from a ive, and clearly it  forus to do ot couldn’t  to get going, but ttengoing, it seemed in very little o move on.

    Consider t about t visible organisms on Eartamong t ambitious. t ticularly ts ops and arctic es,  rock and rain and cold, and almostno competition. In areas of Antarctica  expanses of licypes of tedly to every wind-whipped rock.

    For a long time, people couldn’t understand . Because lic evident nouris or tion of seeds, many people—educatedpeople—believed tones caugs. “Spontaneously,inorganic stone becomes living plant!” rejoiced one observer, a Dr. homschuch, in 1819.

    Closer inspection s liceresting ta partnerse acids t dissolve t t into food sufficient to sustain bot is not avery exciting arrangement, but it is a conspicuously successful one. ty thousand species of lichens.

    Like most t ts, lic may take alicury to attain t button. tes, es David Attenborougo be t ence. “t,” Attenborougestifying to t t life even at its simplest leveloccurs, apparently, just for its own sake.”

    It is easy to overlook t t life just is. As o feel t lifemust . e ions and desires. e  to take constantadvantage of all toxicating existence  o alic its impulse to exist, to be, is every bit as strong as ours—arguably even stronger.

    If I old t I o spend decades being a furry groually all living t, for a moment’s additional existence. Life, in s, justs to be. But—and eresting point—for t part it doesn’t  to bemuch.

    ttle odd because life y of time to develop ambitions. If youimagine tory compressed into a normal eart 4A.M.,  simple, single-celledorganisms, but t sixteen  until almost 8:30 into s arestless skin of microbes. t sea plants appear, folloy minuteslater by t jellyfisic Ediacaran fauna first seen by Reginald Sprigg inAustralia. At 9:04P.M. trilobites so tely bytures of t before 10P.M. plants begin to pop up on ter,  land creatures follow.

    to ten minutes or so of balmy carboniferous forests s areevident. Dinosaurs plod onto t before 11P.M. and  ters of an  ty-one minutes to midnige and seventeen seconds before midnigory, on timebarely an instant. t tly speeded-up day continents slide about and bangtoget a clip t seems positively reckless. Mountains rise and melt a t timesevery minute, some marking t ofa Manson-sized meteor or one even larger. It’s a  anyt all can survive insuctled environment. In fact, not many things do for long.

    Perive reme recentness as a part of ture is to stretco t extent and imagine tire ory of to Joance from tips of one o t of ther is Precambrian.

    All of complex life is in one roke e ory.”

    Fortunately, t moment   t it erject a note of gloom just at t, but t is t tremely pertinent quality about life on Eart goes extinct. Quite regularly. For all trouble take to assemble and preserve tinely. And t, to go extinct.  terribly ambitious.

    So anytime life does somet is quite an event, and feful to t stage in our narrative and came out of the sea.

    Land : , dry, batense ultraviolet radiation,lacking t makes movement in er comparatively effortless. to live onland, creatures o undergo s backbone too o support it. to survive out of er, marinecreatures needed to come up ernal arcecture—not t ofadjustment t . Above all and most obviously, any land creature o take its oxygen directly from ter it from er.

    t trivial co overcome. On tive to leave ter: it ting dangerous doinents into a single landmass, Pangaea, meant tline tal at. So competition tling neype of predator on tly designed forattack t it s emergence: tious time to find an alternative environment to er.

    Plants began tion about 450 million years ago, accompanied ofnecessity by tiny mites and ot to break doter on took a little longer to emerge, but by about 400million years ago turing out of ter, too. Popular illustrations o envision t venturesome land dious fiso puddle duringdroug, t visible mobile residents ondry land tle bugs (crustaceans, in fact) t are commonly toconfusion wurn a rock or log.

    For t learned to breatimes errestrial life first bloomed, o nearer 20 percent noo grow remarkablylarge remarkably quickly.

    And ists kno ingenious fieldknoope geocry. tiny plankton t iny protective son created tmosp s (carbon especially) to form durable compounds suce. It’s trick t goes on in (and is discussed elseo) term carbon cycle—a process t doesn’t make for terribly exciting narrative butis vital for creating a livable planet.

    Eventually in tiny organisms die and drift to ttom of to limestone. Among tiny atomic structures ton take to table isotopes—oxygen-16 and oxygen-18.

    (If you ten ope is, it doesn’t matter, t’s an atomrons.) ts come in, for topes accumulate at different rates depending on mosp time of tion. By comparing t ratios, ts can cunningly read conditions in t emperatures, tent and timing of ice ages, and mucopefindings ists can, e entire landscapes t no human eye ever saw.

    to build up so robustly t terrestrial life  muced by giant treeferns and vast sed tead of completely rotting doative matteraccumulated in ric sediments, o t coal bedst sustain mucivity even now.

    tsized groion of asurface animal yet found is a track left 350 million years ago by a millipede-like creature on arock in Scotland. It  long. Before t some millipedes wouldreac.

    itures on t is per surprising t insects in trick t could keep t of tongue s: to fly. Some tookto tion y t t cecime since. t up to ty-fivemiles an antly stop,  far more proportionately tator ten, “ tunnels to see , and despaired.” too, gorged on ts dragonflies grerees and otation liketained outsized proportions. ails and tree ferns greo s of fifty feet, clubmosses to a y.

    t terrestrial vertebrates— land animals from ly because of a sage of relevantfossils, but partly also because of an idiosyncratic Sations and secretive manner ion for almost ury. Jarvik  of a team of Scandinavian sc to Greenland in ticular t lobe-finned fisypet presumably ral to us and all otures, knoetrapods.

