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单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,HOWLIFEBEGANONTHEEARTH,地球上生命的起源,UNIT 4,课文讲解,1.Nooneknowsexactly,howtheearthbegan,asithappenedso,longago.,没有人确切知道地球是怎样开始形成的，因为在很早很早以前地球就形成了。,2.However,accordingtoawidelyaccepted,theory,theuniverse,beganwith,aBigBang,thatthrewmatter,inalldirections,.,然而，一种普遍为人们所接受的理论是：宇宙起源于一次大爆炸，这次大爆炸将物质投射到四面八方。,3.Afterthat,atomsbegantoformandcombinetocreatestarsandotherbodies.,然后，原子开始形成并结合成恒星和其他天体。,Par 1,1.ForseveralbillionyearsaftertheBigBang,theearthwasstilljustacloudofdust.,大爆炸后好几十亿年过去了，但地球还只是一团尘埃。,2.Whatit,wastobecome,wasuncertainuntilbetween4.5and3.8billionyearsago,whenthedustsettledintoasolidglobe,.,随后它会变成什么没人能知道，直到,3845,亿年前，这团尘埃才慢慢地形成一个固体的球状物。,3.Theearthbecameso,violent,thatitwasnotclearwhethertheshapewouldlastornot.,地球（开始）变得激烈动荡，不知道这个固体形状是否会继续存在下去。,Par 2,4.It,exploded,loudlywithfireandrock.Theywereintimeto,producecarbon,nitrogen,watervapourandothergases,whichweretomaketheearths,atmosphere.,它（地球）猛烈地爆炸喷出了烈火与岩石，最终产生了碳，氮气，水蒸气和其他多种气体，从而形成了地球的大气层。,5.,Whatisevenmoreimportant,is,thatastheearthcooleddown,waterbegantoappearonitssurface,.,更为重要的是，地球冷却了下来，地球的表面就开始出现了水。,1.,WaterhadalsoappearedonotherplanetslikeMarsbut,unliketheearth,ithaddisappearedlater.,在火星这些其他行星上都出现了水，但和地球不同的是，这些水后来都消失了。,2.,It,wasnotimmediatelyobvious,thatwaterwastobefundamentaltothedevelopmentoflife,.,水对于生命的发展会起关键作用，这一点在当时并不明显。,3.Whatmanyscientistsbelieve,is,thatthecontinuedpresenceofwaterallowedtheearthtodissolveharmfulgasesandacidsintotheoceansandseas.,很多科学家相信，由于地球上长期有水存在，使得地球得以把有害气体和酸性物质溶解在海洋里。,4.Thisproducedachainreaction,whichmadeitpossibleforlifetodevelop.,这就产生了一系列的反应，使得生命就有可能开始发展了。,Par 3,1.Manymillionsofyearslater,thefirstextremelysmallplants,begantoappearonthesurfaceofthewater.,好几百万年后，水的表面开始出现了极小的植物。,2.They,multiplied,andfilledtheoceansandseaswithoxygen,whichencouragedthelaterdevelopmentofearlyshellfishandallsortsoffish.,他们繁殖起来就使得海洋充满了氧气，这为早期的贝类及其他各种鱼类后来的进化创造了条件。,3.Next,greenplantsbegantogrowonland.Theywerefollowedintimebylandanimals.,接着，绿色植物开始出现在陆地上。,4.Somewereinsects.Others,calledamphibians,wereableto,liveonlandaswellasinthewater.,随后，陆地上出现了动物，其中有一些是昆虫，另外一些叫两栖动物，他们既能在陆上生活，也能在水里生存。,Par 4,5.Later,whentheplantsgrewintoforests,reptilesappearedfor,thefirsttime.,后来，当植物长成了森林的时候，爬行动物首次出现了。,6.Theyproducedyoungentirelybylayingeggs.,他们一般是通过孵蛋而繁衍后代的。,7.Afterthat,somehugeanimals,calleddinosaurs,developed.,再后来一些叫做恐龙的巨型动物出现了。,8.Theylaideggstooand,existed,ontheearthformorethan140millionyears.,他们也生蛋，在地球上生存了一亿四千多万年。,9.However,65millionyearsagotheageofthedinosaursended.,然而，六千五百万年前，恐龙时代结束了。