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TPO阅读文本和题目正式.docx

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TPO36-1 TheFirstEye Puttingadateonthefirstappearanceofeyesdependsonwhatonemeansbyeye.Ifthetermreferstoamulticellularorgan,evenifithasjustafewcells,thenbydefinition,eyescouldnotformbeforethereweremulticellularanimals.Butmanyprotists(animal-like,plantlike,orfungus-likeunicellularorganismsthatrequireawater-basedenvironment)candetectlightbyusingaggregationsofpigmentmolecules,andtheyusethisinformationtomodifytheirmetabolicactivityormotility(theabilitytomovespontaneouslyandindependently).Oneofthefamiliarlivingexamples,probablyknowntoanyonewhohastakenabiologyclass,istheaquaticprotozoanEuglena,whichhasaneyespotnearitsmotilefIagellum(hairlikestructure).Somelivingprotistsareveryliketheirancestralformsembeddedinancientsedimentaryrocks,andthissimilaritysuggeststhattheabilitytodetectlightandmodifybehaviorinresponsetolighthasbeenaroundforaverylongtime.Animalsarosefromoneofsuchunicellularcreatures,perhapsfromonealreadyspecializedforaprimitivekindofvision. Aneyeisacollectionofcellsthatarespecializedforlightdetectionthroughthepresenceofphotosensitivepigmentaswellasameansofrestrictingthedirectionofincominglightthatwillstrikethephotosensitivecells.Thisdefinitionsaysnothingaboutimageformation,lenses,eyemovements,oranyoftheotherfeaturesweassociatewithourowneyes,butitdoesrecognizethesimplestformoffunctionalandanatomicalspecialisationnamely,detectionoflight.Everythingelsecanbebuiltupfromthissimplebeginning,andsomeanimalsappeartohavehadeyesalmostfromthebeginningoftheanimalkingdom. Animalswerescarce600millionyearsagointhegeologicaleracalledthePrecambrian.Thereareveryfewfossilremainsfromthattime(thoughmorekeepturningup),andmostevidenceofthepresenceofanimalsisindirect,suchassmalltunnelsinrockthatcouldbeancientwormburrowings.Butjust50millionyearsorsolater,fossilizedbitsandpiecesofanimalsabound,suggestingthatagreatburstofevolutionarycreativityoccurredinthe50-million-yearinterval.Thissurgeofnewlife,markedbyanabundanceofanimals,iscalledtheCambrianexplosion. Thefirstdirectevidencefortheearlyor