WEBVTT

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Prof: Then today we're
going to take our third look at

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the history of life on this
planet,

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and it's going to be about the
fossil record and the major

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groups of life.

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You remember,
the first look was major

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transitions, and the issues
involved in them.

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The second look was how life
shaped the planet and how the

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planet shaped life;
so it was a description of the

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geological theater in which
evolution has occurred.

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And today we'll actually look
at the fossil record,

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which has its own unique and
important messages.

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So I'm going to again give you
another view of geological time.

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This is something that's
important to build up in your

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head.

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It takes awhile.

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The names are unfamiliar,
the depth of time is

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astonishing.

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But it's a very necessary
framework for understanding

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evolution on this planet.

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We'll talk about a few big
events, the major radiations,

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the groups that are still
expanding,

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the ones that are vanishing or
gone,

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vanishing communities,
extraordinary extinct

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creatures.

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Then I'll mention stasis and
I'll also mention Cope's law.

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So here is another way of
looking at geological time.

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Last time I showed you the
24-hour clock.

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This time I'm showing you a
series of blowups,

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a three-panel blowup.

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So here is the origin of the
planet.

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Here is today, up here.

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And the pre-Cambrian is in red.

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So this is everything before
the major animal groups and

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fossils with hard parts appear.

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And, as you can see,
that's most of life.

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Roughly speaking,
life begins here,

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and becomes eukaryotic
somewhere around here,

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and multi-cellular somewhere
around here,

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and we start picking fossils up
at the beginning of the

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Cambrian,
in any kind of numbers;

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there were a few before then.

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And then if we take everything
after the Cambrian--it's called

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the Phanerozoic;
that's this column here--and we

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blow that up,
and you can see it falls into

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the Paleozoic,
the Mesozoic and the Cenozoic,

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with all of these eras in it.

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And some of these eras are
actually marked,

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their endings are marked by
mass extinctions,

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and the way that the geologists
could tell,

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around the world,
looking at different rocks that

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they were dealing with the same
rocks,

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is that there are
characteristic fossils found in

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them that disappeared all over
the world at a certain time.

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And so, for example,
the disappearance--the

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trilobites appear in the
Cambrian and they disappear at

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the end of the Permian.

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Any rock in the world that you
see that has a trilobite in it

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is going to be in the Paleozoic.

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The ammonites,
you'll see in a few minutes,

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appear and disappear a number
of times, but they finally

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disappear at the end of the
Cretaceous.

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Any rock in it that's got a
complex ammonite in it is

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Mesozoic.

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So these geological eras are
actually,

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in part, defined by fossils,
and the coordination of them

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across the planet is done by
matching types of fossils.

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In the late twentieth century
we had radiometric dating that

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helped a great deal with this,
and that's gotten better and

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better.

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But the original layout was
done with fossils.

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And if we then take everything
that's happened since the

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end-Cretaceous mass extinction,
that's called the Cenozoic.

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So this, the Mesozoic is more
or less the Age of Reptiles;

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the Cenozoic is the Age of
Mammals;

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and we blow the Cenozoic up,
this is what we get.

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We get Paleocene,
Eocene, Oligocene,

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Miocene, Pliocene,
Pleistocene.

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And the last 10,000 years is
the Holocene;

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that's since the glaciers
melted, that's the period we

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call the Holocene.

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And roughly speaking the world
restocks itself with

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biodiversity in the Paleocene
and Eocene.

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We have roughly modern levels
of biodiversity since the

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Oligocene, in terms of mammal
families and things like that.

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And most of the mammal orders
have their roots in the Eocene

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and Paleocene;
as you'll see in a bit.

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Now if we look at large-scale
events, one of the most

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interesting is well when did
multi-cellular life really get

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going?

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And for that the tiny fossils
that are preserved in phosphate

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beds in China are absolutely
astonishing.

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These things have been
discovered within the last ten

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years.

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They come from a number of
places in China,

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but this spot,
Chengjiang, in Yunnan Province,

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is--I think it's on Yunnan;
it might be just on the border

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with another province there;
no province lines in the

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map--are certainly some of the
earliest and most intriguing.

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So the Cambrian starts at about
550.

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So this is 20 million years
before the Cambrian;

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we're in the Vendian,
we're in the late-Proterozic

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Era.

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And a lake, or an inlet,
dried up and the salts in it

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crystallized and they perfectly
preserved the algae that were in

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it.

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So these are micrographs of
microfossils showing

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multi-cellular algae,
and in some of them you can

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even see the spindles in the
mitotic divisions.

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In formations in China of the
same age, there are

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multi-cellular,
bilateral animals.

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These look like early-stage
cell divisions of Crustacea.

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So this is again 20 million
years before the Cambrian,

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and the implication that there
might be a Crustacean 20 million

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years before the Cambrian is a
very interesting one,

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as you'll see in a minute.

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So our molecular phylogenies
suggest,

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looking not at the fossils but
at the molecules,

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that the eukaryotic
radiation--so that's before

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multi-cellularity;
this is just the eukaryotic

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cells making Protista--that was
underway about a billion years

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ago.

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These microfossils support the
idea that many groups may have

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diverged before the Cambrian,
but we have no trace of them in

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the fossils.

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We just have this marker;
we have these Crustacean-like

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embryos.

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Now if that's really true,
then the first fossils of large

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animals,
animals that you could see with

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the naked eye,
that had hard body parts,

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that had endoskeletons or
exoskeletons,

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these things crop up in the
Cambrian,

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and they may simply then be
recording the fact that formerly

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soft-bodied things started to
acquire skeletons.

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So the groups existed before
then, they just couldn't be

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fossilized, and that that may
very well have been because of

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co-evolution with predators.

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So that's the picture that
seems to be emerging.

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I just want to remind you that
the Tree of Life has these three

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big groups in it,
and we're now going to blow up

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this part of it.

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Here we are.

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The Chinese microfossils look
like they're right about here,

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and they are at 570 million
years ago.

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And if we just walk out around
the Tree of Life,

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and say, "Oh,
anything that has that branch

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length from what we think is the
origin,

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was probably there at the same
time,

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even though we don't have a
fossil of it."

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That's the implication of the
molecular phylogeny;

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it's that everything else
that's about that far out from

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the common ancestor was probably
there at the time.

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That means that all these other
branches were there too.

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Now most of these other things
are single-celled organisms,

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and we wouldn't expect them to
leave fossils.

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Okay?

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I mean, we've got stuff like
slime molds and amoebas and

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euglenas, the ancestors
of--let's see,

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where is malaria and stuff like
that?

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We've got all kinds of algae
out here.

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Those things were probably all
there.

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We just don't have fossils of
them.

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And that's why it's really
important to be able to deal

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with both the molecular
phylogenies and the fossils,

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because they complement each
other and they allow a kind of

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inference that's not available
from either alone.

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Now, what happens in the
Cambrian?

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That's when we really start-
when the fossil record really

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gets going.

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The idea that there was an
explosion of biodiversity in the

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Cambrian seems to be well
supported by the fossils.

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Okay?

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This is the number of orders
that can be observed,

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of animal groups.

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So these are fairly large
clades of marine invertebrates

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that start--
and some, by the way,

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by the end of the Cambrian,
we start picking up vertebrates

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as well--
and they start getting added on

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at a pretty high rate.

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And the interesting thing is no
major body plans appear in the

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fossil record,
in animals, after that.

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They do in the plants,
but in the animals it's as

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though there's one burst of
diversity,

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550 million years ago,
and then all the major body

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plans get frozen,
and we don't get new kinds of

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animals after that.

