Evolution: Great Transformations (PBS Documentary) 2/7
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0:01 - 0:05Downloaded From www.AllSubs.org
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0:05 - 0:12MAN:
It's a basic human need
to ask, "Who are we?" -
0:12 - 0:15"Where do we come from?"
-
0:15 - 0:17"How did we get here?"
-
0:17 - 0:24"Why do we look the way we do?"
-
0:24 - 0:26NARRATOR:
The story of our evolution -
0:26 - 0:29is just a small chapter
in a much larger story: -
0:29 - 0:33the evolution
of all living things. -
0:33 - 0:35(trumpeting)
-
0:35 - 0:38MAN:
Evolution shows us
that we're much more connected -
0:38 - 0:41to the rest of the world,
the rest of animal life -
0:41 - 0:43than we could ever
have imagined. -
0:43 - 0:47NARRATOR:
We can recognize the connection
to our closest relatives -
0:47 - 0:49but when we know how to look
-
0:49 - 0:52we can also find it
in other mammals: -
0:52 - 0:54birds...
-
0:54 - 0:55reptiles...
-
0:55 - 0:57fish...
-
0:57 - 1:02even insects.
-
1:02 - 1:05The deeper we dig,
the farther back we go -
1:05 - 1:09the more we see that
everything alive has evolved -
1:09 - 1:12from a single starting point.
-
1:12 - 1:14The tree of life
has been branching -
1:14 - 1:19for four billion years.
-
1:19 - 1:22And we can now follow its
branches back to their roots. -
1:22 - 1:25MAN:
When we look back over time -
1:25 - 1:28we find certain signposts,
certain key events -
1:28 - 1:31the great transformations,
the big evolutionary steps. -
1:31 - 1:36NARRATOR:
In the history of our planet,
a few great transformations -
1:36 - 1:39have opened the door
for new ways of life -
1:39 - 1:43and new forms of life.
-
1:43 - 1:4550 million years ago
-
1:45 - 1:49land mammals evolved
into sea creatures. -
1:49 - 1:55Long before that,
fish colonized land. -
1:55 - 1:59At the dawn
of animal life itself -
1:59 - 2:01the first bodies appeared.
-
2:01 - 2:09These are some of the chapters
in life's story... our story. -
2:09 - 2:12MAN:
And... and part of the fun
of studying this -
2:12 - 2:14is understanding each
different chapter -
2:14 - 2:16because by understanding
those chapters -
2:16 - 2:19we can begin to see
the unity of life, -
2:19 - 3:01the common history
of all life on Earth. -
3:01 - 3:08NARRATOR:
Human civilization stretches
back thousands of years. -
3:08 - 3:11But compared to the age
of the earth -
3:11 - 3:14we humans have only
just arrived. -
3:14 - 3:17MAN:
The earth is really old. -
3:17 - 3:19If you take the entire history
of the earth -
3:19 - 3:22from 4.6 billion years ago
to the present -
3:22 - 3:24and sort of call that an hour...
-
3:24 - 3:26(clock chime dings)
-
3:26 - 3:29SHUBIN:
The first 50 minutes
are largely spent -
3:29 - 3:33in a world of microbes,
single-celled organisms. -
3:33 - 3:35(clock ticking)
-
3:35 - 3:39Animal life appeared in the
last ten minutes of that hour. -
3:39 - 3:41(clock ticking)
-
3:41 - 3:45All of human history, our
civilization, our evolution, -
3:45 - 3:49happened in the last hundredth
of a second of that hour. -
3:49 - 3:51(clock chime dings)
-
3:51 - 3:53CHILDREN:
? Ring-around-a-rosy... ? -
3:53 - 3:55NARRATOR:
We've come quite late
to the party -
3:55 - 3:58but we've been shaped
by the same forces -
3:58 - 4:00that have helped shape
all life on Earth. -
4:00 - 4:03CHILDREN:
? ...We all fall down. ? -
4:03 - 4:07NARRATOR:
To understand how we fit in,
we need to look back -
4:07 - 4:09to long before our own origins
-
4:09 - 4:13to see how other
living things evolved. -
4:13 - 4:16(whales singing)
-
4:16 - 4:19NARRATOR:
Whales are the largest
living animals. -
4:19 - 4:23Like us, whales and dolphins
-
4:23 - 4:27took on their present forms
relatively recently. -
4:27 - 4:30For a long time, the origin
of these marine mammals -
4:30 - 4:32was a scientific mystery.
-
4:32 - 4:35MAN:
Whales are so different
from every other kind of mammal -
4:35 - 4:39that we can't easily relate them
to anything else -
4:39 - 4:43and so they're off by themself
as a branch of mammal evolution. -
4:43 - 4:48NARRATOR:
Mammals first appeared on Earth
around 200 million years ago... -
4:48 - 4:50on land.
-
4:50 - 4:53Mammals are warm-blooded
-
4:53 - 5:00they give birth to living young
and they breathe air. -
5:00 - 5:07These are all adaptations
to living on land. -
5:07 - 5:08(dolphin clicking)
-
5:08 - 5:12NARRATOR:
But whales and dolphins
are mammals, too. -
5:12 - 5:15SHUBIN:
They're mammals that live
in the water -
5:15 - 5:19but we know that mammals
evolved on land. -
5:19 - 5:24So it's a real puzzle
how whales originally evolved. -
5:24 - 5:26By understanding
how that happens -
5:26 - 5:29we'll begin to understand
how these big jumps -
5:29 - 5:34these big transformations
happen generally. -
5:34 - 5:37GINGERICH:
People are interested in whales,
and I can understand. -
5:37 - 5:39They're so beautiful.
-
5:39 - 5:43Their origin is such a mystery.
-
5:43 - 5:46Whales are one of
the few groups of mammals -
5:46 - 5:50that have really large,
complicated brains like we do. -
5:50 - 5:54And so in a sense, they're our
alter egos living in the sea -
5:54 - 5:56while we live on land
-
5:56 - 5:59dominating the sea
while we dominate land. -
5:59 - 6:00And I think for that reason
-
6:00 - 6:03we're very interested
in what goes on there -
6:03 - 6:07how they got there, as a
reflection of our own history -
6:07 - 6:10through geological time.
-
6:10 - 6:14NARRATOR:
When Phil Gingerich began
his career 30 years ago -
6:14 - 6:20he knew nothing about whales,
and that was just fine with him. -
6:20 - 6:23He was drawn to geology
mostly because -
6:23 - 6:26he couldn't imagine a career
spent behind a desk. -
6:26 - 6:29GINGERICH:
I think I was interested
in geology -
6:29 - 6:31because it was a science
outdoors. -
6:31 - 6:34And in geology, I became
interested in paleontology -
6:34 - 6:38because it was about life
and the history of life. -
6:38 - 6:41NARRATOR:
Gingerich's early interest
in primitive land mammals -
6:41 - 6:47eventually took him to Pakistan.
-
6:47 - 6:50It was there that he made
the kind of find -
6:50 - 6:53most paleontologists
only dream about: -
6:53 - 6:57a fossil that would rewrite one
of evolution's greatest stories. -
6:57 - 7:02GINGERICH:
I found the back of a skull
that I couldn't identify. -
7:02 - 7:05It had a very good,
well-preserved ear region -
7:05 - 7:09and that offered the clue
to what it was. -
7:09 - 7:11NARRATOR:
The shape was familiar -
7:11 - 7:16but in other ways it was like
nothing Gingerich had ever seen. -
7:16 - 7:18This is the original specimen.
