We are so used to the notion of our own inevitability as life’s dominant species that it ishard to grasp that we are here only because of timely extraterrestrial bangs and other randomflukes. The one thing we have in common with all other living things is that for nearly fourbillion years our ancestors have managed to slip through a series of closing doors every timewe needed them to. Stephen Jay Gould expressed it succinctly in a well-known line: “Humansare here today because our particular line never fractured—never once at any of the billionpoints that could have erased us from history.”
But come we did, though reaching our present state of eminence has not of course alwaysbeen straightforward. Since life on land began, it has consisted of four megadynasties, as theyare sometimes called. The first consisted of primitive, plodding but sometimes fairly heftyamphibians and reptiles. The best-known animal of this age was the Dimetrodon, a sail-backed creature that is commonly confused with dinosaurs (including, I note, in a picturecaption in the Carl Sagan book Comet). 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.
Plants began the process of land colonization about 450 million years ago, accompanied ofnecessity by tiny mites and other organisms that they needed to break down and recycle deadorganic matter on their behalf. Larger animals took a little longer to emerge, but by about 400million years ago they were venturing out of the water, too. Popular illustrations haveencouraged us to envision the first venturesome land dwellers as a kind of ambitious fish—something like the modern mudskipper, which can 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 known as pillbugsor sow bugs. These are the little bugs (crustaceans, in fact) that are commonly thrown intoconfusion when you upturn a rock or log.
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 shortage of relevantfossils, but partly also because of an idiosyncratic Swede named Erik Jarvik whose oddinterpretations and secretive manner held back progress on this question for almost half acentury. Jarvik was part of a team of Scandinavian 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 ancestral to us and all other walking creatures, known as tetrapods.
Eventually, this would prove their salvation, but they would have to wait nearly 150 millionyears for Megadynasty 3, the Age of Dinosaurs, to come to an abrupt end and make room forMegadynasty 4 and our own Age of Mammals.
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 once they 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 vacancy in the predatory chain, mammals rose(often literally) to fill it. Early members of the raccoon family migrated to South America,discovered a vacancy, and evolved into creatures the size and ferocity of bears. Birds, too,prospered disproportionately. For millions of years, a gigantic, flightless, carnivorous birdcalled Titanis was possibly the most ferocious creature in North America. Certainly it was themost daunting bird that ever lived. It stood ten feet high, weighed over eight hundred pounds,and had a beak that could tear the head off pretty much anything 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.
Like most things that thrive in harsh environments, lichens are slow-growing. It may take alichen more than half a century to attain the dimensions of a shirt button. Those the size ofdinner plates, writes David Attenborough, are therefore “likely to be hundreds if notthousands of years old.” It would be hard to imagine a less fulfilling existence. “They simplyexist,” Attenborough 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 inclined to feel that lifemust have a point. We have plans and aspirations and desires. We want to take constantadvantage of all the intoxicating existence we’ve been endowed with. But what’s life to alichen? Yet its impulse to exist, to be, is every bit as strong as ours—arguably even stronger.
WHEN YOU CONSIDER 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.
(I should perhaps note that we are speaking here of extinction as a natural, long-term process.
Perhaps an even more effective way of grasping our extreme recentness as a part of this4.5-billion-year-old picture is to stretch your arms to their fullest extent and imagine thatwidth as the entire history of the Earth. On this scale, according to John McPhee in Basin andRange, the distance from the fingertips of one hand to the wrist of the other is Precambrian.
One of the reasons it is so hard to produce convincing explanations for extinctions is that itis so very hard to exterminate life on a grand scale. As we have seen from the Manson impact,you can 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 itwas positively enormous. It struck with the force of 100million megatons. Such an outburst is not easily imagined, but as James Lawrence Powell haspointed out, if you exploded one Hiroshima-sized bomb for every person alive on earth 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 percent of Earth’s life, dinosaurs included.
Which brings us to another reason for our uncertainty about extinctions: the paltriness ofthe fossil record. We have touched already on the unlikelihood of any set of bones becomingfossilized, but the record is actually worse than you might think. Consider dinosaurs.
