A Short History of Nearly Everything-第87章

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and　these；　you　may　recall；　were　men　who　thought　science　was　nearly　at　an　end。

。d　xs　



26THE　STUFF　OF　LIFE

小？说网
if　your　two　parents　hadn’t　bonded　just　when　they　did—possibly　to　the　second；　possiblyto　the　nanosecond—you　wouldn’t　be　here。　and　if　their　parents　hadn’t　bonded　in　a　preciselytimely　manner；　you　wouldn’t　be　here　either。　and　if　their　parents　hadn’t　done　likewise；　andtheir　parents　before　them；　and　so　on；　obviously　and　indefinitely；　you　wouldn’t　be　here。

push　backwards　through　time　and　these　ancestral　debts　begin　to　add　up。　go　back　just　eightgenerations　to　about　the　time　that　charles　darwin　and　abraham　lincoln　were　born；　andalready　there　are　over　250　people　on　whose　timely　couplings　your　existence　depends。

continue　further；　to　the　time　of　shakespeare　and　the　mayflower　pilgrims；　and　you　have　nofewer　than　16；384　ancestors　earnestly　exchanging　genetic　material　in　a　way　that　would；eventually　and　miraculously；　result　in　you。

at　twenty　generations　ago；　the　number　of　people　procreating　on　your　behalf　has　risen　to1；048；576。　five　generations　before　that；　and　there　are　no　fewer　than　33；554；432　men　andwomen　on　whose　devoted　couplings　your　existence　depends。　by　thirty　generations　ago；　yourtotal　number　of　forebears—remember；　these　aren’t　cousins　and　aunts　and　other　incidentalrelatives；　but　only　parents　and　parents　of　parents　in　a　line　leading　ineluctably　to　you—is　overone　billion　（1；073；741；824；　to　be　precise）。　if　you　go　back　sixty…four　generations；　to　the　time　ofthe　romans；　the　number　of　people　on　whose　cooperative　efforts　your　eventual　existencedepends　has　risen　to　approximately　1；000；000；000；000；000；000；　which　is　several　thousandtimes　the　total　number　of　people　who　have　ever　lived。

clearly　something　has　gone　wrong　with　our　math　here。　the　answer；　it　may　interest　you　tolearn；　is　that　your　line　is　not　pure。　you　couldn’t　be　here　without　a　little　incest—actually　quitea　lot　of　incest—albeit　at　a　genetically　discreet　remove。　with　so　many　millions　of　ancestors　inyour　background；　there　will　have　been　many　occasions　when　a　relative　from　your　mother’sside　of　the　family　procreated　with　some　distant　cousin　from　your　father’s　side　of　the　ledger。　infact；　if　you　are　in　a　partnership　now　with　someone　from　your　own　race　and　country；　thechances　are　excellent　that　you　are　at　some　level　related。　indeed；　if　you　look　around　you　on　abus　or　in　a　park　or　café　or　any　crowded　place；　most　of　the　people　you　see　are　very　probablyrelatives。　when　someone　boasts　to　you　that　he　is　descended　from　william　the　conqueror　orthe　mayflower　pilgrims；　you　should　answer　at　once：　“me；　too！”　in　the　most　literal　andfundamental　sense　we　are　all　family。

we　are　also　uncannily　alike。　pare　your　genes　with　any　other　human　being’s　and　onaverage　they　will　be　about　99。9　percent　the　same。　that　is　what　makes　us　a　species。　the　tinydifferences　in　that　remaining　0。1　percent—“roughly　one　nucleotide　base　in　every　thousand；”

to　quote　the　british　geneticist　and　recent　nobel　laureate　john　sulston—are　what　endow　uswith　our　individuality。　much　has　been　made　in　recent　years　of　the　unraveling　of　the　humangenome。　in　fact；　there　is　no　such　thing　as　“the”　human　genome。　every　human　genome　isdifferent。　otherwise　we　would　all　be　identical。　it　is　the　endless　rebinations　of　ourgenomes—each　nearly　identical；　but　not　quite—that　make　us　what　we　are；　both　as　individualsand　as　a　species。

but　what　exactly　is　this　thing　we　call　the　genome？　and　what；　e　to　that；　are　genes？

well；　start　with　a　cell　again。　inside　the　cell　is　a　nucleus；　and　inside　each　nucleus　are　thechromosomes—forty…six　little　bundles　of　plexity；　of　which　twenty…three　e　from　yourmother　and　twenty…three　from　your　father。　with　a　very　few　exceptions；　every　cell　in　yourbody—99。999　percent　of　them；　say—carries　the　same　plement　of　chromosomes。　（theexceptions　are　red　blood　cells；　some　immune　system　cells；　and　egg　and　sperm　cells；　which　forvarious　organizational　reasons　don’t　carry　the　full　genetic　package。）　chromosomes　constitutethe　plete　set　of　instructions　necessary　to　make　and　maintain　you　and　are　made　of　longstrands　of　the　little　wonder　chemical　called　deoxyribonucleic　acid　or　dna—“the　mostextraordinary　molecule　on　earth；”　as　it　has　been　called。

dna　exists　for　just　one　reason—to　create　more　dna—and　you　have　a　lot　of　it　inside　you：

