It's 55 million years ago in North America, and the continent is draped in a canopy of swampy cypress trees and humid broadleaf forests.

Bizarre predators, like the hoofed carnivore Mesonyx, lurk in the undergrowth, ready to nab an animal called Eohippus.

Eohippus may be small -- just half a meter tall -- but it's also fast, and well-adapted to its environment.

It runs on padded toes, each capped with a cute lil' hoof, perfect for making a quick getaway on the soft forest floor.

But the future has big changes in store for little Eohippus.

The climate is about to get cooler and drier, and soon the landscape of North America will become unrecognizable.

Trees will give way to vast carpets of grass.

Mesonyx and other forest hunters will go extinct.

And powerful, new, pack-hunting predators will evolve.

To adapt to life on these newly forming plains, the descendents of Eohippus will undergo some dramatic changes.

They'll grow to be over 30 times its size.

They'll change their diet entirely.

And they'll all start running ... on one giant toe.

In time, these animals will become so successful that they'll spread all over the world, to Europe, Asia, South America, and Africa.

But in their native range of North America, they'll vanish for 10,000 years...until another strange mammal brings them back.

Back at the start of the Eocene Epoch, the world was in the clutches of the Paleocene-Eocene Thermal Maximum, and warm, damp forests covered the land from pole to pole.

This was the environment that gave rise to the very first perissodactyls, hoofed mammals with an odd number of toes.

Today they include rhinos, tapirs and horses.

One of the first perissodactyls we know of was Hyracotherium, a small ungulate whose fossils were first discovered in England and named, for some reason, for its supposed resemblance to the rock hyrax.

I mean, what?

Anyway, there were lots of different Hyracotherium-like animals, known as hyracotheres, that were widespread all over the Northern Hemisphere.

And one of them, found only in North America, was Eohippus or "dawn horse."

It was named by American paleontologist O.C.

Marsh, whose facial hair I really admire, and who found the first complete fossil of the animal in New Mexico in 1876.

It was about the size of a dog, around 35 centimeters at the shoulder.

And, like other hyracotheres, it had short, low-crowned teeth that were good for browsing on leaves in the dense forest.

Eohippus also had feet unlike any animal alive today, with toes that were each capped with separate hooves: four on its front feet and three on its hind feet.

Is it me or does that just sound really cute?

Now, this unassuming little forest creature might not seem like much, but it gave rise to the most successful family of Perissodactyls of all time, the Equidae: the family that includes the modern horse.

At the height of its diversity, the Equid family would include more than a dozen genera that roamed the northern hemisphere.

Today, however, only one genus remains: Equus, which includes modern horses, donkeys, and zebras.

Thanks to an abundance of fossils found throughout North America, we can reconstruct much of the horse family tree, from the tiny, browsing, multi-toed Eohippus at its roots, to one of the crowning branches that includes the large, grazing, single-toed Equus.

The story of horse evolution is one of constant adaptation and radiation in response to changes in North America's climate.

And all of these adaptations were in some way driven by the appearance of grasslands, which weren't widespread in North America when Eohippus was around.

But about 49 million years ago, in the mid-Eocene, the climate began to get cooler and drier.

By the late Eocene, the Thermal Maximum was history, and North America's dense forests began to give way to a mosaic of dry grasslands.

It was in this patchwork environment that a descendant of Eohippus first appeared.

Mesohippus begins to show up about 38 million years ago, first described, again by O. C. Marsh, in 1875.

And in very short order -- at least geologically speaking -- one lineage of Mesohippus quickly diversified into yet another genus, Miohippus, within about 3 or 4 million years.

So, Mesohippus and Miohippus roamed the continent together at the start of the Oligocene.

But they were both different from Eohippus in some important ways -- showing that they were starting to adapt to North America's changing landscape.

For one thing, they both had more molars than Eohippus did.

And their teeth had higher crests, for grinding more fibrous, abrasive food, like grass.

Both were also slightly larger than Eohippus, and with longer legs.

Mesohippus weighed around 23 kilograms while Miohippus averaged about twice that.

And, both had lost their fourth front toe, while their middle toe had grown larger and was bearing more weight.

By the mid-Oligocene, the smaller Mesohippus had disappeared.

But the larger Miohippus survived and radiated into many different species that flourished in the following epoch, the Miocene.

By this time, the mosaic of swamps, forests, and grasslands wasn't really a mosaic any more.

The forests and marshes had given way to wide stretches of dry, open prairie.

And one of the most important likely descendants of Miohippus was an equid that was even better adapted to life on the plains: Parahippus.

Paleontologists have been able to learn a lot about one particular species of this horse, Parahippus leonensis, thanks to a quarry in Florida where nearly 90 specimens have been found.

It first appears in the fossil record about 23 million years ago, and its adaptations to prairie life went further than just having pointy molars.

