1. The Earth was formed about 4.6 billion years ago .
2. The first life appeared about billion years later .
3. The first plants didn’t appear until around 2 billion years ago .
4.Until about 580 million years ago life forms were small and microscopic.
microscopic /,maɪkrə'skɑpɪk/ 显微镜看的
5.Since that time , life has evolved into manydifferent types ,or species.
evolve into 进化成
species 生物物种
6. However ,scientists estimate that more than 99% of them are now extinct .
estimate /'ɛstə,met/ 估计，估价；判断，看法
extinct /ɪk'stɪŋkt/ 灭绝的，绝种的；熄灭的
7. There have been several extinction events in the history of the Earth .
8.In an extinction event many life forms ,or species of life ,died out .
9.The cause of mass extinction event are unsolved mysteries.
mysteries /'mistəriz/ 神秘的事物；难以理解的事
10.These events are not predictable and can happen at any time .
predictable /prɪ'dɪktəbl/ 可预言的
11. Scientists think that some of them were started by asteroid impacts or volcanic eruptions .
asteroid /'æstərɔɪd/ 小行星
volcanic /vɑl'kænɪk/ eruption /ɪ'rʌpʃən/ 火山喷发
12.Events like these can change the global conditions that life depends on .
13. One major extinction event happened about 66 million years ago.
14. That event may have been started by a largea steroid that hit the Earth.
15.As a result, the dinosaurs and about half of all species on the planet died out.
16.When the asteroid hit the earth it started achain of events.
17. A chain of events is a series of events linked together, like a chain.
18.This particular chain of events happened over a period of months and years.
19. When the asteroid hit the Earth, it caused a huge cloud of dust to enter the atmosphere.
atmosphere /'ætməsfɪr/ 大气
20.This cloud of dust blocked the sun and darkened the Earth for many months.
21.Without enough sunlight, plants and plant-eating animals quickly died.
22.With the deaths of so many plants andanimals, the food chain collapsed.
23.This led to the deaths of many more species.
24.Mass extinction events have played animportant part in the evolution of life.
25.In fact, some extinction events have helpedto accelerate evolution.
26.For example, some mammals lived through the age of the dinosaurs, but they couldn't compete.
27.When the dinosaurs were removed, the mammals were able to compete more successfully.
28.Once they could compete, mammals evolved to a higher state.
29. Without extinction events, perhaps we wouldn't be here today.