-
7:18 - 7:20It's the one we found
in about 1978. -
7:20 - 7:22There's several things
that strike you. -
7:22 - 7:24One is it's very similar
in size and shape -
7:24 - 7:29to the back of a skull
of a wolf. -
7:29 - 7:33NARRATOR:
But there was something odd
about this skull. -
7:33 - 7:36On its underside
was a walnut-sized bump. -
7:36 - 7:38GINGERICH:
If this wasn't here -
7:38 - 7:42I would think that this was
an archaic carnivorous mammal -
7:42 - 7:47or what we call a creodont,
but it is here. -
7:47 - 7:49NARRATOR:
It was part of
the animal's inner ear -
7:49 - 7:52and it had a distinctive shape,
-
7:52 - 8:01a shape found today in only
one kind of animal: whales. -
8:01 - 8:05What was the ear of a whale
doing on the skull of an animal -
8:05 - 8:09that resembled a wolf?
-
8:09 - 8:13Gingerich was intrigued,
so he constructed a model -
8:13 - 8:18of what the creature's full
skull might have looked like. -
8:18 - 8:22He wondered, was his find
a crucial missing link -
8:22 - 8:25the first fossil evidence
ever found -
8:25 - 8:28for one of Darwin's
most daring claims, -
8:28 - 8:35that whales had evolved
from land mammals? -
8:35 - 8:37To know for sure
-
8:37 - 8:41Gingerich would need
to find more fossils... -
8:41 - 8:47ones that would show each stage
of the whale transformation, -
8:47 - 8:51what scientists call
"transitional forms." -
8:51 - 8:53I want to line them all up.
-
8:53 - 8:55I want anyone
to be able to see it -
8:55 - 9:01and believe it
because they've seen it. -
9:01 - 9:05NARRATOR:
Gingerich tried to return
to Pakistan to resume his search -
9:05 - 9:10but war had broken out,
and the borders were closed. -
9:10 - 9:14Frustrated, Gingerich decided
to look elsewhere. -
9:14 - 9:19He had heard stories
about whale skeleton sightings -
9:19 - 9:21in a very unlikely place.
-
9:21 - 9:27So he decided to check it out
for himself. -
9:27 - 9:32The Sahara Desert is one
of the driest places on Earth. -
9:32 - 9:38But 40 million years ago, things
here were quite different. -
9:38 - 9:42GINGERICH:
This used to be the sea. -
9:42 - 9:44Just think of this being
-
9:44 - 9:47the current Mediterranean
coast of Egypt -
9:47 - 9:50backed up about 40 million years
-
9:50 - 9:57but about 100 kilometers to
the south of where it is today. -
9:57 - 10:01NARRATOR:
Here, in what had once been
the Southern Mediterranean Sea -
10:01 - 10:05is a 100-square-mile stretch
of layered sandstone -
10:05 - 10:10with a surprising name...
-
10:10 - 10:16Valley of the Whales.
-
10:16 - 10:20The name is well suited.
-
10:20 - 10:23Scattered everywhere
across this arid landscape -
10:23 - 10:27are what look like heaps
of rose-colored stones. -
10:27 - 10:29Here's
aBasilosaurus. -
10:29 - 10:32NARRATOR:
But they're not stones... -
10:32 - 10:35You can see how big
the vertebrae are. -
10:35 - 10:38Here's a lumbar
partly weathered away. -
10:38 - 10:44NARRATOR:
They're whale skeletons
40 million years old. -
10:44 - 10:47There's another one
back here coming out
of the mushroom. -
10:47 - 10:51There's one
over here. -
10:51 - 10:56And back over there
is one. -
10:56 - 11:01This whole place
is full of whales. -
11:01 - 11:07NARRATOR:
Why were there so many whales
concentrated in this one spot? -
11:07 - 11:09Gingerich believes
that Whale Valley -
11:09 - 11:11was once a protected bay,
-
11:11 - 11:19a lagoon hidden from the open
sea by underwater sandbars. -
11:19 - 11:23Perhaps the whales
birthed their young here -
11:23 - 11:28and came here to die.
-
11:28 - 11:32But even with hundreds
of whale bones at his feet -
11:32 - 11:34Gingerich was disappointed.
-
11:34 - 11:37Nearly all of the skeletons
-
11:37 - 11:40belonged to a whale
called"Basilosaurus--" -
11:40 - 11:46a 40-million-year-old creature
already known to science. -
11:46 - 11:50Basilosauruslived full time
in the water. -
11:50 - 11:51This isBasilosaurus.
-
11:51 - 11:53I got all excited...
-
11:53 - 11:56NARRATOR:
If whales had evolved
from land mammals -
11:56 - 12:01they had done so
long beforeBasilosaurus. -
12:01 - 12:06So Gingerich didn't think the
bones would be of much interest -
12:06 - 12:10but he couldn't
have been more wrong. -
12:10 - 12:14After only a few days
of digging -
12:14 - 12:18he made his second amazing find.
-
12:18 - 12:21It turned out thatBasilosaurus
-
12:21 - 12:25had something modern whales
have long since lost. -
12:25 - 12:29For the first time,
we've got whales that have legs. -
12:29 - 12:34NARRATOR:
The bones were small
but unmistakable. -
12:34 - 12:38A pelvis.
-
12:38 - 12:42A kneecap.
-
12:42 - 12:45Even toes.
-
12:45 - 12:51This whale had a complete set
of leg bones. -
12:51 - 12:53Gingerich brought back
as much of the skeleton -
12:53 - 12:56as he could carry.
-
12:56 - 12:57It was dramatic evidence
-
12:57 - 13:05that whales had once
been four-legged animals. -
13:05 - 13:06Since Gingerich's finds
-
13:06 - 13:13he and others have filled in
more of this fantastic story. -
13:13 - 13:15Scientists now think
-
13:15 - 13:18that the earliest ancestor
of whales was similar -
13:18 - 13:25to this 50 million-year-old
wolflike mammal called sinonyx. -
13:25 - 13:27Sinonyx was
a predatory scavenger -
13:27 - 13:32that lived and hunted along
the shores of an ancient sea. -
13:32 - 13:35Perhaps its descendants
found the water -
13:35 - 13:40a source of abundant food,
and a haven from competition. -
13:40 - 13:44Over millions of years
front legs became fins -
13:44 - 13:47rear legs disappeared,
bodies lost fur -
13:47 - 13:54and took on their familiar
streamlined shape. -
13:54 - 13:58Since Gingerich's first find,
named Pakicetus -
13:58 - 14:02the list of known
transitional whales has grown. -
14:02 - 14:08It now includes Ambulocetus...
-
14:08 - 14:11Rhodocetus...
-
14:11 - 14:15Durodon...
-
14:15 - 14:20as well as Basilosaurus.
-
14:20 - 14:22They reveal another element
of whale evolution -
14:22 - 14:25the gradual migration of
nostrils to the top of the head -
14:25 - 14:28as whales adapted to breathing
in the water. -
14:28 - 14:29(exhaling)
-
14:29 - 14:32GINGERICH:
How did whales
lose their legs? -
14:32 - 14:39As the years went by, they
evolved into newer types of... -
14:39 - 14:43NARRATOR:
Gingerich's work demonstrates
what Darwin himself insisted -
14:43 - 14:47that the evidence for evolution
is all around us -
14:47 - 14:53if we choose to look for it.
