♪ MAN: I was taken out to this site in the middle of the desert.

I had no idea what awaited me.

And then I saw them, and I just started crying.

NARRATOR: These trackways in the Arabian Desert are footprints in time.

They were made seven million years ago by prehistoric elephants.

They take us back to a bygone world and its vanished creatures.

How did elephants from so long ago give rise to the magnificent animals we know today?

How did any of the animals in this African landscape become what they are?

For generations, it was a mystery.

Now scientists are revealing the answers... WOMAN: You can see the exceptional preservation of soft tissues.

NARRATOR: ...piecing together an epic story.

Fossils are the key.

MAN: Fossils are the messengers of the past.

I believe that.

MAN: Imagine I'm taking you on a safari, but a safari back in time.

You'll be confronted with these magical creatures, a slice of Africa that is now gone.

NARRATOR: This film unlocks the evolution of four of the world's most spectacular creatures-- crocodiles, birds, whales... [whale singing] [roars] and elephants.

Like every animal alive today, they have a deep time history, a lineage full of twists and turns, shaped by strange ancestors from long ago-- bizarre ancient crocs... MAN: This thing was built like a greyhound.

NARRATOR: ...feathered dinosaurs... WOMAN: Dinosaurs never went extinct.

In fact, birds are dinosaurs.

NARRATOR: ...a whale ancestor that lived on land.

MAN: It more looks like a dog with a long snout.

NARRATOR: With new tools at their fingertips, researchers are filling in the gaps in the story of life, charting the rise and fall of ancient animals, revealing why some died out while others survived.

MAN: How did these ancient worlds flourish, and why did they disappear?

So, this is to me, really sort of the ultimate story.

NARRATOR: It's not just a story about the past.

It tells us about the world today, its creatures, and what they may be facing in the future.

WOMAN: There used to be dozens of species of elephants, and now there are just three.

And if we're not careful, we will lose them as well, and that will be the end of the lineage of elephants.

NARRATOR: It's a story millions of years in the making, of lost creatures rediscovered and the surprising lessons of deep time.

♪ ♪ Madagascar, 250 miles off the coast of Africa.

This island is a perfect laboratory for the study of evolutionary change.

Isolated here for millions of years, species have transformed in unique ways.

Scientists have long studied the evolution of the island's chameleons, bats, and lemurs.

Now the search is on for the ancient history of one of Earth's strangest animals-- the crocodile.

♪ In flooded underground caves, researchers are looking for the remains of an extinct croc species.

It may have disappeared just 2,000 years ago, a recent twist in the story of crocs.

EVON HEKKALA: Some people see crocodiles as this animal that's unchanging through time.

And, actually, we know now that that's not true, but they look like it.

I'm Dr. Evon Hekkala, and I use DNA to look at how crocodiles have changed in the recent past and how they are related to each other in deeper time.

Most of the living species of crocodile are really, really hard to tell apart, unless you're somebody who spends all your time thinking about crocodiles, like me.

NARRATOR: Cold-blooded, ferocious, but strangely beautiful, the 14 living crocodile species are all very similar.

They all have the same reptilian body plan... low to the ground and armored from head to toe in thick scales.

♪ They seem like living fossils, some hangover from the age of dinosaurs.

But are they?

Is this really an animal frozen in time?

Perhaps the mysterious species in the cave will provide some answers about recent crocodile evolution.

HEKKALA: So, there's this crocodile that existed on the island of Madagascar until relatively recently, and it disappears about the same time that humans got here, and that crocodile was called the horned crocodile of Madagascar.

And so, one of the big questions is, what was that crocodile?

Who were its relatives?

And what happened to it?

It's one of the biggest mysteries.

♪ NARRATOR: Today there is only one species on Madagascar... the famous Nile crocodile... one of the fiercest of Africa's predators.

♪ [wildebeest bellowing] ♪ Scientists think it squeezed out the smaller horned croc, but there's a lot they don't know.

There may be answers in the flooded caves.

A dive team sets out to search for the bones of the horned croc.

The caves are a time capsule, where the remains of ancient animals have been preserved for thousands of years.

HEKKALA: I knew from speaking with some other people that there were supposedly crocodile skulls in this cave of this species that no one has seen for at least 200 years, but more likely 1,000 years.

NARRATOR: Evon waits, hoping the skulls are there.

HEKKALA: And I'm sitting there and I'm thinking, just the anticipation is like, "Any minute, I'm going to get to see this thing."

It must be so exciting when they bring things in.

You have no idea what they're going to bring.

WOMAN: Yeah, yeah, it's like Christmas every time.

NARRATOR: Deep in the caves, divers have to squeeze through tight spaces.

If they brush the bottom, they can stir up clouds of silt and lose their way.

♪ A half hour in, they see it-- a skull.

♪ ♪ WOMAN: What do you think?

HEKKALA: It looks like a crocodile!

WOMAN: Does it?

HEKKALA: So excited.

WOMAN: A skull?

HEKKALA: Yeah.

WOMAN: Look at that, it looks perfect.

HEKKALA: Oh, it's beautiful.

Wow!

WOMAN: Oh, my God!

It is a crocodile.

HEKKALA: You know, half the collections are just pieces, so... WOMAN: That is so gorgeous.

You got it?

HEKKALA: Yeah, I've got it.

This is a skull of the extinct horned crocodile of Madagascar, and when researchers first started coming to Madagascar from Europe, they saw these crocodile skulls, and they weren't sure exactly what they were.

They didn't look exactly like Nile crocodiles.

They realized it was a separate species.

This individual is the first individual I've seen that came out of a cave, a water-filled cave, so that is pretty exciting.

NARRATOR: If Evon's lucky, she'll be able to extract its DNA and learn where the horned croc fits into the story of crocodile evolution.

The results will take time, but one thing is clear-- apart from its small but distinctive horns, this animal was very similar to the crocs alive today-- armored, low to the ground, with short legs and large, powerful jaws.

To us, this body plan is what defines a crocodile, but millions of years in the past, there was more than one way to be a croc.

HEKKALA: One of the cool things that's happening these days is that we're finding out that the ancient crocodilians were incredibly diverse, and there are all different forms of them.

NARRATOR: Scientists are now discovering the earliest ancient crocs.

They were nothing like the animals alive today.

To meet the very first crocs, we must trace one branch of the vast tree of life far back in deep time, 230 million years into the geologic period called the Triassic.

It's hard to imagine such a vast expanse of time.

HANS SUES: We throw around numbers as scientists, but at the same time, do we really understand what it means that 100 million years have passed?

I'm Hans Sues.

I'm a paleontologist at the Smithsonian Institution in Washington, D.C., and I study fossils.

If you look, for instance, at your historical time, you can think back to your grandparents and that time frame that people are still comfortable with.

But you go further back, go, say, back thousands of years when much of North America was covered by ice, a very different world.

But go further back yet.

Go back to 100 million years, and this world would have been entirely unrecognizable, would have been like going to another planet and seeing life forms that were, for the most part, utterly alien to our experience, and this is really difficult to grasp.

NARRATOR: There is little about the planet of the Triassic we would recognize.

The continents we know today had not yet formed.

They were all contained in one huge land mass surrounded by water--Pangaea.

Most of Pangaea was dry, with blazing hot summers and cold winters.

From Triassic period rocks, scientists know much of the interior was desert.

Fossilized plant remains tell them that closer to the sea, there were open, fern-filled woodlands.

It was here, along with the dinosaurs, that the first crocs evolved.

In the beginning, they were even more successful than the dinosaurs, and to our eyes, they look totally bizarre.

ANJAN BHULLAR: Well, the Triassic was, in many ways, actually the age of crocodiles, and in any Triassic landscape, you would have seen this vast diversity of crocodile relatives with many body forms.

My name is Bhart-Anjan Bhullar.

I go by Anjan.

I'm a paleontologist, and I would say that I am a historian of life.

NARRATOR: Anjan Bhullar has been unearthing the bones of the very first crocodile relatives.

Two of them were found together near some sandstone cliffs in southern Utah.