Most animals are tetrapods, and all living tetrapods have one thing in common: four limbsthat end in a maximum of five fingers or toes. Dinosaurs, whales, birds, humans, even fish—all are tetrapods, which clearly suggests they come from a single common ancestor. The clueto this ancestor, 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 and kept at it for the nextforty-eight years. Unfortunately, Jarvik refused to let anyone study his tetrapod. The world’spaleontologists had to be content with two sketchy interim papers in which Jarvik noted thatthe creature had five fingers in each of four limbs, confirming its ancestral importance.
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.
If I were told that I had to spend decades being a furry growth on a rock in the woods, Ibelieve I would lose the will to go on. Lichens don’t. Like virtually all living things, they willsuffer any hardship, endure any insult, for a moment’s additional existence. Life, in short, justwants to be. But—and here’s an interesting point—for the most part it doesn’t want to bemuch.
These in turn were in an ideal position to send colonists onto dry land when another blowoutin the late Devonian period gave life another sound shaking. And so it has gone at 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.
Extinction is always bad news for the victims, of course, but it appears to be a good thingfor a dynamic planet. “The alternative to extinction is stagnation,” says Ian Tattersall of theAmerican Museum of Natural History, “and stagnation is seldom a good thing in any realm.”
But on the whole, it is true, most of the survivors were small and furtive. Indeed, with theworld dark and hostile, it was a perfect time to be small, warm-blooded, nocturnal, flexible indiet, and cautious by nature—the very qualities that distinguished our mammalian forebears.
These were not trivial challenges to overcome. On the other hand, there was a powerfulincentive to leave the water: it was getting dangerous down there. The slow fusion of thecontinents into a single landmass, Pangaea, meant there was much, much less coastline thanformerly and thus much less coastal habitat. So competition was fierce. There was also anomnivorous and unsettling new type of predator on the scene, one so perfectly designed forattack that it has scarcely changed in all the long eons since its emergence: the shark. Neverwould there be a more propitious time to find an alternative environment to water.
Over time, each of these principal groupings split into further subdivisions, of which someprospered and some faltered. Anapsids gave rise to the turtles, which for a time, perhaps atouch improbably, appeared poised to predominate as the planet’s most advanced and deadlyspecies, before an evolutionary lurch let them settle for durability rather than dominance. Thesynapsids divided into four streams, only one of which survived beyond the Permian.
Trilobites vanished altogether. Clams and sea urchins nearly went. Virtually all other marineorganisms were staggered. Altogether, on land and in the water, it is thought that Earth lost 52percent of its families—that’s the level above genus and below order on the grand scale of life(the subject of the next chapter)—and perhaps as many as 96 percent of all its species. Itwould be a long time—as much as eighty million years by one reckoning—before speciestotals recovered.
This last is a particularly intriguing possibility. Nobody knows how big solar flares can getbecause we have only been watching them since the beginning of the space age, but the Sun isa mighty engine and its storms are commensurately enormous. A typical solar flare—something we wouldn’t even notice on 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 magnetosphere and atmosphere between them normally swat these back intospace or steer them safely toward the poles (where they produce the Earth’s comely auroras),but it is thought that an unusually big blast, say a hundred times the typical flare, couldoverwhelm our ethereal defenses. The 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, according to Bruce Tsurutani of the NASA Jet Propulsion Laboratory, “it wouldleave no trace in history.”
Closer inspection showed that lichens were more interesting 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 convert into food sufficient to sustain both. It is not avery exciting arrangement, but it is a conspicuously successful one. The world has more thantwenty thousand species of lichens.
Even for the period of the late Cretaceous—the most studied prehistoric period there is,thanks to our long interest in dinosaurs and their extinction—some three quarters of allspecies that lived may yet be undiscovered. Animals bulkier than the Diplodocus or moreforbidding than tyrannosaurus may have roamed the Earth in the thousands, and we maynever know it. Until very recently everything known about the dinosaurs of this period camefrom only about three hundred specimens representing just sixteen species. The scantiness ofthe record led to the widespread belief that dinosaurs were on their way out already when theKT impact occurred.
