Don: Hey Yaël, ready for a quiz?

Yaël: Ready, Don.

D: Which of these is a tetrapod: a spider, elephant or goldfish?

Y: Hmm, well I think the prefix "tetra" means four, and "pod" is Greek for feet, so I'll      guess "elephant," since it has four feet.

D: You're right! Tetrapods are vertebrates with limbs. Spiders don't have backbones, so      they aren't vertebrates. Fish are vertebrates, but don't have limbs.

     Before about 360 million years ago, no vertebrates had limbs. They all shared the      same fish-like body shape. No limbs, just fins. But around that time something      exciting happened. The fleshy lobed paired fins of some prehistoric fish evolved into      limbs. The modified fins gradually evolved into legs capable of supporting the      animal's weight, and allowing it to walk out of the sea. Tetrapods evolved not only      limbs, but also digits.

Y: Fingers and toes?

D: Exactly. One false notion used in attempts to discredit the theory of evolution, is that      there aren't any so-called "transitional fossils." In other words, there aren't examples      of intermediate forms between two branches on the evolutionary tree, such as      between prehistoric fish and tetrapods.

Y: Are there any?

D: Lots! In fact a particularly exciting discovery was made in 2008. Scientists used CT      scans to look at the bones hidden inside the intact fossilized pectoral fins of a 385-      million year old lobe-finned fish discovered in Latvia. Inside the fish's stumpy limb-      like fins were rudimentary digits, like a tetrapod.

Y: Cool, a fish with fingers!

D: Yep, this fossil is just one of many that illustrate the gradual evolution of legs that      allowed early tetrapods to leave the water and colonize the land.  

To hear this program click here

A Moment of Science^®