In 1739, Captain Charles Le Moyne was marching four hundred French and Indian troops down the Ohio River when he came across a sulphurous marsh where, as Elizabeth Kolbert puts it, ‘hundreds – perhaps thousands – of huge bones poked out of the muck, like spars of a ruined ship.’ The captain and his soldiers had no idea what sort of creatures the bones had supported, whether any of their living kin were nearby and, if so, what sort of threat they presented. The bones were similar to an elephant’s, but no one had seen anything like an elephant near the Ohio River, or indeed anywhere in the New World. Perhaps the animals had wandered off to the uncharted wilds out west? No one could say. The captain packed up a massive circular tusk, a three-foot-long femur and some ten-pound teeth, carried them around for several months as he went about the tricky task of eradicating the Chickasaw nation, and finally delivered the relics, after a stopover in New Orleans, to Paris, where they confounded naturalists for several decades.
A contemporary reader may guess, correctly, that the bones belonged to a species of animal that had long since ceased to exist – in fact, they came from the Mammut americanum, the American mastodon – but at the time such an imaginative leap would have been very difficult, because it hadn’t yet occurred to anyone that an entire species could cease to exist. ‘Aristotle wrote a ten-book History of Animals without ever considering the possibility that animals actually had a history,’ Kolbert writes, and in Linnaeus’s Systema Naturae, published four years before Le Moyne’s discovery, ‘there is really only one kind of animal – those that exist.’ The French naturalist Georges-Louis Leclerc thought the bones might belong to a species that, uniquely in history and for reasons unknown, had disappeared from the Earth, but his conjecture was widely rejected. Thomas Jefferson put forward the consensus view in 1781, in his Notes on the State of Virginia: ‘Such is the economy of nature, that no instance can be produced of her having permitted any one race of her animals to become extinct; of her having formed any link in her great work so weak as to be broken.’
In 1796, Georges Cuvier presented a new theory: nature did permit links to be broken, sometimes a lot of them all at once. Cuvier, just 27, was teaching at the Paris Museum of Natural History, one of the few institutions to survive the Terror, and had spent many hours studying its collection of fossils and bones. He noticed that the teeth of Le Moyne’s incognitum had unusual little bumps on them, like nipples. He was convinced they couldn’t be elephant teeth. He called their owner mastodonte, ‘breast tooth’. Other remains were similarly unmatched to the contemporary world: the elephant-sized ground sloth, called megatherium, bones of which had been discovered near Buenos Aires and reassembled in Madrid (Cuvier worked from sketches); the meat-eating aquatic lizard (now called mosasaurus), whose massive fossilised jaw had been picked out of a quarry near Maastricht; the woolly mammoth, whose frozen remains were everywhere in Siberia. Such creatures must have populated a lost world. ‘But what was this primitive earth?’ Cuvier asked. ‘And what revolution was able to wipe it out?’
These were interesting questions, but Cuvier’s contemporaries were slow to consider answers. More and more they were coming to accept that occasionally species might disappear – Darwin would soon propose that ‘the appearance of new forms and the disappearance of old forms’ were procedurally ‘bound together’ by natural selection – but evolution was a gradual process. Mass extinction, ‘revolution’, was something else. The claim that nature could undergo a sudden radical shift seemed not just historically unfounded but scientifically (and perhaps politically) untenable. Charles Lyell countered Cuvier’s anarchic ‘catastrophism’ with stately ‘uniformitarianism’. All change, geological or biological, took place gradually, steadily. Any talk of catastrophe, Lyell admonished, was ‘unphilosophical’.
The evidence of catastrophe accrued nonetheless. Geologists have understood since the 17th century that sedimentary layers of rock and soil mark the passage of time, the youngest layers at the top, the oldest at the bottom, and that sometimes geological forces will push up a slice of the world that contains several aeons’ worth of fossil-rich strata, which we can read like the lines of a census report. The fossils in most layers did indeed demonstrate a uniform degree of biodiversity, but some indicated a massive decline. Where once there were many different forms of life, suddenly there were few.
Palaeontologists and geologists now generally agree that the Earth has endured five major extinctions, and more than a dozen lesser ones. The first took place 450 million years ago, during the late Ordovician period, and the most lethal 200 million years later, during the Permian-Triassic – ‘the great dying’, when nine out of ten marine species vanished. The most terrifying of the mass extinctions, though, was surely the fifth, the Cretaceous-Palaeogene incident, which began 65 million years ago when an asteroid the size of Manhattan smashed into the Yucatan Peninsula with the explosive impact of a hundred million hydrogen bombs. The palaeobiologist Peter Ward, writing last year in Nautilus, calls it
life’s worst day on Earth, when the world’s global forest burned to the ground, absolute darkness from dust clouds encircled the earth for six months, acid rain burned the shells off of calcareous plankton, and a tsunami picked up all of the dinosaurs on the vast, Cretaceous coastal plains, drowned them, and then hurled their carcasses against whatever high elevations finally subsided the monster waves.