This is perhaps a little odd because life has had plenty of time to develop ambitions. If youimagine the 4,500-billion-odd years of Earth’s history compressed into a normal earthly day,then life begins very early, about 4A.M., with the rise of the first simple, single-celledorganisms, but then advances no further for the next 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 minuteslater by the first jellyfish and the enigmatic Ediacaran fauna first seen by Reginald Sprigg inAustralia. At 9:04P.M. trilobites swim onto the scene, 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.
As Flannery notes, the period immediately after the dinosaur extinction could well be knownas the Age of Turtles. Sixteen species survived in North America and three more came intoexistence soon after.
Clearly it helped to be at home in water. The KT impact wiped out almost 90 percent ofland-based species but only 10 percent of those living in fresh water. Water obviously offeredprotection against heat and flame, but also presumably provided more sustenance in the leanperiod that followed. All the land-based animals that survived had a habit of retreating to asafer environment during times of danger—into water or underground—either of whichwould have provided considerable shelter against the ravages without. Animals thatscavenged 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.
With such creatures on the prowl, it is perhaps not surprising that insects in the periodevolved a trick that could keep them safely out of tongue shot: they learned to fly. Some tookto this new means of locomotion with such uncanny facility that they haven’t changed theirtechniques in all the time since. Then, as now, dragonflies could cruise at up to thirty-fivemiles an hour, instantly stop, hover, fly backwards, and lift far more proportionately than anyhuman flying machine. “The U.S. Air Force,” one commentator has written, “has put them inwind tunnels to see how they do it, and despaired.” They, too, gorged on the rich air. InCarboniferous forests dragonflies grew as big as ravens. Trees and other vegetation likewiseattained outsized proportions. Horsetails and tree ferns grew to heights of fifty feet, clubmosses to a hundred and thirty.
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.
Happily, that was the stream we belonged to, and it evolved into a family of protomammalsknown as therapsids. These formed Megadynasty 2.
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 in thelast few years to corals dying from changes in sea temperature of only a degree or so. If theyare that vulnerable to small changes, how did they survive the long impact winter?
Fortunately, that moment hasn’t happened, but the chances 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 one otherextremely pertinent quality about life on Earth: it goes extinct. Quite regularly. For all thetrouble they take to assemble and preserve themselves, species crumple and die remarkablyroutinely. And the more complex they get, the more quickly they appear to go extinct. Whichis perhaps one reason why so much of life isn’t terribly ambitious.
And how, you may reasonably wonder, can scientists 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 wrapped themselves inside tiny protective shells. Then, asnow, the plankton created their shells by drawing oxygen from the atmosphere and combiningit with other elements (carbon especially) to form durable compounds such as calciumcarbonate. It’s the same chemical trick that goes on in (and is discussed elsewhere in relationto) the long-term carbon cycle—a process that doesn’t make for terribly exciting narrative butis vital for creating a livable planet.
In a sense, an even greater question than that of what wiped out 70 percent of the speciesthat were existing at the time is how did the remaining 30 percent 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 can tell no species of toad,newt, salamander, or other amphibian went extinct in North America. “Why should thesedelicate creatures have emerged unscathed from such an unparalleled disaster?” asks TimFlannery in his fascinating prehistory of America, Eternal Frontier.
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.
These are difficult inconsistencies. 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 insectsurvivors become difficult to account for. “Some insects, 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.”
Museums give the impression that we have a global abundance of dinosaur fossils. In fact,overwhelmingly museum displays are artificial. The giant Diplodocus that dominates theentrance hall of the Natural History Museum in London and has delighted and informedgenerations of visitors is made of plaster—built in 1903 in Pittsburgh and presented to themuseum by Andrew Carnegie. The entrance hall of the American Museum of Natural Historyin New York is dominated by an even grander tableau: 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 ancient bones.
“It was, truly, a mass extinction, a carnage of a magnitude that had never troubled the Earthbefore,” says Richard Fortey. The Permian event was particularly devastating to sea creatures.
