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1、oneknowsexactlyhowtheearthbegan,asithappenedsolongago.However,accordingtoawidelyacceptedtheory,theuniversebeganwitha“BigBang“thatthrewmatterinalldirections.Afterthat,atomsbegantoformandcombingtocreatestarsandotherbodies. Forseveralbillionyearsafterthe“BigBang“,theearthwasstilljustacloudofdust.Whati
2、twastobecomewasuncertainuntilbetween4.5and3.8billionyearsagowhenthedustsettledintoasolidglobe.Theearthenwarebecamesoviolentthatitwasnotclearwhethertheshapewouldlastornot.Itexplodedloudlywithfireandrock.Theywereintimetoproducecarbon,nitrogen,watervapourandothergases,whichweretomaketheearth’satmosphe
3、re.Whatisevenmoreimprtantisthatastheearthcooleddown,waterbegantoappearonitssurface. WaterhadalsoappearedonotherplanetslikeMarsbut,unliketheearth,ithaddisappearedlater.Itwasnotimmediatelyobviousthatwaterwastobefundamentaltothedevelopmentoflife.Whatmanyscientistsbelieveisthatthecontinuedpersenceofwat
4、erallowedtheearthtodissolveharmfulgasesandacidsintotheoceansandseas.Thisproducedachainreaction,whichmadeitpossibleforlifetodevelop. Manymillionsofyearslater,thefirstextremelysmallplantsbegantoappearonthesurfaceofthewater.Theymultipliedandfilledtheoceansandseaswithoxygen,whichencouragedthelaterdevel
5、opmentofearlyshellfishandallsortsoffish.Next,greenplantsbegantogrowonland.Theywerefollowedintimebylandanimals.Somewereinsects.Others,calledamphibians,wereabletoliveonlandaswellasinthewater.Laterwhentheplantsgrewintoforests,reptilesappearedforthefirsttime.Theyproducedyounggenerallybylayingeggs.After
6、that,somehugeanimals,calleddinosaurs,developed.Theylaideggstooandexistedontheearthformorethan140millionyears.However,65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappearedstillremainsamystery.Thisdisappearancemadepossiblethiefriseofmammalsontheearth.Theseanimalsweredifferentfromallli
7、feformsinthepast,becausetheyGavinbirthtoyoungbabyanimalsandprodecedmilktofeedthem. Finnallyabout2.6millionyearsagosomesmallcelveranimals,nowwithhandsandfeet,appearedandspreadallovertheearth.Thustheyhave,int
