22 GOOD-BYE TO ALL THAT
WHENYOUSIDERitfromahumanperspective,andclearlyitwouldbedifficultforustodootherwise,lifeisanoddthing.Itcouldn’twaittogetgoing,butthen,havinggottengoing,itseemedinverylittlehurrytomoveon.
sidertheli.Lisarejustaboutthehardiestvisibleanismsoh,butamongtheleastambitious.Theywillgroilyenoughinasunnychurchyard,buttheyparticularlythriveinenviroswherenoanismwouldgo—onblowymountaintopsandarcticwastes,whereverthereislittlebutrodrainandcold,andalmostnopetition.InareasofAntarcticawherevirtuallynothingelsewillgrow,youfindvastexpansesofli—fourhuypesofthem—adheriedlytoeverywind-whippedrock.
Foralongtime,peoplecouldn’tuandhowtheydidit.Becauselisgrewonbarerockwithoutevidentnourishmentortheproduofseeds,manypeople—educatedpeople—believedtheywerestonescaughtintheprocessofbeingplants.“Spontaneously,inaniebeeslivingplant!”rejoieobserver,aDr.Homschu1819.
Closeriionshowedthatlisweremoreiingthanmagical.Theyareinfactapartnershipbetweenfungiandalgae.Thefungiexcreteacidsthatdissolvethesurfaceoftherock,freeingmineralsthatthealgaevertintofoodsuffittosustainboth.Itisnotaveryexgarra,butitisaspicuouslysuccessfuloheworldhasmorethahousandspeciesoflis.
Likemostthingsthatthriveinharshenviros,lisareslow-growing.Itmaytakealimorethanhalfaturytoattainthedimensionsofashirtbutton.Thosethesizeofdinnerplates,writesDavidAttenbh,aretherefore“likelytobehundredsifnotthousandsofyearsold.”Itwouldbehardtoimaginealessfulfillieheysimplyexist,”Attenbhadds,“testifyingtothemovingfactthatlifeevenatitssimplestleveloccurs,apparently,justforitsownsake.”
Itiseasytooverlookthisthoughtthatlifejustis.Ashumansweareinedtofeelthatlifemusthaveapoint.lansandaspirationsanddesires.Wewanttotakestantadvantageofalltheintoxigexistencewe’vebeeneh.Butwhat’slifetoali?Yetitsimpulsetoexist,tobe,iseverybitasstrongasours—arguablyevenstronger.
IfIweretoldthatIhadtospenddecadesbeingafurrygrowthonarothewoods,IbelieveIwouldlosethewilltogoon.Lisdon’t.Likevirtuallyalllivingthings,theywillsufferanyhardship,endureanyinsult,foramoment’sadditioence.Life,inshort,justwantstobe.But—andhere’saingpoint—forthemostpartitdoesn’twanttobemuch.
Thisisperhapsalittleoddbecauselifehashadplentyoftimetodevelopambitions.Ifyouimagihe4,500-billion-oddyearsofEarth’shistorypressedintoanormalearthlyday,thenlifebeginsveryearly,about4A.M.,withtheriseofthefirstsimple,single-celledanisms,butthenadvanofurtherforthesixteenhours.Notuntilalmost8:30intheevening,withthedayfive-sixthsover,hasEarthanythingtoshowtheuniversebutarestlessskinofmicrobes.Then,finally,thefirstseaplantsappear,followedtwentymierbythefirstjellyfishandtheenigmaticEdiafaunafirstseenbyReginaldSprigginAustralia.At9:04P.M.trilobitesswimontothese,followedmoreorlessimmediatelybytheshapelycreaturesoftheBurgessShale.Justbefore10P.M.plaopopupontheland.Soonafter,withlessthantwohoursleftintheday,thefirstlauresfollow.
Thankstotenminutesorsoofbalmyweather,by10:24theEarthiscoveredinthegreatcarboniferousforestswhoseresiduesgiveusallourcoal,andthefirstwingedisareevident.Dinosaursplodontothesejustbefore11P.M.andholdswayforaboutthree-quartersofanhour.Attwenty-onemiomidnighttheyvanishandtheageofmammalsbegins.Humansemergeoneminuteaeensedsbeforemidnight.Thewholeofourrecordedhistory,onthisscale,wouldbenomorethanafewseds,asinglehumaimebarelyaninstant.Throughoutthisgreatlyspeeded-updaytisslideaboutandbangtogetherataclipthatseemspositivelyreckless.Mountainsriseaaway,obasinseandgo,icesheetsadvandwithdraw.Andthroughoutthewhole,aboutthreetimeseveryminute,somewhereontheplahereisaflashbulbpopoflightmarkingtheimpaanson-sizedmeteororoneevenlarger.It’sawohatanythingatallsurviveinsuchapummeledanduledenviro.Infaanythingsd.
Perhapsanevenmoreeffectivewayofgraspingourextremereessasapartofthis4.5-billion-year-oldpictureistostretchyourarmstotheirfullestextentandimagiwidthastheeoryoftheEarth.Onthisscale,acctoJohnMcPheeinBasinandRahedistanthefiipsofonehandtothewristoftheotherisPrecambrian.
Allofplexlifeisinonehand,“andinasirokewithamedium-grainednailfileyoucoulderadicatehumanhistory.”
Fortuhatmomenthasn’thappened,butthecesaregoodthatitwill.Idon’twishtointerjeoteofgloomjustatthispoint,butthefactisthatthereisoherextremelypertiqualityaboutlifeoh:itgoesextinct.Quiteregularly.Forallthetroubletheytaketoassembleandpreservethemselves,speciescrumpleanddieremarkablyroutinely.Andthemoreplextheyget,themorequicklytheyappeartogoextinct.Whichisperhapsonereasonwhysomuchoflifeisn’tterriblyambitious.
Soanytimelifedoessomethingbolditisquitea,andfewoccasionsweremoreeventfulthanwhenlifemovedontotheageinournarrativeandcameoutofthesea.
Landwasaformidableenviro:hot,dry,bathedininteravioletradiation,lagthebuoyancythatmakesmovementinwaterparativelyeffortless.Toliveonland,creatureshadtoundergowholesalerevisionsoftheiranatomies.Holdafishateadanditsagsinthemiddle,itsbaetooweaktosupportit.Tosurviveoutofwater,mariuresoeupwithnewload-bearinginternalarchitecture—notthesortofadjustmentthathappensht.Aboveallandmostobviously,anylaurewouldhavetodevelopawaytotakeitsoxygelyfromtheairratherthanfilteritfromwater.
Thesewerenottrivialchalleoovere.Oherhand,thereowerfuliivetoleavethewater:itwasgettingdangerousdowheslowfusionofthetisintoasinglelandmass,Pangaea,meanttherewasmuch,muchlesscoastlihanformerlyandthusmuchlesscoastalhabitat.Sopetitionwasfierce.Therewasalsoanomnivorousanduliypeofpredatoronthese,onesoperfectlydesignedforattackthatithasscarcelygedinallthelongeonssisemergeheshark.Neverwouldtherebeamorepropitioustimetofindaiveenviroowater.
