WALTER ALVAREZ: I think
most people are fascinated
by murder mysteries.
Well, about 30 years ago,
I stumbled onto
a great murder mystery
and the victims
were the dinosaurs.
And the question was,
what had caused them
to become extinct?
Well, I was working
on a different project in Italy
at that time
and sort of stumbled on the...
on the level in the rock record
where the dinosaurs
became extinct.
Only there weren't
any dinosaurs.
There were tiny little
single-celled organisms
called forams.
This is a piece of our beautiful
pink limestone from Italy.
And, if you look
really closely down here,
you can see there are lots
of little specs
about the size of sand grains,
and those are forams--
single-celled fossils-- fossils
of single-celled organisms
that almost became extinct
at the same time
the dinosaurs did.
And here's what it looks like
in a piece of rock in the field.
So this is the last bed
of the Cretaceous,
which is the time
that the dinosaurs were around.
And if you look closely again,
you can see these little
sand-sized specs
which are microfossils.
This is the first bed
of the tertiary
which is the time that was
after the dinosaur extinction
and after the almost extinction
of the forams
because the-- once that you
can see with your naked eye
became extinct, up here,
there are forams,
but they're too small to see.
And this is a level of
clay in between the two.
Now, it's hard now
because this piece of rock
has been encased in plastic.
But we got interested in that
clay and what it might tell us.
And we, as a group here at
Berkeley, it was my father,
Luis Alvarez
who was a physicist,
and I'm the geologist
in the group
and Frank Asaro and Helen Michel
who were two nuclear chemists
who could measure elements
at extremely low concentrations.
And we decided to measure
the element iridium
which is a marker
for extraterrestrial material.
And we had expected
to find either no iridium
or a very small amount
of iridium in that level.
But I'll never forget the day
that I got a telephone call
from Frank, and Frank said,
"Something is wrong.
There is far more
iridium in this clay
"than we had been--
than any of our ideas
would have predicted."
Something was wrong and
it turned out to be the clue
that led to solving
the extinction
of the dinosaur murder.
But it took quite a while
to figure out
what that clue meant.
What we finally figured out
was that it was telling us
that a comet or an asteroid
the size of Mount Everest
had hit the earth
on a particular day,
65 million years ago
and caused the extinction,
not only of dinosaurs
and the forams,
but of many other different
kinds of plants and animals.
For example, like this
coiled-shelled ammonite.
This group also became
extinct at that same time.
And we gradually came to realize
how much energy there
is in a very big rock
falling from the sky.
There would have been
enough energy to blow debris
and rocks and dusts
and chunks of things
from the impact site
all over the entire earth.
And when it landed
on the top of the atmosphere,
there were so much of
it that it would have made
the earth cold and
dark for probably months
so that plants
would stop growing
and animals wouldn't have food
and many groups became extinct.
Not all, obviously,
because we're still around.
Well, this was a
catastrophic explanation
for the extinction
and that did not sit
well with most geologists
and paleontologist
at the time about 1980
because we were used to
thinking of all changes
in the Earth's history
as being slow and gradual
and this was the exact opposite.
So there was a huge debate.
It went on for ten years,
from 1980 to 1990.
And meanwhile, people
looked for other evidence
and they found other
evidence of impact,
like, spherules
and shocked quartz.
And all that time,
all during that ten years,
the big unanswered question was,
where was the crater
that would have been produced
when this comet or
asteroid fell from the sky?
Finally, in 1991, the great
breakthrough took place.
We learned about a huge crater
below the surface
of the Yucatan Peninsula
that the Mexican oil geologists
had discovered.
It's the biggest impact crater
that's formed on this planet
in the last billion years.
And the Mexicans drilled it
and here's what they found.
So this is a thin
slice out of a circular core.
And this is rock that was melted
by the heat of impact.
And you can tell
that it was melted
by impact because the mineral
grains in it have features
that only form with
very great shock effects
And so we then had
evidence for a giant impact.
We could tell that it was
approximately the right age
but you couldn't be sure
of exactly the right age.
So some of us went and
looked at other outcrops
in other parts of Mexico
and we found the debris
from this impact--
from this crater--
at exactly the level
of the extinction,
right at the boundary
between the Cretaceous
and the tertiary.
And so that was the evidence
that can...
has convinced almost all
geologists and paleontologists
that it was indeed an impact
that caused the mass extinction.
Well, this is a pretty good
example of how science works
and how geologists
and paleontologists
figure out what
happened in earth history.
You start with some unexpected
evidence like the iridium.
Then you make up a theory
to try to explain it.
And then other people
get interested,
and they look for other
evidence like spherules
and shocked quartz.
And there's a great debate
because that's the way we work
these things out in science.
And finally,
along comes the most
convincing evidence of all--
in this case the crater, which
convinces virtually everyone.
And so that's the story
of how geologists
and paleontologists
solved what may have been
the greatest murder mystery
in all of Big History.
