They know by studying the remains of pastlifepreserved 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(jù)。
These rocks are formed from particles of mudandsand, typically when they settle to the bottom of abody 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'tappear in sedimentary rock older than about 600million years,
生物體中較硬的身體部分所形成的化石不會出現(xiàn)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(yù)計已經(jīng)有46億年之久了。
To find the oldest life, paleontologists looked for it in the oldest sedimentary rock they couldfind.
為了尋找最古老的生命，古生物學(xué)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(xué)者發(fā)現(xiàn)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.
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(jù)。
These rocks are formed from particles of mudandsand, typically when they settle to the bottom of abody 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'tappear in sedimentary rock older than about 600million years,
生物體中較硬的身體部分所形成的化石不會出現(xiàn)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(yù)計已經(jīng)有46億年之久了。
To find the oldest life, paleontologists looked for it in the oldest sedimentary rock they couldfind.
為了尋找最古老的生命，古生物學(xué)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(xué)者發(fā)現(xiàn)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.