Plaheprocessoflandizationabout450millionyearsago,apaniedofybytinymitesandanismsthattheyobreakdownandrecycledeadanicmatterontheirbehalf.Largeranimalstookalittleloe,butbyabout400millionyearsagotheywereventuringoutofthewater,too.Popularillustrationshaveencedustoenvisionthefirstventuresomelanddwellersasakindofambitiousfish—somethinglikethemodernmudskipper,whihopfrompuddletopuddleduringdroughts—orevenasafullyformedamphibian.Infact,thefirstvisiblemobileresidentsondrylandwereprobablymuchmorelikemodernwoodlietimesalsoknoillbugsorsowbugs.Thesearethelittlebugs(crustas,infact)thatareonlythrownintofusionwhenyouupturnar.
Forthosethatlearobreatheoxygenfromtheair,timesweregood.OxygenlevelsintheDevonianandCarboniferousperiods,whenterrestriallifefirstbloomed,wereashighas35pert(asopposedtonearer20perow).Thisallowedanimalstogrowremarkablylargeremarkablyquickly.
Andhow,youmayreasonablywonder,stistsknowwhatoxygenlevelswerelikehundredsofmillionsofyearsago?Theanswerliesinaslightlyobscurebutingeniousfieldknownasisotopegeochemistry.Thelong-agoseasoftheCarboniferousandDevonianswarmedwithtinyplanktonthatedthemselvesiinyprotectiveshells.Then,asnow,theplanktoedtheirshellsbydrawingoxygenfromtheatmosphereandbiningitwithotherelements(carbonespecially)toformdurablepoundssuchascalciumcarbo’sthesamechemicaltrickthatgoesonin(andisdiscussedelsewhereiionto)thelong-termcarboncycle—aprocessthatdoesn’tmakeforterriblyexgnarrativebutisvitalforcreatingalivablepla.
Eventuallyinthisprocessallthetinyanismsdieanddrifttothebottomofthesea,wheretheyareslowlypressedintolimestone.Amoinyatomicstructurestheplanktohegravewiththemaretwoverystableisotopes—oxygen-16andoxygen-18.
(Ifyouhavefottenwhatanisotopeis,itdoesn’tmatter,thoughfortherecordit’sanatomwithanabnormalnumberofrons.)Thisiswherethegeochemistsein,fortheisotopesaccumulateatdifferentratesdependingonhowmuchoxygenorcarbondioxideisimosphereatthetimeoftheircreation.Byparingthesearatios,thegeochemistsinglyreadditionsintheaworld—oxygenlevels,airaemperatures,theextentandtimingoficeages,andmuchelse.Bybiningtheirisotopefindingswithotherfossilresidues—pollenlevelsandsoon—stists,withsiderablefidence,re-createentirelandscapesthatnohumaneyeeversaw.
Theprincipalreasonoxygenlevelswereabletobuildupsorobustlythroughouttheperiodofearlyterrestriallifewasthatmuchoftheworld’slandscapewasdominatedbygianttreefernsandvastss,whichbytheirboggynaturedisruptedthenormalcarbonrecygprocess.Insteadofpletelyrottingdown,fallingfrondsandotherdeadvegetativematteraccumulatedinrich,wetsediments,whichwereeventuallysqueezedintothevastcoalbedsthatsustainmuicactivityevennow.
Theheadylevelsofoxygenclearlyencedoutsizedgrowth.Theoldestindicationofasurfaimalyetfoundisatrackleft350millionyearsagobyamillipede-likecreatureonaroScotland.Itwasoverthreefeetlong.Beforetheerawasoutsomemillipedeswouldreagthsmorethandoublethat.
Withsuchcreaturesontheprowl,itisperhapsnotsurprisingthatisintheperiodevolvedatrickthatcouldkeepthemsafelyoutofto:theylearofly.Sometooktothisnewmeansoflootionwithsuyfacilitythattheyhaven’tgedtheirteiquesinallthetimesihen,asnonfliescouldcruiseatuptothirty-fivemilesanhour,instantlystop,hover,flybackwards,andliftfarmoreproportiohananyhumanflyingmae.“TheU.S.AirForce,”oneentatorhaswritten,“hasputtheminwindtuoseehowtheydoit,anddespaired.”They,toedontherichair.InCarboniferousforestsdragonfliesgrewasbigasravens.Treesandetationlikewiseattaisizedproportions.Horsetailsandtreeferoheightsoffiftyfeet,clubmossestoahundredandthirty.
Thefirstterrestrialvertebrates—whichistosay,thefirstlandanimalsfromwhichwewouldderive—aresomethingofamystery.Thisispartlybecauseofasheofrelevantfossils,butpartlyalsobecauseofanidiosyncraticSwedenamedErikJarvikwhoseoddinterpretationsaivemannerheldbackprogressonthisquestionforalmosthalfatury.JarvikartofateamofSdinavianscholarswhowenttoGreenlandinthe1930sand1940slookingforfossilfish.Inparticulartheysoughtlobe-finnedfishofthetypethatpresumablywerearaltousandallotherwalkiures,knowrapods.
Mostanimalsaretetrapods,andalllivirapodshaveohinginon:fourlimbsthatendinamaximumoffivefingersortoes.Dinosaurs,whales,birds,humans,evenfish—allaretetrapods,whichclearlysuggeststheyefromasingleoor.Thecluetothisaor,itwasassumed,wouldbefoundintheDevonianera,fromabout400millionyearsago.Beforethattimenothingwalkedonland.Afterthattimelotsofthingsdid.Luckilytheteamfoundjustsuchacreature,athree-foot-longanimalcalledanIchthyostega.TheanalysisofthefossilfelltoJarvik,whobeganhisstudyin1948aitfortheforty-eightyears.Unfortunately,Jarvikrefusedtoletaudyhistetrapod.Theworld’spaleontologistshadtobetentwithtwosketterimpapersinwhichJarvikhatthecreaturehadfivefingersineachoffourlimbs,firmingitsaralimportance.
Jarvikdiedin1998.Afterhisdeath,otherpaleontologistseagerlyexamihespeandfoundthatJarvikhadseverelymistedthefingersaherewereactuallyeightoneachlimb—andfailedtoobservethatthefishcouldnotpossiblyhavewalked.Thestructureofthefinwassuchthatitwouldhavecollapsedusow.Needlesstosay,thisdidnotdoagreatdealtoadvanceouruandingofthefirstlandanimals.Todaythreeearlytetrapodsareknownandnonehasfivedigits.Inshort,wedon’tknowquitewherewecamefrom.
