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Passage One

A new study by US researchers suggests that the ocean, in particular the epic ebbs and flows of sea level and sediment（沉积） over the course of geological time, is the primary cause of the world’s periodic mass extinctions over the past 500 million years.

“The expansion and contractions of those environments have pretty profound effects on life on earth,” says assistant professor of geology and geophysics at the University of Wisconsin-Madison, who is the author of the new report published in the latest issue of the journal Nature.

In short, says the professor, changes in ocean environments related to sea level exert a driving influence on rates of extinction, which animals and plants survive or vanish, and generally determine the composition（结构） of life in the oceans.

Since the advent of life on Earth 3.5 billion years ago, scientists think there may have been as many as 23 mass extinction events. During the past 540 million years, there have been five well-documented mass extinctions, primarily of marine plants and animals, with as many as 75～ 95 percent of species lost.

For the most part, scientists have been unable to pin down the causes of such dramatic events. In the case of the disappearance of the dinosaurs, scientists have a smoking gun, an impact crater（火山口） that suggests dinosaurs were wiped out as the result of a large asteroid（行星） crashing into the planet. But the causes of other mass extinction events have been murky（模糊的） , at best.

Arnold Miller, a professor of geology at the University of Cincinnati, says the new study is striking because it establishes a clear relationship between the tempo of mass extinction events and changes in sea level and sediment.

The new Wisconsin study, Peters says, does not preclude other influence upon extinction such as physical events like volcanic eruptions or killer asteroids, or biological influences such as disease and competition among species. But what it does do, he argues, is to provide a common link for mass extinction events over a significant stretch of Earth history.