原新闻网址： http://www.nytimes.com/2011/02/15/science/15obfrog.html?ref=science 原文: Dollo’s Law, a theory proposed by the scientist Louis Dollo in the 1800s, states that when a particular trait is lost in a species, it never comes back. Dollo这位科学家曾在1800年提供了一套理论，说明了当一个物种的特徵 若是消失了，此特徵将不会再出现。 It’s one explanation for why humans no longer have tails, birds and turtles are toothless and snakes have stayed limbless. 这也是说为什麽人们已经不会有尾巴，鸟类与乌龟没有牙齿，蛇维持着没有 脚。 But a new analysis, done by a researcher at Stony Brook University, found that while frogs lost teeth in the lower jaw at least 200 million years ago, a particular type of marsupial tree frog regained those lower teeth about 20 million years ago. 但是最近有一份新的研究指出，在两亿年前青蛙已经失去了下颚的牙齿，但是 有一种特殊型态的树蛙却在两千万年前再次获得牙齿。 “It’s a very clear-cut case of re-evolution because of the large time span,” said John Wiens, the Stony Brook biologist who authored the paper in the journal Evolution. 这将是一个非常明显的再次演进，因为有非常大的time span。(time span不知道 如何翻译?) Dr. Wiens analyzed DNA samples of 170 modern and fossilized frogs to approximate the dates of loss and re-evolution of the teeth. Dr.Wiens分析一百七十种近代的DNA试片(觉得有点怪怪的翻译!!)，与化石蛙 去估计失去牙齿的日期与观察牙齿的再次演进。 Most frogs have teeth on their upper jaws, which may have made the re-evolution in the tree frog, known as Gastrotheca guentheri, easier, Dr. Wiens said. 大多数的青蛙在上颚都有牙齿，这样会促使树蛙在下颚也衍生出牙齿。 “They already had teeth in the upper jaw, so they had the enamel, dentine and other necessities,” he said. “There was a way to facilitate new teeth after 200 million years.” 这些青蛙在上颚已经有了牙齿，因此他们拥有了砝郎质、象牙质，还有其他 的材料。两亿年後，有一个方法让他们重新配置他们的牙齿。 The species is the only known modern frog species with lower teeth, though certain other species with upper teeth do have toothlike structures on the lower jaw. 这些拥有下颚牙齿特徵的蛙只出现在近代的青蛙上，其他拥有上颚牙齿的 青蛙并不一定会有此特徵。 “That’s a big question now: What’s keeping the other frogs from developing real teeth on the lower jaw?” Dr. Wiens said. 问题在於，甚麽会让某些品种的青蛙在下颚无法长出牙齿? 阅读获得的心单字: enamel:砝琅质 dentine:象牙质 应该还有很多要修正地方，请各位大大多给建议。 --

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