Butewedid,thoughreagourpresentstateofeminencehasnotofcoursealwaysbeenstraightforward.Sincelifeonlandbegan,ithassistedoffadynasties,astheyaresometimescalled.Thefirstsistedofprimitive,ploddingbutsometimesfairlyheftyamphibiansailes.Thebest-knownanimalofthisagewastheDimetrodon,asail-backedcreaturethatisonlyfusedwithdinosaurs(including,Inote,inapicturecaptionintheCarlSaganbooket).TheDimetrodonwasinfactasynapsid.So,onceuponatime,werewe.Synapsidswereohefourmaindivisionsofearlyreptiliaheothersbeinganapsids,euryapsids,anddiapsids.Thenamessimplyrefertothenumberandlocationofsmallholestobefoundinthesidesoftheirowners’skulls.Synapsidshadoneholeintheirlowertemples;diapsidshadtwo;euryapsidshadasingleholehigherup.
Overtime,eachoftheseprincipalgroupingssplitintofurthersubdivisions,ofwhieprosperedandsomefaltered.Anapsidsgaverisetotheturtles,whichforatime,perhapsatouchimprobably,appearedpoisedtopredomihepla’smostadvanddeadlyspecies,beforeanevolutionarylurchletthemsettlefordurabilityratherthandomihesynapsidsdividedintofourstreams,onlyoneofwhichsurvivedbeyondthePermian.
Happily,thatwasthestreamwebeloo,anditevolvedintoafamilyofprotomammalsknownastherapsids.TheseformedMegadynasty2.
Unfortunatelyforthetherapsids,theircousinsthediapsidsroductivelyevolving,intheircaseintodinosaurs(amongotherthings),whichgraduallyprovedtoomuchforthetherapsids.Uopeteheadtoheadwiththeseaggressivenewcreatures,thetherapsidsbyandlargevanishedfromtherecord.Averyfew,however,evolvedintosmall,furry,burrowingbeingsthatbidedtheirtimeforaverylongwhileaslittlemammals.Thebiggestofthemgrewnerthanahousecat,andmostwerenobiggerthanmice.
Eventually,thiswouldprovetheirsalvation,buttheywouldhavetowaitnearly150millionyearsfadynasty3,theAgeofDinosaurs,toetoanabruptendandmakeroadynasty4andourownAgeofMammals.
Eachofthesemassivetransformations,aswellasmanysmalleroweenandsince,eonthatparadoxicallyimportantmotorress:extin.Itisacuriousfactthatohspeciesdeathis,iliteralsense,awayoflife.Nooneknowshoeciesanismshaveexistedsincelifebegan.Thirtybillionisaonlycitedfigure,butthenumberhasbeenputashighas4,000billion.Whatevertheactualtotal,99.99pertofallspeciesthathaveeverlivedarenolohus.“Toafirstapproximation,”asDavidRaupoftheUyofChicagolikestosay,“allspeciesareextinct.”Forplexanisms,theaveragelifespanofaspeciesisonlyaboutfourmillionyears—roughlyaboutwherewearenow.
Extinisalwaysbadnewsforthevictims,ofcourse,butitappearstobeagoodthingforadynamiet.“Thealternativetoextinisstagnation,”saysIanTattersalloftheAmeriMuseumofNaturalHistory,“andstagnationisseldomagoodthinginanyrealm.”
(Ishouldperhapseakinghereofextinasanatural,long-termprocess.
Extinbroughtaboutbyhumancarelessnessisateraltogether.)Crisesih’shistoryareinvariablyassociatedwithdramaticleapsafterward.ThefalloftheEdiafaunawasfollowedbythecreativeoutburstoftheCambrianperiod.TheOrdoviextinof440millionyearsagoclearedtheosofalotofimmobilefilterfeedersand,somehow,createdditionsthatfavoreddartingfishandgiantaquaticreptiles.
TheseinturnwereinanidealpositiontosendistsontodrylandwhenanotherblowoutieDevonianperiodgavelifeanothersoundshaking.Andsoithasgoscatteredintervalsthroughhistory.Ifmostoftheseeventshadn’thappenedjustastheydid,justwhentheydid,wealmostcertainlywouldn’tbeherenow.
Earthhasseenfivemajorextinepisodesinitstime—theOrdovi,Devonian,Permian,TriassidCretaceous,inthatorder—andmanysmalleroheOrdovi(440millionyearsago)andDevonian(365million)eachwipedoutabout80to85pertofspecies.TheTriassiillionyearsago)aaceous(65millionyears)eachwipedout70to75pertofspecies.ButtherealwhopperwasthePermiainofabout245millionyearsago,whichraisedthecurtainonthelohedinosaurs.InthePermian,atleast95pertofanimalsknownfromthefossilrecordcheckout,oreturn.Evenaboutathirdofispecieswent—theonlyoccasiononwhichtheywerelostenmasse.Itisascloseaswehaveeveretototalobliteration.
“Itwas,truly,amassextin,aageofamagnitudethathadroubledtheEarthbefore,”saysRichardFortey.ThePermiaarticularlydevastatingtoseacreatures.
Trilobitesvaogether.Clamsandseaursnearlywent.Virtuallyallothermarineanismswerestaggered.Altogether,onlandandier,itisthoughtthatEarthlost52pertofitsfamilies—that’sthelevelabovegenusandbeloworderonthegrandscaleoflife(thesubjectofthechapter)—andperhapsasmanyas96pertofallitsspecies.Itwouldbealongtime—asmuchaseightymillionyearsbyonereing—beforespeciestotalsrecovered.
Twopoiobekeptinmind.First,thesearealljustinfuesses.EstimatesforthenumberofanimalspeciesaliveattheendofthePermianrangefromaslowas45,000toashighas240,000.Ifyoudon’tknowhoecieswerealive,youhardlyspecifywithvitheproportionthatperished.Moreover,wearetalkingaboutthedeathofspecies,notindividuals.Forindividualsthedeathtollcouldbemuchhigher—inmanycases,practicallytotal.Thespeciesthatsurvivedtothephaseoflife’slotteryalmostcertainlyowetheirexisteoafewscarredandlimpingsurvivors.
Iweenthebigkill-offs,therehavealsobeenmanysmaller,lesswell-kinepisodes—theHemphillian,Frasnian,Famennian,Rancholabrean,andadozenorsoothers—whichwerenotsodevastatingtototalspeumbers,butoftencriticallyhitcertainpopulations.Grazinganimals,includinghorses,werenearlywipedoutintheHemphilliaaboutfivemillionyearsago.Horsesdeedtoasinglespecies,whichappearssosporadicallyinthefossilrecordastosuggestthatforatimeitteeteredonthebrinkofoblivion.Imagineahumanhistorywithouthorses,withoutgrazinganimals.