,10.Whytheysuddenlydisappeared,still,remains,amystery.,他们为什么会突然灭绝至今仍是个谜。,11.Thisdisappearancemadepossibletheriseofmammalsontheearth.,恐龙的灭绝使地球上哺乳动物的增多有了可能。,12.Theseanimalsweredifferentfromalllifeformsinthepast,becausetheygavebirthtoyoungbabyanimalsandproducedmilktofeedthem,.,这些动物不同于以往所有的生物形态，因为他们能从体内生产出幼仔并给幼仔哺乳。,1.Finallyabout2.6millionyearsagosomesmallcleveranimals,nowwithhandsandfeet,appearedandspreadallovertheearth.,最后，大约,260,万年前，一些小巧聪明，长着手脚的动物出现了，他们散布在地球的各个地方。,2.Thustheyhave,intheirturn,becomethemostimportant,animalsontheplanet.,于是，他们接着成为这个行星上最重要的动物。,3.Buttheyarenotlookingaftertheearthverywell.,然而，他们对地球却不怎么在意，,Par 5,4.Theyareputtingtoomuchcarbondioxideintotheatmosphere,whichpreventsheatfromescapingfromtheearthintospace,.,他们把过多的二氧化碳释放到大气层中，这使得地球上的热不能释放到太空中去。,5.Asaresultofthis,manyscientistsbelieve,theearthmay,becometoohottoliveon,.,因此，许多科学家相信地球可能会变得太热而不适合生物生存。,6.So,whetherlifewillcontinueontheearthformillionsofyearstocome,will,dependon,whetherthisproblemcanbesolved,.,所以，在未来的数百万年中，生命能否在地球上延续，取决于这个问题能否得到解决。,主语从句,一个句子在复合句中充当主语叫主语从句。,常用引导词：,连接词,：,that(,无词义,)whether,That he will come and help us i,s,certain.,Whether we stay or not makes no,d,ifference.,连接代词,:,who(ever)whom what(ever),which(ever)whose,Who will go to the energy conference is not,important.,连接副词：,when where how why,When and where the test will be given is not decided.,1.,引导主语从句的连词是不能省略的。,That the earth is round,is true,2.That-,从句作主语通常用,it,作先行词，而将,that-,从句置于句末，例如：,It is true,that the earth is round,.,结构：,Attention,a.It+be+,形容词,+that-,从句,It is,necessary,that,有必要,It is,important,that,重要的是,It is obvious that,很明显,b.It+be+p.p.+that-,从句,It is believed that,人们相信,It is known to all that,从所周知,It has been decided that,已决定,c.It+be+,名词,+that-,从句,It is common knowledge that ,是常识,It is a surprise that,令人惊奇的是,It is a fact that,事实是,d.It+,不及物动词,+that-,分句,It appears that,似乎,It happens that,碰巧,It occurred to me that,我突然想起,2.,表示“是否”意义时，一般用,whether,而不用,if,引导主语从句，,Whether,she will come or not,is still a question,3.,有时可用形式主语,it,代替主语从句：,e,.g.It is still a question,whether she will come or not,.,It has not been decided yet,when they will start.,4.,无论主语从句有多长，一般做单数处理。,e,.g.When we will start,has,not been decided yet.,注意：,以,what,开始的主语从句如果后面的表语是复数名词，系动词用复数形式。,What he wants to buy,are,three books,and,two pens,.,1.,I,t is obvious to the students _ they should get well prepared for their future.,A.as B.which,C.whether D.that,2._ wants to stay in a hotel has to pay their own way.,A.Anyone B.The one,C.Whoever D.Who,
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