Narrator: This is Science Today. We take oxygen for granted, but the amount of it in the air wasn't always the same. Three hundred million years ago, for example, in what's called the Carboniferous era, the air was 35 percent oxygen, according to biologist Jeffrey Graham of the University of California, San Diego.
Graham: To put that in scale, the present atmospheric oxygen level's about 21 percent oxygen. And so a 35 percent level of oxygen is roughly one and a half times as much oxygen as there is now.
Narrator: Graham says that much oxygen had an effect on evolution. The air was so thick and rich that flying insects could be a lot bigger than they are today. Dragonflies, for instance...
Graham: And these dragonflies were immense compared to modern day dragonflies. For example, the biggest dragonfly that we find in nature today might have a wingspan of about six inches. Whereas the late Paleozoic or Carboniferous dragonflies had wingspans of close to over two feet. So they were huge animals, the size of -- probably of seagulls.
Narrator: For Science Today, I'm Steve Tokar.