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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： 鈥渢his 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。鈥�
although you鈥檇 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鈥檚 atmosphere to 20 percent； preparing the way for the next； more plex chapter inlife鈥檚 history。
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。 鈥渁nimals could not summonup the energy to work；鈥潯s fortey has put it。 it took about two billion years； roughly 40percent of earth鈥檚 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鈥攐ne with a nucleus and other little bodies collectively calledorganelles （from a greek word meaning 鈥渓ittle tools鈥潱！he 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鈥攜ou could pack a billion into the spaceoccupied by a grain of sand鈥攂ut also very hungry。 almost every nutriment you absorb goesto feeding them。
we couldn鈥檛 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鈥檛 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 鈥渢ruly nucleated鈥潱弧s contrastedwith the old type； which is known as a prokaryote （鈥減renucleated鈥潱弧nd 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 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 i