The Earth was formed about 4.6 billion years ago. The first life appeared about a billion years later. The first plants didn't appear until around 2 billion years ago. Until about 580 million years ago, life forms were small and microscopic. Since that time, life has evolved into many different types, or species. However, scientists estimate that more than 99% of them are now extinct.
地球形成于大约46亿年前，大约10亿年后首次出现生命，首批植物直到20亿年前才出现。5.8亿年之前，生命形式一直都是极其微小的。从那以后，生命演化成多种不同的形式或物种，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. In an extinction event many life forms, or species of life, die out. The causes of mass extinction events are unsolved mysteries. These events are not predictable and can happen at any time. Scientists think that some of them were started by asteroid impacts or volcanic eruptions. Events like these can change the global conditions that life depends on.
在地球的历史上发生过多次灭绝事件，每次灭绝事件都造成多种生命形式或物种灭绝。灭绝事件的起因仍是未解之谜，这些事件是不可预测的，可能随时发生。科学家认为其中一些是由小行星撞击或火山爆发引起的，像这样的事件可以改变生命所依赖的地球环境。

One major extinction event happened about 66 million years ago. That event may have been started by a large asteroid that hit the Earth. As a result, the dinosaurs and about half of all species on the planet died out. When the asteroid hit the Earth, it started a chain of events. A chain of events is a series of events linked together, like a chain.
大约6600万年前发生了一次大灭绝，这一事件可能是由一颗小行星撞击地球引起。结果，恐龙和地球上约一半的物种灭绝了。当小行星撞击地球时，引发了一连串的事件。一连串的事件是一系列相互关联的事件，像链条一样。

This particular chain of events happened over a period of months and years. When the asteroid hit the Earth, it caused a huge cloud of dust to enter the atmosphere. This cloud of dust blocked the sun and darkened the Earth for many months. Without enough sunlight, plants and plant-eating animals quickly died. With the deaths of so many plants and animals, the food chain collapsed. This led to the deaths of many more species.
这次特别的事件链延续了几个月，甚至几年。当小行星撞击地球时，巨大的尘埃云进入大气层。尘埃云挡住了阳光，地球因此好几个月陷入黑暗。没有足够的阳光，植物和食草动物很快死亡。随着这么多动植物的死亡，食物链崩溃，这导致了更多物种的死亡。

Mass extinction events have played an important part in the evolution of life. In fact, some extinction events have helped to accelerate evolution. For example, some mammals lived through the age of the dinosaurs, but they couldn't compete. When the dinosaurs were removed, the mammals were able to compete more successfully. Once they could compete, mammals evolved to a higher state. Without extinction events, perhaps we wouldn't be here today.
灭绝事件在生命进化中扮演了重要角色，事实上，一些灭绝事件有助于加速进化。比如，一些生活在恐龙时代的哺乳动物不能充分进化。当恐龙灭绝后，这些哺乳动物的进化变得顺利。一旦能够充分进化，哺乳动物就进入一个更高的状态。如果没有灭绝事件，也许我们人类今天就不会在这里。