♪ ♪ NARRATOR: Every new life on Earth is here thanks to a fundamental drive to reproduce.

The game of life is all about reproduction.

STUART SUMIDA: If you don't reproduce, your genetic line goes extinct.

NARRATOR: But while there are many ways to do it... SARAH OTTO: Across the tree of life, we see so many different ways of reproducing, it's mind-boggling.

NARRATOR: ...one way took evolution to the next level.

DANIEL FIELD: The evolution of sex was one of the most important steps in the entire history of life.

♪ ♪ NARRATOR: Sexual reproduction turbocharged the engine of evolution.

Sex gives us variety, variety, variety.

STEVE BRUSATTE: Every new generation is a roll of the genetic dice.

NARRATOR: Leading to astonishing innovations... BHART-ANJAN BHULLAR: Eggs are protective capsules more complex than anything that could be produced today using machine technology.

NARRATOR: ...and eye-catching transformations.

RILEY BLACK: Evolution doesn't just create what needs to survive, it can create what's beautiful.

BRUSATTE: Sex fueled the color, the beauty, and the downright bizarre stuff we see in the world today.

BHULLAR: Sex made the modern world.

Sex made us.

NARRATOR: How did sex create the wondrous world we see around us?

"Evolution: The Mating Game," right now, on "NOVA."

♪ ♪ ♪ ♪ ♪ ♪ (chirps) ♪ ♪ NARRATOR: Standing nine feet tall and weighing up to 300 pounds... (moans) ...the ostrich is the largest bird on Earth and the fastest animal on two legs.

BRUSATTE: Ostriches are remarkable.

They can reach up to 43 miles an hour.

A single stride when they're running at full speed is, like, the length of a car.

(chirping) BHULLAR: The legs of an ostrich are so powerful, they can deliver kicks that can kill with a single blow.

♪ ♪ NARRATOR: But the ostrich isn't just a fighter.

It's also a lover.

♪ ♪ After an elaborate courtship ritual, females can lay up to 18 eggs.

Ostrich eggs are so cool.

ALISON PISCHEDDA: I marvel at the size of it-- it is huge.

SUMIDA: This egg right here weighs about as much as two dozen chicken eggs.

BHULLAR: It's the largest egg laid by any animal alive today.

(chick chirping) NARRATOR: In about 40 days, a single fertilized egg cell transforms into a two-pound, ten-inch-tall chick.

(chirping) Its DNA is a mixture of its parents'.

No chick exactly like it has ever existed before or will ever exist again.

So, how did the ostrich and its astonishing egg come to be?

♪ ♪ To answer that question, we have to go back.

Way back.

♪ ♪ To meet the ancient ancestors of the ostrich... ♪ ♪ ...who lived and died and reproduced in an unbroken chain of life... ...stretching back billions of years.

♪ ♪ From its powerful legs to its unique feathers and giant eggs, the ostrich tells the epic story of how sex unleashed the full creative power of evolution.

♪ ♪ (thunder rumbling) ♪ ♪ These are some of the world's earliest living things.

Simple single cells.

And there's one thing they are especially good at: reproducing.

But nearly four billion years ago, there are no sexes, no mating, no eggs.

These cells reproduce simply by creating a copy of themselves.

A clone.

SUMIDA: Cloning is a great way to make a perfect copy, as if you were going to a photocopying machine.

RIKA ANDERSON: All it does is make a copy of its DNA and then it makes a copy of all the other contents of the cell, and then it splits itself in half and creates two cells, and those two cells are almost identical to each other.

NARRATOR: Cloning is how bacteria and many other microbes reproduce.

And it still works incredibly well for them.

ANDERSON: They have a tremendous diversity of ways of living.

There are microbes that live in hot vents at the bottom of the ocean.

There are microbes that can eat methane and microbes that live in sea ice in the Arctic.

NARRATOR: Through cloning, single-celled life forms like this one prosper and multiply.

And they might well have remained the only form of life on Earth.

But then, around two billion years ago, a new type of cell emerges.

♪ ♪ On the surface, this cell might seem similar to all the microbes that came before it.

But in fact, it's the beginning of a new branch on the tree of life, the common ancestor of every animal, plant, and fungus alive today.

And it has a superpower.

