[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. 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]
