22 GOOD-BYE TO ALL THAT

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    WHEN YOU SIDER it from a human perspective, and clearly it would be difficult forus to do otherwise, life is an odd thing. It couldn’t wait to get going, but then, having gottengoing, it seemed in very little hurry to move on.

    sider the li. Lis are just about the hardiest visible anisms oh, butamong the least ambitious. They will groily enough in a sunny churchyard, but theyparticularly thrive in enviros where no anism would go—on blowymountaintops and arctic wastes, wherever there is little but rod rain and cold, and almostno petition. In areas of Antarctica where virtually nothing else will grow, you  findvast expanses of li—four huypes of them—adheriedly to every wind-whipped rock.

    For a long time, people couldn’t uand how they did it. Because lis grew on barerock without evident nourishment or the produ of seeds, many people—educatedpeople—believed they were stones caught in the process of being plants. “Spontaneously,inanic stone bees living plant!” rejoiced one observer, a Dr. Homschuch, in 1819.

    Closer iion showed that lis were more iing than magical. They are in facta partnership between fungi and algae. The fungi excrete acids that dissolve the surface of therock, freeing minerals that the algae vert into food suffit to sustain both. It is not avery exg arra, but it is a spicuously successful ohe world has more thay thousand species of lis.

    Like most things that thrive in harsh enviros, lis are slow-growing. It may take ali more than half a tury to attain the dimensions of a shirt button. Those the size ofdinner plates, writes David Attenbh, are therefore “likely to be hundreds if notthousands of years old.” It would be hard to imagine a less fulfilliehey simplyexist,” Attenbh adds, “testifying to the moving fact that life even at its simplest leveloccurs, apparently, just for its own sake.”

    It is easy to overlook this thought that life just is. As humans we are ined to feel that lifemust have a point. lans and aspirations and desires. We want to take stantadvantage of all the intoxig existence we’ve been endowed with. But what’s life to ali? Yet its impulse to exist, to be, is every bit as strong as ours—arguably even stronger.

    If I were told that I had to spend decades being a furry growth on a ro the woods, Ibelieve I would lose the will to go on. Lis don’t. Like virtually all living things, they willsuffer any hardship, endure any insult, for a moment’s additioence. Life, in short, justwants to be. But—and here’s an iing point—for the most part it doesn’t want to bemuch.

    This is perhaps a little odd because life has had plenty of time to develop ambitions. If youimagihe 4,500-billion-odd years of Earth’s history pressed into a normal earthly day,then life begins very early, about 4A.M., with the rise of the first simple, single-celledanisms, but then advano further for the  sixteen hours. Not until almost 8:30 inthe evening, with the day five-sixths over, has Earth anything to show the universe but arestless skin of microbes. Then, finally, the first sea plants appear, followed twenty mier by the first jellyfish and the enigmatic Edia fauna first seen by Reginald Sprigg inAustralia. At 9:04P.M. trilobites swim onto the se, followed more or less immediately bythe shapely creatures of the Burgess Shale. Just before 10P.M. plants begin to pop up on theland. Soon after, with less than two hours left in the day, the first land creatures follow.

    Thanks to ten minutes or so of balmy weather, by 10:24 the Earth is covered in the greatcarboniferous forests whose residues give us all our coal, and the first winged is areevident. Dinosaurs plod onto the se just before 11P.M. and hold sway for about three-quarters of an hour. At twenty-one mio midnight they vanish and the age of mammalsbegins. Humans emerge one minute aeen seds before midnight. The whole of ourrecorded history, on this scale, would be no more than a few seds, a single human lifetimebarely an instant. Throughout this greatly speeded-up day tis slide about and bangtogether at a clip that seems positively reckless. Mountains rise a away, o basinse and go, ice sheets advand withdraw. And throughout the whole, about three timesevery minute, somewhere on the plahere is a flashbulb pop of light marking the impaanson-sized meteor or one even larger. It’s a wohat anything at all  survive insuch a pummeled and uled enviro. In faot many things do for long.

