[MUSIC PLAYING]
SEAN CARROLL: Picture a world of giant reptiles,
flying pterosaurs with 40-foot wingspans.
While, beneath the waves, fierce predators called
mosasaurs prowled the seas, and dinosaurs roamed the land.
For more than 100 million years, creatures like these
ruled the planet.
And then they were gone, lost in the shadows of time.
Their extinction-- a mystery for the ages.
[MUSIC PLAYING]
This is all we have left of those magnificent creatures--
bones, and a lot of them.
We've collected enormous numbers of bones.
And from them, we can tell how these creatures
lived, what they looked like, when they lived.
But we don't know why they disappeared.
To solve that mystery required some of the greatest
scientific detective work ever.
And the trail began almost halfway around the world
from here.
[BELLS RINGING]
This is Gubbio, a sleepy Italian town.
Not a single giant reptile in sight.
Still, the past is everywhere you look--
in the medieval palaces and churches, the homes
and narrow streets, while an even deeper past lies nearby.
[MUSIC PLAYING]
Just outside of Gubbio, Alessandro Montanari
is looking for that distant past.
He's a time traveler moving towards the lost
world of the dinosaurs.
It's ancient history written in these limestone cliffs.
ALESSANDRO MONTANARI: Millions of years
ago, these mountains were at the bottom of a deep sea,
collecting layers of sediment and deposited it slowly
through time, and eventually, pushed by tectonic forces,
folded and uplifted.
SEAN CARROLL: So all of this rock and all of these layers
were once at the bottom of the ocean.
ALESSANDRO MONTANARI: Exactly.
SEAN CARROLL: And then all these other forces
have brought this older rock up to you.
ALESSANDRO MONTANARI: Yes.
The reconnection is starting layer by layer,
like pages of a book, the history of the earth.
[MUSIC PLAYING]
SEAN CARROLL: Gubbio's deep-sea limestone,
now exposed by the side of the road,
became a magnet for scientists, especially this site that may
seem ordinary, but holds one of the strangest features
to confront a geologist--
a thin layer of dark clay.
In the 1970s, the clay captured the attention
of an American geologist, Walter Alvarez.
He was trying to determine the relative ages
of the limestone layers by analyzing the fossils left
by tiny shelled sea creatures.
Called foraminifera, or foram for short,
they're among the most common ocean plankton.
When they die, their shells become part of the sediment.
Since many different species have evolved over the ages,
they can serve as markers of geologic time.
But as Alvarez studied them, he noticed something striking.
ALESSANDRO MONTANARI: Well, what he sees,
and he was puzzled by, after the very top of that white layer,
all these very diverse microfossils
species would disappear.
SEAN CARROLL: OK.
So Sandro, when Walter Alvarez looked in these rocks,
he saw all these species of forams.
They're plentiful.
There are many species for thousands and thousands
and thousands of years.
But after this little scene of clay here,
all the way up through these rock layers,
those forams are missing.
ALESSANDRO MONTANARI: Those species are gone forever.
SEAN CARROLL: Disappeared.
ALESSANDRO MONTANARI: Disappeared.
SEAN CARROLL: And that's a fundamental mystery.
What would make these little creatures disappear?
ALESSANDRO MONTANARI: Yes.
SEAN CARROLL: So Alvarez was stumped.
What did this thin line of clay represent?
It was laid down 65 million years ago, the same time
when the dinosaurs disappeared.
Was there any link?
The same questions were being asked
1,500 kilometers away in Spain.
[MUSIC PLAYING]
On the Atlantic Coast, outside the town of Zumaia,
Dutch geologist Jan Smit was studying the forams
from a different ancient sea.
Their fossilized shells formed these limestone rocks,
which, like Gubbio's mountains, were once
at the bottom of the ocean.
Now exposed, they represent over 1 million years
of a geological time period known as the Cretaceous.
Like Alvarez in Italy, Smit had also
found a strange clay layer formed at the same time, which
told a dramatic story.
Jan, what can we tell about earth's history
from just looking at these rocks.
JAN SMIT: Well, you can see the end of the one world
and the beginning of the next one.
You see there is a very sharp, extremely sharp,
dividing line between the two.
SEAN CARROLL: That gray band.
JAN SMIT: That's that gray band over there.
At the bottom of the gray band, you see it's razor-sharp.
SEAN CARROLL: And all this reddish-maroon rock, the oceans
are healthy and [INAUDIBLE].
JAN SMIT: They're healthy.
They're steady.
They don't change at all.
And all of a sudden, we see a dramatic change,
which we call the K/T boundary.