BHULLAR: Well, these are two of the most extraordinary skeletons from the crocodile line that have ever been found.

This large animal here is something called a Poposaur.

It is the only complete skeleton of one of these animals that's ever been found.

All of the subtle features on it, I mean, it's just, it's the find of a lifetime.

These animals show us what crocodiles were like at the beginning of their evolution, and they're very, very different.

The extraordinary thing about this animal is that it was really trying to be a dinosaur before dinosaurs were dominant, in that it was actually walking around on two legs.

You see how large and how heavily built the legs of this animal are compared to these tiny little arms.

And so, this animal couldn't rest any of its weight on its arms.

NARRATOR: Poposaurus was a fierce, fast-moving land predator, acting for all the world like a dinosaur.

And it wasn't the only strange Triassic croc to be found in Utah.

BHULLAR: In fact, the Poposaur, this large skeleton, was the first thing we found, and we dug out this big skeleton.

Then right underneath it was a tiny foot.

And underneath that foot was this animal here, which is the only complete specimen of what we call a sphenosuchian-grade stem crocodile that's ever been found.

NARRATOR: They called the sphenosuchian "Little Foot."

It was even stranger than Poposaurus.

BHULLAR: And so, the Poposaur is something like a mountain lion, whereas the sphenosuchian, you look at this animal, and it's got a slender little body, it's got extraordinarily long legs, extraordinarily long arms, and these arms were taking its weight, unlike those of the Poposaur.

And so this thing was built almost like a greyhound with a heavy head and a long tail.

It was an animal that was utterly built for speed.

♪ NARRATOR: It's hard to imagine an animal less like the lumbering, heavy-set modern crocodiles.

How did Anjan even know they were related?

BHULLAR: There are many features, subtle features of the skeleton that tell us it's actually from the crocodile line.

NARRATOR: One tell-tale clue is the structure of the feet and ankles.

Another is in the skull.

BHULLAR: Among the crocodile relatives, there are a few iconic features.

One of those is the heavy reinforcement of the skull bones, especially the jaws.

These animals had strong skulls and a very powerful bite.

NARRATOR: By comparing details like these, scientists can figure out if fossils are related to each other and arrange them on a family tree.

BHULLAR: As our ways of thinking about evolution have improved, we've realized that instead of a sort of general overall similarity, we should really be focusing on unique shared characters.

NARRATOR: If two fossils share unique characteristics, the odds are that they once shared a common ancestor, too.

BHULLAR: This spread on the table in front of me encompasses more than 220 million years of evolution.

And in fact, if I look at this group, I can actually start to see that a couple of these animals share that reinforced skull form.

NARRATOR: That tells Anjan they belong in the crocodile line, whereas other fossils, superficially similar, don't.

BHULLAR: These fossils actually have a much more lightly built skull, almost a bird-like lightly built skull, so these two don't pertain to the ancestry of crocodiles.

NARRATOR: By focusing on details like ankles and skulls, scientists can build a family tree of crocs and reveal the twists and turns of their deep time history.

The strange creatures of the Triassic were part of the first great flowering of the crocodile family tree, but 200 million years ago, much of that tree was cut back... [rumbling] ...when a mass extinction wiped out the vast majority of the animals of land and sea.

As the supercontinent Pangaea began to break up, volcanoes pumped vast amounts of carbon dioxide into the air, acidifying the oceans.

The planet warmed, and habitats were transformed.

Most of the Triassic crocodile relatives, including Poposaurus and Little Foot, died out.

SUES: Extinctions are of great interest to evolutionary biologists because they basically reset the evolutionary game.

We know that organisms, over time, try to adapt to particular ecological circumstances, however, every once in a while during the history of life, there have been catastrophic events of such magnitude that thousands or millions of species were wiped out in what basically in deep time is a single moment.

NARRATOR: There have been five major extinctions in Earth's history.

Through the lens of deep time, we can see they caused profound destruction and changed the history of life on Earth.

The mass extinction that ended the Triassic period ushered in a new chapter in the story of crocs.

In the Jurassic and Cretaceous periods, crocs flourished again, but in new and even stranger ways.

A record of those wondrous creatures is preserved in one of the most spectacular landscapes on Earth--Patagonia.

♪ DIEGO POL: After the Triassic- Jurassic extinction event, many groups diversified in the Jurassic.

Dinosaurs were certainly the most conspicuous because they were big and they were very abundant, but other groups began to diversify in a very, very impressive way, and crocs were one of them.

I'm Diego Pol.

I'm a paleontologist.

I work here in Patagonia in the eastern coast, in the southern tip of South America, and I study crocs.

I really love to drive, you know, in places like this because it's really like you are driving through time.

NARRATOR: For the duration of the Jurassic and Cretaceous periods, almost 150 million years, crocs vied with dinosaurs for dominance.

They stopped being only the land animals they were in the Triassic and experimented with radically new ways of being.

POL: So, the Triassic is over.

We're moving into the Jurassic, and dinosaurs were evolving and diversifying, but also crocs were diversifying at the same time.

You start seeing animals like Dakosaurus, is the swimming marine croc that adapted their forelimbs into paddle-like flippers.

And then you have freshwater gigantic crocs like Sarcosuchus that's about 12 meters long.

It was eating dinosaurs.

And then you have herbivorous crocs like Notosuchians.

Look at this animal.

It, it really looks nothing like a croc.

It has a short snout, very high, the eye sockets are pointing to the sides, and the, the crazy, crazy thing is that it moved the lower jaw back and forth.

And this was a way of the upper teeth and the lower teeth to slide against, against each other, and in that way, this animal was processing the plant matter.

NARRATOR: So, here was an ancient croc that chewed like a goat.

Diego has spent much of the last 20 years excavating ancient crocs from the barren outcrops of Patagonia.

But back when those creatures lived, the world was a very different place.

The continents were forming, and it was hot.

POL: It was much warmer planet.

There were no ice caps on the poles.

There was, uh, a time when conifers dominated the ecosystem.

NARRATOR: In the warming greenhouse world of the Jurassic and Cretaceous, crocs prospered.

This was their golden age.

POL: They became so diverse that in some places you go, and you basically only find crocs.

So, we can see a really, really crazy diversity in the Cretaceous.

And then about 66 million years ago, it all went away.

NARRATOR: What happened to the many Cretaceous crocs?

It seems most of them perished the same way as the dinosaurs.

SUES: It was long realized that around 66 million years ago, there was a major extinction of animals and plants on land and in the oceans.

On land, the most famous casualty were the dinosaurs.

[crash] NARRATOR: Scientists agree that an asteroid strike brought on the global devastation, but many now argue it was not the only cause of the mass extinction.

From the fossil record, they can see that climate changes were already pushing many species to extinction before the asteroid impact.

SUES: So basically, this impact is sort of thought, if not the sole cause of the extinction, certainly the coup de grâce for a great many lineages.

[growls] NARRATOR: Ancient crocs were reduced to a fraction of what they once had been.

POL: So, out of this wonderful diversity, only a few species made it through.

And the ones that actually made it through were very particular in many ways.

They were certainly adapted to living in the freshwater environment.

They were adapted to, uh, feeding in water.

They were predators.

They were not herbivores.

They were not land crocodiles.

They were not marine crocodiles.

So only a tiny fraction of that diversity made it through.

NARRATOR: The crocodiles that made it through were the shoreline predators that lived half in water and half on land.

Their low, tank-like body plan may have been one secret of their survival.

It allows them to lie semi-submerged in the shallows and ambush their prey with ferocious speed.

[splash] The success of that lethal design is why crocodiles today all seem so similar.

HEKKALA: So, crocodiles have this form that's very, very successful.

They have sort of an armored body plan that allows them to be a successful predator in aquatic environments.

Aquatic environments can be more stable than other kinds of environments, like terrestrial environments, and so it makes sense that they've retained this body plan from deep time, from the ancestral crocodilians that were around the globe millions of years ago.

It works really well.

Why change it?

NARRATOR: Of course, that classic body plan is no guarantee of survival.

The horned croc of Madagascar died out just centuries ago, likely pushed out by the giant Nile croc.

