Directions:
Inthefollowingarticle, somesentenceshavebeenremoved. ForQuestions 21-25, choosethemostsuitableonefromthelistA-Gtofitintoeachofthenumberedblank. Thereisoneextrachoicethatdoesnotfitinanyofthegaps. MarkyouranswersonANSWERSHEET 1. (10 points)
Between 5,000 millionand 4,000 millionyearsagotheEarthwasformed. By 3,000 millionyearsagolifehadarisenandwehavefossilsofmicroscopicbacterialikecreaturestoproveit. (21) ————. Nobodyknowswhathappened, buttheoristsagreethatthekeywasthespontaneousarisingofselfreplicatingentities, i.e. somethingequivalentto “genes” inthegeneralsense.
TheatmosphereoftheearlyEarthprobablycontainedgasesstillabundanttodayonotherplanetsinthesolarsystem. Chemistshaveexperimentallyreconstructedtheseancientconditionsinthelaboratory. Ifplausiblegasesaremixedinaflaskwithwater, andenergyisaddedbyanelectricdischarge (simulatedlightning), organicsubstancesarespontaneouslysynthesized. TheseincludethebuildingblocksofRNAandDNA. ItseemsprobablethatsomethinglikethishappenedontheearlyEarth. Consequently, theseawouldhavebecomea “soup” ofprebiologicalorganiccompounds. (22)————.
TodaythemostfamousselfreplicatingmoleculeisDNA (deoxyribonucleicacid), butitiswidelythoughtthatDNAitselfcouldnothavebeenpresentattheoriginoflifebecauseitsreplicationistoodependentonsupportfromspecializedmachinery, whichcouldnothavebeenavailablebeforeevolutionitselfbegan. DNAhasbeendescribedasa “hightech” moleculewhichprobablyarosesometimeaftertheoriginoflifeitself. PerhapstherelatedmoleculeRNA, whichstillplaysvariousvitalrolesinlivingcells, wastheoriginalselfreplicatingmolecule. Orperhapstheprimordialreplicatorwasadifferentkindofmoleculealtogether. (23) ————. Variantsthatwereparticularlygoodatreplicationwouldautomaticallyhavecometopredominateintheprimevalsoup. Varietiesthatdidnotreplicate, orthatdidsoinaccurately, wouldhavebecomerelativelylessnumerous. Thisledtoeverincreasingefficiencyamongreplicatingmolecules.
Asthecompetitionbetweenreplicatingmoleculeswarmedup, successmusthavegonetotheonesthathappenedtohituponspecialtricksordevicesfortheirownselfpreservationandtheirownrapidreplication. Therestofevolutionmayberegardedasacontinuationofthenaturalselectionofreplicatormolecules, nowcalledgenes, byvirtueoftheircapacitytobuildforthemselvesefficientdevices (cellsandmulticellularbodies) fortheirownpreservationandreproduction. (24)————.
FossilswerenotlaiddownonmorethanasmallscaleuntiltheCambrianera, nearly 600 millionyearsago. Thefirstvertebratesmaydateback 530 millionyears, accordingtofossilevidence—primitive, jawlessfisheswithfins, gills, andfishlikemusclepatterns—foundinChinain 1999. Vertebratesappearabundantlyinfossilbedsbetween 300 and 400 millionyearsago. (25)————. Mammalsand, later, birds, arosefromtwodifferentbranchesofreptiles. Therapiddivergenceofmammalsintotherichvarietyoftypesthatweseetoday, fromopossumstoelephants, fromanteaterstomonkeys, seemstohavebeenunleashedintothevacuumleftbythecatastrophicextinctionof
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| 重点单词 | 查看全部解释 | |||
| abundant | [ə'bʌndənt] | 想一想再看 adj. 丰富的，充裕的 |
联想记忆
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单词abundant 联想记忆： ab加强+und波动+ant→[多得]象波浪一样→丰富的 |
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| machinery | [mə'ʃi:nəri] | 想一想再看 n. (总称)机器，机械 |
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| consequently | ['kɔnsikwəntli] | 想一想再看 adv. 所以，因此 |
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| vertebrate | ['və:tibreit] | 想一想再看 adj. 脊椎的，脊椎动物的 n. 脊椎动物 |
联想记忆
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单词vertebrate 联想记忆： vert转，ebr，ate吃：能转动身体还能吃－脊椎动物。 |
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| replicate | ['replikeit] | 想一想再看 v. 折叠，复制，模写 n. 同样的样品 adj. 转折 |
联想记忆
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单词replicate 联想记忆： re重新+plic重叠，折叠+ate→再重叠[一份]→复制 |
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| specialized | ['speʃəlaizd] | 想一想再看 专门的 专科的 |
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| efficiency | [i'fiʃənsi] | 想一想再看 n. 效率，功率 |
联想记忆
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单词efficiency 联想记忆： ef出+fic做…做出事情…有效率+iency表名词“性质，状态”，ence的变体→效率 |
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| species | ['spi:ʃiz] | 想一想再看 n. （单复同）物种，种类 |
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| emphasize | ['emfəsaiz] | 想一想再看 vt. 强调，着重 |
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| dependent | [di'pendənt] | 想一想再看 adj. 依靠的，依赖的，从属的 |
联想记忆
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单词dependent 联想记忆： de加强+pend=hang悬挂+ent→依赖的 |