    Most animals are tetrapods, and all living tetrapods  end in a maximum of five fingers or toes. Dinosaurs, rapods, or. to tor, it  400 millionyears ago. Before t time noter t time lots of team found just sucure, a t-long animal called an Icega. to Jarvik,  at it for tforty-eigunately, Jarvik refused to let anyone study etrapod. tologists o be content cerim papers in ture s ancestral importance.

    Jarvik died in 1998. After ologists eagerly examined t Jarvik ed toes—tually eigo observe t t possibly ructure of t it  do a great deal to advance our understanding of t land animals. todaytetrapods are kno knoe wherewe came from.

    But come  state of eminence  of course alraig ed of four megadynasties, as times called. t consisted of primitive, plodding but sometimes fairly yampiles. t-knorodon, a sail-backed creature t is commonly confused e, in a picturecaption in t). trodon  a synapsid. So, onceupon a time, ilian life,to tion of small o be found in temples; diapsids wo; euryapsids had a single hole higher up.

    Over time, eac into furtered. Anapsids gave rise to turtles, e as t’s most advanced and deadlyspecies, before an evolutionary lurc ttle for durability rato four streams, only one of whe Permian.

    ream o, and it evolved into a family of protomammalsknoy 2.

    Unfortunately for tively evolving,in to dinosaurs (among otoo muco compete o ures, to small,furry, burro bided time for a very long  of t, and most han mice.

    Eventually, tion, but to  nearly 150 millionyears for Megadynasty 3, to come to an abrupt end and make room forMegadynasty 4 and our own Age of Mammals.

    Eacransformations, as  on t paradoxically important motor of progress: extinction. It is a curiousfact t on Eart literal sense, a y billion is a commonly citedfigure, but t as ever tual total, 99.99percent of all species t o a first approximation,” asDavid Raup of ty of Co say, “all species are extinct.” For complexorganisms, t four million years—rougwhere we are now.

    Extinction is alims, of course, but it appears to be a good t. “ternative to extinction is stagnation,” says Ian tattersall of tural ory, “and stagnation is seldom a good thing in any realm.”

    (I se t ion as a natural, long-term process.

    Extinction broug by ter altogetory are invariably associated ic leaps afterive outburst of tinction of 440 million years ago cleared t of immobile filterfeeders and, someed conditions t favored darting fis aquatic reptiles.

    turn ion to send colonists onto dry land e Devonian period gave life anot  scatteredintervals tory. If most of ts   as t ainly  be here now.

    Eartinction episodes in its time—triassic, and Cretaceous, in t order—and many smaller ones. t about 80 to 85 percent ofspecies. triassic (210 million years ago) and Cretaceous (65 million years) eac 70 to 75 percent of species. But tinction of about 245million years ago, least 95 percent of animals kno, never to return. Evenabout a t species —t en masse. It isas close as otal obliteration.

    “It ruly, a mass extinction, a carnage of a magnitude t roubled tey. t icularly devastating to sea creatures.

    trilobites vanisoget. Virtually all otaggered. Altogeter, it is t t Eart 52percent of its families—t’s t of t cer)—and per of all its species. Itime—as mucy million years by one reckoning—before speciestotals recovered.

    ts need to be kept in mind. First, t informed guesses. Estimates fort toas  knoion t perisalking about t individuals. For individuals toll could be mucically total. t survived to t ptery almost certainlyoence to a few scarred and limping survivors.

    In betinctionepisodes—t so devastating to total species numbers, but often critically  certainpopulations. Grazing animals, including  in t about five million years ago. o a single species,  t for a time it teetered on tory   grazing animals.

    In nearly every case, for botinctions and more modest ones, le idea of er stripping out t notions till more t caused tinction events ts. At leasttential culprits ified as causes or prime contributors: globalionkno leaks of meteor and cometimpacts, runarophic solar flares.

    t is a particularly intriguing possibility. Nobody knoy engine and its storms are commensurately enormous. A typical solar flare—somet even notice on Eart of a billiono space a ons or so of murderous icles. tospmosp tospace or steer to it is t t an unusually big blast, say a imes typical flare, couldover certainly kill a very ion of all t basked in its gloo Bruce tsurutani of t Propulsion Laboratory, “it race in ory.”

    all t it, is “tons of conjecture and verylittle evidence.” Cooling seems to be associated  least tinctionevents—t beyond t little is agreed, includingists can’t agree, for instance,inction—t t ebrates movingonto thousands of years or in one lively day.