SEAN CARROLL: The K/T boundary, found
at the bottom of this gray layer, marks, not only
the end of a period, the Cretaceous,
but also of the Mesozoic era, a much longer stretch of time.
The history of animal life on earth
has been divided into three such eras--
early life, or Paleozoic; the age of dinosaurs, or Mesozoic;
and the era of mammals, or Cenozoic.
The Cretaceous tertiary boundary lies
right in between these two eras, marked here
by this layer of clay.
What drew you to this boundary, Jan?
JAN SMIT: Well, Sean, we're looking here
at the very top-most rocks of the Cretaceous.
These rocks are literally packed with foraminifera.
So we call this the Cretaceous, and these rocks here
are the tertiary.
And the base of the tertiary consists of a dark gray clay
layer.
And there, the life has almost disappeared.
And the K/T boundary is just in between.
I can point at it.
It's right here where you see a contrast between the purple
and the dark.
It's an extremely sharp boundary.
SEAN CARROLL: And just across that thin little boundary,
there's this huge change in what you see in the forams.
And why is that so stunning to you?
JAN SMIT: It's so stunning because there
is no preceding evidence of anything happening there.
So it doesn't matter if I take a piece here,
a piece there, or just underneath what
we call the K/T boundary.
The foraminifera will remain the same.
So they don't change overnight.
And then, bang, they're gone.
SEAN CARROLL: So what's that tell you?
JAN SMIT: That tells me the base of the food chain
of the oceans disappeared.
And everything which is dependent on it
is totally zapped right at the boundary.
[MUSIC PLAYING]
SEAN CARROLL: In Amsterdam, change at the boundary
becomes clearer when Smit takes a closer look at the evidence.
In his lab, he analyzes forams extracted from the limestone.
Under the microscope, the rich diversity
of fossilized shells from the Cretaceous snaps into focus.
Some four dozen species appear below the boundary.
But above it, there's a different world.
JAN SMIT: And these are the foraminifera
from just below the boundary.
And these are from just above the boundary.
SEAN CARROLL: Only a few species have
survived into the tertiary, and they're much smaller.
JAN SMIT: As soon as you see the extinction,
and you realize, at the same time,
that the dinosaurs are disappearing,
you know you're looking at something very important.
But you see that nobody has witnessed it.
So we're looking for silent witnesses in the rock.
And the first thing that comes to mind
is this beautiful clay layer.
SEAN CARROLL: A layer varying in color and thickness
and found around the world.
Walter Alvarez also believed it was a silent witness
to the end of an era.
The key question was, how long did it
take to form, for the world to change?
To find out, he sought help at the University of California
at Berkeley from a Nobel Prize-winning physicist,
someone he knew pretty well, his father, Louis Alvarez.
Louis loved a good mystery, no matter what field it was in.
And that's how physics joined geology in the quest
to explain the K/T extinction.
RICHARD MULLER: Alvarez looked at this layer,
tried to figure out how you could determine the time scale.
He brought in his knowledge of astrophysics,
his knowledge of nuclear physics,
and realized that there's an element that's
relatively rare in the crust of the earth that
occurs in meteorites.
SEAN CARROLL: The element was iridium,
which falls steadily in an invisible rain of cosmic space
dust.
If the layer had taken thousands of years to form,
Alvarez thought there might be just enough iridium to measure.
But when the clay was tested, scientists
were stunned to find it contained 30 times more iridium
than the surrounding rock.
Moreover, samples from other K/T sites had similar levels--
too much to come from ordinary space dust.
What could explain so much iridium deposited
around the world?
Perhaps, a catastrophic event in outer space.
Alvarez wondered if a supernova exploding nearby
might be responsible.
CHRISTOPHER MCKEE: So he asked me if that was possible.
And I concluded that there was only 1 chance in 1 billion
that such a supernova would occur that close in 100
million years.
SEAN CARROLL: A supernova would have also deposited
a rare isotope, plutonium 244.
But testing revealed there wasn't any.
CHRISTOPHER MCKEE: Well, I suggested, alternatively,
that it could have been an asteroid or a comet.
SEAN CARROLL: There are hundreds of asteroids whose paths cross
the earth's orbit.
Their sizes range from a few meters to hundreds
of kilometers across.
RICHARD MULLER: So Louis Alvarez had this hypothesis
that an asteroid or comet would cause this destruction.
He had the clue, the amount of iridium at Gubbio.
Under this hypothesis, it would be spread all around the world.
So now he could calculate how much
iridium there had been laid down over the entire earth.
Now, he also knew how much iridium there
is in asteroids and comets.
So he can now calculate the size of the object.
SEAN CARROLL: The answer was sobering.