    Perhaps an even more effective way of grasping our extreme reess as a part of this4.5-billion-year-old picture is to stretch your arms to their fulbbr>?</abbr>lest extent and imagihatwidth as the entire history of the Earth. On this scale, acc to John McPhee in Basin andRahe distance from the fiips of one hand to the wrist of the other is Precambrian.

    All of plex life is in one hand, “and in a siroke with a medium-grained nail file youcould eradicate human history.”

    Fortunately, that moment hasn’t happened, but the ces are good that it will. I don’twish to interject a note of gloom just at this point, but the fact is that there is oherextremely perti quality about life oh: it goes extinct. Quite regularly. For all thetrouble they take to assemble and preserve themselves, species crumple and die remarkablyroutinely. And the more plex they get, the more quickly they appear to go extinct. Whichis perhaps one reason why so much of life isn’t terribly ambitious.

    So anytime life does something bold it is quite a, and few occasions were moreeventful than when life moved on to the  stage in our narrative and came out of the sea.

    Land was a formidable enviro: hot, dry, bathed in interaviolet radiation,lag the buoyancy that makes movement in water paratively effortless. To live onland, creatures had to undergo wholesale revisions of their anatomies. Hold a fish at eadand it sags in the middle, its bae too weak to support it. To survive out of water, mariures o e up with new load-bearing internal architecture—not the sort ofadjustment that happens ht. Above all and most obviously, any land creature wouldhave to develop a way to take its oxygen directly from the air rather than filter it from water.

    These were not trivial challeo overe. Oher hand, there owerfuliive to leave the water: it was getting dangerous down there. The slow fusion of thetis into a single landmass, Pangaea, meant there was much, much less coastlihanformerly and thus much less coastal habitat. So petition was fierce. There was also anomnivorous and uliype of predator on the se, one so perfectly designed forattack that it has scarcely ged in all the long eons sis emergehe shark. Neverwould there be a more propitious time to find an alternative enviroo water.

    Plants began the process of land ization about 450 million years ago, apanied ofy by tiny mites and anisms that they o break down and recycle deadanic matter on their behalf. Larger animals took a little loo emerge, but by about 400million years ago they were venturing out of the water, too. Popular illustrations haveenced us to envision the first venturesome land dwellers as a kind of ambitious fish—something like the modern mudskipper, which  hop from puddle to puddle duringdroughts—or even as a fully formed amphibian. In fact, the first visible mobile residents ondry land were probably much more like modern wood lice, sometimes also knoillbugsor sow bugs. These are the little bugs (crustas, in fact) that are only thrown intofusion when you upturn a rock .

    For those that learo breathe oxygen from the air, times were good. Oxygen levels inthe Devonian and Carboniferous periods, when terrestrial life first bloomed, were as high as35 pert (as opposed to nearer 20 pert now). This allowed animals to grow remarkablylarge remarkably quickly.

    And how, you may reasonably wonder,  stists know what oxygen levels were likehundreds of millions of years ago? The answer lies in a slightly obscure but ingenious fieldknown as isotope geochemistry. The long-ago seas of the Carboniferous and Devonianswarmed with tiny plankton that ed themselves iiny protective shells. Then, asnow, the planktoed their shells by drawing oxygen from the atmosphere and biningit with other elements (carbon especially) to form durable pounds such as calciumcarbo’s the same chemical trick that goes on in (and is discussed elsewhere iionto) the long-term carbon cycle—a process that doesn’t make for terribly exg narrative butis vital for creating a livable pla.

    Eventually in this process all the tiny anisms die and drift to the bottom of the sea,where they are slowly pressed into limestone. Among the tiny atomic structures theplankton take to the grave with them are two very stable isotopes—oxygen-16 and oxygen-18.

