NOVA

NARRATOR: In the sands of the Egyptian Desert, experts are uncovering clues to a lost past.

Look at this!

Right here.

NARRATOR: From a time long before the pharaohs, when this place was underwater and whales... Had legs?

HESHAM SALLAM: Here's the hind limb of this beast.

It's just like T. rex hand, do nothing.

NARRATOR: Whales are the world's biggest mammals.

But how did they end up in the ocean?

This is so awesome.

They're doing everything mammals do, but in the water.

NARRATOR: Now new discoveries are revealing clues about their evolutionary past.

Wow!

Look at this!

SALLAM: Oh, my word!

PHILIP GINGERICH: When I first saw it, I had no idea what it was.

NARRATOR: From prehistoric predators to the largest animal that has ever lived.

How did the whale's journey begin?

It's one of the greatest stories of evolution.

"When Whales Could Walk."

Right now, on "NOVA."

NARRATOR: Wadi Hitan, in Egypt's Sahara Desert.

Hidden beneath these rocks are secrets from a time long before humans.

SALLAM: Many people think of Egypt like ancient Egyptian civilization.

Like pharaohs, Sphinx, and Romans, even.

But what I'm studying is way beyond this time.

Prehistoric life.

NARRATOR: Paleontologist Hesham Sallam is on a mission to uncover his country's prehistoric past.

SALLAM: The place that we are heading toward is one of the most important place in Egypt, if not in the world, in term of paleontology.

NARRATOR: Hesham is searching for clues to an extraordinary evolutionary mystery.

Here, it's the middle of nowhere, it seems like empty place, but there is tons of evidence that you can see.

Fossils are everywhere, telling you what life looked like 40 million years ago.

NARRATOR: Across more than 75 square miles, fossils litter the desert.

There are so many, Hesham has to remove his shoes to avoid crushing them.

Just look at this, for example.

There's a really nice shell.

Really wonderful, well-preserved.

It's look like living one, but it was living 40 million years ago.

NARRATOR: But this desert also hides much larger fossils.

SALLAM: This is typically what you see in Wadi Hitan: the bone sticking out from the cliff, calling you to come and see it.

And indeed, this is a really huge animal.

The vertebra would be that big.

And not only the vertebrae, but also, you can find ribs all over the place.

Given the size of the vertebrae, it might be getting up to 20 meter long.

NARRATOR: The size and shape of an animal around 60 feet long encased in this rock has led scientists to a remarkable conclusion.

SALLAM: I think we have a complete skeleton of the prehistoric whale that lived here in Egypt, long, long, long time ago.

NARRATOR: Since the first whale fossils were discovered here in 1902, experts have found around 1,000 individuals.

This is the biggest ancient whale graveyard known on Earth.

That's why paleontologists named it Wadi Hitan-- the Valley of the Whales.

But what are these sea creatures doing here in a desert over 100 miles from the coast?

Shorouq Al-Ashqar is one of Egypt's first female vertebrate paleontologists.

AL-ASHQAR: These are shark teeth.

Also, look at these shells.

These animals live at the bottom of the sea.

NARRATOR: Nearby, Shorouq finds another clue.

AL-ASHQAR: Look at this structure.

Many scientists do believe that these are mangrove roots, as they found a lot of mangrove seeds all over the area.

But other scientists believe that these might be crustacean burrows.

Crabs, snails lived there and burrowed in the soft sand.

NARRATOR: Whether these are fossilized mangrove roots or burrows dug by prehistoric crustaceans, it's clear this area was once underwater.

Today's excavation site was at the bottom of the sea 40 million years ago.

Back then, the Mediterranean was part of a much larger ocean, the Tethys.

It stretched from Europe to India, and was full of marine life.

But when sea levels dropped, they left behind a seabed rich in fossils.

Today, this desert is the resting place of some of the earliest whales ever found.

They may hold the key to how today's ocean giants evolved.

6,000 miles away, in the Dominican Republic, living whales gather in these tropical waters early in the year.

It's humpback whale breeding season.

Comparative anatomist Joy Reidenberg has come to study them.

REIDENBERG: Oh, there's a blow.

Wow-- oh, and just the other side!

Did you see that?

There's this tiny little fin-- that's the baby!

We've got a mother and her calf right here together, swimming side by side.

NARRATOR: Humpbacks are one of around 90 different species of whale living today that include toothed whales, like orcas, dolphins, and porpoises, and baleen whales, like these humpbacks.

