NARRATOR: Sprinting.

Swooping.

Pouncing.

In the race for survival, movement is key to success.

STUART SUMIDA: Speed is important.

Speed is power, speed is survival, and speed is feeding.

(splashing) SHEILA PATEK: Movement is fundamental to life on Earth.

There is no question about it.

NARRATOR: All of this activity began with a single evolutionary leap, more than half a billion years ago.

STEVE BRUSATTE: The development of muscle was groundbreaking because it unleashed incredible potential.

KARMA NANGLU: Muscles are such a flexible, powerful tool, the Swiss army knives of evolution.

NARRATOR: Muscles powered the first steps... BRUSATTE: You are looking at Neil Armstrong's footsteps on the moon here, an evolutionary equivalent to that.

NARRATOR: ...and drummed life's first beat.

(booming) BROOKE FLAMMANG: The heart is this constant ticking that is running through our entire lifetime.

NARRATOR: Life found ways to get a leg up.

ZOË SELF DAVIES: The muscle is all bunched up at the top here, because that's where all of the power is coming from.

KORY EVANS: They really threaded that line between speed and power.

NARRATOR: Evolution took the ability to move, and ran with it.

MATT WILKINSON: Movement is one of the most important things that evolution ever did.

NARRATOR: So, how has movement driven the story of life on Earth?

(squawking) ♪ ♪ "Evolution: On the Run."

Right now, on "NOVA."

♪ ♪ ♪ ♪ NARRATOR: The natural world is constantly in motion, but one animal in particular is synonymous with power, speed, and endurance.

BRUSATTE: When I think of the power and beauty and grace of movement, my mind automatically goes to a horse.

(neighing, splashing) FLAMMANG: When I think of horses, the first word that comes to mind is majestic.

Speed.

Athlete.

Versatility.

One word?!

You're talking to an academic!

(laughs) (splashing) ERIC SCOTT: I would actually pick two words.

Tremendous machine.

Man, I could watch 'em all day.

(laughs) DAVIES: So much of the horse is geared entirely towards running.

♪ ♪ NARRATOR: More than 700 muscles dedicated to movement.

SUMIDA: The amount of power they can generate with those bodies is tremendous.

NARRATOR: Huge, pumping hearts... BRUSATTE: The horse's heart is about 1% of its entire body.

That's much, much, much more than it is for us.

NARRATOR: ...and skeletons built for speed.

Horses are essentially running on the tip of their toes.

DAVIES: When a racehorse is running at maximum speed, their leg is experiencing about 2,000 pounds worth of force.

NARRATOR: The horse has evolved for a life on the move and harnessing its power has changed the course of human history.

NANGLU: Horses have evolved alongside us.

They're an extremely useful animal and they've pervaded all sorts of different parts of our society.

SCOTT: When you think about fast cars, horsepower is how that's measured.

NARRATOR: So, how did movement drive evolution, and turn the horse into one of nature's greatest running machines?

To find out... we must go back in time, to meet the ancient relatives of the horse.

♪ ♪ Before hooves evolved from toes, or limbs from fins.

Before hearts began to beat.

Back to a time when almost no animal life could move at all.

This is the story of how the horse became a master of movement.

♪ ♪ Across the world, animals have evolved to move in a stunning variety of ways.

BRUSATTE: My favorite movement that any animal does is when a bird takes off from a perch.

FLAMMANG: Without a question, swimming has to be one of them.

But fishes also fly and walk and ride on other animals.

My absolute favorite would be pronking, which is what gazelles and deer do.

I think it looks like they're having an amazing time.

For me, it would be napping.

But my favorite type of animal movement would have to be running.

WILKINSON: Most of what we know and love about the living world is there because of movement.

And it's completely dominated the history of life since it arose.

(spouting) NARRATOR: But before animals could swim, fly and run... (cawing) their lives were much more... stationary.

♪ ♪ (birds chirping) On a windswept hilltop in rural England, an unassuming slab of rock was once home to an ancient community.

FRANKIE DUNN: This is Charnwood Forest.

For me, this is absolutely one of the most significant paleontological sites in the world, because it allows us to reach back into the geological past and see the very first stages of animal evolution.

NARRATOR: Paleontologist Frankie Dunn studies some of the oldest animal fossils ever found.

DUNN: What we observe in Charnwood is like the ghost or the shadow of animals that once were.

