知识点百日百篇 早期的地球以及生命的形成

第39天20XX年11月16日北美考试机经EarlyEarthandtheFormationofLife①Earthwasformedabout4.5billionyearsagofromtheswirlingdustandgasremnantsofanoldstar’ssupernovaexplosion.AsthemoltenmassfromEarth’sinteriorsettledandcooled,asolidcrustsoonformed,probablywithinaslittleasabout150millionyears,alongwitharudimentaryatmospherecomposedlargelyofcarbondioxide,watervaporandnitrogen.Oxygenwasthoughttobenearlyabsentinthisearlyatmosphere.②Thenanear-catastrophiccollisionwithanotherplanethappened(whichcreatedtheMoonintheprocess),whichmanyscientistsbelievedhavecausedtheEarthtocooldown.Asthetemperaturedropped,watervaporfromvolcanoescondensedintheatmosphere,fellasrain,andcollectedontheEarth’ssurfacetograduallyformtheearlyocean.Thisperhapsoccurredwithin750millionyearsofEarth’sformation(about3.8billionyearsago).Atthispoint,eventhoughtheenvironment(includingtheconstantbombardmentbyasteroidsandprodigiousvolcanicactivity)wouldhavebeenhighlyhazardoustolife,thenecessaryingredientswereallpresentinsomeformoranother:liquidwater,chemicalbuildingblocks(usuallytakentobethesixelements:oxygen,hydrogen,carbon,nitrogen,sulphurandphosphorus)andsomekindofenergysource.③Liquidwaterisconsideredessentialtotheinitialdevelopmentoflifeforseveralreasons.First,itofferstherighttemperatureforchemicalreactionstohappen;second,manychemicalsdissolveeasilyinwater,allowingthemtomixtogetherandreact;finally,manychemicalshavepartswhichareattractedtowaterandpartswhicharerepelledbyit,whichalsohelpsreactionshappen.Carbonisimportantbecauseofitsabilitytoformlongchain-likemolecules(carbonchainsformthebackboneoforganicmolecules).Hydrogenandoxygenaswellasnitrogencanallbondwithcarboninmanydifferentways,andlargemoleculesmadefromcarbon,hydrogen,oxygenandnitrogenalsotendtobeverystable.Allchemicalreactionsneedanenergysourcetodrivethem,whetheritbeultravioletlightfromtheSunorelectricalenergyfromlightningorchemicalenergyfromdeep-seavents,allofwhichwouldhavebeenavailableontheearlyEarth.④Cyanobacteria(alsoknownasblue-greenalgae),oneoftheearliesttypesofprokaryoticbacteria,formedintocoloniesormatscalledstromatolites,andfossilizedremainsofthesehavebeenfoundinAustraliadatingbacktobetween3.4and2.8billionyearsago.Ancientastheiroriginsare,thesebacteria(whicharestillaroundtoday)werealreadybiologicallycomplex,withcellwallsprotectingtheirprotein-producingDNA,soscientiststhinkitlikelythatlifeactuallybeganmuchearlier,perhapsasearlyas3.8billionyearsago.⑤Theseearlycyanobacteriawerethefirstoxygen-producing,evolving,phototropicorganisms.TheirexistencehadamajorimpactontheearlyEarth’senvironmentastheyfedoffatmosphericcarbondioxideandconvertedmuchofitintomarinesedimentsconsistingoftheshellsofseacreatures.Apartfromreducingthecarbondioxidecontentoftheatmosphere,italsostartedtoproduceoxygen.Otherphotosynthesizingplantsevolvedlatercontinuedthisprocess,leadingtothebuildupofincreasinglevelsofoxygenintheatmosphere,aswellasthereleaseofnitrogenintotheatmosphereastheoxygenreactedwithammonia.Eventually,alayerofozoneformedintheatmosphere,whichbetterprotectedearlylifeformsfromultravioletradiation.WhileoxygenwasapparentlynotneededfortheoriginationoflifeonEarth,therapidexplosionoflifebeganonlyafteroxygenbecameabundant.⑥Thefirsteukaryoticcells(organismswithoneormorecomplexcells,eachofwhichcontainsanucleusandissurroundedbyamembranethatholdsthecell’sgeneticmaterial)evolvedsometimebetween2.5and1.7billionyearsago,perhapscoincidentwiththeriseinatmosphericoxygentoalevelabletosupportthismorecomplexlife.ThenucleusinthesecellswasabletoholdandprotectcomplexmoleculessuchasRNAandDNA.Asrecentlyasthe1970s,awholenewgroupofsingle-celledorganismsknownasarchaeawasdiscovered,whichisnowrecognizedasathirddomainoflife,completelyseparatefrombothprokaryotesandeukaryotes.Manyscientistsbelievearchaeatobethecommonancestorofbothprokaryotesandeukaryotes,andmayrepresenttheoldestformoflifeonearth.——20XX年11月16日北美考试机经WhichofthesentencesbelowbestexpresstheessentialinformationinthehighlightedsentenceinParagraph5?Incorrectchoiceschangethemeaninginimportantwaysorleaveoutessentialinformation.A.Becausetherewaslittleoxygenwhenlifeoriginated,manylifeonlybecameexistentwhentherewasenoughoxygen.B.Althoughoxygenwasnotpresentwhenlifeoriginated,thequickgrowthoflifeonlybeganwhenoxygenbecamelargeinnumber.C.Earlylifedidn’tneedoxygentocomeintoexistence,whichcausedoxygentobecomeveryabundant.D.Thoughearlylifedidn’tneedoxygentocomeintoexistence,theformationoflargenumberoflifelaterneededabundantoxygen.答案：B早期的地球以及生命的形成约45亿年前，地球在扬尘和一颗老恒星的超新星爆炸后的气体残留物中形成。随着地球内部熔体的沉淀和冷却，一个固体地壳迅速形成，可能只有短短1.5亿年的时间，同时伴有一个主要由二氧化碳、水蒸气和氮气构成的基本的大气层。在这个早期的大气层中，氧气被认为基本上不存在。接着发生与另一个星球的近乎灾难性的碰撞（在此过程中月亮产生），许多科学家认为这次碰撞引起地球降温。随着温度的下降，火山爆发产生的水蒸气凝结在大气中，以雨的形式降落，并聚集在地球表面逐渐形成早期的海洋。这可能发生在地球形成的7.5亿年的时间段内（约38亿年前）。这时候，尽管周围环境（包括经常遭到小行星的撞击和巨大的火山活动）将会对生命高度危险，但必要的成分都以某种或其他形式出现了：液态水、化学基本成分（通常采取六种元素：氧、氢、碳、氮、硫和磷）和某种能量来源。液态水在生命最初的发展阶段是必不可少的，有这样几个原因。首先，它提供了化学反应发生的适当的温度。第二，许多化学物质溶解在水中，容易使它们混合在一起发生反应。最后，许多化学物有溶于水的部分也有不溶的部分，这也有助于反应的发生。碳很重要，因其有形成长的链状分子（碳链形成的有机分子的基础）的能力。氢、氧和氮都能以不同的方式和碳结合，由碳、氢、氧和氮形成的大分子也往往是非常稳定的。所有的化学反应都需要一个能量源来推动，无论是来自太阳的紫外线，或者来自闪电的电能，还是深海喷口