Whales can swim thousands of miles, dive thousands of feet, and stay underwater for over an hour.

REIDENBERG: I love whales, I really do.

Such magnificent, huge creatures.

And so well-adapted for being in the water.

NARRATOR: And yet, they're mammals.

They breathe air using lungs.

Just like us.

REIDENBERG: We've got a mother and calf over here.

The mother is nursing its baby.

But these animals not only breathe air and give milk, but they're doing everything mammals do, but in the water.

They have to mate in the water.

They have to carry their pregnancy in the water.

They have to find food in the water.

And then they give birth in the water.

All of this because these animals have evolved to be an aquatic mammal.

NARRATOR: How did air-breathing mammals end up in the ocean?

For centuries, many people confused whales with fish, including the characters in the novel "Moby-Dick."

Even though, in the 1750s, the Swedish naturalist Carl Linnaeus had already classified whales as mammals, recognizing that, among other traits, they have lungs and produce milk.

But where whales came from caused even Charles Darwin to scratch his beard.

Inspired by a bear seen feeding while it swam, he imagined how whales could evolve from land mammals.

But faced with ridicule, he removed this idea from his later writings.

Perhaps Darwin's hunch had legs.

Could it be that whales hadn't evolved in the water, but were actually descended from mammals that once walked on land?

It almost seems like evolution had taken a backwards step.

It's a question that fascinates Hesham Sallam.

He believes clues can be found in the huge fossils that lie scattered in this desert.

This is one of the most complete skeleton that we find in Wadi Hitan.

In the middle of nowhere, you find a lot of really huge vertebrae lined up.

The skull would be over there in that rock, and the ribs on both side.

This is really spectacularly huge animal that lived here in Wadi Hitan 40 million years ago, the Basilosaurus.

NARRATOR: Basilosaurus fossils have been discovered in many parts of the world, including around 600 here at Wadi Hitan.

This one has been laid out in the place it was found.

When the first Basilosaurus was studied in 1834, experts were baffled.

SALLAM: When the early scientists found this, they thought it's belonged to a kind of gigantic marine reptile.

And the, this is why they give it name "Basilosaurus," which means "king lizard."

NARRATOR: But the skull contains a clue to Basilosaurus's true identity.

This is the skull upside down, and you can see all the teeth are sticking up.

And this, actually, they have incisors, canine, premolars, and molar.

Really, like our teeth, which is actually a really good indication for, this is not a marine reptile, this is actually mammal.

So Basilosaurus, the king lizard, it's actually ancient whale.

NARRATOR: 40 million years ago, Basilosaurus was the apex predator of its day.

It could grow up to 60 feet long, the length of a bowling alley, and weigh more than seven tons.

Powerful jaws filled with sharp teeth made it a killing machine.

Scientists estimate its bite had a force of nearly two tons, enough to crush the bones of other whales.

Basilosaurus was a prehistoric king of beasts.

But its discovery didn't initially shed much light on whale evolution.

Or how these mammals ended up in the ocean.

It's a mystery that has intrigued Philip Gingerich for almost 50 years.

A pioneer in the field of whale evolution, he was one of the first paleontologists to excavate at Wadi Hitan.

So this is where I keep the fossils I'm working on.

So here is the skull of Basilosaurus.

Uh, it's upside down.

And this is a model of it, a cast of it.

These are all from Egypt, from Wadi Hitan.

Collected in 2005.

Here we are, I'm still working on them.

(chuckling): Takes time.

NARRATOR: When Philip started his career, he studied land mammals.

At the time, paleontologists had very little idea about the origins of whales.

I grew up in the Midwest and I wasn't near the ocean, and... I didn't know anything about whales.

I knew so little that I wasn't interested.

NARRATOR: In 1978, Philip went to Pakistan to search for prehistoric horses.

Instead, his team unearthed the remains of a mysterious creature.

The first thing we found was this skull.

Back of a skull-- it's not complete.

The front, the part with the eyes and the teeth and everything, has broken off.

When I first saw it, I had no idea what it was.

I was probably disappointed, because I was looking for horses, and it clearly wasn't a horse.

But what it was, I couldn't figure out.

NARRATOR: The team named the strange animal Pakicetus.

It's about 50 million years old.

When Philip took a closer look, he spotted something unexpected in the creature's ear.

So when you look at this covering bone, covering the ear, it's very dense, it's thickened, it has a sloping surface on this side.