-
14:53 - 14:58And bones aren't the only
evidence of whale evolution. -
14:58 - 15:05Their ancestry is also visible
in the way they move. -
15:05 - 15:12Frank Fish studies how today's
marine mammals swim. -
15:12 - 15:15He looks for their
evolutionary heritage -
15:15 - 15:18in the way they move
through the water. -
15:18 - 15:20FISH:
The big question is: -
15:20 - 15:23How do you go
from a terrestrial mammal -
15:23 - 15:25that ran around
on all four legs -
15:25 - 15:27to something such as a dolphin
-
15:27 - 15:30which now doesn't have
any legs at all -
15:30 - 15:34and is well adapted
to swimming in the oceans? -
15:34 - 15:40NARRATOR:
Even though whales look like
fish, they don't swim like them. -
15:40 - 15:44Fish swim by flexing
their spines from side to side -
15:44 - 15:47like this shark.
-
15:47 - 15:52But mammals swim differently.
-
15:52 - 15:58This otter swims by undulating
its spine up and down... -
15:58 - 16:02in exactly the same way
that whales do. -
16:02 - 16:05And, as it turns out
in the same way -
16:05 - 16:12that land mammals use
their spines when running. -
16:12 - 16:15Whales took with them
into the water -
16:15 - 16:17their ancestral way of moving
-
16:17 - 16:25and we can still see it...
50 million years later. -
16:25 - 16:30SHUBIN:
In one sense, evolution didn't
invent anything new with whales; -
16:30 - 16:32it was just tinkering
with land mammals. -
16:32 - 16:34It's using
the old to make the new -
16:34 - 16:36and we call that tinkering.
-
16:36 - 16:38And it does this
in every animal group -
16:38 - 16:43at every time during
evolutionary history. -
16:43 - 16:47NARRATOR:
The starting point for whales
was a four-legged land animal -
16:47 - 16:50that lived
over 50 million years ago. -
16:50 - 16:55But land animals were also the
product of a transformation, -
16:55 - 16:59a much earlier one.
-
16:59 - 17:01Hundreds of millions
of years ago -
17:01 - 17:03there were no animals on land.
-
17:03 - 17:06SHUBIN:
Before then -
17:06 - 17:09all our distant ancestors
lived in the water. -
17:09 - 17:13So at some point you had
this shift from life in water -
17:13 - 17:15to life on land.
-
17:15 - 17:17That's a huge change.
-
17:17 - 17:20NARRATOR:
It was the moment when fish
crawled out of the water -
17:20 - 17:22and onto land.
-
17:22 - 17:25WOMAN:
If these early animals hadn't
made the transition -
17:25 - 17:27we wouldn't be here today.
-
17:27 - 17:31And it's important
to understand how and when -
17:31 - 17:34and possibly, where
that transition took place. -
17:34 - 17:40NARRATOR:
The first creatures to leave the
water really started something. -
17:40 - 17:45Their descendants eventually
evolved into today's reptiles... -
17:45 - 17:47birds...
-
17:47 - 17:50and mammals.
-
17:50 - 17:53And these creatures'
common origins -
17:53 - 17:58are still visible
in their bodies. -
17:58 - 18:03Just like us, they all have
bodies with four limbs, -
18:03 - 18:06they're all tetrapods.
-
18:06 - 18:11SHUBIN:
What that means is that all
these different creatures -
18:11 - 18:13are descended
from a common ancestor -
18:13 - 18:19whichhadsomething very similar
or akin to limbs. -
18:19 - 18:23NARRATOR:
Just what was
that common ancestor? -
18:23 - 18:27And how did it leave the water
370 million years ago? -
18:27 - 18:29(men conversing)
-
18:29 - 18:33Those are the questions that
paleontologists Neil Shubin -
18:33 - 18:36and Ted Daeschler
are trying to answer. -
18:36 - 18:40They think that the cliffs
here in Central Pennsylvania -
18:40 - 18:47may offer some clues.
-
18:47 - 18:50DAESCHLER:
All right, I think
it's a good day for fossils. -
18:50 - 18:51What do you say?
-
18:51 - 18:52Great day;
let's find some. -
18:52 - 18:53Hey, Doug.
-
18:53 - 18:54Hey, Doug.
-
18:54 - 18:56Good trip up?
-
18:56 - 18:57What you say
we go over here? -
18:57 - 18:59That's good.
-
18:59 - 19:01Get some good digging in today.
-
19:01 - 19:04NARRATOR:
An unlikely spot to hunt
for early tetrapod fossils. -
19:04 - 19:08But they're here because
the rocks in these hills -
19:08 - 19:09are just the right age,
-
19:09 - 19:12laid down during the period
in Earth's history -
19:12 - 19:13called the Devonian.
-
19:13 - 19:15(men conversing)
-
19:15 - 19:19SHUBIN:
Back in the Devonian,
this place was very different. -
19:19 - 19:20It was south of the Equator,
-
19:20 - 19:23remember the continents are
continually moving around, -
19:23 - 19:25and back this time
we're actually dealing with -
19:25 - 19:28a much more tropical climate
in Pennsylvania. -
19:28 - 19:30NARRATOR:
Hundreds of millions
of years ago -
19:30 - 19:33the fossils and sediments
in these layers -
19:33 - 19:40were collecting
on the bottom of a stream. -
19:40 - 19:42SHUBIN:
What we have here -
19:42 - 19:44is a snapshot
of life in a stream -
19:44 - 19:46about 370 million years ago.
-
19:46 - 19:53These are fossilized... broken
fossils of scales, of teeth. -
19:53 - 19:55This little bone here,
-
19:55 - 19:58it's a spine of a creature
known as a spiny shark. -
19:58 - 20:04NARRATOR:
Most of the fossils are too
fragmented to be of much value. -
20:04 - 20:06But in 1995, right at this spot
-
20:06 - 20:13Daeschler came across something
he had never seen before. -
20:13 - 20:20It was a small shoulder bone,
but not from a fish. -
20:20 - 20:30It was a tetrapod shoulder,
370 million years old. -
20:30 - 20:33Shubin and Daeschler
had unearthed the remains -
20:33 - 20:37of one of life's first
four-legged creatures. -
20:37 - 20:41DAESCHLER:
It was the first evidence
of these early tetrapods -
20:41 - 20:48from all of North America,
and that made it very exciting. -
20:48 - 20:51NARRATOR:
And there was another surprise. -
20:51 - 20:53Since it was found
in the stream bed -
20:53 - 20:56this tetrapod most likely
livedin the water. -
20:56 - 20:58SHUBIN:
And it's
a very surprising discovery. -
20:58 - 20:59It's not something
-
20:59 - 21:01we necessarily
would have predicted. -
21:01 - 21:05NARRATOR:
Why would an animal with limbs
live in the water? -
21:05 - 21:08Limbs were thought
to have evolved -
21:08 - 21:10for getting around on land.
-
21:10 - 21:13The old idea was that
the fish came on shore first -
21:13 - 21:15and then developed the legs.
-
21:15 - 21:17And what we now think
-
21:17 - 21:21is that the tetrapods
developed the fingers first -
21:21 - 21:23and then left the water.