Innearlyeverycase,forbothbigextinsandmoremodestones,wehavebewilderinglylittleideaofwhatthecausewas.Everippingoutthemorecrackpotnotionstherearestillmoretheoriesforwhatcausedtheextineventsthantherehavebees.Atleasttwodozeialculpritshavebeeifiedascausesorprimetributlobalwarming,globalcooling,gingsealevels,oxygeionoftheseas(aditionknownasanoxia),epidemics,giantleaksofmethanegasfromtheseafloor,meteorandetimpacts,runawayhurriesofatypeknoeres,hugevolicupwellings,catastrophicsolarflares.
Thislastisaparticularlyintriguingpossibility.Nobodyknowshowbigsolarflaresgetbecausewehaveonlybeenwatgthemsihebeginningofthespaceage,buttheSunisamightyengineanditsstormsareensuratelyenormous.Atypicalsolarflare—somethingwewouldn’tevennotiEarth—willreleasetheenergyequivalentofabillionhydrogenbombsandflingintospaceahundredbilliontonsorsoofmurderoushigh-energyparticles.Themagosphereandatmospherebetweenthemnormallyswatthesebatospaceorsteerthemsafelytowardthepoles(wheretheyproducetheEarth’selyauroras),butitisthoughtthatanunusuallybigblast,sayahuimesthetypicalflare,couldoverwhelmouretherealdefehelightshowwouldbeagloriousoitwouldalmostcertainlykillaveryhighproportionofallthatbaskedinitsglow.Moreover,andratherchillingly,acctoBruceTsurutanioftheNASAJetPropulsionLaboratory,“itwouldleavenotrahistory.”
Whatallthisleavesuswith,asoneresearcherhasputit,is“tonsofjectureandverylittleevidence.”Cooliobeassociatedwithatleastthreeofthebigextinevents—theOrdovi,Devonian,andPermian—butbeyondthatlittleisagreed,includiheraparticularepisodehappenedswiftlyorslowly.Stists’tagree,forinstance,whetherthelateDevoin—theeventthatwasfollowedbyvertebratesmovingontotheland—happenedovermillionsofyearsorthousandsofyearsorinonelivelyday.
Ohereasonsitissohardtoproduvingexplanationsforextinsisthatitissoveryhardtoexterminatelifeonagrandscale.AswehaveseenfromtheMansonimpact,youreceiveaferociousblowandstillstageafull,ifpresumablysomewhatwobbly,recovery.Sowhy,outofallthethousandsofimpactsEarthhasendured,wastheKTeventsosingularlydevastating?Well,firstitositivelyenormous.Itstruckwiththeforceof100millioons.Suoutburstisnoteasilyimagined,butasJamesLawrencePowellhaspoi,ifyouexplodedoneHiroshima-sizedbombforeverypersonaliveohtodayyouwouldstillbeaboutabillionbombsshortofthesizeoftheKTimpact.Buteventhatalonemaynothavebeenenoughtowipeout70pertofEarth’slife,dinosaursincluded.
TheKTmeteorhadtheadditionaladvantage—advantageifyouareamammal,thatis—thatitlandedinashallowseajustteersdeep,probablyatjusttherightaatimewhenoxygenlevelswere10perthigherthanatpresentandsotheworldwasmorebustible.Abovealltheflooroftheseawhereitlandedwasmadeofrockrisulfur.
TheresultactthatturnedanareaofseafloorthesizeofBelgiumintoaerosolsofsulfuricacid.Formonthsafterward,theEarthwassubjectedtorainsaoughtoburnskin.
Inasense,aneveerquestionthanthatofedout70pertofthespeciesthatwereexistingatthetimeishowdidtheremaining30pertsurvive?Whywastheeventsoirremediablydevastatingtoeverysingledinosaurthatexisted,whileotherreptiles,likesnakesandcrocodiles,passedthroughunimpeded?Sofaraswetellnospeciesoftoad,,salamander,orotheramphibiaextinorthAmerica.“Whyshouldthesedelicatecreatureshaveemergedunscathedfromsuunparalleleddisaster?”asksTimFlanneryinhisfasatioryofAmerica,EternalFrontier.
Intheseasitwasmuchthesamestory.Alltheammonitesvanished,buttheircousiiloids,wholivedsimilarlifestyles,swamon.Amongplankton,somespecieswerepracticallywipedout—92pertofforaminiferans,forinstance—whileanismslikediatoms,desigoasimilarplanandlivingalongside,wereparativelyunscathed.
Thesearedifficultinsistencies.AsRichardForteyobserves:“Somehowitdoesnotseemsatisfyingjusttocallthem‘lues’andleaveitatthat.”If,asseemsentirelylikely,theeventwasfollowedbymonthsofdarkandchokingsmoke,thenmanyoftheisurvivorsbeedifficulttoatfor.“Someis,likebeetles,”Forteynotes,“couldliveonwoodorotherthingslyingaround.Butwhataboutthoselikebeesthatnavigatebysunlightandneedpollen?Explainingtheirsurvivalisn’tsoeasy.”
Aboveall,therearethecorals.Coralsrequirealgaetosurviveandalgaerequiresunlight,andbothtogetherrequiresteadyminimumtemperatures.Muchpublicityhasbeehelastfewyearstocoralsdyingfromgesiemperatureofonlyadegreeorso.Iftheyarethatvulnerabletosmallges,howdidtheysurvivethelongimpater?
Therearealsomanyhard-to-explainregionalvariatioinsseemtohavebeenfarlesssevereihernhemispherethahern.NewZealandinparticularappearstohaveethroughlargelyunscathedeventhoughithadalmostnoburrowiures.Evenitsvegetationwasoverwhelminglyspared,ahescaleofflagrationelsewheresuggeststhatdevastationwasglobal.Inshort,thereisjustagreatdealwedon’tknow.
Someanimalsabsolutelyprospered—including,alittlesurprisingly,theturtlesonceagain.
AsFlanneryheperiodimmediatelyafterthedinosaurextincouldwellbeknownastheAgeofTurtles.SixteenspeciessurvivedinNorthAmeridthreemorecameiencesoonafter.
ClearlyithelpedtobeathomeiheKTimpactwipedoutalmost90pertofland-basedspeciesbutonly10pertofthoselivinginfreshwater.Waterobviouslyofferedproteagaiandflame,butalsopresumablyprovidedmoresusteheleanperiodthatfollowed.Alltheland-basedanimalsthatsurvivedhadahabitofretreatingtoasaferenviroduringtimesofdanger—intowaterorundergrouherofwhichwouldhaveprovidedsiderableshelteragainsttheravageswithout.Animalsthatsgedforalivingwouldalsohaveenjoyedanadvantage.Lizardswere,andare,largelyimpervioustothebacteriainrottingcarcasses.Indeed,oftentheyarepositivelydrawntoit,andforalongwhiletherewereclearlyalotofputridcarcassesabout.
ItisoftenwronglystatedthatonlysmallanimalssurvivedtheKTevent.Infact,amongthesurvivorswerecrocodiles,whichwerenotjustlargebutthreetimeslargerthantheyaretoday.
