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They know by studying the remains of past lifepreserved in rocks, particularly sedimentary rocks.
研究人員在研究巖石特別是沉積巖中保存著的生物遺骸時找到了證據。
These rocks are formed from particles of mudand sand, typically when they settle to the bottom of a body of water.
這些巖石是由泥土和沙礫沉積在水底形成的。
Fossils of organisms withhard body parts don't appear in sedimentary rock older than about 600 million years,
生物體中較硬的身體部分所形成的化石不會出現在6億年以上的沉積巖中，
but theEarth is estimated to be 4.6 billion years old.
但是地球預計已經有46億年之久了。
To find the oldest life, paleontologists looked for it in the oldest sedimentary rock they could find.
為了尋找最古老的生命，古生物學者找到了他們可以找到的最古老的沉積巖。
Sedimentary rocks dating back to Earth's earliest days are rare.
這些可以追溯到形成于地球初期的沉積巖是十分罕見的。
Some of the oldest known areabout three and a half billion years old.
目前所知的一些最古老的沉積巖大約有35億年的歷史。
In these very old rocks, paleontologists have found something that looked familiar.
在這些古老的巖石中，古生物學者發現了一些熟悉的東西。
Certaincommunities of microbes can form layered, rock-like mounds called stromatolites.
某些菌落可以形成一層一層，巖石狀的土丘，被叫做疊層石。
Stromatolitesgrow in shallow bodies of water in many places around the world.
世界上有很多地方的淺水區都存在著疊層石。
In the 1980′s paleontologistsfound the remains of ancient stromatolites in 3.5 billion year old sedimentary rocks.
在20世紀80年代的時候，古生物學家在35億年的沉積巖里發現了古疊層石的遺骸。
Besides stromatolites, possible microscopic fossils have been found in rocks up to 3.5 billion yearsold.
除了疊層石外，其他目前在巖石中已經找到的可能的微觀化石已經有長達35億年的歷史了。
While the origin of these tiny structures is still intensely debated, many scientists areconvinced that they are the remains of microorganisms.
雖然關于這些微小結構的起源還存在著嚴重的分歧，但很多科學家已經確認，他們是微生物的殘骸。
It appears that by at least 3.5 billion yearsago, the oceans were teeming with life, and that the land was also colonized by microbes veryearly in Earth's history.
似乎在地球演變的早期過程中，至少在35億年以前，海洋里存在著大量生體命，而陸地是微生物的主要棲息地。
