There are many forms（形式） of life on earth ,including human beings.myg
Life extists in a variety of conditions.
some forms of life live in a watery environment（环境）,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 dead out.
one extinction event happened about 250 million years ago.
These was the largest extinction event of all time.
many forms of life became extinct（已灭绝）.
96% of all life in the oceans died out.
Most insects（昆虫） also became extinct.
This event happened over a period of several million years.
The causes of this extinction event are still unknown.
Possible causes inculde large volcanic（火山） eruptions（爆发） and global（全球） warming.
Some scientists belive that there were several causes.
They believe that a series（系列） of events caused the extinctions.
Scientists are working to better understand what really happened.
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