhouse gases and the snowball thawed out. Cold-tolerant forms of life managed to survive the crisis. And some organisms learned to not only put up with oxygen, but to use it to make energy. They became aerobic life forms, but many anaerobic organisms had to either find a place to hide or snuff it.

Life is easy, but surviving is hard, and so is becoming any more complex than a single, simple cell. 

Life stayed single-celled for 2 billion years or so, and it only changed because something really weird happened. That something weird was endosymbiosis. Sometime in the Proterozoic, before 2.1 billion years ago, an anaerobic cell ate an aerobic bacterium, one that used oxygen to produce energy. And the bigger cell never digested the smaller one. The smaller one kept on living, using oxygen and producing energy, and there was so much surplus energy that the bigger cell benefited from having the little guy in there.

Eukaryotic cells -- the ones with nuclei -- have the descendants of these aerobic bacteria inside them. They’re our mitochondria. That convenient energy arrangement paved the way for life to become more complex, and all because of a weird fluke. Except... maybe it wasn’t that weird, because it happened again. The ancestor of plants engulfed a cyanobacterium, which kept right on doing its photosynthesis thing, and now plants have chloroplasts.

The earliest evidence for multicellular life is 2.1 billion years old. But it could have just been a colony of single-celled organisms. The earliest multicellular eukaryotes we can be fairly confident about are 1.5 billion years old. Animals didn’t show up until 600-800 million years ago. Late in the Proterozoic and into the next geologic time period, animal life established the groundwork for every kind of animal body plan that still exists today.

Stefan: So we've got life, although it is still fairly simple, but in the next stage of Earth's history, things start to get really interesting as evolution takes off and we see a huge explosion in the diversity of life.  

Hank: 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 out.

Stefan: So we almost didn't make it there, but mass extinctions cleared the way for new forms of life to dominate, and in this case, it was the dinosaurs.  

Hank: Life had been brought to its knees by a mass extinction at the end of the Paleozoic era. The Mesozoic era, from 251 to 65 million years ago, followed the great extinction and would produce some of the weirdest and most fascinating animals of all time, including the dinosaurs! 

It also led to most of the major land animal groups we know today. Like other eras, the Mesozoic is divided into periods -- in this case, three of them.

The first period, the Triassic, lasted from 251 to 199 million years ago. It was a time of transition, when the dominant vertebrates of the late Paleozoic, the therapsids, pretty much disappeared. A new group of reptiles, the dinosaurs, then rose to become Earth’s new dominant land vertebrates.

Throughout the Mesozoic, Earth was warmer than it is now, and had no polar ice caps. At the beginning of the Triassic, Earth’s landmasses were lumped together into the dry supercontinent Pangaea. Slowly, life started to repopulate the place.

Repopulating meant diversifying. The therapsids were declining, but a new group of vertebrates was starting to take over -- the archosaurs. Archosaurs descended from one of the earliest major groups of land vertebrates, the diapsids, at the end of the Paleozoic.

These major animal groups -- the archosaurs and the diapsids -- are defined by the holes in their skulls, which attach to muscles and mean that the big, heavy bones weigh a little less. It might seem like kind of a strange way to tell animals apart, but it’s actually a very clear marker: Diapsids have two openings behind their eyes. Archosaurs have two extra openings, one in front of the eye and one in the lower jaw.

Dinosaurs are archosaurs, but so is another group that almost took over instead of the dinosaurs: the pseudosuchians, which evolved very similar body plans to the dinosaurs that came later -- including standing on two legs. They came in a lot of different shapes and sizes, but if you want know what a pseudosuchian looked like, picture something a bit like a crocodile, but about twelve times more terrifying, because it’s able to stand up straight on its long legs and run really fast over land.

Pseudosuchians were nearly wiped out in another mass extinction at the end of the Triassic. Only one lineage survived, one that took to living in swamps and gave rise to modern crocodiles and alligators. Although thankfully, the modern versions don’t run around on two legs. 

The only other archosaurs around today are birds.  So the archosaurs took over from the therapsids, which had mostly died out at the end of the Paleozoic. But some therapsids hung on through the Triassic, and some didn’t die out at all -- luckily for us. The therapsids are the descendants of another major branch of land vertebrates, the synapsids, who are also classified by their skull holes.

Synapsids have one skull opening, not four, and they’re the ancestors of mammals. The earliest mammals appeared in the middle of the Triassic -- at practically the same time as dinosaurs. Yes -- we mammals started out as dinosaur buddies, and we still hang out with dinosaurs now, just in bird form.

