TPO-5 Reading 3 阅读译文

The CambrianExplosion寒武纪大爆发The geologictimescale ismarked bysignificant geologicand biologicalevents,including the origin of Earth about

4.6billion yearsago,the origin of lifeabout

3.5billionyears ago,the origin of eukaryoticlife-forms livingthings thathave cellswith truenucleiabout

1.5billion yearsago,and theoriginofanimals about

0.6billion yearsago.The lasteventmarks thebeginning of the Cambrian period.Animals originatedrelatively latein thehistory ofEarth-in onlythe last10percent ofEarths history.During ageologically brief100-million-year period,all modern animal groupsalong withother animals that arenowextinct evolved.This rapidorigin anddiversification ofanimals isoften referredto as“theCambrian explosion.”地质年代是由重大地质事件和生物事件标记的，包括46亿年前地球的形成、35亿年前生命的起源、15亿年前真核生物细胞中有真核的生命体的起源以及6亿年前动物的起源；其中最后的一个事件标志着寒武纪的开始动物的起源相对处于地球历史的晚期仅存在于地球历史时间的1/10o在短暂的1亿年地质学周期中，所有现代动物群包括现在已经灭绝的生物进化了这次快速的动物起源和分化常常被称为“寒武纪大爆发”Scientists haveasked importantquestions aboutthis explosionfor morethan acentury.Why didit occurso latein thehistoryofEarth Theoriginofmulticellular formsof lifeseemsa relativelysimple stepcompared to theoriginof lifeitself.Why doesthe fossilrecordnot documentthe seriesof evolutionarychanges duringthe evolutionof animalsWhydid animallife evolveso quicklyPaleontologists continueto searchthe fossilrecordfor answersto thesequestions.一个多世纪以来，科学家们对这次大爆发一直有疑惑为什么它发生得这么晚？毕竟多细胞生物的出现相对于生命的出现而言只是一次相对简单的进化为什么化石没有记录下动物演化的一系列变化呢？为什么动物生命进化得如此迅速呢？古生物学家们仍旧在研究化石记录以期回答这些问题One interpretationregarding theabsence offossils duringthis important100-million-year periodis thatearly animalswere softbodied andsimply didnot fossilize.■Fossilization of soft-bodied animalsis lesslikely thanfossilization ofhard-bodied animals,but itdoes occur.■Conditions thatpromote fossilizationof soft-bodied animals includevery rapidcovering bysediments thatcreate anenvironment thatdiscouragesdecomposition.・In foct,fossil bedscontaining soft-bodied animalshave beenknown fbrmanyyears.■关于这重要的1亿年内化石的缺失，有一种解释是早期的动物都是软体动物，它们很难形成化石软体动物的化石比硬体动物化石少见得多，但是也是存在的促使软体动物成为化石的条件是沉积物的迅速覆盖以形成一个抑制分解的环境事实上，含有软体动物的化石层在很多年以前就已经为人们所知了The Ediacarafossil formation,which containsthe oldestknown animalfossils,consistsexclusively ofsoft-bodied forms.Although named after asite inAustralia,the Ediacaraformationis worldwidein distributionand datesto Precambriantimes.This700-million-year-old formationgives fewclues tothe originsof modern animals,however,because paleontologistsbelieve itrepresents anevolutionary experimentthat failed.Itcontains noancestors of modernanimal groups.