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That's kind of a puzzling and
not completely solved problem.

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Why was it that way?

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Now let's take a look at one of
these communities.

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They contain some organisms
that are profoundly weird.

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By the way, they weren't very
big.

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The giant among them,
the sperm whale of the Cambrian

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seas was this guy,
up here.

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Okay?

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That's Anomalocaris,
and that is an arthropod

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predator,
arthropod-like predator,

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and it's got some funny sort of
quasi-tentacle antennae,

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and a mouth right here,
and it swims around,

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and it's the biggest,
nastiest thing in the ocean,

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and it's about this big.

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So if you are skin-diving in a
Cambrian sea,

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you don't need to worry about
white sharks.

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You are actually the biggest,
meanest thing around.

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Okay?

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And that's an interesting
observation.

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Again and again,
in fossil history,

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things start small and get big.

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Things start small and have
short generation times and short

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lives, and get to be big and
have long generation times and

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long lives.

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And I don't mean by that that
the small things are replaced by

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the big things;
the big things add onto them.

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It's like a community would be
dominated initially by small

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things, and they would continue
to be there, but big things

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would evolve.

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So this is that process
starting to happen.

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There are a few things that
were running around,

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in these oceans,
that we don't have anymore.

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There are trilobites here,
of course.

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There is this profoundly
puzzling creature.

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Okay?

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And we're going to--that's
Opabinia--we're going to take a

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good look at it,
in a few minutes.

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That's one of the favorite
animals of Derek Briggs,

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who's now the director of the
Peabody Museum.

11:56.350 --> 11:59.950
Derek, by the way,
has great BBC cartoons of the

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way these things swam and moved.

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He's done the functional
morphology of the Cambrian

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community.

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So if you're interested in
that, maybe you could talk Derek

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into having a showing.

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So that is something that's not
around anymore.

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But something like this is.

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That's a priapulid worm,
and there are still priapulid

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worms that look pretty much like
that.

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So that thing is now a living
fossil.

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And here's an Onychophoran,
and Onychophorans that look

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just like that are running
around the Australian

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rainforests now;
instead of living on reefs,

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they're running around the
rainforest, but they look pretty

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much like that.

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Okay?

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So the things that we get in
the Cambrian are at least three

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of the mollusk classes.

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So these are the chitons,
these are the snails,

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and these are the squids,
octopuses and ammonites.

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We get the polychaetes,
which are the biggest group of

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the annelids--the ones that are
most familiar to you are

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probably earthworms;
those are oligochaetes.

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But the polychaetes--I think
there are 43 families of

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polychaetes.

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They're a very dominant group
in the ocean,

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and have been for 550 million
years.

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We start getting arthropods;
so we get the trilobites.

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The chelicerates are the
horseshoe crabs and the spiders

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and their relatives,
and we start picking up some

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Crustacea.

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We get the brachiopods,
the lampshells,

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which are still around.

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If you go diving on a reef in
Malaysia, you can see

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brachiopods.

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There are deep-water
brachiopods around the world,

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but they've mostly been in
retreat for a long time.

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And we get echinoderms,
and the fact that we get

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echinoderms is interesting
because they're the sister group

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of the chordates,
and that implies that the

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chordates had diverged from the
echinoderms,

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at that point,
and they just weren't

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fossilizing.

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And we know,
from the first fossils that we

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can get of things like
Amphioxus,

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that if you have a tiny,
little, one-inch long,

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translucent,
tadpole-like,

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fish-like chordate,
that's the ancestor of the

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vertebrates,
it's probably not going to

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fossilize.

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So our best evidence that that
divergence had occurred is the

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existence of the echinoderms.

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And by the way,
the echinoderms went through an

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explosive radiation.

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They made many classes.

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The different classes of the
echinoderms now are things like

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the asteroids,
which are the starfish;

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the holothuroids,
which are the sea cucumbers,

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and so forth.

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There are, I think,
six or seven classes currently

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of echinoderms;
but back in the Cambrian there

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were about twenty-five or
thirty.

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Most of them have now gone
extinct.

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And some of those things that
you saw in that earlier picture

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were extinct classes of
echinoderms.

14:52.750 --> 14:56.660
Okay, so for the animals
there's this explosion 550 to

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500 million years ago in the
Cambrian.

14:59.620 --> 15:01.380
It's very different for the
plants.

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The plants had a much steadier,
more measured evolution of

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diversity.

15:07.620 --> 15:08.990
Okay?

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The major groups of plants
arrive later because plants got

15:13.056 --> 15:14.336
onto land later.

15:14.340 --> 15:16.650
Most of the animal groups,
all the animal groups

15:16.650 --> 15:19.160
originated in the ocean,
but much of plant diversity

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originated on land;
so they had to get onto land.

15:22.350 --> 15:27.140
The mosses and the ferns appear
in the fossil record in the

15:27.143 --> 15:30.453
Devonian, about 400 million
years ago.

15:30.450 --> 15:33.630
The gymnosperms,
which is pines and firs and

15:33.631 --> 15:36.081
their relatives,
they actually are

15:36.075 --> 15:37.995
350-million-years-old.

15:38.000 --> 15:40.320
So they appear in the early
Carboniferous,

15:40.320 --> 15:43.830
and they undergo continuing
evolution up to the present day.

15:43.830 --> 15:47.020
So they keep getting-
diversifying and becoming more

15:47.019 --> 15:48.019
sophisticated.

15:48.019 --> 15:52.169
But there are recognizable
gymnosperms 350 million years

15:52.167 --> 15:52.617
ago.

15:52.620 --> 15:56.930
And when the flowering plants
evolve depends on whether you're

15:56.928 --> 15:59.328
looking at molecules or fossils.

15:59.330 --> 16:02.830
The molecules suggest that it
might be as old as

16:02.825 --> 16:06.965
Carboniferous-Permian-Triassic;
that is, 200 to 300 million

16:06.971 --> 16:07.661
years ago.

16:07.659 --> 16:11.109
Some people don't believe that.

16:11.110 --> 16:13.960
The really solid evidence,
of course, is the fossil,

16:13.961 --> 16:15.861
at a certain age,
and that's in the

16:15.860 --> 16:16.980
late-Cretaceous.

16:16.980 --> 16:23.090
So you can see angiosperms that
are 75-million-years-old in the

16:23.091 --> 16:24.671
fossil record.

16:24.668 --> 16:27.868
This is what the first plant on
land might have looked like,

16:27.871 --> 16:30.751
and the first plant on land
might actually have been a

16:30.750 --> 16:31.510
liverwort.

16:31.509 --> 16:34.869
So this is a thalloid liverwort.

16:34.870 --> 16:38.960
And when you look at it,
it looks a fair amount like

16:38.961 --> 16:43.221
algae that we are familiar with
and that we see in the

16:43.215 --> 16:44.815
intertidal zone.

16:44.820 --> 16:49.840
It doesn't really look that
different in its structure from

16:49.842 --> 16:53.742
a marine alga,
but it is adapted for living on

16:53.740 --> 16:54.520
land.

16:54.519 --> 16:57.549
And to get onto land this is
what you need.

16:57.548 --> 17:00.328
If you're an animal,
you're going to have to come up

17:00.327 --> 17:01.797
with an impermeable skin.

17:01.798 --> 17:04.828
If you want to locomote on
land, you'll need limbs,

17:04.828 --> 17:08.038
and for that you'll need
shoulder and hip supports.