In the late 1980s a paleontologist from the Milwaukee Public Museum, Peter Sheehan,decided to conduct an experiment. Using two hundred volunteers, he made a painstakingcensus of a well-defined, but also well-picked-over, area of the famous Hell Creek formationin Montana. Sifting meticulously, the volunteers collected every last tooth and vertebra andchip of bone—everything that had been overlooked by previous diggers. The work took threeyears. When finished they found that they had more than tripled the global total of dinosaurfossils from the late Cretaceous. The survey established that dinosaurs remained numerousright 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,” Sheehan reported.
For millions of years through the Age of Dinosaurs not a single fossil has yet been found.
Jarvik died in 1998. After his death, other paleontologists eagerly examined the specimenand found that Jarvik had severely miscounted 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 under its own weight. Needless tosay, this did not do a great deal to advance our understanding 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.
Each of these massive transformations, as well as many smaller ones between and since,was dependent on that paradoxically important motor of progress: extinction. It is a curiousfact that on Earth species death is, in the most literal sense, a way of life. No one knows howmany species of organisms have existed since life began. Thirty billion is a commonly citedfigure, but the number has been put as high as 4,000 billion. Whatever the actual total, 99.99percent of all species that have ever lived are no longer with us. “To a first approximation,” asDavid Raup of the University of Chicago likes to say, “all species are extinct.” For complexorganisms, the average lifespan of a species is only about four million years—roughly aboutwhere we are now.
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 percent of foraminiferans, for instance—while other organisms likediatoms, designed to a similar plan and living alongside, were comparatively unscathed.
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 swamps, which by their boggy nature disrupted the normal carbon recyclingprocess. Instead of completely rotting down, falling fronds and other dead vegetative matteraccumulated in rich, wet sediments, which were eventually squeezed into the vast coal bedsthat sustain much economic activity even now.
In between the big kill-offs, there have also been many smaller, less well-known extinctionepisodes—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 Hemphillianevent about five million years ago. Horses declined 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.
There are also many hard-to-explain regional variations. Extinctions seem to have been farless severe in the southern hemisphere than the northern. New Zealand in particular appears tohave come through largely unscathed even though it had almost no burrowing creatures. Evenits vegetation was overwhelmingly spared, and yet the scale of conflagration 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.
All of complex life is in one hand, “and in a single stroke with a medium-grained nail file youcould eradicate human history.”
Consider the lichen. Lichens are just about the hardiest visible organisms on Earth, butamong the least ambitious. They will grow happily enough in a sunny churchyard, but theyparticularly thrive in environments where no other organism would go—on blowymountaintops and arctic wastes, wherever there is little but rock and rain and cold, and almostno competition. In areas of Antarctica where virtually nothing else will grow, you can findvast expanses of lichen—four hundred types of them—adhering devotedly to every wind-whipped rock.
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 been identified (almost half of them knownfrom a single specimen), which is about a quarter of the number of mammal species alivenow. Dinosaurs, bear in mind, ruled the Earth for roughly 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).
(If you have forgotten what an isotope is, it doesn’t matter, though for the record it’s an atomwith an abnormal number of neutrons.) This is where the geochemists come in, for theisotopes accumulate at different rates depending on how much oxygen or carbon dioxide is inthe atmosphere at the time of their creation. By comparing these ancient ratios, thegeochemists can cunningly read conditions in the ancient world—oxygen levels, air and oceantemperatures, the extent and timing of ice ages, and much else. By combining their isotopefindings with other fossil residues—pollen levels and so on—scientists can, with considerableconfidence, re-create entire landscapes that no human eye ever saw.
So anytime life does something bold it is quite an event, and few occasions were moreeventful than when life moved on to the next stage in our narrative and came out of the sea.
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.
For a long time, people couldn’t understand how they did it. Because lichens grew on barerock without evident nourishment or the production of seeds, many people—educatedpeople—believed they were stones caught in the process of becoming plants. “Spontaneously,inorganic stone becomes living plant!” rejoiced one observer, a Dr. Homschuch, in 1819.
For those that learned to breathe oxygen from the air, times were good. Oxygen levels inthe Devonian and Carboniferous periods, when terrestrial life first bloomed, were as high as35 percent (as opposed to nearer 20 percent now). This allowed animals to grow remarkablylarge remarkably quickly.
