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Evolution: Great Transformations (PBS Documentary) 2/7

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

Broadcast (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.

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Duration:
56:38
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