17:08.038 --> 17:10.048
And if you want to reproduce on
land,

17:10.048 --> 17:11.658
rather than in the water--which
is,

17:11.660 --> 17:13.990
of course, what many of the
amphibians have continued to

17:13.994 --> 17:15.574
do--
then you'll need an egg that

17:15.565 --> 17:16.265
won't dry out.

17:16.269 --> 17:18.859
So you need a shell and an
amnion, and this basically is

17:18.856 --> 17:21.206
something that happened between
the amphibians,

17:21.210 --> 17:24.960
and then everything that came
later in the tetrapods.

17:24.960 --> 17:27.260
If you're a plant,
you need an impermeable leaf.

17:27.259 --> 17:30.659
That means you need to invent
the biochemistry and the

17:30.662 --> 17:33.682
developmental biology to make a
waxy cuticle.

17:33.680 --> 17:35.730
You need a means of gas
exchange.

17:35.730 --> 17:39.280
So you're going to have to
invent all of the neat stuff

17:39.282 --> 17:42.842
about stomata and stomatal
regulation of carbon dioxide

17:42.836 --> 17:45.136
coming in and oxygen going out.

17:45.140 --> 17:48.470
And you'll need to have roots,
resistant spores;

17:48.470 --> 17:51.020
eventually you'll need seeds.

17:51.019 --> 17:54.129
So there's really quite a bit
of stuff to evolve,

17:54.133 --> 17:55.823
when you come onto land.

17:55.819 --> 17:57.149
This is a major event.

17:57.150 --> 17:59.860
It was complicated and it took
some time.

17:59.858 --> 18:03.718
If we look at the vertebrates
coming onto land,

18:03.718 --> 18:07.408
here are some late-Devonian
lobe-fin fish.

18:07.410 --> 18:11.330
So the group that seems to have
spawned the tetrapods is related

18:11.334 --> 18:13.954
to the Coelacanths,
the lobe-fin fishes.

18:13.950 --> 18:18.000
I'll show you a picture of
Eustenopteron and Ichthyostega

18:17.998 --> 18:19.008
in a moment.

18:19.009 --> 18:24.079
And these things start--this
creature, Eustenopteron,

18:24.080 --> 18:26.910
is actually a pelagic fish.

18:26.910 --> 18:32.450
It's not really crawling around
in the drying up lagoon;

18:32.450 --> 18:34.380
it appears to be swimming in
open water.

18:34.380 --> 18:38.590
But, as you'll see in a minute,
it has really pretty good

18:38.586 --> 18:41.136
beginnings of the tetrapod limb.

18:41.140 --> 18:45.070
So it looks like some of the
structural elements in the

18:45.067 --> 18:47.467
skeleton,
that were needed for things to

18:47.472 --> 18:49.952
come on land,
probably developed for other

18:49.949 --> 18:52.869
reasons,
in another environment,

18:52.869 --> 18:56.079
as an exaptation,
something that happened for

18:56.083 --> 18:57.873
other reasons earlier in
evolution,

18:57.868 --> 19:02.928
and that could then be co-opted
and used to get onto land.

19:02.930 --> 19:04.470
And these are some of the
relatives.

19:04.470 --> 19:05.320
Okay?

19:05.318 --> 19:09.258
So you can see Coelacanths in
the fossil record at 360 million

19:09.263 --> 19:13.143
years, and you can see them from
a submersible off Madagascar

19:13.143 --> 19:13.793
today.

19:13.789 --> 19:16.809
They're a nice living fossil.

19:16.809 --> 19:19.939


19:19.940 --> 19:21.480
Here's Eustenopteron.

19:21.480 --> 19:25.700
The skeletons are recovered
from Miguasha in Quebec;

19:25.700 --> 19:28.910
385-million-years-old.

19:28.910 --> 19:33.140
It was a pelagic fish,
and you can see that it already

19:33.142 --> 19:37.452
is getting, in its hind limbs,
many of the identifiable

19:37.453 --> 19:40.173
elements of a vertebrate limb.

19:40.170 --> 19:44.850
So this is a blowup of the
pectoral of Eustenopteron.

19:44.849 --> 19:47.679
It appears to have a humerus.

19:47.680 --> 19:49.330
This is Ichthyostega.

19:49.328 --> 19:54.568
This thing is a transition form
between fish and amphibia.

19:54.569 --> 19:57.849
It's late-Devonian;
it's 20 million years later.

19:57.848 --> 20:01.098
These usually come from eastern
Greenland;

20:01.098 --> 20:03.088
that's where the fossil
deposits are.

20:03.088 --> 20:06.618
And this guy already has most
of the elements of the

20:06.623 --> 20:07.873
vertebrate limb.

20:07.868 --> 20:11.108
So this developed in a swimming
environment.

20:11.108 --> 20:14.808
This guy arguably was crawling
around in shallow water,

20:14.806 --> 20:17.746
but he can't support himself as
an adult.

20:17.750 --> 20:22.340
That shoulder girdle and the
hip girdle are not strong enough

20:22.340 --> 20:27.010
for that animal to actually walk
on land, if it's an adult.

20:27.009 --> 20:29.539
The larvae could.

20:29.538 --> 20:34.238
So perhaps the first stage of
coming onto land was the kids

20:34.242 --> 20:37.892
went exploring,
and then they went back in the

20:37.891 --> 20:40.731
water and grew up to be adults.

20:40.730 --> 20:43.770
The parents couldn't go into
the new habitat because they

20:43.766 --> 20:47.016
didn't have limbs that were
strong enough to support them.

20:47.019 --> 20:49.219
I think that's kind of a cool
idea.

20:49.220 --> 20:52.010
So it might have been that,
just like with computers,

20:52.008 --> 20:54.688
the young were showing the old
which way was up.

20:54.690 --> 20:57.980


20:57.980 --> 21:01.620
If we look at the plant
radiation,

21:01.618 --> 21:06.078
there's a whole series of
acquisitions of major elements

21:06.080 --> 21:11.880
of what it means to be a plant,
and they occur at a pretty

21:11.883 --> 21:18.443
steady pace between about 450
and 75 million years ago.

21:18.440 --> 21:22.820
So chlorophyll B is quite old.

21:22.818 --> 21:28.728
I would guess that chlorophyll
B is on the order of maybe 1 to

21:28.726 --> 21:31.046
1.5 billion years old.

21:31.048 --> 21:35.608
You get plant cell structure
probably at the level of about a

21:35.613 --> 21:36.833
billion years.

21:36.828 --> 21:41.098
You get alternation of
generations, haploid/diploid

21:41.102 --> 21:44.352
generations, coming in pretty
early.

21:44.348 --> 21:50.178
Then you have,
as you move out of the mosses,

21:50.180 --> 21:55.000
and move towards the club
mosses, you can see that the

21:55.003 --> 21:59.423
water delivery system,
of plants, starts to develop.

21:59.420 --> 22:03.580
So they're developing roots and
they're developing all of the

22:03.576 --> 22:07.866
plumbing that will allow water
to move and bring nutrients from

22:07.869 --> 22:10.779
the roots up into a growing
structure.

22:10.778 --> 22:15.378
Wood starts to develop right
about in here,

22:15.380 --> 22:20.700
and by the time you get up into
the Equisitifolia and the

22:20.695 --> 22:25.635
precursors to the gymnosperms,
you're getting pretty well

22:25.643 --> 22:28.863
developed xylem;
so you're getting phloem,

22:28.855 --> 22:32.095
complex xylem,
and a pretty good delivery

22:32.098 --> 22:32.908
system.

