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The environments of the sea and land are so starkly

different that a transition from one to the other

seems virtually impossible.

Nevertheless, they are the dual stage of a grand drama

which took place over the course of 4 billion years.

Some creatures birthed in the sea climbed up onto the land,

while others returned to the oceans from whence they came.

This odyssey from sea to land, and then back to the sea,

is an integral part of the history of life.

It is thanks to the pioneering creatures that first set foot

on land that human beings exist today.

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This is a desert located 150 kilometers southwest

of Cairo, the capital of Egypt.

It is a vast sea of sand which seems to

be utterly bereft of life.

Yet, it is this barren land that harbors vestiges of the

greatest mystery in the history of life.

To paleontologists who trace the history of life,

this is a treasure trove of information.

Buried within the hot desert sand are ancient bones.

Thirty years ago, Philip Gingerich of the University of

Michigan made a startling discovery here.

He discovered the fossil of an ancient whale.

And this means it's an experiment in whale evolution

that didn't happen before and was very sucessful

but didn't happen after.

You can see it looks like recent bone.

It looks like it died yesterday but here it is in

the sandstone for 37 million years.

So that's why Wadi El-Hitan is important.

He unearthed over a thousand fossils in this place called

Wadi El-Hitan or Valley of the Whales.

Thirty-seven million years ago,

these parched lands were covered by the Tethys Ocean.

Before Professor Gingerich discovered this fossil,

not much was known about the origins of the modern whale.

Whales are a rare instance where a terrestrial animal

returned to the sea, but until then,

the exact reasons for this remained a mystery.

The key to unraveling this mystery is Basilosaurus,

the ancient forerunner of the modern whale.

Thirty-seven million years ago,

Basilosaurus ruled the Tethys ocean.

It was 16 meters long, with a gaping

mouth and serrated teeth.

And unlike most other aquatic creatures,

it had a pair of hind legs.

Today, vertebrates dominate the world.

And the hind legs of the Basilosaurus are a

characteristic that is unique to land-dwelling animals.

Man is one of them.

In order find the link between Basilosaurus and

land-dwelling animals, we must travel back eons

to the origins of life itself.

4.5 billion years ago, Earth in its infancy was an

environment hostile to life.

The atmosphere was over 80 degrees Celsius

and devoid of any oxygen.

But after millions of years, a miracle occurred.

The first living organisms appeared on the Earth,

birthed by the ocean.

3.5 billion years ago, the ocean was rich with oxygen,

and the horizon glowed with the sapphire hue of the water.

It was at this time that Stromatolite,

the first form of life, appeared.

This primitive organism was in fact colonies of blue-green

algae which survived through photosynthesis and produced

abundant amounts of oxygen.

Thanks to this organism, Earth's atmosphere became

oxygen-rich, creating an environment

that was conducive to life.

500 million years ago, the seas began to teem with an

endless variety of strange-looking creatures.

They possessed long tentacles and fearsome spines,

and were protected by stone-hard armor.

This sudden proliferation of life is called the

Cambrian explosion of life.

Scientists believe that the ancestors of most of the

animals in existence today appeared at this time.

This creature swimming through the water using its wing-like

appendages is Anomalocaris.

Over a meter long, this animal was the largest and most

fearsome predator of the Cambrian oceans.

Thanks to its powerful jaws, it could pierce through the

hard armor of a trilobite with a single bite.

In the seas, competition for survival grew increasingly

fierce, and creatures were forced to modify

themselves in order to survive.

During this time, a transformation occurred which

would alter the course of evolutionary history.

This change occurred in the tiny Pikaia,

a creature no larger than the size of a thumb,

which was the constant target of larger predators.

When we look at Pikaia, we are really stepping into the

door of vertebrate evolution.

We are there, as we say in English, on the ground floor.

These are the opening moments of this extraordinary story

which is going to take another half a billion years.

The Pikaia developed a stiff rod on its back called a notochord.

It was the precursor of the spine.

When we look closely, we actually see that there are

quite a lot of details.

