A Short History of Nearly Everything-第51章

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s　to　a　small　window。

on　another　occasion；　while　poisoning　himself　with　elevated　levels　of　oxygen；　haldane　hada　fit　so　severe　that　he　crushed　several　vertebrae。　collapsed　lungs　were　a　routine　hazard。

perforated　eardrums　were　quite　mon；　but；　as　haldane　reassuringly　noted　in　one　of　hisessays；　“the　drum　generally　heals　up；　and　if　a　hole　remains　in　it；　although　one　is　somewhatdeaf；　one　can　blow　tobacco　smoke　out　of　the　ear　in　question；　which　is　a　socialacplishment。”

what　was　extraordinary　about　this　was　not　that　haldane　was　willing　to　subject　himself　tosuch　risk　and　disfort　in　the　pursuit　of　science；　but　that　he　had　no　trouble　talkingcolleagues　and　loved　ones　into　climbing　into　the　chamber；　too。　sent　on　a　simulated　descent；his　wife　once　had　a　fit　that　lasted　thirteen　minutes。　when　at　last　she　stopped　bouncing　acrossthe　floor；　she　was　helped　to　her　feet　and　sent　home　to　cook　dinner。　haldane　happily　employedwhoever　happened　to　be　around；　including　on　one　memorable　occasion　a　former　primeminister　of　spain；　juan　negrín。　dr。　negrín　plained　afterward　of　minor　tingling　and　“acurious　velvety　sensation　on　the　lips”　but　otherwise　seems　to　have　escaped　unharmed。　he　mayhave　considered　himself　very　lucky。　a　similar　experiment　with　oxygen　deprivation　lefthaldane　without　feeling　in　his　buttocks　and　lower　spine　for　six　years。

among　haldane’s　many　specific　preoccupations　was　nitrogen　intoxication。　for　reasons　thatare　still　poorly　understood；　beneath　depths　of　about　a　hundred　feet　nitrogen　bees　apowerful　intoxicant。　under　its　influence　divers　had　been　known　to　offer　their　air　hoses　topassing　fish　or　decide　to　try　to　have　a　smoke　break。　it　also　produced　wild　mood　swings。　inone　test；　haldane　noted；　the　subject　“alternated　between　depression　and　elation；　at　onemoment　begging　to　be　depressed　because　he　felt　‘bloody　awful’　and　the　next　minutelaughing　and　attempting　to　interfere　with　his　colleague’s　dexterity　test。”　in　order　to　measurethe　rate　of　deterioration　in　the　subject；　a　scientist　had　to　go　into　the　chamber　with　thevolunteer　to　conduct　simple　mathematical　tests。　but　after　a　few　minutes；　as　haldane　laterrecalled；　“the　tester　was　usually　as　intoxicated　as　the　testee；　and　often　forgot　to　press　thespindle　of　his　stopwatch；　or　to　take　proper　notes。”　the　cause　of　the　inebriation　is　even　now　amystery。　it　is　thought　that　it　may　be　the　same　thing　that　causes　alcohol　intoxication；　but　as　noone　knows　for　certain　what　causes　that　we　are　none　the　wiser。　at　all　events；　without　thegreatest　care；　it　is　easy　to　get　in　trouble　once　you　leave　the　surface　world。

which　brings　us　back　（well；　nearly）　to　our　earlier　observation　that　earth　is　not　the　easiestplace　to　be　an　organism；　even　if　it　is　the　only　place。　of　the　small　portion　of　the　planet’ssurface　that　is　dry　enough　to　stand　on；　a　surprisingly　large　amount　is　too　hot　or　cold　or　dry　orsteep　or　lofty　to　be　of　much　use　to　us。　partly；　it　must　be　conceded；　this　is　our　fault。　in　terms　ofadaptability；　humans　are　pretty　amazingly　useless。　like　most　animals；　we　don’t　much　likereally　hot　places；　but　because　we　sweat　so　freely　and　easily　stroke；　we　are　especiallyvulnerable。　in　the　worst　circumstances—on　foot　without　water　in　a　hot　desert—most　peoplewill　grow　delirious　and　keel　over；　possibly　never　to　rise　again；　in　no　more　than　six　or　sevenhours。　we　are　no　less　helpless　in　the　face　of　cold。　like　all　mammals；　humans　are　good　atgenerating　heat　but—because　we　are　so　nearly　hairless—not　good　at　keeping　it。　even　in　quitemild　weather　half　the　calories　you　burn　go　to　keep　your　body　warm。　of　course；　we　cancounter　these　frailties　to　a　large　extent　by　employing　clothing　and　shelter；　but　even　so　theportions　of　earth　on　which　we　are　prepared　or　able　to　live　are　modest　indeed：　just　12　percentof　the　total　land　area；　and　only　4　percent　of　the　whole　surface　if　you　include　the　seas。

yet　when　you　consider　conditions　elsewhere　in　the　known　universe；　the　wonder　is　not　thatwe　use　so　little　of　our　planet　but　that　we　have　managed　to　find　a　planet　that　we　can　use　even　abit　of。　you　have　only　to　look　at　our　own　solar　system—or；　e　to　that；　earth　at　certainperiods　in　its　own　history—to　appreciate　that　most　places　are　much　harsher　and　much　lessamenable　to　life　than　our　mild；　blue　watery　globe。

so　far　space　scientists　have　discovered　about　seventy　planets　outside　the　solar　system；　outof　the　ten　billion　trillion　or　so　that　are　thought　to　be　out　there；　so　humans　can　hardly　claim　tospeak　with　authority　on　the　matter；　but　it　appears　that　if　you　wish　to　have　a　planet　suitable　forlife；　you　have　to　be　just　awfully　lucky；　and　the　more　advanced　the　life；　the　luckier　you　have　tobe。　various　observers　have　identified　about　two　dozen　particularly　helpful　breaks　we　havehad　on　earth；　but　this　is　a　flying　survey　so　we’ll　distill　them　down　to　the　principal　four。　theyare：