about　six　feet　of　it　squeezed　into　almost　every　cell。　each　length　of　dna　prises　some　3。2billion　letters　of　coding；　enough　to　provide　103；480；000；000possible　binations；　“guaranteed　tobe　unique　against　all　conceivable　odds；”　in　the　words　of　christian　de　duve。　that’s　a　lot　ofpossibility—a　one　followed　by　more　than　three　billion　zeroes。　“it　would　take　more　than　fivethousand　average…size　books　just　to　print　that　figure；”　notes　de　duve。　look　at　yourself　in　themirror　and　reflect　upon　the　fact　that　you　are　beholding　ten　thousand　trillion　cells；　and　thatalmost　every　one　of　them　holds　two　yards　of　densely　pacted　dna；　and　you　begin　toappreciate　just　how　much　of　this　stuff　you　carry　around　with　you。　if　all　your　dna　werewoven　into　a　single　fine　strand；　there　would　be　enough　of　it　to　stretch　from　the　earth　to　themoon　and　back　not　once　or　twice　but　again　and　again。　altogether；　according　to　onecalculation；　you　may　have　as　much　as　twenty　million　kilometers　of　dna　bundled　up　insideyou。

your　body；　in　short；　loves　to　make　dna　and　without　it　you　couldn’t　live。　yet　dna　is　notitself　alive。　no　molecule　is；　but　dna　is；　as　it　were；　especially　unalive。　it　is　“among　the　mostnonreactive；　chemically　inert　molecules　in　the　living　world；”　in　the　words　of　the　geneticistrichard　lewontin。　that　is　why　it　can　be　recovered　from　patches　of　long…dried　blood　or　semenin　murder　investigations　and　coaxed　from　the　bones　of　ancient　neandertals。　it　also　explainswhy　it　took　scientists　so　long　to　work　out　how　a　substance　so　mystifyingly　low　key—so；　in　aword；　lifeless—could　be　at　the　very　heart　of　life　itself。

as　a　known　entity；　dna　has　been　around　longer　than　you　might　think。　it　was　discoveredas　far　back　as　1869　by　johann　friedrich　miescher；　a　swiss　scientist　working　at　the　universityof　tübingen　in　germany。　while　delving　microscopically　through　the　pus　in　surgicalbandages；　miescher　found　a　substance　he　didn’t　recognize　and　called　it　nuclein　（because　itresided　in　the　nuclei　of　cells）。　at　the　time；　miescher　did　little　more　than　note　its　existence；　butnuclein　clearly　remained　on　his　mind；　for　twenty…three　years　later　in　a　letter　to　his　uncle　heraised　the　possibility　that　such　molecules　could　be　the　agents　behind　heredity。　this　was　anextraordinary　insight；　but　one　so　far　in　advance　of　the　day’s　scientific　requirements　that　itattracted　no　attention　at　all。

for　most　of　the　next　half　century　the　mon　assumption　was　that　the　material—nowcalled　deoxyribonucleic　acid；　or　dna—had　at　most　a　subsidiary　role　in　matters　of　heredity。　itwas　too　simple。　it　had　just　four　basic　ponents；　called　nucleotides；　which　was　like　havingan　alphabet　of　just　four　letters。　how　could　you　possibly　write　the　story　of　life　with　such　arudimentary　alphabet？　（the　answer　is　that　you　do　it　in　much　the　way　that　you　create　plexmessages　with　the　simple　dots　and　dashes　of　morse　code—by　bining　them。）　dna　didn’tdo　anything　at　all；　as　far　as　anyone　could　tell。　it　just　sat　there　in　the　nucleus；　possibly　bindingthe　chromosome　in　some　way　or　adding　a　splash　of　acidity　on　mand　or　fulfilling　someother　trivial　task　that　no　one　had　yet　thought　of。　the　necessary　plexity；　it　was　thought；had　to　exist　in　proteins　in　the　nucleus。

there　were；　however；　two　problems　with　dismissing　dna。　first；　there　was　so　much　of　it：

two　yards　in　nearly　every　nucleus；　so　clearly　the　cells　esteemed　it　in　some　important　way。　ontop　of　this；　it　kept　turning　up；　like　the　suspect　in　a　murder　mystery；　in　experiments。　in　twostudies　in　particular；　one　involving　the　pneumonococcus　bacterium　and　another　involvingbacteriophages　（viruses　that　infect　bacteria）；　dna　betrayed　an　importance　that　could　only　beexplained　if　its　role　were　more　central　than　prevailing　thought　allowed。　the　evidencesuggested　that　dna　was　somehow　involved　in　the　making　of　proteins；　a　process　vital　to　life；yet　it　was　also　clear　that　proteins　were　being　made　outside　the　nucleus；　well　away　from　thedna　that　was　supposedly　directing　their　assembly。

no　one　could　understand　how　dna　could　possibly　be　getting　messages　to　the　proteins。　theanswer；　we　now　know；　was　rna；　or　ribonucleic　acid；　which　acts　as　an　interpreter　betweenthe　two。　it　is　a　notable　oddity　of　biology　that　dna　and　proteins　don’t　speak　the　samelanguage。　for　almost　four　billion　years　they　have　been　the　living　world’s　great　double　act；　andyet　they　answer　to　mutually　inpatible　codes；　as　if　one　spoke　spanish　and　the　other　hindi。

to　municate　they　need　a　mediator　in　the　form　of　rna。　working　with　a　kind　of　chemicalclerk　called　a　ribosome；　rna　translates　information　from　a　cell’s　dna　into　terms　proteinscan　understand　and　act　upon。

however；　by　the　early　1900s；　where　we　resume　our　story；　we　were　still　a　very　long　wayfrom　understanding　that；　or　indeed　almost　anything　else　to　do　with　the　confused　business　ofheredity。

clearly　there　was　a　need　for　some　inspired　and　clever　experimentation；　and　happily　the　ageproduced　a　young　person　with　t