NARRATOR：

Now listen to part of a lecture in a biology class.

MALE PROFESSOR：

OK, so a good example of this-

found right here in North America-

is something an animal called the American [slowly, with emphasis] pronghorn does.

Pronghorns, as you may know, are a kinda deer-like animal...

they live out on the open, grassy plains...

somewhat in the middle of North America....

And they are super fast.

Pronghorns are, in fact, noted for being the fastest animal in the Western Hemisphere.

Once a pronghorn starts running-

zoom!-none of it's present-day predators,

like the... bobcat... or coyote, can even hope to catch up with it...

it's off ina flash!

OK, so why then do pronghorns run so fast?

That's the question.

Well ... it turns out that quite a long time ago ...

I'm talking tens of thousands of years ...

things on the grassy plains used to be very different for the pronghorns.

Because back then lions used to live on the plains...

chasing and preying upon the pronghorns.

And lions, of course, are a very swift-moving mammal...

much faster than the bobcat or coyote, or other predators that you find on the plains today....

But, now, however, lions are all extinct in North America.

They're no longer a predator of the pronghorn.

Tens of thousands of years ago, though, the lions were there, chasing the pronghorns.

So, back then, the pronghorn's speed was critical to its survival.