Narrator: This is Science Today. Once every two hundred million years, most of life on earth goes extinct -- usually around the same time as the impact of a huge meteor. The last time was 60 million years ago, when the dinosaurs disappeared. Paleontologist Kevin Padian of the University of California, Berkeley says that even with no meteor, the dinosaurs were in trouble already, and had been for millions of years. Earth's climate was changing, volcanoes were sending ash and dust into the air, and the planet's ecology was in turmoil.

Padian: The dinosaurs weren't really happy, it seems, with these changes. All these things are going on, and then this asteroid comes in.

Narrator: But whatever additional damage was caused by the asteroid, says Padian, it didn't make any difference to the dinosaurs -- because they'd already gone extinct.

Padian: It seems like a massive coincidence that the incredible asteroid which comes once every two hundred million years or so would choose that moment to strike and set up greater havoc. But in fact, that's what the record tells us.

Narrator: For Science Today, I'm Steve Tokar.