NARRATOR: An ancient volcano covered this area in ash, capturing an underwater ecosystem in rock.

DUNN: When you get the light just right, the whole surface lights up, and you can see all of these animals in the community that they once lived in.

It's really magical.

This is a fossil called Bradgatia, and there are several Bradgatia specimens on this fossil surface.

And what I hope you can see is that there are several branches which kind of swirl in this direction.

Bradgatia would have been a cabbage-shaped organism, probably about a foot in length.

And as it's been flattened, it's kind of splayed out, and that's what we can see on the rock.

NARRATOR: But Bradgatia isn't just frozen here in death.

It was also trapped in life.

Over half a billion years ago, this rock was deep at the bottom of the ocean.

♪ ♪ This seafloor is full of strange and unfamiliar animals.

But these creatures have a serious limitation.

NANGLU: If you took a time machine back to the Ediacaran Period, probably the first difference you'd notice is how still animal life seems to be compared to today.

WILKINSON: Everything that was big enough for you to see with the naked eye would be sessile, which means that they just kind of stick in one spot.

Essentially a still and silent world.

NANGLU: Some of the smaller organisms during the Ediacaran Period likely did move.

WILKINSON: But these are microscopic organisms, moving using cilia or flagella, which work like little oars or little propellers, sometimes little corkscrews.

And that works fine.

But the big problem with these is that they just don't scale up.

NARRATOR: For the larger animals of this time, standing still is a huge stumbling block.

They are entirely dependent on the current to bring them what they need, and are powerless when it doesn't.

♪ ♪ But one animal found in Charnwood stands apart from the rest.

This fossil is called Auroralumina.

This is probably the most evolutionarily significant fossil I'll ever have the pleasure of working on.

NARRATOR: Auroralumina had something its neighbors did not.

DUNN: Sticking out of the top of the fossil you have this kind of crown of tentacles.

It would have been using these tentacles to draw prey, whatever it was eating, into its mouth, which would have been here.

So, this is a close relative of things like sea anemones and corals and jellyfish.

And so, we can make the inference that like living corals and jellyfish, Auroralumina has muscle tissue.

So, this is some of the first evidence in the fossil record for muscle tissue.

♪ ♪ NARRATOR: The development of muscle allowed animals to take an evolutionary leap that would change them forever.

A leap that's recorded in another remarkable Ediacaran fossil.

This fossil is almost uniquely important in the evolutionary history of animals.

You should be able to make out this linear trail, and within this trail, there are a series of semi-circles, little half-moons, and that's evidence of an anemone-like creature, so something that you'd find in rock pools, shuffling along the sea floor.

Fossils of muscular movement from this age are astoundingly rare.

This is the only one that we have.

So this is the oldest footstep known to science.

NARRATOR: Some scientists think the story of this fossil went like this: These animals are stuck to the Ediacaran sea bed and feed on particles that drift through the water.

♪ ♪ But now, the currents have changed, carrying the stream of food just out of reach.

♪ ♪ Unable to feed, the only option for this stationary population is to sit and wait for their luck, and the currents, to change.

Otherwise, they might starve.

But this animal is different.

This tiny, fragile creature can do something that others can't.

Using its muscles to create a ripple of contraction throughout its entire body, it launches itself forward... ♪ ♪ and slowly moves across the sea floor.

One small shuffle for an anemone-like blob, one giant leap for animal kind.

With every heave, it leaves behind a trail of "footprints."

♪ ♪ A record which will wait over half a billion years to be found.

BRUSATTE: This is a trailblazing moment in evolution.

You are looking at Neil Armstrong's footsteps on the moon here, an evolutionary equivalent to that.

(splashing) NARRATOR: When animals started to move, it kicked evolution into a higher gear.

SCOTT: Once movement enters the picture, suddenly you're able to interact with other organisms.

(splashing) WILKINSON: You can escape from predators, you can catch your prey, you can disperse to find a new home.

So this ability to move accelerates the evolutionary process.

It's a turbo boost for evolution.

NARRATOR: The evolution of muscle led to a movement revolution.

♪ ♪ (birds squawking) Despite the variety of movement it unleashed, all muscle depends on just one simple mechanism: contraction.

BRUSATTE: A really good animal to illustrate muscles and how they work and why they're so important is a leech.

And I have one here.

It really is a dynamic creature here.

We can see that it's using two basic sets of muscles.

Some muscles circle the body like rings, and then other muscles run along the length of the whole body.