    One of t is so o produce convincing explanations for extinctions is t itis so very o exterminate life on a grand scale. As ill stage a full, if presumably somes Eart event sosingularly devastating? ell, first itively enormous. It struck ons. Sucburst is not easily imagined, but as James La, if you exploded one odayyou ill be about a billion bombs s of t impact. But even talone may not o  70 percent of Earth’s life, dinosaurs included.

    t meteor ional advantage—advantage if you are a mammal, t is—t it landed in a s ten meters deep, probably at just t angle, at a time and so tible. Above all t landed was made of rock rich in sulfur.

    t  t turned an area of seafloor to aerosols ofsulfuric acid. For montered to rains acid enougo burn skin.

    In a sense, an even greater question t of  70 percent of t ing at time is  survive? so irremediably devastating to every single dinosaur t existed, ell no species of toad,ne extinct in Norte creatures er?” asks timFlannery in ing preory of America, Eternal Frontier.

    In t es vanis tiloids, on, some species ically —92 percent of foraminiferans, for instance—o a similar plan and living alongside, ively unscathed.

    t inconsistencies. As Ricey observes: “Some does notseem satisfying just to call t at t.” If, as seems entirely likely,t survivors become difficult to account for. “Some insects, like beetles,” Fortey notes, “couldlive on   t navigate bysunlig so easy.”

    Above all, to survive and algae require sunligogeteady minimum temperatures. Mucy  feo corals dying from cemperature of only a degree or so. If t vulnerable to small c er?

    to-explain regional variations. Extinctions seem to icular appears to  no burroures. Evenits vegetation ion else devastation  a great deal  know.

    Some animals absolutely prospered—including, a little surprisingly, turtles once again.

    As Flannery notes, tely after tinction could urtles. Sixteen species survived in Nortoexistence soon after.

    Clearly it o be at er. t impact  almost 90 percent ofland-based species but only 10 percent of ter. ater obviously offeredprotection against  and flame, but also presumably provided more sustenance in t follo survived  of retreating to asafer environment during times of danger—into er or underground—eiter against t. Animals tscavenged for a living o teria in rotting carcasses. Indeed, often tively drao it,and for a long rid carcasses about.

    It is often ated t only small animals survived t event. In fact, among t just large but times larger today.

    But on t is true, most of tive. Indeed, ile, it  time to be small, urnal, flexible indiet, and cautious by nature—ties t distinguished our mammalian forebears.

    ion been more advanced, ead,mammals found to hing alive.

    as if mammals so fill every nicion mayabe t Steven M. Stanley, “but it often takes a long timeto fill it.” For peren million years mammals remained cautiously small. Intertiary, if you  you could be king.

    But once t going, mammals expanded prodigiously—sometimes to an almostpreposterous degree. For a time, tory ory cen literally) to fill it. Early members of ted to Souto creatures ty of bears. Birds, too,prospered disproportionately. For millions of years, a gigantic, fligitanis  ferocious creature in Nortainly it  daunting bird t ever lived. It stood ten feet   could tear tty muc irked it. Its familysurvived in formidable fasy million years, yet until a skeleton  ed.

    o anotainty about extinctions: triness oftouc of bones becomingfossilized, but tually  think. Consider dinosaurs.

    Museums give t  Diplodocus t dominates trance ural ory Museum in London and ed and informedgenerations of visitors is made of plaster—built in 1903 in Pittsburged to trance ural oryin Need by an even grander tableau: a skeleton of a large Barosaurusdefending tack by a darting and toot is a y feet to alsoentirely fake. Every one of t. Visit almost anylarge natural ory museum in t, Buenos Aires,Mexico City—and  you are antique models, not ancient bones.

    t is,  really kno deal about tified (almost  a quarter of times as long as mammalsive of species or ibly apt cliché).

    For millions of years t a single fossil  been found.

    Even for te Cretaceous—t studied preoric period to our long interest in dinosaurs and tinction—some ters of allspecies t lived may yet be undiscovered. Animals bulkier tyrannosaurus may . Until very recently everyt t ting just sixteen species. tiness ofto t dinosaurs  already  occurred.

    In te 1980s a paleontologist from ter So conduct an experiment. Using t also ionin Montana. Sifting meticulously, teers collected every last tootebra andc ook t tripled total of dinosaurfossils from te Cretaceous. tablis dinosaurs remained numerousrigo time of t impact. “to believe t t gradually during t taceous,” Sed.

    e are so used to tion of our oability as life’s dominant species t it iso grasp t raterrestrial bangs and ot for nearly fourbillion years our ancestors o slip timeo. Step succinctly in a icular line never fractured—never once at any of ts t could ory.”

    e started ter s: Life s to be; life doesn’t al to bemucime to time goes extinct. to ten, as  are decidedly amazing.