NARRATOR：

Now Listen to part of a lecture in an environmental studies class.

FEMALE PROFESSOR：

What happens, biologically speaking,

when a major climate change occurs?

Well, scientists generally agree that the extinction of various animal or plant species

is one very likely effect.

This has happened in the past.

An example is the disappearance 35,000 years ago

of a giant Australian bird called the thunder bird.

Now these birds were so big that they couldn't fly.

[finding the thread again] But as I was saying, uh...right...many scientists are convinced

that it was a change in climate that caused this species to become extinct.

Here's the evidence,

here's why scientists think it was climate change:

[pause] researchers have discovered an enormous number of 35,000 year-old thunder bird bones,

all together in one spot¡ª

the bones were found near an ancient, dried up lake.

Now it's really rare to find so many bones from the same species in one place,

but there it is: you have all

these 35,000-year-old thunder bird bones, all together near this dried up lake.

So what's the explanation?

Well, many scientists believe

that a change in the Australian climate

may be behind this ...

Their hypothesis is...

they think that during a very long dry period,

when there was no rainfall,

the birds may have flocked together at this lake.

You see, during a drought, animals tend to gravitate

toward the last few remaining water sources.

But then, if it still doesn't rain, if the drought lasts too long,

the water source

may dry up, too,

and if that happens, the animals that have gathered there, well, they die.

Scientists think this

ancient lake where they found the bones was one of the last remaining sources of fresh water

during the drought, so

that's where all these birds gathered.

They survived there for a time,

but eventually the lake dried up,

and well, that was it.

By the time the drought ended, the species was extinct.