era at a time.

If you haven't been on that trip with us yet, those videos, they're all in a playlist down

below.

And by now, you're probably tired of hearing us tell you that you're related to all of

these bizarre organisms that look nothing like you.

Like, in the Mesozoic Era, we introduced you to the Megazostrodon, a little insectivore that

lived among the dinosaurs as one of the earliest known mammals.

At least that's a mammal, so you can see the connection, right?

But what about Dimetrodon?

It lived in the era before the Mesozoic, in the Paleozoic.

It's not our direct ancestor, but it was a stem mammal, part of the group of animals

that descended from reptiles to give rise to mammals.

And when you look at it, well … it's not exactly like looking in a mirror, is it?

By the time we follow our lineage back even farther, to LECA, the ancestor of all

eukaryotes, and LUCA, the single-celled ancestor of everything that's alive today, we're

talking about forms of life whose lives and structures we can only speculate about.

But, now you've arrived at the Cenozoic Era.

And in fact, you've always been there!

Because that's the era we're in now!

And the Cenozoic is when many organisms took shapes and behaviors that you could actually

recognize.

Most of the mammals and birds that you can think of appeared during this era.

And reptiles went through some surprising changes, but they eventually settled into

the ranges they inhabit today.

But perhaps more importantly--for us at least-

the Cenozoic marks the rise of organisms that look a lot

like you and me.

OK to be fair, if you traveled back to the start of the Cenozoic Era, 66 million years

ago, there would still be a lot that you would not recognize.

It was so warm that the whole world was full of tropical and subtropical forests, even

at the poles.

And for about the first 10 million years of the Cenozoic, the world was still recovering

from the K-Pg extinction event that wiped out the non avian dinosaurs.

This was the very beginning of the Paleogene Period, and the world was … kind of empty.

Along with the dinosaurs, almost all other large land vertebrates had vanished.

Many terrestrial plants were gone too, and in the oceans, the giant marine reptiles and

even most of the plankton had disappeared.

Because of this scarcity of life, during the first chapter of this period, known as Paleocene

Epoch, there were plenty of open ecological niches, and the surviving forms of life began

to fill them.

The last remaining dinosaurs -- birds -- had begun to diversify into some pretty familiar

forms.

For example, around this time, we begin to see the likes of Waimanu, a small, flightless

waterbird from New Zealand that's one of the earliest known penguins.

Likewise, in New Mexico, the appearance of Tsidiiyazhi tells us that the ancestors of

mousebirds, found today all over sub-Saharan Africa, were already on the scene.

Meanwhile, on the forest floor, some early, ungulate-like mammals began to take over.

At first, these mammals had it pretty easy, because there weren't many predators.

But plenty of insectivorous mammals had survived the extinction.

And it didn't take very long for some of them to start developing a taste for bigger

prey.

These were the creodonts, predators that first appeared in North America like the small,

kinda dog-like Galecyon as well as, Oxyaena, which looked more like a cat.

For a long time, scientists thought that these small meat-eaters were the direct ancestors

of today's modern carnivores.

But in recent years we've learned that they were actually a separate lineage, one that

happened to converge on the same strategies and general body plans of the carnivores we

know today.

Now, other mammals made their homes in the trees, including some of the first primate-like

species: the Plesiadapiforms.

They first showed up in Europe and North America, and even though most researchers think they

weren't direct ancestors of primates, they can still tell us a lot about what the earliest

primates might have looked like.

Purgatorius, for example, looks a lot like a rat.

But it had long, grasping fingers, useful for life in the trees, and wide teeth for

chewing things like fruits and leaves.

But like other plesiadapiforms, Purgatorius had claws instead of nails, and it was missing

one of the key features of a true primate — its eyes didn't face forward.

So, by the middle of the Paleocene Epoch, animal life was on the rebound.

And then it started to get really warm.

About 55 million years ago, the average temperature on land went up by 5 to 8 degrees Celsius

… in less than 20,000 years.

This spate of global warming marks the transition to the next epoch, the Eocene, and it's

known as the Paleocene-Eocene Thermal Maximum, or PETM.

Remember this one

If you've seen our episode on this phenomenon already, then you know that we're not totally

sure what caused it.

It might've had to do with volcanic eruptions or melting methane ice on the ocean floor

that released greenhouse gases into the atmosphere.

Either way, as a result, a world that was already warmer today's started to get even

hotter.

At the poles, lush ecosystems took hold, while parts of western North America became arid.

This was great for animals that thrived in the heat, like reptiles.

How great?

Well, this is when the world saw the biggest snake that ever lived.

Titanoboa slithered through South America during this hot spell -- all 13 meters of

it, about twice the size of a modern anaconda.

It feasted on fish but also crocodilians and, also anything else it could get its jaws

around--which was most things

Also in the water were reptiles like the giant turtle Carbonemys, which, unlike our shelled

friends we know today, were about 3 meters long.

And it fed on mammals and other reptiles

While all this was going on, some of the first true primates were appearing.

The tiny Omomyids for example, had grasping fingers with nails instead of claws, and giant

eyes like tarsiers'.

Then, about 49 million years ago, this warming trend shifted, and the world began its long

journey from a greenhouse to an icehouse.

The shift may have been caused, at least in part, by what's known as the Azolla event,

where massive amounts of the small, moss-like Azolla fern grew in the Arctic.

These plants took up to half of the carbon dioxide in the atmosphere — enough that

the climate began to cool.

