19 THE RISE OF LIFE
IN1953,STANLEYMiller,agraduatestudentattheUyofChicago,tooktwoflasks—oainingalittlewatertorepresentaprimevalo,theotherholdingamixtureofmethane,ammonia,andhydrogensulphidegasestorepreseh’searlyatmosphere—ectedthemwithrubbertubes,andintrodueelectricalsparksasastand-infhtning.Afterafewdays,thewaterintheflaskshadturnedgreenandyellowiybrothofaminoacids,fattyacids,sugars,andanipounds.“IfGoddidn’tdoitthisway,”observedMiller’sdelightedsupervisor,theNobellaureateHaroldUrey,“Hemissedagoodbet.”
Pressreportsofthetimemadeitsoundasifaboutallthatwasneedednowwasforsomebodytogivethewholeagoodshakeandlifewouldcrawlout.Astimehasshown,itwasn’tnearlysosimple.Despitehalfaturyoffurtherstudy,wearenoosynthesizinglifetodaythanwewerein1953andmuchfurtherawayfromthinkingwe.
StistsarenowprettycertainthattheearlyatmospherewasnothinglikeasprimedfordevelopmentasMillerandUrey’sgaseousstew,butratherwasamuchlessreactiveblendofnitrogenandcarbondioxide.RepeatingMiller’sexperimentswiththesemorechallenginginputshassofarprodulyonefairlyprimitiveaminoacid.Atallevents,creatingaminoacidsisnotreallytheproblem.Theproblemisproteins.
Proteiyougetwhenyaminoacidstogether,andweneedalotofthem.
Noonereallyknows,buttheremaybeasmanyasamilliontypesofproteininthehumanbody,andeaeisalittlemiracle.Byallthelawsofprobabilityproteinsshould.
Tomakeaproteinyouoassembleaminoacids(whichIamobligedbylongtraditioohereas“thebuildingblocksoflife”)inaparticularorder,inmuchthesamewaythatyouassemblelettersinaparticularordertospellaword.Theproblemisthatwordsintheaminoacidalphabetareoftenexceedinglylong.Tospellcollagen,thenameofaontypeofprotein,youeeightlettersintherightorder.Buttomakecollagen,youe1,055aminoacidsinpreciselytherightseque—andhere’sanobviousbutcrucialpoint—youdon’tmakeit.Itmakesitself,spontaneously,withoutdire,andthisiswheretheunlikelihoodsein.
Thecesofa1,055-sequencemoleculelikecollagenspontaneouslyself-assemblingare,frankly,nil.Itjustisn’tgoingtohappen.Tograspwhatalongshotitsexistenceis,visualizeastandardLasVegasslotmaebutbroadenedgreatly—toaboutyfeet,tobeprecise—toaodate1,055spinningwheelsinsteadoftheusualthreeorfour,andwithtwentysymbolsoneachwheel(oneforeaonaminoacid).
1Howlongwouldyouhavetopullthehandlebeforeall1,055symbolscameupintherightorder?Effectivelyforever.Evenifyoureducedthenumberofspinniotwohundred,whichisactuallyamoretypiumberofaminoacidsforaprotein,theoddsagainstalltwohundredingupina1Thereareactuallytwenty-twonaturallyaminoacidsknowh,andmoremayawaitdiscovery,butonlytwentyofthemarenecessarytoproduceusandmostotherlivingthings.Thetwenty-sed,calledpyrrolysine,wasdiscoveredin2002byresearchersatOhioStateUyandisfoundonlyinasiypeofarchaean(abasioflifethatwewilldiscussalittlefurtheroniory)calledMethanosarabarkeri.
prescribedsequenceare1in10260(thatisa1followedby260zeroes).Thatinitselfisalargerhanalltheatomsintheuniverse.
Proteins,inshort,areplexentities.Hemoglobinisonly146aminoacidslong,aruntbyproteinstandards,yetevenitoffers10190possibleaminoabinations,whichiswhyittooktheCambridgeUychemistMaxPerutztwenty-threeyears—acareer,moreorless—tou.Forrastoproduceevenasieinwouldseemastunningimprobability—likeawhirlwindspinningthroughajunkyardandleavingbehindafullyassembledjumbojet,inthecolorfulsimileoftheastronomerFredHoyle.
Yetwearetalkingaboutseveralhuhousandtypesofprotein,perhapsamillion,eaiqueandeach,asfarasweknow,vitaltothemaintenanceofasoundandhappyyou.Anditgoesonfromthere.Aproteintobeofusemustnotonlyassembleaminoacidsintherightsequethenmustengageinakindofchemicalamiandfolditselfintoaveryspecificshape.Evenhavingachievedthisstructuralplexity,aproteinisnogoodtoyouifit’treproduceitself,andproteins’t.ForthisyouneedDNA.DNAisalig—itmakeacopyofitselfinseds—butdovirtuallynothingelse.Soaradoxicalsituation.Proteins’texistwithoutDNA,andDNAhasnopurposewithoutproteins.Arewetoassumethenthattheyarosesimultaneouslywiththepurposeofsuppeachother?Ifso:wow.
Andthereismorestill.DNA,proteins,aherposoflifecouldn’tprosperwithoutsomesortofmembraotainthem.Noatomormoleculehaseverachievedlifeindepely.Pluyatomfromyourbody,anditisnomorealivethanisagrainofsand.
Itisonlywhentheyetogetherwithiuringrefugeofacellthatthesediversematerialstakepartintheamazingdawecalllife.Withoutthecell,theyarenothingmorethaingchemicals.Butwithoutthechemicals,thecellhasnopurpose.
AsthephysicistPaulDaviesputsit,“Ifeverythingneedseverythingelse,howdidtheunityofmoleculeseverariseinthefirstplace?”Itisratherasifalltheingredientsinyourkitsomehowgottogetherandbakedthemselvesintoacake—butacakethatoreoverdividewhennecessarytoproducemorecakes.Itislittlewohatwecallitthemiracleoflife.Itisalsolittlewohatwehavebarelybeguntouandit.
Sowhatatsforallthiswondrousplexity?Well,onepossibilityisthatperhapsitisn’tquite—notquite—sowondrousasatfirstitseems.Takethoseamazinglyimprobableproteins.Thewonderweseeintheirassemblyesinassumingthattheyarrivedonthesefullyformed.Butwhatiftheproteinsdidn’tassembleallatonce?Whatif,inthegreatslotmaeofcreation,someofthewheelscouldbeheld,asagamblermightholdanumberofpromisingcherries?Whatif,inotherwords,proteinsdidn’tsuddenlyburstintobeing,butevolved.