We can see the front end and we can see the tail.

And then most interesting, roughly along here,

there is a distinctly different zone.

The notochord proved to be a revolutionary improvement.

The notochord and the V-shaped muscles around it called

myomeres, worked in concert to propel this creature forward,

giving it unprecedented speed and power.

This is how Pikaia probably looked as it swam.

Its propulsion method is similar to that of modern fish.

Despite being an apex predator,

Anomalocaris soon became extinct.

But Pikaia managed to survive.

While it was armed with neither eyes nor a protective

shell, it could elude even the fiercest of predators

using its superior speed.

Pikaia's legacy is significant in the history of evolution.

Once you got a notochord, once you got the myomeres,

then you're on the way to becoming a fish.

Once you're on the way to becoming a fish,

you're on the way to becoming a vertebrate.

And once you are on the way to becoming a vertebrate,

then you're on the way to becoming us.

Had the Pikaia become extinct, the history of life on earth

would have ended 5 hundred million years ago in the sea.

Instead, these tiny proto-vertebrates were slowly

preparing to become fish.

470 million years ago, the primitive notochords become

fully developed vertebral columns.

This is the fossilized ancestor of all

vertebrates including man.

And the black pieces you can see on the surface here are

pieces of an Arandaspis fish called Sacabambaspis.

Now these are some of the very earliest fishes where the

scales and armor was on the surface of fishes.

While the fossil is incomplete,

it shows the distinct traces of scales,

a feature that is unique to fish.

This is Arandaspis, the first fish.

It was covered in scales like the countless fish species

that came after it, but because it did not have fins,

it was an awkward and painstakingly slow creature.

Also, it was a jawless fish which sucked water through an

opening in its belly, and subsisted on the various

microorganisms which it was able to strain.

After another hundred million years,

a new fish appeared.

This is a fishing village in Indonesia.

Here, fishermen caught a strange fish which

they had never before seen in their lives.

The fish that had become tangled in their nets was none

other than a living fossil.

So, what does this fish look like?

They open the lid and clear the ice away

to reveal the coelacanth.

Scientists had previously thought that the species had

already gone extinct during the Cretaceous period when

dinosaurs still roamed the earth.

It takes the strength of three men to

lift the fish onto a table.

These animals can grow up to 2 meters long and

reach 90 kilograms in weight.

Like most other ancient fish species,

they are armed with sharp teeth and thick scales.

Also, they carry their eggs with them and give

birth to fully developed young.

But the feature that makes this fish truly unique is its fins.

The fish is equipped with large, powerful fins.

This foreshadowed an important new evolutionary development.

This is Miguasha National Park located in Quebec, Canada.

The sedimentary layers here are up to 380 million years old.

Although this is a northern coastal area,

it was a tropical region located near the equator

hundreds of millions years ago.

At the time, a river flowed through this area.

And it was populated by a diverse array of fauna that

comprised a complicated food chain.

It is here, in the geological strata of the cliff,

that traces of a significant evolutionary leap can be found.

On exhibit in the museum is a fossil of a fish that is

strikingly similar to the coelacanth.

The species first appeared in the Devonian period.

It was a large fish that breathed through its lungs and

possessed a set of powerful fins.

Called Eusthenopteron, its name literally

means "Powerful Fins."

The fins of the fish have a feature that had not been

found in previous species.

The fins contain bones.

We have one major element that is articulating with the

shoulder girdle and this element is the humerus,

exactly the same element that we're finding in our arm or in

the arms of the earliest tetrapods.

The boned fins of Eusthenopteron are proof that

the limbs of terrestrial animals originated in fish.

But why did this fish develop bones in its fins?

These most likely will be used to move in a very swampy

environment, just to move among the plants.

So that is giving not just a very thin fin but that will

give a very strong paddle that the animal will be able to

push the plant to get into an environment to find its food.

Eusthenopteron was a skilled swimmer.

It used its powerful fins to propel itself

freely through the water.

