高一英语人教版必修三Unit-4-课文内容

Unit4Astronomy:thescienceofthestarsHOWLIFEBEGANONTHEEARTHNooneknowsexactlyhowtheearthbegan,asithappenedsolongago.However,accordingtoawidelyacceptedtheory,theuniversebeganwitha“BigBang”thatthrewmatterinalldirections.Afterthat,atomsbegantoformandcombinetocreatestarsandotherbodies.Forseveralbillionyearsafterthe“BigBang”,theearthwasstilljustacloudofdust.Whatitwastobecomewasuncertainuntilbetween4.5and3.8billionyearsagowhenthedustsettledintoasolidglobe.Theearthbecamesoviolentthatitwasnotclearwhethertheshapewouldlastornot.Itexplodedloudlywithfireandrock.Theywereintimetoproducecarbondioxide,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.Theyproducedyounggenerallybylayingeggs.Afterthat,somehugeanimals,calleddinosaurs,developed.Theylaideggstooandexistedontheearthformorethan140millionyears.However,65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappearedstillremainsamystery.Thisdisappearancemadepossibletheriseofmammalsontheearth.Theseanimalsweredifferentfromalllifeformsinthepast,becausetheygavebirthtoyoungbabyanimalsandproducedmilktofeedthem.Finallyabout2.6millionyearsagosomesmallcleveranimals,nowwithhandsandfeet,appearedandspreadallovertheearth.Thustheyhave,intheirturn,becomethemostimportantanimalsontheplanet.Buttheyarenotlookingaftertheearthverywell.Theyareputtingtoomuchcarbondioxideintotheatmosphere,whichpreventsheatfromescapingfromtheearthintospace.Asaresultofthis,manyscientistsbelievetheearthmaybecometoohottoliveon.Sowhetherlifewillcontinueontheearthformillionsofyearstocomewilldependonwhetherthisproblemcanbesolved.AVISITTOTHEMOONLastmonthIwasluckyenoughtohaveachancetomakeatripintospacewithmyfriendLiYanping,anastronomer.Wevisitedthemooninourspaceship!Beforeweleft,LiYanpingexplainedtomethattheforceofgravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheair,wewerepushedbackintoourseatsbecauseweweretryingtoescapethepulloftheearth’sgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.“Whyisthespaceshipnotfallingbacktotheearth?OntheearthifIfallfromatreeIwillfalltotheground.”Iasked.“Wearetoofarfromtheearthnowtofeelitspull,”heexplained,“sowefeelasifnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth’s.”Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger.Whenwegotthere,Iwantedtoexploreimmediately.“Comeon,”Isaid.“Ifyouareright,myweightwillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!”Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforward.IfoundIwascarriedtwiceasfarasontheearthandfellover.“Ohdear,”Icried,“walkingdoesneedabitofpracticenowthatgravityhaschanged.”AfterawhileIgotthehangofitandwebegantoenjoyourselves.Leavingthemoon’sgravitywasnotaspainfulasleavingtheearth’s.Butreturningtotheearthwasveryfrightening.Wewatched,amazedasfirebrokeoutontheoutsideofthespaceshipastheearth’sgravityincreased.Againwewerepushedhardintoourseatsaswecamebacktoland.“Thatwasveryexhaustingbutveryexcitingtoo,”Isaid.“NowIknowmuchmoreaboutgravity!Doyouthinkwecouldvisitsomestarsnexttime?”“Ofcourse,”hesmiled,“whichstarwouldyouliketogoto?”