1、掌握英语，从听开始 中国最受欢迎英语学习网站 Searching For Earths Oldest LifeHave you wondered why it is scientists have concluded that our planet is billions of years old?Sedimentary FossilsOne of the reasons is what researchers have discovered studying the remains of past life preserved in rocks, particularly sedimenta
2、ry rocks. These rocks are formed from particles of mud and sand, typically when they settle to the bottom of a body of water. Fossils of organisms with hard body parts dont appear in sedimentary rock older than about 600 million years, but the Earth is estimated to be 4.6 billion years old.To find t
3、he oldest life, paleontologists looked for it in the oldest sedimentary rock they could find. Sedimentary rocks dating back to Earths earliest days are rare. Some of the oldest known are about three and a half billion years old.StromatolitesIn these very old rocks, paleontologists have found somethi
4、ng that looked familiar. Certain communities of microbes can form layered, rock-like mounds called stromatolites. Stromatolites grow in shallow bodies of water in many places around the world. In the 1980s paleontologists found the remains of ancient stromatolites in 3.5 billion year old sedimentary
5、 rocks.Besides stromatolites, possible microscopic fossils have been found in rocks up to 3.5 billion years old. While the origin of these tiny structures is still intensely debated, many scientists are convinced that they are the remains of microorganisms. It appears that by at least 3.5 billion ye
6、ars ago, the oceans were teeming with life, and that the land was also colonized by microbes very early in Earths 译文：你有没有想过为什么科学家会认为地球已经有数十亿年的历史了？沉积化石理由之一便是研究人员在研究岩石特别是沉积岩中保存着的生物遗骸时找到了证据。这些岩石是由泥土和沙砾沉积在水底形成的。生物体中较硬的身体部分掌握英语，从听开始 中国最受欢迎英语学习网站 所形成的化石不会出现在6亿年以上的沉积岩中，但是地球预计已经有46亿年之久了。为了寻找最古老的生命，古生物学者找到了他
7、们可以找到的最古老的沉积岩。这些可以追溯到形成于地球初期的沉积岩是十分罕见的。目前所知的一些最古老的沉积岩大约有35亿年的历史。叠层石在这些古老的岩石中，古生物学者发现了一些熟悉的东西。某些菌落可以形成一层一层，岩石状的土丘，被叫做叠层石。世界上有很多地方的浅水区都存在着叠层石。在20世纪80年代的时候，古生物学家在 35亿年的沉积岩里发现了古叠层石的遗骸。除了叠层石外，其他目前在岩石中已经找到的可能的微观化石已经有长达35亿年的历史了。虽然关于这些微小结构的起源还存在着严重的分歧，但很多科学家已经确认，他们是微生物的残骸。似乎在地球演变的早期过程中，至少在35亿年以前，海洋里存在着大量生体命
8、，而陆地是微生物的主要栖息地。【本译文由普特网友 kevinde 提供】PS：西澳大利亚皮尔巴拉地区发现了至今为止最久远的生命证据。美国与澳大利亚的研究者们在皮尔巴拉的沉积岩中，发现了距今约34.5亿年前的复杂的细菌生态系统的痕迹。这项发现又将地球上的生命证据提前了数百万年。微生物和细菌喜欢集群生活，人类胃里的细菌群就是一例。细菌会根据环境中不同的化学梯度分层次生活，例如喜欢光的细菌生活在上层，而厌光的生活在下层。皮尔巴拉地区有着地球上最古老且最无损的沉积岩，研究者们在这些沉积岩中发现了菌群的痕迹。35亿年前的地球和现在差别很大，气温和海平面都远高于今天的水平。在更复杂的生命形式被进化出来前，诸如皮尔巴拉地区的菌群这样的细菌是地球上最为先进的生命形式，它们对地球的统治长达十多亿年。那时，地球上多数是海洋，火山活动频繁，空气中充满硫磺。而直到25亿年前，真核生物才刚刚开始进化出来，然后再经过漫长的岁月，才进化出多细胞生物直至动物和人类。科学公园