    (If you have fotten what an isotope is, it doesn’t matter, though for the record it’s an atomwith an abnormal number of rons.) This is where the geochemists e in, for theisotopes accumulate at different rates depending on how much oxygen or carbon dioxide is imosphere at the time of their creation. By paring these a ratios, thegeochemists  ingly read ditions in the a world—oxygen levels, air and otemperatures, the extent and timing of ice ages, and much else. By bining their isotopefindings with other fossil residues—pollen levels and so on—stists , with siderablefidence, re-create entire landscapes that no human eye ever saw.

    The principal reason oxygen levels were able to build up so robustly throughout the periodof early terrestrial life was that much of the world’s landscape was dominated by giant treeferns and vast ss, which by their boggy nature disrupted the normal carbon recygprocess. Instead of pletely rotting down, falling fronds and other dead vegetative matteraccumulated in rich, wet sediments, which were eventually squeezed into the vast coal bedsthat sustain much eic activity even now.

    The heady levels of oxygen clearly enced outsized growth. The oldest indication of asurfaimal yet found is a track left 350 million years ago by a millipede-like creature on aro Scotland. It was over three feet long. Before the era was out some millipedes wouldreach lengths more than double that.

    With such creatures on the prowl, it is perhaps not surprising that is in the periodevolved a trick that could keep them safely out of tongue shot: they learo fly. Some tookto this new means of lootion with suy facility that they haven’t ged theirteiques in all the time sihen, as now, dragonflies could cruise at up to thirty-fivemiles an hour, instantly stop, hover, fly backwards, and lift far more proportiohan anyhuman flying mae. “The U.S. Air Force,” one entator has written, “has put them inwind tuo see how they do it, and despaired.” They, too, ged on the rich air. InCarboniferous forests dragonflies grew as big as ravens. Trees and etation likewiseattained outsized proportions. Horsetails and tree ferns grew to heights of fifty feet, clubmosses to a hundred and thirty.

    The first terrestrial vertebrates—which is to say, the first land animals from which wewould derive—are something of a mystery. This is partly because of a she of relevantfossils, but partly also because of an idiosyncratic Swede named Erik Jarvik whose oddinterpretations aive manner held back progress on this question for almost half atury. Jarvik art of a team of Sdinavian scholars who went to Greenland in the1930s and 1940s looking for fossil fish. In particular they sought lobe-finned fish of the typethat presumably were aral to us and all other walking creatures, known as tetrapods.

    Most animals are tetrapods, and all livirapods have ohing in on: four limbsthat end in a maximum of five fingers or toes. Dinosaurs, whales, birds, humans, even fish—all are tetrapods, which clearly suggests they e from a single on aor. The clueto this aor, it was assumed, would be found in the Devonian era, from about 400 millionyears ago. Before that time nothing walked on land. After that time lots of things did. Luckilythe team found just such a creature, a three-foot-long animal called an Ichthyostega. Theanalysis of the fossil fell to Jarvik, who began his study in 1948 a at it for the forty-eight years. Unfor<u></u>tunately, Jarvik refused to let audy his tetrapod. The world’spaleontologists had to be tent with two sketchy interim papers in which Jarvik hatthe creature had five fingers in each of four limbs, firming its aral importance.

    Jarvik died in 1998. After his death, other paleontologists eagerly examihe speand found that Jarvik had severely misted the fingers and toes—there were actually eighton each limb—and failed to observe that the fish could not possibly have walked. Thestructure of the fin was such that it would have collapsed us ow. Needless tosay, this did not do a great deal to advance our uanding of the first land animals. Todaythree early tetrapods are known and none has five digits. In short, we don’t know quite wherewe came from.

    But e we did, though reag our present state of eminence has not of course alwaysbeen straightforward. Since life on land began, it has sisted of fadynasties, as theyare sometimes called. The first sisted of primitive, plodding but sometimes fairly heftyamphibians ailes. The best-known animal of this age was the Dimetrodon, a sail-backed creature that is only fused with dinosaurs (including, I note, in a picturecaption in the Carl Sagan book et). The Dimetrodon was in fact a synapsid. So, onceupon a time, were we. Synapsids were one of the four main divisions of early reptilian life,the others being anapsids, euryapsids, and diapsids. The names simply refer to the number andlocation of small holes to be found in the sides of their owners’ skulls. Synapsids had one holein their lower temples; diapsids had two; euryapsids had a single hole higher up.

