万物简史英文版_比尔·布莱森-第47章

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for　a　long　time；　astronomers　assumed　that　the　moon　and　earth　either　formed　together　orthat　the　earth　captured　the　moon　as　it　drifted　by。　we　now　believe；　as　you　will　recall　from　anearlier　chapter；　that　about　4。5　billion　years　ago　a　mars…sized　object　slammed　into　earth；blowing　out　enough　material　to　create　the　moon　from　the　debris。　this　was　obviously　a　verygood　thing　for　us鈥攂ut　especially　so　as　it　happened　such　a　long　time　ago。　if　it　had　happened　in1896　or　last　wednesday　clearly　we　wouldn鈥檛　be　nearly　so　pleased　about　it。　which　brings　us　toour　fourth　and　in　many　ways　most　crucial　consideration：

timing。the　universe　is　an　amazingly　fickle　and　eventful　place；　and　our　existence　within　itis　a　wonder。　if　a　long　and　unimaginably　plex　sequence　of　events　stretching　back　4。6billion　years　or　so　hadn鈥檛　played　out　in　a　particular　manner　at　particular　times鈥攊f；　to　take　justone　obvious　instance；　the　dinosaurs　hadn鈥檛　been　wiped　out　by　a　meteor　when　they　were鈥攜oumight　well　be　six　inches　long；　with　whiskers　and　a　tail；　and　reading　this　in　a　burrow。

we　don鈥檛　really　know　for　sure　because　we　have　nothing　else　to　pare　our　own　existenceto；　but　it　seems　evident　that　if　you　wish　to　end　up　as　a　moderately　advanced；　thinking　society；you　need　to　be　at　the　right　end　of　a　very　long　chain　of　outes　involving　reasonable　periodsof　stability　interspersed　with　just　the　right　amount　of　stress　and　challenge　（ice　ages　appear　tobe　especially　helpful　in　this　regard）　and　marked　by　a　total　absence　of　real　cataclysm。　as　weshall　see　in　the　pages　that　remain　to　us；　we　are　very　lucky　to　find　ourselves　in　that　position。

and　on　that　note；　let　us　now　turn　briefly　to　the　elements　that　made　us。

there　are　ninety…two　naturally　occurring　elements　on　earth；　plus　a　further　twenty　or　so　thathave　been　created　in　labs；　but　some　of　these　we　can　immediately　put　to　one　side鈥攁s；　in　fact；chemists　themselves　tend　to　do。　not　a　few　of　our　earthly　chemicals　are　surprisingly　littleknown。　astatine；　for　instance；　is　practically　unstudied。　it　has　a　name　and　a　place　on　theperiodic　table　（next　door　to　marie　curie鈥檚　polonium）；　but　almost　nothing　else。　the　problemisn鈥檛　scientific　indifference；　but　rarity。　there　just　isn鈥檛　much　astatine　out　there。　the　mostelusive　element　of　all；　however；　appears　to　be　francium；　which　is　so　rare　that　it　is　thought　thatour　entire　planet　may　contain；　at　any　given　moment；　fewer　than　twenty　francium　atoms。

altogether　only　about　thirty　of　the　naturally　occurring　elements　are　widespread　on　earth；　andbarely　half　a　dozen　are　of　central　importance　to　life。

as　you　might　expect；　oxygen　is　our　most　abundant　element；　accounting　for　just　under　50percent　of　the　earth鈥檚　crust；　but　after　that　the　relative　abundances　are　often　surprising。　whowould　guess；　for　instance；　that　silicon　is　the　second　most　mon　element　on　earth　or　thattitanium　is　tenth？　abundance　has　little　to　do　with　their　familiarity　or　utility　to　us。　many　of　themore　obscure　elements　are　actually　more　mon　than　the　better…known　ones。　there　is　morecerium　on　earth　than　copper；　more　neodymium　and　lanthanum　than　cobalt　or　nitrogen。　tinbarely　makes　it　into　the　top　fifty；　eclipsed　by　such　relative　obscurities　as　praseodymium；samarium；　gadolinium；　and　dysprosium。

abundance　also　has　little　to　do　with　ease　of　detection。　aluminum　is　the　fourth　mostmon　element　on　earth；　accounting　for　nearly　a　tenth　of　everything　that鈥檚　underneath　yourfeet；　but　its　existence　wasn鈥檛　even　suspected　until　it　was　discovered　in　the　nineteenth　centuryby　humphry　davy；　and　for　a　long　time　after　that　it　was　treated　as　rare　and　precious。　congressnearly　put　a　shiny　lining　of　aluminum　foil　atop　the　washington　monument　to　show　what　aclassy　and　prosperous　nation　we　had　bee；　and　the　french　imperial　family　in　the　sameperiod　discarded　the　state　silver　dinner　service　and　replaced　it　with　an　aluminum　one。　thefashion　was　cutting　edge　even　if　the　knives　weren鈥檛。