Imagineifyoutookallthepohatmakeupahumanbeing—carbon,hydrogen,oxygen,andsoon—andputtheminataihsomewater,gaveitavigorousstir,andoutsteppedapletedperson.Thatwouldbeamazihat’sessentiallywhatHoyleandothers(includingmanyardeionists)arguewhentheysuggestthatproteinsspontaneouslyformedallatoheydidn’t—they’thave.AsRichardDawkinsarguesinTheBlindWatchmaker,theremusthavebeensomekindofcumulativeseleprocessthatallowedaminoacidstoassembleinks.Perhapstwoorthreeaminoacidslinkedupforsomesimplepurposeaeratimebumpedintosomeothersimilarsmallclusterandinsodoing“discovered”someadditionalimprovement.
Chemicalreasofthesortassociatedwithlifeareactuallysomethingofaonplace.Itmaybebeyondustocookthemupinalab,àlaStanleyMillerandHaroldUrey,buttheuniversedoesitreadilyenough.Lotsofmoleculesinnaturegettogethertoformlongscalledpolymers.Sugarsstantlyassembletoformstarches.Crystalsberoflifelikethings—replicate,respondtoenviroalstimuli,takeonapatternedplexity.They’veneverachievedlifeitself,ofcourse,buttheydemonstraterepeatedlythatplexityisanatural,spontaneous,entirelyonplaceevent.Theremayormaynotbeagreatdealoflifeintheuniverseatlarge,butthereisneoforderedself-assembly,ihingfromthetransfixingsymmetryofsnowflakestotheelyringsofSaturn.
Sopowerfulisthisnaturalimpulsetoassemblethatmanystistsnowbelievethatlifemaybemoreiablethahink—thatitis,inthewordsoftheBelgianbiochemistandNobellaureateChristiandeDuve,“anobligatorymaionofmatter,boundtoarisewhereverditionsareappropriate.”DeDuvethoughtitlikelythatsuditionswouldbeenteredperhapsamilliontimesineverygalaxy.
Certainlythereisnothingterriblyexotithechemicalsthatanimateus.Ifyouwishedtocreateanotherlivingobject,whetheragoldfishoraheadoflettuceorahumanbeing,youwouldneedreallyonlyfourprincipalelements,carbon,hydrogen,oxygen,andnitrogen,plussmallamountsofafewothers,principallysulfur,phosphorus,calcium,andiron.Putthesetogetherinthreedozenorsobinationstoformsomesugars,acids,andotherbasipoundsandyoubuildanythingthatlives.AsDawkinshereisnothingspecialaboutthesubstancesfromwhichlivingthingsaremade.Livingthingsarecollesofmolecules,likeeverythingelse.”
Thebottomlihatlifeisamazingandgratifying,perhapsevenmiraculous,buthardlyimpossible—aswerepeatedlyattestwithourowexisteobesure,manyofthedetailsoflife’sbeginningsremaiyimponderable.Everysarioyouhaveeverreadingtheditionsnecessaryforlifeinvolveswater—fromthe“warmlittlepond”
whereDarwinsupposedlifebegantothebubblingseaventsthatarenowthemostpopulardidatesforlife’sbeginnings—butallthisoverlooksthefactthattoturnmonomersintopolymers(whichistosay,tobegintocreateproteins)involveswhatisknowntobiologyas“dehydrationlinkages.”Asoneleadingbiologytextputsit,withperhapsjustatinyhintofdisfort,“Researchersagreethatsuchreaswouldnothavebeeicallyfavorableintheprimitivesea,orindeedinanyaqueousmedium,becauseofthemassalaw.”Itisalittlelikeputtingsugarinaglassofwaterandhavingitbeeacube.Itshouldn’thappen,butsomehowinnatureitdoes.Theactualchemistryofallthisisalittleareforourpurposeshere,butitisenoughtoknowthatifyoumakemonomerswettheydon’tturnintopolymers—exceptwheinglifeoh.Howandwhyithappensthenandnototherwiseisoneofbiology’sgreatunansweredquestions.
Ohebiggestsurprisesihsretdecadeswasthediscoveryofjusthowearlyih’shistorylifearose.Wellintothe1950s,itwasthoughtthatlifewaslessthan600millionyearsold.Bythe1970s,afewadventuroussoulsfeltthatmaybeitwentback2.5billionyears.Butthepresentdateof3.85billionyearsisstunninglyearly.Earth’ssurfacedidn’tbeesoliduntilabout3.9billionyearsago.
“Welyinferfromthisrapiditythatitisnot‘difficult’forlifeofbacterialgradetoevolveonplasropriateditions,”StephenJayGouldobservedintheNewYorkTimesin1996.Orasheputitelsewhere,itishardtoavoidtheclusionthat“life,arisingassoonasitcould,waschemicallydestiobe.”
Lifeemergedsoswiftly,infact,thatsomeauthoritiesthinkitmusthavehadhelp—perhapsagooddealofhelp.Theideathatearthlylifemighthavearrivedfromspacehasasurprisinglylongandevenoccasionallydistinguishedhistory.ThegreatLordKelvinhimselfraisedthepossibilityaslongagoas1871atameetingoftheBritishAssociationfortheAdvaofScewhenhesuggestedthat“thegermsoflifemighthavebeenbroughttotheearthbysomemeteorite.”ButitremaitlemorethanafriionuntiloneSundayiember1969whentensofthousandsofAustraliaartledbyaseriesofsonisandthesightofafireballstreakingfromeasttowestacrossthesky.Thefireballmadeastrangecragsoundasitpassedabehindasmellthatsomelikeomethylatedspiritsandothersdescribedasjustawful.
ThefireballexplodedaboveMurchison,atownofsixhundredpeopleintheGoulburnValleynorthofMelbourne,andcamerainingdowninks,someweighinguptotwelvepounds.Fortunately,noonewashurt.Themeteoritewasofararetypeknownasacarbonaceousdrite,aownspeoplehelpfullycollectedandbroughtiwohundredpoundsofit.Thetimingcouldhardlyhavebeeer.Lessthantwomonthsearlier,theApollo11astronautshadreturoEarthwithabagfulloflunarrocks,solabsthroughouttheworldweregearedup—indeedclam—forrocksofextraterrestrialin.
TheMureteoritewasfoundtobe4.5billionyearsold,anditwasstuddedwithaminoacids—seventy-fourtypesinall,eightofwhichareinvolvedintheformatiohlyproteins.Inlate2001,morethanthirtyyearsafteritcrashed,ateamattheAmesResearterinCaliforniaannouhattheMurchisonrockalsotainedplexstringsofsugarscalledpolyols,whichhadnotbeenfoundofftheEarthbefore.
Afewothercarbonaceousdriteshavestrayedih’spathsinethatlandedagishLakeinada’sYukoninJanuary2000wasseenepartsofNorthAmeridtheyhavelikewisefirmedthattheuniverseisactuallyrianipounds.Halley’set,itisnowthought,isabout25pertanicmolecules.Getenoughofthosecrashingintoasuitableplace—Earth,forinstandyouhavethebasicelementsyouneedforlife.
Therearetwoproblemswithnotionsofpanspermia,asextraterrestrialtheoriesareknown.