    Over time, each of these principal groupings split into further subdivisions, of whieprospered and some faltered. Anapsids gave rise to the turtles, which for a time, perhaps atouch improbably, appeared poised to predominate as the pla’s most advanced and deadlyspecies, before an evolutionary lurch let them settle for durability rather than domihesynapsids divided into four streams, only one of which survived beyond the Permian.

    Happily, that was the stream we beloo, and it evolved into a family of protomammalsknown as therapsids. These formed Megadynasty 2.

    Unfortunately for the therapsids, their cousins the diapsids were also productively evolving,in their case into dinosaurs (among other things), which gradually proved too much for thetherapsids. Uo pete head to head with these aggressive new creatures, thetherapsids by and large vanished from the record. A very few, however, evolved into small,furry, burrowing beings that bided their time for a very long while as little mammals. Thebiggest of them grew ner than a house cat, and most were no bigger than mice.

    Eventually, this would prove their salvation, but they would have to wait nearly 150 millionyears fadynasty 3, the Age of Dinosaurs, to e to an abrupt end and make room fadynasty 4 and our own Age of Mammals.

    Each of these massive transformations, as well as many smaller ones between and since,was depe on that paradoxically important motor ress: extin. It is a curiousfact that oh species death is, in the most literal sense, a way of life. No one knows howmany species anisms have existed since life began. Thirty billion is a only citedfigure, but the number has been put as high as 4,000 billion. Whatever the actual total, 99.99pert of all species that have ever lived are no longer with us. “To a first approximation,” asDavid Raup of the Uy of Chicago likes to say, “all species are extinct.” For plexanisms, the average lifespan of a species is only about four million years—roughly aboutwhere we are now.

    Extin is always bad news for the victims, of course, but it appears to be a good thingfor a dynamic pla. “The alternative to extin is stagnation,” says Ian Tattersall of theAmeri Museum of Natural History, “and stagnation is seldom a good thing in any realm.”

    (I should perhaps hat we are speaking here of extin as a natural, long-term process.

    Extin brought about by human carelessness is another matter altogether.)Crises ih’s history are invariably associated with dramatic leaps afterward. The fall ofthe Edia fauna was followed by the creative outburst of the Cambrian period. TheOrdovi extin of 440 million years ago cleared the os of a lot of immobile filterfeeders and, somehow, created ditions that favored darting fi<var></var>sh and giant aquatic reptiles.

    These in turn were in an ideal position to send ists onto dry land when another blowoutie Devonian period gave life another sound shaking. And so it has go scatteredintervals through history. If most of these events hadn’t happened just as they did, just whenthey did, we almost certainly wouldn’t be here now.

    Earth has seen five major extin episodes in its time—the Ordovi, Devonian,Permian, Triassid Cretaceous, in that order—and many smaller ohe Ordovi(440 million years ago) and Devonian (365 million) each wiped out about 80 to 85 pert ofspecies. The Triassic (210 million years ago) and Cretaceous (65 million years) each wipedout 70 to 75 pert of species. But the real whopper was the Permiain of about 245million years ago, which raised the curtain on the long age of the dinosaurs. In the Permian, atleast 95 pert of animals known from the fossil record check out, o return. Evenabout a third of i species went—the only occasion on which they were lost en masse. It isas close as we have ever e to total obliteration.

    “It was, truly, a mass extin, a age of a magnitude that had roubled the Earthbefore,” says Richard Fortey. The Permia articularly devastating to sea creatures.