Results from Evon's work suggest the species are more closely related than anybody realized.

♪ It's a great example of how DNA analysis has become a vital tool for paleontologists.

♪ HEKKALA: One of the exciting things about the new tool kit we have today, the ability to use DNA to look at evolutionary history, is we can use it as sort of a metric for change over time and lineages.

So, with living species, we can take their DNA and we can calculate back how long it takes to accumulate the number of changes we see in the genome.

And we can say this corresponds to a split three million years ago or six million years ago or 20 million years ago.

NARRATOR: DNA has shown that both crocodiles and alligators are descended from a common ancestor that lived 80 million years ago.

HEKKALA: The ancestral lineage that the true crocodiles came from diverged from the group that includes all the alligators about 60 to 80 million years ago.

And then, even more recently, this thing that we've always thought as the sort of primordial crocodile, the Nile crocodile, we recently have found, using molecular clock dating, that the Nile crocodile is just a baby.

It just arose probably within the last four to six million years.

It's a young'un on the landscape of, uh, crocodilians in the modern world.

NARRATOR: So, despite their prehistoric appearance, it turns out that over their 230-million-year history, crocodiles have been in constant evolution.

They have proved themselves to be one of the most resilient lineages on the face of the planet.

HEKKALA: For me, these crocodiles sort of represent an organism that, through the history of life on Earth, it's found a way to persist.

NARRATOR: The story of the crocodile lineage is of deep time transformations that produced a wild diversity of croc species, followed by a steep decline.

Today there are just a handful of crocs, and the strange animals that gave rise to them are all gone.

The surviving crocs are all very similar, all shoreline predators of the tropics.

But while the crocodile lineage has bottlenecked, another lineage is experiencing a wild explosion of diversity.

[squawking] ♪ [cawing] [honks] [quacking] [honking] Birds have colonized every environment on Earth.

And they come in an astonishing variety of shapes, colors, and sizes.

[whistling] JULIA CLARKE: There are more species of birds than any other group of vertebrates that lives on land.

♪ They can cross the Himalayas on wing.

They can dive into a part of the ocean where sunlight does not reach.

They can migrate between continents.

So, they are truly remarkable.

I'm Julia Clarke, and I'm a professor of vertebrate paleontology at the University of Texas at Austin.

[ducks quacking] Well, I think the captivation of birds is that they have this intelligence that we don't really understand.

It's sort of like this foreign or alien intelligence that is in our everyday spaces, which is the premise of, like, a lot of sci-fi movies, if you think about it, you know, they're among us.

And, if you look closely, I think there's a lot to be fascinated with, even in everyday birds.

NARRATOR: Birds have colonized not only the natural environments of the planet, but also the urban spaces created by humans.

CLARKE: There are more than 10,000 species of birds, but even more striking than that, there are more than half that number is just within one lineage, songbirds, so that means that the birds that are in your backyard are part of what is truly an extraordinary evolution.

NARRATOR: So, what's the story behind the spread of birds across the planet?

How did they come to be everywhere and so diverse?

CLARKE: Things that can seem so commonplace, crows or pigeons in a park, are the leavings of an amazing history that stretches back hundreds of millions of years.

NARRATOR: Our understanding of that evolutionary history began with one extraordinary fossil discovered in the 1860s in Germany--Archaeopteryx.

CLARKE: This fossil, to many people, might just look like roadkill or something that hit your window.

But in fact, to me, these bones, they come to life.

And the wings are moving, covered in feathers, but with mobile claws at their tips.

Most striking is a long, bony tail with feathers.

NARRATOR: 150 million years old, archaeopteryx was a bird.

It had feathers, and it could fly.

But with its claws, tail, and toothed beak, there was something almost dinosaur-like about it.

It led proponents of the new theory of evolution to make a sensational claim: Birds must have evolved from dinosaurs.

CLARKE: When I look at this, I see an icon of evolution.

It was one of the first key and totally unavoidable pieces of evidence consistent with evolution.

NARRATOR: But the theory that birds evolved from dinosaurs met intense opposition.

How could something so huge and heavy evolve into something so small and light?

One of the biggest objections was that no dinosaur had ever been found with a wishbone, in birds, the crucial brace for the chest that makes flight possible.

The search was on.

If scientists could find a dinosaur with a wishbone, they would clinch the case.

But for a century, they failed.

♪ Then, in the 1960s, paleontologist John Ostrom hit pay dirt... a dinosaur fossil with a wishbone.

He called it Deinonychus, terrible claw.

JACQUES GAUTHIER: Here we have Deinonychus antirrhopus, the fossil that changed everything that we know about the origin of birds and fundamentally altered our understanding of how flight evolved.

NARRATOR: Deinonychus was a ferocious predator with wing-like arms and all the bones and muscles necessary for flight, but it couldn't fly.

GAUTHIER: Here's an animal with forelimbs much too short and much too heavy in the body to be able to fly, yet it has all the bells and whistles that we associate with the flight stroke.

[squawking] NARRATOR: Not only that-- it had feathers, too.

But all this had nothing to do with flight.

Its feathers were for warmth, and its clawed wings were for killing.

[squawks] GAUTHIER: The arms up against the body and shoot it down and forward, grab your prey, drag it up and back.

Down and forward, up and back, down and forward, up and back.

That's the flight stroke.

So, all the details, the basic architecture, at least, of the flight stroke is evolved in an animal that is not using it to fly.

NARRATOR: Amazingly, these tools built for killing would eventually power flight.

[squawks] GAUTHIER: So, you have feathers and you have the flight stroke colliding in one animal, so the skies were no longer a barrier to the evolution of dinosaurs.

And that's why we think that birds are living dinosaurs.

NARRATOR: Deinonychus was powerful evidence that dinosaurs gave rise to birds.

But the many stages of that evolution were still unknown.

To complete the story, scientists needed more fossils.

Another long wait began.

♪ Then, in the 1990s, farmers in a remote province of northeastern China blew the story of bird evolution wide-open.

[clucking] They had been turning up rocks with the outlines of birds in them for as long as they could remember.

They had no idea these were the bird/dinosaur fossils scientists had been waiting for.

By the late 1990s, fossil fever had broken out in Liaoning Province.

[man speaking Chinese] [rooster crows] NARRATOR: A single fossil could bring a year's income.

As farmers scoured the landscape, the fossils started to pour in, hundreds of ancient birds from the time of the dinosaurs.

JINGMAI O'CONNOR: Bird fossils are extremely rare, typically, in the fossil record.

And up until the eighties, all we really had was Archaeopteryx.

So, without anything in between this bird that looks very much like a dinosaur and birds that look very much like living birds, without any fossils to fill in this gap, we had no idea how the modern bird evolved.

But the fossils from Western Liaoning started to show us the transitional forms.

They showed us the earliest birds with beaks, the earliest birds with short tails, like living birds.

My name is Jingmai O'Connor, and I am a vertebrate paleontologist.

You know, I have a very strong personality, a bit unusual, well, at least for a scientist.

To be honest, my bird obsession grew with my studies.

I've developed this deep fascination and love for these, this incredible group of animals.

NARRATOR: Jingmai's obsession with birds was cemented by the fossils of Liaoning.

They are priceless.

Bird fossils are extremely rare because bird bones are so delicate.

But here in northeastern China, 130 million years ago, a unique combination of circumstances created the perfect conditions for their preservation.

O'CONNOR: If you were able to come back here 131 to 120 million years ago and looked out, you would have seen lakes as far as the eye could see, with active volcanoes going off around them and a forested environment growing by the shores of these lakes, and this forest would have been teeming with small feathered dinosaurs.

[rumbling] NARRATOR: As those small feathered dinosaurs died and fell or were washed into lakes, they were quickly buried in layers of volcanic ash.

♪ The result?

An extraordinary record of Cretaceous bird/dinosaurs perfectly preserved in stone.

O'CONNOR: The amazing thing about these fossils is the exceptional preservation of soft tissues which reveal these extinct animals in a level of detail that we paleontologists never previously thought possible.

NARRATOR: Fossilized along with the birds were plants and seeds, allowing scientists to reconstruct the forested environment of the time.