Thefirstisthatitdoesn’tansweraionsabouthowlifearose,butmerelymovesresponsibilityforitelsewhere.Theotheristhatpanspermiasometimesexciteseverespectableadherentstolevelsofspeculationthatbesafelycalledimprudent.FrancisCrick,codiscovererofthestructureofDNA,andhiscolleagueLeslieelhavesuggestedthatEarthwas“deliberatelyseededwithlifebyintelligentaliens,”ahatGribbincalls“attheveryfringeofstificrespectability”—or,putanotherway,anotionthatwouldbesideredwildlylunatiotvoicedbyaNobellaureate.FredHoyleandhiscolleaguedraWickramasihererodedenthusiasmforpanspermiabysuggestingthatouterspacebroughtusnotonlylifebutalsomanydiseasessuchasfluandbubonicplague,ideasthatwereeasilydisprovedbybiochemists.Hoyle—anditseemsnecessarytoiareminderherethathewasohegreatstifidsofthetwehtury—alsooncesuggested,asmentionedearlier,thatournosesevolvedwiththenostrilsunderhasawayofkeepingicpathogensfromfallingintothemastheydrifteddownfromspace.
Whateverpromptedlifetobegin,ithappenedjustohatisthemostextraordinaryfabiology,perhapsthemostextraordinaryfactweknow.Everythingthathaseverlived,plantoranimal,datesitsbeginningsfromthesameprimordialtwitch.Atsomepointinanunimaginablydistantpastsomelittlebagofchemicalsfidgetedtolife.Itabsorbedsomenutrients,gentlypulsed,hadabriefexistehismuchmayhavehappenedbefore,perhapsmanytimes.Butthisaralpacketdidsomethingadditionalaraordinary:itcleaveditselfandproduheir.Atinybundleofgeicmaterialpassedfromoneliviytoanother,andhasoppedmovingsiwasthemomentofcreationforusall.
BiologistssometimescallittheBigBirth.
“Whereveryougointheworld,whateveranimal,plant,bug,orblobyoulookat,ifitisalive,itwillusethesamediaryandknowthesamecode.Alllifeisone,”saysMattRidley.Wearealltheresultofasinglegerideddeiontogeionnearlyfourbillionyears,tosuextentthatyoutakeafragmentofhumaistru,patchitintoafaultyyeastcell,andtheyeastcellwillputittoworkasifitwereitsown.Inaveryrealseisitsown.
Thedawnoflife—orsomethingverylikeit—sitsonashelfintheofficeofafriendlyisotopegeochemistnamedVictoriaBeihScesbuildingoftheAustralianNationalUyinberra.AnAmeri,Ms.BecametotheANUfromCaliforniaonatwo-yeartra1989andhasbeenthereeversince.WhenIvisitedher,inlate2001,shehandedmeamodestlyheftyhunkofroposedofthiingstripesofwhitequartzandagray-greenmaterialcalledopyroxeherockcamefromAkiliaIslandinGreenland,whereunusuallyarockswerefoundin1997.Therocksare3.85billionyearsoldandrepresenttheoldestmarinesedimentseverfound.
“We’tbecertainthatwhatyouareholdingoainedlivinganismsbecauseyou’dhavetopulverizeittofindout,”Beoldme.“Butitesfromthesamedepositwheretheoldestlifewasexcavated,soitprobablyhadlifeinit.”Norwouldyoufindactualfossilizedmicrobes,howevercarefullyyousearched.Anysimpleanisms,alas,wouldhavebeenbakedawaybytheprocessesthatturnedomudtostone.Insteadouldseeifwecheduptherodexamimicroscopicallywouldbethechemicalresiduesthattheanismsleftbehind—carbonisotopesandatypeofphosphatecalledapatite,whichtogetherprovidestrongevidetherotainediesoflivingthings.“Welyguesswhattheanismmighthavelookedlike,”Besaid.“Itrobablyaboutasbasicaslifeget—butitwaslifeheless.Itlived.Itpropagated.”
Auallyitledtous.
Ifyouareintoveryoldrocks,atindubitablyis,theANUhaslongbeenaprimeplacetobe.ThisislargelythankstotheiyofamannamedBillpston,whoisiredbutinthe1970sbuilttheworld’sfirstSensitiveHighResolutionIonMicroProbe—orSHRIMP,asitismoreaffeatelyknownfromitsinitialletters.Thisisamaethatmeasuresthedecayrateofuraniumintinymineralscalledzirs.Zirsappearinmostrocksapartfrombasaltsaremelydurable,survivingeverynaturalprocessbutsubduostoftheEarth’scrusthasbeenslippedbatotheovenatsomepoint,butjustoccasionally—iernAustraliaandGreenland,forexample—geologistshavefoundoutcropsofrocksthathaveremainedalwaysatthesurfapston’smaeallowedsuchrockstobedatedwithunparalleledprecisiootypeSHRIMPwasbuiltandmaedihScedepartment’sownworkshops,andlookedlikesomethingthathadbeenbuiltfromsparepartsonabudget,butitwreat.Onitsfirstformaltest,in1982,itdatedtheoldestthingeverfound—a4.3-billion-year-oldroWesternAustralia.
“Itcausedquiteastiratthetime,”Beoldme,“tofindsomethingsoimportantsoquicklywithbraeology.”
Shetookmedownthehalltoseethecurrentmodel,SHRIMPII.Itwasabigheavypieceofstainless-steelapparatus,perhapstwelvefeetlongandfivefeethigh,andassolidlybuiltasadeep-seaprobe.Atasoleinfrontofit,keepinganeyeonever-gingstringsoffiguresonas,wasamannamedBobfromterburyUyinNewZealand.Hehadbeentheresince4A.M.,hetoldme.SHRIMPIIruy-fourhoursaday;there’sthatmanyrockstodate.Itwasjustafter9A.M.andBobhadthemaetillnoon.Askapairofgeochemistshowsomethinglikethisworks,andtheywillstarttalkingaboutisotopidandionizationlevelswithahusiasmthatismoreendearingthanfathomable.
Theupshotofit,however,wasthatthemae,bybasampleofrockwithstreamsofchargedatoms,isabletodetectsubtlediffereheamountsofleadanduraniuminthezirples,bywhichmeanstheageofrocksbeaccuratelyadduced.
Bobtoldmethatittakesaboutseventeenmioreadonezirditisnecessarytoreaddozensfromeachroakethedatareliable.Inpractice,theprocessseemedtoinvolveaboutthesamelevelofscatteredactivity,andaboutasmuchstimulation,asatriptoalaundromat.Bobseemedveryhappy,however;butthenpeoplefromNewZealandverygenerallydo.
