[CRICKETS]
[BING]
[TIGER ROARING]
[MUSIC PLAYING]
[WATER RUSHING]
NEIL SHUBIN: Our planet is teeming
with many kinds of animal life, including groups with defining
structures, such as the four legs of land animals,
the feathered wings of birds, and the dexterous hands
of primates.
Understanding the origins of these structures
and of the groups that possess them has long been
a central quest of biology.
Charles Darwin asserted that each kind of animal
must have evolved from pre-existing, earlier
kinds of animals that lacked those structures.
He boldly predicted that buried in the crust of the earth
were animals that connected one major group to another.
Such transitional fossils would be
intermediate in form between earlier and later groups.
What made Darwin's prediction so bold
was at the time he stated it in The Origin of Species,
no such fossils had been found.
His critics immediately latched onto the admission
that transitional animals are somehow
missing from the fossil record.
Over the decades, it's become a standard talking point
for those who have closed their mind
to the science of evolution.
SPEAKER 1: There you go.
NEIL SHUBIN: Yeah, this is the stuff.
Right, so this was the--
But, in fact, since Darwin, paleontologists
have unearthed hundreds of transitional creatures
that have enabled us to reconstruct
the origins of many groups.
And even so, searching for such fossils
remains a challenging adventure, and finding the right one
can change the way we think about the origins of living
creatures.
[THEME MUSIC]
For me, paleontology has always been
about filling the gaps in the story of life.
And for a long time, one of the biggest ones
was understanding the origin of animal limbs,
with fingers and toes.
[MUSIC PLAYING]
Paired limbs are a feature of many animals,
but there's little at first glance that
suggests the limbs of different species are related.
On frogs, they're springy.
On elephants, not so much.
They're feathered on eagles, not on bats.
But inside the limbs of mammals, amphibians, reptiles,
and birds, one finds a common architecture.
Here's a dog.
Dogs run and jump.
What do you have?
One bone, two bones, little bones, and then the digits--
the equivalents of the fingers or toes.
And, of course, here's a bird.
It flies.
Its limb has been modified into a wing.
And it has one bone, two bones, lots of bones, and then digits.
The amazing fact is every four-limbed animal
walking the earth today has this fundamental pattern.
One bone, two, bones, little bones, fingers.
That pattern suggests a connection
between these very different groups of animals.
And it's not the only feature they share.
They also all have a backbone.
They're vertebrates.
The history of vertebrates has been captured in rock
we can accurately date.
Fossils reveal when each of these groups of animals
first emerged.
The youngest group is the birds.
Go further back in time, and you'll
find the first mammals, and then the first reptiles,
and the first amphibians.
And then you get to 370 million years ago.
Suddenly, there are no four-limbed creatures,
or tetrapods, anywhere to be found.
Where the first pods came from has always
been one of the great mysteries of biology.
I mean, it's not like there weren't animals or vertebrates
around 400 million years ago.
There were, but they were all fish.
Did four-legged animals come from fish?
Fish might seem unlikely candidates
to be the earliest ancestors of frogs, horses, and humans.
They don't even have limbs.
They have fins.
Despite their different external appearances,
there are revealing similarities.
First, fish and tetrapods are vertebrates.
And early in life, when they are embryos,
they look remarkably similar.
Finally, DNA analysis shows that fish are tetrapods' closest
relatives.
All of this suggests four-legged animals did indeed
come from fish.
But how did a fish with fins give rise
to tetrapods with four legs?
As a young scientist, I wanted to find
fossils that could help answer that question.
I knew it wasn't going to be easy.
The world's a big place, the earth is a giant planet,
and fossils are very small.
So how do you find those things?
Well, there's a checklist we run through.
We look for places in the world that
have rocks of the right age.
You know, if you're interested in the origin of dinosaurs,
there's one age of rock to look at.
If you're interested in the origin of land living
creatures, there's another age of rocks.
Then you look for places in the world that
have rocks of the right type.
The kinds of rocks that are likely to hold fossils.
A lot of things have to come together for an animal
to be fossilized.
For starters, it has to be in the right kind of setting
where sediments form.
