A Short History of Nearly Everything-第63章

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　had　the　machine　till　noon。　ask　a　pair　ofgeochemists　how　something　like　this　works；　and　they　will　start　talking　about　isotopicabundances　and　ionization　levels　with　an　enthusiasm　that　is　more　endearing　than　fathomable。

the　upshot　of　it；　however；　was　that　the　machine；　by　bombarding　a　sample　of　rock　withstreams　of　charged　atoms；　is　able　to　detect　subtle　differences　in　the　amounts　of　lead　anduranium　in　the　zircon　samples；　by　which　means　the　age　of　rocks　can　be　accurately　adduced。

bob　told　me　that　it　takes　about　seventeen　minutes　to　read　one　zircon　and　it　is　necessary　toread　dozens　from　each　rock　to　make　the　data　reliable。　in　practice；　the　process　seemed　toinvolve　about　the　same　level　of　scattered　activity；　and　about　as　much　stimulation；　as　a　trip　to　alaundromat。　bob　seemed　very　happy；　however；　but　then　people　from　new　zealand　verygenerally　do。

the　earth　sciences　pound　was　an　odd　bination　of　things—part　offices；　part　labs；part　machine　shed。　“we　used　to　build　everything　here；”　bennett　said。　“we　even　had　our　ownglassblower；　but　he’s　retired。　but　we　still　have　two　full…time　rock　crushers。”　she　caught　mylook　of　mild　surprise。　“we　get　through　a　lot　of　rocks。　and　they　have　to　be　very　carefullyprepared。　you　have　to　make　sure　there　is　no　contamination　from　previous　samples—no　dustor　anything。　it’s　quite　a　meticulous　process。”　she　showed　me　the　rock…crushing　machines；which　were　indeed　pristine；　though　the　rock　crushers　had　apparently　gone　for　coffee。　besidethe　machines　were　large　boxes　containing　rocks　of　all　shapes　and　sizes。　they　do　indeed　getthrough　a　lot　of　rocks　at　the　anu。

back　in　bennett’s　office　after　our　tour；　i　noticed　hanging　on　her　wall　a　poster　giving　anartist’s　colorfully　imaginative　interpretation　of　earth　as　it　might　have　looked　3。5　billion　yearsago；　just　when　life　was　getting　going；　in　the　ancient　period　known　to　earth　science　as　thearchaean。　the　poster　showed　an　alien　landscape　of　huge；　very　active　volcanoes；　and　asteamy；　copper…colored　sea　beneath　a　harsh　red　sky。　stromatolites；　a　kind　of　bacterial　rock；filled　the　shallows　in　the　foreground。　it　didn’t　look　like　a　very　promising　place　to　create　andnurture　life。　i　asked　her　if　the　painting　was　accurate。

“well；　one　school　of　thought　says　it　was　actually　cool　then　because　the　sun　was　muchweaker。”　（i　later　learned　that　biologists；　when　they　are　feeling　jocose；　refer　to　this　as　the“chinese　restaurant　problem”—because　we　had　a　dim　sun。）　“without　an　atmosphereultraviolet　rays　from　the　sun；　even　from　a　weak　sun；　would　have　tended　to　break　apart　anyincipient　bonds　made　by　molecules。　and　yet　right　there”—she　tapped　the　stromatolites—“youhave　organisms　almost　at　the　surface。　it’s　a　puzzle。”

“so　we　don’t　know　what　the　world　was　like　back　then？”

“mmmm；”　she　agreed　thoughtfully。

“either　way　it　doesn’t　seem　very　conducive　to　life。”

she　nodded　amiably。　“but　there　must　have　been　something　that　suited　life。　otherwise　wewouldn’t　be　here。”

it　certainly　wouldn’t　have　suited　us。　if　you　were　to　step　from　a　time　machine　into　thatancient　archaean　world；　you　would　very　swiftly　scamper　back　inside；　for　there　was　no　moreoxygen　to　breathe　on　earth　back　then　than　there　is　on　mars　today。　it　was　also　full　of　noxiousvapors　from　hydrochloric　and　sulfuric　acids　powerful　enough　to　eat　through　clothing　andblister　skin。　nor　would　it　have　provided　the　clean　and　glowing　vistas　depicted　in　the　poster　invictoria　bennett’s　office。　the　chemical　stew　that　was　the　atmosphere　then　would　haveallowed　little　sunlight　to　reach　the　earth’s　surface。　what　little　you　could　see　would　beillumined　only　briefly　by　bright　and　frequent　lightning　flashes。　in　short；　it　was　earth；　but　anearth　we　wouldn’t　recognize　as　our　own。

anniversaries　were　few　and　far　between　in　the　archaean　world。　for　two　billion　yearsbacterial　organisms　were　the　only　forms　of　life。　they　lived；　they　reproduced；　they　swarmed；but　they　didn’t　show　any　particular　inclination　to　move　on　to　another；　more　challenging　levelof　existence。　at　some　point　in　the　first　billion　years　of　life；　cyanobacteria；　or　blue…green　algae；learned　to　tap　into　a　freely　available　resource—the　hydrogen　that　exists　in　spectacularabundance　in　water。　they　absorbed　water　molecules；　supped　on　the　hydrogen；　and　releasedthe　oxygen　as　waste；　and　in　so　doing　invented　photosynthesis。　as　margulis　and　sagan　note；photosynthesis　is　“undoubtedly　the　most　important　single　metabolic　innovation　in　the　historyof　life　on　the　planet”—and　it　was　invented　not　by　plants　but　by　bacteria。