TheEarthSpoundwasanoddbinationofthings—partoffices,partlabs,partmaeshed.“Weusedtobuildeverythinghere,”Besaid.“Weevenhadlassblower,buthe’sretired.Butwestillhavetwofull-timerockcrushers.”Shecaughtmylookofmildsurprise.“Wegetthroughalotofrocks.Andtheyhavetobeverycarefullyprepared.Youhavetomakesurethereisnoinationfromprevioussamples—nodustoranything.It’squiteameticulousprocess.”Sheshowedmetherock-crushingmaes,whichwereindeedpristihoughtherockcrushershadapparentlygoneforcoffee.Besidethemaeswerelargeboxestainingrocksofallshapesandsizes.TheydoithroughalotofrocksattheANU.
BaBe’sofficeafterourtour,Inoticedhangingonherivinganartist’scolorfullyimaginativeinterpretatiohasitmighthavelooked3.5billionyearsago,justwhenlifewasgettinggoing,intheaperiodknowhsceastheArchaeaershowedanalienlandscapeofhuge,veryactivevoloes,andasteamy,copper-coloredseabehaharshredsky.Stromatolites,akindofbacterialrock,filledtheshallowsinthefround.Itdidn’tlooklikeaverypromisingplacetocreateandnurturelife.Iaskedherifthepaintingwasaccurate.
“Well,oneschoolofthoughtsaysitwasactuallycoolthenbecausethesunwasmuchweaker.”(Ilaterlearhatbiologists,whentheyarefeelingjocose,refertothisasthe“eserestaurantproblem”—becausewehadadimsun.)“Withoutanatmosphereultravioletraysfromthesun,evenfromaweaksun,wouldhaveteobreakapartanyincipientbondsmadebymolecules.Arightthere”—shetappedthestromatolites—“youhaveanismsalmostatthesurface.It’sapuzzle.”
“Sowedon’tknowwhattheworldwaslikebackthen?”
“Mmmm,”sheagreedthoughtfully.
“Eitherwayitdoesn’tseemveryducivetolife.”
Shenoddedamiably.“Buttheremusthavebeehingthatsuitedlife.Otherouldn’tbehere.”
Itcertainlywouldn’thavesuitedus.IfyouweretostepfromatimemaeintothataArchaeanworld,youwouldveryswiftlyscamperbaside,fortherewasnomoreoxygentobreatheohbackthenthanthereisonMarstoday.Itwasalsofullofnoxiousvaporsfromhydrochloridsulfuricacidspowerfulenoughtoeatthroughclothingandblisterskin.NorwouldithaveprovidedtheandglowingvistasdepictedierinVictoriaBe’soffice.ThechemicalstewthatwastheatmospherethenwouldhaveallowedlittlesunlighttoreachtheEarth’ssurface.Whatlittleyoucouldseewouldbeilluminedonlybrieflybybrightandfrequentlightningflashes.Inshort,itwasEarth,butahwewouldn’treizeasourown.
AnniversarieswerefewandfarbetweenintheArchaeanworld.Fortwobillionyearsbacterialanismsweretheonlyformsoflife.Theylived,theyreproduced,theyswarmed,buttheydidn’tshoarticularinationtomoveontoanother,morechallenginglevelofexistesomepointinthefirstbillionyearsoflife,obacteria,orblue-greenalgae,learapintoafreelyavailableresource—thehydrogenthatexistsiacularabundanwater.Theyabsorbedwatermolecules,suppedonthehydrogen,andreleasedtheoxygenaswaste,andinsodoingiedphotosynthesis.AsMargulisandSagannote,photosynthesisis“undoubtedlythemostimportantsiaboliovationioryoflifeonthepla”—anditwasiednotbyplantsbutbybacteria.
AsobacteriaproliferatedtheworldbegantofillwithO2tothesternationofthanismsthatfounditpoisonous—whithosedayswasallofthem.Inananaerobic(oranon-oxygen-using)world,oxygeremelypoisonous.Ourwhitecellsactuallyuseoxygentokillinvadingbacteria.Thatoxygenisfuallytoxicoftenesasasurprisetothoseofuswhofinditsovivialtoourwell-being,butthatisonlybecausewehaveevolvedtoexploitit.Tootherthingsitisaterror.Itiswhatturnsbutterrandmakesironrust.Eveolerateitonlyuptoapoint.Theoxygenlevelinourcellsisonlyaboutatenththelevelfoundimosphere.
Thenewoxygen-usinganismshadtwoadvantages.Oxygenwasamoreeffitroduergy,anditvanquishedpetitanisms.Someretreatedintotheoozy,anaerobicworldofbogsandlakebottoms.Othersdidlikewisebutthenlater(muchlater)migratedtothedigestivetractsofbeingslikeyouandme.Quiteaheseprimevalentitiesarealiveinsideyourbhtnow,helpingtodigestyourfood,butabheveihintofO2.Untoldnumbersofothersfailedtoadaptanddied.
Theobacteriawerearunawaysuccess.Atfirst,theextraoxygentheyproduceddidn’taccumulateimosphere,butbihirontoformferricoxides,whiktothebottomofprimitiveseas.Formillionsofyears,theworldliterallyrusted—aphenomenonvividlyrecordedinthebandedirondepositsthatprovidesomuchoftheworld’sirooday.Formanytensofmillionsofyearsnotagreatdealmorethanthishappened.IfyouwentbacktothatearlyProterozoicworldyouwouldn’tfindmanysignsofpromiseforEarth’sfuturelife.Perhapshereandthereieredpoolsyou’denterafilmoflivingscumoracoatingofglossygreensandbrownsonshorelinerocks,butotherwiseliferemainedinvisible.
Butabout3.5billionyearsagosomethingmoreemphaticbecameapparent.Wherevertheseaswereshallow,visiblestructuresbegantoappear.Astheywentthroughtheirchemicalroutiheobacteriabecameveryslightlytacky,andthattaesstrappedmicroparticlesofdustandsand,whichbecameboundtogethertoformslightlyweirdbutsolidstructures—thestromatolitesthatwerefeaturedintheshallowsoftheposteronVictoriaBe’sofficewall.Stromatolitescameinvariousshapesandsizes.Sometimestheylookedlikeenormouscauliflowers,sometimeslikefluffymattresses(stromatoliteesfromtheGreekfor“mattress”),sometimestheycameintheformofns,risingtensofmetersabovethesurfaceofthewater—sometimesashighasahuers.Inalltheirmaions,theywereakindoflivingrodtheyrepreseheworld’sfirstcooperativevehsomevarietiesofprimitiveanismlivingjustatthesurfadotherslivingjustunderh,eachtakingadvantageofditioedbytheother.Theworldhaditsfirstecosystem.
Formanyyears,stistsknewaboutstromatolitesfromfossilformations,butin1961theygotarealsurprisewiththediscoveryofaunityoflivingstromatolitesatSharkBayontheremotenorthwestcoastofAustralia.Thiswasmostued—soued,infact,thatitwassomeyearsbeforestistsrealizedquitewhattheyhadfound.Today,however,SharkBayisatouristattra—oratleastasmuchofatouristattraasapladredsofmilesfromanywheremuddozensofmilesfromaalleverbe.