And in modern mammals, those are only found in whales.

And why?

To enable them to hear in water.

NARRATOR: This ear bone, unique to whales and dolphins, helps them locate the direction of sounds underwater.

It's proof of Pakicetus's pedigree.

GINGERICH: This bone was the key to understanding that Pakicetus is a whale.

That made it the oldest fossil whale anybody ever found.

NARRATOR: It was groundbreaking.

And as they discover more Pakicetus fossils, they realized something else.

This whale could walk.

Pakicetus is an animal a little bigger than a wolf.

Probably built approximately like a wolf.

It has teeth like a carnivorous mammal.

NARRATOR: But unlike a wolf, that has claws on the ends of its toes, Pakicetus had tiny hooves.

Pakicetus was a carnivore that hunted on land.

But its anatomy suggests it had adapted to living in water.

Its long snout, full of sharp teeth, also allowed it to probe shallow riverbeds for prey.

Its eyes were squeezed onto the top of its head, so it could keep watch while swimming.

And some scientists think markings on its foot bones are evidence it had webbing between its toes, allowing it to hunt underwater.

Why was Pakicetus spending so much time in the water?

I think it was because the water was full of fish.

And judging from its teeth, it's pretty clear that they were taking advantage of that, going in the water, feeding on the fish, and didn't have much competition.

And of course, it didn't take long until they moved into the water more permanently.

NARRATOR: Pakicetus marks the beginning of an eventful journey from land animals to today's gigantic whales.

For Philip, it was the start of a lifelong passion.

It changed the course of my entire career because I got interested in this as an example of evolution.

And it's especially interesting because it seems like it's backwards-- it's back to the sea, not out of the sea.

NARRATOR: The oceans are thought to be where life started.

Around 400 million years ago, some fish left the water to live on land.

Over time, their descendants evolved into amphibians, dinosaurs, and mammals.

Then, about 50 million years ago, something incredible happened.

Some mammals found their way back into the water.

They spread to all the world's oceans, evolving into the whales we know today, from huge blue and sperm whales to orcas and porpoises.

How did this transformation happen?

To find out, scientists examine anatomical clues in modern whales, as well as ancient fossils.

By doing a dissection, they hope to reveal more secrets of their ancestry.

REIDENBERG: Okay, let's unwrap.

NARRATOR: Comparative anatomist Joy Reidenberg is investigating a young beaked whale that was found dead on the coast of the U.S.A.

Joining her is evolutionary biologist Michael McGowen.

It's always really sad when you have a stranded whale.

Yeah.

But for us, this is a gold mine.

We have an opportunity here to learn something about an animal that's quite rare.

These particular species are rarely sighted at the surface, because they just come up, take a quick breath, and go back down.

REIDENBERG: What's really cool, I think, is, beaked whales are really adapted to stay at depth, and that's their norm.

Yes.

NARRATOR: Cutting into the animal's abdomen, they reveal something curious: an important clue about the origins of whales.

That's the stomach.

Uh-huh.

Now, look at, look how weird this is.

These animals are carnivores-- you expect them to be like a cat or a dog, and have one stomach chamber, but in fact, they don't.

They have multiple stomach chambers, kind of like hoofed animals, like cattle or deer or sheep.

So you got one, two, three, four, five, six, seven, eight... Wow.

I think there's eight or nine chambers, which is crazy, when you think about it!

It's crazy-- it's nuts.

NARRATOR: All whales and dolphins have this unexpected feature of multiple stomach chambers.

It's a trait they inherited from their ancient relatives that walked on land.

MCGOWEN: Just like cows have multiple stomachs to digest their plant matter, whales have multiple stomachs to digest what they're eating, which is completely different-- fishes and squid-- but it's still coming from the same structure as a terrestrial mammal.

It's just a throwback to their terrestrial ancestry of having a multi-chambered stomach because their ancestor did.

NARRATOR: This anatomy is more evidence that whales are related to hoofed mammals.

And this terrestrial heritage can even be revealed in their genes.

In his lab, Michael uses modern whales' DNA to map their past.

It's opening up a vast new world of information about the origins of whales.

So if you think about different marine mammals, such as a manatee or a seal or sea lion, they all swim and live in the ocean and have similar adaptations to whales and dolphins.

But we can look at the DNA to see whether whales and dolphins are closely related to those other groups or whether they're related to another species entirely.

NARRATOR: Scientists wanted to identify the whale's closest living relative.

