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必修3Unit4HOWLIFEBEGANONTHEEARTHNooneknowsexactlyhowtheearthbeganasithappenedsolongago.HoweveraccordingtoawidelyacceptedtheorytheuniversebeganwithaBigBangthatthrewmatterinalldirections.Afterthatatomsbegantoformandcombinetocreatestarsandotherbodies.ForseveralbillionyearsaftertheBigBangtheearthwasstilljustacloudofdust.Whatitwastobecomewasuncertainuntilbetween
4.5and
3.8billionyearsagowhenthedustsettledintoasolidglobe.Theearthbecamesoviolentthatitwasnotclearwhethertheshapewouldlastornot.Itexplodedloudlywithfireandrock.Theywereintimetoproducecarbonnitrogenwatervapourandothergaseswhichweretomaketheearthsatmosphere.Whatisevenmoreimportantisthatastheearthcooleddownwaterbegantoappearonitssurface.WaterhadalsoappearedonotherplanetslikeMarsbutunliketheearthithaddisappearedlater.Itwasnotimmediatelyobviousthatwaterwastobefundamentaltothedevelopmentoflife.Whatmanyscientistsbelieveisthatthecontinuedpresenceofwaterallowedtheearthtodissolveharmfulgasesandacidsintotheoceansandseas.Thisproducedachainreactionwhichmadeitpossibleforlifetodevelop.Manymillionsofyearslaterthefirstextremelysmallplantsbegantoappearonthesurfaceofthewater.Theymultipliedandfilledtheoceansandseaswithoxygenwhichencouragedthelaterdevelopmentofearlyshellfishandallsortsoffish.Nextgreenplantsbegantogrowonland.Theywerefollowedintimebylandanimals.Somewereinsects.Otherscalledamphibianswereabletoliveonlandaswellasinthewater.Laterwhentheplantsgrewintoforestsreptilesappearedforthefirsttime.Theyproducedyoungenerallybylayingeggs.Afterthatsomehugeanimalscalleddinosaursdeveloped.Theylaideggstooandexistedontheearthformorethan140millionyears.However65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappearedstillremainsamystery.Thisdisappearancemadepossibletheriseofmammalsontheearth.Theseanimalsweredifferentfromalllifeformsinthepastbecausetheygavebirthtoyoungbabyanimalsandproducedmilktofeedthem.Finallyabout
2.6millionyearsagosomesmallcleveranimalsnowwithhandsandfeetappearedandspreadallovertheearth.Thustheyhaveintheirturnbecomethemostimportantanimalsontheplanet.Buttheyarenotlookingaftertheearthverywell.Theyareputtingtoomuchcarbondioxideintotheatmospherewhichpreventsheatfromescapingfromtheearthintospace.Asaresultofthismanyscientistsbelievetheearthmaybecometoohottoliveon.Sowhetherlifewillcontinueontheearthformillionsofyearstocomewilldependonwhetherthisproblemcanbesolved.AVISITTOTHEMOONLastmonthIwasluckyenoughtohaveachancetomakeatripintospacewithmyfriendLiYanpinganastronomer.Wevisitedthemooninourspaceship!BeforeweleftLiYanpingexplainedtomethattheforceofgravitywouldchangethreetimesonourjourneyandthatthefirstchangewouldbethemostpowerful.Thenwewereoff.Astherocketroseintotheairwewerepushedbackintoourseatsbecauseweweretryingtoescapethepulloftheearthsgravity.Itwassohardthatwecouldnotsayanythingtoeachother.GraduallytheweightlessenedandIwasabletotalktohim.WhyisthespaceshipnotfallingbacktotheearthOntheearthifIfallfromatreeIwillfalltotheground.Iasked.Wearetoofarfromtheearthnowtofeelitspullheexplainedsowefeelasifthereisnogravityatall.Whenwegetclosertothemoonweshallfeelitsgravitypullingusbutitwillnotbeasstrongapullastheearths.Icheeredupimmediatelyandfloatedweightlesslyaroundinourspaceshipcabinwatchingtheearthbecomesmallerandthemoonlarger.WhenwegotthereIwantedtoexploreimmediately.ComeonIsaid.IfyouarerightmymasswillbelessthanontheearthbecausethemoonissmallerandIwillbeabletomovemorefreely.ImightevengrowtallerifIstayherelongenough.Ishallcertainlyweighless!Ilaughedandclimbeddownthestepsfromthespaceship.ButwhenItriedtostepforwardIfoundIwascarriedtwiceasfarasontheearthandfellover.OhdearIcriedwalkingdoesneedabitofpracticenowthatgravityhaschanged.AfterawhileIgotthehangofitandwebegantoenjoyourselves.Leavingthemoonsgravitywasnotaspainfulasleavingtheearths.Butreturningtotheearthwasveryfrightening.Wewatchedamazedasfirebrokeoutontheoutsideofthespaceshipastheearthsgravityincreased.Againwewerepushedhardintoourseatsaswecamebacktoland.ThatwasveryexhaustingbutveryexcitingtooIsaid.NowIknowmuchmoreaboutgravity!DoyouthinkwecouldvisitsomestarsnexttimeOfcoursehesmiledwhichstarwouldyouliketogoto。