Life on the Earth is wiped out every 27million years---and we have about 16 million years left until the next extinction, according to scientists.
Research into so-called ‘extinction events’ for our planet over the past 500 million years---twice as long as any previous studies ----has proved that they crop up with metronomic(有节奏的) regularity .
Scientists from the university of Kansas and the Smithsonian Institute in Washington DC are 99 percent confident that there are extinction every 27 million years.
In the 1980s scientists believed that Earth’s regular extinctions could be the result of a distant dark twin of the Sun, called Nemesis.
The theory was that Nemesis crashed through the Oort cloud every 27 million years and sent a shower of comets in our direction.
The Ocrt cloud is a vast belt of dust and ice that is believed to lie around one light year from the Sun and is the origin of many of the comets that pass through our solar system.
But now scientists claim that the regularity of the mass extinctions actually disproves the Nemesis theory because its orbit would have change over time as it interacted with other stars.
“Fossil data, which motivated the idea of Nemesis, now militate against(妨碍) it,” said the researchers.
The last extinctions event, 11 millions years ago, saw 10 percent of the Earth’s inhabitants (居民) wiped out.
This means there is around 16 million years until the next event takes place, although the graph shows that it occasionally the event takes place up to 10 million years early.
Asteroids(小行星) crashing into the Earth are commonly believed to be one of the main reasons behind mass extinctions like that suffered by the dinosaurs ---the Cretaceous-Tertiary (KT) extinction (白垩-第三纪灭绝事件).
The extinction wiped out more than half of all species on the planet clearing the way for mammals to become the dominant species(优势物种) on Earth.
The extinction was caused by a massive asteroid slamming into Earth at Chicxulub in Mexico.
The asteroid, which was around 15 kilometers wide, is believed to have hit Earth with a force one billion times more powerful than the atomic bomb at Hiroshima.
小题1: How many years are left until the next extinctions comes? (No more 6 words) (2′)
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小题2:What resulted in the Earth’s regular extinctions according to scientists in the 1980s? (No more than 10words) (2′)
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小题3: What did the last extinction event cause to the Earth? (No more than 10 words)(3′)
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小题4:What’s believed to be one of the main reasons behind mass extinctions like that suffered by the dinosaurs? (No more than 7 words) (3′)
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