万物简史英文版_比尔·布莱森- 第78节

pared　with　the　new　eukaryotes　the　old　prokaryotes　were　little　more　than　鈥渂ags　ofchemicals；鈥潯�in　the　words　of　the　geologist　stephen　drury。　eukaryotes　were　bigger鈥攅ventuallyas　much　as　ten　thousand　times　bigger鈥攖han　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鈥攐rganisms　that　expel　oxygen　（like　plants）　and　those　that　take　it　in　（you　andme）。

single…celled　eukaryotes　were　once　called　protozoa　（鈥減re…animals鈥潱�；　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鈥攅nough；　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鈥攁billion　years　or　so鈥攂ut　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。

xiaoshuot锛竧锛宑o锛�



20　　　　SMALL　WORLD

灏彙！ｈ�础！�t。xt。。澶�。鍫�
it鈥檚　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鈥檛　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鈥攁bout　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鈥檚　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鈥檛　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鈥檛　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鈥檚　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。　鈥済iven　an　adequatesupply　of　nutrients；　a　single　bacterial　cell　can　generate　280；000　billion　individuals　in　a　singleday；鈥潯�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鈥攃hange　is　always　risky　for　an　organism鈥攂ut　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鈥檚rather　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鈥攖iny；　dispersed；　but　invincible。

they　will　live　and　thrive　on　almost　anything　you　spill；　dribble；　or　shake　loose。　just　givethem　a　little　moisture鈥攁s　when　you　run　a　damp　cloth　over　a　counter鈥攁nd　they　will　bloom　asif　created　from　nothing。　they　will　eat　wood；　the　glue　in　wallpaper；　the　metals　in　hardenedpaint。　scientists　in　australia　found　microbes　known　as　thiobacillus　concretivorans　that　livedin鈥攊ndeed；　could　not　live　without鈥攃oncentrations　of　sulfuric　acid　strong　enough　to　dissolvemetal。　a　species　called　micrococcus　radiophilus　was　found　living　happily　in　the　waste　tanksof　nuclear　reactors；　gorging　itself　on　plutonium　and　whatever　else　was　there。　some　bacteriabreak　down　chemical　materials　from　which；　as　far　as　we　can　tell；　they　gain　no　benefit　at　all。

they　have　been　found　living　in　boiling　mud　pots　and　lakes　of　caustic　soda；　deep　insiderocks；　at　the　bottom　of　the　sea；　in　hidden　pools　of　icy　water　in　the　mcmurdo　dry　valleys　ofantarctica；　and　seven　miles　down　in　the　pacific　ocean　where　pressures　are　more　than　athousand　times　greater　than　at　the　surface；　or　equivalent　to　being　squashed　beneath　fiftyjumbo　jets。　some　of　them　seem　to　be　practically　indestructible。　deinococcus　radiodurans　is；according　to　theeconomist　；　鈥渁lmost　immune　to　radioactivity。鈥潯�blast　its　dna　with　radiation；and　the　pieces　immediately　reform　鈥渓ike　the　scuttling　limbs　of　an　undead　creature　from　ahorror　movie。鈥�

perhaps　the　most　extraordinary　survival　yet　found　was　that　of　a　streptococcus　bacteriumthat　was　recovered　from　the　sealed　lens　of　a　camera　that　had　stood　on　the　moon　for　two　years。

in　short；　there　are　few　environments　in　which　bacteria　aren鈥檛　prepared　to　live。　鈥渢hey　arefinding　now　that　when　they　push　probes　into　ocean　vents　so　hot　that　the　probes　actually　startto　melt；　there　are　bacteria　even　there；鈥潯�victo