Boardwalkshavebeenbuiltoutintothebaysothatvisitorsstrolloverthewatertogetagoodlookatthestromatolites,quietlyrespiringjustbehesurface.Theyarelusterlessandgrayandlook,asIrecordedinanearlierbook,likeverylargeco松语文学www.16sy.coM免费小说阅读
Pressreportsofthetimemadeitsoundasifaboutallthatwasneedednowwasforsomebodytogivethewholeagoodshakeandlifewouldcrawlout.Astimehasshown,itwasn’tnearlysosimple.Despitehalfaturyoffurtherstudy,wearenoosynthesizinglifetodaythanwewerein1953andmuchfurtherawayfromthinkingwe.
StistsarenowprettycertainthattheearlyatmospherewasnothinglikeasprimedfordevelopmentasMillerandUrey’sgaseousstew,butratherwasamuchlessreactiveblendofnitrogenandcarbondioxide.RepeatingMiller’sexperimentswiththesemorechallenginginputshassofarprodulyonefairlyprimitiveaminoacid.Atallevents,creatingaminoacidsisnotreallytheproblem.Theproblemisproteins.
Proteiyougetwhenyaminoacidstogether,andweneedalotofthem.
Noonereallyknows,buttheremaybeasmanyasamilliontypesofproteininthehumanbody,andeaeisalittlemiracle.Byallthelawsofprobabilityproteinsshould.
Tomakeaproteinyouoassembleaminoacids(whichIamobligedbylongtraditioohereas“thebuildingblocksoflife”)inaparticularorder,inmuchthesamewaythatyouassemblelettersinaparticularordertospellaword.Theproblemisthatwordsintheaminoacidalphabetareoftenexceedinglylong.Tospellcollagen,thenameofaontypeofprotein,youeeightlettersintherightorder.Buttomakecollagen,youe1,055aminoacidsinpreciselytherightseque—andhere’sanobviousbutcrucialpoint—youdon’tmakeit.Itmakesitself,spontaneously,withoutdire,andthisiswheretheunlikelihoodsein.
Thecesofa1,055-sequencemoleculelikecollagenspontaneouslyself-assemblingare,frankly,nil.Itjustisn’tgoingtohappen.Tograspwhatalongshotitsexistenceis,visualizeastandardLasVegasslotmaebutbroadenedgreatly—toaboutyfeet,tobeprecise—toaodate1,055spinningwheelsinsteadoftheusualthreeorfour,andwithtwentysymbolsoneachwheel(oneforeaonaminoacid).
1Howlongwouldyouhavetopullthehandlebeforeall1,055symbolscameupintherightorder?Effectivelyforever.Evenifyoureducedthenumberofspinniotwohundred,whichisactuallyamoretypiumberofaminoacidsforaprotein,theoddsagainstalltwohundredingupina1Thereareactuallytwenty-twonaturallyaminoacidsknowh,andmoremayawaitdiscovery,butonlytwentyofthemarenecessarytoproduceusandmostotherlivingthings.Thetwenty-sed,calledpyrrolysine,wasdiscoveredin2002byresearchersatOhioStateUyandisfoundonlyinasiypeofarchaean(abasioflifethatwewilldiscussalittlefurtheroniory)calledMethanosarabarkeri.
prescribedsequenceare1in10260(thatisa1followedby260zeroes).Thatinitselfisalargerhanalltheatomsintheuniverse.
Proteins,inshort,areplexentities.Hemoglobinisonly146aminoacidslong,aruntbyproteinstandards,yetevenitoffers10190possibleaminoabinations,whichiswhyittooktheCambridgeUychemistMaxPerutztwenty-threeyears—acareer,moreorless—tou.Forrastoproduceevenasieinwouldseemastunningimprobability—likeawhirlwindspinningthroughajunkyardandleavingbehindafullyassembledjumbojet,inthecolorfulsimileoftheastronomerFredHoyle.
Yetwearetalkingaboutseveralhuhousandtypesofprotein,perhapsamillion,eaiqueandeach,asfarasweknow,vitaltothemaintenanceofasoundandhappyyou.Anditgoesonfromthere.Aproteintobeofusemustnotonlyassembleaminoacidsintherightsequethenmustengageinakindofchemicalamiandfolditselfintoaveryspecificshape.Evenhavingachievedthisstructuralplexity,aproteinisnogoodtoyouifit’treproduceitself,andproteins’t.ForthisyouneedDNA.DNAisalig—itmakeacopyofitselfinseds—butdovirtuallynothingelse.Soaradoxicalsituation.Proteins’texistwithoutDNA,andDNAhasnopurposewithoutproteins.Arewetoassumethenthattheyarosesimultaneouslywiththepurposeofsuppeachother?Ifso:wow.
Andthereismorestill.DNA,proteins,aherposoflifecouldn’tprosperwithoutsomesortofmembraotainthem.Noatomormoleculehaseverachievedlifeindepely.Pluyatomfromyourbody,anditisnomorealivethanisagrainofsand.
Itisonlywhentheyetogetherwithiuringrefugeofacellthatthesediversematerialstakepartintheamazingdawecalllife.Withoutthecell,theyarenothingmorethaingchemicals.Butwithoutthechemicals,thecellhasnopurpose.
AsthephysicistPaulDaviesputsit,“Ifeverythingneedseverythingelse,howdidtheunityofmoleculeseverariseinthefirstplace?”Itisratherasifalltheingredientsinyourkitsomehowgottogetherandbakedthemselvesintoacake—butacakethatoreoverdividewhennecessarytoproducemorecakes.Itislittlewohatwecallitthemiracleoflife.Itisalsolittlewohatwehavebarelybeguntouandit.
Sowhatatsforallthiswondrousplexity?Well,onepossibilityisthatperhapsitisn’tquite—notquite—sowondrousasatfirstitseems.Takethoseamazinglyimprobableproteins.Thewonderweseeintheirassemblyesinassumingthattheyarrivedonthesefullyformed.Butwhatiftheproteinsdidn’tassembleallatonce?Whatif,inthegreatslotmaeofcreation,someofthewheelscouldbeheld,asagamblermightholdanumberofpromisingcherries?Whatif,inotherwords,proteinsdidn’tsuddenlyburstintobeing,butevolved.
Imagineifyoutookallthepohatmakeupahumanbeing—carbon,hydrogen,oxygen,andsoon—andputtheminataihsomewater,gaveitavigorousstir,andoutsteppedapletedperson.Thatwouldbeamazihat’sessentiallywhatHoyleandothers(includingmanyardeionists)arguewhentheysuggestthatproteinsspontaneouslyformedallatoheydidn’t—they’thave.AsRichardDawkinsarguesinTheBlindWatchmaker,theremusthavebeensomekindofcumulativeseleprocessthatallowedaminoacidstoassembleinks.Perhapstwoorthreeaminoacidslinkedupforsomesimplepurposeaeratimebumpedintosomeothersimilarsmallclusterandinsodoing“discovered”someadditionalimprovement.
