A Short History of Nearly Everything-第62章

按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！


’s　great　unanswered　questions。

one　of　the　biggest　surprises　in　the　earth　sciences　in　recent　decades　was　the　discovery　ofjust　how　early　in　earth’s　history　life　arose。　well　into　the　1950s；　it　was　thought　that　life　wasless　than　600　million　years　old。　by　the　1970s；　a　few　adventurous　souls　felt　that　maybe　it　wentback　2。5　billion　years。　but　the　present　date　of　3。85　billion　years　is　stunningly　early。　earth’ssurface　didn’t　bee　solid　until　about　3。9　billion　years　ago。

“we　can　only　infer　from　this　rapidity　that　it　is　not　‘difficult’　for　life　of　bacterial　grade　toevolve　on　planets　with　appropriate　conditions；”　stephen　jay　gould　observed　in　the　new　yorktimes　in　1996。　or　as　he　put　it　elsewhere；　it　is　hard　to　avoid　the　conclusion　that　“life；　arising　assoon　as　it　could；　was　chemically　destined　to　be。”

life　emerged　so　swiftly；　in　fact；　that　some　authorities　think　it　must　have　had　help—perhapsa　good　deal　of　help。　the　idea　that　earthly　life　might　have　arrived　from　space　has　a　surprisinglylong　and　even　occasionally　distinguished　history。　the　great　lord　kelvin　himself　raised　thepossibility　as　long　ago　as　1871　at　a　meeting　of　the　british　association　for　the　advancement　ofscience　when　he　suggested　that　“the　germs　of　life　might　have　been　brought　to　the　earth　bysome　meteorite。”　but　it　remained　little　more　than　a　fringe　notion　until　one　sunday　inseptember　1969　when　tens　of　thousands　of　australians　were　startled　by　a　series　of　sonicbooms　and　the　sight　of　a　fireball　streaking　from　east　to　west　across　the　sky。　the　fireball　madea　strange　crackling　sound　as　it　passed　and　left　behind　a　smell　that　some　likened　to　methylatedspirits　and　others　described　as　just　awful。

the　fireball　exploded　above　murchison；　a　town　of　six　hundred　people　in　the　goulburnvalley　north　of　melbourne；　and　came　raining　down　in　chunks；　some　weighing　up　to　twelvepounds。　fortunately；　no　one　was　hurt。　the　meteorite　was　of　a　rare　type　known　as　acarbonaceous　chondrite；　and　the　townspeople　helpfully　collected　and　brought　in　some　twohundred　pounds　of　it。　the　timing　could　hardly　have　been　better。　less　than　two　months　earlier；the　apollo　11　astronauts　had　returned　to　earth　with　a　bag　full　of　lunar　rocks；　so　labsthroughout　the　world　were　geared　up—indeed　clamoring—for　rocks　of　extraterrestrial　origin。

the　murchison　meteorite　was　found　to　be　4。5　billion　years　old；　and　it　was　studded　withamino　acids—seventy…four　types　in　all；　eight　of　which　are　involved　in　the　formation　of　earthlyproteins。　in　late　2001；　more　than　thirty　years　after　it　crashed；　a　team　at　the　ames　researchcenter　in　california　announced　that　the　murchison　rock　also　contained　pletrings　ofsugars　called　polyols；　which　had　not　been　found　off　the　earth　before。

a　few　other　carbonaceous　chondrites　have　strayed　into　earth’s　path　since—one　that　landednear　tagish　lake　in　canada’s　yukon　in　january　2000　was　seen　over　large　parts　of　northamerica—and　they　have　likewise　confirmed　that　the　universe　is　actually　rich　in　organicpounds。　halley’s　et；　it　is　now　thought；　is　about　25　percent　organic　molecules。　getenough　of　those　crashing　into　a　suitable　place—earth；　for　instance—and　you　have　the　basicelements　you　need　for　life。

there　are　two　problems　with　notions　of　panspermia；　as　extraterrestrial　theories　are　known。

the　first　is　that　it　doesn’t　answer　any　questions　about　how　life　arose；　but　merely　movesresponsibility　for　it　elsewhere。　the　other　is　that　panspermia　sometimes　excites　even　the　mostrespectable　adherents　to　levels　of　speculation　that　can　be　safely　called　imprudent。　franciscrick；　codiscoverer　of　the　structure　of　dna；　and　his　colleague　leslie　orgel　have　suggestedthat　earth　was　“deliberately　seeded　with　life　by　intelligent　aliens；”　an　idea　that　gribbin　calls“at　the　very　fringe　of　scientific　respectability”—or；　put　another　way；　a　notion　that　would　beconsidered　wildly　lunatic　if　not　voiced　by　a　nobel　laureate。　fred　hoyle　and　his　colleaguechandra　wickramasinghe　further　eroded　enthusiasm　for　panspermia　by　suggesting　that　outerspace　brought　us　not　only　life　but　also　many　diseases　such　as　flu　and　bubonic　plague；　ideasthat　were　easily　disproved　by　biochemists。　hoyle—and　it　seems　necessary　to　insert　areminder　here　that　he　was　one　of　the　great　scientific　minds　of　the　twentieth　century—alsoonce　suggested；　as　mentioned　earlier；　that　our　noses　evolved　with　the　nostrils　underneath　as　away　of　keeping　cosmic　pathogens　from　falling　into　them　as　they　drifted　down　from　space。

