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.
地球形成于大約46億年前，大約10億年后首次出現(xiàn)生命，首批植物直到20億年前才出現(xiàn)。5.8億年之前，生命形式一直都是極其微小的。從那以后，生命演化成多種不同的形式或物種，然而，科學(xué)家估計其中99%以上已經(jīng)滅絕。

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.
在地球的歷史上發(fā)生過多次滅絕事件，每次滅絕事件都造成多種生命形式或物種滅絕。滅絕事件的起因仍是未解之謎，這些事件是不可預(yù)測的，可能隨時發(fā)生?？茖W(xué)家認(rèn)為其中一些是由小行星撞擊或火山爆發(fā)引起的，像這樣的事件可以改變生命所依賴的地球環(huán)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.
大約6600萬年前發(fā)生了一次大滅絕，這一事件可能是由一顆小行星撞擊地球引起。結(jié)果，恐龍和地球上約一半的物種滅絕了。當(dāng)小行星撞擊地球時，引發(fā)了一連串的事件。一連串的事件是一系列相互關(guān)聯(lián)的事件，像鏈條一樣。

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.
這次特別的事件鏈延續(xù)了幾個月，甚至幾年。當(dāng)小行星撞擊地球時，巨大的塵埃云進入大氣層。塵埃云擋住了陽光，地球因此好幾個月陷入黑暗。沒有足夠的陽光，植物和食草動物很快死亡。隨著這么多動植物的死亡，食物鏈崩潰，這導(dǎo)致了更多物種的死亡。

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.
滅絕事件在生命進化中扮演了重要角色，事實上，一些滅絕事件有助于加速進化。比如，一些生活在恐龍時代的哺乳動物不能充分進化。當(dāng)恐龍滅絕后，這些哺乳動物的進化變得順利。一旦能夠充分進化，哺乳動物就進入一個更高的狀態(tài)。如果沒有滅絕事件，也許我們?nèi)祟惤裉炀筒粫谶@里。