Speaking of dinosaurs: there probably weren’t many Triassic dinos in the coloring books you had when you were six, because they hadn’t developed much yet. But they had some advantages that would eventually make them the ruling reptiles.

Like I mentioned earlier, pseudosuchians and dinosaurs had very similar body plans, but dinosaurs had a slight physiological edge: their breathing was more efficient, and while both groups evolved legs that were positioned straight under them instead of sprawling to the sides, dinosaurs were the stronger movers. The earliest dinosaurs that we can be confident about go back 230 million years.

But there are some very dino-like animals from 10 million years before that. An animal called Nyasasaurus may or may not be a true dinosaur, depending on who you ask, but it’s definitely close. And it comes from 243 million years ago in present-day Tanzania.

As is often the case with evolution, it’s hard to draw the line between true dinosaurs and their immediate ancestors. But somewhere between Nyasasaurus and later dinosaurs like Eoraptor, they had officially evolved, ready to take over the world.

A couple of other animal groups turned up during the Triassic. One was the ichthyosaurs, the first reptile group to become fully aquatic again after evolving a land-based lifestyle. They’re also one of only two groups to evolve a fish-shaped body from a four-footed animal body. The other group being the whales.

Finally, toward the end of Triassic, a group of archosaurs that were closely related to the dinosaurs -- but weren’t dinosaurs themselves -- evolved the power of flight: the pterosaurs. 

After the Triassic came a period that you might have heard of: the Jurassic period, which lasted from 199 to 146 million years ago. Although I do feel like I need to point out that some of the dinosaurs in the movie franchise are partly or totally made up, and others aren’t from the Jurassic at all.

During the Jurassic, Pangaea was beginning to separate into two continents, Laurasia and Gondwana. Shallow seas covered parts of the land. This is when dinosaurs diversified into their more familiar forms.

The Jurassic was a great time to be a sauropod, for example: a huge, long-necked plant-eater that walked on four legs. Diplodocus, Brachiosaurus, and Apatosaurus all lived during the Jurassic. Then there were the theropods, the meat-eaters that walked upright.

Allosaurus was one major predator, but even bigger and meaner theropods were yet to come. The Stegosaurus also evolved during the Jurassic. It was a big plant-eater with plates all along its back and a spiked tail weapon called a thagomizer, because if you’re going to pick a name for a giant spiky tail-weapon you might as well make it awesome.

Meanwhile, the Plesiosaurs, a group of reptiles not closely related to dinosaurs, joined ichthyosaurs in the oceans. There was one other major group of dinosaurs that appeared in the Jurassic: birds. We know that birds are descended from dinosaurs because of the similarities of their skeletons, and the fact that many dinosaurs had feathers.

And because people who study evolution like to include all of a group’s descendents in that group, birds technically are dinosaurs. So, if you’ve ever fed a chicken nugget that’s shaped like a dinosaur to a child: that’s a weird experience. That’s a whole, strange thing.

Archaeopteryx, which is usually considered the earliest bird, dates back to the Jurassic, and so do lots of other early birds. They, along with the pterosaurs, were the two kinds of flying archosaurs during the Jurassic -- and during the next period, the Cretaceous. The Cretaceous, which means chalk-bearing, lasted from 146 to 65 million years ago and was even warmer than the earlier Mesozoic.

The continents continued to drift apart, heading for where they are now. As the seafloor spread, it released carbon trapped in the Earth’s crust and caused some serious global warming. 

Ichthyosaurs disappeared sometime during the Cretaceous.

But a new type of marine reptile appeared: the mosasaurs, aquatic lizards related to the monitor lizards we have today -- but not closely related to dinosaurs. Another new arrival? Flowering plants, which were excellent at getting animals to spread their pollen. That’s why, at the same time as flowers, we see pollinators like bees appearing in the fossil record.

Whether flowers or pollinators came first is a kind of evolutionary chicken-and-egg question. Probably neither one of them came first, exactly. The flowers and pollinators influenced each other’s evolution and became more interdependent as time went on.

Mammals -- which, you’ll remember, had been around since the Triassic -- evolved into the major lineages alive today: placental mammals like us, marsupials like the opossum, and monotremes like the platypus. The Cretaceous also meant even more dinosaurs! Like the frilled ceratopsians, the duck-billed hadrosaurs... and, of course, Tyrannosaurus rex. I don’t know about you, but I think T. rex is pretty cool. I’m also very glad predators that size aren’t around today to snack on us.

