Don: Time now for another episode of A Moment of Science's "Did You Know..." So,Ya?l, did you know that there have been about twenty-three mass extinctions since life began on earth?

Ya?l: I know there've been a few extinctions, Don, but not that many!

D: Yep. Many were of prehistoric bacteria and other single-celled microorganisms. But in the past 540 million years or so there have been about five mass extinctions, mainly of marine plants and animals but also of land organisms.

Y: Well, I know that the dinosaurs disappeared about sixty-five million years ago,possibly due to a large meteor that crashed into earth and blotted out the sun. But what caused all those other extinctions?

D: That's been a mystery for a long time. But now some scientists think that most mass extinctions have been driven by the rise and fall of sea level. That may sound strange, but consider that millions of years ago a shallow sea covered the entire middle   section of North America. That sea grew and shrank back several times. Each time it drained it caused entire species of sea plants and creatures to die off. The last time it  drained was around sixty-five million years ago--when the dinosaurs disappeared.

Y: OK, I understand how a sea vanishing would kill off marine life. But did the sea's disappearance also have something to do with the dinosaurs going extinct?

D: Maybe. When a sea disappears it has a big effect on climate. All that water provides heat and moisture. And when a sea drains, the climate becomes drier and colder. So the change in climate may have combined with the meteor impact to kill off the dinosaurs.