A Short History of Nearly Everything-第74章

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e　into　dinosaurs　（among　other　things）；　which　gradually　proved　too　much　for　thetherapsids。　unable　to　pete　head　to　head　with　these　aggressive　new　creatures；　thetherapsids　by　and　large　vanished　from　the　record。　a　very　few；　however；　evolved　into　small；furry；　burrowing　beings　that　bided　their　time　for　a　very　long　while　as　little　mammals。　thebiggest　of　them　grew　no　larger　than　a　house　cat；　and　most　were　no　bigger　than　mice。

eventually；　this　would　prove　their　salvation；　but　they　would　have　to　wait　nearly　150　millionyears　for　megadynasty　3；　the　age　of　dinosaurs；　to　e　to　an　abrupt　end　and　make　room　formegadynasty　4　and　our　own　age　of　mammals。

each　of　these　massive　transformations；　as　well　as　many　smaller　ones　between　and　since；was　dependent　on　that　paradoxically　important　motor　of　progress：　extinction。　it　is　a　curiousfact　that　on　earth　species　death　is；　in　the　most　literal　sense；　a　way　of　life。　no　one　knows　howmany　species　of　organisms　have　existed　since　life　began。　thirty　billion　is　a　monly　citedfigure；　but　the　number　has　been　put　as　high　as　4；000　billion。　whatever　the　actual　total；　99。99percent　of　all　species　that　have　ever　lived　are　no　longer　with　us。　“to　a　first　approximation；”　asdavid　raup　of　the　university　of　chicago　likes　to　say；　“all　species　are　extinct。”　for　plexorganisms；　the　average　lifespan　of　a　species　is　only　about　four　million　years—roughly　aboutwhere　we　are　now。

extinction　is　always　bad　news　for　the　victims；　of　course；　but　it　appears　to　be　a　good　thingfor　a　dynamic　planet。　“the　alternative　to　extinction　is　stagnation；”　says　ian　tattersall　of　theamerican　museum　of　natural　history；　“and　stagnation　is　seldom　a　good　thing　in　any　realm。”

（i　should　perhaps　note　that　we　are　speaking　here　of　extinction　as　a　natural；　long…term　process。

extinction　brought　about　by　human　carelessness　is　another　matter　altogether。）crises　in　earth’s　history　are　invariably　associated　with　dramatic　leaps　afterward。　the　fall　ofthe　ediacaran　fauna　was　followed　by　the　creative　outburst　of　the　cambrian　period。　theordovician　extinction　of　440　million　years　ago　cleared　the　oceans　of　a　lot　of　immobile　filterfeeders　and；　somehow；　created　conditions　that　favored　darting　fish　and　giant　aquatic　reptiles。

these　in　turn　were　in　an　ideal　position　to　send　colonists　onto　dry　land　when　another　blowoutin　the　late　devonian　period　gave　life　another　sound　shaking。　and　so　it　has　gone　at　scatteredintervals　through　history。　if　most　of　these　events　hadn’t　happened　just　as　they　did；　just　whenthey　did；　we　almost　certainly　wouldn’t　be　here　now。

earth　has　seen　five　major　extinction　episodes　in　its　time—the　ordovician；　devonian；permian；　triassic；　and　cretaceous；　in　that　order—and　many　smaller　ones。　the　ordovician（440　million　years　ago）　and　devonian　（365　million）　each　wiped　out　about　80　to　85　percent　ofspecies。　the　triassic　（210　million　years　ago）　and　cretaceous　（65　million　years）　each　wipedout　70　to　75　percent　of　species。　but　the　real　whopper　was　the　permian　extinction　of　about　245million　years　ago；　which　raised　the　curtain　on　the　long　age　of　the　dinosaurs。　in　the　permian；　atleast　95　percent　of　animals　known　from　the　fossil　record　check　out；　never　to　return。　evenabout　a　third　of　insect　species　went—the　only　occasion　on　which　they　were　lost　en　masse。　it　isas　close　as　we　have　ever　e　to　total　obliteration。

“it　was；　truly；　a　mass　extinction；　a　carnage　of　a　magnitude　that　had　never　troubled　the　earthbefore；”　says　richard　fortey。　the　permian　event　was　particularly　devastating　to　sea　creatures。

trilobites　vanished　altogether。　clams　and　sea　urchins　nearly　went。　virtually　all　other　marineorganisms　were　staggered。　altogether；　on　land　and　in　the　water；　it　is　thought　that　earth　lost　52percent　of　its　families—that’s　the　level　above　genus　and　below　order　on　the　grand　scale　of　life（the　subject　of　the　next　chapter）—and　perhaps　as　many　as　96　percent　of　all　its　species。　itwould　be　a　long　time—as　much　as　eighty　million　years　by　one　reckoning—before　speciestotals　recovered。

two　points　need　to　be　kept　in　mind。　first；　these　are　all　just　informed　guesses。　estimates　forthe　number　of　animal　species　alive　at　the　end　of　the　permian　range　from　as　low　as　45；000　toas　high　as　240；000。　if　you　don’t　know　how　many　species　were　alive；　you　can　hardly　specifywith　conviction　the　proportion　that　perished。　moreover；　we　are　talking　about　the　death　ofspecies；　not　individuals。　for　individuals　the　death　toll　could　be　much　higher—in　many　cases；practically　total。　the　species　that　survived　to　the　next　phase　of　life’s　lottery　almost　certainlyowe　their　existence　to　a　few　scarred　and　limping　survivors。