So they compared whale DNA with a range of other animals.

They came up with a really surprising finding.

And the finding was that the whale's closest relative, using DNA, was... ...the hippo.

NARRATOR: Whales and hippos both descended from a common hoofed ancestor that lived about five million years before Pakicetus.

The family resemblance is striking.

Some of the earliest whales, like Pakicetus, may have lived like hippos.

Also, hippos give birth underwater.

They nurse underwater.

Their skin is also very thick.

So it's interesting to think that maybe the common ancestor of whales and dolphins had these particular features.

NARRATOR: But life in the ocean is very different from life on the riverbank.

Over time, whales' ancestors adapted to this new environment.

(hippos croaking) Scientists compared the DNA of hippos and whales to find out how.

(hippos croaking) When we look at the genome of whales, we see that whales still have a lot of the genes from when they used to live on land.

So they still have genes involved in smelling, sweat glands, color vision, producing saliva.

But these genes are inactivated, and they gradually degrade.

But I think this is incredibly powerful evidence that shows that whales come from land ancestors, that they still have these genes in their genomes, even though they're inactivated.

NARRATOR: Over millions of years, whales lost many traits beneficial on land that had no use in water.

But what happened to that most vital land animal feature, legs?

In Egypt, Hesham Sallam's mission is to find fossils that can tell us more about how whales became fully aquatic.

He's leaving Wadi Hitan to search a nearby unexplored area with older rock deposits.

SALLAM: Here we are.

(brake engages, engine stops) Ready to find fossils?

AL-ASHQAR: Sure.

NARRATOR: Joining him are fellow paleontologists Shorouq Al-Ashqar and Abdullah Gohar.

From geological maps, they know this part of the ancient seabed is about two million years older than the deposits where the giant Basilosaurus was found.

They're searching for intermediate fossils that might shed light on how four-legged land mammals evolved into their fully aquatic descendants.

SALLAM: I'm hoping to find maybe a full skeleton of, uh, some ancient whales that we didn't know much about.

If we're lucky to find that, this prehistoric animal here might have sturdy legs that can actually lift the body out of the water.

So this is what I'm trying to do, is find more primitive whales, and this is going to happen in, in maybe, in a few hours, few weeks, few months.

It depends how luck we have.

NARRATOR: And it's not long before fossils start to appear.

Wow!

Look at this!

SALLAM: Hey, what have you found?

We have a monster here.

Oh, my word.

A giant... This is incredible.

Yeah.

Look at this beast.

Yes.

This is a shark tooth.

This is really huge shark.

NARRATOR: But there's still no sign of a whale.

And then... SALLAM: Hey!

NARRATOR: They've found something.

Look at this.

Right here.

A small vertebra.

GOHAR: Oh.

AL-ASHQAR: Wow.

SALLAM: Right there, another one here.

GOHAR: Amazing.

SALLAM: Another one there.

So it seems like a... Um, a complete skeleton here.

Yeah.

Look at this line up.

Could be the vertebral column going that way.

But this is definitely not Basilosaurus.

The size of the vertebra is quite small.

NARRATOR: With his expert eye, Hesham can recognize the shape of the bones immediately.

I think this is a, a skeleton, and I do believe might be the skull over there.

Really exciting.

NARRATOR: Could it provide clues to how whales lost their legs?

So this is definitely older than Basilosaurus, because the Basilosaurus come later, in the younger deposits.

So this might be a kind of amphibious lifestyle whale, but with really sturdy hind limb.

If we are lucky enough, we might find the pelvis somewhere and the hind limbs right here.

NARRATOR: Before they can find out more, they hit a problem.

(wind roaring) If it's actually getting too windy, we just have to leave the site and come when the nature calm down.

I think we should stop.

NARRATOR: They mark the spot so they can return to it later.

Until then, this whale's place in the family tree will remain unknown.

Meanwhile, in their lab at Mansoura University, Hesham and Abdullah are investigating another new piece of the puzzle.

This is one of the most important discovery.

Here we have nearly complete skull, a lower jaw, some of the vertebrae, and some of the broken ribs.

NARRATOR: This is a previously unknown species of ancient whale.

Abdullah has named it Phiomicetus anubis, after Anubis, the ancient Egyptian god of death.

GOHAR: We have a clear indication of very powerful predator that hunt everything around him.

So we clearly can consider him the god of death for all living animal that lived by him 43 million years ago.