Butonthewhole,itistrue,mostofthesurvivorsweresmallandfurtive.Indeed,withtheworlddarkandhostile,iterfecttimetobesm松语文学www.16sy.coM免费小说阅读
sidertheli.Lisarejustaboutthehardiestvisibleanismsoh,butamongtheleastambitious.Theywillgroilyenoughinasunnychurchyard,buttheyparticularlythriveinenviroswherenoanismwouldgo—onblowymountaintopsandarcticwastes,whereverthereislittlebutrodrainandcold,andalmostnopetition.InareasofAntarcticawherevirtuallynothingelsewillgrow,youfindvastexpansesofli—fourhuypesofthem—adheriedlytoeverywind-whippedrock.
Foralongtime,peoplecouldn’tuandhowtheydidit.Becauselisgrewonbarerockwithoutevidentnourishmentortheproduofseeds,manypeople—educatedpeople—believedtheywerestonescaughtintheprocessofbeingplants.“Spontaneously,inaniebeeslivingplant!”rejoieobserver,aDr.Homschu1819.
Closeriionshowedthatlisweremoreiingthanmagical.Theyareinfactapartnershipbetweenfungiandalgae.Thefungiexcreteacidsthatdissolvethesurfaceoftherock,freeingmineralsthatthealgaevertintofoodsuffittosustainboth.Itisnotaveryexgarra,butitisaspicuouslysuccessfuloheworldhasmorethahousandspeciesoflis.
Likemostthingsthatthriveinharshenviros,lisareslow-growing.Itmaytakealimorethanhalfaturytoattainthedimensionsofashirtbutton.Thosethesizeofdinnerplates,writesDavidAttenbh,aretherefore“likelytobehundredsifnotthousandsofyearsold.”Itwouldbehardtoimaginealessfulfillieheysimplyexist,”Attenbhadds,“testifyingtothemovingfactthatlifeevenatitssimplestleveloccurs,apparently,justforitsownsake.”
Itiseasytooverlookthisthoughtthatlifejustis.Ashumansweareinedtofeelthatlifemusthaveapoint.lansandaspirationsanddesires.Wewanttotakestantadvantageofalltheintoxigexistencewe’vebeeneh.Butwhat’slifetoali?Yetitsimpulsetoexist,tobe,iseverybitasstrongasours—arguablyevenstronger.
IfIweretoldthatIhadtospenddecadesbeingafurrygrowthonarothewoods,IbelieveIwouldlosethewilltogoon.Lisdon’t.Likevirtuallyalllivingthings,theywillsufferanyhardship,endureanyinsult,foramoment’sadditioence.Life,inshort,justwantstobe.But—andhere’saingpoint—forthemostpartitdoesn’twanttobemuch.
Thisisperhapsalittleoddbecauselifehashadplentyoftimetodevelopambitions.Ifyouimagihe4,500-billion-oddyearsofEarth’shistorypressedintoanormalearthlyday,thenlifebeginsveryearly,about4A.M.,withtheriseofthefirstsimple,single-celledanisms,butthenadvanofurtherforthesixteenhours.Notuntilalmost8:30intheevening,withthedayfive-sixthsover,hasEarthanythingtoshowtheuniversebutarestlessskinofmicrobes.Then,finally,thefirstseaplantsappear,followedtwentymierbythefirstjellyfishandtheenigmaticEdiafaunafirstseenbyReginaldSprigginAustralia.At9:04P.M.trilobitesswimontothese,followedmoreorlessimmediatelybytheshapelycreaturesoftheBurgessShale.Justbefore10P.M.plaopopupontheland.Soonafter,withlessthantwohoursleftintheday,thefirstlauresfollow.
Thankstotenminutesorsoofbalmyweather,by10:24theEarthiscoveredinthegreatcarboniferousforestswhoseresiduesgiveusallourcoal,andthefirstwingedisareevident.Dinosaursplodontothesejustbefore11P.M.andholdswayforaboutthree-quartersofanhour.Attwenty-onemiomidnighttheyvanishandtheageofmammalsbegins.Humansemergeoneminuteaeensedsbeforemidnight.Thewholeofourrecordedhistory,onthisscale,wouldbenomorethanafewseds,asinglehumaimebarelyaninstant.Throughoutthisgreatlyspeeded-updaytisslideaboutandbangtogetherataclipthatseemspositivelyreckless.Mountainsriseaaway,obasinseandgo,icesheetsadvandwithdraw.Andthroughoutthewhole,aboutthreetimeseveryminute,somewhereontheplahereisaflashbulbpopoflightmarkingtheimpaanson-sizedmeteororoneevenlarger.It’sawohatanythingatallsurviveinsuchapummeledanduledenviro.Infaanythingsd.
Perhapsanevenmoreeffectivewayofgraspingourextremereessasapartofthis4.5-billion-year-oldpictureistostretchyourarmstotheirfullestextentandimagiwidthastheeoryoftheEarth.Onthisscale,acctoJohnMcPheeinBasinandRahedistanthefiipsofonehandtothewristoftheotherisPrecambrian.
Allofplexlifeisinonehand,“andinasirokewithamedium-grainednailfileyoucoulderadicatehumanhistory.”
Fortuhatmomenthasn’thappened,butthecesaregoodthatitwill.Idon’twishtointerjeoteofgloomjustatthispoint,butthefactisthatthereisoherextremelypertiqualityaboutlifeoh:itgoesextinct.Quiteregularly.Forallthetroubletheytaketoassembleandpreservethemselves,speciescrumpleanddieremarkablyroutinely.Andthemoreplextheyget,themorequicklytheyappeartogoextinct.Whichisperhapsonereasonwhysomuchoflifeisn’tterriblyambitious.
Soanytimelifedoessomethingbolditisquitea,andfewoccasionsweremoreeventfulthanwhenlifemovedontotheageinournarrativeandcameoutofthesea.
Landwasaformidableenviro:hot,dry,bathedininteravioletradiation,lagthebuoyancythatmakesmovementinwaterparativelyeffortless.Toliveonland,creatureshadtoundergowholesalerevisionsoftheiranatomies.Holdafishateadanditsagsinthemiddle,itsbaetooweaktosupportit.Tosurviveoutofwater,mariuresoeupwithnewload-bearinginternalarchitecture—notthesortofadjustmentthathappensht.Aboveallandmostobviously,anylaurewouldhavetodevelopawaytotakeitsoxygelyfromtheairratherthanfilteritfromwater.
Thesewerenottrivialchalleoovere.Oherhand,thereowerfuliivetoleavethewater:itwasgettingdangerousdowheslowfusionofthetisintoasinglelandmass,Pangaea,meanttherewasmuch,muchlesscoastlihanformerlyandthusmuchlesscoastalhabitat.Sopetitionwasfierce.Therewasalsoanomnivorousanduliypeofpredatoronthese,onesoperfectlydesignedforattackthatithasscarcelygedinallthelongeonssisemergeheshark.Neverwouldtherebeamorepropitioustimetofindaiveenviroowater.