as　cyanobacteria　proliferated　the　world　began　to　fill　with　o2to　the　consternation　of　thoseorganisms　that　found　it　poisonous—which　in　those　days　was　all　of　them。　in　an　anaerobic　（or　anon…oxygen…using）　world；　oxygen　is　extremely　poisonous。　our　white　cells　actually　useoxygen　to　kill　invading　bacteria。　that　oxygen　is　fundamentally　toxic　often　es　as　a　surpriseto　those　of　us　who　find　it　so　convivial　to　our　well…being；　but　that　is　only　because　we　haveevolved　to　exploit　it。　to　other　things　it　is　a　terror。　it　is　what　turns　butter　rancid　and　makes　ironrust。　even　we　can　tolerate　it　only　up　to　a　point。　the　oxygen　level　in　our　cells　is　only　about　atenth　the　level　found　in　the　atmosphere。

the　new　oxygen…using　organisms　had　two　advantages。　oxygen　was　a　more　efficient　way　toproduce　energy；　and　it　vanquished　petitor　organisms。　some　retreated　into　the　oozy；anaerobic　world　of　bogs　and　lake　bottoms。　others　did　likewise　but　then　later　（much　later）migrated　to　the　digestive　tracts　of　beings　like　you　and　me。　quite　a　number　of　these　primevalentities　are　alive　inside　your　body　right　now；　helping　to　digest　your　food；　but　abhorring　eventhe　tiniest　hint　of　o2。　untold　numbers　of　others　failed　to　adapt　and　died。

the　cyanobacteria　were　a　runaway　success。　at　first；　the　extra　oxygen　they　produced　didn’taccumulate　in　the　atmosphere；　but　bined　with　iron　to　form　ferric　oxides；　which　sank　to　thebottom　of　primitive　seas。　for　millions　of　years；　the　world　literally　rusted—a　phenomenonvividly　recorded　in　the　banded　iron　deposits　that　provide　so　much　of　the　world’s　iron　oretoday。　for　many　tens　of　millions　of　years　not　a　great　deal　more　than　this　happened。　if　youwent　back　to　that　early　proterozoic　world　you　wouldn’t　find　many　signs　of　promise　forearth’s　future　life。　perhaps　here　and　there　in　sheltered　pools　you’d　encounter　a　film　of　livingscum　or　a　coating　of　glossy　greens　and　browns　on　shoreline　rocks；　but　otherwise　life　remainedinvisible。

but　about　3。5　billion　years　ago　something　more　emphatic　became　apparent。　wherever　theseas　were　shallow；　visible　structures　began　to　appear。　as　they　went　through　their　chemicalroutines；　the　cyanobacteria　became　very　slightly　tacky；　and　that　tackiness　trappedmicroparticles　of　dust　and　sand；　which　became　bound　together　to　form　slightly　weird　but　solidstructures—the　stromatolites　that　were　featured　in　the　shallows　of　the　poster　on　victoriabennett’s　office　wall。　stromatolites　came　in　various　shapes　and　sizes。　sometimes　they　lookedlike　enormous　cauliflowers；　sometimes　like　fluffy　mattresses　（stromatolite　es　from　thegreek　for　“mattress”）；　sometimes　they　came　in　the　form　of　columns；　rising　tens　of　metersabove　the　surface　of　the　water—sometimes　as　high　as　a　hundred　meters。　in　all　theirmanifestations；　they　were　a　kind　of　living　rock；　and　they　represented　the　world’s　firstcooperative　venture；　with　some　varieties　of　primitive　organism　living　just　at　the　surface　andothers　living　just　underneath；　each　taking　advantage　of　conditions　created　by　the　other。　theworld　had　its　first　ecosystem。

for　many　years；　scientists　knew　about　stromatolites　from　fossil　formations；　but　in　1961they　got　a　real　surprise　with　the　discovery　of　a　munity　of　living　stromatolites　at　sharkbay　on　the　remote　northwest　coast　of　australia。　this　was　most　unexpected—so　unexpected；in　fact；　that　it　was　some　years　before　scientists　realized　quite　what　they　had　found。　today；however；　shark　bay　is　a　tourist　attraction—or　at　least　as　much　of　a　tourist　attraction　as　a　placehundreds　of　miles　from　anywhere　much　and　dozens　of　miles　from　anywhere　at　all　can　ever　be。

boardwalks　have　been　built　out　into　the　bay　so　that　visitors　can　stroll　over　the　water　to　get　agood　look　at　the　stromatolites；　quietly　respiring　just　beneath　the　surface。　they　are　lusterlessand　gray　and　look；　as　i　recorded　in　an　earlier　book；　like　very　large　cow…pats。　but　it　is　acuriously　giddying　moment　to　find　yourself　staring　at　living　remnants　of　earth　as　it　was　3。5billion　years　ago。　as　richard　fortey　has　put　it：　“this　is　truly　time　traveling；　and　if　the　worldwere　attuned　to　its　real　wonders　this　sight　would　be　as　well…known　as　the　pyramids　of　giza。”

although　you’d　never　guess　it；　these　dull　rocks　swarm　with　life；　with　an　estimated　（well；obviously　estimated）　three　billion　individual　organisms　on　every　square　yard　of　rock。

sometimes　when　you　look　carefully　you　can　see　tiny　strings　of　bubbles　rising　to　the　surface　asthey　give　up　their　oxygen。　in　two　billion　years　such　tiny　exertions　raised　the　level　of　oxygenin　earth’s　atmosphere　to　20　percent；　preparing　the　way　for　the　next；　more　plex　chapter　inlife’s　history。

it　has　been　suggested　that　the　cyanobacteria　at　shark　bay　are　perhaps　the　slowest…evolvingorganisms　on　earth；　and　cert