WILKINSON: So, by alternating contractions-- so, circular muscles, then the front-to-back muscles-- the leech can move forwards pretty fast.

NARRATOR: And this contraction relies on an ancient protein that has existed for more than a billion years: myosin.

Myosin is what we call a motor protein.

Its job is to generate movement.

It can be used for dividing cells, to pinch cells away from each other, to create new cells.

WILKINSON: It can be used to traffic various packages of materials around the cell.

PATEK: It's also important for changing the shape of cells.

It's essential for cellular movement.

NARRATOR: But at least 560 million years ago, some myosin was put to novel use in a new type of animal cell.

WILKINSON: What happened with muscle cells was taking that very ancient system and creating this absolutely regimented array of myosin protein.

It's like looking at a beehive.

You see these hexagons of proteins, incredible level of organization.

As the myosin binds and pulls, it's creating a very unified shape change of the entire cell.

NARRATOR: The myosin within these muscle cells could then generate powerful, unified contractions.

And this structure had an additional benefit: it could be scaled up.

BRUSATTE: One of the great benefits of muscles is that they are stackable, they are repeatable, they're like Lego blocks.

You can build bigger systems out of them.

In a small animal like a leech, you might have thousands of muscle fibers.

But in a large animal like a horse, you might have millions and millions of muscle fibers all working together throughout the animal.

NARRATOR: Leeches can also demonstrate the versatility of muscle, with a way of moving that would come to dominate the oceans.

WILKINSON: So, the way the leech is moving at the moment is standard worm locomotion.

But leeches have a little trick up their sleeves, which had great evolutionary significance.

So, if I'm just grabbing this pot of water, and if I drop it in... there, it starts to swim.

♪ ♪ NARRATOR: Over the next 40 million years, evolution gallops on.

♪ ♪ Animals with muscle take over the oceans, and more and more time is spent on the move.

♪ ♪ By the Cambrian, animals are slithering, scuttling, and swimming.

But muscle is expensive to maintain, because every contraction consumes energy in the form of oxygen and nutrients.

So, the more muscle an animal has, the more energy it needs.

PATEK: In order for animals to get bigger, to get faster, they needed to evolve mechanisms that would allow more efficient delivery of essential things like oxygen.

NARRATOR: Some unusual animals alive today offer a glimpse of how movement was "powered up" by the evolution of a vital structure.

Tunicates are a group of animals that you can find basically all around the world's oceans.

EVANS: It looks a bit like a coral or like a sea sponge at first, but they're actually more closely related to us than they are other invertebrates.

NANGLU: Tunicates have, inside their bodies, a really interesting structure.

It looks kind of like a short, thin tube.

A tube that is made of muscle.

EVANS: This specialized muscle is able to circulate those nutrients to get all the tissues the nutrients that they need.

♪ ♪ NARRATOR: And in the Cambrian seas, it is a similar system that pumps oxygen and nutrients around the body of this early swimmer.

It has a primitive backbone, so when its muscles flex, its body bends.

And this undulation thrusts it forward.

♪ ♪ But to keep its muscles moving, the supply of energy needs to be regular and reliable.

NANGLU: The way most muscle cells work is they need to receive an external electrical signal, usually from the nervous system, in order to know, "now it's time to actually activate and contract."

About half a billion years ago, a new mechanism for controlling muscle contractions evolved.

NARRATOR: Some muscle cells specialized, and were able to better coordinate those vital contractions independently of the nervous system.

They're known as pacemaker cells.

PATEK: Imagine this is all muscle, and the red is the contracting muscle.

This simulation shows how a pacemaker muscle cell can generate an electrical signal and start a contraction that spreads to an entire set of muscles.

It's a spectacular innovation.

♪ ♪ NARRATOR: Around 500 million years ago, this innovation helped pump oxygen and nutrients around this animal... ♪ ♪ ...providing its muscles with a continuous supply of energy... ♪ ♪ ...and allowing it to swim farther, faster, and for longer than its competitors.

♪ ♪ This simple creature has a pulse.

(pulse beats) It passes this innovation on to its offspring, and over the course of generations, the muscle walls around the pacemaker thicken.

Cells stack upon cells, communicating with incredible speed, forming valves and chambers, becoming a powerful pumping organ that sends blood around the body: (blood pumping) a heart.

NANGLU: The evolution of the heart allowed animals to do all sorts of things they couldn't do before.