Chemicalreasofthesortassociatedwithlifeareactuallysomethingofaonplace.Itmaybebeyondustocookthemupinalab,àlaStanleyMillerandHaroldUrey,buttheuniversedoesitreadilyenough.Lotsofmoleculesinnaturegettogethertoformlongscalledpolymers.Sugarsstantlyassembletoformstarches.Crystalsberoflifelikethings—replicate,respondtoenviroalstimuli,takeonapatternedplexity.They’veneverachievedlifeitself,ofcourse,buttheydemonstraterepeatedlythatplexityisanatural,spontaneous,entirelyonplaceevent.Theremayormaynotbeagreatdealoflifeintheuniverseatlarge,butthereisneoforderedself-assembly,ihingfromthetransfixingsymmetryofsnowflakestotheelyringsofSaturn.
Sopowerfulisthisnaturalimpulsetoassemblethatmanystistsnowbelievethatlifemaybemoreiablethahink—thatitis,inthewordsoftheBelgianbiochemistandNobellaureateChristiandeDuve,“anobligatorymaionofmatter,boundtoarisewhereverditionsareappropriate.”DeDuvethoughtitlikelythatsuditionswouldbeenteredperhapsamilliontimesineverygalaxy.
Certainlythereisnothingterriblyexotithechemicalsthatanimateus.Ifyouwishedtocreateanotherlivingobject,whetheragoldfishoraheadoflettuceorahumanbeing,youwouldneedreallyonlyfourprincipalelements,carbon,hydrogen,oxygen,andnitrogen,plussmallamountsofafewothers,principallysulfur,phosphorus,calcium,andiron.Putthesetogetherinthreedozenorsobinationstoformsomesugars,acids,andotherbasipoundsandyoubuildanythingthatlives.AsDawkinshereisnothingspecialaboutthesubstancesfromwhichlivingthingsaremade.Livingthingsarecollesofmolecules,likeeverythingelse.”
Thebottomlihatlifeisamazingandgratifying,perhapsevenmiraculous,buthardlyimpossible—aswerepeatedlyattestwithourowexisteobesure,manyofthedetailsoflife’sbeginningsremaiyimponderable.Everysarioyouhaveeverreadingtheditionsnecessaryforlifeinvolveswater—fromthe“warmlittlepond”
whereDarwinsupposedlifebegantothebubblingseaventsthatarenowthemostpopulardidatesforlife’sbeginnings—butallthisoverlooksthefactthattoturnmonomersintopolymers(whichistosay,tobegintocreateproteins)involveswhatisknowntobiologyas“dehydrationlinkages.”Asoneleadingbiologytextputsit,withperhapsjustatinyhintofdisfort,“Researchersagreethatsuchreaswouldnothavebeeicallyfavorableintheprimitivesea,orindeedinanyaqueousmedium,becauseofthemassalaw.”Itisalittlelikeputtingsugarinaglassofwaterandhavingitbeeacube.Itshouldn’thappen,butsomehowinnatureitdoes.Theactualchemistryofallthisisalittleareforourpurposeshere,butitisenoughtoknowthatifyoumakemonomerswettheydon’tturnintopolymers—exceptwheinglifeoh.Howandwhyithappensthenandnototherwiseisoneofbiology’sgreatunansweredquestions.
Ohebiggestsurprisesihsretdecadeswasthediscoveryofjusthowearlyih’shistorylifearose.Wellintothe1950s,itwasthoughtthatlifewaslessthan600millionyearsold.Bythe1970s,afewadventuroussoulsfeltthatmaybeitwentback2.5billionyears.Butthepresentdateof3.85billionyearsisstunninglyearly.Earth’ssurfacedidn’tbeesoliduntilabout3.9billionyearsago.
“Welyinferfromthisrapiditythatitisnot‘difficult’forlifeofbacterialgradetoevolveonplasropriateditions,”StephenJayGouldobservedintheNewYorkTimesin1996.Orasheputitelsewhere,itishardtoavoidtheclusionthat“life,arisingassoonasitcould,waschemicallydestiobe.”
Lifeemergedsoswiftly,infact,thatsomeauthoritiesthinkitmusthavehadhelp—perhapsagooddealofhelp.Theideathatearthlylifemighthavearrivedfromspacehasasurprisinglylongandevenoccasionallydistinguishedhistory.ThegreatLordKelvinhimselfraisedthepossibilityaslongagoas1871atameetingoftheBritishAssociationfortheAdvaofScewhenhesuggestedthat“thegermsoflifemighthavebeenbroughttotheearthbysomemeteorite.”ButitremaitlemorethanafriionuntiloneSundayiember1969whentensofthousandsofAustraliaartledbyaseriesofsonisandthesightofafireballstreakingfromeasttowestacrossthesky.Thefireballmadeastrangecragsoundasitpassedabehindasmellthatsomelikeomethylatedspiritsandothersdescribedasjustawful.
ThefireballexplodedaboveMurchison,atownofsixhundredpeopleintheGoulburnValleynorthofMelbourne,andcamerainingdowninks,someweighinguptotwelvepounds.Fortunately,noonewashurt.Themeteoritewasofararetypeknownasacarbonaceousdrite,aownspeoplehelpfullycollectedandbroughtiwohundredpoundsofit.Thetimingcouldhardlyhavebeeer.Lessthantwomonthsearlier,theApollo11astronautshadreturoEarthwithabagfulloflunarrocks,solabsthroughouttheworldweregearedup—indeedclam—forrocksofextraterrestrialin.
TheMureteoritewasfoundtobe4.5billionyearsold,anditwasstuddedwithaminoacids—seventy-fourtypesinall,eightofwhichareinvolvedintheformatiohlyproteins.Inlate2001,morethanthirtyyearsafteritcrashed,ateamattheAmesResearterinCaliforniaannouhattheMurchisonrockalsotainedplexstringsofsugarscalledpolyols,whichhadnotbeenfoundofftheEarthbefore.
Afewothercarbonaceousdriteshavestrayedih’spathsinethatlandedagishLakeinada’sYukoninJanuary2000wasseenepartsofNorthAmeridtheyhavelikewisefirmedthattheuniverseisactuallyrianipounds.Halley’set,itisnowthought,isabout25pertanicmolecules.Getenoughofthosecrashingintoasuitableplace—Earth,forinstandyouhavethebasicelementsyouneedforlife.
Therearetwoproblemswithnotionsofpanspermia,asextraterrestrialtheoriesareknown.