-
21:23 - 21:26NARRATOR:
Jenny Clack
of Cambridge University -
21:26 - 21:31suspected that the theory taught
in many textbooks was wrong. -
21:31 - 21:34The story that you will find
in many of the old textbooks -
21:34 - 21:36and the pictures
that you will see -
21:36 - 21:39in children's books
and museum galleries -
21:39 - 21:44is a picture of a fish
stranded in a drying pool -
21:44 - 21:47trying to support itself
out of water. -
21:47 - 21:51And it looks really odd if you
look at it objectively. -
21:51 - 21:55NARRATOR:
Clack thought there
had to be a better explanation -
21:55 - 21:57but where to look?
-
21:57 - 22:01Only a handful of early tetrapod
fossils had ever been found, -
22:01 - 22:04most of those
in a remote part of Greenland -
22:04 - 22:07at the turn of the century.
-
22:07 - 22:12All she had to guide her was
a note scribbled in a journal -
22:12 - 22:16from a scouting trip
to Greenland years earlier. -
22:16 - 22:23It referred to tetrapod fossils
on an unnamed mountain. -
22:23 - 22:28Clack flew to Greenland
to search for those bones. -
22:28 - 22:33CLACK:
Eventually we found the spot,
800 meters up on a hillside. -
22:33 - 22:38NARRATOR:
Clack returned
with four tons of rock... -
22:38 - 22:45And spent
the next four years drilling. -
22:45 - 22:47At the end
-
22:47 - 22:50she had the most complete early
tetrapod skeleton ever found; -
22:50 - 22:53and it forever
changed the textbooks. -
22:53 - 22:56CLACK:
One of the first things -
22:56 - 23:00that we found was this forelimb.
-
23:00 - 23:04NARRATOR:
At the end of the animal's limb -
23:04 - 23:08was an unmistakable
array of bones. -
23:08 - 23:11This was a hand.
-
23:11 - 23:12CLACK:
This is a life reconstruction.. -
23:12 - 23:15The artist is using imagination
on the color scheme -
23:15 - 23:17and on the eyes
-
23:17 - 23:20but we think this is
as accurate as you can get. -
23:20 - 23:24NARRATOR:
The creature,
named Acanthostega -
23:24 - 23:26was clearly a water-dweller:
-
23:26 - 23:34It had a fishlike tail and gills
for breathing in the water. -
23:34 - 23:38But the ends of its arms
were petal-shaped... -
23:38 - 23:41possibly the first hands
on Earth. -
23:41 - 23:46CLACK:
This was a swimming creature. -
23:46 - 23:48We don't know whether it could
ever have come out on land -
23:48 - 23:50but it certainly
wouldn't havewalked -
23:50 - 23:52in the conventional sense.
-
23:52 - 23:56Basically, it's...
a fish with fingers. -
23:56 - 24:01NARRATOR:
Clack's find was
a scientific breakthrough. -
24:01 - 24:09It proved that some fish had
arms and legs in the water. -
24:09 - 24:16So tetrapods didn't need to grow
limbs after they got onto land. -
24:16 - 24:21The limbs had already evolved
-
24:21 - 24:23and helped them survive
out of the water. -
24:23 - 24:26The basic pattern for limbs
had been in place -
24:26 - 24:29for millions of years.
-
24:29 - 24:31SHUBIN:
Here we have the fin -
24:31 - 24:36of a 370-million-year-old fossil
fish and an arm of a human. -
24:36 - 24:40In a human arm, what
you have is one bone... -
24:40 - 24:46then two bones,
the wrist and the digits. -
24:46 - 24:48In this fin what do you have?
-
24:48 - 24:53You have one bone,
two bones... -
24:53 - 24:57even little bones that can be
compared to a wrist -
24:57 - 24:59and then rods that face away
-
24:59 - 25:01from the rest
of the appendage itself -
25:01 - 25:04just like our fingers or toes.
-
25:04 - 25:06So you have, in a fish fin
-
25:06 - 25:09already set up
about 370 million years ago -
25:09 - 25:13many of the bones that are used
in a tetrapod limb. -
25:13 - 25:17NARRATOR:
With the basic pattern
already there -
25:17 - 25:19the fin-to-limb transition
took place -
25:19 - 25:22in a series of small changes
-
25:22 - 25:24occurring over
millions of years. -
25:24 - 25:26SHUBIN:
There's really no goal
to evolution. -
25:26 - 25:29Evolution wasn'ttrying
to make limbs -
25:29 - 25:31it wasn'ttrying
to push our distant ancestors -
25:31 - 25:33out of the water.
-
25:33 - 25:38What was happening
was a series of experiments. -
25:38 - 25:40NARRATOR:
In the crowded,
freshwater streams -
25:40 - 25:42where tetrapods first evolved
-
25:42 - 25:46the competition for survival
was intense. -
25:46 - 25:49SHUBIN:
These small streams
were like an engine -
25:49 - 25:53or a crucible
of evolutionary change. -
25:53 - 25:59NARRATOR:
Fish experimented with all sorts
of survival strategies. -
25:59 - 26:01Some became predators.
-
26:01 - 26:07The owner of this jaw
was a 12-foot-long killer. -
26:07 - 26:12Its teeth were the size
of railroad spikes. -
26:12 - 26:20Smaller fish developed elaborate
defenses, like this heavy armor. -
26:20 - 26:24Others packed weaponry,
like this sharp spike. -
26:24 - 26:31It protruded from behind
its owner's neck. -
26:31 - 26:34These armaments
were all tools for survival -
26:34 - 26:37in a dangerous world.
-
26:37 - 26:41Perhaps their new arms and legs
gave the first tetrapods -
26:41 - 26:44another way to survive.
-
26:44 - 26:47SHUBIN:
It was to get out of the way;
it was to get onto land. -
26:47 - 26:49And what enabled those animals
-
26:49 - 26:52to get out of the way,
that is, to get out of the water -
26:52 - 26:56were these new features,
like limbs. -
26:56 - 27:00NARRATOR:
Those that did escape
found a new world -
27:00 - 27:09filled with opportunity.
-
27:09 - 27:11The transformation
from water to land -
27:11 - 27:13was only the latest example
-
27:13 - 27:20of evolution experimenting
with radically new forms. -
27:20 - 27:22An earlier transformation,
-
27:22 - 27:25perhaps the most
significant of all, -
27:25 - 27:31occurred just over
half a billion years ago... -
27:31 - 27:37And it led
to all animals as we know them. -
27:37 - 27:40Evolution tinkered with fish
to make limbs. -
27:40 - 27:43But fish carry the baggage
of their own past. -
27:43 - 27:46Think of a fish:
-
27:46 - 27:50It has a head, it has a tail
and a bunch of fins in between. -
27:50 - 27:54That's a body plan,
the way the body's put together. -
27:54 - 27:57But that's just one of many ways
of putting animals together. -
27:57 - 28:01I mean, some animals are
like disks, like jellyfish. -
28:01 - 28:04Other animals have
lots of little legs. -
28:04 - 28:05The question is
-
28:05 - 28:11what sort of tinkering led
to these body plans? -
28:11 - 28:13I mean, really
what we're dealing with here -
28:13 - 28:16is the origin of animals.
-
28:16 - 28:18NARRATOR:
According to the fossil record -
28:18 - 28:21animals appeared upon the earth
in a short burst -
28:21 - 28:26around 570 million years ago.
-
28:26 - 28:30Scientists call
this crucial transformation -
28:30 - 28:33the Cambrian Explosion.
-
28:33 - 28:35MAN:
The Cambrian Explosion
was effectively -
28:35 - 28:36one of the greatest
breakthroughs -
28:36 - 28:39in the history of life.
