Chris Mooney in Slate (photo via PBS):
We all know the Darwin fish, the car-bumper send-up of the Christian ichthys symbol, or Jesus fish. Unlike the Christian symbol, the Darwin fish has, you know, legs.
But the Darwin fish isn't merely a clever joke; in effect, it contains a testable scientific prediction. If evolution is true, and if life on Earth originated in water, then there must have once been fish species possessing primitive limbs, which enabled them to spend some part of their lives on land. And these species, in turn, must be the ancestors of four-limbed, land-living vertebrates like us.
Sure enough, in 2004, scientists found one of those transitional species: Tiktaalik roseae, a 375-million-year-old Devonian period specimen discovered in the Canadian Arctic by paleontologist Neil Shubin and his colleagues. Tiktaalik, explains Shubin on the latest episode of the Inquiring Minds podcast, is an “anatomical mix between fish and a land-living animal.”
“It has a neck,” says Shubin, a professor at the University of Chicago. “No fish has a neck. And you know what? When you look inside the fin, and you take off those fin rays, you find an upper arm bone, a forearm, and a wrist.” Tiktaalik, Shubin has observed, was a fish capable of doing a push-up. It had both lungs and gills. In sum, it's quite the transitional form.