excellent　location。we　are；　to　an　almost　uncanny　degree；　the　right　distance　from　the　right　sortof　star；　one　that　is　big　enough　to　radiate　lots　of　energy；　but　not　so　big　as　to　burn　itself　outswiftly。　it　is　a　curiosity　of　physics　that　the　larger　a　star　the　more　rapidly　it　burns。　had　our　sunbeen　ten　times　as　massive；　it　would　have　exhausted　itself　after　ten　million　years　instead　of　tenbillion　and　we　wouldn’t　be　here　now。　we　are　also　fortunate　to　orbit　where　we　do。　too　muchnearer　and　everything　on　earth　would　have　boiled　away。　much　farther　away　and　everythingwould　have　frozen。

in　1978；　an　astrophysicist　named　michael　hart　made　some　calculations　and　concluded　thatearth　would　have　been　uninhabitable　had　it　been　just　1　percent　farther　from　or　5　percentcloser　to　the　sun。　that’s　not　much；　and　in　fact　it　wasn’t　enough。　the　figures　have　since　beenrefined　and　made　a　little　more　generous—5　percent　nearer　and　15　percent　farther　are　thoughtto　be　more　accurate　assessments　for　our　zone　of　habitability—but　that　is　still　a　narrow　belt。

1to　appreciate　just　how　narrow；　you　have　only　to　look　at　venus。　venus　is　only　twenty…fivemillion　miles　closer　to　the　sun　than　we　are。　the　sun’s　warmth　reaches　it　just　two　minutesbefore　it　touches　us。　in　size　and　position；　venus　is　very　like　earth；　but　the　smalldifference　in　orbital　distance　made　all　the　difference　to　how　it　turned　out。　it　appears　thatduring　the　early　years　of　the　solar　system　venus　was　only　slightly　warmer　than　earth　andprobably　had　oceans。　but　those　few　degrees　of　extra　warmth　meant　that　venus　could　not　holdon　to　its　surface　water；　with　disastrous　consequences　for　its　climate。　as　its　water　evaporated；the　hydrogen　atoms　escaped　into　space；　and　the　oxygen　atoms　bined　with　carbon　to　forma　dense　atmosphere　of　the　greenhouse　gas　co2。　venus　became　stifling。　although　people　ofmy　age　will　recall　a　time　when　astronomers　hoped　that　venus　might　harbor　life　beneath　itspadded　clouds；　possibly　even　a　kind　of　tropical　verdure；　we　now　know　that　it　is　much　toofierce　an　environment　for　any　kind　of　life　that　we　can　reasonably　conceive　of。　its　surfacetemperature　is　a　roasting　470　degrees　centigrade　（roughly　900　degrees　fahrenheit）；　which　ishot　enough　to　melt　lead；　and　the　atmospheric　pressure　at　the　surface　is　ninety　times　that　ofearth；　or　more　than　any　human　body　could　withstand。　we　lack　the　technology　to　make　suitsor　even　spaceships　that　would　allow　us　to　visit。　our　knowledge　of　venus’s　surface　is　based　ondistant　radar　imagery　and　some　startled　squawks　from　an　unmanned　soviet　probe　that　wasdropped　hopefully　into　the　clouds　in　1972　and　functioned　for　barely　an　hour　beforepermanently　shutting　down。

so　that’s　what　happens　when　you　move　two　light　minutes　closer　to　the　sun。　travel　fartherout　and　the　problem　bees　not　heat　but　cold；　as　mars　frigidly　attests。　it；　too；　was　once　amuch　more　congenial　place；　but　couldn’t　retain　a　usable　atmosphere　and　turned　into　a　frozenwaste。

but　just　being　the　right　distance　from　the　sun　cannot　be　the　whole　story；　for　otherwise　themoon　would　be　forested　and　fair；　which　patently　it　is　not。　for　that　you　need　to　have：

the　right　kind　of　planet。i　don’t　imagine　even　many　geophysicists；　when　asked　to　counttheir　blessings；　would　include　living　on　a　planet　with　a　molten　interior；　but　it’s　a　pretty　nearcertainty　that　without　all　that　magma　swirling　around　beneath　us　we　wouldn’t　be　here　now。

apart　from　much　else；　our　lively　interior　created　the　outgassing　that　helped　to　build　anatmosphere　and　provided　us　with　the　magnetic　field　that　shields　us　from　cosmic　radiation。　italso　gave　us　plate　tectonics；　which　continually　renews　and　rumples　the　surface。　if　earth　wereperfectly　smooth；　it　would　be　covered　everywhere　with　water　to　a　depth　of　four　kilometers。

there　might　be　life　in　that　lonesome　ocean；　but　there　certainly　wouldn’t　be　baseball。

in　addition　to　having　a　beneficial　interior；　we　also　have　the　right　elements　in　the　correctproportions。　in　the　most　literal　way；　we　are　made　of　the　right　stuff。　this　is　so　crucial　to　ourwell…being　that　we　are　going　to　discuss　it　more　fully　in　a　minute；　but　first　we　need　to　considerthe　two　remaining　factors；　beginning　with　another　one　that　is　often　overlooked：

1the　discovery　of　extremophiles　in　the　boiling　mudpots　of　yellowstone　and　similar　organisms　found　elsewheremade　scientists　realize　that　actually　life　of　a　type　could　range　much　farther　than　that…even；　perhaps；　beneath　the