Narrator: This is Science Today. We take the air we breathe for granted, but it wasn't always 21 percent oxygen. Four hundred million years ago it was down to 15 percent. A little later, when amphibians, the first land animals, emerged from the ocean, oxygen shot up to 35 percent. Biologist Jeff Graham of the University of California, San Diego says it made the air thick enough to support dragonflies the size of seagulls...

Graham: ... and also sufficiently present in the atmosphere to enable the increased metabolic requirements for animals that were leaving water and coming on to land. An example of this is just to think about the added weight that an animal has when it leaves water.

Narrator: In water, an early amphibian would have been practically weightless.

Graham: But the point there is that as animals leave water and come ashore their weight basically -- their mass increases by a factor of a thousand.

Narrator: And thus early amphibians would have needed that much more energy to get around -- energy that would have been supplied by the greater amount of oxygen in the air. For Science Today, I'm Steve Tokar.