    Trilobites vanished altogether. Clams and sea urs nearly went. Virtually all other marineanisms were staggered. Altogether, on land and ier, it is thought that Earth lost 52pert of its families—that’s the level above genus and below order on the grand scale of life(the subject of the  chapter)—and perhaps as many as 96 pert of all its species. Itwould be a long time—as much as eighty million years by one reing—before speciestotals recovered.

    Two points o be kept in mind. First, these are all just informed guesses. Estimates forthe number of animal species alive at the end of the Permian range from as low as 45,000 toas high as 240,000. If you don’t know how many species were alive, you  hardly specifywith vi the proportion that perished. Moreover, we are talking about the death ofspecies, not individuals. For individuals the death toll could be much higher—in many cases,practically total. The species that survived to the  phase of life’s lottery almost certainlyowe their existeo a few scarred and limping survivors.

    Iween the big kill-offs, there have also been many smaller, less well-knowinepisodes—the Hemphillian, Frasnian, Famennian, Rancholabrean, and a dozen or so others—which were not so devastating to total species numbers, but often critically hit certainpopulations. Grazing animals, including horses, were nearly wiped out in the Hemphillia about five million years ago. Horses deed to a single species, which appears sosporadically in the fossil record as to suggest that for a time it teetered on the brink ofoblivion. Imagine a human history without horses, without grazing animals.

    In nearly every case, for both big extins and more modest ones, we have bewilderinglylittle idea of what the cause was. Even after stripping out the more crackpot notions there arestill more theories for what caused the extin events than there have bees. At leasttwo dozen potential culprits have beeified as causes or prime tributlobalwarming, global cooling, ging sea levels, oxygeion of the seas (a ditionknown as anoxia), epidemics, giant leaks of methane gas from the seafloor, meteor and etimpacts, runaway hurries of a type known as hyperes, huge volic upwellings,catastrophic solar flares.

    This last is a particularly intriguing possibility. Nobody knows how big solar flares  getbecause we have only been watg them sihe beginning of the space age, but the Sun isa mighty engine and its storms are ensurately enormous. A typical solar flare—something we wouldn’t even noti Earth—will release the energy equivalent of a billionhydrogen bombs and fling into space a hundred billion tons or so of murderous high-energyparticles. The magosphere and atmosphere between them normally swat these batospace or steer them safely toward the poles (where they produce the Earth’s ely auroras),but it is thought that an unusually big blast, say a huimes the typical flare, couldoverwhelm our ethereal defehe light show would be a glorious one, but it would almostcertainly kill a very high proportion of all that basked in its glow. Moreover, and ratherchillingly, acc to Bruce Tsurutani of the NASA Jet Propulsion Laboratory, “it wouldleave no tra history.”

    What all this leaves us with, as one researcher has put it, is “tons of jecture and verylittle evidence.” Cooling seems to be associated with at least three of the big extinevents—the Ordovi, Devonian, and Permian—but beyond that little is agreed, includiher a particular episode happened swiftly or slowly. Stists ’t agree, for instance,whether the late Devoniain—the event that was followed by vertebrates movingonto the land—happened over millions of years or thousands of years or in one lively day.

    One of the reasons it is so hard to produce ving explanations for extins is that itis so very hard to exterminate life on a grand scale. As we have seen from the Manson impact,you  receive a ferocious blow and still stage a full, if presumably somewhat wobbly,recovery. So why, out of all the thousands of impacts Earth has endured, was the KT event sosingularly devastating? Well, first itositively enormous. It struck with the force of 100millioons. Su outburst is not easily imagined, but as James Lawrence Powell haspointed out, if you exploded one Hiroshima-sized bomb for every person alive oh todayyou would still be about a billion bombs short of the size of the KT impact. But even thatalone may not have been enough to wipe out 70 pert of Earth’s life, dinosaurs included.

    The KT meteor had the additional advantage—advantage if you are a mammal, that is—that it landed in a shallow sea just teers deep, probably at just the right a a timewhen oxygen levels were 10 pert higher than at present and so the world was morebustible. Above all the floor of the sea where it landed was made of rock ri sulfur.

