【高中英语必修三第四单元第一篇课文原文】-查字典问答网
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  【高中英语必修三第四单元第一篇课文原文】

  高中英语必修三第四单元第一篇课文原文

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2019-10-29 03:42
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何玉

  HOWLIFEBEGANONTHEEARTH

  Nooneknowsexactlyhowtheearthbegan,asithappenedsolongago.However,accordingtoawidelyacceptedtheory,theuniversebeganwitha"BigBang"thatthrewmatterinalldirections.Afterthat,atomsbegantoformandcombinetocreatestarsandotherbodies.

  Forseveralbillionyearsafterthe"BigBang",theearthwasstilljustacloudofdust.Whatitwastobecomewasuncertainuntilbetween4.5and3.8billionyearsagowhenthedustsettledintoasolidglobe.Theearthbecamesoviolentthatitwasnotclearwhethertheshapewouldlastornot.Itexplodedloudlywithfireandrock.Theywereintimetoproducecarbon,nitrogen,watervapourandothergases,whichweretomaketheearth'satmosphere.Whatisevenmoreimportantisthatastheearthcooleddown,waterbegantoappearonitssurface.

  WaterhadalsoappearedonotherplanetslikeMarsbut,unliketheearth,ithaddisappearedlater.Itwasnotimmediatelyobviousthatwaterwastobefundamentaltothedevelopmentoflife.Whatmanyscientistsbelieveisthatthecontinuedpresenceofwaterallowedtheearthtodissolveharmfulgasesandacidsintotheoceansandseas.Thisproducedachainreaction,whichmadeitpossibleforlifetodevelop.

  Manymillionsofyearslater,thefirstextremelysmallplantsbegantoappearonthesurfaceofthewater.Theymultipliedandfilledtheoceansandseaswithoxygen,whichencouragedthelaterdevelopmentofearlyshellfishandallsortsoffish.Next,greenplantsbegantogrowonland.Theywerefollowedintimebylandanimals.Somewereinsects.Others,calledamphibians,wereabletoliveonlandaswellasinthewater.Laterwhentheplantsgrewintoforests,reptilesappearedforthefirsttime.Theyproducedyoungentirelybylayingeggs.Afterthat,somehugeanimals,calleddinosaurs,developed.Theylaideggstooandexistedontheearthformorethan140millionyears.However,65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappearedstillremainsamystery.Thisdisappearancemadepossibletheriseofmammalsontheearth.Theseanimalsweredifferentfromalllifeformsinthepast,becausetheygavebirthtoyoungbabyanimalsandproducedmilktofeedthem.

  Finallyabout2.6millionyearsagosomesmallcleveranimals,nowwithhandsandfeet,appearedandspreadallovertheearth.Thustheyhave,intheirturn,becomethemostimportantanimalsontheplanet.Buttheyarenotlookingaftertheearthverywell.Theyareputtingtoomuchcarbondioxideintotheatmosphere,whichpreventsheatfromescapingfromtheearthintospace.Asaresultofthis,manyscientistsbelievetheearthmaybecometoohottoliveon.Sowhetherlifewillcontinueontheearthformillionsofyearstocomewilldependonwhetherthisproblemcanbesolved.

  给分.

2019-10-29 03:45:05

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