含有最古老的动物化石的伊迪卡拉化石群就全部由软体动物化石组成尽管伊迪卡拉是以澳大利亚的一处地名而命名，但是伊迪卡拉沉积层的分布却遍及世界各地，并且可以追溯到前寒武纪时期这些7亿年前形成的地层为现代动物的起源提供了一些新的线索因为古生物学家们认为它代表着一次失败的进化试验，其中并没有包含任何现代动物的祖先A slightlyyounger fossil formation containinganimal remainsis theTommotianformation,namedaftera localein Russia.It datestothevery early Cambrianperiod,and italsocontains onlysoft-bodied forms.At onetime,the animalspresent in these fossil bedswere assignedto vaiiousmodem animalgioups,but mostpaleontologists nowagiee thatallTommotian fossilsrepresent uniquebody formsthat arosein theearlyCambrianperiod anddisappearedbefore theend ofthe period,leaving nodescendants inmodem animalgioups.以俄罗斯的一处地名而命名的Tommotian是一层包含动物残骸的较年轻的化石层它形成于寒武纪的早期，并且同样只含有软体动物化石在一段时间内，人们认为这些化石中的动物分化出了各种各样的现代动物但是古生物学家们现在却认为，所有的Tommotian化石都仅代表在寒武纪初期出现但到寒武纪结束时就消失了的特别生物所以它们没有在现在动物中留下后代A thirdfossilformationcontaining bothsoft-bodied andhard-bodied animalsprovidesevidence ofthe resultofthe Cambrian explosion.This fossilformation,called theBurgessShale,is inYoho NationalPark inthe CanadianRocky Mountainsof BritishColumbia.Shortly aftertheCambrian explosion,mud slidesrapidly buriedthousands ofmarineanimals underconditions thatfavored fossilization.These fossilbeds provideevidence ofabout32modernanimalgroups,plus about20other animalbody formsthat areso differentfromany modernanimals thatthey cannotbe assignedto anyone ofthe moderngroups.These unassignableanimalsincludea largeswimming predatorcalled Anomalocaris and asoft-bodied animalcalled Wiwaxia,which atedetritus oralgae.The Burgess Shaleformation alsohas fossilsof manyextinct representativesofmodernanimalgroups.Forexample,a well-known BurgessShale animalcalled Sidneyiaisarepresentative ofapreviously unknowngroup ofarthropods acategory ofanimalsthatincludes insects,spiders,mites,and crabs.第三种化石层既包含了软体动物也包含了硬体动物，它为寒武纪大爆发提供了证据这种叫做伯吉斯页岩的化石群就在加拿大的大不列颠哥伦比亚石山上的约霍国家公园内在寒武纪大爆发后不久，滑落的泥土迅速掩埋了成千上万的海洋动物，形成了极有利于化石形成的环境这些化石层含有大约32种现代动物，还有大约20种与现在动物截然不同以至于不可能分类为任何一种现代动物的其他动物体这些无法划分的动物包含一中叫做Anomalocaris的肉食动物和一^中叫做Wiwaxia的以岩屑和藻类为食的软体动物伯吉斯页岩化石群也含有很多现在已经灭绝了的动物化石例如伯吉斯页岩化石群中的一种著名动物Sidneyia,就是一种典型的以前还不为人知的节肢动物（一种动物分类，它包括昆虫、蜘蛛、蹒虫和螃蟹）Fossil formationslike theBurgessShaleshow thatevolution cannotalways bethoughtof asa slowprogression.The Cambrianexplosion involvedrapid evolutionarydiversification,followed bythe extinctionof manyunique animals.Why wasthis evolutionsorapid Noone reallyknows.Many zoologistsbelieve thatit wasbecause somanyecological nicheswere availablewith virtuallyno competitionfrom existingspecies.Willzoologists everknow theevolutionary sequencesintheCambrianexplosionPerhapsanother ancientfossilbedofsoft-bodied animalsfrom600-million-year-old seasisawaiting discovery.像伯吉斯页岩化石群这样的化石层表明进化不能总是被认为是缓慢的过程寒武纪大爆涉及到了快速的进化分化，接着就是很多独特动物的灭绝为什么这种进化如此迅速呢？没有人真正明白很多动物学家认为这是很多几乎没有任何竞争性物种的环境使然动物学家们是否知道寒武纪大爆发的动物的进化顺序呢？或许另一些含有来自于6亿年前的海洋动物的化石亟待发现。