-
28:39 - 28:43About half a billion years ago,
suddenly, things change -
28:43 - 28:47and we have this extraordinary
explosion of diversity. -
28:47 - 28:50And this sudden appearance
of the fossils led to this term -
28:50 - 28:51the Cambrian Explosion.
-
28:51 - 28:54And Darwin, as ever,
was extremely candid. -
28:54 - 28:57He said, "Look, this is
a problem for my theory. -
28:57 - 29:00How is it that suddenly, animals
seem to come out of nowhere?" -
29:00 - 29:04And to a certain extent, that is
still something of a mystery. -
29:04 - 29:08NARRATOR:
Most of what we know
of the Cambrian Explosion -
29:08 - 29:10is a result
of a single discovery -
29:10 - 29:15probably the greatest
fossil find in history. -
29:15 - 29:19In 1913, while climbing
in the Canadian Rockies -
29:19 - 29:23paleontologist Charles Walcott
discovered a layer of shale -
29:23 - 29:29containing thousands
of exquisitely detailed fossils. -
29:29 - 29:33These animals, all sea dwellers
-
29:33 - 29:39were caught in a catastrophic
underwater mudslide. -
29:39 - 29:43Over the next 500 million years
-
29:43 - 29:45the sea floor
which entombed them -
29:45 - 29:49rose to become
the top of a mountain. -
29:49 - 29:53Walcott removed over 60,000
fossils from the site -
29:53 - 29:59which he named
the Burgess Shale. -
29:59 - 30:05Simon Conway Morris has studied
the fossils for over 30 years. -
30:05 - 30:07It's almost as if
you've gone to another planet. -
30:07 - 30:08You've been given a fishing boat
and a net -
30:08 - 30:10and you've been allowed
to throw that net -
30:10 - 30:12over into the deep ocean
-
30:12 - 30:13and you'd no idea
what was going to come up. -
30:13 - 30:17NARRATOR:
Some of the Burgess Shale
creatures were familiar. -
30:17 - 30:19MORRIS:
And here, we've got
one of the trilobites. -
30:19 - 30:22We see the delicate soft parts,
also preserved. -
30:22 - 30:26NARRATOR:
Trilobites
are extinct arthropods -
30:26 - 30:29creatures
with external skeletons. -
30:29 - 30:33Today's arthropods,
like crabs, lobsters -
30:33 - 30:35insects and spiders
-
30:35 - 30:38are all descendants
of creatures like these. -
30:38 - 30:44Other Burgess Shale animals
were bizarre, alien-seeming. -
30:44 - 30:50An animal with five eyes
and a long retractable nozzle. -
30:50 - 30:56One with long, sharp spines
protruding from its back. -
30:56 - 31:04Another with a circle of prongs
around its mouth. -
31:04 - 31:07And yet, as alien
as these creatures seem -
31:07 - 31:13they are also surprisingly
familiar. -
31:13 - 31:15Like living animals,
they have bodies -
31:15 - 31:20with heads, tails, appendages,
-
31:20 - 31:25specialized segments performing
specialized functions. -
31:25 - 31:32All the basic body plans found
in nature today are here. -
31:32 - 31:35Every animal that has lived
for the last half billion years -
31:35 - 31:43has come from tinkering
with these initial designs. -
31:43 - 31:46We might even see
our own ancestor here. -
31:46 - 31:50MORRIS:
Maybe this is the crown
of the Burgess Shale. -
31:50 - 31:52This isPikaia.
-
31:52 - 31:56NARRATOR:
A tiny creature,Pikaiais one
of the rarest fossils -
31:56 - 31:57from the Burgess Shale.
-
31:57 - 32:01It's the only one
with an internal nerve cord -
32:01 - 32:04resembling a spine.
-
32:04 - 32:06That might mean
that creatures likePikaia -
32:06 - 32:11were the earliest ancestors
of all animals with skeletons. -
32:11 - 32:15MORRIS:
The idea is that this might be
the precursor of the fish -
32:15 - 32:17and so, ultimately
-
32:17 - 32:20through a long evolutionary
story, ourselves. -
32:20 - 32:22The Cambrian Explosion matters
for lots of reasons. -
32:22 - 32:24Basically,
it's part of our history. -
32:24 - 32:27It's where we came from
and that matters very much. -
32:27 - 32:30This is the time
when the animals first appear. -
32:30 - 32:38We look back and we can see
part of our history unfolding. -
32:38 - 32:40So what do we learn by looking
-
32:40 - 32:43at 600 million years
of animal history? -
32:43 - 32:47Evolution's tinkering
with mammalness to make whales. -
32:47 - 32:49In the same way,
it's tinkering with fishiness -
32:49 - 32:52to make tetrapods
-
32:52 - 32:54and it's tinkering
with animalness -
32:54 - 32:57to make all the different
body plans that we see. -
32:57 - 33:03All these different creatures
are variations of the same theme -
33:03 - 33:07restated over and over again.
-
33:07 - 33:12The question was, what was
evolution tinkering with? -
33:12 - 33:14One of the remarkable
discoveries -
33:14 - 33:15of the last 20 years
-
33:15 - 33:20is that evolution's not
tinkering with the bodies. -
33:20 - 33:22It's tinkering with the recipe
-
33:22 - 33:24the machinery
that builds bodies. -
33:24 - 33:26What is that recipe?
-
33:26 - 33:27What is that machinery?
-
33:27 - 33:31It's the genes.
-
33:31 - 33:36NARRATOR:
Fossils record the changes
in animals' bodies over time -
33:36 - 33:42but just how bodies change
was unknown. -
33:42 - 33:45The search for the genetic
mechanism of evolution -
33:45 - 33:48took most of this century.
-
33:48 - 33:51When scientists finally found it
-
33:51 - 33:54they were astonished...
-
33:54 - 34:00by just how simple it was.
-
34:00 - 34:04One of the key players
was Mike Levine. -
34:04 - 34:07LEVINE:
I was, um, I guess,
kind of a weird kid. -
34:07 - 34:09I always liked bugs.
-
34:09 - 34:12We had a nice, big backyard,
and I could go back there. -
34:12 - 34:13It was kind of a sanctuary.
-
34:13 - 34:16And, uh, I played with bugs...
-
34:16 - 34:19dissected them,
manipulated them. -
34:19 - 34:21That's really
the most pleasant memory I have. -
34:21 - 34:26NARRATOR:
Levine's affinity for bugs led
to his study of biology. -
34:26 - 34:31One insect in particular
became an object of fascination. -
34:31 - 34:35LEVINE:
They have
a quick generation time -
34:35 - 34:37and they have lots of pattern.
-
34:37 - 34:39I mean, you wouldn't know it
if you look at a distance -
34:39 - 34:41but when you look
under a microscope -
34:41 - 34:43at an adult fruit fly
-
34:43 - 34:47you'd be astounded
by the number of bristles -
34:47 - 34:52the intricacies of their wings,
the patterns of their eyes. -
34:52 - 34:55But the embryos
are something else. -
34:55 - 34:56I do love the embryos.
-
34:56 - 34:58NARRATOR:
Scientists had long suspected -
34:58 - 35:07that embryos held clues
to how animals evolve. -
35:07 - 35:09All embryos start out
as clusters -
35:09 - 35:15of nearly identical cells.
-
35:15 - 35:18But soon,
an embryo partitions itself -
35:18 - 35:21into specialized segments
-
35:21 - 35:26which develop into
the final form of the animal. -
35:26 - 35:29What controlled this process?