Namely, it was the first early horse to be a true hypsodont.

That means its teeth were not only really long, but they also kept erupting out of the gums as they wore down, to reveal new chewing surface.

This proved to be a huge advantage for living on the plains.

Because if you're going to make a living by eating grass, which is really abrasive, then you need large teeth that can withstand a lot of wear.

This adaptation helped Parahippus, and its descendants, take full advantage of North America's fastest-growing niche.

And by about 17 million years ago, the fossil record shows that a new, distinct genus had evolved from the Parahippus lineage, called Merychippus.

And Merychippus is another important benchmark in the evolution of the horse, because it's the first true equine.

Equines are the subfamily of equids that includes modern horses.

And, like Merychippus, they all have important traits that earlier horses lacked.

For one thing, Merychippus was a lot bigger.

It stood about a meter tall, the height of a small pony, and had the long head that we'd now call horse-like.

And even moreso than its ancestors, its legs were especially well adapted to running on hard ground.

It stood on tip-toe, with all of its weight on three toes supported by springy ligaments.

Its leg bones were also longer, and the bones in the foreleg were fused together, making them stronger and able to withstand more force.

With the help of these adaptations, equines like Merychippus were able to grow larger than forest-dwelling horses, reaching up to 450 kilograms.

Now, being so big probably helped them deal with large Miocene predators, like saber-toothed barbourofelids.

But their size also put a lot more stress on their legs and toes.

That is, until yet another new adaptation appeared: Several descendants of Merychippus became monodactyls, meaning they had only one toe.

Like Dinohippus, which appeared about 10 million years ago.

Some horses in this genus still had three toes.

But others only had one!

And, counterintuitively enough, having only one, large toe reduced the stress caused by the horse's weight.

To make up for the stability that was lost, over time, horses began to rely more on their ligaments.

So, as they became increasingly adapted to their grassy new environment, equines thrived throughout the Miocene.

Then, as the Pliocene Epoch opened about 4 million years ago, one of the single-toed groups of equines finally gave rise to Equus, the genus of the modern horse.

We don't know which one it was exactly, but Dinohippus seems to be the monodactyl that's most closely related to the horses we know today.

Now, the oldest-known species of Equus is Equus simplicidens.

It appeared about 4 million years ago, and has been found from Florida to the Hagerman Fossil Beds in Idaho.

It was about the size of a modern horse, with similar teeth, a long face and neck, fully fused leg bones, and a well-developed "stay mechanism," which horses use to lock their legs in place -- an adaption for spending most of their time on their feet.

But unlike their predecessors, species of Equus didn't stay confined to North America.

3 million years ago, they crossed into South America as part of the Great American Biotic Interchange.

And about a half-million years after that, they crossed the Bering Strait land bridge to spread into Asia, Europe, and Africa.

During this time, all of the three-toed horses and most other monodactyls died out.

Then, about 10,000 years ago, at the end of the Pleistocene, most of North America's large mammals - including Equus - went extinct.

Now, there's a big healthy scientific debate about what caused this extinction event, but it's safe to say that it was probably a combination of factors.

As you can imagine, the end of the last ice age brought a lot of major changes to the continent, like changes in habitat and vegetation patterns.

Plus, populations of bison began to grow and spread, competing with horses for food.

The fossil record shows that horses' ranges were shrinking, and that some horses themselves may have gotten smaller to help deal with the lack of resources.

And then, humans showed up.

Scientists have found evidence that these early humans hunted horses, probably putting more pressure on animals that were already struggling.

But, the horses that had migrated out of North America survived the extinction on other continents.

Over in Asia, horses traversed the grasslands of the Eurasian Steppes.

And about 6,000 years ago, humans began to realize that horses could be pretty useful.

People like members of the Botai culture in ancient Kazakhstan began to domesticate the survivors of the Equus lineage.

Today, all modern horses all over the world are descendents of domesticated horses from this region.

Over the next 6,000 years, horses shaped the course of human history.

They proved to be a game-changer for everything from hunting and agriculture to war and transportation.

Humans, in turn, shaped horses, selectively breeding them to grow even larger, faster, and in my opinion, even more beautiful.

Then, in the late 1400s, Spanish settlers put their horses on ships and brought them across the Atlantic.

Much of America was still covered in those open grasslands that horses were so well suited for, and it didn't take long for them to take over their former range a second time.

The first known feral horses escaped Mexico City around 1550.

More escaped from ranches in New Mexico in the 1600s.

Native Americans began to capture and ride the horses, spreading them further across the continent.

Thanks to humans, horses have become one of the most numerous and widespread mammals of all time.

The continent has changed a lot since the days when Eohippus first picked its way through vast, humid forests.

But today, millions of domestic horses live all over the world, including 67,000 wild horses that are back to roaming their native plains -- members of that second, successful wave of horses, that took over North America.