And soon after it dies, it has to be
buried before its remains are ravaged by decay, weather,
or scavengers.
The dirt and mud burying it has to harden sufficiently
to protect what's left for thousands, or more
likely millions, of years.
After which, something, say erosion,
has to bring the embedded remains to the surface.
And then someone who cares about such things,
like me or my longtime colleague, Ted Daeschler,
has to wander by and find it.
The fossils I wanted to find would
have been alive in the Devonian era,
between 365 and 385 million years ago.
But where could we find the right kinds of rocks
from that era?
But you could still see one of these ledges--
I remember sitting in the office,
and we were doing the sort of usual banter one day
about something geological.
We had a college textbook.
We were just thumbing through the diagrams
in the book and, boom, there was this figure
that changed our lives.
I remember seeing that and saying to myself, holy cow!
This is what we're looking for.
It was a map of North America which
highlighted three areas of Devonian rock of just
the right type to hold fossil fish moving onto land.
Two of those areas had already been
worked on, so we focused on the third, the Canadian Arctic.
My heart was racing when I saw that because--
and I'm sure yours was too.
I mean, no--
TED DAESCHLER: Yeah.
NEIL SHUBIN: --paleontologist had worked on that expressly
looking for early tetrapods.
Then you dug out the aerial photos,
and that's when I got kind of terrified.
I remember seeing this for the first time thinking,
you got to be kidding me.
Yeah, look at all this snow.
How do you work there?
[MUSIC PLAYING]
The Arctic presents some unusual challenges.
You're far from help so you have to bring
everything you'll need.
And you have to move fast because the season is short.
And you don't want to be there when the weather turns.
So when the helicopter drops you off
in the Arctic for the first time,
you're standing here saying, what am I doing here?
You know, you're thinking, oh, polar bears.
That's the first thing you look for.
Is there anything on the landscape?
Everything white becomes a polar bear when you're first here.
The last thing on your mind are fossils.
[MUSIC PLAYING]
It's hard to believe when you look out
across this frozen terrain that, once, this
was a warm, watery world, swimming with life.
There's this huge disconnect between the present
and the past.
What we see today is a valley with red and green rocks
that are tilted, stacked one on top of the other.
But that's not how it was in the past.
These valleys have been carved by glaciers
that have moved back and forth, and those red and green rocks
actually at one point extended across the valley.
And they were straight, they weren't tilted.
Now, look inside the rocks.
And what those rocks tell us, that this valley,
375 million years ago, was a giant flood plain.
And that flood plain was filled with rivers
that swelled their banks and sometimes shrunk,
but in those conditions formed swamps and streams
of all different sizes.
And inside those streams was diverse life.
Including, we suspected, a fish with features
that would ultimately enable animals to walk on land.
[MUSIC PLAYING]
But even if it had been there, could we
find evidence of it buried on one
of the nameless hillsides that had built up and eroded
over the past 375 million years?
So how do you find fossils?
I pick up a lot of stuff, right?
Sometimes it's just a piece of rock.
Sometimes it's bird poop.
Sometimes it's a leaf.
But, occasionally, you know, it's a jaw with teeth in it.
So what you begin to learn is to tell
the difference between white which is not bone and white
which is bone, teeth, or scale.
Once you have that in your mind, then you
start to apply that search image to other rocks.
Here's another scale here.
It stands out like a sore thumb.
And then we go--
if you look around, right here.
OK.
So this is the underside of a skull.
So you never know where you're going to hit it around here.
You know, that's why we keep on looking.
[MUSIC PLAYING]
But that first expedition ended without finding
what we had come for.
And as our second trip drew to a close, we were still searching.
Then a bit before our scheduled departure, we had a real scare.
The team had separated.
And the idea is everybody needs to return back to camp
by radio call.
Hey, you guys see Jason?
No, I didn't see Jason.
Did you see Jason?
I said, I asked you the question.
You didn't see Jason?
And then all of a sudden, it became where's Jason?
This is our youngest member.
We were looking out for him the entire season, and no Jason.
I mean, my heart was really beginning to race.
Then I hear footsteps outside the tent.
There's Jason.
His eyes are like globes.