in　between　the　big　kill…offs；　there　have　also　been　many　smaller；　less　well…known　extinctionepisodes—the　hemphillian；　frasnian；　famennian；　rancholabrean；　and　a　dozen　or　so　others—which　were　not　so　devastating　to　total　species　numbers；　but　often　critically　hit　certainpopulations。　grazing　animals；　including　horses；　were　nearly　wiped　out　in　the　hemphillianevent　about　five　million　years　ago。　horses　declined　to　a　single　species；　which　appears　sosporadically　in　the　fossil　record　as　to　suggest　that　for　a　time　it　teetered　on　the　brink　ofoblivion。　imagine　a　human　history　without　horses；　without　grazing　animals。

in　nearly　every　case；　for　both　big　extinctions　and　more　modest　ones；　we　have　bewilderinglylittle　idea　of　what　the　cause　was。　even　after　stripping　out　the　more　crackpot　notions　there　arestill　more　theories　for　what　caused　the　extinction　events　than　there　have　been　events。　at　leasttwo　dozen　potential　culprits　have　been　identified　as　causes　or　prime　contributors：　globalwarming；　global　cooling；　changing　sea　levels；　oxygen　depletion　of　the　seas　（a　conditionknown　as　anoxia）；　epidemics；　giant　leaks　of　methane　gas　from　the　seafloor；　meteor　and　etimpacts；　runaway　hurricanes　of　a　type　known　as　hypercanes；　huge　volcanic　upwellings；catastrophic　solar　flares。

this　last　is　a　particularly　intriguing　possibility。　nobody　knows　how　big　solar　flares　can　getbecause　we　have　only　been　watching　them　since　the　beginning　of　the　space　age；　but　the　sun　isa　mighty　engine　and　its　storms　are　mensurately　enormous。　a　typical　solar　flare—something　we　wouldn’t　even　notice　on　earth—will　release　the　energy　equivalent　of　a　billionhydrogen　bombs　and　fling　into　space　a　hundred　billion　tons　or　so　of　murderous　high…energyparticles。　the　magnetosphere　and　atmosphere　between　them　normally　swat　these　back　intospace　or　steer　them　safely　toward　the　poles　（where　they　produce　the　earth’s　ely　auroras）；but　it　is　thought　that　an　unusually　big　blast；　say　a　hundred　times　the　typical　flare；　couldoverwhelm　our　ethereal　defenses。　the　light　show　would　be　a　glorious　one；　but　it　would　almostcertainly　kill　a　very　high　proportion　of　all　that　basked　in　its　glow。　moreover；　and　ratherchillingly；　according　to　bruce　tsurutani　of　the　nasa　jet　propulsion　laboratory；　“it　wouldleave　no　trace　in　history。”

what　all　this　leaves　us　with；　as　one　researcher　has　put　it；　is　“tons　of　conjecture　and　verylittle　evidence。”　cooling　seems　to　be　associated　with　at　least　three　of　the　big　extinctionevents—the　ordovician；　devonian；　and　permian—but　beyond　that　little　is　agreed；　includingwhether　a　particular　episode　happened　swiftly　or　slowly。　scientists　can’t　agree；　for　instance；whether　the　late　devonian　extinction—the　event　that　was　followed　by　vertebrates　movingonto　the　land—happened　over　millions　of　years　or　thousands　of　years　or　in　one　lively　day。

one　of　the　reasons　it　is　so　hard　to　produce　convincing　explanations　for　extinctions　is　that　itis　so　very　hard　to　exterminate　life　on　a　grand　scale。　as　we　have　seen　from　the　manson　impact；you　can　receive　a　ferocious　blow　and　still　stage　a　full；　if　presumably　somewhat　wobbly；recovery。　so　why；　out　of　all　the　thousands　of　impacts　earth　has　endured；　was　the　kt　event　sosingularly　devastating？　well；　first　itwas　positively　enormous。　it　struck　with　the　force　of　100million　megatons。　such　an　outburst　is　not　easily　imagined；　but　as　james　lawrence　powell　haspointed　out；　if　you　exploded　one　hiroshima…sized　bomb　for　every　person　alive　on　earth　todayyou　would　still　be　about　a　billion　bombs　short　of　the　size　of　the　kt　impact。　but　even　thatalone　may　not　have　been　enough　to　wipe　out　70　percent　of　earth’s　life；　dinosaurs　included。

the　kt　meteor　had　the　additional　advantage—advantage　if　you　are　a　mammal；　that　is—that　it　landed　in　a　shallow　sea　just　ten　meters　deep；　probably　at　just　the　right　angle；　at　a　timewhen　oxygen　levels　were　10　percent　higher　than　at　present　and　so　the　world　was　morebustible。　above　all　the　floor　of　the　sea　where　it　landed　was　made　of　rock　rich　in　sulfur。

the　result　was　an　impact　that　turned　an　area　of　seafloor　the　size　of　belgium　into　aerosols　ofsulfuric　acid。　for　months　afterward；　the　earth　was　subjected　to　rains　acid　enough　to　burn　skin。

in　a　sense；　an　even　greater　question　than　that　of　what　wiped　out　70　percent　of　the　speciesthat　were　existing　at　the　time　is　how　did　the　remaining　30　percent　survive？　why　was　the　eventso　irremediably　devastating　to　every　single　dinosaur　that　existed；　while　other　reptiles；　likesnakes　and　crocodiles；　passed　through　unimpeded？　so　far　as　we　can　tell　no　species　of　toad；newt；　salamander；　or　other　amphibian　went　extinct　in　north　america。　“why　should　thesedelica