A Short History of Nearly Everything-第61章

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he　problem　is　proteins。

proteins　are　what　you　get　when　you　string　amino　acids　together；　and　we　need　a　lot　of　them。

no　one　really　knows；　but　there　may　be　as　many　as　a　million　types　of　protein　in　the　humanbody；　and　each　one　is　a　little　miracle。　by　all　the　laws　of　probability　proteins　shouldn’t　exist。

to　make　a　protein　you　need　to　assemble　amino　acids　（which　i　am　obliged　by　long　tradition　torefer　to　here　as　“the　building　blocks　of　life”）　in　a　particular　order；　in　much　the　same　way　thatyou　assemble　letters　in　a　particular　order　to　spell　a　word。　the　problem　is　that　words　in　theamino　acid　alphabet　are　often　exceedingly　long。　to　spell　collagen；　the　name　of　a　montype　of　protein；　you　need　to　arrange　eight　letters　in　the　right　order。　but　to　make　collagen；　youneed　to　arrange　1；055　amino　acids　in　precisely　the　right　sequence。　but—and　here’s　anobvious　but　crucial　point—you　don’t　make　it。　it　makes　itself；　spontaneously；　withoutdirection；　and　this　is　where　the　unlikelihoods　e　in。

the　chances　of　a　1；055…sequence　molecule　like　collagen　spontaneously　self…assembling　are；frankly；　nil。　it　just　isn’t　going　to　happen。　to　grasp　what　a　long　shot　its　existence　is；　visualize　astandard　las　vegas　slot　machine　but　broadened　greatly—to　about　ninety　feet；　to　be　precise—to　acmodate　1；055　spinning　wheels　instead　of　the　usual　three　or　four；　and　with　twentysymbols　on　each　wheel　（one　for　each　mon　amino　acid）。

1how　long　would　you　have　topull　the　handle　before　all　1；055　symbols　came　up　in　the　right　order？　effectively　forever。　evenif　you　reduced　the　number　of　spinning　wheels　to　two　hundred；　which　is　actually　a　moretypical　number　of　amino　acids　for　a　protein；　the　odds　against　all　two　hundred　ing　up　in　a1there　are　actually　twenty…two　naturally　occurring　amino　acids　known　on　earth；　and　more　may　await　discovery；but　only　twenty　of　them　are　necessary　to　produce　us　and　most　other　living　things。　the　twenty…second；　calledpyrrolysine；　was　discovered　in　2002　by　researchers　at　ohio　state　university　and　is　found　only　in　a　single　type　ofarchaean　（a　basic　form　of　life　that　we　will　discuss　a　little　further　on　in　the　story）　called　methanosarcina　barkeri。

prescribed　sequence　are　1　in　10260（that　is　a　1　followed　by　260　zeroes）。　that　in　itself　is　a　largernumber　than　all　the　atoms　in　the　universe。

proteins；　in　short；　are　plex　entities。　hemoglobin　is　only　146　amino　acids　long；　a　runt　byprotein　standards；　yet　even　it　offers　10190possible　amino　acid　binations；　which　is　why　ittook　the　cambridge　university　chemist　max　perutz　twenty…three　years—a　career；　more　orless—to　unravel　it。　for　random　events　to　produce　even　a　single　protein　would　seem　astunning　improbability—like　a　whirlwind　spinning　through　a　junkyard　and　leaving　behind　afully　assembled　jumbo　jet；　in　the　colorful　simile　of　the　astronomer　fred　hoyle。

yet　we　are　talking　about　several　hundred　thousand　types　of　protein；　perhaps　a　million；　eachunique　and　each；　as　far　as　we　know；　vital　to　the　maintenance　of　a　sound　and　happy　you。　andit　goes　on　from　there。　a　protein　to　be　of　use　must　not　only　assemble　amino　acids　in　the　rightsequence；　but　then　must　engage　in　a　kind　of　chemical　origami　and　fold　itself　into　a　veryspecific　shape。　even　having　achieved　this　structural　plexity；　a　protein　is　no　good　to　you　ifit　can’t　reproduce　itself；　and　proteins　can’t。　for　this　you　need　dna。　dna　is　a　whiz　atreplicating—it　can　make　a　copy　of　itself　in　seconds—but　can　do　virtually　nothing　else。　so　wehave　a　paradoxical　situation。　proteins　can’t　exist　without　dna；　and　dna　has　no　purposewithout　proteins。　are　we　to　assume　then　that　they　arose　simultaneously　with　the　purpose　ofsupporting　each　other？　if　so：　wow。

and　there　is　more　still。　dna；　proteins；　and　the　other　ponents　of　life　couldn’t　prosperwithout　some　sort　of　membrane　to　contain　them。　no　atom　or　molecule　has　ever　achieved　lifeindependently。　pluck　any　atom　from　your　body；　and　it　is　no　more　alive　than　is　a　grain　of　sand。

it　is　only　when　they　e　together　within　the　nurturing　refuge　of　a　cell　that　these　diversematerials　can　take　part　in　the　amazing　dance　that　we　call　life。　without　the　cell；　they　arenothing　more　than　interesting　chemicals。　but　without　the　chemicals；　the　cell　has　no　purpose。

as　the　physicist　paul　davies　puts　it；　“if　everything　needs　everything　else；　how　did　themunity　of　molecules　ever　arise　in　the　first　place？”　it　is　rather　as　if　all　the　ingredients　inyour　kitchen　somehow　got　together　and　baked　themselves　into　a　cake—but　a　cake　that　couldmoreover　divide　when　necessary　to　produce　more　cakes。　it　is　little　wonder　that　we　call　it　themiracle　of　life。　it　is　also　little　wonder　that　we　have　barely　begun　to　understand　it。