Plaheprocessoflandizationabout450millionyearsago,apaniedofybytinymitesandanismsthattheyobreakdownandrecycledeadanicmatterontheirbehalf.Largeranimalstookalittleloe,butbyabout400millionyearsagotheywereventuringoutofthewater,too.Popularillustrationshaveencedustoenvisionthefirstventuresomelanddwellersasakindofambitiousfish—somethinglikethemodernmudskipper,whihopfrompuddletopuddleduringdroughts—orevenasafullyformedamphibian.Infact,thefirstvisiblemobileresidentsondrylandwereprobablymuchmorelikemodernwoodlietimesalsoknoillbugsorsowbugs.Thesearethelittlebugs(crustas,infact)thatareonlythrownintofusionwhenyouupturnar.
Forthosethatlearobreatheoxygenfromtheair,timesweregood.OxygenlevelsintheDevonianandCarboniferousperiods,whenterrestriallifefirstbloomed,wereashighas35pert(asopposedtonearer20perow).Thisallowedanimalstogrowremarkablylargeremarkablyquickly.
Andhow,youmayreasonablywonder,stistsknowwhatoxygenlevelswerelikehundredsofmillionsofyearsago?Theanswerliesinaslightlyobscurebutingeniousfieldknownasisotopegeochemistry.Thelong-agoseasoftheCarboniferousandDevonianswarmedwithtinyplanktonthatedthemselvesiinyprotectiveshells.Then,asnow,theplanktoedtheirshellsbydrawingoxygenfromtheatmosphereandbiningitwithotherelements(carbonespecially)toformdurablepoundssuchascalciumcarbo’sthesamechemicaltrickthatgoesonin(andisdiscussedelsewhereiionto)thelong-termcarboncycle—aprocessthatdoesn’tmakeforterriblyexgnarrativebutisvitalforcreatingalivablepla.
Eventuallyinthisprocessallthetinyanismsdieanddrifttothebottomofthesea,wheretheyareslowlypressedintolimestone.Amoinyatomicstructurestheplanktohegravewiththemaretwoverystableisotopes—oxygen-16andoxygen-18.
(Ifyouhavefottenwhatanisotopeis,itdoesn’tmatter,thoughfortherecordit’sanatomwithanabnormalnumberofrons.)Thisiswherethegeochemistsein,fortheisotopesaccumulateatdifferentratesdependingonhowmuchoxygenorcarbondioxideisimosphereatthetimeoftheircreation.Byparingthesearatios,thegeochemistsinglyreadditionsintheaworld—oxygenlevels,airaemperatures,theextentandtimingoficeages,andmuchelse.Bybiningtheirisotopefindingswithotherfossilresidues—pollenlevelsandsoon—stists,withsiderablefidence,re-createentirelandscapesthatnohumaneyeeversaw.
Theprincipalreasonoxygenlevelswereabletobuildupsorobustlythroughouttheperiodofearlyterrestriallifewasthatmuchoftheworld’slandscapewasdominatedbygianttreefernsandvastss,whichbytheirboggynaturedisruptedthenormalcarbonrecygprocess.Insteadofpletelyrottingdown,fallingfrondsandotherdeadvegetativematteraccumulatedinrich,wetsediments,whichwereeventuallysqueezedintothevastcoalbedsthatsustainmuicactivityevennow.
Theheadylevelsofoxygenclearlyencedoutsizedgrowth.Theoldestindicationofasurfaimalyetfoundisatrackleft350millionyearsagobyamillipede-likecreatureonaroScotland.Itwasoverthreefeetlong.Beforetheerawasoutsomemillipedeswouldreagthsmorethandoublethat.
Withsuchcreaturesontheprowl,itisperhapsnotsurprisingthatisintheperiodevolvedatrickthatcouldkeepthemsafelyoutofto:theylearofly.Sometooktothisnewmeansoflootionwithsuyfacilitythattheyhaven’tgedtheirteiquesinallthetimesihen,asnonfliescouldcruiseatuptothirty-fivemilesanhour,instantlystop,hover,flybackwards,andliftfarmoreproportiohananyhumanflyingmae.“TheU.S.AirForce,”oneentatorhaswritten,“hasputtheminwindtuoseehowtheydoit,anddespaired.”They,toedontherichair.InCarboniferousforestsdragonfliesgrewasbigasravens.Treesandetationlikewiseattaisizedproportions.Horsetailsandtreeferoheightsoffiftyfeet,clubmossestoahundredandthirty.
Thefirstterrestrialvertebrates—whichistosay,thefirstlandanimalsfromwhichwewouldderive—aresomethingofamystery.Thisispartlybecauseofasheofrelevantfossils,butpartlyalsobecauseofanidiosyncraticSwedenamedErikJarvikwhoseoddinterpretationsaivemannerheldbackprogressonthisquestionforalmosthalfatury.JarvikartofateamofSdinavianscholarswhowenttoGreenlandinthe1930sand1940slookingforfossilfish.Inparticulartheysoughtlobe-finnedfishofthetypethatpresumablywerearaltousandallotherwalkiures,knowrapods.
Mostanimalsaretetrapods,andalllivirapodshaveohinginon:fourlimbsthatendinamaximumoffivefingersortoes.Dinosaurs,whales,birds,humans,evenfish—allaretetrapods,whichclearlysuggeststheyefromasingleoor.Thecluetothisaor,itwasassumed,wouldbefoundintheDevonianera,fromabout400millionyearsago.Beforethattimenothingwalkedonland.Afterthattimelotsofthingsdid.Luckilytheteamfoundjustsuchacreature,athree-foot-longanimalcalledanIchthyostega.TheanalysisofthefossilfelltoJarvik,whobeganhisstudyin1948aitfortheforty-eightyears.Unfortunately,Jarvikrefusedtoletaudyhistetrapod.Theworld’spaleontologistshadtobetentwithtwosketterimpapersinwhichJarvikhatthecreaturehadfivefingersineachoffourlimbs,firmingitsaralimportance.
Jarvikdiedin1998.Afterhisdeath,otherpaleontologistseagerlyexamihespeandfoundthatJarvikhadseverelymistedthefingersaherewereactuallyeightoneachlimb—andfailedtoobservethatthefishcouldnotpossiblyhavewalked.Thestructureofthefinwassuchthatitwouldhavecollapsedusow.Needlesstosay,thisdidnotdoagreatdealtoadvanceouruandingofthefirstlandanimals.Todaythreeearlytetrapodsareknownandnonehasfivedigits.Inshort,wedon’tknowquitewherewecamefrom.
