知識(shí)點(diǎn)百日百篇 早期的地球以及生命的形成

第39天20XX年11月16日北美考試機(jī)經(jīng)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日北美考試機(jī)經(jīng)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約45億年前，地球在揚(yáng)塵和一顆老恒星的超新星爆炸后的氣體殘留物中形成。隨著地球內(nèi)部熔體的沉淀和冷卻，一個(gè)固體地殼迅速形成，可能只有短短1.5億年的時(shí)間，同時(shí)伴有一個(gè)主要由二氧化碳、水蒸氣和氮?dú)鈽?gòu)成的基本的大氣層。在這個(gè)早期的大氣層中，氧氣被認(rèn)為基本上不存在。接著發(fā)生與另一個(gè)星球的近乎災(zāi)難性的碰撞（在此過程中月亮產(chǎn)生），許多科學(xué)家認(rèn)為這次碰撞引起地球降溫。隨著溫度的下降，火山爆發(fā)產(chǎn)生的水蒸氣凝結(jié)在大氣中，以雨的形式降落，并聚集在地球表面逐漸形成早期的海洋。這可能發(fā)生在地球形成的7.5億年的時(shí)間段內(nèi)（約38億年前）。這時(shí)候，盡管周圍環(huán)境（包括經(jīng)常遭到小行星的撞擊和巨大的火山活動(dòng)）將會(huì)對(duì)生命高度危險(xiǎn)，但必要的成分都以某種或其他形式出現(xiàn)了：液態(tài)水、化學(xué)基本成分（通常采取六種元素：氧、氫、碳、氮、硫和磷）和某種能量來源。液態(tài)水在生命最初的發(fā)展階段是必不可少的，有這樣幾個(gè)原因。首先，它提供了化學(xué)反應(yīng)發(fā)生的適當(dāng)?shù)臏囟?。第二，許多化學(xué)物質(zhì)溶解在水中，容易使它們混合在一起發(fā)生反應(yīng)。最后，許多化學(xué)物有溶于水的部分也有不溶的部分，這也有助于反應(yīng)的發(fā)生。碳很重要，因其有形成長的鏈狀分子（碳鏈形成的有機(jī)分子的基礎(chǔ)）的能力。氫、氧和氮都能以不同的方式和碳結(jié)合，由碳、氫、氧和氮形成的大分子也往往是非常穩(wěn)定的。所有的化學(xué)反應(yīng)都需要一個(gè)能量源來推動(dòng)，無論是來自太陽的紫外線，或者來自閃電的電能，還是深海噴口