A Short History of Nearly Everything- 第64部分

it　has　been　suggested　that　the　cyanobacteria　at　shark　bay　are　perhaps　the　slowest…evolvingorganisms　on　earth；　and　certainly　now　they　are　among　the　rarest。　having　prepared　the　way　formore　plex　life　forms；　they　were　then　grazed　out　of　existence　nearly　everywhere　by　thevery　organisms　whose　existence　they　had　made　possible。　（they　exist　at　shark　bay　becausethe　waters　are　too　saline　for　the　creatures　that　would　normally　feast　on　them。）one　reason　life　took　so　long　to　grow　plex　was　that　the　world　had　to　wait　until　thesimpler　organisms　had　oxygenated　the　atmosphere　sufficiently。　“animals　could　not　summonup　the　energy　to　work；”　as　fortey　has　put　it。　it　took　about　two　billion　years；　roughly　40percent　of　earth’s　history；　for　oxygen　levels　to　reach　more　or　less　modern　levels　ofconcentration　in　the　atmosphere。　but　once　the　stage　was　set；　and　apparently　quite　suddenly；　anentirely　new　type　of　cell　arose—one　with　a　nucleus　and　other　little　bodies　collectively　calledorganelles　（from　a　greek　word　meaning　“little　tools”）。　the　process　is　thought　to　have　startedwhen　some　blundering　or　adventuresome　bacterium　either　invaded　or　was　captured　by　someother　bacterium　and　it　turned　out　that　this　suited　them　both。　the　captive　bacterium　became；　itis　thought；　a　mitochondrion。　this　mitochondrial　invasion　（or　endosymbiotic　event；　asbiologists　like　to　term　it）　made　plex　life　possible。　（in　plants　a　similar　invasion　producedchloroplasts；　which　enable　plants　to　photosynthesize。）mitochondria　manipulate　oxygen　in　a　way　that　liberates　energy　from　foodstuffs。　withoutthis　niftily　facilitating　trick；　life　on　earth　today　would　be　nothing　more　than　a　sludge　ofsimple　microbes。　mitochondria　are　very　tiny—you　could　pack　a　billion　into　the　spaceoccupied　by　a　grain　of　sand—but　also　very　hungry。　almost　every　nutriment　you　absorb　goesto　feeding　them。

we　couldn’t　live　for　two　minutes　without　them；　yet　even　after　a　billion　years　mitochondriabehave　as　if　they　think　things　might　not　work　out　between　us。　they　maintain　their　own　dna。

they　reproduce　at　a　different　time　from　their　host　cell。　they　look　like　bacteria；　divide　likebacteria；　and　sometimes　respond　to　antibiotics　in　the　way　bacteria　do。　in　short；　they　keep　theirbags　packed。　they　don’t　even　speak　the　same　genetic　language　as　the　cell　in　which　they　live。

it　is　like　having　a　stranger　in　your　house；　but　one　who　has　been　there　for　a　billion　years。

the　new　type　of　cell　is　known　as　a　eukaryote　（meaning　“truly　nucleated”）；　as　contrastedwith　the　old　type；　which　is　known　as　a　prokaryote　（“prenucleated”）；　and　it　seems　to　havearrived　suddenly　in　the　fossil　record。　the　oldest　eukaryotes　yet　known；　called　grypania；　werediscovered　in　iron　sediments　in　michigan　in　1992。　such　fossils　have　been　found　just　once；　andthen　no　more　are　known　for　500　million　years。

pared　with　the　new　eukaryotes　the　old　prokaryotes　were　little　more　than　“bags　ofchemicals；”　in　the　words　of　the　geologist　stephen　drury。　eukaryotes　were　bigger—eventuallyas　much　as　ten　thousand　times　bigger—than　their　simpler　cousins；　and　carried　as　much　as　athousand　times　more　dna。　gradually　a　system　evolved　in　which　life　was　dominated　by　twotypes　of　form—organisms　that　expel　oxygen　（like　plants）　and　those　that　take　it　in　（you　andme）。

single…celled　eukaryotes　were　once　called　protozoa　（“pre…animals”）；　but　that　term　isincreasingly　disdained。　today　the　mon　term　for　them　is　protists　。　pared　with　thebacteria　that　had　gone　before；　these　new　protists　were　wonders　of　design　and　sophistication。

the　simple　amoeba；　just　one　cell　big　and　without　any　ambitions　but　to　exist；　contains　400million　bits　of　genetic　information　in　its　dna—enough；　as　carl　sagan　noted；　to　fill　eightybooks　of　five　hundred　pages。

eventually　the　eukaryotes　learned　an　even　more　singular　trick。　it　took　a　long　time—abillion　years　or　so—but　it　was　a　good　one　when　they　mastered　it。　they　learned　to　formtogether　into　plex　multicellular　beings。　thanks　to　this　innovation；　big；　plicated；visible　entities　like　us　were　possible。　planet　earth　was　ready　to　move　on　to　its　next　ambitiousphase。

but　before　we　get　too　excited　about　that；　it　is　worth　remembering　that　the　world；　as　we　areabout　to　see；　still　belongs　to　the　very　small。