♪ ♪ When it gets close to another of its species... ♪ ♪ ...their cell membranes can fuse.

They combine and mix their DNA together and eventually divide into new offspring, different from their parents.

This is the earliest form of sex.

And life would never be the same again.

Imagine this is one parent-- it's all red.

And if it cloned, that's the only type of offspring it would have.

Maybe, occasionally, mutations.

If it were this individual, it'd be all black.

And if it cloned, that's all you'd get.

What sex does is, it allows those cards to be mixed up, to shuffle them, to have offspring... (deck shuffles) ...that are different from any of the parents.

And that introduces new varieties, combinations that weren't there in the parents.

NARRATOR: Through sex, there is an almost limitless number of possible combinations of the parents' genes.

Meaning the offspring of sexual reproduction are completely unique.

BHULLAR: That reshuffling of genes produces an enormous amount of variation upon which evolution can work.

And so it can accelerate the entire evolutionary process enormously.

We can never rerun the tape of evolution, but I do think that if sexual reproduction never evolved, then complex life would not have evolved.

And by that, I mean the type of life form that we are.

Lots of different body parts, trillions of cells all working together.

When sexual reproduction came on the scene, it enabled organisms to shuffle around their genes to evolve new structures.

That changed everything.

NARRATOR: This pioneering new way to reproduce sends evolution into overdrive.

♪ ♪ And as the ages pass... ...some cells join forces... ...building bigger, more complex organisms made up of thousands, millions, and billions of cells.

Until eventually, after around one-and-a-half billion years of sexual reproduction... ...the Cambrian seas are teeming with complex life.

From primitive fish, algae, and sponges to ancient relatives of insects, the drive to reproduce is as strong as ever.

♪ ♪ But as life got more complicated, so did sex.

ENELE TWALA: South Africa is a fantastic place to be a paleobiologist, because we have a wide range of different types of fossils.

NARRATOR: Enele Twala studies fossils to understand how ancient creatures lived.

TWALA: A great example is a trilobite.

So, this specific trilobite was found right here in South Africa, and this is one of 20,000 species of trilobites that have been found so far.

And we can see that its body is made up of multiple segments and, by looking at these features and how they're structured, we can infer that trilobites were most likely walking on the seafloor.

This one here is called an eocrinoid, and it is 510 million years old.

We can tell a lot about how it lived based on how it looks.

So it was firmly attached to the seafloor, and it has these finger-like protrusions that most likely helped it catch its food.

NARRATOR: But the elaborate bodies of Cambrian creatures had a catch.

Some were anchored to the seafloor, away from potential mates, and some had hard shells.

With complex forms like these, fusing entire bodies to exchange DNA, like their single-celled ancestors, is simply impossible.

They could only evolve in the first place thanks to an entirely different way of having sex: using specialized cells to do the fusing for them.

By the Cambrian, some life had already been doing this for around 500 million years.

These tiny specks are eggs.

And these even tinier ones are sperm.

BRUSATTE: Evolution can and often does favor specialization.

Basically a way to divide roles up and to hyper-focus on doing specific things.

We can think about it at the cellular level.

Egg cells, sperm cells-- those are hyper-specialized cells.

Really, their only job is for reproduction.

♪ ♪ NARRATOR: As life grew larger and more complex, this division of labor allowed the parents to live on separately, while contributing some of their genetic code to each sex cell, or gamete.

OTTO: A gamete is a single cell that carries half of the information from your parents and combines with another gamete through sex, sexual reproduction, to make the baby offspring that then goes on to start a new life.

NARRATOR: Just like the first sexually reproducing organisms, these individual gamete cells could fuse membranes and combine DNA to create new, unique offspring.

PISCHEDDA: The earliest gametes were probably similar in size and shape.

They didn't differ that much.

But then over time, they evolved to be separate eggs and sperm, and this allowed them to specialize for different functions.

OTTO: And what did they specialize on?

Well, we think that the sperm, the smaller cell, specialized on moving around and increasing the chances of encountering an egg, whereas the egg specialized on being nutritious and feeding the young.

♪ ♪ NARRATOR: But the evolution of eggs and sperm did not necessarily mean that every creature was either female or male.

FIELD: This snail is extraordinary, because any individual can produce both sperm and eggs.