22:32.910 --> 22:38.180
Then the seeds evolved with the
gymnosperms;

22:38.180 --> 22:40.550
gymnosperm means naked seed.

22:40.548 --> 22:43.828
And this is the radiation here
of the gymnosperms.

22:43.828 --> 22:50.338
The Pinales would be the pine
trees, and firs,

22:50.336 --> 22:52.936
and all of that.

22:52.940 --> 22:54.970
And the Gingkoes,
of course, are the familiar

22:54.968 --> 22:57.178
Gingko trees;
there's one down here at the

22:57.180 --> 22:58.810
corner of the Peabody Museum.

22:58.809 --> 23:00.359
And that makes a clade.

23:00.358 --> 23:02.668
And that's where seeds were
invented.

23:02.670 --> 23:05.920
Then as we go up further,
we get into pollen grains that

23:05.917 --> 23:09.047
have a distal aperture,
and then finally we get to the

23:09.048 --> 23:10.288
flowering plants.

23:10.288 --> 23:13.128
And at the base of the
angiosperms,

23:13.130 --> 23:18.250
down here, there are some
wonderful and weird plants,

23:18.250 --> 23:21.480
and the only one that I'd
really like to mention now,

23:21.480 --> 23:23.400
time permitting,
is Welwitschia,

23:23.395 --> 23:24.875
which is the Gnetales.

23:24.880 --> 23:29.170
And Welwitschia is a plant that
basically is a root with two

23:29.173 --> 23:33.683
leaves, and the two leaves can
grow to be up to 100 or 200 feet

23:33.684 --> 23:34.344
long.

23:34.338 --> 23:36.448
It lives in the sand dunes of
Namibia,

23:36.450 --> 23:42.080
and because sand drifts and
makes dunes that grow,

23:42.078 --> 23:45.038
Welwitschia can keep growing to
keep its leaves on top of the

23:45.035 --> 23:45.425
dunes.

23:45.430 --> 23:47.980
And so some Welwitschias are
actually 100 or 200 feet high;

23:47.980 --> 23:50.930
it's just that they're all
below ground and they just have

23:50.933 --> 23:53.063
these big leaves that come out
the top.

23:53.058 --> 23:58.328
So there are wonderful things
that are represented in the

23:58.334 --> 24:00.034
plant radiation.

24:00.028 --> 24:03.918
Okay, so the theme of that
basically--let me just go back

24:03.923 --> 24:06.223
and reinforce these two points.

24:06.220 --> 24:08.850
Some of the stuff that you need
to get on land was developed

24:08.849 --> 24:10.839
earlier in the water,
for other reasons,

24:10.836 --> 24:13.026
and then was co-opted to get
you on land,

24:13.028 --> 24:17.188
and that's what probably
happened with the vertebrate

24:17.190 --> 24:17.750
limb.

24:17.750 --> 24:21.750
The plants developed much of
their diversity after they had

24:21.749 --> 24:23.059
gotten onto land.

24:23.058 --> 24:26.838
And you can see that they are
adding things like vascular

24:26.842 --> 24:30.762
canals and water delivery
systems and things like that--

24:30.759 --> 24:35.899
wood--at a fairly steady pace,
as you go up through a period

24:35.898 --> 24:39.818
between about 450 and 75 million
years ago.

24:39.818 --> 24:43.978
If we then look at large
patterns in the history of life,

24:43.981 --> 24:47.631
to see what kinds of messages
the fossils give us,

24:47.625 --> 24:50.445
this is one of the classical
ones.

24:50.450 --> 24:55.400
This is how many different
families of ammonites there

24:55.404 --> 24:56.064
were.

24:56.059 --> 24:57.059
Okay?

24:57.058 --> 25:03.388
And you can think of each of
these radiations,

25:03.390 --> 25:07.380
that are presented as kind of a
leaf with grey coloring around

25:07.376 --> 25:10.496
it,
as being roughly at the level

25:10.503 --> 25:11.733
of an order.

25:11.730 --> 25:16.430
So an order of mammals would
be--to make it familiar to

25:16.431 --> 25:20.351
you--would be something like the
ungulates.

25:20.348 --> 25:23.358
An order of birds would be
something like the albatrosses

25:23.357 --> 25:24.537
and their relatives.

25:24.538 --> 25:27.648
Fairly big groups with a lot of
species in them.

25:27.650 --> 25:30.730
And within each of these groups
you can see that there are lots

25:30.732 --> 25:31.432
of families.

25:31.430 --> 25:35.620
Now look what happens to them.

25:35.618 --> 25:39.088
At the end of the--they start
to radiate,

25:39.088 --> 25:41.608
back in the Devonian--at the
end of the Devonian there's a

25:41.608 --> 25:44.518
mass extinction,
lots of them get cut off,

25:44.517 --> 25:46.747
two lineages come through.

25:46.750 --> 25:50.930
This lineage radiates,
makes a whole lot of different

25:50.932 --> 25:53.832
species and families of
ammonites.

25:53.828 --> 25:57.368
At the end of the Permian,
they all go extinct.

25:57.368 --> 26:01.258
This line here manages to get
two of them through--two

26:01.256 --> 26:05.726
lineages, maybe three--through
the Permian mass extinction.

26:05.730 --> 26:07.790
One of them goes out in the
Triassic;

26:07.789 --> 26:09.609
the other radiates.

26:09.608 --> 26:12.038
At the end of the Triassic
there's a mass extinction.

26:12.038 --> 26:15.398
Almost all the ammonites
disappear again.

26:15.400 --> 26:20.310
One or two lineages get
through, into the Jurassic,

26:20.307 --> 26:25.307
and at the end of the
Cretaceous these two surviving

26:25.313 --> 26:28.163
branches both go extinct.

26:28.160 --> 26:31.940
So people looked at that,
and what they saw was this

26:31.938 --> 26:34.928
continuing extinction,
and then re-radiation,

26:34.934 --> 26:36.514
and extinction,
and re-radiation,

26:36.510 --> 26:40.120
and they asked themselves,
"Can the world hold only

26:40.116 --> 26:42.496
so many kinds of ammonites?

26:42.500 --> 26:46.110
Does it kind of fill up,
and then when it's wiped clean,

26:46.109 --> 26:50.179
does that create a space for
the others to re-radiate?"

26:50.180 --> 26:52.730
Well the pattern is consistent
with that interpretation.

26:52.730 --> 26:56.060


26:56.058 --> 26:58.268
Consistency is a very weak
logical criterion;

26:58.269 --> 27:01.639


27:01.640 --> 27:03.160
but it's evocative.

27:03.160 --> 27:07.310
So I leave it at that.

27:07.308 --> 27:12.958
Now that consistency comment's
going to apply to this as well.

27:12.960 --> 27:15.110
So this is the mammal radiation.

27:15.109 --> 27:16.229
Okay?

27:16.230 --> 27:19.750
And when you look at it,
the first thing you notice is

27:19.750 --> 27:23.340
oh, mammals started to radiate
back in the Triassic.

27:23.338 --> 27:26.018
If we were back in the Triassic
we might not have called them

27:26.019 --> 27:29.809
mammals yet,
but they have already split off

27:29.811 --> 27:34.171
from other ancestors,
and it looks like things like

27:34.172 --> 27:38.472
the Monotremes have their roots
at about that level in time.

27:38.470 --> 27:40.950
So we're looking back about 200
million years.