An asteroid 10 kilometers in diameter,
as large as Mount Everest, and weighing hundreds of billions
of tons.
Still, how could something that size wreak havoc
on a large planet?
Because traveling through the vacuum of space,
it would have slammed into the Earth's atmosphere
at 80,000 kilometers per hour, 20 times faster than a bullet,
heating the air to several times the temperature of the sun.
At impact, the energy released would
be equal to about 100 million nuclear bombs exploding
at once.
A huge mass of pulverized debris would
have been blasted into space, some
of it orbiting the earth before raining back down.
The debris may have blocked out the sun for months.
Photosynthesis would have stopped.
Plants, plant-eaters and then meat-eaters would have died.
This was the asteroid impact hypothesis
for how the Mesozoic era ended--
a big idea that was just too big for some.
When the Alvarez hypothesis was first proposed,
it was difficult for many scientists to accept.
Because, for almost 2 centuries, geologists
had crafted their worldview around a gradual picture,
a slow but steady change in the earth
without major catastrophes.
Now, they're hearing a proposal that something
could come from outer space and rewritten the history of life
in almost an instant.
RICHARD MULLER: Louis Alvarez got very frustrated
when the paleontologists didn't say, yes, sir.
Thank you for solving our problem.
But many of the paleontologists just
looked on him as someone who didn't know their field
and was stepping into this just because it
was such a big, important, famous problem.
SEAN CARROLL: The controversy would continue for years.
To convince the skeptics, more evidence was needed.
[MUSIC PLAYING]
One criticism of the K/T hypothesis
was the lack of a crater the right age, type, and size.
Alvarez thought it should be 200 kilometers across--
bigger than the state of Connecticut.
How could you miss that?
RICHARD MULLER: They looked.
They looked for craters that were 65 million years old
all over the earth.
Many of these things had been discovered
and had been measured--
didn't find any.
SEAN CARROLL: 2/3 of the planet is covered by water.
If the asteroid landed in the ocean,
the crater might never be found.
Even so, there would still be a trail of debris, ejecta,
blasted from the crater.
So attention focused on finding this evidence.
Geologist Jan Smit discovered glass-like beads
in K/T boundaries called spherules
formed when vaporized rock cools and rains back down.
Another key clue was rock that had been so shocked
by the impact, it had criss-crossing bands
of dislocated minerals.
This was shocked quartz.
JAN SMIT: And we know, if you set off a nuclear bomb,
the damage done to the surrounding rocks
will produce you a shocked quartz.
So if you put two and two together,
we find shocked quartz at the K/T boundary
and shocked quartz in nuclear craters,
you know it is an explosion which
deforms your quartz crystals.
And quartz is only found on land.
So that was the big clue.
We have to look for a crater somewhere on land.
SEAN CARROLL: The search for fresh clues
led here, to Texas, along the Brazos
river, some 300 kilometers from the Gulf of Mexico.
65 million years ago, this was the bottom
of the sea instead of grazing land.
In the early 1980s, scientists noted unusual deposits
of sediments across the river basin.
Intrigued, Alan Hildebrand, then,
a graduate student in geology, came to investigate.
ALAN HILDEBRAND: It's actually easy.
There's fresh tracks.
SEAN CARROLL: Along the banks of the river and its tributaries,
he examined exposures at the K/T boundary different from those
seen elsewhere and saw something on top of the Cretaceous mud
and rock that it once accumulated on the seafloor.
So this is the typical mud is found in this area?
ALAN HILDEBRAND: Exactly.
All we see is this green, Cretaceous mud.
And there's 7 million years of this mud here.
So we're talking boring, in other words, nothing going on.
But--
SEAN CARROLL: So what's--
ALAN HILDEBRAND: You see it right here.
The sea floor has been eroded right at this point.
SEAN CARROLL: An untrained eye might see this sediment
and never look twice at the protruding boulders.
But Hildebrand saw evidence of a catastrophic event.
ALAN HILDEBRAND: So something happened here
that eroded the sea floor.
And we started seeing these very coarse sediments.
And this first unit is really quite extraordinary
because, you trace round over here,
you see here is a boulder in it.
It's like 50 centimeters across.
And you come over here, this is another one.
It was weak, so it's weathered out, but it's even bigger.
Here's another boulder.
But notice this boulder is different stuff than these.
SEAN CARROLL: So you had this really regular mud layer,
and then, all of a sudden, this area that's
just full of this whole mixture of boulders and--
ALAN HILDEBRAND: And from different places.
It isn't just the rock that was here in the sea floor.
Maybe some of it got pulled from deep water.
Maybe some get pulled from shallow.
It all got jumbled together and dumped here.
SEAN CARROLL: All right.
