A Short History of Nearly Everything- 第72部分

alas；　it　turns　out　the　cambrian　explosion　may　not　have　been　quite　so　explosive　as　all　that。

the　cambrian　animals；　it　is　now　thought；　were　probably　there　all　along；　but　were　just　toosmall　to　see。　once　again　it　was　trilobites　that　provided　the　clue—in　particular　that　seeminglymystifying　appearance　of　different　types　of　trilobite　in　widely　scattered　locations　around　theglobe；　all　at　more　or　less　the　same　time。

on　the　face　of　it；　the　sudden　appearance　of　lots　of　fully　formed　but　varied　creatures　wouldseem　to　enhance　the　miraculousness　of　the　cambrian　outburst；　but　in　fact　it　did　the　opposite。

it　is　one　thing　to　have　one　well…formed　creature　like　a　trilobite　burst　forth　in　isolation—thatreally　is　a　wonder—but　to　have　many　of　them；　all　distinct　but　clearly　related；　turning　upsimultaneously　in　the　fossil　record　in　places　as　far　apart　as　china　and　new　york　clearlysuggests　that　we　are　missing　a　big　part　of　their　history。　there　could　be　no　stronger　evidencethat　they　simply　had　to　have　a　forebear—some　grandfather　species　that　started　the　line　in　amuch　earlier　past。

and　the　reason　we　haven’t　found　these　earlier　species；　it　is　now　thought；　is　that　they　weretoo　tiny　to　be　preserved。　says　fortey：　“it　isn’t　necessary　to　be　big　to　be　a　perfectlyfunctioning；　plex　organism。　the　sea　swarms　with　tiny　arthropods　today　that　have　left　nofossil　record。”　he　cites　the　little　copepod；　which　numbers　in　the　trillions　in　modern　seas　andclusters　in　shoals　large　enough　to　turn　vast　areas　of　the　ocean　black；　and　yet　our　totalknowledge　of　its　ancestry　is　a　single　specimen　found　in　the　body　of　an　ancient　fossilized　fish。

“the　cambrian　explosion；　if　that’s　the　word　for　it；　probably　was　more　an　increase　in　sizethan　a　sudden　appearance　of　new　body　types；”　fortey　says。　“and　it　could　have　happened　quiteswiftly；　so　in　that　sense　i　suppose　it　was　an　explosion。”　the　idea　is　that　just　as　mammalsbided　their　time　for　a　hundred　million　years　until　the　dinosaurs　cleared　off　and　then　seeminglyburst　forth　in　profusion　all　over　the　planet；　so　too　perhaps　the　arthropods　and　other　triploblastswaited　in　semimicroscopic　anonymity　for　the　dominant　ediacaran　organisms　to　have　theirday。　says　fortey：　“we　know　that　mammals　increased　in　size　quite　dramatically　after　thedinosaurs　went—though　when　i　say　quite　abruptly　i　of　course　mean　it　in　a　geological　sense。

we’re　still　talking　millions　of　years。”

incidentally；　reginald　sprigg　did　eventually　get　a　measure　of　overdue　credit。　one　of　themain　early　genera；　spriggina；　was　named　in　his　honor；　as　were　several　species；　and　the　wholebecame　known　as　the　ediacaran　fauna　after　the　hills　through　which　he　had　searched。　by　thistime；　however；　sprigg’s　fossil…hunting　days　were　long　over。　after　leaving　geology　he　foundeda　successful　oil　pany　and　eventually　retired　to　an　estate　in　his　beloved　flinders　range；where　he　created　a　wildlife　reserve。　he　died　in　1994　a　rich　man。

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22GOOD…BYE　TO　ALL　THAT

。^生。网
when　you　consider　it　from　a　human　perspective；　and　clearly　it　would　be　difficult　forus　to　do　otherwise；　life　is　an　odd　thing。　it　couldn’t　wait　to　get　going；　but　then；　having　gottengoing；　it　seemed　in　very　little　hurry　to　move　on。

consider　the　lichen。　lichens　are　just　about　the　hardiest　visible　organisms　on　earth；　butamong　the　least　ambitious。　they　will　grow　happily　enough　in　a　sunny　churchyard；　but　theyparticularly　thrive　in　environments　where　no　other　organism　would　go—on　blowymountaintops　and　arctic　wastes；　wherever　there　is　little　but　rock　and　rain　and　cold；　and　almostno　petition。　in　areas　of　antarctica　where　virtually　nothing　else　will　grow；　you　can　findvast　expanses　of　lichen—four　hundred　types　of　them—adhering　devotedly　to　every　wind…whipped　rock。

for　a　long　time；　people　couldn’t　understand　how　they　did　it。　because　lichens　grew　on　barerock　without　evident　nourishment　or　the　production　of　seeds；　many　people—educatedpeople—believed　they　were　stones　caught　in　the　process　of　being　plants。　“spontaneously；inorganic　stone　bees　living　plant！”　rejoiced　one　observer；　a　dr。　homschuch；　in　1819。

closer　inspection　showed　that　lichens　were　more　interesting　than　magical。　they　are　in　facta　partnership　between　fungi　and　algae。　the　fungi　excrete　acids　that　dissolve　the　surface　of　therock；　freeing　minerals　that　the　algae　convert　into　food　sufficient　to　sustain　both。　it　is　not　avery　exciting　arrangement；　but　it　is　a　conspicuously　successful　one。　the　world　has　more　thantwenty　thousand　species　of　lichens。

