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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—or at least the little bit of it that was paying attention—byredrawing the tree of life to incorporate not five main divisions； but twenty…three。 these hegrouped under three new principal categories—bacteria； archaea； and eukarya （sometimesspelled eucarya）—which he called domains。
woese’s 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； “felt bitterly disappointed。” but slowly his new scheme began to catchon among microbiologists。 botanists and zoologists were much slower to admire its virtues。
it’s not hard to see why。 on woese’s 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。
“these folks were brought up to classify in terms of gross morphological similarities anddifferences；” woese told an interviewer in 1996。 “the idea of doing so in terms of molecularsequence is a bit hard for many of them to swallow。” in short； if they couldn’t see a differencewith their own eyes； they didn’t like it。 and so they persisted with the traditional five…kingdom division—an arrangement that woese called “not very useful” in his mildermoments and “positively misleading” much of the rest of the time。 “biology； like physicsbefore it；” woese wrote； “has moved to a level where the objects of interest and theirinteractions often cannot be perceived through direct observation。”
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—“empires”
he called them。 in a paper published in the proceedings of the national academy of sciences；mayr said that woese’s findings were interesting but ultimately misguided； noting that“woese was not trained as a biologist and quite naturally does not have an extensivefamiliarity with the principles of classification；” which is perhaps as close as onedistinguished scientist can e to saying of another that he doesn’t know what he is talkingabout。
the specifics of mayr’s criticisms are too technical to need extensive airing here—theyinvolve issues of meiotic sexuality； hennigian cladification； and controversial interpretationsof the genome of methanobacterium thermoautrophicum； among rather a lot else—butessentially he argues that woese’s 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—“but hardlymore than that。” by contrast； the eukaryotic realm—that is； the plicated organisms withnucleated cells； like us—numbers already in the millions。 for the sake of “the principle ofbalance；” mayr argues for bining the simple bacterial organisms in a single category；prokaryota； while placing the more plex and “highly evolved” remainder 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 “iswhere the great break is in the living world。”
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’s 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—in a word； toward us。 we flatter ourselves。 most of the real diversity inevolution has been small…scale。 we large things are just flukes—an interesting side branch。 ofthe twenty…three main divisions of life； only three—plants； animals； and fungi—are 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—every living thing； plants included—microbes would account for at least 80 percent of all there is； perhapsmore。 the world belongs to the very small—and 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’t hurt humans at all—or； e to that； any othervertebrates—but 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 —though； 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 d