Also, it was an excellent hunter with

well-developed jaws and teeth.

But the Devonian seas were the stage for a

fierce battle for survival.

And Dunkleosteus ruled supreme.

At 6 meters in length, Dunkleosteus

was a true behemoth.

It was a placoderm which possessed powerful jaws

and a thick plate of armor.

Equipped with razor sharp teeth,

it often preyed on Eusthenopteron.

In order to avoid this vicious predator,

Eusthenopteron undertook a risky journey.

It left the ocean in search of a new habitat.

It used its powerful fins in order to swim into a river,

and then to other bodies of freshwater.

In the water, there are huge fish,

some of them about 16 feet long,

of all different kinds, armored fish,

all kinds of predatory fish.

It was a remarkable period.

During the Devonian period 370 million years ago,

the land was a place full of opportunity.

Trees appeared, foresting the barren terrain,

and rivers began to flow.

The fish which populated these rivers heralded a new era in

evolutionary history.

Paleontologist Neil Shubin discovered a fossil which would

shock the scientific world and be called the "Missing Link."

He discovered this heretofore unknown species on Ellesmere

Island in Northern Canada.

It was named Tiktaalik, or "Large Freshwater Fish" in

Inuktitut, the language of the local Native American tribe.

Although it had a flat, lizard-like head,

its scales showed that it was clearly a fish.

And for the first time, we're seeing the kinds of fish that

can actually leave the water, that can make the transition

from life in water to life on land.

So all kinds of new things are happening.

Tiktaalik harbors clues to the gradual evolution of

land-dwelling animals from the sea.

Unlike most other fish, the Tiktaalik had a flat head with

eyes on the top of its skull.

Another distinctive feature is this.

Tiktaalik possessed a neck.

This was a new feature that wasn't

present in previous fish.

Tiktaalik use this neck to rotate its head freely and

scour its surroundings.

But the real reason that Tiktaalik is called the

"Missing Link" is its fins.

Its fins are more powerful and intricately designed

than that of Eusthenopteron.

Tiktaalik had shoulders, elbows and even wrists.

Now what's very special about this is not only that these

are bones that compare to our own arms,

but if you look at the surfaces of this,

there's surfaces for all kinds of muscles.

And not just any kinds of muscles,

muscles that would've helped the animals do a kind of

a push-up to support itself against the ground.

Tiktaalik was a revolutionary fish.

It could move its neck freely, and scour its surroundings

with its eyes located on top of its skull.

Its fins were equipped with elbows and wrists,

and were strong enough to support its gigantic body.

The appearance of this fish signaled a new

movement towards the land.

Thus began the long and arduous process of adjusting

to the new terrestrial environment.

So, how did Tiktaalik use its versatile fins?

Mudskippers which live in the tidelands of Korea,

give us an idea of how Tiktaalik must have moved.

The mudskipper uses its pectoral fins to support

itself while crawling around on its belly.

Tiktaalik must have climbed onto the land using its fins

in much the same manner.

Water was loaded with competitors.

Large fish, small fish, everything competing for food.

Some of those fish would eat you.

If you look at land, there are plants there,

there's food there and these early insect-like creatures.

So there are advantages for creatures leaving the water to

go to land, to remove themselves from the

competition for food resources,

but also to escape some of the predation that was

going on in the water as well.

After Tiktaalik, the evolution of the fish

became even more accelerated.

Paleontologist Jennifer Clack shows us a sketch of

a small lizard-like creature.

This is Acanthostega, a species which marked another

turning point in the history of evolution.

Acanthostega is a freshwater fish which lived 360 million

years ago during the Devonian period,

It had four almost fully formed legs and

breathed air using its lungs.

Also, it had something very unique

which Tiktaalik did not have.

One of the first things that we found was that on each

limb, there were eight fingers or toes,

which was a big surprise.

It was the first creature in the history of evolution to

have fingers and toes.

Previously people had thought that the primitive number for

fingers and toes was five because that's what we have.

But it turns out that when tetrapods with limbs first

evolved, they had more than five.

