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1、必修3Unit4HOWLIFEBEGANONTHEEARTHNooneknows1.（确切地）howtheearthbegan,2.ithappenedsolongago.However,3.（根据）awidely』（accept）theory,theuniverse5.（起源于）a”BigBang”thatthrewmatter6.7.（从那以后）,atomsbegan&（fonn）and9.（结合成）createstarsandotherbodies.Forseveral10.（十亿）yearsafterthenBigBang11,theeart
2、hwasstilljustll.（—团尘埃）.12.itwastobecomewasuncertainuntilbetween4.5and3.8billionyearsagowhenthedust13.（慢慢地形成）asolidglobe.Theearthbecamesoviolent14.itwasnotclear15.theshapewouldlastornot.It16.（爆炸）loudlywithfireandrock.Theywere17.（及时/终于）toproduce18.（碳），nitrogen,watervapourandother
3、gases,19.weretomaketheearth'satmosphere•Whatisevenmoreimportantis20.astheearth21.（冷却），waterbegantoappearonitssurface.WaterhadalsoappearedonotherplanetslikeMarsbut,22.theeartth,it23-（disappear）later.Itwasnot24.（immediate）obviousthatwaterwastobe25.（fundament）tothedevelopmentoflif
4、e.Whatmanyscientistsbelieveisthatthe26.（continue）presenceofwater27.（allow）theearthtodissolve2&（harm）gasesandacidsinto29•（海洋）.Thisproducedachainreaction,whichmade30.possibleforlifetodevelop.Manymillionsofyears31.（以后），thefirstextremelysmallplantsbegan32.（appear）onthesurfaceofthew
5、ater.They33.（繁殖）andfilledtheoceansandseaswith34.（氧气），which35.（鼓励）thelaterdevelopmentofearlyshellfishand36.（各种各类）fish.Next,greenplantsbegantogrowonland.They37.（follow）intimebylandanimals.Somewereinsects.Others,3&（call）amphibians,wereabletoliveonland39.（也）inthewater.Laterwhenthep
6、lants40.（长成）forests,reptilesappearedforthefirsttime.Theyproducedyounggenerallyby4L（生蛋）•Afterthat,somehugeanimals,calleddinosaurs,developed.Theylaideggstooand42.（生存,存在）ontheearthformorethan140millionyears.However.65millionyearsagotheageofthedinosaursended.Whytheysuddenlydisappea
7、redstill43.（仍然）amystery.Thisdisappearancemadepossiblethe44.（出现）ofmammalsontheearth.Theseanimalsweredifferentfromalllifeformsinthepast,becausethey45.（产牛,分娩）youngbabyanimalsandproducedmilktofeedthem.Finallyabout2.6millionyearsagosomesmallcleveranimals,nowwithhandsandfeet,appeared
8、andspreadallovertheearth.46.（因此）theyhave,47.（接着），becom