Together, the wealth of Liaoning fossils reveal a world where creatures representing every stage of dinosaur-to-bird evolution lived side by side.

They show how nature experimented with every aspect of what would finally make a bird-- beaks, wings, feathers, and flight.

Here was a sort of evolutionary laboratory in which many different combinations of dinosaur and bird characteristics were tried out.

There was Caudipteryx, a small feathered dinosaur that had wings but couldn't fly.

O'CONNOR: You can see, with its very robust hind limbs and its very small forelimbs, this was definitely not a flying dinosaur.

NARRATOR: Living alongside it were creatures that flew but still had many dinosaur characteristics-- teeth, long tails, and claws on their wings.

O'CONNOR: Here we have a primitive bird named Jeholornis.

It has lost almost all its teeth.

It has a shoulder girdle that looks almost like that of living birds, but then it has this tail that is actually longer than that of Archaeopteryx.

♪ NARRATOR: And there were creatures like Confuciusornis, almost identical to modern birds.

O'CONNOR: If you were in northeastern China during the early Cretaceous, you would see flocks of Confuciusornis flying overhead.

NARRATOR: The feathered dinosaurs of Liaoning are a reminder that evolution does not move in a straight line.

SUES: There's a common perception that evolution proceeds in a very linear fashion.

So, you have a little dinosaur that somehow decides, quote, unquote, to become a bird.

But actually, the many bird features were gradually acquired.

There were a lot of dead ends in this evolutionary process.

[squawking] NARRATOR: Just like Deinonychus, the Liaoning fossils show that wings and feathers evolved separately and for reasons that have nothing to do with flight.

[squawks] It was only after millions of years that these things came together to make the creature we call a bird.

O'CONNOR: In fact, what we've learned from this huge diversity of fossils is that these features that we associate with modern birds actually evolved multiple times.

And I'm not only talking about flight, I'm talking about beaks, I'm talking about the abbreviated tail.

NARRATOR: The Liaoning fossils opened a window on the profusion of feathered dinosaurs that populated the whole world many millions of years ago.

What happened to them all?

Most died out in the same extinction event that killed the big terrestrial dinosaurs and most crocs.

[crash] But like the crocs, a few birds squeaked through.

Those few survivors gave rise to all the birds on Earth today.

[honking] But how did that amazing story of survival play out?

Remarkably, scientists are solving that mystery not from bones, but from DNA.

ERICH JARVIS: So, what happened is that this mass extinction occurs, maybe as a result of a big, giant meteorite, then climate change and so forth.

A few groups of birds survive, only five percent of the species survive.

My name is Erich Jarvis.

I'm a professor at the Rockefeller University in New York.

I study how the brain produces learned behaviors, specifically like language.

[chirping] JARVIS: See, that's a call.

NARRATOR: Erich is especially interested in how bird songs evolved.

But songs don't fossilize.

So, along with other scientists, Erich hoped the answer might come from genetics.

JARVIS: And we all wanted to know, is the bird family tree correct?

Because there are many papers that were coming out, changing the tree from one year to another, and it keeps screwing up our experiments, basically.

NARRATOR: It was an international consortium of scientists working to pin down the avian tree of life.

And in the process, they would answer the big question-- how did the birds that survived the asteroid go on to take over the planet?

Erich and his colleagues compared the genomes of different bird species to calculate their degrees of similarity.

They could then figure out when they last shared common ancestors.

JARVIS: Fossils is anatomical structures with shapes, of course, and that anatomical structure you can measure, you can quantify the volume, the size, the shape, and so forth and use that information to infer relatedness, whereas the genome is pieces of DNA, the genetic code, that you can also use to infer relatedness.

The genome has millions, hundreds of millions of pieces of information in the sequence.

Which one is more accurate?

Most people trust the DNA to be more accurate.

NARRATOR: DNA sleuthing can't replace the work of paleontologists with fossils, but it made for a big break in the case of recent bird evolution.

It showed that just a handful of birds survived the mass extinction to give rise to the amazing diversity of birds today.

JARVIS: First thing we figured out is that those ones that survived, with the deepest branches in the tree that we generated, were the ones that were shorebirds, that can survive in water, that can survive in land, that can survive in different habitats.

NARRATOR: Along with shorebirds like ducks and geese, the ancestors of ostriches and emus also survived.

JARVIS: Then those few four or five lineages that survive, in a 15-million-year window, gave rise to every species that we are looking at now today.

NARRATOR: How exactly did that small group of survivors give rise to modern birds?

Scientists are still working that out.

CLARKE: I think that why birds are so species-rich is a big question, and it's not one that's going to be easily answered.

There's so many innovations that have been proposed to explain this incredible number of species we have today.

And some of them are, of course, flight.

And some of them are rapid growth to adult-size, so baby birds grow to full-size superfast.

Other strategies are things like bright colors or complex color patterns, complex modes of communication that can be deployed in attracting a mate or defending a territory.

These kinds of behaviors are thought to be linked to rapid speciation rates.

So, these are all big, open questions, and there is, as yet, no simple answer.

NARRATOR: Whatever the reason for the amazing diversity of birds, scientists are finally in a position to see the whole vast avian tree of life.

Arising with the dinosaurs, the bird lineage experimented for millions of years with feathers, beaks, and wings.

It barely survived a major extinction event before finally flowering in the 10,000 species alive today.

[honking] But while the bird lineage exploded and birds colonized every habitat on Earth, another lineage concentrated on colonizing just the seas.

♪ For as long as humans have known them, whales have inspired fear and wonder.

How could these giants of the deep have grown so huge?

How could air-breathing creatures like us have come to live in the depths of the ocean?

In the next hour, we explore the wonders of whale evolution and the astonishing rise and fall of the empire of the elephants.

♪ No animal embodies the beauty and awe of the seas more than whales.

[whale singing] What science now knows about these leviathans of the deep is amazing.

[singing] Bowhead whales can live for up to 200 years, longer than any other mammal on Earth.

Sperm whales can dive to depths of over 3,000 feet and stay underwater for an hour and a half without breathing.

Blue whales are the biggest creatures to ever have lived on Earth, bigger than even the largest dinosaurs.

♪ Up to 100 feet long and 200 tons in weight, they have a heart the size of a small car.

And these giants of the deep sing to each other, communicating over vast distances in a language of song that researchers still don't fully understand.

It's just one of the many mysteries still surrounding whales.

JACKIE HILDERING: What I've learned more than anything else as a scientist is how little we know.

There's this presumption that we know so much.

We don't.

I'm Jackie Hildering.

[camera shutter clicks] I'm a whale researcher and educator.

♪ [whale blows] [laughs] Mom and calf, Obsidian and Ripple.

Yes, yes.

[clicking] It's mind-blowing to see the number of humpbacks here, considering what a rarity it was to used to see them.

[whale blows] And I'm trying to figure out who they all are.

And this is... [click] Claw.

It's a female.

She's had a baby.

NARRATOR: Jackie has been working with the humpback whale population off northern Vancouver Island for almost 20 years.

She knows each of them individually by the distinctive marks on their tail flukes.

HILDERING: What I can't express properly is what it feels like to know who they are.

That the one that just went by I've known since he was a calf and was here with his mother in 2007.

NARRATOR: With others, Jackie is working to understand humpback family groups and their epic migrations.

Every year, they swim up to 6,000 miles, mating in the warm waters of the equator, feeding in the cold food-rich waters nearer the poles.

How did these gargantuan animals and their remarkable behavior evolve?

For so long, it was a mystery.

MARK UHEN: We've known that whales are mammals since the 18th and 19th centuries because they're warm blooded and they suckle their young, but we didn't know where they came from.

I'm Mark Uhen, and I work at George Mason University, and I study the evolution of whales and other marine mammals.

[click] [click] [click] Darwin talked about the origin of whales in his classic book "The Origin of Species," where he said someone he knew had observed a bear swimming around, skimming insects off the surface of the water, and he suggested that perhaps something like a bear could have evolved into a whale, the kind of whale that filter-feeds by doing something similar and getting better and better at it over time.

