Source: Evolution: "Great Transformations"
NARRATOR: The first creatures to leave the water really started something. Their descendants eventually evolved into today's reptiles...birds...and mammals.
And these creatures' common origins are still visible in their bodies. Just like us, they all have bodies with four limbs—they're all tetrapods.
JENNY CLACK, Cambridge University: The old idea was that the fish came on shore first and then developed the legs. And what we now think is that the tetrapods developed the fingers first and then left the water.
NARRATOR: Jenny Clack of Cambridge University suspected that the theory taught in many textbooks was wrong.
CLACK: The story that you will find in many of the old textbooks and the pictures that you will see in children's books and museum galleries is a picture of a fish stranded in a drying pool, trying to support itself out of water. And it looks really odd if you look at it objectively.
NARRATOR: Clack thought there had to be a better explanation, but where to look? Only a handful of early tetrapod fossils had ever been found—most of those in a remote part of Greenland at the turn of the century. All she had to guide her was a note scribbled in a journal from a scouting trip to Greenland years earlier.
It referred to tetrapod fossils on an unnamed mountain. Clack flew to Greenland to search for those bones.
CLACK: Eventually we found the spot, 800 meters up on a hillside.
NARRATOR: Clack returned with four tons of rock...and spent the next four years drilling. At the end, she had the most complete early tetrapod skeleton ever found; and it forever changed the textbooks.
CLACK: One of the first things that we found was this forelimb.
NARRATOR: At the end of the animal's limb was an unmistakable array of bones. This was a hand.
CLACK: This is a life reconstruction. The artist is using imagination on the color scheme and on the eyes, but we think this is as accurate as you can get.
NARRATOR: The creature, named Acanthostega, was clearly a water-dweller: It had a fish-like tail and gills for breathing in the water. But the ends of its arms were paddle-shaped...possibly the first hands on Earth.
CLACK: This was a swimming creature. We don't know whether it could ever have come out on land, but it certainly wouldn't have walked in the conventional sense.
Basically, it's...a fish with fingers.
NARRATOR: Clack's find was a scientific breakthrough. It proved that some fish had arms and legs in the water. So tetrapods didn't need to grow limbs after they got onto land. The limbs had already evolved and helped them survive out of the water.
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