Get larger, move faster, and support all the complex musculature throughout the body.

(purring) PATEK: And it's something that we share with countless species of animals.

NARRATOR: Today, hearts beat across the Earth, pumping oxygen around trillions and trillions of bodies.

And each one is a collection of muscle cells dancing to the same beat.

EVANS: There are some animals who have hearts that are so small that you can't see them with the naked eye.

And then you have things like the blue whale, where the heart is all the way up to 1,000 pounds.

(spouting) NARRATOR: In a typical human, around two-and-a-half billion heartbeats supply our muscles with the energy they need, from before we even take our first breath.

SUMIDA: I can tell you, the-- the first time I saw an ultrasound of my own son's heartbeat, it was, it was... life-changing.

NARRATOR: It's a rhythm that connects all of us to those first beats more than 500 million years ago.

As evolution powers onward, the descendants of those energetic swimmers grow to be bigger, stronger, and faster.

(lightning crashing) Their bodies shift and change through countless generations.

And after 140 million years... they have become a little more familiar.

This is the age of fishes.

Animals with pumping hearts and powerful muscles dominate the Devonian seas.

And here, it's not just about having the right types of muscles, but also the right type of body.

KIERSTEN FORMOSO: I love morphology, which is the shape of living organisms.

There are rules that govern the way an organism is shaped.

(distant honking) And those same rules also govern the world around us, and that's physics.

(water churning) NARRATOR: Paleobiologist Kiersten Formoso studies how fundamental laws of physics determine the ways animals move.

FORMOSO: An animal that swims around in the ocean today is constrained by the same physics of an animal that swam around in the oceans of the past.

Water is a challenging environment to live in, because animals have to find a way to not sink, to not float, and to not get battered by the currents of that moving fluid.

♪ ♪ NARRATOR: In order to better navigate their watery world, fishes have evolved fins made of fine bone, cartilage, and muscle... ♪ ♪ providing lift, stability, and maneuverability.

♪ ♪ But not all fins are built the same.

Around the start of the Devonian, bony fish split into two different groups.

And one of those groups had fins that were a little unusual.

FORMOSO: This is Eusthenopteron, and Eusthenopteron is a lobe-finned fish.

You can see very clearly the lobe fin right here, and these are the rays, what we normally think of as a fish fin, but you'll notice all this other stuff going on here.

You can see this bone here, and this bone here, they are pretty hefty-shaped bones, and so we can infer that the muscles that were attached to them were large muscles, too.

NARRATOR: With their extra bones and powerful muscles, these lobe fins were full of possibility.

FORMOSO: A lot of people think evolution has a purpose, it's directed, but that's not necessarily the case.

This lobe fin may not have evolved for any specific reason, but it didn't harm this lineage of fish and instead, it gave them all this evolutionary potential.

NARRATOR: Some of the potential of these strange fins might have been realized in this ancient predator.

Onychodus patrols along the shallow coastlines of the Devonian and is equipped with muscular, bone-filled lobefins.

Up to ten feet long, its size makes it a formidable hunter.

But one downside of being so big, is that when Onychodus swims along the sea bed, it can stir up lots of water and silt, and that's a dead giveaway.

So, how does this marine giant get around the tricky physics of water to catch unsuspecting prey?

♪ ♪ A hint can be found in a basic mechanism hidden inside their fins: the lever.

FORMOSO: A lever is a simple machine which allows for more powerful and potentially faster movements.

NARRATOR: By pivoting around a fixed point called a fulcrum, a lever can turn small, high-force movements at one end into something much bigger at the other.

You can see that in action... ...with the motion of these oars.

Where the force that I apply... ...is translated across the fulcrum... ...into the end of the oar and allows me to translate... smaller movements into larger movements... ...at the ends of these oars.

♪ ♪ NARRATOR: Using their fins as levers, fish can amplify the movements of their muscles.

But lobe-finned fishes had far more potential.

FORMOSO: All of the little bones inside those lobe fins are effectively tiny little levers that eventually would have joints between them.

♪ ♪ And it's the evolution of those joints that really allowed that fin, which is a single, simple paddle, not too dissimilar from these oars, to be really dynamic appendages for movement.

♪ ♪ NARRATOR: And it's those multi-levered appendages that scientists think allowed Onychodus to get around in a very un-fishlike way.

With its four lobefins concealing bones, joints and powerful muscles... ...this hulking predator can move with surprising delicacy.