Now, Darwin was not correct about that, but at least he could conceptualize of a way that whales had evolved.

But basically, we didn't know where whales came from until extremely recently.

NARRATOR: The fascination with whale evolution began in the 19th century.

Stories of sea monsters filled the popular imagination.

When the first fossil whale was unearthed, its discoverers thought they'd found a gigantic sea serpent.

It was called Basilosaurus.

UHEN: It was discovered in North America along the Gulf Coast in the 1830s, and it was actually the first fossil whale ever to be scientifically named and studied.

The gentleman who named it, a man named Harlan, had read about large sea serpents being discovered elsewhere in the world, and so he thought that this was something similar, and thus he named this animal Basilosaurus, which means king lizard.

NARRATOR: Up to 50 feet long with ferocious teeth, Basilosaurus was a formidable marine predator.

Finally, in 1841, it was identified as a whale and dated to about 35 million years ago.

For almost a century after its discovery, Basilosaurus remained the oldest known whale.

It was assumed to be the ancestor of all living whales.

UHEN: So, after the discovery of Basilosaurus, there were other whales found that were just a little bit earlier and more primitive, but not by very much, maybe five million years older than Basilosaurus.

And it didn't push the origin back in time, and we really didn't discover a whole lot more about the origin of whales.

NARRATOR: One of those other early whales was Dorudon, a little bit smaller than Basilosaurus.

It would turn out to play a bigger role in whale evolution than its discoverers imagined.

But where did those early whales come from?

They must have evolved from something, but what?

The deep origins of the whale lineage were a mystery until 1975, when a paleontologist went on a fossil hunting expedition to Pakistan.

He wasn't looking for early whales at all.

PHILIP GINGERICH: I was interested in how archaic mammals changed into modern mammals.

Things like the first horses, that's what I was interested in.

My name is Philip Gingerich.

I'm a professor emeritus of paleontology at the University of Michigan.

I took several students with me and colleagues from Paris, and we looked first in Punjab Province, then south in Sindh.

We still hadn't found anything very interesting.

And we went to the Northwest Frontier Province.

And there, high on a hill, we found a little jaw of a land mammal.

NARRATOR: Later they found a piece of skull, and its strange ear bones finally unlocked the secrets of early whale evolution.

GINGERICH: This is what was left, the back of a skull, and if you look at the underside, it has the covering of the ear on the right side, and that covering is missing on the left side.

NARRATOR: Those ear bones turned out to be the key.

Philip thought he was looking at the skull of something like a primitive deer, but it unmistakably had the ear bones of a whale.

GINGERICH: Whales have a special structure of the ear to be able to hear in water.

It turns out that they effectively see in water by using sound, and so to do this, the ears have become modified.

[whale singing] UHEN: The ear bones of whales are very dense, and that density helps them to hear sound in the water.

Mammal ears originally evolved in terrestrial animals, and so their structure is optimal for hearing in air.

And in water, sound behaves really differently.

So, for example, if you go into a pool or under a lake, you can hear sound, but it's a little muffled, but the one thing you can't do is you can't figure out where the sound is coming from around you.

And that's because the way mammals do this is they use the difference in time between when a sound hits your right ear and your left ear to figure out what direction the sound is coming from, so if the sound is off to my right, it hits my right ear first, then my left ear, and so my brain says, "The sound is over to my right."

But in water, the tissue of your face and skull is about the same density as water, so the sound, rather than going around my skull to my left ear, goes right through it, so it gets to my left ear at almost the same time as my right ear.

And I can't tell where the sound is coming from.

So, the added density to whale ears, that's reestablishing their ability to hear directionally underwater.

NARRATOR: Philip's fossil had that same distinctive ear, and it was 49 million years old.

That could only mean one thing-- here at last was one of the very first whales.

Philip named it Pakicetus.

GINGERICH: Once we knew it was a whale, we knew it was the oldest whale anyone had ever found.

NARRATOR: Pakicetus pushed the origins of whales back 15 million years to the time when the Indian subcontinent was slowly crashing into Asia.

Where Pakicetus was found was once an ocean shoreline.

Its ears made it clear Pakicetus spent time underwater, but other fragments of skeleton also clearly showed it walked on four legs.

How did this strange beast give rise to the giants we know today?

♪ At the Museum of Natural History in Paris, one of the birthplaces of paleontology, they have been assembling the skeletons of prehistoric animals for over 200 years.

They now have one of the few complete reconstructions of the extraordinary whale ancestor, Pakicetus.

CHRISTIAN DE MUIZON: You see it's a quadrupedal animal with the higher forelimbs and hind limbs.

It means that this animal was, uh, definitely partly terrestrial.

I am Christian de Muizon.

I'm a paleontologist working at the Natural History Museum in Paris.

And obviously it's quite a strange animal, very small, doesn't whale-looking at all.

It more looks like a dog with a long snout.

NARRATOR: Pakicetus is one of the strangest surprises of evolution, a whale ancestor that looks like a small wolf with webbed feet for swimming.

GINGERICH: The key thing about it is it has elongated finger and toe bones, so, clearly, it's already semiaquatic.

[sniffing] NARRATOR: Pakicetus was a creature of the shoreline, hunting for fish and perhaps other small animals in the shallows.

UHEN: And we think that it was using its longer snout to probe for aquatic prey in the water, and so it was feeding in the water while almost certainly going out on land to breed and have their young.

NARRATOR: Once they adapted to life in the shallows, it took ten million years for the descendants of Pakicetus to become fully aquatic.

Why did it take so long?

Because to live underwater, they had to change.

It is one of the most remarkable stories of total physical transformation in the annals of evolution.

[whale singing] UHEN: After Pakicetus, whales take about 10 to 12 million years to evolve into fully aquatic forms.

And during that time, their hind limbs tend to get smaller and their skulls tend to get longer, and the naris, which is the hole in the skull where the nose is, moves up the skull.

In addition, their forelimbs tend to turn into flippers, and they get more vertebrae in their back, which makes their bodies longer.

NARRATOR: When Basilosaurus arrived on the scene 35 million years ago, along with its evolutionary cousin, Dorudon, there'd be no mistaking that these were whales that lived their whole lives in water.

But there was still one great transformation to take place in whales-- baleen feeding.

CARLOS PEREDO: So, near the base of the whale family tree, there's this major split into the two groups that we have today.

We have toothed whales, and then we have baleen whales that actually lose their teeth entirely, and they grow what are called baleen plates that are made of keratin, so they're actually more like hair or fingernails.

And these whales use it to filter their food, so they'll take big gulps of water, and they'll actually filter their prey, little tiny microscopic organisms out of that water using the baleen plates.

♪ NARRATOR: The split into toothed whales, like orcas and dolphins, and baleen whales, like blue whales and humpbacks, happened about 30 million years ago.

It turns out it occurred in the descendants not of Basilosaurus but of Dorudon.

UHEN: Basilosaurus is sort of a sideline.

We don't think it gave rise to anything alive today.

NARRATOR: The toothed whales remained, like their ancient ancestors, predators living mostly in coastal waters.

There they could feed on fish or, like some of the orcas, seals and baby sea lions.

♪ The baleen feeders underwent a much greater transformation.

With a radically new way of filter-feeding, they moved into the deep oceans and became huge.

Why?

For a long time, the massive size of some baleen whales was a puzzle.

[blowing] But it turns out that in the ocean, great size is an advantage.

UHEN: If you look at the energy budget of these animals, they use fewer calories per unit body mass when you get bigger, and so they're more efficient when they're huge.

They're also more efficient when they move, when they swim at large body size, so there's advantages to being large in the ocean.

[blowing] NARRATOR: The huge size of baleen whales is also linked to one of the last great planetary transformations.

PEREDO: Argentina and South America become completely separate from Antarctica, and that changes the currents in the ocean system, and it seems to really have a profound impact on what whales are doing.

NARRATOR: The cold current that began to circle Antarctica led to a vast upsurge in the krill population that baleen whales feed on.

With large amounts of food and the efficient baleen filter-feeding system, there was simply nothing to stop baleen whales from becoming huge.