NARRATOR: It's one of the oldest whale fossils ever unearthed in Africa.

But could it walk on land, like its ancestor Pakicetus?

The team hasn't found leg bones, so they must look for other evidence.

Abdullah has found a clue in a bony projection on one of the vertebrae.

This is a thoracic vertebra from the rib cage region, and you can see, this sticking-out bone here is called neural spine.

It's very short in human, here, but this is very clear evidence of walking lifestyle, because the longer this sticking-out bone, the more ability to hold massive muscles that support the walking on four legs.

So, land mammals have this very long neural spine here.

This is a cow.

SALLAM: But if you look at the modern dolphin, you can see clearly this sticking neural spine is much, much shorter comparing to the walking terrestrial animal.

So this is fully aquatic, this is fully terrestrial, and anubis in between.

NARRATOR: The team believes that if anubis had strong back muscles, it probably used them for walking.

Anubis, the god of death, was a formidable marine predator.

It measured around ten feet long and weighed over half a ton.

It spent extended periods in the sea, where it hunted fish and turtles.

But from the bones the team has found, they think it was able to come back to land, perhaps to breed.

And it didn't wriggle out of the water like a seal.

Anubis was probably a walking whale.

Where can I put it?

(man speaking off mic) Thank you.

NARRATOR: To find out more about anubis's place in the whale family tree, Hesham and Abdullah take the skull to a nearby hospital for a CT scan.

Shall we get outside now?

MOHAMMED: Yes, yes, yes, outside.

Can you go inside?

NARRATOR: Hesham wants to take a closer look at the position of anubis's nostrils... Okay, can you zoom out, please?

NARRATOR: ...and spots something striking.

So here in anubis, the tip of the snout is broken off, and it could be up to here, but the nasal opening would be right here, one-third the way back in the snout.

Comparing to the modern whale, the blowhole would be on the top of the skull.

NARRATOR: Land mammals' nostrils are at the tip of the nose.

Modern whales' nostrils have moved to the top of their heads to become blowholes.

But anubis's nostrils are a third of the way back on its snout.

It's one step closer to becoming fully aquatic.

So, how did walking whales lose their legs to become modern whales?

Back at Wadi Hitan, Hesham takes a closer look at Basilosaurus.

This huge marine predator evolved about three million years after anubis.

But could it support itself on land?

Here is the arm of this beast.

It's actually wasn't like a regular arm of a mammal's, but flippers, to allow it to swim in the sea.

NARRATOR: Basilosaurus's front legs have turned into flippers.

And at the back of the animal is something even more intriguing.

This is really something very cool in the whale evolution.

This is the hind limb of this monster, and this is a complete pelvis.

Would be underneath one of these vertebrae, like that.

And this is the whole legs of this really huge animal.

This is the right leg-- the thigh bone, the shinbone, and the foot.

NARRATOR: These casts show that Basilosaurus's legs were smaller than the arms of a human.

Definitely Basilosaurus cannot walk, given the size of these hind limbs comparing to the whole body, 20 meter long.

Just totally cannot support walking on land.

Just like T. rex hand, do nothing.

NARRATOR: Unlike its walking whale ancestors, Basilosaurus was fully aquatic.

But in modern whales, are there any remnants of their walking past?

Back in the lab, Joy Reidenberg and Michael McGowen are looking for more clues inside the beaked whale.

We're now looking at the flank area, and I'm going to show you something really cool in this area.

Right in here is a small bone.

Something that is really a throwback to the ancestral condition of having hind legs.

All that's left is a little remnant of a pelvis.

NARRATOR: This tiny pelvic bone is hidden in the whale's hind quarters, where hips would once have been.

The only thing it doesn't have is the connection back to the spine.

So ours is connected in the back to the spine.

This one is just free-floating.

It's tiny-- it's actually a lot smaller than I thought it was gonna be.

This pelvis is so interesting, because it's a throwback to the land animals that were using a pelvis for walking.

But whales aren't walking.

They don't have hind legs.

NARRATOR: All modern whales still have a pelvis, and some have tiny hidden leg bones, too.

But why would a whale need a pelvis?

It still has function.

Part of that function is to anchor the muscles of the belly for swimming, so it's still being used in locomotion, just not with legs.

So think about, in the front, you know, we have the six-pack muscles?

Yeah.

These help to bend the body in this downward motion like that.

So that's part of the swimming action.

Oh, wow, okay.

They anchor on this bone.