It uses the physics of levers to turn the strength of its muscles into stealthy movement... ...and tiptoes towards its prey.

This is a fish that can walk.

And this way of moving allows it to sneak unnoticed through the murky waters, preparing for one final, powerful thrust.

Thanks to the control and range of movement afforded by its lobefins, Onychodus is a master of the ambush.

But despite their effectiveness in the ancient seas, lobe-finned fishes would be even more successful in a very different environment.

EVANS: So, the lobefin, even though it evolved underwater initially, was really uniquely specialized and uniquely positioned to allow them to evolve the adaptations necessary to support their weight out of water.

NARRATOR: Because this novel method of locomotion and multi-levered fins... ...are passed down through the generations.

Over millions of years, their hidden potential is realized.

Thick bridges of bone form, and the muscles surrounding them shift and alter.

Creating strong, versatile appendages with complex joints.

No longer fins at all... ...but limbs.

EVANS: Even though lobe-finned fishes have mostly disappeared from the oceans in their original forms... ...they went on to be very, very successful in terrestrial habitats.

FLAMMANG: The truth is, lobefin fishes are everywhere.

All of us.

You and I, your pet dog, your favorite horse, every terrestrial vertebrate you can think of that walks on land, or flies, or climbs is really a fish.

A lobe-finned fish.

NARRATOR: Today, there are more than 30,000 species descended from those lobe-finned pioneers.

♪ ♪ Putting their additional joints and levers to use in a surprising variety of ways.

From legs, to wings, to flippers.

They are called the tetrapods, a diverse group of animals which includes the horse.

For many of today's tetrapods, moving on land is second nature.

But for their trailblazing ancestors, it was a very different story.

STEVE BRUSATTE: Those first fishes that moved onto the land, they stepped into a new world, a very different world.

NARRATOR: Those early tetrapods were masters of neither land nor sea.

And some present-day amphibians can show us what this major transition might have looked like.

EVANS: Newts are really an example of, like, a bad fish, but an even worse reptile.

SUMIDA: They still move very slowly, very deliberately, you will never see a newt running.

They still walk in that sort of undulatory fashion similar to fishes.

They simply pivot on their hands and feet.

They can survive in the water, can survive on the land, but they're not perfectly adapted to either, and that is a great example of how evolution works.

PATEK: It's not like one day a fish needed to grow legs and walk off and munch leaves on trees.

Major evolutionary transitions are really actually rather subtle.

NARRATOR: Evolution isn't about being the best.

It's about being good enough to survive.

FLAMMANG: Evolution is opportunistic and experimental, and no animal is perfectly adapted to do anything.

NARRATOR: Newts get away with their ungainly movement on land because when threatened, they can retreat to the water.

But as tetrapods left the water behind, their bodies evolved to meet the demands of their new environment.

SUMIDA: This was Diadectes.

When you look at an animal like this, we see skeletal structures that help this animal live independent of the water.

We have a very, very robust backbone that helped hold its belly up off the ground very, very well, and then if we take a look at the other side of this cast, you can see the upper arm bone, the same bone that you have in your arm that runs from your shoulder to your elbow.

But the end of the bone is very thick and very robust, and that's for the attachment of very powerful forearm and hand muscles, which help this animal to move in an incredibly efficient manner.

NARRATOR: But it was something subtler than bulked up bones that allowed animals like these to move with increased fluidity.

BRUSATTE: When you look at a newt with its pretty awkward, ungainly movement, it doesn't move with a whole lot of purpose.

But look at a horse or look at us or look at a bird.

There's purpose in those movements.

There's precision in those movements.

NARRATOR: That precision came from the evolution of a new structure, once again made of muscle cells: muscle spindles.

BRUSATTE: Muscles spindles are sensory structures, you can think of them as a muscular equivalent of our eyes or our ears or our nose.

WILKINSON: Essentially, it wraps around a muscle fiber.

And whenever that muscle fiber gets longer or shorter, these sense organs fire.

So, an animal that has these will have a lot more information about what's happening to its muscular system.

It can fine tune what it's doing and then move well and move efficiently.

BRUSATTE: Muscle spindles allow precise movements.

Whether that is a gymnast doing a routine, whether it's me, you know, snapping my fingers.

You can think of all the different precise movements that animals do and muscle spindles allow it to happen.

NARRATOR: Thanks to this new feature, animals could put one foot in front of the other with greater coordination.