[blowing] UHEN: It was suggested early on that the reason that terrestrial mammals can't be that big is because of gravity.

Anything much bigger than the largest mammals we see in the fossil record would be crushed due to the force of gravity, like their limbs couldn't hold their bodies up.

But whales are approximately neutrally buoyant, and so gravity doesn't affect them.

And so freed from that constraint, the baleen whales keep getting bigger and bigger and bigger over time because the largest ones are the most efficient.

[whale grunting] NARRATOR: Along with baleen, whales have developed sophisticated fishing behaviors to trap their prey.

Humpbacks rely on the help of gulls.

[squawking] HILDERING: And what's happening is the diving birds are forcing the herring together, that form a huge ball, that gives the gulls at the surface a chance to propel themselves down and try to grab a herring, and then somehow the humpbacks know that there's a concentration of feed there.

And they come with their huge mouths, gulp it down, get rid of the salt water through their baleen, and swallow.

[camera shutter clicking] NARRATOR: Jackie's work has led her to marvel at the delicate web of relationships linking humpbacks to every aspect of the marine ecosystem.

HILDERING: It's perfection.

It's been going on longer than we can understand.

Knowing whales keeps me in a state of humility and mystery... which I think is how a human life is well lived.

It makes me feel connected.

It makes me feel appropriately small in the presence of giants.

[whale singing] NARRATOR: Over millions of years, whales grew huge as they explored the seas, but another mammal grew huge as it explored the earth, evolving its own remarkable tools for life on land.

[roars] ♪ [trumpets] ♪ For millennia, these stately creatures have patrolled the African savannas, transforming the land as they go.

♪ ♪ PAULA KAHUMBU: We're driving along tracks that the elephants have made.

Uh, they've been walking on these trails probably for hundreds of years.

They create paths that they then use for generations.

♪ My name is Paula Kahumbu.

I'm an ecologist, and I run WildlifeDirect.

It's a Kenyan-based conservation organization to really change hearts and minds and the laws to protect our incredible wildlife.

♪ Elephants are such extraordinary animals.

The more we learn about them, the more we realize that we actually are only just scratching the surface.

Every new discoveries emerge that elephants can understand things that we couldn't have dreamed possible.

And I think we'll never uncover their secrets.

NARRATOR: The intelligence and memory of these enigmatic giants is legendary.

[grumbling] Their complex elephant mind has allowed them to evolve a social life with many parallels to our own.

KAHUMBU: Elephants are incredibly social animals.

They live in family groups.

And the family groups will have everybody from the grandmothers, the daughters, the mothers, the aunts, the cousins, the nieces, the nephews, everybody all together, so huge range of different-sized elephants all walking together and feeding together.

NARRATOR: Scientists have shown that elephants communicate through calls, touch, and even low frequency vibrations, which can travel underground for miles.

But there's still a lot that they don't fully understand.

KAHUMBU: It would be amazing if we could actually uncover what it is that they know.

I really think that we are blundering about right now because we don't even have the tools to measure how they communicate.

NARRATOR: And Paula is certain that along with their ability to talk to each other, they experience many of the same emotions we feel-- grief, anger, and empathy.

She senses a mysterious inner life.

KAHUMBU: When you sit with them for hours, this, like, wealth of sensations just keep coming and coming and coming, and it just seems to be endless.

I feel like I can really sense what is going on with those elephants.

And the longer I spend with elephants, the more I feel in tune with them.

NARRATOR: As much as she admires them, Paula is keenly aware that elephants today are in trouble.

[roaring] KAHUMBU: There used to be dozens of species of elephants, and now there are just three.

And if we're not careful, we will lose them as well, and that will be the end of the lineage of elephants.

NARRATOR: Could the ancient lineage of elephants really be coming to an end?

How did these magnificent animals go from myriad species to just three today?

And how did the ancient histories of humans and elephants become so intertwined?

[trumpets] As with every lineage, it's a story that begins in deep time, a story told by fossils, most of them found in the birthplace of elephants-- Africa.

♪ The deep origins of elephants lie in the Turkana Basin in Northern Kenya.

In a place called Buluk, scientists are finding fossils that point to a magnificent age of elephants.

They reveal the ancient history of the mammals with trunks called proboscideans.

BILL SANDERS: This is from the right side of the jaw.

Fossils are the messengers of the past.

I believe that.

I think that we, as paleontologists, are trying to be the interpreters of the messengers from the past.

They give you the opportunity to see the kind of incredible journey that elephants had to make to become elephants.

It took 60 million years to make an elephant.

I'm Bill Sanders.

I'm a paleontologist at the University of Michigan.

I specialize on studying the evolution of proboscideans, including elephants.

NARRATOR: Today, Buluk is one of the hottest, driest places on Earth, but 17 million years ago, it was a lush forest with rivers and wetlands.

SANDERS: It's a very wet climate.

It's a very equable climate, very warm climate.

It's a great place to be a browser.

It's a great place to go out and look for vegetation, a great place to go get a salad, Africa in the early Miocene.

NARRATOR: How can scientists reconstruct those vanished environments?

It turns out ancient landscapes leave traces just like ancient animals do.

ELLEN MILLER: We often find fossil wood and fossil seeds, so that's telling us something about the environment, and, uh, the red sediments around us are ancient soils, they're actually ancient floodplain deposits, so the animals would have been living out on these floodplains in these forested areas.

I'm Ellen Miller.

I'm a paleontologist associated with the Turkana Basin Institute.

We're trying to reconstruct the whole environment that these animals were living in.

If you go in and just pick up the bones, it's like, it's like taking the chocolate chips out of the cookies.

So, we work with geologists and climate scientists and isotope specialists and all kinds of people, geochemists.

NARRATOR: The painstaking work of Ellen and her colleagues is allowing them to reconstruct the whole vanished world of ancient Buluk.

MILLER: So, if you were to be transported back to the early Miocene of Buluk, 17 million years ago, it would have been a mature meandering river system and a woodland, and there would have been a whole host of different kinds of elephants.

So, you would have had the Deinotheres.

They're very, very primitive.

They're about a third or a half the size of a modern elephant, and they would have been kind of snuffling along the riverbanks because they seemed to really like a closed canopy forest and a wet environment.

But at the same time, you have the Amebelodons, the shovel-tuskers.

The lower incisors are these big, long shovel-like tusks, so they would have used them to scoop up their dinner.

NARRATOR: As scientists excavate, they are astonished by the diversity of strange beasts they are discovering.

♪ Ancient Buluk was a sort of Jurassic Park of elephants.

ISAIAH NENGO: Imagine I'm taking you on a safari, but a safari back in time.

You'll be confronted with these magical creatures, a slice of Africa that is now gone.

You'd be confronted here with amazing herds of elephants.

♪ You have a herd of, of Deinotheres.

You, you have a herd of these elephant ancestors called the Gomphotheres.

It would be mind-blowing to, to, to confront this scene.

I think the Miocene 16 million years ago at Buluk, that would have been the center of the empire of the age of the elephants.

[blows] My name is Isaiah Nengo, and I am the Associate Director for the Turkana Basin Institute.

When you're out there and you pick up a piece of bone, you pick up a fragment of a skull or you pick up a tooth, and it, it strikes you that "Man, I'm the first person to ever look at this thing.

I am the first person to know that this exists."

There's an immense joy.

There's, uh, there's no way to describe it.

NARRATOR: The ancient elephants here bear witness to a glorious flowering of the elephant lineage about 17 million years ago.

But what were their origins, and how did they come by their magnificent tusks and trunks?

30 million years ago, there was a species called Palaeomastodon.

It was probably the first elephant ancestor to actually look like an elephant.

SANDERS: If you saw them on the landscape, you would say, "Yeah, that thing is like an elephant."

They walk around like a walking card table, like elephants do today, probably very carefully with three legs on the ground at any one time.

We believe from the shape of where their nose is and the anatomy around it, they must have had a trunk, which is very cool.

So, it's at 30 million years of Palaeomastodons that we begin to get that framework for elephants, the beginning of recognizability for elephants.