nor　does　abundance　necessarily　relate　to　importance。　carbon　is　only　the　fifteenth　mostmon　element；　accounting　for　a　very　modest　0。048　percent　of　earth鈥檚　crust；　but　we　wouldbe　lost　without　it。　what　sets　the　carbon　atom　apart　is　that　it　is　shamelessly　promiscuous。　it　isthe　party　animal　of　the　atomic　world；　latching　on　to　many　other　atoms　（including　itself）　andholding　tight；　forming　molecular　conga　lines　of　hearty　robustness鈥攖he　very　trick　of　naturenecessary　to　build　proteins　and　dna。　as　paul　davies　has　written：　鈥渋f　it　wasn鈥檛　for　carbon；　lifeas　we　know　it　would　be　impossible。　probably　any　sort　of　life　would　be　impossible。鈥潯�yetcarbon　is　not　all　that　plentiful　even　in　humans；　who　so　vitally　depend　on　it。　of　every　200atoms　in　your　body；　126　are　hydrogen；　51　are　oxygen；　and　just　19　are　carbon。

2other　elements　are　critical　not　for　creating　life　but　for　sustaining　it。　we　need　iron　tomanufacture　hemoglobin；　and　without　it　we　would　die。　cobalt　is　necessary　for　the　creation　ofvitamin　b12。　potassium　and　a　very　little　sodium　are　literally　good　for　your　nerves。

molybdenum；　manganese；　and　vanadium　help　to　keep　your　enzymes　purring。　zinc鈥攂less　it鈥攐xidizes　alcohol。

we　have　evolved　to　utilize　or　tolerate　these　things鈥攚e　could　hardly　be　here　otherwise鈥攂ut　even　then　we　live　within　narrow　ranges　of　acceptance。　selenium　is　vital　to　all　of　us；　buttake　in　just　a　little　too　much　and　it　will　be　the　last　thing　you　ever　do。　the　degree　to　whichorganisms　require　or　tolerate　certain　elements　is　a　relic　of　their　evolution。　sheep　and　cattlenow　graze　side　by　side；　but　actually　have　very　different　mineral　requirements。　modern　cattleneed　quite　a　lot　of　copper　because　they　evolved　in　parts　of　europe　and　africa　where　copperwas　abundant。　sheep；　on　the　other　hand；　evolved　in　copper…poor　areas　of　asia　minor。　as　arule；　and　not　surprisingly；　our　tolerance　for　elements　is　directly　proportionate　to　their2of　the　remaining　four；　three　are　nitrogen　and　the　remaining　atom　is　divided　among　all　the　other　elements。

abundance　in　the　earth鈥檚　crust。　we　have　evolved　to　expect；　and　in　some　cases　actually　need；the　tiny　amounts　of　rare　elements　that　accumulate　in　the　flesh　or　fiber　that　we　eat。　but　step　upthe　doses；　in　some　cases　by　only　a　tiny　amount；　and　we　can　soon　cross　a　threshold。　much　ofthis　is　only　imperfectly　understood。　no　one　knows；　for　example；　whether　a　tiny　amount　ofarsenic　is　necessary　for　our　well…being　or　not。　some　authorities　say　it　is；　some　not。　all　that　iscertain　is　that　too　much　of　it　will　kill　you。

the　properties　of　the　elements　can　bee　more　curious　still　when　they　are　bined。

oxygen　and　hydrogen；　for　instance；　are　two　of　the　most　bustion…friendly　elements　around；but　put　them　together　and　they　make　inbustible　water。

3odder　still　in　bination　aresodium；　one　of　the　most　unstable　of　all　elements；　and　chlorine；　one　of　the　most　toxic。　drop　asmall　lump　of　pure　sodium　into　ordinary　water　and　it　will　explode　with　enough　force　to　kill。

chlorine　is　even　more　notoriously　hazardous。　though　useful　in　small　concentrations　forkilling　microorganisms　（it鈥檚　chlorine　you　smell　in　bleach）；　in　larger　volumes　it　is　lethal。

chlorine　was　the　element　of　choice　for　many　of　the　poison　gases　of　the　first　world　war。　and；as　many　a　sore…eyed　swimmer　will　attest；　even　in　exceedingly　dilute　form　the　human　bodydoesn鈥檛　appreciate　it。　yet　put　these　two　nasty　elements　together　and　what　do　you　get？　sodiumchloride鈥攎on　table　salt。

