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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 couldbe forgiven for thinking that that is quite enough。 even if mostly benign； microbes are still thenumber…three killer in the western world； and even many less lethal ones of course make usdeeply rue their existence。
making a host unwell has certain benefits for the microbe。 the symptoms of an illnessoften help to spread the disease。 vomiting； sneezing； and diarrhea are excellent methods ofgetting out of one host and into position for another。 the most effective strategy of all is toenlist the help of a mobile third party。 infectious organisms love mosquitoes because themosquito鈥檚 sting delivers them directly to a bloodstream where they can get straight to workbefore the victim鈥檚 defense mechanisms can figure out what鈥檚 hit them。 this is why so manygrade…a diseases鈥攎alaria； yellow fever； dengue fever； encephalitis； and a hundred or soother less celebrated but often rapacious maladies鈥攂egin with a mosquito bite。 it is afortunate fluke for us that hiv； the aids agent； isn鈥檛 among them鈥攁t least not yet。 any hivthe mosquito sucks up on its travels is dissolved by the mosquito鈥檚 own metabolism。 whenthe day es that the virus mutates its way around this； we may be in real trouble。
it is a mistake； however； to consider the matter too carefully from the position of logicbecause microorganisms clearly are not calculating entities。 they don鈥檛 care what they do toyou any more than you care what distress you cause when you slaughter them by the millionswith a soapy shower or a swipe of deodorant。 the only time your continuing well…being is ofconsequence to a pathogen is when it kills you too well。 if they eliminate you before they canmove on； then they may well die out themselves。 this in fact sometimes happens。 history；jared diamond notes； is full of diseases that 鈥渙nce caused terrifying epidemics and thendisappeared as mysteriously as they had e。鈥潯e cites the robust but mercifully transientenglish sweating sickness； which raged from 1485 to 1552； killing tens of thousands as itwent； before burni