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didn鈥檛 fitanywhere else。 it included （depending on which text you consulted） slime molds； amoebas；and even seaweed； among much else。 by one calculation it contained as many as 200；000different species of organism all told。 that鈥檚 a lot of odd socks。
ironically； just as whittaker鈥檚 five…kingdom classification was beginning to find its wayinto textbooks； a retiring academic at the university of illinois was groping his way toward adiscovery that would challenge everything。 his name was carl woese （rhymes with rose）； andsince the mid…1960s鈥攐r about as early as it was possible to do so鈥攈e had been quietlystudying genetic sequences in bacteria。 in the early days； this was an exceedingly painstakingprocess。 work on a single bacterium could easily consume a year。 at that time； according towoese； only about 500 species of bacteria were known； which is fewer than the number ofspecies you have in your mouth。 today the number is about ten times that； though that is stillfar short of the 26；900 species of algae； 70；000 of fungi； and 30；800 of amoebas and relatedorganisms whose biographies fill the annals of biology。
it isn鈥檛 simple indifference that keeps the total low。 bacteria can be exasperatingly difficultto isolate and study。 only about 1 percent will grow in culture。 considering how wildlyadaptable they are in nature； it is an odd fact that the one place they seem not to wish to live isa petri dish。 plop them on a bed of agar and pamper them as you will； and most will just liethere； declining every inducement to bloom。 any bacterium that thrives in a lab is bydefinition exceptional； and yet these were； almost exclusively； the organisms studied bymicrobiologists。 it was； said woese； 鈥渓ike learning about animals from visiting zoos。鈥�
genes； however； allowed woese to approach microorganisms from another angle。 as heworked； woese realized that there were more fundamental divisions in the microbial worldthan anyone suspected。 a lot of little organisms that looked like bacteria and behaved likebacteria were actually something else altogether鈥攕omething that had branched off frombacteria a long time ago。 woese called these organisms archaebacteria； later shortened toarchaea。
it has be said that the attributes that distinguish archaea from bacteria are not the sort thatwould quicken the pulse of any but a biologist。 they are mostly differences in their lipids andan absence of something called peptidoglycan。 but in practice they make a world ofdifference。 archaeans are more different from bacteria than you and i are from a crab orspider。 singlehandedly woese had discovered an unsuspected division of life； so fundamentalthat it stood above the level of kingdom at the apogee of the universal tree of life； as it israther reverentially known。
in 1976； he startled the world鈥攐r at least the little bit of it that was paying attention鈥攂yredrawing the tree of life to incorporate not five main divisions； but twenty…three。 these hegrouped under three new principal categories鈥攂acteria； archaea； and eukarya （sometimesspelled eucarya）鈥攚hich he called domains。
woese鈥檚 new divisions did not take the biological world by storm。 some dismissed them asmuch too heavily weighted toward the microbial。 many just ignored them。 woese； according to frances ashcroft； 鈥渇elt bitterly disappointed。鈥潯ut slowly his new scheme began to catchon among microbiologists。 botanists and zoologists were much slower to admire its virtues。
it鈥檚 not hard to see why。 on woese鈥檚 model； the worlds of botany and zoology are relegatedto a few twigs on the outermost branch of the eukaryan limb。 everything else belongs tounicellular beings。
鈥渢hese folks were brought up to classify in terms of gross morphological similarities anddifferences；鈥潯oese told an interviewer in 1996。 鈥渢he idea of doing so in terms of molecularsequence is a bit hard for many of them to swallow。鈥潯n short； if they couldn鈥檛 see a differencewith their own eyes； they didn鈥檛 like it。 and so they persisted with the traditional five…kingdom division鈥攁n arrangement that woese called 鈥渘ot very useful鈥潯n his mildermoments and 鈥減ositively misleading鈥潯uch of the rest of the time。 鈥渂iology； like physicsbefore it；鈥潯oese wrote； 鈥渉as moved to a level where the objects of interest and theirinteractions often cannot be perceived through direct observation。鈥�
in 1998 the great and ancient harvard zoologist ernst mayr （who then was in his ninety…fourth year and at the time of my writing is nearing one hundred and still going strong） stirredthe pot further by declaring that there should be just two prime divisions of life鈥斺�渆mpires鈥�
he called them。 in a paper published in the proceedings of the national academy of sciences；mayr said that woese鈥檚 findings were interesting but ultimately misguided； noting that鈥渨oese was not trained as a biologist and quite naturally does not have an extensivefamiliarity with the principles of classification；鈥潯hich is perhaps as close as onedistinguished scientist can e to saying of another that he doesn鈥檛 know what he is talkingabout。
the specifics of mayr鈥檚 criticisms are too technical to need extensive airing here鈥攖heyinvolve issues of meiotic sexuality； hennigian cladification； and controversial interpretationsof the genome of methanobacterium thermoautrophicum； among rather a lot else鈥攂utessentially he argues that woese鈥檚 arrangement unbalances the tree of life。 the bacterialrealm； mayr notes； consists of no more than a few thousand species while the archaean has amere 175 named specimens； with perhaps a few thousand more to be found鈥斺�渂ut hardlymore than that。鈥潯y contrast； the eukaryotic realm鈥攖hat is； the plicated organisms withnucleated cells； like us鈥攏umbers already in the millions。 for the sake of 鈥渢he principle ofbalance；鈥潯ayr argues for bining the simple bacterial organisms in a single category；prokaryota； while placing the more plex and 鈥渉ighly evolved鈥潯emainder in the empireeukaryota； which would stand alongside as an equal。 put another way； he argues for keepingthings much as they were before。 this division between simple cells and plex cells 鈥渋swhere the great break is in the living world。鈥�
the distinction between halophilic archaeans and methanosarcina or between flavobacteriaand gram…positive bacteria clearly will never be a matter of moment for most of us； but it isworth remembering that each is as different from its neighbors as animals are from plants。 ifwoese鈥檚 new arrangement teaches us anything it is that life really is various and that most ofthat variety is small； unicellular； and unfamiliar。 it is a natural human impulse to think ofevolution as a long chain of improvements； of a never…ending advance toward largeness andplexity鈥攊n a word； toward us。 we flatter ourselves。 most of the real diversity inevolution has been small…scale。 we large things are just flukes鈥攁n interesting side branch。 ofthe twenty…three main divisions of life； only three鈥攑lants； animals； and fungi鈥攁re largeenough to be seen by the human eye； and even they contain species that are microscopic。
indeed； according to woese； if you totaled up all the biomass of the planet鈥攅very living thing； plants included鈥攎icrobes would account for at least 80 percent of all there is； perhapsmore。 the world belongs to the very small鈥攁nd it has for a very long time。
so why； you are bound to ask at some point in your life； do microbes so often want to hurtus？ what possible satisfaction could there be to a microbe in having us grow feverish orchilled； or disfigured with sores； or above all expire？ a dead host； after all； is hardly going toprovide long…term hospitality。
to begin with； it is worth remembering that most microorganisms are neutral or evenbeneficial to human well…being。 the most rampantly infectious organism on earth； abacterium called wolbachia； doesn鈥檛 hurt humans at all鈥攐r； e to that； any othervertebrates鈥攂ut if you are a shrimp or worm or fruit fly； it can make you wish you had neverbeen born。 altogether； only about one microbe in a thousand is a pathogen for humans；according to national geographic 鈥攖hough； knowing what some of them can do； we cou