27:40.950 --> 27:47.430


27:47.430 --> 27:51.000
During the whole time that
dinosaurs were the dominant

27:50.998 --> 27:56.628
large creatures on the planet,
and the most diverse tetrapods,

27:56.630 --> 28:00.370
the mammals continued to
radiate.

28:00.368 --> 28:02.948
There were multituberculates,
there were triconodonts;

28:02.950 --> 28:05.260
there were all sorts of things,
back there.

28:05.259 --> 28:09.159
They tended to be rather small,
but they were perking along.

28:09.160 --> 28:12.840
Then there's the end-Cretaceous
extinction.

28:12.838 --> 28:17.578
Everything bigger than five
kilos that lives on land gets

28:17.582 --> 28:21.142
wiped out, and the mammals then
radiate.

28:21.140 --> 28:25.000
Well that is consistent with
the idea that the extinction of

28:25.000 --> 28:28.930
the dinosaurs was a necessary
pre-condition for the radiation

28:28.925 --> 28:30.165
of the mammals.

28:30.170 --> 28:34.350
And it looks like a reiteration
of the pattern we saw with the

28:34.345 --> 28:37.485
ammonites: clean the planet out
and make space,

28:37.493 --> 28:39.893
and then they can evolve again.

28:39.890 --> 28:44.480
So this is really kind of a
tetrapod recapitulation of what

28:44.481 --> 28:46.701
we saw with the ammonites.

28:46.700 --> 28:51.550
But, as I said,
consistency is a weak logical

28:51.546 --> 28:58.046
criterion, and the problem is
we're dealing with one planet,

28:58.045 --> 29:01.675
and we don't have replicates.

29:01.680 --> 29:02.890
>

29:02.890 --> 29:05.830
It would be nice if we could
replicate this experiment a

29:05.826 --> 29:09.236
hundred times and see that every
time the dinosaurs went extinct,

29:09.243 --> 29:10.583
the mammals radiated.

29:10.578 --> 29:10.988
Okay?

29:10.990 --> 29:13.450
We have a sample size of one.

29:13.450 --> 29:16.010
So it's a very interesting
pattern.

29:16.009 --> 29:17.849
It might very well be true.

29:17.848 --> 29:20.128
It sounds plausible,
and you can't demonstrate it

29:20.131 --> 29:20.941
experimentally.

29:20.940 --> 29:24.340


29:24.338 --> 29:27.848
So what are the groups that are
still radiating?

29:27.848 --> 29:31.498
If we just look around the
planet right now,

29:31.497 --> 29:32.937
what do we see?

29:32.940 --> 29:38.080
Well the beetles are still
going like gangbusters.

29:38.078 --> 29:40.138
We don't actually know how many
beetles there are.

29:40.140 --> 29:46.760
The number of beetles that have
been named is I think about

29:46.758 --> 29:47.898
350,000.

29:47.900 --> 29:53.500
The number of beetle species
that might exist could be on the

29:53.500 --> 29:55.460
order of 5,000,000.

29:55.460 --> 29:58.070
When J.B.S. Haldane,
who was an atheist Communist,

29:58.070 --> 30:01.430
was having dinner with the wife
of the Archbishop of Canterbury,

30:01.428 --> 30:03.128
she asked him,
"Mr. Haldane,

30:03.134 --> 30:06.384
what do you conclude about the
nature of the creator from your

30:06.384 --> 30:08.254
study of biology?"

30:08.250 --> 30:10.310
And he turned to her and said,
"Madame,

30:10.307 --> 30:12.317
an inordinate fondness of
beetles."

30:12.319 --> 30:13.809
>

30:13.808 --> 30:18.378
So there are a lot of beetles,
and they're still radiating.

30:18.380 --> 30:21.370
The Diptera,
the flies and the mosquitoes,

30:21.373 --> 30:24.733
are a young group,
and they are still producing

30:24.730 --> 30:25.900
new species.

30:25.900 --> 30:30.450
Among the mammals,
it's the bats that are probably

30:30.451 --> 30:34.631
the most impressive producers of
biodiversity,

30:34.631 --> 30:37.141
along with the rodents.

30:37.140 --> 30:40.990
And the place where the bats
and the rodents are doing the

30:40.987 --> 30:43.347
most of this is in South
America.

30:43.348 --> 30:45.298
So if you really are a
mammalogist,

30:45.298 --> 30:49.348
and you want to study recent
evolution and see things that

30:49.353 --> 30:52.343
are still in the process of
speciating,

30:52.338 --> 30:54.508
South America is certainly one
good place,

30:54.509 --> 30:58.189
and the groups to look at are
bats and rodents.

30:58.190 --> 31:01.860
In the flowering plants,
there is really impressive

31:01.855 --> 31:06.395
biodiversity in the composites,
the orchids and the grasses.

31:06.400 --> 31:10.590
There are about 12,000 species
of orchids I think.

31:10.588 --> 31:14.198
And I have forgotten--Jeremy,
do you know the figures for the

31:14.198 --> 31:14.798
grasses?

31:14.799 --> 31:16.259
Teaching Assistant:  Yes.

31:16.259 --> 31:18.519
I think there's somewhere
around 15,000.

31:18.519 --> 31:21.359
Prof: 15,000 species of
grasses and composites.

31:21.358 --> 31:23.828
Teaching Assistant:
There's around 30,000.

31:23.828 --> 31:25.968
Prof: There are about
30,000, and they probably don't

31:25.965 --> 31:27.265
even call them composites
anymore.

31:27.269 --> 31:27.979
Teaching Assistant:  No.

31:27.980 --> 31:28.920
Prof: What are they
called?

31:28.920 --> 31:30.030
Teaching Assistant:
Asteraceae.

31:30.028 --> 31:31.488
Prof: They're called
Asteraceae.

31:31.490 --> 31:34.270
Okay, see, the phylogeneticists
are busy, they're on their game,

31:34.266 --> 31:35.276
they're naming stuff.

31:35.279 --> 31:40.419


31:40.420 --> 31:44.260
Now that's---those are the
clades that are currently

31:44.259 --> 31:46.519
filling the world with life.

31:46.519 --> 31:48.369
What about the stuff that's
been wiped out?

31:48.368 --> 31:51.138
Well all of those exotic things
in the Burgess Shale,

31:51.138 --> 31:53.588
they're gone forever,
and they've been gone for

31:53.588 --> 31:55.398
hundreds of millions of years.

31:55.400 --> 31:58.640


31:58.640 --> 32:00.890
The trilobites,
the ammonites,

32:00.893 --> 32:03.693
the dinosaurs,
those are all gone.

32:03.690 --> 32:05.900
There was a wonderful group
called the glossopterids.

32:05.900 --> 32:11.280
They were Jurassic tongue-ferns;
they were ferns that looked

32:11.275 --> 32:12.925
tongue-like.

32:12.930 --> 32:16.590
There's a great story about how
when South America got connected

32:16.587 --> 32:18.767
to North America,
at the Isthmus of Panama,

32:18.768 --> 32:21.358
about 10 million years ago,
a bunch of tough,

32:21.358 --> 32:24.758
North American hoodlums
migrated south,

32:24.759 --> 32:26.979
over the Isthmus,
and ate up everything in South

32:26.978 --> 32:27.448
America.

32:27.450 --> 32:31.800
They were called things like
pumas and wolves and stuff like

32:31.803 --> 32:36.013
that, and they ate up the South
American notoungulates.

