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1、选修7U20李仕才一、阅读理解AOldFossilsScientistshavefoundwhattheythinkisprobablytheoldestfossilonEarth,aremnantoflifefrom3.7billionyearsagowhenEarth’sskieswereorangeanditsoceansgreen.InanewlymeltedpartofGreenland,Australianscientistsfoundtheleftoverstructurefromacommunityof
2、microbes（微生物）thatlivedonanancientseafloor.Basedontheiranalysisofthefossils,theresearchersdeterminedthattheyare220millionyearsolderthanthosediscoveredinWesternAustralia,whichwere3.48billionyearsold.Thediscoveryshowslifemayhaveformedquickerandmoreeasilythanoncetho
3、ught,abouthalfabillionyearsafterEarthformed.Andthatmayalsogivehopeforlifeformingelsewhere,suchasMars,saidstudyco-authorMartinVanKranendonkoftheUniversityofNewSouthWalesanddirectoroftheAustralianCenterforAstrobiology.＂Itgivesusanideahowourplanetevolvedandhowlifeg
4、ainedafoothold,＂VanKranendonksaid.ScientistshadthoughtitwouldtakeatleasthalfabillionyearsforlifetoformafterthemoltenEarthstartedtocoolabit,butthisshowsitcouldhavehappenedquicker,hesaid.That’sbecausethenewlyfoundfossilisfartoocomplextohavedevelopedsoonafterthepla
5、net’sfirstlifeforms,hesaid.Inanoutcropofrocksthatusedtobecoveredwithiceandsnowwhichmeltedafteranexceptionallywarmspring,theAustralianteamfoundstromatolites（叠层石）,whicharelayeredstructuresthatareoftenproducedbyacommunityofmicrobes.Thestromatoliteswereabout1to4cent
6、imetershigh.＂Itislikethehouseleftbehindmadebythemicrobes,＂VanKranendonksaid.＂Scientistsusedthelayersofashfromvolcanoesandtinyzircon（锆石）withuraniumandestimatedthattheydatebackto3.7billionyearsago.＂Thedatingseemsaboutright,saidAbigailAllwood,aNASAastrobiologistwho
7、foundthepreviousoldestfossil,from3.48billionyearsago,inAustralia.ButAllwoodsaidsheisnotcompletelyconvincedthatwhatVanKranendonk’steamfoundoncewasalive.Shesaidtheevidencewasn’tconclusiveenoughthatitwaslifeandnotageologicquirk（地质巧合）.＂Itwouldbenicetohavemoreevidenc
8、e,butintheserocksthat’salottoask,＂Allwoodsaidinanemail.1.Theunderlinedwords＂gainedafoothold＂inParagraph3mean______.A.continuedB.changedC.increasedD.Started2.Wecanlear
