正文：
The Earth was formed about 4.6 billion years ago.
地球大约在46亿年前形成。
The first life appeared about a billion years later.
大约十亿年后才出现第一个生命。
The first plants didn't appear until around 2 billion years ago.
最早的植物直到大约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，生命形式仍然很小且微观。
Since that time, life has evolved into many different types, or species.
从那时起，生命已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.
但是，科学家估计其中超过99％的物种已经灭绝。
There have been several extinction events in the history of the Earth.
地球历史上曾发生过几次灭绝事件。
In an extinction event many life forms, or species of life, die out.
在灭绝事件中，许多生命形式或生命种类死亡。
The causes of mass extinction events are unsolved mysteries.
大规模灭绝事件的原因尚未解决。
These events are not predictable and can happen at any time.
这些事件是不可预知的，并且随时可能发生。
Scientists think that some of them were started by asteroid impacts or volcanic eruptions.
科学家认为，其中一些起因于小行星撞击或火山爆发。
Events like these can change the global conditions that life depends on.
诸如此类的事件可以改变生活所依赖的全球条件。
One major extinction event happened about 66 million years ago.
一场重大的灭绝事件发生在大约6600万年前。
That event may have been started by a large asteroid that hit the Earth.
那个事件可能是由一颗撞击地球的小行星开始的。
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，它开始了一系列事件。
A chain of events is a series of events linked together, like a chain.
事件链是一系列链接在一起的事件，就像一个链。
This particular chain of events happened over a period of months and years.
这种特殊的事件链发生在几个月和几年的时间里。
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.
尘埃云阻挡了太阳并使地球变暗了许多个月。
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会死亡。
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.
这导致更多物种的死亡。
Mass extinction events have played an important part in the evolution of life.
大规模灭绝事件在生命的演变中发挥了重要作用。
In fact, some extinction events have helped to accelerate evolution.
实际上，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，有些哺乳动物在恐龙时代就生活了，但它们却无法与恐龙竞争。
When the dinosaurs were removed, the mammals were able to compete more successfully.
恐龙灭绝后，哺乳动物便能够更加成功地竞争。
Once they could compete, mammals evolved to a higher state.
一旦它们能够从竞争中胜出，哺乳动物就会进化到更高的状态。
Without extinction events, perhaps we wouldn't be here today.
没有灭绝事件，也许我们今天不会在这里。
正文2：
Scientists estimate that there have been at least 5 mass extinctions in the last 540 million years.
科学家估计在过去的5亿4千万年间至少发生了5次大灭绝。
They also believe that we have entered a new extinction event.
他们还认为我们已经进入了新的灭绝事件。
Species are becoming extinct at a much faster rate than before.
物种灭绝的速度比以前快得多。
Instead of asteroids or volcanic eruptions, the current event is the result of human activities.
当前的事件不是小行星或火山爆发，而是人类活动的结果。
Human activities, for example, have led to global warming.
例如，人类活动导致了全球变暖。
Global warming is the result of an increase in carbon gases in our atmosphere.
全球变暖是我们大气中碳气体增加的结果。
These gases trap heat from the sun, so the Earth is getting warmer.
这些气体会吸收来自太阳的热量，因此地球正在变暖。
As a result, sea levels are rising and weather patterns are changing.
结果，海平面上升，天气模式也在变化。
In addition, we humans are destroying the habitats of many plants and animals.
此外，我们人类正在破坏许多动植物的栖息地。
Cutting down forests and polluting rivers destroys the habitats of many forms of life.
砍伐森林和污染河流破坏了许多生命形式的栖息地。
There is no longer any doubt that this is happening.
毫无疑问，这正在发生。
Species of life are disappearing at an increasing rate.
生命的物种正在以越来越快的速度消失。
Global temperatures are rising and weather patterns are changing.
全球温度在上升，天气模式也在变化。
Scientists are warning that fast action is necessary to save our future.
科学家警告说，必须采取迅速行动来挽救我们的未来。
If we don't act, we may go the way of the dinosaurs.
如果我们不采取行动，我们可能会重蹈恐龙的覆辙。