-
35:29 - 35:35How did the embryos know
what shape to take? -
35:35 - 35:37One of the first people
to study these questions -
35:37 - 35:44was a 19th-century naturalist
named William Bateson. -
35:44 - 35:46Bateson wrote
that animals' skeletons revealed -
35:46 - 35:59an underlying structure
of repeating segments. -
35:59 - 36:02He also observed that animals
occasionally developed -
36:02 - 36:07with some segments
in the wrong places. -
36:07 - 36:10MAN:
Insects with legs
in the wrong place. -
36:10 - 36:13Crabs where a claw
was transformed into a leg. -
36:13 - 36:15Pythons with extra ribs
-
36:15 - 36:17or frogs
with extra cervical vertebrae -
36:17 - 36:20and all these sorts of things.
-
36:20 - 36:23NARRATOR:
To Bateson,
these developmental errors meant -
36:23 - 36:26that the underlying blueprint
for the animal -
36:26 - 36:29was being disrupted.
-
36:29 - 36:31He had no idea how it happened
-
36:31 - 36:34but he suspected
that these random changes -
36:34 - 36:41might provide the fuel
for evolution. -
36:41 - 36:44By the 1940s, scientists working
with fruit flies -
36:44 - 36:47had learned
how to cause disruptions -
36:47 - 36:50in the developmental blueprint:
-
36:50 - 36:56by dousing growing embryos
with radiation and poison. -
36:56 - 36:57MAN:
And so when they did that -
36:57 - 37:01they found flies with changed
wing structures, changed legs -
37:01 - 37:03and these very special flies
-
37:03 - 37:07that have one part of the body
in the wrong place -
37:07 - 37:17or a copy of a normal part
of the body in another place. -
37:17 - 37:19NARRATOR:
The scientists
had triggered the changes -
37:19 - 37:25by damaging the flies' DNA.
-
37:25 - 37:27Within each cell
of the developing embryo -
37:27 - 37:31is a chainlike molecule
called DNA. -
37:31 - 37:34The experiments showed
that DNA was somehow -
37:34 - 37:40causing the embryo
to divide into segments. -
37:40 - 37:43But how?
-
37:43 - 37:45Scientists were just beginning
to grasp -
37:45 - 37:53that the DNA itself was made up
of segments, called genes. -
37:53 - 38:01The question was: how did
the genes shape the body? -
38:01 - 38:04One researcher,
Dr. Ed Lewis of Caltech -
38:04 - 38:07studied this question
for 30 years -
38:07 - 38:12by crossbreeding
thousands of flies. -
38:12 - 38:16Lewis's work led him
to a controversial idea. -
38:16 - 38:19He proposed that a surprisingly
simple mechanism -
38:19 - 38:23was shaping embryos.
-
38:23 - 38:26He wrote that each segment
of the fly -
38:26 - 38:30was being directed to grow
by a single gene. -
38:30 - 38:34A small set of genes,
a kind of genetic toolkit -
38:34 - 38:38appeared to be laying out
the entire body. -
38:38 - 38:40And as he looked at these genes,
he said -
38:40 - 38:42"This one affects
this part of the body. -
38:42 - 38:43"This affects
the next part of the body. -
38:43 - 38:45And this affects
the next part of the body." -
38:45 - 38:50That was
an astonishing observation. -
38:50 - 38:53NARRATOR:
It was astonishing
because it seemed too simple. -
38:53 - 38:56Nobody else thought single genes
were powerful enough -
38:56 - 39:02to control something as complex
as the structure of the body. -
39:02 - 39:06Skeptics argued
that Lewis's idea was guesswork. -
39:06 - 39:10Of course, he had never seen
the genes -
39:10 - 39:15because the techniques to do so
didn't exist. -
39:15 - 39:16From the 1920s to the 1970s
-
39:16 - 39:19it was not possible
to physically isolate -
39:19 - 39:21any specific gene.
-
39:21 - 39:24That opportunity first became
available, fortunately for me -
39:24 - 39:26at the time
that I was a student. -
39:26 - 39:30And so, many of us
thought, "Wow. -
39:30 - 39:33"We can finally dig in there
-
39:33 - 39:38and identify
these really mysterious genes." -
39:38 - 39:44NARRATOR:
Levine enlisted his friend and
fellow scientist Bill McGinnis. -
39:44 - 39:49The first gene they went after
had an unusual name. -
39:49 - 39:55Antennapedia,
which means "antenna leg." -
39:55 - 40:01The gene was thought
to control the growth of legs. -
40:01 - 40:03When the gene misfired
-
40:03 - 40:07flies grew legs
in the wrong place: -
40:07 - 40:11on their heads,
in place of antennae. -
40:11 - 40:15In normal flies,
legs grow from the midsection -
40:15 - 40:18the area called the thorax.
-
40:18 - 40:22So Levine and McGinnis decided
to hunt for the gene -
40:22 - 40:27in the thorax
of a normal embryo. -
40:27 - 40:28LEVINE:
The expectation -
40:28 - 40:31is that antennapedia
would be active -
40:31 - 40:33expressed in the thorax
-
40:33 - 40:34the developing thorax,
of the embryo. -
40:34 - 40:36But who knew?
-
40:36 - 40:39NARRATOR:
Levine and McGinnis had
to do something -
40:39 - 40:42no one had ever done before.
-
40:42 - 40:48They had to find a way to see
a gene in action. -
40:48 - 40:50LEVINE:
We wanted to light up the gene -
40:50 - 40:53and it was very
painstaking work. -
40:53 - 40:59NARRATOR:
The project called
for new and untested methods. -
40:59 - 41:01McGINNIS:
At first, it didn't work
very well -
41:01 - 41:07and there were a number
of technical problems to solve. -
41:07 - 41:10NARRATOR:
The team had to find
a delicate balance -
41:10 - 41:16of radioactive probes
and toxic enzymes. -
41:16 - 41:22Too much of either
would destroy the embryos. -
41:22 - 41:24LEVINE:
The process
was not very gratifying -
41:24 - 41:27on a day-by-day basis.
-
41:27 - 41:32Unbelievably tedious.
-
41:32 - 41:40NARRATOR:
It took months
of trial and error. -
41:40 - 41:43McGINNIS:
People often said, "You know,
you should try something else. -
41:43 - 41:48"You know,
this is too long-shot. -
41:48 - 41:51You know, you're going to...
you're just wasting your time." -
41:51 - 41:56But we kept going.
-
41:56 - 42:11NARRATOR:
Finally, late one night,
all the work paid off. -
42:11 - 42:13LEVINE:
And there was this moment... -
42:13 - 42:18when we saw that the gene
was turned on in a band -
42:18 - 42:21in the middle
of a very early embryo. -
42:21 - 42:23This had never been seen before.
-
42:23 - 42:29NARRATOR:
The antennapedia gene was acting
like a master switch -
42:29 - 42:31turning on the segment
of the embryo -
42:31 - 42:34that would become the thorax.
-
42:34 - 42:37The implications
were mind-boggling: -
42:37 - 42:41if single genes like
antennapedia could define -
42:41 - 42:43whole segments of an animal
-
42:43 - 42:49these genes were acting
like architects of the body. -
42:49 - 42:54And if one of these genes
turned on in the wrong place -
42:54 - 42:58striking changes
to the body could result. -
42:58 - 43:02It seemed that Levine and
McGinnis had uncovered -
43:02 - 43:06the genes responsible
for the evolution of bodies. -
43:06 - 43:09But there were still doubts.