And a lot of mammals from the Paleocene couldn't handle the colder, drier weather of the late

Eocene, so, by about 40 million years ago, many of them had gone extinct.

At least 45 groups disappeared from Asia, like the Eurymylids, a family thought to be

closely related to early rodents.

In North America and Europe, the changing forests caused problems for mammals that lived

in the trees — including primates.

By the late Eocene, practically all of the primates on those continents died out.

But at the same time, some modern mammal groups start to show up in the fossil record, like

familiar rodents and the odd-toed ungulates — the group that includes today's horses,

rhinos, and tapirs.

And it's in the late Eocene that some of the simians, the clade that includes monkeys

and apes, begin to appear in the fossil record.

By the time the next Epoch, the Oligocene beings around 34 million years ago

We start to see the likes of Aegyptopithecus in northern Africa.

An early member of the group that includes what are known as the Old World Monkeys

and eventually the apes

But now the global temperature took an even steeper plunge.

With ice sheets beginning to form in Antarctica

And soon, another extinction event began — this one mostly in Europe.

It's known as the Grande Coupure, and like most other extinctions, we haven't figured

out all the details of what caused it.

But we know the drop in temperature would have made it hard for some of the older groups

of mammals to survive.

It might have also lowered sea levels enough to allow for more migration from Asia and

therefore, more competition.

Almost all of the tree-dwelling European mammals were wiped out, and all kinds of new creatures

moved in, like true carnivores, and artiodactyls — the group of ungulates that includes animals

like today's pigs, deer, and cattle.

Then, in the cooler, drier climate of the Oligocene, a new habitat appeared: grassland.

And this was an enormous deal, because the fibrous grass was much harder to digest than

softer vegetation like leaves, which meant that animals had to adapt, or die.

Among the herbivores, the even-toed ungulates known as ruminants had the advantage, because

they had an extra stomach chamber where grass could be fermented and partly digested, then

sent back to the mouth to be chewed again.

In the late Oligocene, there was also a major split among the simians.

Around 26 million years ago, the first so-called New World monkeys appear in the fossil record,

in South America.

We're not really sure how they got there, although the lower sea levels might have helped

them get around.

These New World monkeys retained a lot of the traits of earlier monkeys, like their

small size and fruit-based diet.

Meanwhile, the larger Old World monkeys in Africa and Asia began to take a different

route.

Many of these Old World monkeys had a broader diet, and some started to spend more of their

lives on the ground.

Now, the transition from the Paleogene to the next period of the Cenozoic, the Neogene,

is a subtle one, usually recognized by changes in microscopic fossils of things like algae

and foraminifera.

But the events that unfolded during this period, starting 23 million years ago, were hard to

miss.

The Neogene opened with the Miocene Epoch as continental plates were on the move, kicking

off era of mountain-building that continues today.

The Himalayas were forming, as the Indian plate rammed into Asia, while collisions in

Europe started to create the Pyrenees and the Alps.

Meanwhile, in Africa, another transition was underway, as the first apes evolved from the

Old World monkeys.

We're not completely sure what the first ape was, but a transitional genus called Proconsul

first appeared around the start of the Neogene that may have been close to the start of the

ape lineage.

The most obvious trait of these animals was their lack of a tail.

But they also had more flexible shoulder joints; broad, flat rib cages; and a shorter spine.

Among other things, these traits combined to make it easier for apes to swing from the

trees and — eventually — to balance on two legs.

For the most part, the rest of the Miocene continued the trends that began in the Oligocene.

As the world continued to cool, forests began to shrink while grasslands spread.

For animals that grazed instead of browsing on trees, they had vast new expanses to disperse

to

But almost all of the herbivores that couldn't survive on grass -- like browsing horses -- began

to disappear.

In the oceans, the bizarre marine mammals like Desmostylia had disappeared.

But new forms of life start showing up in the fossil record -- like sea otters and other

animals that made their homes in the world's first kelp forests.

And in the meantime, the apes continued to diversify.

Over the next several million years, the ancestors of each of the great apes split off from the

ancestors of humans.

The ancestors of orangutans diverged from our lineage first, about 13 million years

ago.

The ancestors of gorillas were next, around 10 million years ago; followed by the ancestors

of chimpanzees and bonobos, around 7 million years ago.

By the time the Miocene ended 5.33 million years ago, the lineage that would lead to

humans was established.

In fact, most of the groups of animals around today had evolved.

The world was getting closer to something we would truly recognize.

Sure, there was still the occasional gomphothere or three-toed horse.

But other groups, like canids, bears, and whales, were fully fledged.

The transition to the next epoch, the Pliocene, involved a brief period of warming, followed

by an even faster drop in temperature.

South America, which had detached from Antarctica in the Oligocene, finally bumped into North

America, cutting off the Atlantic Ocean from the warm currents circling the equator.

And as the climate continued to cool, our ancestors — the hominins — were taking

over the expanding grasslands.

The first known hominins, Australopithecus, appear in the fossil record around 4 million

years ago.

Researchers think some of these early hominins were actually able to digest grass, which

would have made it easier for them to find food no matter what the climate was.

Over time, they became better runners and more skilled hunters.

And there's evidence that toward the end of the Pliocene, they had begun using stone

tools.

By 2.8 million years ago — just a couple hundred thousand years before the end of the

epoch — a new genus, Homo, appeared on the scene in the form of a lower mandible found