Thefirstisthatitdoesn’tansweraionsabouthowlifearose,butmerelymovesresponsibilityforitelsewhere.Theotheristhatpanspermiasometimesexciteseverespectableadherentstolevelsofspeculationthatbesafelycalledimprudent.FrancisCrick,codiscovererofthestructureofDNA,andhiscolleagueLeslieelhavesuggestedthatEarthwas“deliberatelyseededwithlifebyintelligentaliens,”ahatGribbincalls“attheveryfringeofstificrespectability”—or,putanotherway,anotionthatwouldbesideredwildlylunatiotvoicedbyaNobellaureate.FredHoyleandhiscolleaguedraWickramasihererodedenthusiasmforpanspermiabysuggestingthatouterspacebroughtusnotonlylifebutalsomanydiseasessuchasfluandbubonicplague,ideasthatwereeasilydisprovedbybiochemists.Hoyle—anditseemsnecessarytoiareminderherethathewasohegreatstifidsofthetwehtury—alsooncesuggested,asmentionedearlier,thatournosesevolvedwiththenostrilsunderhasawayofkeepingicpathogensfromfallingintothemastheydrifteddownfromspace.
Whateverpromptedlifetobegin,ithappenedjustohatisthemostextraordinaryfabiology,perhapsthemostextraordinaryfactweknow.Everythingthathaseverlived,plantoranimal,datesitsbeginningsfromthesameprimordialtwitch.Atsomepointinanunimaginablydistantpastsomelittlebagofchemicalsfidgetedtolife.Itabsorbedsomenutrients,gentlypulsed,hadabriefexistehismuchmayhavehappenedbefore,perhapsmanytimes.Butthisaralpacketdidsomethingadditionalaraordinary:itcleaveditselfandproduheir.Atinybundleofgeicmaterialpassedfromoneliviytoanother,andhasoppedmovingsiwasthemomentofcreationforusall.
BiologistssometimescallittheBigBirth.
“Whereveryougointheworld,whateveranimal,plant,bug,orblobyoulookat,ifitisalive,itwillusethesamediaryandknowthesamecode.Alllifeisone,”saysMattRidley.Wearealltheresultofasinglegerideddeiontogeionnearlyfourbillionyears,tosuextentthatyoutakeafragmentofhumaistru,patchitintoafaultyyeastcell,andtheyeastcellwillputittoworkasifitwereitsown.Inaveryrealseisitsown.
Thedawnoflife—orsomethingverylikeit—sitsonashelfintheofficeofafriendlyisotopegeochemistnamedVictoriaBeihScesbuildingoftheAustralianNationalUyinberra.AnAmeri,Ms.BecametotheANUfromCaliforniaonatwo-yeartra1989andhasbeenthereeversince.WhenIvisitedher,inlate2001,shehandedmeamodestlyheftyhunkofroposedofthiingstripesofwhitequartzandagray-greenmaterialcalledopyroxeherockcamefromAkiliaIslandinGreenland,whereunusuallyarockswerefoundin1997.Therocksare3.85billionyearsoldandrepresenttheoldestmarinesedimentseverfound.
“We’tbecertainthatwhatyouareholdingoainedlivinganismsbecauseyou’dhavetopulverizeittofindout,”Beoldme.“Butitesfromthesamedepositwheretheoldestlifewasexcavated,soitprobablyhadlifeinit.”Norwouldyoufindactualfossilizedmicrobes,howevercarefullyyousearched.Anysimpleanisms,alas,wouldhavebeenbakedawaybytheprocessesthatturnedomudtostone.Insteadouldseeifwecheduptherodexamimicroscopicallywouldbethechemicalresiduesthattheanismsleftbehind—carbonisotopesandatypeofphosphatecalledapatite,whichtogetherprovidestrongevidetherotainediesoflivingthings.“Welyguesswhattheanismmighthavelookedlike,”Besaid.“Itrobablyaboutasbasicaslifeget—butitwaslifeheless.Itlived.Itpropagated.”
Auallyitledtous.
Ifyouareintoveryoldrocks,atindubitablyis,theANUhaslongbeenaprimeplacetobe.ThisislargelythankstotheiyofamannamedBillpston,whoisiredbutinthe1970sbuilttheworld’sfirstSensitiveHighResolutionIonMicroProbe—orSHRIMP,asitismoreaffeatelyknownfromitsinitialletters.Thisisamaethatmeasuresthedecayrateofuraniumintinymineralscalledzirs.Zirsappearinmostrocksapartfrombasaltsaremelydurable,survivingeverynaturalprocessbutsubduostoftheEarth’scrusthasbeenslippedbatotheovenatsomepoint,butjustoccasionally—iernAustraliaandGreenland,forexample—geologistshavefoundoutcropsofrocksthathaveremainedalwaysatthesurfapston’smaeallowedsuchrockstobedatedwithunparalleledprecisiootypeSHRIMPwasbuiltandmaedihScedepartment’sownworkshops,andlookedlikesomethingthathadbeenbuiltfromsparepartsonabudget,butitwreat.Onitsfirstformaltest,in1982,itdatedtheoldestthingeverfound—a4.3-billion-year-oldroWesternAustralia.
“Itcausedquiteastiratthetime,”Beoldme,“tofindsomethingsoimportantsoquicklywithbraeology.”
Shetookmedownthehalltoseethecurrentmodel,SHRIMPII.Itwasabigheavypieceofstainless-steelapparatus,perhapstwelvefeetlongandfivefeethigh,andassolidlybuiltasadeep-seaprobe.Atasoleinfrontofit,keepinganeyeonever-gingstringsoffiguresonas,wasamannamedBobfromterburyUyinNewZealand.Hehadbeentheresince4A.M.,hetoldme.SHRIMPIIruy-fourhoursaday;there’sthatmanyrockstodate.Itwasjustafter9A.M.andBobhadthemaetillnoon.Askapairofgeochemistshowsomethinglikethisworks,andtheywillstarttalkingaboutisotopidandionizationlevelswithahusiasmthatismoreendearingthanfathomable.
Theupshotofit,however,wasthatthemae,bybasampleofrockwithstreamsofchargedatoms,isabletodetectsubtlediffereheamountsofleadanduraniuminthezirples,bywhichmeanstheageofrocksbeaccuratelyadduced.
Bobtoldmethatittakesaboutseventeenmioreadonezirditisnecessarytoreaddozensfromeachroakethedatareliable.Inpractice,theprocessseemedtoinvolveaboutthesamelevelofscatteredactivity,andaboutasmuchstimulation,asatriptoalaundromat.Bobseemedveryhappy,however;butthenpeoplefromNewZealandverygenerallydo.
TheEarthSpoundwasanoddbinationofthings—partoffices,partlabs,partmaeshed.“Weusedtobuildeverythinghere,”Besaid.“Weevenhadlassblower,buthe’sretired.Butwestillhavetwofull-timerockcrushers.”Shecaughtmylookofmildsurprise.“Wegetthroughalotofrocks.Andtheyhavetobeverycarefullyprepared.Youhavetomakesurethereisnoinationfromprevioussamples—nodustoranything.It’squiteameticulousprocess.”Sheshowedmetherock-crushingmaes,whichwereindeedpristihoughtherockcrushershadapparentlygoneforcoffee.Besidethemaeswerelargeboxestainingrocksofallshapesandsizes.TheydoithroughalotofrocksattheANU.