Why aren’t they around any more? Like the Paleozoic, the Mesozoic ended in a mass extinction. But there were a few differences between the two die-outs. For one thing, the extinction at the end of the Mesozoic wasn’t as bad. Only about 50% of Earth’s species went extinct, which is a lot, but not nearly as many as during the extinction at the end of the Paleozoic, when almost all life died out.

And while we don’t know exactly what caused the earlier Paleozoic extinction, we have a major clue about the event at the end of the Mesozoic. It’s a crater in the Yucatan region of Mexico. Most scientists agree that a meteor impact at this site must have been what wiped out all of the dinosaurs except for birds. There might have been other factors at play, but the meteor didn’t help.

So most of the diversity dinosaurs had to offer is gone for good. Sure, birds are cool, but they only represent one lineage of dinosaurs. Those big four-footed plant eaters and the walking around armored vehicles? They’re not around anymore. But once they were gone, mammals had a chance to take over -- which is what happened during the Cenozoic, the era that we are still in.

Stefan: So dinosaurs are pretty cool, but finally, it's our time to shine.  It took another 65 million years or so, but who's counting?  

Michael: The Cenozoic is divided into three periods. The names of the periods have been switched up a bit recently, but these days the era is divided into the Paleogene, Neogene, and Quaternary periods. And each period is subdivided into even smaller units of time, called epochs.

So, first, the Paleogene period: It covers the time from the extinction of the dinosaurs 65 million years ago to about 23 million years ago, and it’s divided into the Paleocene, Eocene, and Oligocene epochs.

The Paleocene, from about 65 to 56 million years ago, came right after the extinction of the non-avian dinosaurs, and even the ones that didn’t die out – the birds – took a big hit on the diversity front.

The dinosaurs left behind huge ecological shoes to fill. There were lots of feeding strategies and body plans that suddenly weren’t being used. The place an organism fits in its environment is its niche, and usually two animals can’t use the same one at the same time. With the dinosaurs gone, the mammals started exploiting those niches. So that’s why mammals expanded a lot in diversity during the Cenozoic, even though they’d existed since the early Mesozoic.

By the Eocene epoch, from 56 to 33. 9 million years ago, mammals had diversified into some pretty neat forms – including orders that still exist, like rodents and primates, but also some that don’t, like the enormous and bizarre titanotheres and uintatheres. One little antelope-like mammal had even wandered into the sea to become the ancestor of whales.

The Oligocene, from 33.9 to 23 million years ago, saw the introduction of carnivores – but not what we usually mean by “carnivores.” There were already animals that ate meat. I’m talking Carnivora: the taxonomic order of mammals that includes cats and dogs. There were also lots of different kinds of rhinos all over the place.

The Paleogene was warm. Despite the mass extinction, the climate carried on more or less the way it had in the Mesozoic: balmy, with no polar ice caps. But all that was about to change, because the continents were shifting. Antarctica drifted over the South Pole and was surrounded by a cold current, which led to global changes in the circulation of the oceans.

Antarctica started to ice over. The later two periods of the Cenozoic, starting with the Neogene, were characterized by the rise to prominence of one extraordinary life form. It changed the course of evolution for every species that encountered it.

I’m talking, of course, about grass. Grass is so common that most of us probably don’t think about it. It just... belongs on the ground. Always has. But grass is a relative newcomer to the evolutionary scene. The first grasses showed up just before the end of the Mesozoic, but the C4 grasses, so called because of the way they process carbon, only showed up between 25 and 35 million years ago.

Those are the important ones, and the major changes they influenced mostly happened within the last 10 million years. Grass is so important because it’s hard to eat. It’s tough, low in nutrients, and it has little bits of silica incorporated into its tissues specifically to discourage herbivores.

Technically called phytoliths, they’re basically sand. And chewing on sand is less than amazing for your teeth. Rather than not eating it, a lot of mammals just got really good at chewing and digesting grass. They evolved teeth with high crowns more resistant to being ground down. They evolved complex stomachs, like the four-chambered arrangement in cows, to extract as much nutrition as possible. And they evolved long legs adapted to running around in the new, open grassland habitats.

Horses and antelope were the big winners in the Neogene. But it wasn’t just them. Grasses have become so widespread that all kinds of creatures depend on them for food – including us.

We don’t eat the leaves with the sandy bits in them, but most humans depend on grain like corn, wheat, and rice – all grasses. We also feed grass and grain to our livestock. That means, for the rest of the Cenozoic up to the present, that the evolution of mammals was tightly bound to the spread of grassy habitats.