    The result was an impact that turned an area of seafloor the size of Belgium into aerosols ofsulfuric acid. For months afterward, the Earth was subjected to rains acid enough to burn skin.

    In a sense, an eveer question than that of what wiped out 70 pert of the speciesthat were existing at the time is how did the remaining 30 pert survive? Why was the eventso irremediably devastating to every single dinosaur that existed, while other reptiles, likesnakes and crocodiles, passed through unimpeded? So far as we  tell no species of toad,, salamander, or other amphibia extin North America. “Why should thesedelicate creatures have emerged unscathed from su unparalleled disaster?” asks TimFlannery in his fasating prehistory of America, Eternal Frontier.

    In the seas it was much the same story. All the ammonites vanished, but their cousins thenautiloids, who lived similar lifestyles, swam on. Among plankton, some species werepractically wiped out—92 pert of foraminiferans, for instance—while anisms likediatoms, desigo a similar plan and living alongside, were paratively unscathed.

    These are difficult insistencies. As Richard Fortey observes: “Somehow it does notseem satisfying just to call them ‘lucky ones’ and leave it at that.” If, as seems entirely likely,the event was followed by months of dark and choking smoke, then many of the isurvivors bee difficult to at for. “Some is, like beetles,” Fortey notes, “couldlive on wood or other things lying around. But what about those like bees that navigate bysunlight and need pollen? Explaining their survival isn’t so easy.”

    Above all, there are the corals. Corals require algae to survive and algae require sunlight,and both together require steady minimum temperatures. Much publicity has been given i few years to corals dying from ges iemperature of only a degree or so. If theyare that vulnerable to small ges, how did they survive the long impact winter?

    There are also many hard-to-explain regional variatioins seem to have been farless severe in the southern hemisphere than the northern. New Zealand in particular appears tohave e through largely unscathed even though it had almost no burrowing creatures. Evenits vegetation was overwhelmingly spared, ahe scale of flagration elsewheresuggests that devastation was global. In short, there is just a great deal we don’t know.

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

    As Flannery he period immediately after the dinosaur extin could well be knownas the Age of Turtles. Sixteen species survived in North Amerid three more came ience soon after.

    Clearly it helped to be at home in water. The KT impact wiped out almost 90 pert ofland-based species but only 10 pert of those living in fresh water. Water obviously offeredprote against heat and flame, but also presumably provided more sustenan the leanperiod that followed. All the land-based animals that survived had a habit of retreating to asafer enviro during times of danger—into water or undergrouher of whichwould have provided siderable shelter against the ravages without. Animals thatsged for a living would also have enjoyed an advantage. Lizards were, and are, largelyimpervious to the bacteria in rotting carcasses. Indeed, often they are positively drawn to it,and for a long while there were clearly a lot of putrid carcasses about.

    It is often wrongly stated that only small animals survived the KT event. In fact, among thesurvivors were crocodiles, which were not just large but three times larger than they are today.

    But on the whole, it is true, most of the survivors were small and furtive. Indeed, with theworld dark and hostile, it erfect time to be small, warm-blooded, noal, flexible i, and cautious by nature—the very qualities that distinguished our mammalian forebears.

    Had our evolution been more advanced, we would probably have been wiped out. Instead,mammals found themselves in a world to which they were as well suited as anything alive.

    However, it wasn’t as if mammals swarmed forward to fill every niche. “Evolution mayabhor a vacuum,” wrote the paleobiologist Steven M. Stanley, “but it often takes a long timeto fill it.” For perhaps as many as ten million years mammals remained cautiously small. Inthe early Tertiary, if you were the size of a bobcat you could be king.