NARRATOR: But where did Palaeomastodon come from?

Beyond 30 million years ago, the elephant fossil record seemed to disappear.

For decades, the earliest origins of the elephant lineage were a mystery.

But then, in the 1990s, a French paleontologist and his team were excavating in an abandoned phosphate mine in Morocco.

In layers dating to 56 million years ago, he came across fossil remains of a mysterious animal about the size of a small dog.

He called it Phosphatherium.

Later he came across fragments of the jaws and teeth of an older, even smaller creature.

This one was no bigger than a rabbit.

As he puzzled over both fossils, he noticed something very strange.

Their teeth seemed to be miniature replicas of an elephant's teeth.

The more he looked, the more certain he became.

They must be ancestral elephants, the oldest ever discovered.

It was an astonishing conclusion.

The oldest elephant ancestor was the size of a rabbit.

[man speaking French] NARRATOR: Emmanuel brought his fossils back to the Paris Museum of Natural History.

Today he keeps them in a filing cabinet in his office.

SANDERS: I give a lot of credit to my colleague Emmanuel Gheerbrant because it's like a detective story.

It's a real puzzler to figure out what they are, and he found these things, and he started looking at their teeth, and he realized they had some subtle features on their molars, the arrangement of the cusps on their molars that said, "I am a proboscidean."

NARRATOR: Finally, scientists could see the very beginnings of the elephant evolutionary tree.

NARRATOR: It starts with the tiny 60-million-year-old creature with the miniature elephant teeth.

Emmanuel called it Eritherium.

NARRATOR: So much of the story of elephant evolution can be told by teeth.

[grumbling] NARRATOR: As elephants evolved, incisors slowly became tusks.

[roars] They helped those early elephant relatives browse in the ancient African forests.

SANDERS: We see that they have large front teeth, and their jaw has grown forward in their mouth so that their front teeth are a bit separated from their back teeth, so their front teeth are specializing.

They're specializing for grabbing food, for acquiring food, for nipping food, for chopping food.

NARRATOR: As incisors became tusks, they took on the functions they have in elephants today.

SANDERS: They're used for acquiring food, for knocking down plants so you can reach food.

They're used for social display.

Tusks are the defining features of elephants, and it's like their behavior is tied up in having these tusks.

NARRATOR: And as tusks grew, they propelled the evolution of the trunks that would become the hallmark of all later elephants.

SANDERS: So, I think once you start to get tusks, then trunks follow that.

It is important because they have to have something to get past the tusks in order to reach the food in their environment.

NARRATOR: As trunks grew, they slowly became the amazingly sensitive organ of touch and smell they are in modern elephants.

SANDERS: Trunks are made up of something like 72,000 tiny muscle fibers, so they're highly complex, and they can move them around the way that you can move your hand around playing the piano.

Elephants have tremendous control over their trunk.

It's not just flopping around and sucking up water.

They can really be very gentle.

They can pick a penny up off the ground with their trunk.

Those trunks get a big workout.

NARRATOR: Tusks, trunks, and great size were obviously successful elephant adaptations to their forest environment, but they didn't just help elephants respond to their environment, they gave them the capacity to change it, too.

NENGO: So, I think of all the species that we know of, apart from humans, the only other mammals that we know have the capacity to be able to alter the natural ecosystems they live in in a short time in a very big way would be the elephants.

SANDERS: Proboscideans are big animals, and you can imagine any big animal going through a landscape is knocking down trees, moving vegetation out of the way, creating paths.

NARRATOR: For that, Bill believes we humans owe them a debt of gratitude.

He thinks elephants helped create the perfect conditions for a certain group of apes millions of years later to come down out of the trees and begin to explore the savannas.

Those were our ancestors, the australopithecines.

The famous early human Lucy was discovered not far from Buluk.

Bill is convinced that she and our other early human ancestors flourished in a landscape that had been unintentionally prepared for them by elephants.

SANDERS: This idea of elephants opening things up and creating the conditions of success for early hominids might not be an exaggeration to say that we might not be here without elephants.

NARRATOR: And just like our ancestors, elephants did not stay put.

After millions of years in Africa, they started to leave.

SANDERS: Proboscideans leave Africa multiple times.

Different groups leave at different times.

The first major foray out of Africa is around 18 million years.

One suspects that you follow the vegetation and you follow the available land paths, so if Africa is docking with Eurasia as continents move around a bit, and you find familiar plants, you don't really know you've left Africa, and you just keep moving.

"Oh, and there's, over-- there's another pasture over there that seems pretty good.

Oh, well, wait.

Over that hill, that looks pretty green, too."

They get out at about 18 million years in a serious way, out to Eurasia, and they make it all the way to Japan in rapid time, within a period of about a million years after getting out of Africa.

♪ NARRATOR: Until recently, all we knew of those ancient elephant species that left Africa was from a few fossil bones.

But then, a dramatically different kind of discovery brought them to life.

In the deserts of Abu Dhabi, scientists discovered not bones, but footprints.

They were made by ancient elephants, a vivid record of their great migration out of Africa.

FAYSAL BIBI: It's phenomenal because when you're actually on the landscape, it seems like the elephants just passed there yesterday, but we know, geologically speaking, that's impossible.

These are very, very old sediments.

My name's Faysal Bibi.

I'm a paleontologist at the Natural History Museum in Berlin.

So, it was, it was almost an afterthought to image the tracks from the air.

And then we went back to the hotel that night, and we started to put the imagery together.

As we realized what we had, we basically, our jaws dropped.

NARRATOR: What they saw was the footprints of an entire herd of ancient elephants on the move, a snapshot in time of elephant behavior seven million years ago.

Back then, this desert was a lush savanna.

The muddy ground after a rainfall captured their footprints perfectly, all made by one of the strangest of elephant relatives.

♪ BIBI: It's probably ten minutes in the lives of these individuals, this herd that walked across the landscape, and those ten minutes are forever preserved in these rocks for us to see.

NARRATOR: And in that ten-minute snapshot, Faysal can see all the dynamics of the herd-- the adults and calves, a single bull male and a number of females.

BIBI: Yeah, so we have, it's a minimum of 13 individuals actually.

You're walking along, and here was a large individual.

There was probably the matriarch.

They slow down.

They speed up a little.

There was a smaller guy.

He's on the edge, so we're clearly not too worried about any predators coming along on this landscape.

And then, perhaps, just the day before or the day after, we've had the large bull who also crossed this landscape.

NARRATOR: Not long after Faysal and his team made their discovery, they took Bill Sanders to see it.

SANDERS: I had no idea what awaited me.

And then I saw them.

All the footprints of an entire herd going on for about 260 meters, and you can see baby footprints, juvenile footprints, female footprints, and then one great track of a big bull male that must have come along later and crossed that track and sort of checking out the herd.

We rarely get that opportunity.

I work on elephants, I love elephants, and I'm seeing their behavior crystalized in time, all the way back to the very beginnings of elephants.

And I just started crying.

And my colleagues all sort of applauded, and they realized I was not crying out of sadness.

I was crying because I was ecstatic.

NARRATOR: Like skilled trackers, the scientists could read the ancient footprints and reconstruct a remarkably detailed picture of that day seven million years ago.

BIBI: We could estimate the actual size of these individuals based on stride lengths of modern elephants, where their weights are known, and their stride lengths are known.

And their estimated weights go from a few hundred kilos for the small one, up to 5,000 kilos or so for the largest in the group, and possibly 6,000 or so for the solitary individual that was walking at that site.

NARRATOR: That makes them as big as any bull elephant today.

From fossils found nearby, we know that they were magnificent animals, four-tusked beasts called Stegotetrabelodons.

[trumpets] BIBI: And here you are, you're on this landscape, and you can imagine them having just been here, like it was yesterday.

NARRATOR: Over generations, their ancestors made the journey from Africa, thousands of miles away.

[grumbling] BIBI: They're extinct, they're long gone, and they haven't just left us their bones and teeth, this is an imprint of their society.

♪ NARRATOR: The discoveries in Abu Dhabi show that by seven million years ago, the social behavior of elephants had already evolved.