32:36.009 --> 32:37.939
There were a few things that
came north;

32:37.940 --> 32:42.190
possums, armadillos came north,
but most of it was a movement

32:42.192 --> 32:42.762
south.

32:42.759 --> 32:48.759
And so there was a complex
Miocene and Pliocene fossil

32:48.756 --> 32:54.296
fauna in South America that's
vanished forever.

32:54.298 --> 32:58.898
In the last 10,000 years,
mostly on islands in the

32:58.903 --> 33:04.543
Pacific, 25 to 35% of the
world's birds have gone extinct.

33:04.538 --> 33:08.138
And outside of Africa,
most of the Pleistocene

33:08.140 --> 33:09.740
megafauna is gone.

33:09.740 --> 33:11.530
If you want to see what the
Pleistocene looked like,

33:11.528 --> 33:15.058
go to a national park in
Africa, because that's what

33:15.057 --> 33:18.237
North America looked like 10,000
years ago,

33:18.240 --> 33:21.070
when we had 300,400-pound
beavers;

33:21.068 --> 33:24.638
and there was a North American
lion that was bigger than an

33:24.644 --> 33:27.474
African lion;
and of course the mammoths and

33:27.470 --> 33:30.130
the wooly rhinos and all of
those things.

33:30.130 --> 33:35.580
So we are actually missing a
lot of this stuff.

33:35.578 --> 33:40.738
And, on the one hand,
none of us probably ever woke

33:40.741 --> 33:45.081
up in the middle of the night,
in a cold sweat,

33:45.077 --> 33:48.437
worrying about the fact that
the dinosaurs were extinct and

33:48.442 --> 33:50.302
we couldn't see them anymore.

33:50.298 --> 33:53.088
And some ornithologists,
who know about the recent

33:53.089 --> 33:55.709
history of extinction in the
world's birds,

33:55.710 --> 33:58.310
probably do occasionally wake
up in a cold sweat at two

33:58.309 --> 34:00.959
o'clock in the morning and worry
that they were gone.

34:00.960 --> 34:03.430
But most of this stuff,
we regard that as oh,

34:03.433 --> 34:05.913
it's in the drawers of the
Peabody Museum;

34:05.910 --> 34:07.590
it's old, dusty fossils.

34:07.588 --> 34:10.448
But basically what we're
talking about here is vanished

34:10.452 --> 34:13.472
worlds;
vanished complete communities,

34:13.472 --> 34:17.462
vanished profligate,
extravagant radiations that

34:17.456 --> 34:20.846
produced life,
that filled up the planet,

34:20.846 --> 34:23.046
and then disappeared.

34:23.050 --> 34:28.770
And 99% of it's gone;
we see a very small fraction

34:28.768 --> 34:29.988
that remains.

34:29.989 --> 34:34.449
And that's actually just a fact
of life.

34:34.449 --> 34:35.609
Okay?

34:35.610 --> 34:40.940
It doesn't actually necessarily
call for an emotional response,

34:40.938 --> 34:46.008
other than the observation that
hey, that's what happens.

34:46.010 --> 34:48.770
Now let's go back to one of
those places.

34:48.768 --> 34:51.488
This is a vanished community,
this is the Burgess Shale,

34:51.489 --> 34:54.909
and about 500 million years
ago--the Burgess Shale is about

34:54.914 --> 34:58.424
505 million years old,
so it's sort of late-Cambrian.

34:58.420 --> 35:03.960
At that point--this is the
North American craton here;

35:03.960 --> 35:08.390
so looking at its western edge,
it's eastern edge would be

35:08.385 --> 35:11.955
Quebec, and then this would be
northern Canada,

35:11.958 --> 35:12.888
up here.

35:12.889 --> 35:14.019
This is the western edge.

35:14.018 --> 35:16.848
At that point it's slightly
south of the equator.

35:16.849 --> 35:20.399
It's not connected to South
America or to Asia,

35:20.402 --> 35:21.642
at that point.

35:21.639 --> 35:23.549
This is where it is today.

35:23.550 --> 35:28.140
You're up at about 10,000 feet;
you're about two kilometers

35:28.141 --> 35:32.381
above sea level;
maybe 8000 feet.

35:32.380 --> 35:35.860
This is a geologist,
and this is a fossil from the

35:35.855 --> 35:36.985
Burgess Shale.

35:36.989 --> 35:39.239
That one happens to look like a
trilobite.

35:39.239 --> 35:39.599
Okay?

35:39.596 --> 35:41.586
And this is the shale here.

35:41.590 --> 35:46.910
So at this site,
505 million years ago,

35:46.909 --> 35:48.729
on the western edge of the
continent,

35:48.730 --> 35:53.340
there was a shallow water
community that was living on the

35:53.342 --> 35:56.052
edge of a cliff,
and occasionally the cliff

35:56.054 --> 35:58.074
would fall down,
the sediment on the cliff would

35:58.072 --> 36:00.132
fall down,
and it would bury things;

36:00.130 --> 36:01.680
and that's what the shale
consists of.

36:01.679 --> 36:06.479
You're looking basically at a
landslide that buried a lot of

36:06.478 --> 36:07.128
stuff.

36:07.130 --> 36:10.080
And these are the kinds of
things that it buried.

36:10.079 --> 36:11.949
So here is our priapulid.

36:11.949 --> 36:13.809
Here is Opabinia.

36:13.809 --> 36:17.739
Here is Anomolocaris;
it looks like a huge looming

36:17.737 --> 36:19.587
predator in that shot;
but remember,

36:19.585 --> 36:21.515
the biggest thing in the ocean
was that big,

36:21.518 --> 36:23.048
and that's this guy right here.

36:23.050 --> 36:26.830
Here's one cruising in the
background.

36:26.829 --> 36:29.329
And these are some of the
creatures that you can pull out

36:29.326 --> 36:30.036
of that shale.

36:30.039 --> 36:31.369
This is one of the most
abundant.

36:31.369 --> 36:32.399
This is Marella.

36:32.400 --> 36:35.720
This is a primitive arthropod.

36:35.719 --> 36:40.589
Now remember the HOX genes,
remember how to turn an

36:40.594 --> 36:43.134
onychophoran into a fly?

36:43.130 --> 36:45.090
Well this is an intermediate
step.

36:45.090 --> 36:50.410
Basically you take a worm and
you start specifying that the

36:50.411 --> 36:54.451
forward segments are going to
form a head.

36:54.449 --> 36:56.259
So you get cephalization.

36:56.260 --> 37:00.070
You can see that it's putting
out gills and legs on most of

37:00.068 --> 37:03.038
its segments--
but it's kind of stopping to do

37:03.043 --> 37:06.923
that on its back segments--
and it's developed a hard

37:06.918 --> 37:08.028
exoskeleton.

37:08.030 --> 37:15.230
So this is steps on the way to
becoming an arthropod.

37:15.230 --> 37:18.290
And we actually don't know
whether this thing is the

37:18.289 --> 37:21.409
ancestor of Crustaceans or of
the chelicerates or the

37:21.409 --> 37:22.309
trilobites.

37:22.309 --> 37:28.709
It's just an intermediate form
between a worm and an arthropod.

37:28.710 --> 37:30.650
This thing is just totally
bizarre.

37:30.650 --> 37:31.860
This is Opabinia.

37:31.860 --> 37:32.360
Okay?

37:32.355 --> 37:35.815
And when it was first
reconstructed,

37:35.824 --> 37:40.574
it resulted in hilarity;
nobody could believe it.