Acanthostega had eight but we also have other animals from

the same time with seven or six.

This is a shallow lake in the late Devonian period.

Acanthostega uses its webbed feet to dart through the

water, weaving through various obstacles along the way.

It hides among aquatic plants and snatches a passing fish.

Acanthostega which possessed both lungs and gills...

...finally took the first steps onto land.

The adjustment to land did not happen overnight.

The appearance of the first spine,

fin equipped with muscle and bone,

and the first legs and toes was the result of billions

of years of trial and error.

Millions of years passed by after the appearance of

Acanthostega and the competition for survival in

the water grew increasingly fierce.

But it was a different story altogether on land.

The terrestrial ecology was still full of opportunities.

Then, 280 million years ago, the first complete

tetrapod appeared on land.

Descended from Acanthostega, it could only breathe air

through its lungs, and it didn't have to return to the

water in order to spawn.

The adaptation to land was complete.

It was a moment of monumental significance on par

with Man's landing on the moon.

The transition from life in water to life on land is one

of the great events in the history of life.

And it's an event that changed the world forever.

For the first time, we have creatures walking on land,

feeding on land.

But it's not just an event that's captured in the past,

it's actually an event that's inside our own bodies because

many of the features that occurred for the first time in

the fish that walk on land is part of us.

In many cases, the fish that climbed onto land were the

weakest members of the food chain.

They left behind the fiercely competitive underwater

environment in order to survive.

But through change and adaptation,

they achieved the herculean feat of adapting to a

completely new and alien environment.

Once tetrapods had reached the land,

they began to proliferate at a breakneck pace.

They diverged into millions of species including amphibians,

reptiles, birds and even mammals.

Among their ancestors, mammals reared and breastfed their

young, and became the masters of the earth.

Once the weakest creatures of the sea,

they achieved a brilliant success through

their mastery of the land.

But there is a mystery in the history of life

that still remains unsolved.

It is the existence of mammals in the ocean.

This is a marine mammal called the dugong.

It uses its fins like arms and feeds on aquatic plants.

It breathes air through its lungs,

so it must surface regularly.

It also births its young and breastfeeds them.

So, why does the dugong possess characteristics that

are common to land-dwelling mammals?

The dugong swims away without revealing any of its secrets.

We returned to the Egyptian desert in order

to unravel this mystery.

Called Wadi Al-Hitan or Valley of the Whales,

this desert has been designated as a

World Heritage by UNESCO.

There is a line of vertebrae in the sand.

Although this fossil was called the "Lizard King" when

it was first discovered, that was later on

proven to be a misnomer.

Basilosaurus was far from a lizard.

Basilosaurus represents an early stage of whale evolution.

It's one of the first aquatic whales,

fully aquatic whales.

Once the dominant species of the Tethys Ocean,

this primitive whale lived from 42 million

to 34 million years ago.

The largest whale at the time, this creature swam by weaving

its long body through the water.

It had a pair of underdeveloped hind legs which

seem puny in comparison to its gargantuan size.

This is the clue which will help us unravel this mystery.

This is a Basilosaurus fossil unearthed in Wadi Al-Hitan.

This animal was also called the "Walking Whale"

because of its hind legs.

Although they were small, the legs

were fully formed appendages.

They are proof that Basilosaurus once roamed the land.

The general trend, life started in the sea.

This means that the general trend,

from our point of view at least,

is to come out on to the land and the whale has gone back.

And so in that sense, it's a reversal, it's backwards.

There is another mammal that returned to the ocean.

It's Dorudon, a smaller primitive whale that was

around 5 meters long.

The raging desert winds revealed whales that had been

buried for countless millennia.

Professor Philip Gingerich has recently discovered a new

fossil embedded in a cliff in Wadi Al-Hitan.

It's a fossil of Dorudon.

This is the first time that we know how complete is dorudon,

how many backbones, how long it is,

it's the first time we know that these whales have legs and

feet and toes and it's because of excellent preservation.