Butewedid,thoughreagourpresentstateofeminencehasnotofcoursealwaysbeenstraightforward.Sincelifeonlandbegan,ithassistedoffadynasties,astheyaresometimescalled.Thefirstsistedofprimitive,ploddingbutsometimesfairlyheftyamphibiansailes.Thebest-knownanimalofthisagewastheDimetrodon,asail-backedcreaturethatisonlyfusedwithdinosaurs(including,Inote,inapicturecaptionintheCarlSaganbooket).TheDimetrodonwasinfactasynapsid.So,onceuponatime,werewe.Synapsidswereohefourmaindivisionsofearlyreptiliaheothersbeinganapsids,euryapsids,anddiapsids.Thenamessimplyrefertothenumberandlocationofsmallholestobefoundinthesidesoftheirowners’skulls.Synapsidshadoneholeintheirlowertemples;diapsidshadtwo;euryapsidshadasingleholehigherup.
Overtime,eachoftheseprincipalgroupingssplitintofurthersubdivisions,ofwhieprosperedandsomefaltered.Anapsidsgaverisetotheturtles,whichforatime,perhapsatouchimprobably,appearedpoisedtopredomihepla’smostadvanddeadlyspecies,beforeanevolutionarylurchletthemsettlefordurabilityratherthandomihesynapsidsdividedintofourstreams,onlyoneofwhichsurvivedbeyondthePermian.
Happily,thatwasthestreamwebeloo,anditevolvedintoafamilyofprotomammalsknownastherapsids.TheseformedMegadynasty2.
Unfortunatelyforthetherapsids,theircousinsthediapsidsroductivelyevolving,intheircaseintodinosaurs(amongotherthings),whichgraduallyprovedtoomuchforthetherapsids.Uopeteheadtoheadwiththeseaggressivenewcreatures,thetherapsidsbyandlargevanishedfromtherecord.Averyfew,however,evolvedintosmall,furry,burrowingbeingsthatbidedtheirtimeforaverylongwhileaslittlemammals.Thebiggestofthemgrewnerthanahousecat,andmostwerenobiggerthanmice.
Eventually,thiswouldprovetheirsalvation,buttheywouldhavetowaitnearly150millionyearsfadynasty3,theAgeofDinosaurs,toetoanabruptendandmakeroadynasty4andourownAgeofMammals.
Eachofthesemassivetransformations,aswellasmanysmalleroweenandsince,eonthatparadoxicallyimportantmotorress:extin.Itisacuriousfactthatohspeciesdeathis,iliteralsense,awayoflife.Nooneknowshoeciesanismshaveexistedsincelifebegan.Thirtybillionisaonlycitedfigure,butthenumberhasbeenputashighas4,000billion.Whatevertheactualtotal,99.99pertofallspeciesthathaveeverlivedarenolohus.“Toafirstapproximation,”asDavidRaupoftheUyofChicagolikestosay,“allspeciesareextinct.”Forplexanisms,theaveragelifespanofaspeciesisonlyaboutfourmillionyears—roughlyaboutwherewearenow.
Extinisalwaysbadnewsforthevictims,ofcourse,butitappearstobeagoodthingforadynamiet.“Thealternativetoextinisstagnation,”saysIanTattersalloftheAmeriMuseumofNaturalHistory,“andstagnationisseldomagoodthinginanyrealm.”
(Ishouldperhapseakinghereofextinasanatural,long-termprocess.
Extinbroughtaboutbyhumancarelessnessisateraltogether.)Crisesih’shistoryareinvariablyassociatedwithdramaticleapsafterward.ThefalloftheEdiafaunawasfollowedbythecreativeoutburstoftheCambrianperiod.TheOrdoviextinof440millionyearsagoclearedtheosofalotofimmobilefilterfeedersand,somehow,createdditionsthatfavoreddartingfishandgiantaquaticreptiles.
TheseinturnwereinanidealpositiontosendistsontodrylandwhenanotherblowoutieDevonianperiodgavelifeanothersoundshaking.Andsoithasgoscatteredintervalsthroughhistory.Ifmostoftheseeventshadn’thappenedjustastheydid,justwhentheydid,wealmostcertainlywouldn’tbeherenow.
Earthhasseenfivemajorextinepisodesinitstime—theOrdovi,Devonian,Permian,TriassidCretaceous,inthatorder—andmanysmalleroheOrdovi(440millionyearsago)andDevonian(365million)eachwipedoutabout80to85pertofspecies.TheTriassiillionyearsago)aaceous(65millionyears)eachwipedout70to75pertofspecies.ButtherealwhopperwasthePermiainofabout245millionyearsago,whichraisedthecurtainonthelohedinosaurs.InthePermian,atleast95pertofanimalsknownfromthefossilrecordcheckout,oreturn.Evenaboutathirdofispecieswent—theonlyoccasiononwhichtheywerelostenmasse.Itisascloseaswehaveeveretototalobliteration.
“Itwas,truly,amassextin,aageofamagnitudethathadroubledtheEarthbefore,”saysRichardFortey.ThePermiaarticularlydevastatingtoseacreatures.
Trilobitesvaogether.Clamsandseaursnearlywent.Virtuallyallothermarineanismswerestaggered.Altogether,onlandandier,itisthoughtthatEarthlost52pertofitsfamilies—that’sthelevelabovegenusandbeloworderonthegrandscaleoflife(thesubjectofthechapter)—andperhapsasmanyas96pertofallitsspecies.Itwouldbealongtime—asmuchaseightymillionyearsbyonereing—beforespeciestotalsrecovered.
Twopoiobekeptinmind.First,thesearealljustinfuesses.EstimatesforthenumberofanimalspeciesaliveattheendofthePermianrangefromaslowas45,000toashighas240,000.Ifyoudon’tknowhoecieswerealive,youhardlyspecifywithvitheproportionthatperished.Moreover,wearetalkingaboutthedeathofspecies,notindividuals.Forindividualsthedeathtollcouldbemuchhigher—inmanycases,practicallytotal.Thespeciesthatsurvivedtothephaseoflife’slotteryalmostcertainlyowetheirexisteoafewscarredandlimpingsurvivors.
Iweenthebigkill-offs,therehavealsobeenmanysmaller,lesswell-kinepisodes—theHemphillian,Frasnian,Famennian,Rancholabrean,andadozenorsoothers—whichwerenotsodevastatingtototalspeumbers,butoftencriticallyhitcertainpopulations.Grazinganimals,includinghorses,werenearlywipedoutintheHemphilliaaboutfivemillionyearsago.Horsesdeedtoasinglespecies,whichappearssosporadicallyinthefossilrecordastosuggestthatforatimeitteeteredonthebrinkofoblivion.Imagineahumanhistorywithouthorses,withoutgrazinganimals.
Innearlyeverycase,forbothbigextinsandmoremodestones,wehavebewilderinglylittleideaofwhatthecausewas.Everippingoutthemorecrackpotnotionstherearestillmoretheoriesforwhatcausedtheextineventsthantherehavebees.Atleasttwodozeialculpritshavebeeifiedascausesorprimetributlobalwarming,globalcooling,gingsealevels,oxygeionoftheseas(aditionknownasanoxia),epidemics,giantleaksofmethanegasfromtheseafloor,meteorandetimpacts,runawayhurriesofatypeknoeres,hugevolicupwellings,catastrophicsolarflares.