whatever　prompted　life　to　begin；　it　happened　just　once。　that　is　the　most　extraordinary　factin　biology；　perhaps　the　most　extraordinary　fact　we　know。　everything　that　has　ever　lived；　plantor　animal；　dates　its　beginnings　from　the　same　primordial　twitch。　at　some　point　in　anunimaginably　distant　past　some　little　bag　of　chemicals　fidgeted　to　life。　it　absorbed　somenutrients；　gently　pulsed；　had　a　brief　existence。　this　much　may　have　happened　before；　perhapsmany　times。　but　this　ancestral　packet　did　something　additional　and　extraordinary：　it　cleaveditself　and　produced　an　heir。　a　tiny　bundle　of　genetic　material　passed　from　one　living　entity　toanother；　and　has　never　stopped　moving　since。　it　was　the　moment　of　creation　for　us　all。

biologists　sometimes　call　it　the　big　birth。

“wherever　you　go　in　the　world；　whatever　animal；　plant；　bug；　or　blob　you　look　at；　if　it　isalive；　it　will　use　the　same　dictionary　and　know　the　same　code。　all　life　is　one；”　says　mattridley。　we　are　all　the　result　of　a　single　genetic　trick　handed　down　from　generation　togeneration　nearly　four　billion　years；　to　such　an　extent　that　you　can　take　a　fragment　of　humangenetic　instruction；　patch　it　into　a　faulty　yeast　cell；　and　the　yeast　cell　will　put　it　to　work　as　if　itwere　its　own。　in　a　very　real　sense；　it　is　its　own。

the　dawn　of　life—or　something　very　like　it—sits　on　a　shelf　in　the　office　of　a　friendlyisotope　geochemist　named　victoria　bennett　in　the　earth　sciences　building　of　the　australiannational　university　in　canberra。　an　american；　ms。　bennett　came　to　the　anu　fromcalifornia　on　a　two…year　contract　in　1989　and　has　been　there　ever　since。　when　i　visited　her；　inlate　2001；　she　handed　me　a　modestly　hefty　hunk　of　rock　posed　of　thin　alternating　stripesof　white　quartz　and　a　gray…green　material　called　clinopyroxene。　the　rock　came　from　akiliaisland　in　greenland；　where　unusually　ancient　rocks　were　found　in　1997。　the　rocks　are　3。85billion　years　old　and　represent　the　oldest　marine　sediments　ever　found。

“we　can’t　be　certain　that　what　you　are　holding　once　contained　living　organisms　becauseyou’d　have　to　pulverize　it　to　find　out；”　bennett　told　me。　“but　it　es　from　the　same　depositwhere　the　oldest　life　was　excavated；　so　it　probably　had　life　in　it。”　nor　would　you　find　actualfossilized　microbes；　however　carefully　you　searched。　any　simple　organisms；　alas；　would　havebeen　baked　away　by　the　processes　that　turned　ocean　mud　to　stone。　instead　what　we　would　seeif　we　crunched　up　the　rock　and　examined　it　microscopically　would　be　the　chemical　residuesthat　the　organisms　left　behind—carbon　isotopes　and　a　type　of　phosphate　called　apatite；　whichtogether　provide　strong　evidence　that　the　rock　once　contained　colonies　of　living　things。　“wecan　only　guess　what　the　organism　might　have　looked　like；”　bennett　said。　“it　was　probablyabout　as　basic　as　life　can　get—but　it　was　life　nonetheless。　it　lived。　it　propagated。”

and　eventually　it　led　to　us。

if　you　are　into　very　old　rocks；　and　bennett　indubitably　is；　the　anu　has　long　been　a　primeplace　to　be。　this　is　largely　thanks　to　the　ingenuity　of　a　man　named　bill　pston；　who　isnow　retired　but　in　the　1970s　built　the　world’s　first　sensitive　high　resolution　ion　microprobe—or　shrimp；　as　it　is　more　affectionately　known　from　its　initial　letters。　this　is　amachine　that　measures　the　decay　rate　of　uranium　in　tiny　minerals　called　zircons。　zirconsappear　in　most　rocks　apart　from　basalts　and　are　extremely　durable；　surviving　every　naturalprocess　but　subduction。　most　of　the　earth’s　crust　has　been　slipped　back　into　the　oven　at　somepoint；　but　just　occasionally—in　western　australia　and　greenland；　for　example—geologistshave　found　outcrops　of　rocks　that　have　remained　always　at　the　surface。　pston’s　machineallowed　such　rocks　to　be　dated　with　unparalleled　precision。　the　prototype　shrimp　was　builtand　machined　in　the　earth　science　department’s　own　workshops；　and　looked　like　somethingthat　had　been　built　from　spare　parts　on　a　budget；　but　it　worked　great。　on　its　first　formal　test；　in1982；　it　dated　the　oldest　thing　ever　found—a　4。3…billion…year…old　　rock　from　westernaustralia。

“it　caused　quite　a　stir　at　the　time；”　bennett　told　me；　“to　find　something　so　important　soquickly　with　brand…new　technology。”

she　took　me　down　the　hall　to　see　the　current　model；　shrimp　ii。　it　was　a　big　heavy　pieceof　stainless…steel　apparatus；　perhaps　twelve　feet　long　and　five　feet　high；　and　as　solidly　built　asa　deep…sea　probe。　at　a　console　in　front　of　it；　keeping　an　eye　on　ever…changing　strings　offigures　on　a　screen；　was　a　man　named　bob　from　canterbury　university　in　new　zealand。　hehad　been　there　since　4　a。m。；　he　told　me。　shrimp　ii　runs　twenty…four　hours　a　day；　there’s　thatmany　rocks　to　date。　it　was　just　after　9a。m。　and　bob　had　the　machine　till　noon。　ask　a　pair　ofgeochemists　how　something　like　this　works；　and　they　will　st