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Don: Time now for another episode of a Moment of Science. Did you know? So Yael, did you know that there’ve been 23 mass extinctions since life begun on earth.
唐：又到了我们的科学时刻了。耶尔，你知道吗？自从地球出现以来，她已经历了23次大规模的物种灭绝。
Yale: I know there’ve been a few extinctions, Don, but not that many.
耶尔：唐，我知道地球确实经历了几次物种灭绝，但没你说的那么多。
Don: Yep, many were prehistoric bacteria and other single-cell microorganisms, but in the past 540 million years or so, there’ve been about 5 mass extinctions. Many are marine plants and animals but also off-land organisms.
唐：是的。远古时候，地球上存在着大量的细菌和其它单细胞的微生物，但在过去的5亿4000万年内，地球经历了5次大灭绝，灭绝的大多是海洋植物和海洋动物，但也有一些陆地生物。
Yale: I know that the dinosaurs disappeared about 65 million years ago possibly due to a large meteor crushing into earth and block out the sun. But what cause all those other extinctions?
耶尔：嗯，我听说恐龙是在大约6千500万年前灭绝的，可能是流星撞击地球之后太阳光被遮挡造成的。其它几次灭绝是什么原因？
Don: That’s been a mystery for a long time. But now some scientists think that most mass extinctions have been driven by the rise and fall of sea level. That may sound strange. But consider that millions and millions years ago, a shallow sea covered the entire middle section of North America. That sea grew and shrank back several times. Each time it drained, it caused entire species of sea plants and creatures to die off. The last time it drained was around 65 million years ago when the dinosaurs disappeared.
唐：那是一个埋藏许久的谜团。但是，现在的一些科学家认为，是海平面的上升或下降造成了这些大灭绝。这听起来可能有点奇怪。但是，考虑到亿万年前覆盖整个北美大陆中部的只是一层浅海，这片海域几经上升和回落。每次干涸，它都会造成整个海洋植物和动物的消亡。它的最后一次干涸出现在6千500万年前，正是在那个时候，恐龙彻底灭亡了。
Yale: OK, I understand that was sea vanishing would kill of the marine life. But did the sea’s disappearing also have something to do with the dinosaurs going extinct?
耶尔：好吧，我知道了海洋消失的确会使海洋生物灭绝，但是也能使恐龙灭绝吗？
Don: Maybe. When the sea disappears, it has big effect on climate. All that water provides heat and moisture. And when the sea drains, the climate becomes drier and colder. So the changing climate may have combine with meteor impact to kill off the dinosaurs.
唐：也许是这样的。当海洋消失时，会对气候产生巨大影响。海水可以提供热量和湿气。但是当海洋干涸时，气候会变得干燥和寒冷。因此，气候变化和流星撞击地球可能是恐龙灭绝的元凶。