So what could have happened here that would
explain something so dramatic?
ALAN HILDEBRAND: Well, it had to be a hugely energetic wave
in the ocean, a giant tsunami.
SEAN CARROLL: If an asteroid 10 kilometers across
landed in the sea or at the edge of a continent,
it would displace incredible amounts of rock and water,
causing tsunamis over 100 meters high.
These giant waves would have crossed
the sea with the speed of a jet, ripping up the seafloor, moving
tons of sediment.
As that debris came to rest in what,
today, is the Brazos River basin,
it would have mixed with ejecta falling from the sky.
OK.
So now we're here in Texas.
Is this making you think that you're a little closer
to the crater in Texas than you might be in Italy, for example?
ALAN HILDEBRAND: Exactly.
Because we can see the products of the impact mixed
into this tsunami deposit.
So we're getting warm.
SEAN CARROLL: Hildebrand was hot on the trail
of every new piece of evidence.
Next stop-- Haiti where he investigated
a report of volcanic rocks.
As he suspected, they were actually
ejecta full of shock minerals and spherules.
They also contained melted rock called tektites,
another telltale sign--
more evidence that a strike had occurred somewhere
around the Gulf of Mexico.
But somewhere wasn't precise enough.
Ironically, a key clue discovered by another geologist
had long been overlooked.
Years before, Glen Penfield had hunted for oil
on the Yucatan Peninsula in Mexico.
From the air, Penfield saw nothing unusual.
But his instruments measured differences
in gravitational fields and revealed the features
of a giant buried crater.
It was Hildebrand who eventually followed up on Penfield's work.
Rock samples from the area Penfield identified
showed all the signs of a high-energy impact.
ALAN HILDEBRAND: And it was full of shocked quartz too.
So this evidence finally convinced everybody
that, indeed, there was a big crater buried on the Yucatan
Peninsula.
SEAN CARROLL: After years of speculation,
the crater had finally been found.
It was named the Chicxulub Crater
after a village built over its center.
The discovery of the Chicxulub Crater
was the ultimate evidence of the asteroid impact.
And it tied together all of the clues that had been gathered
over the previous decade--
the shocked quartz, the tektites,
the spherules that had fallen across the earth.
Moreover, the crater was the same age as the K/T boundary,
and it was the size predicted by Louis Alvarez.
We now knew, for certain, what happened on that horrible day.
[MUSIC PLAYING]
The asteroid crater had finally been found,
but important questions remained.
Which species were wiped out at the end of the Mesozoic?
Which survived, and why?
The search for those answers led to the Badlands of the Dakotas
and Montana in the Hell Creek Formation.
It's eroding buttes hold fossils of plants and animals, that
lived during the last million years of the Cretaceous
and beyond.
When paleontologist Kirk Johnson discovered this K/T boundary
with its telltale spherules, he found the dividing line
between two vastly different worlds.
KIRK JOHNSON: You're looking at a ball of glass
that used to be the bedrock in Chicxulub, Mexico.
SEAN CARROLL: Kirk, how important
was it to find the K/T boundary up here in North Dakota?
KIRK JOHNSON: If you can't put your finger on the boundary,
like you can right here, what that means is
you can ask the very simple question, how
was life before the impact different from life
after the impact?
All you've got to do is look for the fossils
below and compare them with the fossils above.
And that's what we've done here for the last 30 years.
SEAN CARROLL: This arid landscape
was once a wet, lush forest.
Crack open some rocks, and you'll
find the leaves of plants and trees
that flourished here over 65 million years ago.
KIRK JOHNSON: Look at that.
SEAN CARROLL: Just look at that.
You can even tell what insects ate them.
Holy cow.
KIRK JOHNSON: You can see there's
two different kinds of insect damage on this leaf as well.
There's sort of a whole feeding that's in the leaf.
And there's a margin feeding on the edge.
SEAN CARROLL: Take a close look at the ground,
and you can pick up fossils of small animals
that thrived in lakes, rivers, and forests.
KIRK JOHNSON: So in my hand, I've
got evidence of a turtle, fish, crocodile, fish, and mammal.
SEAN CARROLL: And then there were the dinosaurs.
The challenge-- connecting their fate to the K/T boundary.
The clues-- their fossilized remains.
KIRK JOHNSON: This is an ankle bone
of a small, meat-eating dinosaur.
And you find these bones, identify the animal,
and pretty soon, you start assembling
the list of dinosaurs that are present at any given level.
And the lower you are in the formation, the older you are.
And the higher you are, the younger you are.
And the closer to the boundary you are,
the more we address the question of how long
did the dinosaur survive?
