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Don: Time now for another episode of A Moment of Science's "Did You Know..." So, Yael, did you know that there have been about twenty-three mass extinctions since life began on earth?
唐：現在進入科學時刻另一段情節：“你知道...”那么，雅艾爾，你知道嗎，地球上自有生命以來發生過23起物種大滅絕事件？
Yael: I know there've been a few extinctions, Don, but not that many!
雅艾爾：我知道幾次物種大滅絕，但不知道次數如此多！
Don: Yep. Many were of prehistoric bacteria and other single-celled microorganisms. But in the past 540 million years or so there have been about five mass extinctions, mainly of marine plants and animals but also of land organisms.
唐：是啊。許多都是史前的細菌或其它單細胞微生物。但是在5.4億年前已發生過5次大滅絕，主要包括海生植物和動物，當然也有陸生生物。
Yael: Well, I know that the dinosaurs disappeared about 65 million years ago, possibly due to a large meteor that crashed into earth and blotted out the sun. But what caused all those other extinctions?
雅艾爾：好吧，我知道恐龍大約在六千五百萬年前消失，可能是由于一顆大流星撞擊地球，遮住陽光引起的。但是什么導致其他物種滅絕的呢？
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 of 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：長期以來那一直是個謎。但如今一些科學家們認為大多數的滅絕是由于海平面的上升和下降引起的。這也許聽起來很奇怪，但考慮到百萬年前有一片淺海覆蓋在整個北美洲中部就不足為奇了。那片海歷經幾次擴張和消失，每一次干涸就會導致這個海洋的生物滅絕。最后一次干涸發生在6500萬年前恐龍消失的時候。
Yael: OK, I understand how a sea vanishing would kill off marine life. But did the sea's disappearance also have something to do with the dinosaurs going extinct?
雅艾爾：好吧，我明白了海洋的消失會引起海洋生物消失。但海洋的消失與恐龍的滅絕也有聯系嗎？
Don: Maybe. When a sea disappears it has a big effect on climate. All that water provides heat and moisture. And when a sea drains, the climate becomes drier and colder. So the change in climate may have combined with the meteor impact to kill off the dinosaurs.
唐：也許。海洋消失會對氣候產生深遠的影響。海水能提供熱量和濕度。當海水干枯時，氣候就會變得更加干燥和寒冷。因此氣候變化再加上流星撞擊地球足以將恐龍推向毀滅。
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