BRUSATTE: This is incredible.

This is basically the very first motion picture, the very first movie.

These were a bunch of photos that were taken in the late 1800s and stitched together to show movement and to prove that when horses run, that at some point in that running cycle, all the feet are off the ground at once.

You really just get a sense of the grace and beauty of the horse moving.

NARRATOR: The evolution of muscle spindles transformed what animals could do with those bony levers, once hidden in fleshy fins.

Their limbs could become not just longer, stronger and more powerful, but also faster and more precise.

Their four feet could move with such synchronicity that they could temporarily leave the ground behind.

In other words, they could run.

♪ ♪ Over the course of millions of years, tetrapods scatter across Earth... ...diversifying and multiplying.

And while there are some crises along the way... ...every challenge is also an opportunity.

And the forces of evolution... never stop.

By the Paleogene, muscular, coordinated animals have explored almost every habitat on Earth.

And Phenacodus, a mammal about the size of a large dog, must constantly be on guard.

Because although this world seems serene, it is full of highly adapted movers.

Fearsome predators... ...whose bodies have evolved... ...for the chase.

♪ ♪ So, when the hunt is on, straining muscles, pumping hearts and powerful limbs aren't enough on their own.

But Phenacodus has an extra spring in its step... ...and that might just be the key to its survival.

PATEK: I absolutely love long-distance running.

But humans run in a very odd way compared to most vertebrates.

Anatomically speaking, humans run with the whole of their foot contacting the ground.

So if this was a foot, the heel contacts the ground, the sole of the foot contacts the ground, the ball of the foot contacts the ground, everything below the ankle comes into contact with the ground.

NARRATOR: This is called plantigrade, and even though it is excellent for stability, not all mammals run this way.

DAVIES: Most mammals run in a digitigrade stance.

So, this is how your dog runs.

Their ankle is off the ground and their toes are in contact with the ground.

FLAMMANG: If you look at their hind legs, people tend to think that their knees are bent backwards, but that's not actually what's going on.

The part that people assume is the knee is really the ankle.

Because they're up on their toes, it means that their limb is elongated.

So the longer your leg is, the bigger the strides that you can take.

SUMIDA: Which means you can run faster.

We see that in many types of mammals, but we also saw digitigrade limb posture take place in dinosaurs.

So it means that being able to run quickly, must have been very, very advantageous.

NARRATOR: When it comes to running, digitigrade allows for increased speed and efficiency.

♪ ♪ But Phenacodus is on the verge of something new-- an even more extreme style of running.

Not just bouncing on the balls of its feet, but pushing off from the very tips of its toes.

This gives animals like Phenacodus an edge, and generation by generation... ...the best runners outpace the competition.

♪ ♪ Bones elongate.

Muscles bulk up.

And, in service of efficiency, anything no longer needed is stripped away.

Until all that is left at the end of each leg is a single toe with one flat, hard hoof.

And four million years ago, a new kind of single-hooved animal appeared in North America, with limbs evolved for a life on the move.

Equus.

Comparing this to a human limb, this bone at the top looks quite familiar.

This is a thigh bone.

This is a kneecap, just like ours, and then this is the shin bone.

The muscle is all bunched up at the top here because that's where all of the power is coming from.

And in not having a load of bulky muscle down here, the end becomes really, really light.

So, it's much, much easier to swing because it doesn't weigh very much.

What then starts to look a little bit less familiar is that makes all of this section equivalent to the human foot.

So, this is the ankle.

This long bone here is the bone in the sole of your foot, which makes these three bones equivalent to your middle toe.

NARRATOR: These highly specialized limbs meant Equus could run even more efficiently.

But they also came at a cost.

SCOTT: The trade-off is there's not a lot else the animal can do with that limb.

It can't twist its, its limb from side-to-side, it can't grasp things, it can't pick things up.

Its primary overriding focus is on running.

NARRATOR: And, precisely because of that evolutionary compromise, these single-hooved animals became some of the greatest runners the planet has ever seen.

The many species of Equus used their specialized limbs to spread across the globe.

Among them, one iconic species: Equus ferus.

♪ ♪ Commonly known as the horse.

BRUSATTE: When I look at a horse running, I look at an animal that is so fine tuned to its lifestyle, they move in just the most fluid, dynamic, gorgeous way.

♪ ♪ NARRATOR: With powerful, coordinated muscles... passed down over half-a-billion years from pioneers that changed the world.