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20SMALL　WORLD


it’s　probably　not　a　good　idea　to　take　too　personal　an　interest　in　your　microbes。　louispasteur；　the　great　french　chemist　and　bacteriologist；　became　so　preoccupied　with　them　that　hetook　to　peering　critically　at　every　dish　placed　before　him　with　a　magnifying　glass；　a　habit　thatpresumably　did　not　win　him　many　repeat　invitations　to　dinner。

in　fact；　there　is　no　point　in　trying　to　hide　from　your　bacteria；　for　they　are　on　and　around　youalways；　in　numbers　you　can’t　conceive。　if　you　are　in　good　health　and　averagely　diligent　abouthygiene；　you　will　have　a　herd　of　about　one　trillion　bacteria　grazing　on　your　fleshy　plains—about　a　hundred　thousand　of　them　on　every　square　centimeter　of　skin。　they　are　there　to　dineoff　the　ten　billion　or　so　flakes　of　skin　you　shed　every　day；　plus　all　the　tasty　oils　and　fortifyingminerals　that　seep　out　from　every　pore　and　fissure。　you　are　for　them　the　ultimate　food　court；with　the　convenience　of　warmth　and　constant　mobility　thrown　in。　by　way　of　thanks；　they　giveyou　b。o。

and　those　are　just　the　bacteria　that　inhabit　your　skin。　there　are　trillions　more　tucked　awayin　your　gut　and　nasal　passages；　clinging　to　your　hair　and　eyelashes；　swimming　over　thesurface　of　your　eyes；　drilling　through　the　enamel　of　your　teeth。　your　digestive　system　alone　ishost　to　more　than　a　hundred　trillion　microbes；　of　at　least　four　hundred　types。　some　deal　withsugars；　some　with　starches；　some　attack　other　bacteria。　a　surprising　number；　like　theubiquitous　intestinal　spirochetes；　have　no　detectable　function　at　all。　they　just　seem　to　like　tobe　with　you。　every　human　body　consists　of　about　10　quadrillion　cells；　but　about　100quadrillion　bacterial　cells。　they　are；　in　short；　a　big　part　of　us。　from　the　bacteria’s　point　ofview；　of　course；　we　are　a　rather　small　part　of　them。

because　we　humans　are　big　and　clever　enough　to　produce　and　utilize　antibiotics　anddisinfectants；　it　is　easy　to　convince　ourselves　that　we　have　banished　bacteria　to　the　fringes　ofexistence。　don’t　you　believe　it。　bacteria　may　not　build　cities　or　have　interesting　social　lives；but　they　will　be　here　when　the　sun　explodes。　this　is　their　planet；　and　we　are　on　it　onlybecause　they　allow　us　to　be。

bacteria；　never　forget；　got　along　for　billions　of　years　without　us。　we　couldn’t　survive　a　daywithout　them。　they　process　our　wastes　and　make　them　usable　again；　without　their　diligentmunching　nothing　would　rot。　they　purify　our　water　and　keep　our　soils　productive。　bacteriasynthesize　vitamins　in　our　gut；　convert　the　things　we　eat　into　useful　sugars　andpolysaccharides；　and　go　to　war　on　alien　microbes　that　slip　down　our　gullet。

we　depend　totally　on　bacteria　to　pluck　nitrogen　from　the　air　and　convert　it　into　usefulnucleotides　and　amino　acids　for　us。　it　is　a　prodigious　and　gratifying　feat。　as　margulis　andsagan　note；　to　do　the　same　thing　industrially　（as　when　making　fertilizers）　manufacturers　mustheat　the　source　materials　to　500　degrees　centigrade　and　squeeze　them　to　three　hundred　timesnormal　pressures。　bacteria　do　it　all　the　time　without　fuss；　and　thank　goodness；　for　no　largerorganism　could　survive　without　the　nitrogen　they　pass　on。　above　all；　microbes　continue　toprovide　us　with　the　air　we　breathe　and　to　keep　the　atmosphere　stable。　microbes；　including　themodern　versions　of　cyanobacteria；　supply　the　greater　part　of　the　planet’s　breathable　oxygen。

algae　and　other　tiny　organisms　bubbling　away　in　the　sea　blow　out　about　150　billion　kilos　ofthe　stuff　every　year。

and　they　are　amazingly　prolific。　the　more　frantic　among　them　can　yield　a　new　generationin　less　than　ten　minutes；　clostridium　perfringens；　the　disagreeable　little　organism　that　causesgangrene；　can　reproduce　in　nine　minutes。　at　such　a　rate；　a　single　bacterium　could　theoreticallyproduce　more　offspring　in　two　days　than　there　are　protons　in　the　universe。　“given　an　adequatesupply　of　nutrients；　a　single　bacterial　cell　can　generate　280；000　billion　individuals　in　a　singleday；”　according　to　the　belgian　biochemist　and　nobel　laureate　christian　de　duve。　in　the　sameperiod；　a　human　cell　can　just　about　manage　a　single　division。

about　once　every　million　divisions；　they　produce　a　mutant。　usually　this　is　bad　luck　for　themutant—change　is　always　risky　for　an　organism—but　just　occasionally　the　new　bacterium　isendowed　with　some　accidental　advantage；　such　as　the　ability　to　elude　or　shrug　off　an　attack　ofantibiotics。　with　this　ability　to　evolve　rapidly　goes　another；　even　scarier　advantage。　bacteriashare　information。　any　bacterium　can　take　pieces　of　genetic　coding　from　any　other。

essentially；　as　margulis　and　sagan　put　it；　all　bacteria　swim　in　a　single　gene　pool。　anyadaptive　change　that　occurs　in　one　area　of　the　bacterial　universe　can　spread　to　any　other。　it’srather　as　if　a　human　could　go　to　an　insect　to　get　the　necessary　genetic　coding　to　sprout　wingsor　walk　on　ceilings。　it　means　that　from　a　genetic　point　of　view　bacteria　have　bee　a　singlesuperorganism—tiny；　dispersed；　but　invincible。

they　will　live　and　thrive　on　almost　anything　you　spill；　dribble；　or　shake　lo