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托福阅读作业：TPO5 THE CAMBRIAN EXPLOSION

发布时间：2023-05-25 18:53:15 来源：文档文库

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THECAMBRIANEXPLOSIONThegeologictimescaleismarkedbysignificantgeologicandbiologicalevents,includingtheoriginofEarthabout4.6billionyearsago,theoriginoflifeabout3.5billionyearsago,theoriginofeukaryoticlife-forms(livingthingsthathavecellswithtruenucleiabout1.5billionyearsago,andtheoriginofanimalsabout0.6billionyearsago.ThelasteventmarksthebeginningoftheCambrianperiod.AnimalsoriginatedrelativelylateinthehistoryofEarth–inonlythelast10percentofEarth’shistory.Duringageologicallybrief100-million-yearperiod,allmodernanimalgroups(alongwithotheranimalsthatarenowextinctevolved.Thisrapidoriginanddiversificationofanimalsisoftenreferredtoas“theCambrianexplosion.”Scientistshaveaskedimportantquestionsaboutthisexplosionformorethanacentury.WhydiditoccursolateinthehistoryofEarth?Theoriginofmulticellularformsoflifeseemsarelativelysimplestepcomparedtotheoriginoflifeitself.Whydoesthefossilrecordnotdocumenttheseriesofevolutionarychangesduringtheevolutionofanimals?Whydidanimallifeevolvesoquickly?Paleontologistscontinuetosearchthefossilrecordforanswerstothesequestions.Oneinterpretationregardingtheabsenceoffossilsduringthisimportant100-million-yearperiodisthatearlyanimalsweresoftbodiedandsimplydidnotfossilize.Fossilizationofsoft-bodiedanimalsislesslikelythanfossilizationofhard-bodiedanimals,butitdoesoccur.Conditionsthatpromotefossilizationofsoft-bodiedanimalsincludeveryrapidcoveringbysedimentsthatcreateanenvironmentthatdiscouragesdecomposition.Infact,fossilbedscontainingsoft-bodiedanimalshavebeenknownformanyyears.TheEdiacarafossilformation,whichcontainstheoldestknownanimalfossils,consistsexclusivelyofsoft-bodiedforms.AlthoughnamedafterasiteinAustralia,theEdiacaraformationisworldwideindistributionanddatestoPrecambriantimes.This700-million-year-oldformationgivesfewcluestotheoriginsofmodernanimals,however,becausepaleontologistsbelieveitrepresentsanevolutionaryexperimentthatfailed.Itcontainsnoancestorsofmodernanimalgroups.AslightlyyoungerfossilformationcontaininganimalremainsistheTommotianformation,namedafteralocaleinRussia.ItdatestotheveryearlyCambrianperiod,anditalsocontainsonlysoft-bodiedforms.Atonetime,theanimalspresentinthesefossilbedswereassignedtovariousmodernanimalgroups,butmostpaleontologistsnowagreethatallTommotianfossilsrepresentuniquebodyformsthataroseintheearlyCambrianperiodanddisappearedbeforetheendoftheperiod,leavingnodescendantsinmodernanimalgroups.Athirdfossilformationcontainingbothsoft-bodiedandhard-bodiedanimalsprovidesevidenceoftheresultoftheCambrianexplosion.Thisfossilformation,calledtheBurgessShale,isinYohoNationalParkintheCanadianRockyMountainsofBritishColumbia.ShortlyaftertheCambrianexplosion,mudslidesrapidlyburiedthousandsofmarineanimalsunderconditionsthatfavoredfossilization.Thesefossilbedsprovideevidenceofabout32modernanimalgroups,plusabout20otheranimalbodyformsthataresodifferentfromanymodernanimalsthattheycannotbeassignedtoanyoneofthemoderngroups.TheseunassignableanimalsincludealargeswimmingpredatorcalledAnomalocarisandasoft-bodiedanimalcalledWiwaxia,whichatedetritusoralgae.TheBurgessShaleformationalsohasfossilsofmanyextinctrepresentativesofmodernanimalgroups.Forexample,awell-knownBurgessShaleanimalcalledSidneyiaisarepresentativeofapreviouslyunknowngroupofarthropods(acategoryofanimalsthatincludes
insects,spiders,mites,andcrabs.FossilformationsliketheBurgessShaleshowthatevolutioncannotalwaysbethoughtofasaslowprogression.TheCambrianexplosioninvolvedrapidevolutionarydiversification,followedbytheextinctionofmanyuniqueanimals.Whywasthisevolutionsorapid?Noonereallyknows.Manyzoologistsbelievethatitwasbecausesomanyecologicalnicheswereavailablewithvirtuallynocompetitionfromexistingspecies.WillzoologistseverknowtheevolutionarysequencesintheCambrianexplosion?Perhapsanotherancientfossilbedofsoft-bodiedanimalsfrom600-million-year-oldseasisawaitingdiscovery.Paragraph1:Thegeologictimescaleismarkedbysignificantgeologicandbiologicalevents,includingtheoriginofEarthabout4.6billionyearsago,theoriginoflifeabout3.5billionyearsago,theoriginof

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