This whale with large jaws, serrated teeth and tiny hind

legs is directly linked to land-dwelling mammals.

It was a skilled swimmer that lived in the

oceans 37 million years ago.

But its method of swimming was completely

unlike that of any fish.

This is another clue which hints at the origins of the whale.

The mechanics of dorudon are reminiscent of the manner in

which land mammals use their spines in order to run.

What is going on?

65 million years ago, at the end of the Cretaceous period,

a giant asteroid collided with the Earth, killing off

the dinosaurs which had reigned supreme for many eons.

This was the incident which spurred the

whale to return to the sea.

After the Cretaceous-Paleogene extinction event and dinosaurs

are gone, the marine reptiles are gone,

then there is no top predator in the sea.

And so I see it more as an opportunity,

not something pushing from behind but something

pulling from in front.

So, which species was it exactly that returned to the sea?

This fossil of Pakicetus is the key to

unraveling this mystery.

This creature had 4 powerful legs and seems to have nothing

in common with the whale.

Also, its vertebrae were still attached to each other,

making its spine suitable for terrestrial inhabitation.

Yet, it had a specialized ear bone inside its skull.

Only whales possess this bone which is used to

detect sounds underwater.

This is a sea near modern-day Pakistan 50 million years ago.

Pakicetus was an amphibious, wolf-like creature.

It took to the seas because they were abundant with food

and less populated than land.

It also possessed long webbed toes which allowed

it to swim freely in the water.

And it possessed a specialized ear bone which allowed it to

hear the sounds of its underwater prey.

The animal used this hearing to catch fish with its long

snout and razor sharp teeth.

To hear in water is completely different than to hear in air.

And so the hearing apparatus has to be modified.

This ancestor of the whale transformed its body to adapt

to its new, underwater lifestyle.

Although it could only detect underwater sounds at first,

later developments allowed it to locate the direction from

which the sounds came.

This creature changed everything in

order to adapt to the water.

Its forelegs became its pectoral fins and its

superfluous hind legs devolved.

Also, its tail became wide and flat in order to

propel it through the water.

Lastly, its nostrils moved towards the top of its skull

to facilitate underwater breathing.

In this way, the ancestor of the whale adapted

completely to the ocean.

Pakicetus, which had maintained an amphibious

lifestyle for countless millennia,

finally left the land for good.

It had completed its transformation and become

primitive whales such as Dorudon and Basilosaurus.

This return to the ocean is one of the most amazing

incidents to occur in the history of life on the Earth.

The fish coming out on land very early in vertebrate

history and the whale going back to the sea late in

vertebrate history have this similarity.

I think both are taking advantage of some opportunity.

It's a nice comparison because it shows that evolution is not

determined and directional.

Instead it is opportunist and if the opportunity is behind,

we will go back.

But this is not the end of the story.

34 million years ago, around the end of the Eocene Epoch,

the Tethys Ocean which was heavily populated by primitive

whales, began to dry up, and the temperature

of the earth plummeted.

It was at this time that Basilosaurus,

the largest whale species of the era, and its lesser cousin,

Dorudon, met with different fates.

It was the smaller species, Dorudon that managed to

survive in the frigid waters where food had become scarce.

And it became the forefather of the modern whale.

On the other hand, the large and inefficient Basilosaurus

could not avoid extinction.

In this way, these rare primitive whales have become

buried in the sands of time.

Nevertheless, these prehistoric behemoths leave us

with an unmistakable message.

Some people are interested in stars and outer space

and how far we can see.

I'm interested in time and how far back we can see because

all our understanding and expectation for the future is

conditioned on what we understand about the

present and the past.

The study of deep time, the study of life through time,

understanding where we came from,

how we are a part of the earth,

how we are a part of the history of the earth.

To some, fossils are merely the petrified remains of

creatures from ages past.

But it is these creatures that risked their lives to forge a

pathway into the unknown terrestrial environment.

And their legacy has withstood the test of time,

not only surviving in the new environment,

but thriving in it.

We are their legacy.