[elephant trumpets] KAHUMBU: The fact that, uh, these ancient elephants behaved very similarly to modern elephants in a way confirms that it is a very successful strategy to have these families working together.

They are incredibly cooperative as families.

You'll see females looking after each other's calves.

The young will actually have a very high death rate if there aren't aunts around to look after the babies.

NARRATOR: Their close family bonds have been a key to the success of elephants for generations.

[grunting] Even though Stegotetrabelodons disappeared a few million years ago, their descendants and other elephant species soon populated much of the globe.

Some, like the mammoths, adapted to the cold of Siberia and North America.

Others, like the Gomphotheres, headed for the warmer climes of southern Asia and Central America.

It had taken 60 million years, but the elephant lineage had become one of the most successful on the planet.

Just 50,000 years ago, elephant species were on all continents except Australia, Antarctica, and South America.

So, what happened to them all?

SANDERS: A long debate among my colleagues in my field has been, what are the agencies for the extinction of the elephants that we see, for example, mammoths and mastodons?

NARRATOR: One hypothesis is climate change.

At the end of the last ice age, 10,000 years ago, the world warmed.

The cold-adapted elephants of Siberia and North America just couldn't deal with it.

SANDERS: In the Northern Hemisphere, in northern latitudes, you have all this glaciation, and these changes are happening very, very rapidly.

NARRATOR: But for millions of years, the mammoths and mastodons managed to weather similar climate changes.

What was different about the warming at the end of the last ice age?

Bill believes it was our own ancestors who tipped the balance.

SANDERS: So, we see the great sites in Eurasia where indigenous peoples like 10,000 years ago and 50,000 years ago and 60,000 years ago were slaughtering these elephants in great numbers.

There is a tremendous predation pressure, and imagine if you've got predation pressure hitting you on one side, and now you've got this climate change and what it does to the landscape, what it does to the plants and the available resources.

♪ NARRATOR: By 4,000 years ago, the world was left with just the African, Asian, and forest elephants we know today, and now these are under threat as well.

This time, it has nothing to do with the end of an ice age, just us.

♪ In 1800, there were an estimated 25 million elephants.

Today there are less than a million, and the number is falling fast, thanks to habitat loss and the relentless slaughter of elephants for ivory.

♪ KAHUMBU: Poaching now is very mechanized.

It's industrial.

In some places, we're losing 1,000 elephants in a month.

It's being done with not just weapons, but with aircraft, trains and trucks and ships to move the ivory out of Africa very quickly.

NARRATOR: The tusks that helped elephants survive for millions of years have become a liability.

It's a new kind of evolutionary pressure, human generated, and in Africa, it's causing elephants to change almost overnight.

KAHUMBU: Over the millennia, elephants have evolved to have these huge tusks because the most successful elephants are the ones that had the biggest tusks.

But over the last few hundred years, us human beings have been killing elephants for those tusks.

There are some populations of elephants who have very small tusks because poachers are selectively removing elephants with big tusks, and so the only females that get to breed are the ones which have very small tusks, and so you increasingly see tusklessness in some of these elephant populations.

It would be such a tragedy if these magnificent animals lost the one thing that makes them, you know, so unique--their tusks.

NARRATOR: Even as they adapt, elephants are at risk.

Like so many other creatures, they now face a new era of extinction.

SANDERS: If we lose elephants, we put a big hole in the fabric of our coexistence with other animals on Earth.

We've damaged in a way that we can't fix our ecological web, our interdependent ecological web.

And we like to think of ourselves as being separate from that, but we really are inextricably linked.

NARRATOR: Everywhere the natural world is being transformed.

♪ Our own lineage has become the planet's dominant evolutionary force, shaping the web of life that exists all around us.

This one recently arrived species is now the worldwide presence to which all others must adapt.

Our impact is so huge that our era has been given its own name-- the Anthropocene, the age of humans.

It has seen the extinction rate among natural species soar to hundreds of times what it was before our arrival.

As scientists race to chart the planetary changes, they can look back at lessons from deep time.

They're observing many of the things that happened in earlier extinctions-- rising CO2 levels leading to acidification of the oceans and rapid climate change.

Habitat destruction.

Many believe we are witnessing our planet's sixth mass extinction, but the first one caused by a single species.

Like others, it will reset the evolutionary clock.

We just don't know how.

SUES: Today when we live in a world where human populations are gradually changing the face of the globe by turning natural environments into artificial environments, by pollution and many other ways of interfering with natural systems, we are very much confronted with the question of evolution and extinction.

You can't have evolution without extinction, but extinction really complicates our efforts to get the big picture.

NARRATOR: We now know that mass extinctions are an engine of evolution, clearing out environments, making room for new species to evolve.

But in the past, they have usually taken millions of years.

This one is happening fast, in a matter of generations.

Viewed through the lens of deep time, that is a nanosecond, too fast for many living things to adapt... but maybe not too fast for us to make a difference.

Crocs have lived on Earth for almost 230 million years.

They've survived cataclysmic extinction events, but today, 5 of the 14 crocodile species are critically endangered.

HEKKALA: Most of the living crocodilians were on the verge of extinction by the 1970s.

We were on the verge of losing all of them when we put in place protection.

NARRATOR: Protections have helped.

In Australia, both fresh and salt water crocs have rebounded thanks to strong steps like restrictions on hunting.

But what about the other great survivors of deep history?

10,000 species of birds still cover the globe.

Some have adapted to cities, where they live beside us in seeming harmony.

But that's not the whole story.

CLARKE: We are hugely impacting bird evolution.

This is in habitat loss, consumption or killing, poisons, the use of toxins in our environment.

[chirping] NARRATOR: With 40 percent of bird species in decline, there's reason to worry, but also reason to hope.

We managed to turn things around for iconic species like the bald eagle and California condor.

And innovative programs show that more is possible.

In 1974, only four Mauritius kestrels were left in the wild.

Today, there are 100 times as many, thanks to predator control and captive breeding.

Ocean creatures need protection, too.

It took 50 million years for whales to become the wondrous giants of the deep we know today.

[gunshot] But in just two centuries, industrial whaling brought many of them to the brink of extinction.

In the 20th century alone, almost three million whales were slaughtered.

HILDERING: It's unthinkable now that we exploited whales to the extent of, in the case of humpbacks, driving down their population to 10 percent of what they were globally.

They were almost pushed over the edge.

We only stopped whaling on northern Vancouver Island in 1967.

Humpbacks we stopped in 1965.

So, we thought of them so very differently.

We saw them as a, as a resource.

But with humpbacks, we have a second chance.

NARRATOR: While some whales are critically endangered, whaling bans have made a difference.

Humpbacks have rebounded, and blue whale populations, which fell to just 1,500, seem to be slowly increasing.

HILDERING: One of the many things that the whales do is they remind us how connected we are and of our capacity for change.

[grumbling] NARRATOR: Around the planet, others are heeding that reminder, committing themselves to protect endangered animals and places.

[roaring] In Kenya, Paula Kahumbu admires one of the last true giants left on Earth, a tusker named Tolstoy.

[grumbling] KAHUMBU: I feel very humbled to be able to meet Tolstoy.

He is a giant of giants.

He's not just a big tusker, he's a super tusker.

There are very, very few elephants of that size with tusks of that length left in the world.

You know, when, when you're with Tolstoy, it's, one of the biggest rushes you get is that "I'm alive."

Right?

You just feel this sense of life.

It gives you goosebumps just to know that this elephant is aware of your presence, and you're tiny and he's huge.

It's, he's beautiful.

It's just the most incredible experience.

NARRATOR: To know that Tolstoy is the product of an evolutionary journey that has been going on for eons only makes him more precious.

And Tolstoy is not alone.

Crocodiles and birds, whales and elephants are just four life forms among millions.

The tree of life is vast, encompassing everything that has ever lived.

What will its branches look like after the age of humans?

The answer is up to us.

♪ ANNOUNCER: To order When Whales Walked on DVD, visit ShopPBS or call 1-800-PLAY-PBS.

This program is also available on Amazon Prime Video.

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