We call these sorts of animals hermaphrodites.

PISCHEDDA: What's interesting about garden snails is, when they encounter another member of the same species, they can both produce fertilized eggs.

So this allows them to essentially double the number of offspring that they could have.

It's a very efficient strategy for reproduction.

BLACK: Understanding the origin of what we think of as male and female is really constantly changing.

And because of its efficiency, we think that the earliest animals might have combined sexes in the same body in just the same way.

♪ ♪ NARRATOR: Some scientists believe that for the earliest animals, being a hermaphrodite might have been the norm.

They theorize that during the early Cambrian, something happens to upset the status quo.

A process that likely happens multiple times in many different species.

And it's plausible that one of those species is an animal like this.

♪ ♪ This is a vetulicolian.

About the size of a mouse, it doesn't have eyes, but it does have a primitive spinal cord, which makes it a distant cousin of vertebrates like the ostrich.

♪ ♪ And all of us.

For a hermaphrodite, finding a mate is relatively easy.

Any other member of the species will do.

♪ ♪ But for this pair, there's a problem.

One of them has a mutation that switches off its ability to produce one type of gamete.

In this case, sperm.

The eggs it releases are now its only means of reproduction.

And some of those eggs now carry the same mutation.

Meaning some descendants will also only be able to produce eggs.

♪ ♪ They are no longer hermaphrodites.

Instead, they are something different: females.

BLACK: Biological sex is incredibly complex.

We have male, female, and intersex organisms.

But generally, when we talk about organisms that make eggs, we usually refer to them as female, and when an organism makes sperm, we generally refer to it as male.

OTTO: We know that hermaphrodites occasionally mutate and produce just one type of sexual function, say females.

PISCHEDDA: The evolution of males and females is still a mystery, and the science is still evolving, but there are plausible theories for how this might have occurred.

It's very energetically expensive to make both sperm and eggs.

OTTO: So, by specializing on sperm and eggs, oftentimes, those species have come to be better at it, better at making millions to billions of sperm that are highly able to swim and move.

By specializing on being an egg producer, eggs can be huge-- so huge, for example, that an ostrich egg, when it first forms, it's a single cell.

That's very specialized.

NARRATOR: For modern animals, the balance lies in favor of specialization, meaning that around 95% of animal species have distinct males and females.

But elsewhere on the tree of life, sexual reproduction can look radically different.

OTTO: If you look in the plant world, it's the other way around.

About 95% of plants have both in the same individual, like this hibiscus flower, where, if you open it up, you can see both the parts that'll produce pollen and that will receive pollen, and so they can fertilize themselves if they have failed to find a mate.

And that's really helpful.

If they can't move around and find a mate, then it's better to do both.

♪ ♪ NARRATOR: Even among animals, nature remains flexible.

Some dads guard the eggs and raise the young.

Some fish can change sex.

And occasionally, males even get pregnant.

BLACK: Seahorses are a wonderful example in which the female actually puts the eggs into the male, who then carries them and has those babies.

So even though we have these sex categories, what those mean for the animals can be wildly different.

OTTO: Sexual reproduction, like the world around us, is incredibly diverse.

And species accomplish that process of sexual reproduction in so many different ways, it's mind-boggling.

NARRATOR: Evolution can produce all sorts of quirky outcomes.

But for most animals, the specialization of the sexes proves a winning formula... ...setting the egg on its path to greatness.

♪ ♪ As the generations pass... ...animals continue to evolve, adopting new behaviors, adapting to new habitats, spreading out across the oceans.

♪ ♪ Until, around 150 million years later, some have even ventured into a new frontier: the freshwater streams of the Devonian.

If a time machine dropped you into the Devonian period, it would be a very different world.

EMMA BERNARD: There'd be no mammals, no birds, no flowering plants.

♪ ♪ NARRATOR: But there are plenty of fish.

Like this ancestor of the ostrich, searching for a safe place to lay her eggs.

With a streamlined body, she battles against the current.

But her group is being followed.

BRUSATTE: It was a fish-eat-fish world, and there were small fish all the way up to huge fish.

And those big fish at the top of the food chain, some had what were basically, were scissor blades for jaws.

Going swimming in the Devonian would have been putting your life at risk immediately.