BaBe’sofficeafterourtour,Inoticedhangingonherivinganartist’scolorfullyimaginativeinterpretatiohasitmighthavelooked3.5billionyearsago,justwhenlifewasgettinggoing,intheaperiodknowhsceastheArchaeaershowedanalienlandscapeofhuge,veryactivevoloes,andasteamy,copper-coloredseabehaharshredsky.Stromatolites,akindofbacterialrock,filledtheshallowsinthefround.Itdidn’tlooklikeaverypromisingplacetocreateandnurturelife.Iaskedherifthepaintingwasaccurate.
“Well,oneschoolofthoughtsaysitwasactuallycoolthenbecausethesunwasmuchweaker.”(Ilaterlearhatbiologists,whentheyarefeelingjocose,refertothisasthe“eserestaurantproblem”—becausewehadadimsun.)“Withoutanatmosphereultravioletraysfromthesun,evenfromaweaksun,wouldhaveteobreakapartanyincipientbondsmadebymolecules.Arightthere”—shetappedthestromatolites—“youhaveanismsalmostatthesurface.It’sapuzzle.”
“Sowedon’tknowwhattheworldwaslikebackthen?”
“Mmmm,”sheagreedthoughtfully.
“Eitherwayitdoesn’tseemveryducivetolife.”
Shenoddedamiably.“Buttheremusthavebeehingthatsuitedlife.Otherouldn’tbehere.”
Itcertainlywouldn’thavesuitedus.IfyouweretostepfromatimemaeintothataArchaeanworld,youwouldveryswiftlyscamperbaside,fortherewasnomoreoxygentobreatheohbackthenthanthereisonMarstoday.Itwasalsofullofnoxiousvaporsfromhydrochloridsulfuricacidspowerfulenoughtoeatthroughclothingandblisterskin.NorwouldithaveprovidedtheandglowingvistasdepictedierinVictoriaBe’soffice.ThechemicalstewthatwastheatmospherethenwouldhaveallowedlittlesunlighttoreachtheEarth’ssurface.Whatlittleyoucouldseewouldbeilluminedonlybrieflybybrightandfrequentlightningflashes.Inshort,itwasEarth,butahwewouldn’treizeasourown.
AnniversarieswerefewandfarbetweenintheArchaeanworld.Fortwobillionyearsbacterialanismsweretheonlyformsoflife.Theylived,theyreproduced,theyswarmed,buttheydidn’tshoarticularinationtomoveontoanother,morechallenginglevelofexistesomepointinthefirstbillionyearsoflife,obacteria,orblue-greenalgae,learapintoafreelyavailableresource—thehydrogenthatexistsiacularabundanwater.Theyabsorbedwatermolecules,suppedonthehydrogen,andreleasedtheoxygenaswaste,andinsodoingiedphotosynthesis.AsMargulisandSagannote,photosynthesisis“undoubtedlythemostimportantsiaboliovationioryoflifeonthepla”—anditwasiednotbyplantsbutbybacteria.
AsobacteriaproliferatedtheworldbegantofillwithO2tothesternationofthanismsthatfounditpoisonous—whithosedayswasallofthem.Inananaerobic(oranon-oxygen-using)world,oxygeremelypoisonous.Ourwhitecellsactuallyuseoxygentokillinvadingbacteria.Thatoxygenisfuallytoxicoftenesasasurprisetothoseofuswhofinditsovivialtoourwell-being,butthatisonlybecausewehaveevolvedtoexploitit.Tootherthingsitisaterror.Itiswhatturnsbutterrandmakesironrust.Eveolerateitonlyuptoapoint.Theoxygenlevelinourcellsisonlyaboutatenththelevelfoundimosphere.
Thenewoxygen-usinganismshadtwoadvantages.Oxygenwasamoreeffitroduergy,anditvanquishedpetitanisms.Someretreatedintotheoozy,anaerobicworldofbogsandlakebottoms.Othersdidlikewisebutthenlater(muchlater)migratedtothedigestivetractsofbeingslikeyouandme.Quiteaheseprimevalentitiesarealiveinsideyourbhtnow,helpingtodigestyourfood,butabheveihintofO2.Untoldnumbersofothersfailedtoadaptanddied.
Theobacteriawerearunawaysuccess.Atfirst,theextraoxygentheyproduceddidn’taccumulateimosphere,butbihirontoformferricoxides,whiktothebottomofprimitiveseas.Formillionsofyears,theworldliterallyrusted—aphenomenonvividlyrecordedinthebandedirondepositsthatprovidesomuchoftheworld’sirooday.Formanytensofmillionsofyearsnotagreatdealmorethanthishappened.IfyouwentbacktothatearlyProterozoicworldyouwouldn’tfindmanysignsofpromiseforEarth’sfuturelife.Perhapshereandthereieredpoolsyou’denterafilmoflivingscumoracoatingofglossygreensandbrownsonshorelinerocks,butotherwiseliferemainedinvisible.
Butabout3.5billionyearsagosomethingmoreemphaticbecameapparent.Wherevertheseaswereshallow,visiblestructuresbegantoappear.Astheywentthroughtheirchemicalroutiheobacteriabecameveryslightlytacky,andthattaesstrappedmicroparticlesofdustandsand,whichbecameboundtogethertoformslightlyweirdbutsolidstructures—thestromatolitesthatwerefeaturedintheshallowsoftheposteronVictoriaBe’sofficewall.Stromatolitescameinvariousshapesandsizes.Sometimestheylookedlikeenormouscauliflowers,sometimeslikefluffymattresses(stromatoliteesfromtheGreekfor“mattress”),sometimestheycameintheformofns,risingtensofmetersabovethesurfaceofthewater—sometimesashighasahuers.Inalltheirmaions,theywereakindoflivingrodtheyrepreseheworld’sfirstcooperativevehsomevarietiesofprimitiveanismlivingjustatthesurfadotherslivingjustunderh,eachtakingadvantageofditioedbytheother.Theworldhaditsfirstecosystem.
Formanyyears,stistsknewaboutstromatolitesfromfossilformations,butin1961theygotarealsurprisewiththediscoveryofaunityoflivingstromatolitesatSharkBayontheremotenorthwestcoastofAustralia.Thiswasmostued—soued,infact,thatitwassomeyearsbeforestistsrealizedquitewhattheyhadfound.Today,however,SharkBayisatouristattra—oratleastasmuchofatouristattraasapladredsofmilesfromanywheremuddozensofmilesfromaalleverbe.
Boardwalkshavebeenbuiltoutintothebaysothatvisitorsstrolloverthewatertogetagoodlookatthestromatolites,quietlyrespiringjustbehesurface.Theyarelusterlessandgrayandlook,asIrecordedinanearlierbook,likeverylargeco松语文学www.16sy.coM免费小说阅读
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