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s this may wellhave been the niftiest trick in the universe。
and it doesn’t stop there。 having hauled itself up to a more favorable locale； the slimemold transforms itself yet again； taking on the form of a plant。 by some curious orderlyprocess the cells reconfigure； like the members of a tiny marching band； to make a stalk atopof which forms a bulb known as a fruiting body。 inside the fruiting body are millions ofspores that； at the appropriate moment； are released to the wind to blow away and beesingle…celled organisms that can start the process again。
for years slime molds were claimed as protozoa by zoologists and as fungi by mycologists；though most people could see they didn’t really belong anywhere。 when genetic testingarrived； people in lab coats were surprised to find that slime molds were so distinctive andpeculiar that they weren’t directly related to anything else in nature； and sometimes not evento each other。
in 1969； in an attempt to bring some order to the growing inadequacies of classification； anecologist from cornell university named r。 h。 whittaker unveiled in the journalscience aproposal to divide life into five principal branches—kingdoms； as they are known—calledanimalia； plantae； fungi； protista； and monera。 protista； was a modification of an earlier term； protoctista； which had been suggested a century earlier by a scottish biologist namedjohn hogg； and was meant to describe any organisms that were neither plant nor animal。
though whittaker’s new scheme was a great improvement； protista remained ill defined。
some taxonomists reserved it for large unicellular organisms—the eukaryotes—but otherstreated it as the kind of odd sock drawer of biology； putting into it anything that didn’t 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’s a lot of odd socks。
ironically； just as whittaker’s 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—or about as early as it was possible to do so—he 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’t 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； “like learning about animals from visiting zoos。”
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—something 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—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