WILKINSON: You can look at their bodies, and so much of it is about what makes their movements so effective.

It's almost like the story of the evolution of movement is written into a creature like a horse.

NARRATOR: A huge heart, whose ancient beat began deep beneath the waves.

Long, levered legs that once helped a fish to walk.

And four hooves, adapted for a life on the run.

SCOTT: They're like massive, running ballerinas.

It's power, it's energy, it's also delicacy and precision, all of it meshing together in this beautiful, synchronized movement that is just so wonderful to watch.

NARRATOR: But the horse's story doesn't end there.

Because Equus ferus encountered a new force in evolution.

Us.

(horse nickering) ♪ ♪ LUDOVIC ORLANDO: When I see a horse, I don't just see the animal, but I see human history behind the animal.

NARRATOR: Geneticist Ludovic Orlando studies how the ancient history of the horse intertwines with our own.

ORLANDO: Our relationship with the horse goes back deep in time, and most of the time, what we were doing was hunting them.

So horses provided us with some resources, they fed us for a long time during human prehistory.

But at some point, the relationship that we developed with the animal changed and what is fascinating to me is that that whole story is written in the DNA of the horse.

NARRATOR: Ludovic and his team analyzed DNA from horse fossils around the world, to understand how their genes evolved over the last 700,000 years.

ORLANDO: If you replay the DNA movie of horse history, you would realize that about 5,000 years ago, there were many different kind of wild local lineages existing around Eurasia.

But what happens about 4,000 years ago is a totally different picture.

Is that as opposed to this sort of mosaic of different types of wild horses that were existing, you see now only one type... ...that spread across the whole of Eurasia in no more than 300 years.

NARRATOR: The team tracked changes in the horses' DNA and out of approximately 20,000 genes, two stood out.

One gene is actually a modulator of behavior.

The genetic variation that made the horse maybe more docile and more tamable.

And there is a second one which is even more transformative, and that gene is actually associated with a different shape of the back anatomy of the horse that is more compatible with us sitting on their back.

NARRATOR: Together, these genes turned the wild horse into a tamer, more rideable companion for humans.

ORLANDO: And so generation after generation, those genetic types became the most frequent variants present in the population.

And eventually, pretty much all the horses carried those traits.

So, probably those modifications together created or engineered the modern horses that we can admire today.

♪ ♪ NARRATOR: This new type of horse also got a new name: Equus ferus caballus.

The domestic horse.

A new dominant subspecies with its natural strength, speed and endurance shaped by humans.

BRUSATTE: We are essentially directing evolution ourselves.

We are taking animals that are stronger or faster or more agreeable and we are breeding them to accentuate those traits.

NARRATOR: This is not natural selection, but it is still evolution.

Small changes that add up over generations.

Through our choices, we transformed wild horses into workhorses.

And as we harnessed their power, they also changed us.

SUMIDA: The human-horse relationship is based on ability to move with greater power than we ever could have done ourselves.

BRUSATTE: We use them for movement.

For thousands of years, if we needed to get from one place to another, and we couldn't just walk it ourselves, we would ride a horse.

SUMIDA: They've been used to pull plows, to pull wagons, to run machinery that didn't have an engine.

Horses were the original machines.

Our way of life was built on the shoulders of horses, and there's, there's absolutely no way around that.

♪ ♪ NARRATOR: What started as a predator-prey relationship developed into one of collaboration and cooperation.

A shared history with movement at its heart.

PATEK: Everything, it feels like, comes back to movement in some sense.

BRUSATTE: Running and jumping and gallivanting and flying and flapping and swimming.

I could go on and on and on.

EVANS: Evolution has absolutely created this broad diversity of movements that we see in the natural world.

WILKINSON: We are the living history of four billion years of the evolution of life through that lens of movement.

NARRATOR: But through the twists and turns of life on this planet, movement has become more than just a feature of life, it is key to its resilience.

BRUSATTE: Life is old, life is adaptable, and one of the reasons that underpins that is movement.

Being able to leave somewhere if the weather's gone bad, if the climate's gone bad.

PATEK: The ability to move is a lifeline for many animals to find a different habitat, to find a different location, to find pathways to survival.

NARRATOR: Animals have been on the run for hundreds of millions of years.

SUMIDA: Running has never stopped.

We've never seen running go extinct.

And I see no reason why it ever will.

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