so　what　accounts　for　all　this　wondrous　plexity？　well；　one　possibility　is　that　perhaps　itisn’t　quite—not　quite—so　wondrous　as　at　first　it　seems。　take　those　amazingly　improbableproteins。　the　wonder　we　see　in　their　assembly　es　in　assuming　that　they　arrived　on　thescene　fully　formed。　but　what　if　the　protein　chains　didn’t　assemble　all　at　once？　what　if；　in　thegreat　slot　machine　of　creation；　some　of　the　wheels　could　be　held；　as　a　gambler　might　hold　anumber　of　promising　cherries？　what　if；　in　other　words；　proteins　didn’t　suddenly　burst　intobeing；　but　evolved　。

imagine　if　you　took　all　the　ponents　that　make　up　a　human　being—carbon；　hydrogen；oxygen；　and　so　on—and　put　them　in　a　container　with　some　water；　gave　it　a　vigorous　stir；　andout　stepped　a　pleted　person。　that　would　be　amazing。　well；　that’s　essentially　what　hoyleand　others　（including　many　ardent　creationists）　argue　when　they　suggest　that　proteinsspontaneously　formed　all　at　once。　they　didn’t—they　can’t　have。　as　richard　dawkins　arguesin　the　blind　watchmaker；　there　must　have　been　some　kind　of　cumulative　selection　processthat　allowed　amino　acids　to　assemble　in　chunks。　perhaps　two　or　three　amino　acids　linked　upfor　some　simple　purpose　and　then　after　a　time　bumped　into　some　other　similar　small　clusterand　in　so　doing　“discovered”　some　additional　improvement。

chemical　　reactions　　of　　the　　sort　　associated　with　life　are　actually　something　of　amonplace。　it　may　be　beyond　us　to　cook　them　up　in　a　lab；　à　la　stanley　miller　and　haroldurey；　but　the　universe　does　it　readily　enough。　lots　of　molecules　in　nature　get　together　to　formlong　chains　called　polymers。　sugars　constantly　assemble　to　form　starches。　crystals　can　do　anumber　of　lifelike　things—replicate；　respond　to　environmental　stimuli；　take　on　a　patternedplexity。　they’ve　never　achieved　life　itself；　of　course；　but　they　demonstrate　repeatedly　thatplexity　is　a　natural；　spontaneous；　entirely　monplace　event。　there　may　or　may　not　be　agreat　deal　of　life　in　the　universe　at　large；　but　there　is　no　shortage　of　ordered　self…assembly；　ineverything　from　the　transfixing　symmetry　of　snowflakes　to　the　ely　rings　of　saturn。

so　powerful　is　this　natural　impulse　to　assemble　that　many　scientists　now　believe　that　lifemay　be　more　inevitable　than　we　think—that　it　is；　in　the　words　of　the　belgian　biochemist　andnobel　laureate　christian　de　duve；　“an　obligatory　manifestation　of　matter；　bound　to　arisewherever　conditions　are　appropriate。”　de　duve　thought　it　likely　that　such　conditions　would　beencountered　perhaps　a　million　times　in　every　galaxy。

certainly　there　is　nothing　terribly　exotic　in　the　chemicals　that　animate　us。　if　you　wished　tocreate　another　living　object；　whether　a　goldfish　or　a　head　of　lettuce　or　a　human　being；　youwould　need　really　only　four　principal　elements；　carbon；　hydrogen；　oxygen；　and　nitrogen；　plussmall　amounts　of　a　few　others；　principally　sulfur；　phosphorus；　calcium；　and　iron。　put　thesetogether　in　three　dozen　or　so　binations　to　form　some　sugars；　acids；　and　other　basicpounds　and　you　can　build　anything　that　lives。　as　dawkins　notes：　“there　is　nothingspecial　about　the　substances　from　which　living　things　are　made。　living　things　are　collectionsof　molecules；　like　everything　else。”

the　bottom　line　is　that　life　is　amazing　and　gratifying；　perhaps　even　miraculous；　but　hardlyimpossible—as　we　repeatedly　attest　with　our　own　modest　existences。　to　be　sure；　many　of　thedetails　of　life’s　beginnings　remain　pretty　imponderable。　every　scenario　you　have　ever　readconcerning　the　conditions　necessary　for　life　involves　water—from　the　“warm　little　pond”

where　darwin　supposed　life　began　to　the　bubbling　sea　vents　that　are　now　the　most　popularcandidates　for　life’s　beginnings—but　all　this　overlooks　the　fact　that　to　turn　monomers　intopolymers　（which　is　to　say；　to　begin　to　create　proteins）　involves　what　is　known　to　biology　as“dehydration　linkages。”　as　one　leading　biology　text　puts　it；　with　perhaps　just　a　tiny　hint　ofdisfort；　“researchers　agree　that　such　reactions　would　not　have　been　energeticallyfavorable　in　the　primitive　sea；　or　indeed　in　any　aqueous　medium；　because　of　the　mass　actionlaw。”　it　is　a　little　like　putting　sugar　in　a　glass　of　water　and　having　it　bee　a　cube。　itshouldn’t　happen；　but　somehow　in　nature　it　does。　the　actual　chemistry　of　all　this　is　a　littlearcane　for　our　purposes　here；　but　it　is　enough　to　know　that　if　you　make　monomers　wet　theydon’t　turn　into　polymers—except　when　creating　life　on　earth。　how　and　why　it　happens　thenand　not　otherwise　is　one　of　biology’s　great　unanswered　questions。

one　of　the　biggest　surprises　in　the　earth　sciences　in　recent　deca