by　and　large；　if　an　element　doesn鈥檛　naturally　find　its　way　into　our　systems鈥攊f　it　isn鈥檛soluble　in　water；　say鈥攚e　tend　to　be　intolerant　of　it。　lead　poisons　us　because　we　were　neverexposed　to　it　until　we　began　to　fashion　it　into　food　vessels　and　pipes　for　plumbing。　（notincidentally；　lead鈥檚　symbol　is　pb；　for　the　latin　plumbum；　the　source　word　for　our　modernplumbing。）　the　romans　also　flavored　their　wine　with　lead；　which　may　be　part　of　the　reasonthey　are　not　the　force　they　used　to　be。　as　we　have　seen　elsewhere；　our　own　performance　withlead　（not　to　mention　mercury；　cadmium；　and　all　the　other　industrial　pollutants　with　which　weroutinely　dose　ourselves）　does　not　leave　us　a　great　deal　of　room　for　smirking。　when　elementsdon鈥檛　occur　naturally　on　earth；　we　have　evolved　no　tolerance　for　them；　and　so　they　tend　to　beextremely　toxic　to　us；　as　with　plutonium。　our　tolerance　for　plutonium　is　zero：　there　is　no　levelat　which　it　is　not　going　to　make　you　want　to　lie　down。

i　have　brought　you　a　long　way　to　make　a　small　point：　a　big　part　of　the　reason　that　earthseems　so　miraculously　acmodating　is　that　we　evolved　to　suit　its　conditions。　what　wemarvel　at　is　not　that　it　is　suitable　to　life　but　that　it　is　suitable　to　our　life鈥攁nd　hardlysurprising；　really。　it　may　be　that　many　of　the　things　that　make　it　so　splendid　to　us鈥攚ell…proportioned　sun；　doting　moon；　sociable　carbon；　more　magma　than　you　can　shake　a　stick　at；and　all　the　rest鈥攕eem　splendid　simply　because　they　are　what　we　were　born　to　count　on。　noone　can　altogether　say。

other　worlds　may　harbor　beings　thankful　for　their　silvery　lakes　of　mercury　and　driftingclouds　of　ammonia。　they　may　be　delighted　that　their　planet　doesn鈥檛　shake　them　silly　with　itsgrinding　plates　or　spew　messy　gobs　of　lava　over　the　landscape；　but　rather　exists　in　apermanent　nontectonic　tranquility。　any　visitors　to　earth　from　afar　would　almost　certainly；　atthe　very　least；　be　bemused　to　find　us　living　in　an　atmosphere　posed　of　nitrogen；　a　gassulkily　disinclined　to　react　with　anything；　and　oxygen；　which　is　so　partial　to　bustion　thatwe　must　place　fire　stations　throughout　our　cities　to　protect　ourselves　from　its　livelier　effects。

but　even　if　our　visitors　were　oxygen…breathing　bipeds　with　shopping　malls　and　a　fondness　for3oxygen　itself　is　not　bustible；　it　merely　facilitates　the　bus　tion　of　other　things。　this　is　just　as　well；　for　ifoxygen　were　corn　bustible；　each　time　you　lit　a　match　all　the　air　around　you　would　bur　into　flame。　hydrogen　gas；on　the　other　hand；　is　extremely　corn　bustible；　as　the　dirigible　hindenburg　demonstrated　on　may　6；　193　inlakehurst；　new　jersey；　when　its　hydrogen　fuel　burst　explosive）　into　flame；　killing　thirty…six　people。

action　movies；　it　is　unlikely　that　they　would　find　earth　ideal。　we　couldn鈥檛　even　give　themlunch　because　all　our　foods　contain　traces　of　manganese；　selenium；　zinc；　and　other　elementalparticles　at　least　some　of　which　would　be　poisonous　to　them。　to　them　earth　might　not　seem　awondrously　congenial　place　at　all。

the　physicist　richard　feynman　used　to　make　a　joke　about　a　posteriori　conclusions；　as　theyare　called。　鈥測ou　know；　the　most　amazing　thing　happened　to　me　tonight；鈥潯�he　would　say。　鈥渋saw　a　car　with　the　license　plate　arw　357。　can　you　imagine？　of　all　the　millions　of　licenseplates　in　the　state；　what　was　the　chance　that　i　would　see　that　particular　one　tonight？

amazing！鈥潯�his　point；　of　course；　was　that　it　is　easy　to　make　any　banal　situation　seemextraordinary　if　you　treat　it　as　fateful。

so　it　is　possible　that　the　events　and　conditions　that　led　to　the　rise　of　life　on　earth　are　notquite　as　extraordinary　as　we　like　to　think。　still；　they　were　extraordinary　enough；　and　one　thingis　certain：　they　will　have　to　do　until　we　find　some　better。

銆�



17　　　INTO　THE　TROPOSPHERE

锛佸皬锛犺�达純txt锛勫ぉ锛惧爞锛�
thank　goodness　for　the　atmosphere。　it　keeps　us　warm。　without　it；　earth　would　be　alifeless　ball　of　ice　with　an　average　te