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！
pared with the new eukaryotes the old prokaryotes were little more than 鈥渂ags ofchemicals；鈥潯n 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鈥潱弧ut 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。。t。xt。。澶ｅ爞
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ccording 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last 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icto