like　most　things　that　thrive　in　harsh　environments；　lichens　are　slow…growing。　it　may　take　alichen　more　than　half　a　century　to　attain　the　dimensions　of　a　shirt　button。　those　the　size　ofdinner　plates；　writes　david　attenborough；　are　therefore　“likely　to　be　hundreds　if　notthousands　of　years　old。”　it　would　be　hard　to　imagine　a　less　fulfilling　existence。　“they　simplyexist；”　attenborough　adds；　“testifying　to　the　moving　fact　that　life　even　at　its　simplest　leveloccurs；　apparently；　just　for　its　own　sake。”

it　is　easy　to　overlook　this　thought　that　life　just　is。　as　humans　we　are　inclined　to　feel　that　lifemust　have　a　point。　we　have　plans　and　aspirations　and　desires。　we　want　to　take　constantadvantage　of　all　the　intoxicating　existence　we’ve　been　endowed　with。　but　what’s　life　to　alichen？　yet　its　impulse　to　exist；　to　be；　is　every　bit　as　strong　as　ours—arguably　even　stronger。

if　i　were　told　that　i　had　to　spend　decades　being　a　furry　growth　on　a　rock　in　the　woods；　ibelieve　i　would　lose　the　will　to　go　on。　lichens　don’t。　like　virtually　all　living　things；　they　willsuffer　any　hardship；　endure　any　insult；　for　a　moment’s　additional　existence。　life；　in　short；　justwants　to　be。　but—and　here’s　an　interesting　point—for　the　most　part　it　doesn’t　want　to　bemuch。

this　is　perhaps　a　little　odd　because　life　has　had　plenty　of　time　to　develop　ambitions。　if　youimagine　the　4；500…billion…odd　years　of　earth’s　history　pressed　into　a　normal　earthly　day；then　life　begins　very　early；　about　4a。m。；　with　the　rise　of　the　first　simple；　single…celledorganisms；　but　then　advances　no　further　for　the　next　sixteen　hours。　not　until　almost　8：30　inthe　evening；　with　the　day　five…sixths　over；　has　earth　anything　to　show　the　universe　but　arestless　skin　of　microbes。　then；　finally；　the　first　sea　plants　appear；　followed　twenty　minuteslater　by　the　first　jellyfish　and　the　enigmatic　ediacaran　fauna　first　seen　by　reginald　sprigg　inaustralia。　at　9：04p。m。　trilobites　swim　onto　the　scene；　followed　more　or　less　immediately　bythe　shapely　creatures　of　the　burgess　shale。　just　before　10p。m。　plants　begin　to　pop　up　on　theland。　soon　after；　with　less　than　two　hours　left　in　the　day；　the　first　land　creatures　follow。

thanks　to　ten　minutes　or　so　of　balmy　weather；　by　10：24　the　earth　is　covered　in　the　greatcarboniferous　forests　whose　residues　give　us　all　our　coal；　and　the　first　winged　insects　areevident。　dinosaurs　plod　onto　the　scene　just　before　11p。m。　and　hold　sway　for　about　three…quarters　of　an　hour。　at　twenty…one　minutes　to　midnight　they　vanish　and　the　age　of　mammalsbegins。　humans　emerge　one　minute　and　seventeen　seconds　before　midnight。　the　whole　of　ourrecorded　history；　on　this　scale；　would　be　no　more　than　a　few　seconds；　a　single　human　lifetimebarely　an　instant。　throughout　this　greatly　speeded…up　day　continents　slide　about　and　bangtogether　at　a　clip　that　seems　positively　reckless。　mountains　rise　and　melt　away；　ocean　basinse　and　go；　ice　sheets　advance　and　withdraw。　and　throughout　the　whole；　about　three　timesevery　minute；　somewhere　on　the　planet　there　is　a　flashbulb　pop　of　light　marking　the　impact　ofa　manson…sized　meteor　or　one　even　larger。　it’s　a　wonder　that　anything　at　all　can　survive　insuch　a　pummeled　and　unsettled　environment。　in　fact；　not　many　things　do　for　long。

perhaps　an　even　more　effective　way　of　grasping　our　extreme　recentness　as　a　part　of　this4。5…billion…year…old　picture　is　to　stretch　your　arms　to　their　fullest　extent　and　imagine　thatwidth　as　the　entire　history　of　the　earth。　on　this　scale；　according　to　john　mcphee　in　basin　andrange；　the　distance　from　the　fingertips　of　one　hand　to　the　wrist　of　the　other　is　precambrian。

all　of　plex　life　is　in　one　hand；　“and　in　a　single　stroke　with　a　medium…grained　nail　file　youcould　eradicate　human　history。”

fortunately；　that　moment　hasn’t　happened；　but　the　chances　are　good　that　it　will。　i　don’twish　to　interject　a　note　of　gloom　just　at　this　point；　but　the　fact　is　that　there　is　one　otherextremely　pertinent　quality　about　life　on　earth：　it　goes　extinct。　quite　regularly。　for　all　thetrouble　they　take　to　assemble　and　preserve　themselves；　species　crumple　and　die　remarkablyroutinely。　and　the　more　plex　they　get；　the　more　quickly　they　appear　to　go　extinct。　whichis　perhaps　one　reason　why　so　much　of　life　isn’t　terribly　ambitious。

so　anytime　life　does　something　bold　it　is　quite　an　event；　and　few　occasions　were　moreeventful　than　when　life　moved　on　to　the　next　stage　in　our　narrative　and　came　out　of　the　sea。

land　was　a　formidable　environment：　hot；　dry；　bathed　in　intense　ultraviolet　radiation；lacking　the　buoyancy　that　makes　movement　in　water　paratively　effortless。　to　live　onland；　creatures　had　to　undergo　wholesale　revisions　of　their　anatomies。　hold　a　fish　at　each　endand　it　sags　in　the　middle；　its　backbone　too　weak　to　suppor