Paleogene herbivores had been mostly browsers: animals that eat leaves from trees and shrubs. In the Neogene, they were outnumbered by grazers. The Neogene is divided into two epochs: the Miocene and the Pliocene.

In the Miocene epoch, beginning 23 million years ago, the continents were already close to where they are today. Ocean circulation became more modern too, which meant things were cooling down. The giant shark C. megalodon patrolled the oceans. 

Grazers like horses and camels were all over the place. And toward the end of the epoch, in eastern Africa, a group of apes was learning how to walk on two legs.

At some point before or during the Pliocene epoch, from 5.3 to 2.6 million years ago, North and South America crashed into each other. During the Pliocene, animals crossed the new land bridge and switched up their places on the continents. Opossums colonized North America – and as anyone who’s driven around here knows, they stuck around. Camels and bears moved into South America, and they’re still there too.

And in the Afar region of Africa, there lived the early human relative Australopithecus afarensis. Australopithecus’s upright body plan was adapted to a shifting climate. And by the end of the Neogene, that climate was shifting quite a bit.

Antarctica had already started to freeze into the southern polar ice cap, and in the Pliocene the Arctic began to get chilly too. This was the first time Earth had had ice caps for a long time, possibly since the early Paleozoic. So by the time the Quaternary period came around, starting 2.6 million years ago, things were a little different from the mild times that came before.

The Quaternary period is divided into the Pleistocene and Holocene epochs. You might recognize that last one as the epoch we’re in now. The Pleistocene epoch, from 2.6 million to twelve thousand years ago, is sometimes called the Ice Age. But it was more like a series of ice ages, with ice sheets advancing over the Earth and then receding in dozens of cycles.

The reason the ice sheets advance and retreat in cycles has to do with minor, predictable variations in the Earth’s orbit. When the ice sheets get more sun, they melt more than they freeze, and vice versa. Atmospheric carbon dioxide also tracks closely with global temperature during these cycles. When CO2 drops, the temperature plunges, too.

We coexisted with lots of cold-weather organisms in the Pleistocene, like woolly mammoths, saber-toothed cats, and the actually-real-not-just-from-Game-of-Thrones dire wolf. Many of these are extinct, even though we aren’t.

The post-mortem on the Pleistocene megafauna seems to be some combination of the climate variations that caused the glacial cycles, and the arrival of hungry humans with pointy sticks. So-called archaic humans, also sometimes called Homo heidelbergensis, date to around 400,000 years ago. Then anatomically modern humans showed up, a little less than two hundred thousand years ago.

Homo sapiens – that’s us – also coexisted with other branches of our human family tree, like the Neanderthals. We’re the only ones left, but for a while there were a handful of different species of humans running around at the same time. These early humans hadn’t yet developed the complex cultures and traditions that make us truly ourselves, but they mostly just needed time. There’s evidence for art as old as 40,000 years. Our big brains probably evolved as an adaptation to the unpredictable climate. With the glaciers coming and going, we needed flexibility to survive. Also, tools and fire helped.

We are technically still in the Pleistocene Ice Age, in what’s called an interglacial period, even though we consider the Pleistocene epoch to be over. The ice is supposed to return eventually – just not yet.

The most recent epoch, the Holocene, is a tiny slice of time, covering only the most recent warm interglacial cycle. That’s just shy of 12,000 years ago. It’s not the formal definition of the Holocene or anything, but that period of time also happens to correspond to humans learning to farm and keep animals.

We started this miniseries with life emerging nearly 4 billion years ago. Two hundred thousand years of human history isn’t much compared to that, and our actual recorded history – compared to prehistory – is, on a geological scale, VERY short. We’re basically a blip, but even though we’re a relatively young species, we’ve already had a lot of influence on Earth’s geology through things like nuclear tests and our use of plastic.

A group of scientists has argued that this is enough to define the start of a new epoch within the last century: the Anthropocene, or human epoch. The powers that be in geology haven’t adopted this term yet, but it’s often used informally. So, welcome to the Anthropocene, the latest slice of time in geologic history. 

Stefan: Thanks for revisiting our miniseries on the history of life with us.  It took a long time for it to get to where it is today, but the diversity of life on our planet is amazing, and there's so much more to the history of life on Earth than we were able to cover in this series.  So if you love learning about this stuff as much as we do, you can check out our sister show, Eons, which is all about the most bizarrely fascinating things that have happened on this planet over the last 4.6 billion years.  

From sabre-tooth salmon to dinosaurs with hollow butts, just head over to youtube.com/eons and subscribe.  

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