37:40.570 --> 37:46.770
And the thing that really gets
people about it is that it has

37:46.766 --> 37:49.396
five eyes,
and it's got this proboscis

37:49.396 --> 37:52.156
that's got kind of a grasping
organ out on the end of it.

37:52.159 --> 37:57.319
So it looks sort of like a
cross between a spider and a

37:57.324 --> 37:58.954
vacuum cleaner.

37:58.949 --> 38:00.359
It's probably about this big.

38:00.360 --> 38:00.880
Okay?

38:00.880 --> 38:04.530
It's about one or two inches
long.

38:04.530 --> 38:09.230
And people just couldn't figure
out where Opabinia fits.

38:09.230 --> 38:13.700
So Derek Briggs has made the
study of Opabinia one of his

38:13.701 --> 38:16.251
projects;
he knows a lot about it.

38:16.250 --> 38:19.260
And it appears to be related to
Crustacea.

38:19.260 --> 38:23.500
But again you can see that it
looks like it's intermediate

38:23.501 --> 38:26.181
between a worm and something
else.

38:26.179 --> 38:31.169
So it's probably some kind of
intermediate form,

38:31.168 --> 38:33.928
prior to the arthropods.

38:33.929 --> 38:39.489
Now before I go on to stasis
and Cope's rule,

38:39.489 --> 38:47.449
I just want to comment a little
bit on what it means that entire

38:47.449 --> 38:52.629
communities have completely
vanished.

38:52.630 --> 38:57.410
It really places a very
relative view on the current

38:57.409 --> 38:58.159
world.

38:58.159 --> 39:01.029
When the Atlantic was
opening--and the Connecticut

39:01.034 --> 39:04.734
River Valley might have been the
Atlantic, or it could've been a

39:04.731 --> 39:08.061
river valley on a continent;
it was a rift valley at that

39:08.056 --> 39:11.156
time--there were a series of
rift valley lakes that stretched

39:11.155 --> 39:12.805
across eastern North America.

39:12.809 --> 39:16.029
They run basically from
Pennsylvania up to about

39:16.034 --> 39:16.724
Vermont.

39:16.719 --> 39:19.809
And they opened and closed,
and opened and closed several

39:19.807 --> 39:20.247
times.

39:20.250 --> 39:22.230
Every time one of those lakes
opened,

39:22.230 --> 39:24.510
the fish in them went through a
big adaptive radiation,

39:24.510 --> 39:27.030
like the ammonites did,
and then the lake closed and

39:27.025 --> 39:30.075
all the fish died off,
and then it opened up again and

39:30.077 --> 39:32.537
another radiation of fish went
on in it,

39:32.539 --> 39:36.859
and it closed up;
and this happened again and

39:36.864 --> 39:39.974
again and again,
both spatially and temporally,

39:39.974 --> 39:43.764
across the eastern United
States, about 200 million years

39:43.760 --> 39:44.370
ago.

39:44.369 --> 39:52.439


39:52.440 --> 39:55.610
We're currently in the middle
of a big anthropogenic

39:55.606 --> 39:59.236
extinction crisis,
but it appears like this isn't

39:59.237 --> 40:03.347
something that the planet hasn't
experienced before.

40:03.349 --> 40:08.089
Geological processes have
caused many extinctions of

40:08.092 --> 40:11.532
entire communities,
wiped them completely off the

40:11.530 --> 40:14.710
face of the earth,
and life has re-generated new

40:14.710 --> 40:17.930
ones again and again and again
and again.

40:17.929 --> 40:21.189
So that was one of the messages
I'm hoping that you're getting

40:21.192 --> 40:22.532
from the fossil record.

40:22.530 --> 40:24.920
Now what about stasis?

40:24.920 --> 40:28.800
What about the fact that the
Coelacanth that you catch off

40:28.795 --> 40:32.705
the Comoro Islands today,
looks almost exactly like the

40:32.710 --> 40:36.870
Coelacanth that's in the fossil
record from 360 million years

40:36.873 --> 40:37.363
ago?

40:37.360 --> 40:40.660
What about the fact that the
Onychophorans that you collect

40:40.664 --> 40:43.574
in Australia today are
practically indistinguishable

40:43.572 --> 40:46.882
from the ones that you see in
the Burgess Shale 505 million

40:46.878 --> 40:47.788
years ago?

40:47.789 --> 40:50.309
Why is there stasis?

40:50.309 --> 40:54.019
And I mention this because if
you were to write down a list of

40:54.019 --> 40:57.609
the big intellectual problems
that are posed by fossils,

40:57.610 --> 41:01.050
this is certainly going to be
on everybody's list.

41:01.050 --> 41:03.560
There are others,
but this is going to be a

41:03.561 --> 41:04.521
prominent one.

41:04.518 --> 41:07.988
And this is something that was
called to the attention of the

41:07.990 --> 41:11.290
world's scientific community,
primarily by Steve Gould.

41:11.289 --> 41:14.889
This is one of the take-home
messages of his life.

41:14.889 --> 41:18.639
So stasis basically describes a
long period with no

41:18.643 --> 41:20.373
morphological change.

41:20.369 --> 41:22.899
There's no apparent response to
selection.

41:22.900 --> 41:25.950
Evolution doesn't appear to be
going on.

41:25.949 --> 41:29.569
And it is puzzling,
because we know that every

41:29.565 --> 41:32.935
nucleotide sequence undergoes
mutations.

41:32.940 --> 41:36.710
There is no way that you can
stop the production of genetic

41:36.713 --> 41:38.733
diversity in these organisms.

41:38.730 --> 41:39.050
Okay?

41:39.052 --> 41:42.222
So for 350 million years
Coelacanths don't change,

41:42.222 --> 41:45.912
but probably every nucleotide
in their genome has mutated,

41:45.909 --> 41:47.849
over that period of time.

41:47.849 --> 41:51.399
So there's been opportunity for
change, but they have not

41:51.396 --> 41:52.026
changed.

41:52.030 --> 41:55.180
The examples of this include
club mosses and liverworts,

41:55.182 --> 41:58.282
lungfish, Coelacanths,
the priapulids and phoronids.

41:58.280 --> 42:01.460
You saw the priapulids--I
pointed them out in the

42:01.463 --> 42:04.053
Cambrian- in the Burgess Shale
shot--

42:04.050 --> 42:07.120
tuataras currently still
existing on an island off New

42:07.121 --> 42:09.591
Zealand,
and onychophorans;

42:09.590 --> 42:11.730
there are others.

42:11.730 --> 42:14.220
So here are a couple of
onychophorans.

42:14.219 --> 42:18.699
They're kind of intermediate
between annelids and arthropods;

42:18.699 --> 42:21.309
velvet worms.

42:21.309 --> 42:24.899
And here are two possible
explanations for stasis.

42:24.900 --> 42:28.500
There may very well be others,
but I want you to have at least

42:28.500 --> 42:30.920
these two general ones in your
toolkit.

42:30.920 --> 42:34.270
And one is basically a
selectionist explanation for

42:34.266 --> 42:34.866
stasis.

42:34.869 --> 42:39.859
It says that most of the things
that we're talking about have

42:39.862 --> 42:44.362
some method where either a larva
or a seed can find the

42:44.355 --> 42:48.345
environment in which the adult
will do well.

42:48.349 --> 42:51.389
And so there is a selection of
an environment,

42:51.385 --> 42:54.345
early in life,
and that actually then selects

42:54.353 --> 42:58.403
the selection pressures that
will operate on the adults.