BERNARD: It would have been a constant battle, not only for your own survival, but especially for your eggs.

♪ ♪ NARRATOR: Eggs are packed full of nutrients.

A tempting prospect for hungry mouths.

So, if the fish don't find a safer place to spawn, all their eggs could be eaten.

But these fish have something that others lack.

♪ ♪ Strong fleshy fins, known as lobed fins, that might help them move along the riverbed.

And allow some of them to do something other fish can't do.

♪ ♪ Leave the river and crawl on land.

HOLTZ: If you're a fish and you've lived your entire evolutionary history in the water, why come up on land?

Well, there's a couple different possibilities.

It might have been because they were chasing new prey.

HOLTZ: There are these bugs that are crawling around on land that nothing else is really eating.

BERNARD: But it could have been because they were looking for new areas in order to reproduce.

NARRATOR: Beyond the main river lies an isolated network of ponds and streams, where aquatic predators can't follow.

Perfect for allowing fish to spawn in safety.

HOLTZ: If they can go from pond to pond, they can protect their eggs, and therefore, the next generation are more likely to survive, and very importantly, more likely to reproduce.

NARRATOR: Meaning that the need to find safe places to spawn has profound consequences.

Because fish with the strongest fins are more likely to reach the predator-free pools.

They have more surviving offspring... ...which inherit their strong fins.

Over countless generations, bones get longer.

Digits and joints emerge.

Better for walking on land.

Until eventually, after millions of years... ...fins have become limbs.

♪ ♪ And they are fish no more.

SUMIDA: Paleontology is like a treasure hunt.

It's very exciting.

When you find a fossil of this quality, it's the find of a lifetime.

This is a cast of two individuals of an animal called Seymouria.

NARRATOR: The anatomy of Seymouria represents the early amphibious stage very well.

And some of its features are remarkably familiar.

Here you can see a very well-developed hand.

And you can see, right next to mine, five well-developed digits, just like you and me.

Our pattern of five digits goes back over 300 million years.

BLACK: This is basically a formula that's very widely shared.

And the reason it is is not because it's evolved over and over again, but because it goes back to a common ancestor.

NARRATOR: From lizards to elephants to ostriches to us, all land vertebrates trace their heritage back to those first four-legged pioneers.

OTTO: We have all of this shared evolutionary history.

So we look very different on the outside, but if you figure out on the inside what makes us tick, we're pretty similar.

JULIA CLARKE: And so to me, what's most magical about evolution is that we can have this incredible difference, but we are all connected, and you can't understand evolution without recognizing those two parts of the story.

♪ ♪ NARRATOR: For tens of millions of years... ...amphibians reign as the planet's dominant land animals.

But they have a weakness.

This newt-like creature has four strong legs, inherited from her river-dwelling ancestors.

She can move easily in water and on land.

But like many animals here, she's in trouble.

Drier periods have caused the pool she calls home to become little more than a muddy puddle.

To understand why this is such a problem, we only have to look at amphibians today.

♪ ♪ SUMIDA: What we know of amphibian skin we know mostly from the amphibians that are alive today, and living amphibians like frogs and salamanders must keep their skin moist to survive.

They can't dry out, otherwise it's deadly for them.

NARRATOR: And it's not just their skin.

Their eggs need water, too.

The first vertebrates that moved onto land were almost certainly still laying their eggs in the water.

FIELD: Just like living amphibians, their eggs were somewhat gelatinous, and therefore very prone to drying out if they were to be laid on dry land.

BLACK: So, amphibians are still very much tied to the water.

NARRATOR: In dry conditions, many amphibians struggle.

But this one has managed to hang on.

The survival of this individual's offspring doesn't hinge on luck alone.

Laying eggs in the right spot helps, as does the unique mix of genes she has inherited.

SUMIDA: When the environment changes, that's when the diversity of a population becomes important.

NARRATOR: Because sexual reproduction leads to each individual being genetically unique, it makes a species as a whole more resilient.

HOLTZ: Some people use the phrase "survival of the fittest" to describe evolution.

It doesn't necessarily mean the strongest or the smartest or the most aggressive.

It's simply the ones that are best suited to the environment in which they lived, and so they're likely to have more offspring, and therefore there's more like them in future generations.

