There are many forms of life on Earth, including human beings.

Life exists in a variety of conditions.

Some forms of life live in a watery environment, like the oceans.?

Other forms of life can be found in very dry areas like deserts.?

However, for any form of life to exist, conditions must be right.?

When conditions are not right, that form of life will become extinct.?

To become extinct means to die out completely.

Conditions must be right for life to exist.

Millions of years ago there were forms of life that no longer exist.?

When conditions changed, these forms of life died out.

One extinction event happened about 250 million years ago.

This was the largest extinction event of all time.

Many forms of life became extinct.

96% of all life in the oceans died out.

Most insects also became extinct.

This event happened over a period of several million years.

The causes of this extinction event are still unknown.

Possible causes include large volcanic eruptions and global warming.?

Some scientists believe that there were several causes.

They believe that a series of events caused the extinctions.?

Scientists are working to better understand what really happened.