42:58.400 --> 43:00.970
We see the adults,
we don't see the larvae.

43:00.969 --> 43:03.419
Basically the larvae have been
wandering around the planet,

43:03.420 --> 43:06.760
searching out the environment
in which the adults will grow

43:06.764 --> 43:08.444
up,
for hundreds of millions of

43:08.443 --> 43:10.203
years;
and we know that marine larvae

43:10.202 --> 43:11.522
are extremely good at this.

43:11.518 --> 43:14.398
The Coelacanths,
we know that they're deep-sea

43:14.400 --> 43:17.340
creatures;
they live down at about 600 to

43:17.342 --> 43:18.142
1000 feet.

43:18.139 --> 43:21.759
That's a fairly stable
environment.

43:21.760 --> 43:24.620
The club mosses,
that's a little harder to see

43:24.621 --> 43:26.021
how this would work.

43:26.018 --> 43:27.858
But at any rate,
this is one option.

43:27.860 --> 43:28.770
Okay?

43:28.768 --> 43:33.478
So this is one of our
alternative hypotheses.

43:33.480 --> 43:37.090
The reason things stay the same
is that young life history

43:37.085 --> 43:40.935
stages find the environment in
which adult selection will take

43:40.943 --> 43:43.523
place,
and adult selection is

43:43.518 --> 43:44.628
stabilizing.

43:44.630 --> 43:47.680
Intermediate values are
selected for.

43:47.679 --> 43:49.529
Things don't change.

43:49.530 --> 43:52.000
On the other hand,
there's a contrasting

43:51.996 --> 43:55.156
hypothesis, which is an
internalist explanation.

43:55.159 --> 43:59.219
Basically it is that tradeoffs
are creating the stabilizing

43:59.219 --> 44:02.379
selection--
that's one possibility--so that

44:02.382 --> 44:06.742
instead of having an ecological
explanation for why there's a

44:06.740 --> 44:09.720
long period of stabilizing
selection,

44:09.719 --> 44:13.339
we have an internal
physiological or developmental

44:13.338 --> 44:17.178
explanation of why selection has
been stabilizing.

44:17.179 --> 44:19.279
But there's another part,
another option in the

44:19.284 --> 44:22.934
internalist explanation,
and that is that early on,

44:22.929 --> 44:27.379
both in evolution and early on
in development,

44:27.380 --> 44:31.900
key traits get fixed;
key things get set up.

44:31.900 --> 44:34.570
The development of the eye
depends upon the relationship of

44:34.572 --> 44:37.592
two tissue layers,
so that there will always ever

44:37.588 --> 44:41.888
thereafter be nerves and blood
vessels in front of the retina.

44:41.889 --> 44:42.969
Okay?

44:42.969 --> 44:46.929
So if those things are laid
down early, both in evolution

44:46.925 --> 44:50.525
and then in development,
occur early in development,

44:50.527 --> 44:52.857
there's kind of an embedding.

44:52.860 --> 44:56.870
That means things have been in
place that can't be changed

44:56.873 --> 44:59.693
without destroying normal
development.

44:59.690 --> 45:03.840
Now there are arguments for and
against all of these things.

45:03.840 --> 45:06.990
You can find early
developmental traits that have

45:06.990 --> 45:10.670
undergone a lot of evolution
without destroying the adult

45:10.668 --> 45:11.258
form.

45:11.260 --> 45:14.500
So there are some real issues
with trying to understand the

45:14.496 --> 45:16.446
mechanics of how this would
work;

45:16.449 --> 45:18.229
and we don't know yet.

45:18.230 --> 45:18.610
Okay?

45:18.610 --> 45:22.880
I'm just giving you a few ideas
that bear on the issue.

45:22.880 --> 45:26.630
The other major take-home
message is--that I've already

45:26.632 --> 45:28.372
signaled as Cope's Law.

45:28.369 --> 45:31.009
And again, there are two
options here.

45:31.010 --> 45:35.230
One is that the reason that we
see bigger things is that

45:35.231 --> 45:37.921
there's just a neutral
evolution.

45:37.920 --> 45:41.610
Adaptive radiations have been
creating little things and big

45:41.606 --> 45:42.166
things.

45:42.170 --> 45:45.430
But there was more room on the
upper end than there was on the

45:45.427 --> 45:48.117
lower end;
therefore even though it's been

45:48.121 --> 45:51.231
random, we see an accumulation
of larger things,

45:51.233 --> 45:54.283
just because the upper limits
are far away.

45:54.280 --> 45:55.330
Okay?

45:55.329 --> 45:58.059
The lower limit on body size is
always nearby;

45:58.059 --> 46:01.299
it's one cell,
you don't get smaller than one

46:01.302 --> 46:01.822
cell.

46:01.820 --> 46:04.470
But the upper limit appears to
be redwood trees and blue

46:04.474 --> 46:07.374
whales, and at least at the
outset that's pretty far away.

46:07.369 --> 46:10.909
That's up at about 100 meters,
for redwood trees,

46:10.907 --> 46:13.707
and about 30 meters for blue
whales.

46:13.710 --> 46:15.340
So that's one possibility.

46:15.340 --> 46:19.580
The other is that the reason
that things got bigger is

46:19.579 --> 46:21.019
co-evolutionary.

46:21.018 --> 46:24.818
Co-evolution is shaping prey to
escape and predators to kill,

46:24.820 --> 46:29.240
and prey can escape predators
by getting too big to eat,

46:29.239 --> 46:34.319
and predators can kill big prey
by getting bigger than they are.

46:34.320 --> 46:37.850
So this would be an adaptive
life history hypothesis,

46:37.851 --> 46:42.061
saying that Cope's law results
from a co-evolutionary arms race

46:42.061 --> 46:44.101
between predator and prey.

46:44.099 --> 46:47.809
And we don't really yet have a
powerful method for

46:47.806 --> 46:50.376
disentangling these two effects.

46:50.380 --> 46:52.600
And I think if you look at
their logic, you can see that

46:52.597 --> 46:56.137
they're not mutually exclusive;
they can both be going on at

46:56.143 --> 46:57.333
the same time.

46:57.329 --> 47:01.209
Okay, so what does the fossil
record tell us?

47:01.210 --> 47:05.690
It shows us a lot of stuff that
we couldn't see at shorter time

47:05.688 --> 47:06.338
scales.

47:06.340 --> 47:10.710
We see a lot more detail in the
recent than in the distant past.

47:10.710 --> 47:13.950
It looks like mass extinctions
may open up ecological space,

47:13.945 --> 47:16.135
for the radiation of surviving
groups.

47:16.139 --> 47:20.809
So it may be that you need an
extinction before you can have a

47:20.813 --> 47:22.043
big radiation.

47:22.039 --> 47:26.129
Most things start small and get
big.

47:26.130 --> 47:29.530
And there's a lot of stuff
that's not on the planet at all

47:29.532 --> 47:32.232
anymore;
there are no surviving

47:32.228 --> 47:33.468
descendants.

47:33.469 --> 47:37.249
So the fossil record has a
take-home point,

47:37.250 --> 47:41.170
that's actually a puzzle that
can be attacked experimentally,

47:41.170 --> 47:43.750
in part by people doing
evolutionary developmental

47:43.751 --> 47:46.481
biology and phylogenetics,
and that is,

47:46.476 --> 47:48.566
why is there stasis?

47:48.570 --> 47:51.120
It's common,
and we don't have an

47:51.123 --> 47:52.803
explanation for it.

47:52.800 --> 47:57.800