This is a rush transcript from "The O'Reilly Factor," February 26, 2008. This copy may not be in its final form and may be updated.

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BILL O'REILLY, HOST: In the "Factor Follow-up" segment tonight: Did global warming wipe out the dinosaurs 65 million years ago? And if not, what did?

As you may know, global warming is cyclical and right now is the focus of a ferocious debate, almost as ferocious as a T-Rex. To get a better view on the subject, I spoke with Dr. Terry Gates, a well-respected paleontologist.

(BEGIN VIDEOTAPE)

O'REILLY: So doctor, let's put it in perspective right off the bat. Sixty-five million years ago, the last dinosaurs were running around. Then they disappeared. When did people show up?

DR. TERRY GATES, PALEONTOLOGIST: People showed up about 100,000 years ago. And that's what we know today as modern man. 100,000 years ago.

O'REILLY: All right. So you're telling me that Raquel Welch was not chased around by a dinosaur? Is that what you are saying?

GATES: I'm sorry to say but, yes, that is true. She was not chased around by a dinosaur. They were separated by...

O'REILLY: Barney Rubble and Fred Flintstone, no dinosaur interaction?

GATES: I think that Hanna-Barbera got it wrong.

O'REILLY: Your best educated guess, because we hear all kinds of theories. The dinosaurs were around a long, long time, 230 million or 265 million years ago. That's a long period of time.

GATES: Absolutely.

O'REILLY: What caused them to disappear?

GATES: We know, as the KT extinction, when the dinosaurs went extinct was one of five times in Earth history when massive amounts of animal life that went extinct. In fact, 70 percent of all the life on earth went extinct.

And so right now the leading theory is an asteroid, but an asteroid impact that was very massive. We're talking a six-kilometer-wide asteroid that makes 100-mile-wide crater in the Yucatan Peninsula with tremendous damage.

At the same time, there's lots of other data which we still need to collect that doesn't quite make sense for only an asteroid. So it seems to me that you have to have lots of other factors involved in this. And some of the things that have been proposed is a lowering of ocean levels, which would then start wiping out some sea life. You also have effects from volcanism, which may have occurred. And so there's lots of ideas that probably need to culminate together to get the real answer of what happened.

O'REILLY: The bottom line is you guys don't know why the dinosaurs disappeared. Now, I've heard that the climate change...

GATES: Yes.

O'REILLY: ...could have led to the dinosaurs disappearing, because you found a lot of bones under ice, you know, frozen there. And that it used to be tropical, and then it was frigid. Am I reading it wrong?

GATES: It was very tropical. It was much, much warmer than it was today. There was very high CO2 levels. There were no permanent ice poles. But climate change may have impacted it once again and made one of the factors that contributed to the dinosaur extinction.

O'REILLY: I also hear a theory about dinosaurs and bacteria. Some new bacteria came in.

GATES: It may be true for a very small group of dinosaurs. Bacteria are evolving into new forms all the time. They're one of the most quickly evolving life forms on earth. And you don't get massive whole species-wide deaths because of bacteria today. And we're not talking about one species going extinct. We're talking about an entire group of dinosaurs.

O'REILLY: After the dinosaurs disappeared 65 million years ago, what took their place? I mean, they were gone, they vanished. And then what came up?

GATES: In the case of dinosaurs going extinct, the mammals came up to rise.

O'REILLY: OK. Now, some dinosaurs evolved — because there was something going on on Earth that they didn't like that was killing them — evolved into birds.

GATES: Birds evolved approximately 150 million years ago from small carnivorous dinosaurs. So you walk outside and you see pigeons in the sky. You're looking at dinosaurs. So technically dinosaurs are not extinct.

O'REILLY: Last question, and this is dumb, but I'm known for that. If we, through some time machine time travel thing, which may happen, who knows, go back to the dinosaur era, human beings, would the dinosaurs stalk us? Would these carnivores, would they chase us and try to get us?

GATES: There's no doubt that if you were to go back to the Cretaceous, you know, 65, 75 million years ago, you would have velociraptors chasing after you. You would have T-Rex chasing after you.

O'REILLY: Fascinating, doctor. Thanks very much for coming on. We appreciate it.

GATES: Sure thing, Bill.

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