♪ ♪ NARRATOR: This individual's unique mix of genes determines how likely her eggs are to survive in a drying world.

And hers are a tiny bit better at retaining moisture.

A small difference that can mean everything.

The slightest variation in the structure or composition of the egg is all it takes for her eggs to survive when others' don't.

♪ ♪ Over countless generations, the surface of the egg begins to change.

It gets tougher, thicker, and special membranes form to prevent moisture loss.

The egg gets larger and stronger... ...until, after millions of years, something extraordinary has emerged.

♪ ♪ Something that paleobiologists like Anusuya Chinsamy-Turan begin to see in the ancient rocks of South Africa.

CHINSAMY-TURAN: In the Karoo basin, we have an almost 128-million-year sequence of rocks.

So we can study how organisms evolved through that time.

♪ ♪ NARRATOR: Discovered among these rocks were some of the oldest recorded fossils of what are known as amniotic eggs, an evolutionary innovation that persists to this day.

CHINSAMY-TURAN: So this, I'm sure you will recognize, is a chicken egg.

It has this very hard shell.

You can't see it with your naked eye, but there are almost 20,000 tiny little pores in this eggshell.

And these allow that little developing embryo inside to breathe.

But now, if we crack open the same egg... ...inside, they are just absolutely marvelous structures.

We can see, of course, the yolk, and this is the most nutritious part of the egg.

It provides food for the developing embryo.

And the white of the egg acts as a shock absorber and prevents that embryo from getting injured.

If the egg was fertilized, immediately surrounding the embryo would be what we call the amniotic fluid, and this would keep the embryo developing in its own private little pond.

So, we have a really wonderful innovation.

NARRATOR: Kept moist and protected until they hatch, the young emerge as miniature adults.

CHINSAMY-TURAN: This is the egg that allowed these descendants of amphibians to venture further and further away from water, because, unlike amphibians, they didn't have to return to the water to reproduce.

♪ ♪ (chirping) NARRATOR: The ancestors of the ostrich were amphibians no more.

They'd evolved into something new: amniotes.

♪ ♪ HOLTZ: The amniotes are one of the most important groups of animals that have ever existed, and they represent one of the great branching points in the tree of life.

NARRATOR: One branch of amniotes gave rise to reptiles, dinosaurs, and birds.

♪ ♪ And the other branch went on to become mammals, most of whom now incubate their eggs internally.

Even humans, we have all the trappings of an egg, but we've just modified that to have this amazing gestation.

NARRATOR: Today, there are over 25,000 living species of amniotes.

And all owe their existence to the evolution of the amniotic egg.

(snorts) NARRATOR: But for the amniotes that would evolve into reptiles, one other anatomical breakthrough was key to their success.

BRUSATTE: One of the biggest differences, if you look at a reptile versus an amphibian today, is the skin.

Amphibian skin, it needs water, it, it feels moist.

But a reptile's skin, it feels dry, it's scally, it actually helps keep water in.

NARRATOR: Armed with scaly skin, reptiles began exploring a world beyond the water.

But the amniotic egg that would be pivotal to their success did pose a problem.

HOLTZ: A shelled egg is a great thing to do to lay on land, but there's a trick.

A shelled egg can't be fertilized after it's been laid outside the mother.

So in order to do that, the first fully terrestrial vertebrates needed to have sex in a brand-new way.

♪ ♪ NARRATOR: Part of the solution is found on almost all reptile and mammal species today.

It begins to grow early in development.

A tiny outgrowth, like an extra limb.

It's a penis.

BLACK: The origin of the penis was a major moment in evolution that allowed internal fertilization.

BRUSATTE: The egg and sperm could come together before the fertilized egg got a shell.

HOLTZ: This form of reproduction that is specialized for terrestrial vertebrates, that's what we call copulation.

BLACK: Some of the early evidence of internal fertilization that we have happened amongst armored fish many millions of years earlier.

But it was really important for life thriving on land.

(chirping loudly) NARRATOR: Today, penises come in all shapes and sizes across the animal kingdom.

♪ ♪ The blue whale has the largest, almost ten feet long.

The gorilla has one of the smallest, compared to his body size.

♪ ♪ Duck penises are shaped like corkscrews.

And those of cats are covered in barbs.

But for vertebrates that evolved a penis, it was only half the story.

