how-life-began-on-the-earth-文本

第1页共2页ReadingTitle:____________________________1.Nooneknowsexactlyhowtheearthbegan,asithappenedsolongago.However,accordingtoawidelyacceptedtheory,theuniversebeganwithaBigBangthatthrewmatterinalldirections.Afterthat,atomsbegantoformandcombinetocreatestarsandotherbodies.2.ForseveralbillionyearsaftertheBigBang,theearthwasstilljustacloudofdust.Whatitwastobecomewasuncertainuntilbetween4.5and3.8billionyearsagowhenthedustsettledintoasolidglobe.Theearthbecamesoviolentthatitwasnotclearwhethertheshapewouldlastornot.Itexplodedloudlywithfireandrock.Theywereintimetoproducecarbon,nitrogen,watervapourandothergases,whichweretomaketheearth'satmosphere.Whatisevenmoreimportantisthatastheearthcooleddown,waterbegantoappearonitssurface.3.WaterhadalsoappearedonotherplanetslikeMarsbut,unliketheearth,ithaddisappearedlater.Itwasnotimmediatelyobviousthatwaterwastobefundamentaltothedevelopmentoflife.Whatmanyscientistsbelieveisthatthecontinuedpresenceofwaterallowedtheearthtodissolveharmfulgasesandacidsintotheoceansandseas.Thisproducedachainreaction,whichmadeitpossibleforlifetodevelop.4.Manymillionsofyearslater,thefirstextremelysmallplantsbegantoappearonthesurfaceofthewater.They第2页共2页multipliedandfilledtheoceansandseaswithoxygen,whichencouragedthelaterdevelopmentofearlyshellfishandallsortsoffish.Next,greenplantsbegantogrowonland.Theywerefollowedintimebylandanimals.Somewereinsects.Others,calledamphibians,wereabletoliveonlandaswellasinthewater.Laterwhentheplantsgrewintoforests,reptilesappearedforthefirsttime.Theyproducedyoungenerallybylayingeggs.Afterthat,somehugeanimals,calleddinosaurs,developed.Theylaideggstooandexistedontheearthformorethan140millionyears.However,65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappearedstillremainsamystery.Thisdisappearancemadepossibletheriseofmammalsontheearth.Theseanimalsweredifferentfromalllifeformsinthepast,becausetheygavebirthtoyoungbabyanimalsandproducedmilktofeedthem.5.Finallyabout2.6millionyearsagosomesmallcleveranimals,nowwithhandsandfeet,appearedandspreadallovertheearth.Thustheyhave,intheirturn,becomethemostimportantanimalsontheplanet.Buttheyarenotlookingaftertheearthverywell.Theyareputtingtoomuchcarbondioxideintotheatmosphere,whichpreventsheatfromescapingfromtheearthintospace.Asaresultofthis,manyscientistsbelievetheearthmaybecometoohottoliveon.Sowhetherlifewillcontinueontheearthformillionsofyearstocomewilldependonwhetherthisproblemcanbesolved.