学习目标：1. To learn about the origin of life on the earth.
2. To master the usages of some key language points.
3. To develop the feeling of treasuring the earth and life.
学习重点：1.Words: ①explode ②multiply ③spread
2.Expressions:①whether … or not ②in time to do sth ③allow … to do ④in one’s turn
⑤prevent … from
