Extinction events 1
The Earth was formed about 4.6 billion years ago.
地球形成于46亿年前。
The first life appeared about a billion years later.
第一个生命出现在大约十亿年后。
The first plants didn't appear until around 2 billion years ago.
第一批植物直到20亿年前才出现。
Until about 580 million years ago, life forms were small and microscopic.
直到大约5.8亿年前，生命形式都是微小的，只能用显微镜观察。
Since that time, life has evolved into many different types, or species.
从那时起，生命进化成许多不同的类型或物种。
However, scientists estimate that more than 99% of them are now extinct.
然而，科学家估计其中99%以上已经灭绝。
There have been several extinction events in the history of the Earth.
在地球的历史上发生过几次大灭绝事件。
In an extinction event many life forms, or species of life, die out.
在一次灭绝事件中，许多生命形式或生命物种灭绝。
The causes of mass extinction events are unsolved mysteries.
大灭绝事件的起因仍是未解之谜。
These events are not predictable and can happen at any time.
这些事件是不可预测的，并且会在任何时候发生。
Scientists think that some of them were started by asteroid impacts or volcanic eruptions.科学家认为其中一些是由小行星撞击或火山爆发引起的。
Events like these can change the global conditions that life depends on.
像这样的事件可以改变生活所依赖的全球环境。
One major extinction event happened about 66 million years ago.
大约6600万年前发生了一次大灭绝事件。
That event may have been started by a large asteroid that hit the Earth.
这一事件可能是由一颗撞击地球的小行星引起的。
As a result, the dinosaurs and about half of all species on the planet died out.
结果，恐龙和地球上大约一半的物种灭绝了。
When the asteroid hit the earth it started a chain of events.
当小行星撞击地球时，它引发了一连串的事件。
A chain of events is a series of events linked together, like a chain.
一连串的事件是一系列相互联系在一起的事件，就像一条链。
This particular chain of events happened over a period of months and years.这种特殊的连锁反应发生在几个月甚至几年的时间里。
When the asteroid hit the Earth, it caused a huge cloud of dust to enter the atmosphere.当小行星撞击地球时，它引起了巨大的尘埃云进入大气层。
This cloud of the dust blocked the sun and darkened the Earth for many months.
这片尘埃云挡住了太阳，使地球黑暗了好几个月。
Without enough sunlight, plants and plant-eating animals quickly died.
没有足够的阳光，植物和食草动物很快就死了。
With the deaths of so many plants and animals, the food chain collapsed.
随着这么多动植物的死亡，食物链崩溃了。
This led to the deaths of many more species.
这导致了更多物种的死亡
Mass extinction events have played an important part in the evolution of life.
大灭绝事件在生命进化中扮演了重要的角色
In fact, some extinction events have helped to accelerate evolution.
事实上，一些灭绝事件有助于加速进化。
For example, some mammals lived through the age of the dinosaurs, but they couldn't compete.
例如，一些哺乳动物生活在恐龙时代，但他们不能竞争。
When the dinosaurs were removed, the mammals were able to compete more successfully.
恐龙灭绝后，哺乳动物的竞争更加激烈。
Once they could compete, mammals evolved to a higher state.
一旦它们能够竞争，哺乳动物就进化到一个更高的状态。
Without extinction events, perhaps we wouldn't be here today.
如果没有灭绝事件，也许我们今天就不会在这里。