-
43:09 - 43:13The work had all
been done in fruit flies. -
43:13 - 43:15What about other animals?
-
43:15 - 43:20Did they use the same mechanism
to build their bodies? -
43:20 - 43:24An answer would come
from Switzerland. -
43:24 - 43:28In 1994, Walter Gehring
of the University of Basel -
43:28 - 43:31isolated the gene that triggered
-
43:31 - 43:36the growth of eyes
in fruit flies. -
43:36 - 43:38The gene was called Eyeless
-
43:38 - 43:44because flies without it
developed with no eyes. -
43:44 - 43:51Gehring knew of a gene in mice
that worked in the same way. -
43:51 - 43:55He wondered,
were the two genes the same? -
43:55 - 43:57GEHRING:
And this question we tested -
43:57 - 44:02by taking the mouse gene and
putting it into fruit flies -
44:02 - 44:05to see whether flies
can understand -
44:05 - 44:07the message of the mouse.
-
44:07 - 44:14NARRATOR:
Gehring replaced a fly's gene
for eyes with the mouse gene. -
44:14 - 44:17GEHRING:
And to everybody's surprise -
44:17 - 44:20the mouse gene works
perfectly well -
44:20 - 44:27and can induce a compound eye
in the fruit fly. -
44:27 - 44:31NARRATOR:
The fruit fly grew
normal fruit fly eyes -
44:31 - 44:34using a gene from a mouse.
-
44:34 - 44:38Not only did the two creatures
use the same mechanism; -
44:38 - 44:42they used the same gene.
-
44:42 - 44:47This was the mechanism
behind extra wings -
44:47 - 44:55legs sprouting from heads,
and Bateson's deformed animals. -
44:55 - 44:58The century-long search
was complete. -
44:58 - 45:02The genetic engine
of the body's evolution -
45:02 - 45:06turned out to be a tiny handful
of powerful genes. -
45:06 - 45:07CARROLL:
So what this means is -
45:07 - 45:10in some ways, some sense,
evolution is a simpler process -
45:10 - 45:13than we first thought...
-
45:13 - 45:17when you think about all of the
diversity of forms out there. -
45:17 - 45:20We first believed
that this would involve -
45:20 - 45:22all sorts of novel creations
-
45:22 - 45:25starting from scratch,
again and again and again. -
45:25 - 45:28We now understand
that no, that evolution works -
45:28 - 45:31with packets of information,
and uses them -
45:31 - 45:35in new and different ways and
new and different combinations -
45:35 - 45:38without necessarily
having to invent -
45:38 - 45:42anything fundamentally new,
but new combinations. -
45:42 - 45:47NARRATOR:
Suddenly, the commonality
of form among animals -
45:47 - 45:49was understood:
-
45:49 - 45:53animals resembled each other
because they all used -
45:53 - 45:56the same set of genes
to build their bodies -
45:56 - 46:00a set of genes
inherited from a common ancestor -
46:00 - 46:03that lived long ago.
-
46:03 - 46:07And what we see now among all
the animals are just variations -
46:07 - 46:11on a body plan that existed
half a billion years ago. -
46:11 - 46:13And there's only one
inescapable conclusion -
46:13 - 46:16you can draw from that,
which is -
46:16 - 46:18if all of these branches
have these genes -
46:18 - 46:20then you have to go
to the base of that -
46:20 - 46:23which is the last common
ancestor of all animals -
46:23 - 46:26and you deduce,
itmust have had these genes. -
46:26 - 46:28So the whole radiation
of animals -
46:28 - 46:31the whole spring
of animal diversity -
46:31 - 46:36has been fed by essentially
the same set of genes. -
46:36 - 46:40NARRATOR:
Ed Lewis shared
the Nobel Prize in 1995 -
46:40 - 46:44for the discovery of
the universal set of genes -
46:44 - 46:48that builds the bodies
of animals. -
46:48 - 46:51And so, yes, it came
as a huge surprise -
46:51 - 46:54not only to people
like my mother, who says -
46:54 - 46:56"My God, an earthworm
and a mouse? -
46:56 - 47:00An earthworm and me, you know,
share things in common?" -
47:00 - 47:03But it came as a surprise to
other scientists that there was -
47:03 - 47:07this profound conservation of
mechanism of building embryos -
47:07 - 47:13among all these different
kinds of animals. -
47:13 - 47:15NARRATOR:
What about us? -
47:15 - 47:17Our bodies are built
-
47:17 - 47:21from the same genes that build
all other animals. -
47:21 - 47:25Yet we are different.
-
47:25 - 47:34No other animal designs...
or creates like we do. -
47:34 - 47:38We seem so special
it's hard not to think -
47:38 - 47:42that we're somehow an exception
to evolution... -
47:42 - 47:46but of course, we're not.
-
47:46 - 47:48The transformation
that led to us -
47:48 - 47:54was no different
from other transformations. -
47:54 - 47:58Our crucial turning point
seems to have occurred -
47:58 - 48:01when our ancestors
left the trees -
48:01 - 48:03and began to walk on two legs.
-
48:03 - 48:07MAN:
We don't know exactly when,
or even where -
48:07 - 48:10our ancestors became
upright and bipedal. -
48:10 - 48:13We think it goes back
well over four million years. -
48:13 - 48:16When these ancestors
came out of the trees -
48:16 - 48:19and began to exploit
food sources on the ground -
48:19 - 48:21in terrestrial habitats
-
48:21 - 48:24one of the key elements that
would've been so useful to them -
48:24 - 48:27would've been freeing
their forelimbs, their hands -
48:27 - 48:31to be able to gather and carry
foodstuffs over long distances. -
48:31 - 48:33Once that happened, it opened up
-
48:33 - 48:38an extraordinary breadth of
possibilities and opportunities. -
48:38 - 48:41NARRATOR:
Most bipedal hominids
went extinct -
48:41 - 48:45but one branch went on
to evolve larger brains. -
48:45 - 48:50That branch eventually
led to modern humans. -
48:50 - 48:53So how did this
crucial transition -
48:53 - 48:57to two-legged walking begin?
-
48:57 - 49:00Liza Shapiro of
the University of Texas -
49:00 - 49:02looks for clues
in living primates. -
49:02 - 49:04SHAPIRO:
When you look
at the fossil record -
49:04 - 49:07all you have really
is a pile of bones. -
49:07 - 49:08It's a nonmoving entity.
-
49:08 - 49:10There's not much
you can know about it -
49:10 - 49:15unless you look
for living analogs. -
49:15 - 49:18So if you look at living
animals, you've got the bones -
49:18 - 49:24but you can also look
at how they're moving. -
49:24 - 49:27NARRATOR:
In their movements,
living lemurs resemble -
49:27 - 49:28tree-dwelling primates
-
49:28 - 49:30that lived
up to 50 million years ago. -
49:30 - 49:32We didn't evolve from lemurs
-
49:32 - 49:35but they may be
the best living analog -
49:35 - 49:39for those distant ancestors.
-
49:39 - 49:42SHAPIRO:
When we're trying
to reconstruct the scenario -
49:42 - 49:45about how humans evolved
bipedally from this ancestor -
49:45 - 49:48we have to know
what it was we started from -
49:48 - 49:51if we're going to come up
with an explanation -
49:51 - 49:57for not only how we made
that transition, but why. -
49:57 - 50:00NARRATOR:
Today, Shapiro is gathering data -
50:00 - 50:02on the movement style
of the lemur. -
50:02 - 50:05Small reflectors
have been gently placed -
50:05 - 50:07on the animal's back.
