Y: Whenever I visit the chimpanzees at the zoo, it always blows my mind that humans      evolved from primates.

D: I imagine that visiting the fish must really stun you then.

Y: What do you mean?

D: You mean you didn't know that all land vertebrates--ourselves, chimps, chickens,      snakes, you name it--evolved from fish?

Y: Are you pulling one over on me?

D: Not at all. It happened in the Devonian Period, about 365 million years ago, what's      known as the Age of Fishes. There were no land vertebrates, only aquatic      vertebrates. Then some fish called lobe-finned fish, creatures with bony limb-like fins,      slowly evolved into amphibians, the first vertebrates to roam on land.

Y: How do we really know that's how it happened? Is that just some quack theory or      do scientists have hard evidence?

D: They have magnificent evidence. One of the best pieces of evidence is a fossil called      Acanthostega, discovered in Greenland. What's so remarkable about it is that it's a      transition fossil. It defies categories. The creature isn't a fish exactly, nor an      amphibian, but somewhere in between. It has both lungs and internal gills. It has a tail      like a fish, but limbs something like later land creatures. Before the discovery of      fossils like Acanthostega, scientists hypothesized that the evolution of limbs was a      result of the transition from water to land. However, Acanthostega's limbs weren't      capable of supporting the creature's weight. Although it had hands and feet, it      appears that its limbs evolved for use in water in swamps, and only later in evolution      were adapted for walking on land.

Y: I'm officially stunned.  

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