They anchor in other places, too, but they also anchor on this bone.

MCGOWEN: And that's another remnant of its mammal past, is that they move their spine up and down.

And with fish and even reptiles, they move side to side.

You watch a snake move, you watch a crocodile walking, they're swimming like fish, but on the land, with legs.

Yeah.

When you get to mammals, the legs come underneath, the spine and the body come off the ground, and now they're free to gallop.

And when whales went back into the water, they kept the up and down spinal movement, so they're still galloping.

In the water!

NARRATOR: Losing their legs was just one change whales underwent as they adapted to life in the ocean.

To survive in this underwater environment, whales' limbs grew stronger, making swimming easier.

They grew horizontal flukes on the ends of their tails, and front limbs began to turn into flippers for stability and steering.

As they moved to tail-powered swimming, their bodies became more streamlined, hind limbs shrank, and their spines grew longer.

They had become fully aquatic and were unable to return to the land.

The desert at Wadi Hitan is dotted with the skeletons of some of the first ancient whales.

But around 34 million years ago, they mysteriously vanish from the fossil record here.

In an area about 50 miles from Wadi Hitan, Shorouq Al-Ashqar investigates why the whales disappeared from this region.

These strange objects provide an answer.

AL-ASHQAR: This is fossilized wood.

It seems like wood, but it's rock.

The wood replaced by silica and minerals, so it's so heavy, but it's a good indication that this area one day was a forest.

The trees were very tall, 40 to 50 meters long, colorful birds flying in the area.

Turtles, snakes-- full of life.

NARRATOR: The fossilized wood is around 34 million years old.

It's a sign that this area, once a thriving ocean, underwent a dramatic change.

At that time, Earth's climate began to cool.

The Antarctic ice sheet formed and sea levels dropped.

The Tethys Ocean receded, forming the Mediterranean.

And where whales once swam in warm, shallow waters, a forest grew.

The primitive whales that lived in Wadi Hitan, all of them are, died out.

Fortunately, some of whales adapt with the climatic change.

Whales are mammals, and mammals are warm-blooded bodies, so they can adapt with the cold water and find new places to live in.

NARRATOR: Being warm-blooded, they were able to generate their own heat and grow blubber to insulate their bodies from the cold, allowing them to migrate and thrive all over the world.

But the whale's story doesn't end there.

In a warehouse at the Smithsonian National Museum of Natural History is the world's largest whale bone collection.

It holds remains from nearly 10,000 whales.

Evolutionary biologist Ellen Coombs scans their skulls to investigate how they changed over time.

Studying the skull is really important, because in whales, it's where a lot of changes have happened over their evolutionary history, because it houses a lot of sensory organs-- the eyes, the nose, the brain-- and this can tell us lots of things about how the animal has evolved to eat and live its life.

NARRATOR: The 3D scans allow Ellen to look in detail at the more recent chapters in the whale's evolutionary tale.

When the last of the ancient whales died out, the modern whale's journey began.

And as the family tree continued to grow, something surprising happened.

Some kept their teeth, like orcas, sperm whales, and dolphins.

And some, like the blue, humpback, and right whales, lost their teeth and developed a new filter-feeding tool called baleen.

This is the underside of the mouth of a humpback whale, which is one of the baleen whales.

And what makes it a baleen whale is what's going on here.

So this is baleen.

This sits where teeth would normally sit in something like a killer whale or a dolphin.

But instead, they have these amazing plates of baleen, which are made of keratin, just like our hair or nails.

And you can see that it has these kind of hairs on the end here, which are used to filter out prey.

And what they do is, they suck in a big mouthful of water full of fish or krill, which is what they eat, and then they use their tongue to force out that water and capture all of their prey in these plates of baleen.

And that is sometimes up to half a million calories in one mouthful.

NARRATOR: Why did these whales take such a different path from their toothed cousins?

COOMBS: Here we can see a blue whale skull.

The main thing that we see here is how flat the face is, how wide the mouth is, and these are perfect adaptations for mass filter feeding with baleen.

NARRATOR: Meanwhile, the toothed whale has evolved a very different skull shape.

So, if we take a look at the skull of this killer whale from the side, you can see that the forehead is concave.

This is because it houses a load of organs that are used for echolocation.

(dolphins clicking and chirping) NARRATOR: The highly specialized skill of echolocation has evolved in some animals, like bats and whales.

(dolphins clicking and chirping) Whales "see" by emitting high-frequency sounds and then listening for how they bounce back off objects to find prey in the depths of the ocean.