So just nearby, we found this bone,
which is a bone of a much larger, meat-eating dinosaur--
same bone--
But see the size difference?
SEAN CARROLL: Right.
KIRK JOHNSON: And here's the exact same bone.
So when you find bones of different species
in the same layer, they were living in the same place
at the same time.
SEAN CARROLL: The work takes a great deal
of time and patience.
TYLER LYSON: This is basically how you find a dinosaur.
You're walking around looking in these gullies,
and then you spot a piece of bone.
And you can see it's very porous.
You know that it had to travel down--
force of gravity.
You can see the bone trail.
Here's a bone.
Follow the trail of bone up.
And then, here you have a shinbone
of a duck-bill dinosaur really broken up.
And then here is where it would articulate with the knee joint.
SEAN CARROLL: Scientists know that 22 types of dinosaurs
lived in Hell Creek, including triceratops, tyrannosaurs,
and this duck-bill, some 9 meters or 30 feet long.
The more complete the skeleton, the easier
it is to reconstruct the past.
But discoveries like these are extremely rare.
TYLER LYSON: You walk around in the Badlands out here,
and you'll pick up numerous chunks
of dinosaur, chunkasauraus-- that's what we call it.
And that's about it.
So usually, you find piles of bones that aren't articulated.
They're not in the correct order.
So having the vertebrae in the right order like this
is very, very rare.
And what makes this specimen even more important
is it's articulated, and it's pretty
close to the K/T boundary.
KIRK JOHNSON: We now found several specimens that
are very close to the boundary.
And really, what that's showing us
is that, even if dinosaurs are rare, if you look long enough,
you'll find them when they were living on the planet.
What we have not found yet is any dinosaur skeleton anywhere
in the world above the K/T boundary layer.
SEAN CARROLL: Scientists now knew what lived here
and their ultimate fate.
Within 1,000 kilometers of the impact, death came quickly.
BRIAN TOON: If you turn your oven on broil, open the door,
put your hand under the glow bar,
that's what the dinosaurs felt very soon.
And they were probably broiled alive within an hour or so.
SEAN CARROLL: For dinosaurs further away,
death may have been delayed, but not for long.
Soon, vaporized ejecta and smoke from fires filled the air.
BRIAN TOON: And there may have also
been a lot of sulfur blown into the atmosphere
because the impact site in the Yucatan Peninsula
had a lot of sulfur in it.
And all of those things were abundant enough
to obliterate the sun.
SEAN CARROLL: Without light, anything
dependent on photosynthesis, on land or in the sea,
was vulnerable.
As food chains collapsed, giant reptiles still alive died off.
The Mesozoic era, the age of dinosaurs, was over.
And there were other radical changes.
In Kirk Johnson's Denver lab, you
can see what happened to plant life.
Samples taken from below the K/T boundary
show a high diversity of pollen grains, stained here in red.
But above the boundary, this earlier diversity
disappears, reflecting the extinction
of 60% of all plant species.
SPEAKER: Fern spore, fern spore, fern spore.
SEAN CARROLL: Where flowering plants once thrived,
ferns took over first.
SPEAKER: Fern spore, fern spore, fern spore.
SEAN CARROLL: Unlike pollen, fern spores
can germinate on a barren landscape
devoid of living plants.
KIRK JOHNSON: We have a short period of time
where there's only ferns.
That's the fern spike.
But after that, there's about 1 million years of time
where we had the low-diversity, disaster-recovery flora.
And after that first million years,
then things start to pick back up again.
We start to see animals coming back into the landscape.
SEAN CARROLL: In this new, post-impact world,
a niche that dinosaurs left was waiting to be filled.
BRIAN TOON: And so the survivors of this on the land
were creatures that lived in holes, birds,
mice-like creatures, turtles, frogs, things
that lived in swamps or rivers, or near the seashore.
SEAN CARROLL: And compared to dinosaurs,
they had the advantage of size.
They were small.
KIRK JOHNSON: Small animals have large population size.
Small animals higher reproduction rates.
Now, that's not saying that small animals didn't suffer
mass death, but enough survive.
And that's the key point.
SEAN CARROLL: In that million years of recovery,
the small inherited the earth.
It was the beginning of the age of mammals.
Eventually, larger mammals dominated the land,
just as dinosaurs had done before.
And among them were primitive primates whose evolution
would lead in a very promising direction, at least for humans.
What the asteroid impact taught us about evolution
is that it's not always about survival of the fittest.
Sometimes it's about survival of the luckiest.
And there's a profound point there for our species.
Only after the extinction of the giant reptiles
did mammals flourish including our primate ancestors.
Without the asteroid, there'd be no us.
[MUSIC PLAYING]