Extinction events 1
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.
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.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.
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.
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.
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.
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.