-
50:07 - 50:10An array of infrared cameras
will record the lemur -
50:10 - 50:16as it walks across
this makeshift bridge. -
50:16 - 50:19Of course, getting a lemur to do
just about anything on cue -
50:19 - 50:21takes a bit of doing.
-
50:21 - 50:35There you go.
-
50:35 - 50:39NARRATOR:
Finally, the animal
makes it across. -
50:39 - 50:40Here you go...
oh, good! -
50:40 - 50:42All right.
-
50:42 - 50:43Got that.
-
50:43 - 50:44And he's down.
-
50:44 - 50:48NARRATOR:
The motion of the lemur's spine
can now be analyzed -
50:48 - 50:51in three dimensions.
-
50:51 - 50:57The data reveal that lemurs'
spines are extremely flexible -
50:57 - 51:00capable of many
kinds of movements. -
51:00 - 51:03SHAPIRO:
Lemurs walk quadrupedally -
51:03 - 51:06but they're also
very good at leaping. -
51:06 - 51:11NARRATOR:
Like these lemurs, the early
primates probably moved -
51:11 - 51:12in all sorts of ways:
-
51:12 - 51:19down on all fours,
scampering up trees... -
51:19 - 51:24even leaping in
an upright position. -
51:24 - 51:27They weren't limited
to just one style of movement -
51:27 - 51:30so they could serve
as the starting point -
51:30 - 51:36for a number of evolutionary
experiments. -
51:36 - 51:41And most likely,
that's just what happened. -
51:41 - 51:42We weren't the only ones
-
51:42 - 51:45to evolve
from those early ancestors. -
51:45 - 51:49So did most of today's
living primates. -
51:49 - 51:53Our closest living relative
is the chimpanzee. -
51:53 - 51:56We didn't evolve from chimps
-
51:56 - 51:59but we do share
a recent common ancestor. -
51:59 - 52:00Can you walk
over here? -
52:00 - 52:06NARRATOR:
That's why our DNA is nearly
identical to theirs... -
52:06 - 52:09and why our skeletons
have the same number of bones -
52:09 - 52:13arranged in nearly the same way.
-
52:13 - 52:17But the few physical differences
that set us apart -
52:17 - 52:21seem to have made
a great difference. -
52:21 - 52:25Chimps don't walk on two feet.
-
52:25 - 52:28They've evolved a different
style of getting around -
52:28 - 52:29called knucklewalking.
-
52:29 - 52:31JOHANSON:
Knucklewalking -
52:31 - 52:34is a very specialized adaptation
that we see -
52:34 - 52:37among chimps and gorillas today.
-
52:37 - 52:40It's an adaptation
to walking on the ground. -
52:40 - 52:45NARRATOR:
Knucklewalking was as valid
an evolutionary experiment -
52:45 - 52:46as two-legged walking.
-
52:46 - 52:49But the difference
in our walking styles -
52:49 - 52:52which may have affected
our intellects -
52:52 - 52:59is seen in the few slight
differences in our skeletons. -
52:59 - 53:02Here are two skeletons
of modern primates. -
53:02 - 53:04This skeleton I'm sure
you'll all recognize -
53:04 - 53:07because it's a skeleton
like yours and mine. -
53:07 - 53:08This is a modern human.
-
53:08 - 53:10But this smaller skeleton
-
53:10 - 53:16is one of our closest living
relatives, the chimpanzee. -
53:16 - 53:17We began walking on two legs
-
53:17 - 53:21and that made a whole series of
modifications in the skeleton. -
53:21 - 53:22In humans, the spinal chord
-
53:22 - 53:25comes out of the base
of the skull -
53:25 - 53:27and points straight downwards
-
53:27 - 53:30rather than coming out
of the back of the skull. -
53:30 - 53:32The pelvis is shaped
very differently. -
53:32 - 53:36A chimpanzee has
a long, narrow pelvis. -
53:36 - 53:39Ours is short and squat.
-
53:39 - 53:43We walk with our knees
close together. -
53:43 - 53:46Chimpanzees walk
with their knees wide apart. -
53:46 - 53:48These are minor differences.
-
53:48 - 53:52These are the sorts of tinkering
that evolution did -
53:52 - 53:56to change us
into a modern biped. -
53:56 - 54:01NARRATOR:
What if our ancestors
hadn't stood up? -
54:01 - 54:04What if they had taken
one different turn -
54:04 - 54:10along the path
to becoming human? -
54:10 - 54:15JOHANSON:
One of the great misconceptions
that most people have is that... -
54:15 - 54:19that once our ancestors stood
up, it was almost inevitable -
54:19 - 54:21that we would be here today
-
54:21 - 54:24that the egocentric species,
Homo sapiens -
54:24 - 54:26would evolve in this manner.
-
54:26 - 54:28But what we see is
-
54:28 - 54:31that evolution has worked
the same way with us -
54:31 - 54:35as it has with every, single
organism on this planet. -
54:35 - 54:38We're here through a series
of chance coincidences -
54:38 - 54:42specific adaptations,
chosen opportunities. -
54:42 - 54:46So I think that when we
look at our own origins -
54:46 - 54:48we see that it is extraordinary
-
54:48 - 54:53that humans are here to look
back and ponder their past. -
54:53 - 54:55SHUBIN:
Does that mean we are not unique -
54:55 - 54:56in many ways?
-
54:56 - 54:58Of course not.
-
54:58 - 55:00We're the ones
telling this story. -
55:00 - 55:02And that's very important...
-
55:02 - 55:05that evolution, that life
has gotten to the point -
55:05 - 55:50where it can tell this story.
-
55:50 - 55:51Continue the journey
-
55:51 - 55:53into where we're from
and where we're going -
55:53 - 55:55at the Evolution web site.
-
55:55 - 55:58Visit www.pbs.org.
-
55:58 - 56:01The seven-part
Evolution boxed set -
56:01 - 56:02and the companion book
-
56:02 - 56:05are available
from WGBH Boston Video. -
56:05 - 56:13To place an order, please call:
-
56:13 -Downloaded From www.AllSubs.org
- Title:
- Evolution: Great Transformations (PBS Documentary) 2/7
- Description:
-
Please Subscribe To The Evolution Documentary YouTube Channel:
http://www.youtube.com/EvolutionDocumentaryBroadcast (2001) What underlies the incredible diversity of life on Earth? How have complex life forms evolved? The journey from water to land, the return of land mammals to the sea, and the emergence of humans all suggest that creatures past and present are members of a single tree of life. Evolution determines who lives, who dies, and who passes traits on to the next generation. The process plays a critical role in our daily lives, yet it is one of the most overlooked and misunderstood concepts ever described. The Evolution series goals are to heighten public understanding of evolution and how it works, to dispel common misunderstandings about the process, and to illuminate why it is relevant to all of us.
The Evolution project's eight-hour television miniseries travels the world to examine evolutionary science and the profound effect it has had on society and culture. From the genius and torment of Charles Darwin to the scientific revolution that spawned the tree of life, from the power of sex to drive evolutionary change to the importance of mass extinctions in the birth of new species, the Evolution series brings this fascinating process to life. The series also explores the emergence of consciousness, the origin and success of humans, and the perceived conflict between science and religion in understanding life on Earth.
- Duration:
- 56:38
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