人教版英语必修3-unit4--5课本电子版

HOWLIFEBEGANONTHEEARTHNooneknowsexactlyhowtheearthbegan,asithappenedsolongago.However,accordingtoawidelyacceptedtheory,theuniversebeganwithaBigBangthatthrewmatterinalldirections.Afterthat,atomsbegantoformandcombinetocreatestarsandotherbodies.ForseveralbillionyearsaftertheBigBang,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.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,sowefeelasifthereisnogravityatall.Whenwegetclosertothemoon,weshallfeelitsgravitypullingus,butitwillnotbeasstrongapullastheearth's.Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger.Whenwegotthere,Iwantedtoexploreimmediately.Comeon,Isaid.Ifyouareright,mymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforward,IfoundIwascarriedtwiceasfarasontheearthandfellover.Ohdear,Icried,walkingdoesneedabitofpracticenowthatgravityhaschanged.AfterawhileIgotthehangofitandwebegantoenjoyourselves.Leavingthemoon'sgravitywasnotaspainfulasleavingtheearth's.Butreturningtotheearthwasveryfrightening.Wewatched,amazedasfirebrokeoutontheoutsideofthespaceshipastheearth'sgravityincreased.Againwewerepushedhardintoourseatsaswecamebacktoland.Thatwasveryexhaustingbutveryexcitingtoo,Isaid.NowIknowmuchmoreaboutgravity!Doyouthinkwecouldvisitsomestarsnexttime?Ofcourse,hesmiled,whichstarwouldyouliketogoto?CAUGHTBYABLACKHOLEThespaceshipwarnedLiYanpingandmetobereadyaswewereapproachingtheblackhole.Wecouldseesomefaintlightsaroundwhatlookedlikeanemptyspace.LiYanpingsaid,Thatemptyholeislikeamouthalwaysneedingtobefed.Thoselightsarethingsthatarebeingpulledintoitbythegravityofthehole.Wewatchedwithinterestandamazementatthelightswhichseemedtobegoingfainterandfainterroundtheedgeoftheblackhole.Justthenthelightsonourspaceshipwentoutandthecomputerstoppedworking.Whatwashappening?Itriedtoturnthespaceshipawayfromtheholebutitwouldnotmove.Woulditeatustoo?Suddenlythespaceshipjumpedandbegantomoveroundtheedgeoftheholetoo.Thegravityoftheblackholewaspullingusintoitsmouth.Asthespaceshipmovedroundtheblackhole,Ifeltrathersick.MymouthwentdryandIclosedmyeyes.Allthestoriesaboutwhathappenedwhenyouwerecaughtbythegravityofablackholeseemedtobetrue.Butthenthespaceshipjumpedagain.Thistimeitmovedsidewaysanda