And he's like, I found it!
I mean, every pocket was burgeoning with bones.
He was like, I got these bones-- he's laying them down
on the table, one after another.
It's daylight 24 hours a day, so we ran down to Jason's site.
As soon as we came to this bluff here and looked down,
we saw why Jason was so excited.
Because beneath our feet were fossil fish bones,
fragments of fossil fish-- many of them, thousands of them.
It wasn't just one fish, it was a whole aquarium.
It was different species.
It got better.
Because as we walked up the hill,
and we followed that carpet of fossil fragments, it stopped,
meaning it likely came from one layer.
And if we had any luck at all, we'd find that layer
and see what's inside.
Hard as we tried, we couldn't discover
what was buried in Jason's hill before we had to leave.
But we kept coming back to it in following years
to dig, chip, and search.
The second week of July in 2004, we're
all working and serious in this hole.
You know, where my head is right next to Farrish's feet,
and Farrish's feet is next to Steve Gatesy.
And we're digging together and Steve says, hey,
guys, what's this?
Ted and I go running over to see what Steve was referring to,
and what we saw was this v here.
It was covered with rock.
And as soon as we saw this v and we saw these teeth under it,
it became very clear that this little v we're seeing
is the tip of a snout, and that this was
a snout of a flat-headed fish.
And it was sticking out of the rock.
So if we had any luck whatsoever,
the rest of the creature would be encased in the rock.
And here it is.
What's really wonderful about this specimen
is that we have pretty much the whole thing,
and the whole thing is put together.
That is, the head is connected to the body
and the body is connected to the fins,
so we know that this fin comes from this body.
And when we put it all together, we
see this creature is about four feet long.
And some of the biggest were about nine feet long.
What's really amazing is that this is an animal that Darwin
would have predicted.
A real mix of characteristics-- a combination
of fish-like and tetrapod-like features.
Like a fish, it has scales in its back
and it also has fins with fin rays.
But like a tetrapod, it has a flat head with eyes on top.
And when we look inside the body, what we see
are these huge interlocking ribs that
suggested that it had lungs.
When you put the body and the head together,
you see it had a neck, where the head can move independently
of the body.
What that means is this animal could use the neck
to peer outside the water, find prey, and avoid predators.
So we have many bones of these animals,
including this one, which is a hip bone of these creatures.
And what it reveals is that the hind fins were already evolving
into legs while these animals were living in water.
I get really excited when I see the front fins of tiktaalik.
Here's one from one of the larger specimens.
And what you see is the shoulder, but also
some of the fin bones inside.
What you see is a version of the one bone, two bone pattern
that's inside our own arms.
You have one bone, two bones.
You even have a version of a wrist.
[MUSIC PLAYING]
And once those fins were strong enough
to lift its body out of the water,
a whole new frontier opened.
Over millions of years, the two pairs of fins and fish
like tiktaalik would lead to the two pairs
of limbs in every tetrapod.
So what does this all mean?
What it means is that our arms and legs
are derived from the paired fins of our fishy ancestors.
So how fast did this transition happen?
Well, we know the tiktaalik didn't exist in a vacuum.
There are other creatures, other transitional fossils,
that are more fish-like and others
that are more tetrapod-like, and these creatures
existed for over 15 million years.
So what this means is that this great transition from fish
to tetrapod didn't happen in a single step,
but happened gradually over time.
The discovery of tiktaalik made headlines
because it is one of the earliest in a series of fossils
that illuminate the transition from water to land.
And that's just one key transition that fossils
have shed light on.
One of the very first was the evolution
of birds from feathered dinosaurs.
That's now one of the best documented transitions
in the story of life.
SPEAKER 1: Well, this is skull.
NEIL SHUBIN: What it and other well
studied examples tell us is that what at first seemed
to be huge leaps are almost always
products of a series of smaller evolutionary steps.
That's true for when our fish ancestors first came to land,
when dinosaurs took to the air and when we first
stood upright.
[MUSIC PLAYING]
Fish, and an ancestor who lived 3.2 million years ago.
SPEAKER 2: She's beautiful.
[MUSIC PLAYING]