    But ohey got going, mammals expanded prodigiously—sometimes to an almostpreposterous degree. For a time, there were guinea pigs the size of rhinos and rhinos the sizeof a two-story house. Wherever there was a va the predatory , mammals rose(often literally) to fill it. Early members of the ra family migrated to South America,discovered a vacy, and evolved into creatures the size and ferocity of bears. Birds, too,prospered disproportionately. For millions of years, a gigantic, flightless, ivorous birdcalled Titanis ossibly the most fero></a>ous creature in North America. Certainly it was themost daunting bird that ever lived. It stood te high, weighed ht hundred pounds,and had a beak that could tear the head off pretty muything that irked it. Its familysurvived in formidable fashion for fifty million years, yet until a skeleton was discovered inFlorida in 1963, we had no idea that it had ever existed.

    Which brings us to another reason for our uainty about extins: the paltriness ofthe fossil record. We have touched already on the unlikelihood of a of bones beingfossilized, but the record is actually worse than you might think. sider dinosaurs.

    Museums give the impression that we have a global abundance of dinosaur fossils. In fact,overwhelmingly museum displays are artificial. The giant Diplodocus that domiheentrance hall of the Natural History Museum in London and has delighted and infeions of visitors is made of plaster—built in 1903 in Pittsburgh and preseo themuseum by Andrew egie. The entrance hall of the Ameri Museum of Natural Historyin New York is dominated by an even graableau: a skeleton of a large Barosaurusdefending her baby from attack by a darting and toothy Allosaurus. It is a wonderfullyimpressive display—the Barosaurus rises perhaps thirty feet toward the high ceiling—but alsoentirely fake. Every one of the several hundred bones in the display is a cast. Visit almost anylarge natural history museum in the world—in Paris, Vienna, Frankfurt, Buenos Aires,Mexico City—and what will greet you are antique models, not a bones.

    The fact is, we don’t really know a great deal about the dinosaurs. For the whole of the Ageof Dinosaurs, fewer than a thousand species have beeified (almost half of them knownfrom a single spe), which is about a quarter of the number of mammal species alivenow. Dinosaurs, bear in mind, ruled the Earth fhly three times as long as mammalshave, so either dinosaurs were remarkably unproductive of species or we have barelyscratched the surface (to use an irresistibly apt cliché).

    For millions of years through the Age of Dinosaurs not a single fossil has yet been found.

    Even for the period of the late Cretaceous—the most studied prehistoric period there is,thanks to our long i in dinosaurs and their extin—some three quarters of allspecies that lived may yet be undiscovered. Animals bulkier than the Diplodooreforbidding than tyrannosaurus may have roamed the Earth ihousands, and we maynever know it. Until very retly everything known about the dinosaurs of this period camefrom only about three hundred spes representing just sixteen species. The stiness ofthe record led to the widespread belief that dinosaurs were on their way out already whe impact occurred.

    Ie 1980s a paleontologist from the Milwaukee Public Museum, Peter Sheehan,decided to du experiment. Using two hundred volunteers, he made a painstakingsus of a well-defined, but also well-picked-over, area of the famous Hell Creek formationin Montana. Siftiiculously, the volunteers collected every last tooth aebra andchip of bone—everything that had been overlooked by previous diggers. The work took threeyears. When fihey found that they had more than tripled the global total of dinosaurfossils from the late Cretaceous. The survey established that dinosaurs remained numerht up to the time of the KT impact. “There is no reason to believe that the dinosaurs weredying out gradually during the last three million years of the Cretaceous,” Sheehaed.

    We are so used to the notion of our owability as life’s dominant species that it ishard to grasp that we are here only because of timely extraterrestrial bangs and other randomflukes. The ohing we have in on with all other living things is that for nearly fourbillion years our aors have mao slip through a series of closing doors every timewe hem to. Stephen Jay Gould expressed it suctly in a well-known line: “Humansare here today because our particular line never fractured—never o any of the billionpoints that could have erased us from history.”

    We started this chapter with three points: Life wants to be; life doesn’t always want to bemuch; life from time to time goes extinct. To this we may add a fourth: Life goes on. Andoften, as we shall see, it goes on in ways that are decidedly amazing.

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