Two points need to 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 can hardly specifywith conviction 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 next phase of life’s lottery almost certainlyowe their existence to a few scarred and limping survivors.
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. Unable to compete 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 no larger than a house cat, and most were no bigger than mice.
The KT meteor had the additional advantage—advantage if you are a mammal, that is—that it landed in a shallow sea just ten meters deep, probably at just the right angle, at a timewhen oxygen levels were 10 percent higher than at present and so the world was morecombustible. Above all the floor of the sea where it landed was made of rock rich in sulfur.
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 insects areevident. Dinosaurs plod onto the scene just before 11P.M. and hold sway for about three-quarters of an hour. At twenty-one minutes to midnight they vanish and the age of mammalsbegins. Humans emerge one minute and seventeen seconds before midnight. The whole of ourrecorded history, on this scale, would be no more than a few seconds, a single human lifetimebarely an instant. Throughout this greatly speeded-up day continents slide about and bangtogether at a clip that seems positively reckless. Mountains rise and melt away, ocean basinscome and go, ice sheets advance and withdraw. And throughout the whole, about three timesevery minute, somewhere on the planet there is a flashbulb pop of light marking the impact ofa Manson-sized meteor or one even larger. It’s a wonder that anything at all can survive insuch a pummeled and unsettled environment. In fact, not many things do for long.
Earth has seen five major extinction episodes in its time—the Ordovician, Devonian,Permian, Triassic, and Cretaceous, in that order—and many smaller ones. The Ordovician(440 million years ago) and Devonian (365 million) each wiped out about 80 to 85 percent ofspecies. The Triassic (210 million years ago) and Cretaceous (65 million years) each wipedout 70 to 75 percent of species. But the real whopper was the Permian extinction of about 245million years ago, which raised the curtain on the long age of the dinosaurs. In the Permian, atleast 95 percent of animals known from the fossil record check out, never to return. Evenabout a third of insect species went—the only occasion on which they were lost en masse. It isas close as we have ever come to total obliteration.
Eventually in this process all the tiny organisms die and drift to the bottom of the sea,where they are slowly compressed into limestone. Among the tiny atomic structures theplankton take to the grave with them are two very stable isotopes—oxygen-16 and oxygen-18.
In nearly every case, for both big extinctions 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 extinction events than there have been events. At leasttwo dozen potential culprits have been identified as causes or prime contributors: globalwarming, global cooling, changing sea levels, oxygen depletion of the seas (a conditionknown as anoxia), epidemics, giant leaks of methane gas from the seafloor, meteor and cometimpacts, runaway hurricanes of a type known as hypercanes, huge volcanic upwellings,catastrophic solar flares.
What all this leaves us with, as one researcher has put it, is “tons of conjecture and verylittle evidence.” Cooling seems to be associated with at least three of the big extinctionevents—the Ordovician, Devonian, and Permian—but beyond that little is agreed, includingwhether a particular episode happened swiftly or slowly. Scientists can’t agree, for instance,whether the late Devonian extinction—the event that was followed by vertebrates movingonto the land—happened over millions of years or thousands of years or in one lively day.
The heady levels of oxygen clearly encouraged outsized growth. The oldest indication of asurface animal yet found is a track left 350million years ago by a millipede-like creature on arock in Scotland. It was over three feet long. Before the era was out some millipedes wouldreach lengths more than double that.
Extinction brought about by human carelessness is another matter altogether.)Crises in Earth’s history are invariably associated with dramatic leaps afterward. The fall ofthe Ediacaran fauna was followed by the creative outburst of the Cambrian period. TheOrdovician extinction of 440 million years ago cleared the oceans of a lot of immobile filterfeeders and, somehow, created conditions that favored darting fish and giant aquatic reptiles.
Land was a formidable environment: hot, dry, bathed in intense ultraviolet radiation,lacking the buoyancy that makes movement in water comparatively effortless. To live onland, creatures had to undergo wholesale revisions of their anatomies. Hold a fish at each endand it sags in the middle, its backbone too weak to support it. To survive out of water, marinecreatures needed to come up with new load-bearing internal architecture—not the sort ofadjustment that happens overnight. 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.
