A Short History of Nearly Everything-第52章

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s　realize　that　actually　life　of　a　type　could　range　much　farther　than　that…even；　perhaps；　beneath　theicy　skin　of　pluto。　what　we　are　talking　about　here　are　the　conditions　that　would　produce　reasonably　plexsurface　creatures。

we’re　a　twin　planet。not　many　of　us　normally　think　of　the　moon　as　a　panion　planet；but　that　is　in　effect　what　it　is。　most　moons　are　tiny　in　relation　to　their　master　planet。　themartian　satellites　of　phobos　and　deimos；　for　instance；　are　only　about　ten　kilometers　indiameter。　our　moon；　however；　is　more　than　a　quarter　the　diameter　of　the　earth；　which　makesours　the　only　planet　in　the　solar　system　with　a　sizeable　moon　in　parison　to　itself　（exceptpluto；　which　doesn’t　really　count　because　pluto　is　itself　so　small）；　and　what　a　difference　thatmakes　to　us。

without　the　moon’s　steadying　influence；　the　earth　would　wobble　like　a　dying　top；　withgoodness　knows　what　consequences　for　climate　and　weather。　the　moon’s　steady　gravitationalinfluence　keeps　the　earth　spinning　at　the　right　speed　and　angle　to　provide　the　sort　of　stabilitynecessary　for　the　long　and　successful　development　of　life。　this　won’t　go　on　forever。　themoon　is　slipping　from　our　grasp　at　a　rate　of　about　1。5　inches　a　year。　in　another　two　billionyears　it　will　have　receded　so　far　that　it　won’t　keep　us　steady　and　we　will　have　to　e　up　withsome　other　solution；　but　in　the　meantime　you　should　think　of　it　as　much　more　than　just　apleasant　feature　in　the　night　sky。

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—but　especially　so　as　it　happened　such　a　long　time　ago。　if　it　had　happened　in1896　or　last　wednesday　clearly　we　wouldn’t　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　pleequence　of　events　stretching　back　4。6billion　years　or　so　hadn’t　played　out　in　a　particular　manner　at　particular　times—if；　to　take　justone　obvious　instance；　the　dinosaurs　hadn’t　been　wiped　out　by　a　meteor　when　they　were—youmight　well　be　six　inches　long；　with　whiskers　and　a　tail；　and　reading　this　in　a　burrow。

we　don’t　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—as；　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’s　polonium）；　but　almost　nothing　else。　the　problemisn’t　scientific　indifference；　but　rarity。　there　just　isn’t　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’s　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’s　underneath　yourfeet；　but　its　existence　wasn’t　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’t。

nor　does　abundance　necessarily　relate　to　importance。　carbon　is　only　the　fifteenth　mostmon　element；　accounting　for　a　very　modest　0。048　percent　of　earth’s　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—the　very　trick　of　naturenecessary　to　build　proteins　and　dna。　as　paul　davies　has　written：　“if　it　wasn’t　for　carbon；　lifeas　we　know　it　would　be　impossible。　probably　any　sort　of　life　would　be　impossible。”　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—bless　it—oxidizes　alcohol。

we　have　evolved　to　utilize　or　tolerate　these　things—we　could　hardly　be　here　otherwise—but　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’s　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’s　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’t　appreciate　it。　yet　put　these　two　nasty　elements　together　and　what　do　you　get？　sodiumchloride—mon　table　salt。

by　and　large；　if　an　element　doesn’t　naturally　find　its　way　into　our　systems—if　it　isn’tsoluble　in　water；　say—we　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’s　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。