HOLTZ: A penis by itself doesn't do anyone any good.

It takes two to tango.

And so the female reproductive anatomy had to co-evolve with the evolution of the penis.

And that way, the two of them together could make certain that those eggs could be fertilized and it didn't matter if you were near a pond or up in a tree or out in the desert.

♪ ♪ NARRATOR: With their sophisticated eggs and a new way to fertilize them... ...reptiles spread out across the planet.

But the anatomical changes that allowed them to do so had another consequence.

HOLTZ: It's obvious that the evolution of copulation changed the anatomy of the animals, the way they looked.

But what's less obvious, but at least as important, is, it changed the way that they behaved.

(footstep thuds) NARRATOR: And just 100 million years later... (footstep thuds) ...we can see this in action.

(footstep thuds) (Coelophysis growls) This is Coelophysis, an early dinosaur.

♪ ♪ Like his amphibious ancestors, he has strong legs for walking on land, but he walks on two instead of four.

♪ ♪ And it's thought his scaly reptilian skin might have been covered in a layer of fuzz.

♪ ♪ Right now, he's on the hunt.

Not for a meal... (growls) NARRATOR: ...but for a mate.

(Coelophyses growling) There are plenty of females around, but the tricky part is getting their attention.

♪ ♪ ♪ ♪ As eggs evolved to be larger, each one became more costly to produce, so females laid fewer of them, making each choice of a mate vital.

(Coelophysis growls) So, how does this male make himself stand out from the crowd?

♪ ♪ Scientists suspect that some dinosaurs did what many animals still do today: show off.

(Coelophysis snuffling) It might have started with head bobbing.

Followed by twirls.

But to really impress her, he has one more trick up his sleeve.

(Coelophysis growls) Will this be enough to make her choose him?

HOLTZ: In the game of life, survival is just the first test.

If you make it to adulthood, that's when you can be a contender in the mating game.

PISCHEDDA: If you don't get chosen, your genes represent an evolutionary dead end-- they go nowhere.

♪ ♪ NARRATOR: And the need to get chosen has led to some utterly bizarre features in modern animals.

From chameleons to crabs, we see exaggerated features throughout the animal kingdom.

Think about those weird noses of the proboscis monkeys, the horns of rhinos, the enormous antlers of moose.

PISCHEDDA: The fiddler crab that has a giant claw that can be half of its body weight.

They're bizarre and beautiful.

BRUSATTE: You could go on and on about these structures that just seem so inconceivable, but they are essential for reproduction.

♪ ♪ NARRATOR: Fuzzy arm fringes like this might have been just as essential in the mating game.

If females find them attractive, this Coelophysis is more likely to be chosen, meaning his genes get passed down.

And some of his offspring go on to have fuzzy offspring of their own.

Over countless generations, those fuzzy strands become exaggerated, getting longer and wider.

The strands split into multiple filaments... ♪ ♪ ...making an even bigger surface area for display.

New colors and patterns emerge, attracting even more attention.

Until eventually, after tens of millions of years, those fuzzy strands are so transformed, they've become something new.

♪ ♪ This dinosaur now has more than just fuzz.

He is adorned in elaborate, seductive feathers.

(ostrich chirping) And once these new tools for flirting arrived, they were here to stay.

♪ ♪ FIELD: If you think about birds, the single most distinctive attribute that sets them apart from anything else alive today is the presence of feathers.

NARRATOR: Home to over 850 bird species, South Africa is a paradise for paleontologist and bird enthusiast Daniel Field.

(chirps) FIELD: I love birds.

It's hard to explain why.

I fell in love with them when I was about nine years old.

I've got a feather here, which comes from a species called the helmeted guinea fowl.

Which is a common bird in this sort of open habitat in Southern Africa.

And these beautiful feathers serve a really important function in display.

♪ ♪ This comes from the tail of a common pheasant.

In the U.K., where I live, these birds are extremely abundant.

It's greatly elongated and it's got these beautiful, complex patterns on it, also, and the primary function for a feather like this is, well, sexiness.

♪ ♪ NARRATOR: And scientists have strong evidence that birds' use of feathers to attract females has a long history.

This is an image of a fossil from about 160 million years ago from China.