(dolphins clicking and chirping) (squeaking) Toothed whales do this in a very specialized apparatus in their forehead, the key ones being the phonic lips, which make a high-frequency clicking sound, and the melon, which is a fatty organ, which helps to focus these high-frequency sounds as they leave the animal.

(dolphins chirping) NARRATOR: But how this skill evolved is still a mystery.

COOMBS: So we have ancient whales dying out that we know they could not echolocate.

And then we have the appearance of the early toothed whales that could echolocate.

So there's a gap in there, where there'll be several fossils that have maybe very basic echolocation, and they're the fossils that we really need to find.

(orcas squealing) NARRATOR: Scientists think these early whales survived to pass on their genes to their offspring, and so their abilities gradually improved.

(orcas clicking and squealing) Today, some toothed whales are such efficient predators, they even hunt their baleen cousins.

So these humpbacks have evolved new survival strategies.

Back in the Dominican Republic, Joy Reidenberg and marine biologist Mithriel MacKay are on a whale-watching mission.

There's a blow, around 1:00.

And the third.

Yeah.

And four.

There's four there.

NARRATOR: Most whales live in family groups and have complex social lives.

Mithriel and Joy want to understand how certain behaviors give them an evolutionary advantage.

MACKAY: We look at the behaviors and then we start asking questions.

Why are they doing those things?

Because the answer to the why gives us the reason they evolved this way.

Mm-hmm.

(whirring) REIDENBERG: Oh, that's awesome.

MACKAY: That's beautiful-- beautiful.

REIDENBERG: Look at how she's putting the baby on her, on her... MACKAY: Yep.

She'll scoop under and pick it up.

REIDENBERG: And she's using that, that flat surface of the top of her head to hold the calf there, almost like it's got a cradle surface.

So she can give the calf a rest this way.

Or if she feels like the calf's in danger, she could pull this calf right up out of the water.

And there's the male.

NARRATOR: This mom is being pursued by a male.

She's putting the calf on her head to protect it from his aggressive advances.

Mithriel thinks this protective behavior plays a role in baleen whale survival.

Picture them out, and killer whales coming up and seeing this baby as a good meal.

The moms that are able to use their flat head to scoop the baby up out of the water and get away are the ones whose babies are going to have babies.

We're always taught evolution changes things, but it doesn't.

What it really does is, it eliminates the stuff that doesn't work as well, so what's left is what works.

It's reactive, it's not proactive.

Exactly.

NARRATOR: And these whales have one other vital defense mechanism: their huge size.

How did they get so big?

The ancestors of these whales were actually smaller than our current whales.

That made them more nimble.

They could swim through the water more agilely, picking out fish.

But when we look at these large baleen whales, they are feeding in a completely different way, and that is partly what's allowed them to get so big.

So, having large bodies means they have large mouths, and the large mouths allowed them to get a lot of prey.

And having a large body also allows them to carry a lot of fat reserves, which they are using for swimming to the regions where the prey are.

But having that absolute big size is something that really evolves because they're in water.

You know, that biomass would be very hard to support on the land.

They would just be crushed.

NARRATOR: The whale's transition from four-legged land mammal to the giant of the oceans is one of the most extraordinary stories in the history of evolution.

GINGERICH: Has the mystery of whale evolution been solved?

To some degree.

But when we fill a gap, we make two more, and so, we're always going to want to know more.

NARRATOR: From the first wolf-sized creatures that ventured into freshwater rivers, to the walking whales that were champion swimmers, to fearsome marine predators, to the largest animal that has ever lived.

Today, scientists continue to search for the missing chapters in the whale's story.

SALLAM: There is more to find in Wadi Hitan.

There are so many fossils still hidden inside the rocks, and we hoping someday to find very primitive whale in very ancient deposits that can actually complete the story in whale evolution.

That will be a really huge discovery.

NARRATOR: The whale's evolutionary journey is not over.

Today, they must survive new threats that are driving some species close to extinction.

REIDENBERG: There are a lot of new challenges that the whales are facing today, whether it's fishing, ship traffic, noise, climate change-- anything could be a factor, because if it affects their habitat, it affects their evolution.

Exactly.

And the challenge is, evolution doesn't happen overnight, so we don't really know what the consequences of that will be.

NARRATOR: Whales face a precarious future.

The hope is that they will adapt and survive as they have done for 50 million years.