"173 species is 25 times more extinct species than you would expect under the normal, background, extinction rate," he told CNN in an email. He and his team found that in the past 100 years, more than 400 vertebrate species went extinct. In the normal course of evolution, such extinctions would have taken up to 10,000 years, they said.

他在发送给美国有线电视资讯网的电邮中写道：“这是正常物种灭绝速度下灭绝数量的25倍多。”他和他的团队发现，在过去的100年里，超过400种脊椎动物灭绝了。他们说，在正常的进化过程中，这样的灭绝可能需要长达1万年的时间。

vertebrate [ˈvɜːtɪbrət]：adj.脊椎动物

Mass extinctions are just as severe as their name suggests. There have been five mass extinction events in the Earth's history, each wiping out between 70% and 95% of the species of plants, animals and microorganisms. The most recent, 66 million years ago, saw dinosaurs disappear.

物种大灭绝正如其名字所暗示的那样严重。在地球历史上发生过五次大灭绝事件，每一次都导致70%到95%的植物、动物和微生物物种灭绝。最近的一次发生在6600万年前，恐龙灭绝了。

The past events were caused by catastrophic alterations of the environment, including massive volcanic eruptions or collision with an asteroid. The sixth mass extinction -- the one happening now -- is different: Scientists say it's caused by humans.

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