It's called Epidexipteryx, and it's a fascinating feathered dinosaur.

We can see these bizarre, elongate feathers coming off from its tail area.

The only conceivable function of these particular feathers is that they would have been useful in courtship or display.

And that's quite fascinating, because that is an attribute of living birds that, thanks to Epidexipteryx, we can say may extend all the way back 160 million years.

♪ ♪ NARRATOR: Early barbed feathers had a central shaft... ♪ ♪ ...with equal-length barbs on either side.

But around 150 million years ago, a new type of feather appeared in the fossil record: one with a subtle change that would alter the destiny of the dinosaurs.

FIELD: We see the appearance of an asymmetrical feather.

So this feather comes from a goose, and you can clearly see that one edge of the feather is much narrower and it's also stiffer.

This is probably one of those features that just arose by chance in the first place, but it had really important consequences, because this sort of asymmetrical shape shifts the center of pressure towards the leading edge of the feather, creating a much more stable, aerodynamic surface for flight.

♪ ♪ NARRATOR: From fuzz to flirting to flight... ...over 50 million years of the mating game resulted in feathered dinosaurs filling the skies.

For nearly 100 million years, they thrived alongside their ground-dwelling cousins.

(birds calling) And they were there to witness one of our planet's darkest days.

(sonic boom echoes in distance) (explosion roars) FIELD: After the asteroid impact, the world would have been a terrible place to be.

BRUSATTE: Whole food chains collapsed.

But seeds can stay in the soil even if an island is obliterated by a volcano, and the same would have been true with the asteroid, so if you could eat seeds, that might have been your ticket to surviving.

NARRATOR: Within just a few years, three-quarters of all plant and animal species on Earth died out.

BRUSATTE: One of the greatest myths in all of science is this idea that the dinosaurs went extinct.

But it's not quite accurate, because one type of dinosaur did make it through.

And these were the dinosaurs with feathers and wings and beaks-- the dinosaurs that we call birds.

Today's birds evolved from dinosaurs, they are part of the dinosaur family tree, and they were the only dinosaur survivors of that asteroid.

♪ ♪ NARRATOR: But birds didn't just survive.

They thrived.

Today, there are almost twice as many species of birds as there are of mammals.

Some are so skilled at flying that they can stay airborne for months at a time.

While others have shrunk to become the smallest dinosaurs to ever live.

♪ ♪ But in the aftermath of the extinction, large ground predators were gone.

And with flight being energetically expensive, some birds gave up on it altogether.

♪ ♪ This is Pseudocrypturus.

It's about the size of a pheasant.

Birds like it foraged on the forest floor.

And as the generations passed, some of their descendants grew bigger.

Their legs got longer and better for walking.

Until, 20 million years ago, an iconic animal emerged.

♪ ♪ The largest bird alive today.

(chirps) ♪ ♪ CLARKE: When I see an ostrich, I see that the story of dinosaur evolution is not over, but is ongoing.

(moaning) BLACK: Ostriches are just a fantastic symbol of evolutionary change and innovation.

Here you have a surviving dinosaur covered in feathers that evolved from flighted ancestors.

(chirps softly) NARRATOR: Their flying days are long gone, but their feathers remain, still playing a starring role in the mating game.

BRUSATTE: Instead of being used for flying, the wings are now used in mating displays.

And that's a prime example of how evolution is always tinkering, always changing, always adapting.

NARRATOR: Their vast, nutrient-rich eggs are more than just cradles for new life, they are time capsules, holding the story of countless generations.

BHULLAR: The fact that this is the largest single cell in the animal kingdom is an extraordinary testament to how far evolution has brought us from our single-celled ancestors to the complex organisms we are today.

♪ ♪ NARRATOR: For billions of years, sexual reproduction has shaped life on Earth.

♪ ♪ PISCHEDDA: Without sex, we wouldn't have beautiful feathers, or antlers, or horns, or courtship displays, or bird calls.

OTTO: All this diversity evolved because that sexual creature long ago evolved to mix their genes and enable each evolutionary branch of this tree of life to meander in its own direction.

(chirping) BLACK: When I look at life on our planet, when I look at nature, what I'm just endlessly in awe of is how life continues to create itself.

Every moment, every day, evolution brings us something novel, and I think that's gorgeous.

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