Thislastisaparticularlyintriguingpossibility.Nobodyknowshowbigsolarflaresgetbecausewehaveonlybeenwatgthemsihebeginningofthespaceage,buttheSunisamightyengineanditsstormsareensuratelyenormous.Atypicalsolarflare—somethingwewouldn’tevennotiEarth—willreleasetheenergyequivalentofabillionhydrogenbombsandflingintospaceahundredbilliontonsorsoofmurderoushigh-energyparticles.Themagosphereandatmospherebetweenthemnormallyswatthesebatospaceorsteerthemsafelytowardthepoles(wheretheyproducetheEarth’selyauroras),butitisthoughtthatanunusuallybigblast,sayahuimesthetypicalflare,couldoverwhelmouretherealdefehelightshowwouldbeagloriousoitwouldalmostcertainlykillaveryhighproportionofallthatbaskedinitsglow.Moreover,andratherchillingly,acctoBruceTsurutanioftheNASAJetPropulsionLaboratory,“itwouldleavenotrahistory.”
Whatallthisleavesuswith,asoneresearcherhasputit,is“tonsofjectureandverylittleevidence.”Cooliobeassociatedwithatleastthreeofthebigextinevents—theOrdovi,Devonian,andPermian—butbeyondthatlittleisagreed,includiheraparticularepisodehappenedswiftlyorslowly.Stists’tagree,forinstance,whetherthelateDevoin—theeventthatwasfollowedbyvertebratesmovingontotheland—happenedovermillionsofyearsorthousandsofyearsorinonelivelyday.
Ohereasonsitissohardtoproduvingexplanationsforextinsisthatitissoveryhardtoexterminatelifeonagrandscale.AswehaveseenfromtheMansonimpact,youreceiveaferociousblowandstillstageafull,ifpresumablysomewhatwobbly,recovery.Sowhy,outofallthethousandsofimpactsEarthhasendured,wastheKTeventsosingularlydevastating?Well,firstitositivelyenormous.Itstruckwiththeforceof100millioons.Suoutburstisnoteasilyimagined,butasJamesLawrencePowellhaspoi,ifyouexplodedoneHiroshima-sizedbombforeverypersonaliveohtodayyouwouldstillbeaboutabillionbombsshortofthesizeoftheKTimpact.Buteventhatalonemaynothavebeenenoughtowipeout70pertofEarth’slife,dinosaursincluded.
TheKTmeteorhadtheadditionaladvantage—advantageifyouareamammal,thatis—thatitlandedinashallowseajustteersdeep,probablyatjusttherightaatimewhenoxygenlevelswere10perthigherthanatpresentandsotheworldwasmorebustible.Abovealltheflooroftheseawhereitlandedwasmadeofrockrisulfur.
TheresultactthatturnedanareaofseafloorthesizeofBelgiumintoaerosolsofsulfuricacid.Formonthsafterward,theEarthwassubjectedtorainsaoughtoburnskin.
Inasense,aneveerquestionthanthatofedout70pertofthespeciesthatwereexistingatthetimeishowdidtheremaining30pertsurvive?Whywastheeventsoirremediablydevastatingtoeverysingledinosaurthatexisted,whileotherreptiles,likesnakesandcrocodiles,passedthroughunimpeded?Sofaraswetellnospeciesoftoad,,salamander,orotheramphibiaextinorthAmerica.“Whyshouldthesedelicatecreatureshaveemergedunscathedfromsuunparalleleddisaster?”asksTimFlanneryinhisfasatioryofAmerica,EternalFrontier.
Intheseasitwasmuchthesamestory.Alltheammonitesvanished,buttheircousiiloids,wholivedsimilarlifestyles,swamon.Amongplankton,somespecieswerepracticallywipedout—92pertofforaminiferans,forinstance—whileanismslikediatoms,desigoasimilarplanandlivingalongside,wereparativelyunscathed.
Thesearedifficultinsistencies.AsRichardForteyobserves:“Somehowitdoesnotseemsatisfyingjusttocallthem‘lues’andleaveitatthat.”If,asseemsentirelylikely,theeventwasfollowedbymonthsofdarkandchokingsmoke,thenmanyoftheisurvivorsbeedifficulttoatfor.“Someis,likebeetles,”Forteynotes,“couldliveonwoodorotherthingslyingaround.Butwhataboutthoselikebeesthatnavigatebysunlightandneedpollen?Explainingtheirsurvivalisn’tsoeasy.”
Aboveall,therearethecorals.Coralsrequirealgaetosurviveandalgaerequiresunlight,andbothtogetherrequiresteadyminimumtemperatures.Muchpublicityhasbeehelastfewyearstocoralsdyingfromgesiemperatureofonlyadegreeorso.Iftheyarethatvulnerabletosmallges,howdidtheysurvivethelongimpater?
Therearealsomanyhard-to-explainregionalvariatioinsseemtohavebeenfarlesssevereihernhemispherethahern.NewZealandinparticularappearstohaveethroughlargelyunscathedeventhoughithadalmostnoburrowiures.Evenitsvegetationwasoverwhelminglyspared,ahescaleofflagrationelsewheresuggeststhatdevastationwasglobal.Inshort,thereisjustagreatdealwedon’tknow.
Someanimalsabsolutelyprospered—including,alittlesurprisingly,theturtlesonceagain.
AsFlanneryheperiodimmediatelyafterthedinosaurextincouldwellbeknownastheAgeofTurtles.SixteenspeciessurvivedinNorthAmeridthreemorecameiencesoonafter.
ClearlyithelpedtobeathomeiheKTimpactwipedoutalmost90pertofland-basedspeciesbutonly10pertofthoselivinginfreshwater.Waterobviouslyofferedproteagaiandflame,butalsopresumablyprovidedmoresusteheleanperiodthatfollowed.Alltheland-basedanimalsthatsurvivedhadahabitofretreatingtoasaferenviroduringtimesofdanger—intowaterorundergrouherofwhichwouldhaveprovidedsiderableshelteragainsttheravageswithout.Animalsthatsgedforalivingwouldalsohaveenjoyedanadvantage.Lizardswere,andare,largelyimpervioustothebacteriainrottingcarcasses.Indeed,oftentheyarepositivelydrawntoit,andforalongwhiletherewereclearlyalotofputridcarcassesabout.
ItisoftenwronglystatedthatonlysmallanimalssurvivedtheKTevent.Infact,amongthesurvivorswerecrocodiles,whichwerenotjustlargebutthreetimeslargerthantheyaretoday.
Butonthewhole,itistrue,mostofthesurvivorsweresmallandfurtive.Indeed,withtheworlddarkandhostile,iterfecttimetobesm松语文学www.16sy.coM免费小说阅读
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