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1、CellsThroughaboutthreebillions(3,000,000,000)years,scientistssays,thedevelopmentsofplantsandanimalsonourearthhavebeengoingon.Atfirstbothwereverysmall,muchlessthanapinpointinsize.Wemightsaythatthefirstplantsoranimalsweretoosmallforman’seyetosee.Suchcellstakefoodin（吸进）and
2、letwasteout（排出）throughverysmallopeningsintheirwalls.Theydonotgivebirthto(生育)newcells，but,strangetosay,dividetobecometwodaughtercells.Lifebegan,menthink,inthesaltysea.Neartheland,wherethewaterswerenotdeep,verysmallone-celledorganismscameintobeing(形成、产生).Thecellsinourbodi
3、esallhavewaterinthem,thoughtheyareofdifferentsorts.Thiswaterhasinitaboutthesameamountofsalt,commontablesalt（食盐），asthatinthewaterofseainwhichtheearliestorganismsswam.Thatwaterwaslittlelesssaltythanseawateristoday.Inbillionsofyears,therivershavewasheddownmoreandmoresaltfr
4、omthelandintothesea.Theseahasbecomemoresalty.Thewaterinourbodiesdoesnotbecomemoresalty.Evenifweeatagreatamountofsaltinourfood，thebodyautomaticallycontrolshowmuchitwillkeepinthebloodstream.Andthesameistrueifweletmuchsaltoutthroughtheskinwhenwegetoverheated.Thenwehavetoea
5、tsomethingthatwillgiveusnewsuppliesofsalt.Thefirstlivingthings,thoughtheyweretoosmalltobeseen,wereeating,growingorganismwhichdividedtobecomelikethem.Underamicroscope,acelllookslikeabitofclearjellywithathinwallroundit.Ineverycellthereisapartlikealittleball.Thisisthenucle
6、us,whichorganizestheworkofthecell.Thoughcellswerediscoveredtwoandahalfcenturiesago,itisonlyinthelasthundredyearsthatknowledgeoftheworkofthenucleushasdeveloped.Itisthisnucleuswhichkeepsthecellworkingatwhathastobedone.Inshort,alllivingthingsaredevelopmentsfromtheseone-cel
7、lorganisms.Theyhavedevelopedstepbystepthroughchangeuponchangefromthesefirst,simple,livingthings.Ourbodiesandthebodiesoftheplantsandanimalsweseeaboutusarehighlycomplexintheirstructure.Theyhaveinthemmillionsofmillionsofcells,somedoingonesortofwork,someanother.Everydiffere
8、ntcellhasitsownsortofworktodo.CellsThroughaboutthreebillions(3,000,000,000)years,scientistssays,thedevelopment
