ly suspected。 researchers at the manchesterroyal infirmary in england discovered that a sailor who had died of mysterious； untreatablecauses in 1959 in fact had aids。 but for whatever reasons the disease remained generallyquiescent for another twenty years。
the miracle is that other such diseases haven鈥檛 gone rampant。 lassa fever； which wasn鈥檛first detected until 1969； in west africa； is extremely virulent and little understood。 in 1969； adoctor at a yale university lab in new haven； connecticut； who was studying lassa fevercame down with it。 he survived； but； more alarmingly； a technician in a nearby lab； with nodirect exposure； also contracted the disease and died。
happily the outbreak stopped there； but we can鈥檛 count on such good fortune always。 ourlifestyles invite epidemics。 air travel makes it possible to spread infectious agents across theplanet with amazing ease。 an ebola virus could begin the day in； say； benin； and finish it innew york or hamburg or nairobi； or all three。 it means also that medical authoritiesincreasingly need to be acquainted with pretty much every malady that exists everywhere； butof course they are not。 in 1990； a nigerian living in chicago was exposed to lassa fever on avisit to his homeland； but didn鈥檛 develop symptoms until he had returned to the united states。
he died in a chicago hospital without diagnosis and without anyone taking any specialprecautions in treating him； unaware that he had one of the most lethal and infectious diseaseson the planet。 miraculously； no one else was infected。 we may not be so lucky next time。
and on that sobering note； it鈥檚 time to return to the world of the visibly living。
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21 LIFE GOES ON
灏彛昏…t…xt澶ｅ爞
it isn鈥檛 easy to bee a fossil。 the fate of nearly all living organisms鈥攐ver 99。9percent of them鈥攊s to post down to nothingness。 when your spark is gone； everymolecule you own will be nibbled off you or sluiced away to be put to use in some othersystem。 that鈥檚 just the way it is。 even if you make it into the small pool of organisms； the lessthan 0。1 percent； that don鈥檛 get devoured； the chances of being fossilized are very small。
in order to bee a fossil； several things must happen。 first； you must die in the rightplace。 only about 15 percent of rocks can preserve fossils； so it鈥檚 no good keeling over on afuture site of granite。 in practical terms the deceased must bee buried in sediment； whereit can leave an impression； like a leaf in wet mud； or depose without exposure to oxygen；permitting the molecules in its bones and hard parts （and very occasionally softer parts） to bereplaced by dissolved minerals； creating a petrified copy of the original。 then as thesediments in which the fossil lies are carelessly pressed and folded and pushed about byearth鈥檚 processes； the fossil must somehow maintain an identifiable shape。 finally； but aboveall； after tens of millions or perhaps hundreds of millions of years hidden away； it must befound and recognized as something worth keeping。
only about one bone in a billion； it is thought； ever bees fossilized。 if that is so； itmeans that the plete fossil legacy of all the americans alive today鈥攖hat鈥檚 270 millionpeople with 206 bones each鈥攚ill only be about fifty bones； one quarter of a pleteskeleton。 that鈥檚 not to say of course that any of these bones will actually be found。 bearing inmind that they can be buried anywhere within an area of slightly over 3。6 million squaremiles； little of which will ever be turned over； much less examined； it would be something ofa miracle if they were。 fossils are in every sense vanishingly rare。 most of what has lived onearth has left behind no record at all。 it has been estimated that less than one species in tenthousand has made it into the fossil record。 that in itself is a stunningly infinitesimalproportion。 however； if you accept the mon estimate that the earth has produced 30billion species of creature in its time and richard leakey and roger lewin鈥檚 statement （inthe sixth extinction ） that there are 250；000 species of creature in the fossil record； thatreduces the proportion to just one in 120；000。 either way； what we possess is the merestsampling of all the life that earth has spawned。
moreover； the record we do have is hopelessly skewed。 most land animals； of course； don鈥檛die in sediments。 they drop in the open and are eaten or left to rot or weather down tonothing。 the fossil record consequently is almost absurdly biased in favor of marine creatures。
about 95 percent of all the fossils we possess are of animals that once lived under water；mostly in shallow seas。
i mention all this to explain why on a gray day in february i went to the natural historymuseum in london to meet a cheerful； vaguely rumpled； very likeable paleontologist namedrichard fortey。
fortey knows an awful lot about an awful lot。 he is the author of a wry； splendid bookcalled life： an unauthorised biography； which covers the whole pageant of animate creation。
but his first love is a type of marine creature called trilobites that once teemed in ordovicianseas but haven鈥檛 existed for a long time except in fossilized form。 all shared a basic body planof three parts； or lobes鈥攈ead； tail； thorax鈥攆rom which es the name。 fortey found hisfirst when he was a boy clambering over rocks at st。 david鈥檚 bay in wales。 he was hookedfor life。
he took me to a gallery of tall metal cupboards。 each cupboard was filled with shallowdrawers； and each drawer was filled with stony trilobites鈥攖wenty thousand specimens in all。
鈥渋t seems like a big number；鈥潯e agreed； 鈥渂ut you have to remember that millions uponmillions of trilobites lived for millions upon millions of years in ancient seas； so twentythousand isn鈥檛 a huge number。 and most of these are only partial specimens。 finding aplete trilobite fossil is still a big moment for a paleontologist。鈥�
trilobites first appeared鈥攆ully formed； seemingly from nowhere鈥攁bout 540 million yearsago； near the start of the great outburst of plex life popularly known as the cambrianexplosion； and then vanished； along with a great deal else； in the great and still mysteriouspermian extinction 300；000 or so centuries later。 as with all extinct creatures； there is anatural temptation to regard them as failures； but in fact they were among the most successfulanimals ever to live。 their reign ran for 300 million years鈥攖wice the span of dinosaurs；which were themselves one of history鈥檚 great survivors。 humans； fortey points out； havesurvived so far for one…half of 1 percent as long。
with so much time at their disposal； the trilobites proliferated prodigiously。 most remainedsmall； about the size of modern beetles； but some grew to be as big as platters。 altogetherthey formed at least five thousand genera and sixty thousand species鈥攖hough more turn upall the time。 fortey had recently been at a conference in south america where he wasapproached by an academic from a small provincial university in argentina。 鈥渟he had a boxthat was full of interesting things鈥攖rilobites that had never been seen before in southamerica； or indeed anywhere； and a great deal else。 she had no research facilities to studythem and no funds to look for more。 huge parts of the world are still unexplored。鈥�
鈥渋n terms of trilobites？鈥�
鈥渘o； in terms of everything。鈥�
throughout the nineteenth century； trilobites were almost the only known forms of earlyplex life； and for that reason were assiduously collected and studied。 the big mysteryabout them was their sudden appearance。 even now； as fortey says； it can be startling to go tothe right formation of rocks and to work your way upward through the eons finding no visiblelife at all； and then suddenly 鈥渁 whole profallotaspis or elenellus as big as a crab will popinto your waiting hands。鈥潯hese were creatures with limbs； gills； nervous systems； probingantennae； 鈥渁 brain of sorts；鈥潯n fortey鈥檚 words； and the strangest eyes ever seen。 made of calcite rods； the same stuff that forms limestone； they constituted the earliest visual systemsknown。 more than this； the earliest trilobites didn鈥檛 consist of just one venturesome speciesbut dozens； and