The lesson that mass extinction is normal is hard to accept. Scientists are beginning to recognise that we’re in the middle of another event, perhaps the sixth mass extinction, but that recognition has been slow in coming. In 1963, Colin Bertram, a marine biologist and polar explorer, warned that human expansion could destroy ‘most of the remaining larger mammals of the world, very many of the birds, the larger reptiles, and so many more both great and small’, and in 1979 the biologist Norman Myers published a little-read book called The Sinking Ark, showing with statistics that Bertram had been correct. But it wasn’t until the 1990s that large numbers of biologists began to take such concerns seriously. In 1991, the palaeobiologist David Jablonski published a paper in Science that compared the present rate of loss to that of previous mass extinctions. Other papers followed and by 1998 a survey by the American Museum of Natural History found that seven out of ten biologists suspected another mass extinction was underway. In 2008, two such biologists, David Wake and Vance Vredenburg, asked in a widely discussed paper, ‘Are We in the Midst of the Sixth Mass Extinction?’ The answer arrived in 2012 from a large team of biologists and palaeontologists writing in Nature: we almost certainly are. If we continue at the current rate of destruction, about three-quarters of all living species will be lost within the next few centuries.
Theories about what caused the earlier extinctions have varied – droughts, methane eruptions, volcanic ash, the ongoing problem of asteroids, the orbit of an invisible sun, our motion through the spirals of the Milky Way – but there’s little doubt about the culprit behind the sixth extinction. Wake and Vredenburg list the proximate causes: ‘human population growth, habitat conversion, global warming and its consequences, impacts of exotic species, new pathogens etc’. What most of these causes have in common isn’t just that they are the result of human activity, but that they have been going on for a very long time. Kolbert traces the way the human wake precisely matches the millennial waves of extinction:
The first pulse, about forty thousand years ago, took out Australia’s giants. A second pulse hit North America and South America some 25,000 years later. Madagascar’s giant lemurs, pygmy hippos and elephant birds survived all the way into the Middle Ages. New Zealand’s moas made it as far as the Renaissance. It’s hard to see how such a sequence could be squared with a single climate change event. The sequence of the pulses and the sequence of human settlement, meanwhile, line up almost exactly.
Despite the clear trend, it’s hard to say with any precision how many species are dying, or have died, or will die. One reason, as Darwin said, is that species come and go. The ordinary ‘background’ rate of extinction for mammals is about one every seven hundred years, and for amphibians a little higher. A second reason, not unrelated to the first, is that biologists have no real baseline for the current number of species. They can make good guesses, but the world is a big place, and biologists are getting better and better at finding new species. They may discover dozens in a day, but half of them will be in the process of dying out: it’s as if the biologists were going from room to room flicking on the lights in a house from which much of the life was rapidly scurrying.
They have seen enough, though, to draw a bleak picture. The historical record shows that the European lion, the Labrador duck and the passenger pigeons that once darkened the American prairie have all gone the way of the dodo (and the Pallas cormorant and the white-winged sandpiper and the Carolina parakeet). We know too that in recent years amphibians seem to have become especially endangered. (A 2007 study suggested that the current amphibian extinction rate was 45,000 times greater than the expected background rate.) And we also know, Kolbert writes, that ‘one-third of all reef-building corals, a third of all freshwater molluscs, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles and a sixth of all birds are headed towards oblivion.’ E.O. Wilson calculated that the current rate of extinction for all animals was ten thousand times greater than the background rate, a loss of biodiversity that is helping to create what the nature writer David Quammen memorably described as a ‘planet of weeds’, a simple world where ‘weedy’ animals – pigeons, rats, squirrels – thrive and little else remains.
Most of The Sixth Extinction is about dead or dying animals: the great auk, little brown bats, Neanderthals, sea snails, the Sumatran rhino. Such a book should be depressing, but Kolbert’s isn’t, largely because our attention is drawn not just to the work of destruction but also to the work of discovery. The asteroid impact that wiped out the dinosaurs, for instance: how did we come to know about such an unlikely event? In the 1970s, Walter Alvarez, a geologist, was studying the Gola del Bottaccione, a gorge in Perugia. Tectonic activity had lifted and tilted the ancient Italian limestone 45 degrees, and visitors could hike past a hundred million years of strata in just a few hundred yards. About halfway up, which is to say about 65 million years ago, was a puzzling half-inch clay stratum – the exposed Cretaceous-Palaeogene (K-Pg) boundary. In the limestone below was evidence of bountiful Cretaceous life; in the limestone above, quite a lot less Palaeogene life. A good uniformitarian would argue that the clay marked the passage of a very long period of time, enough for a stately adjustment of the census. Alvarez was a good uniformitarian but he was also curious: how long? He took some samples home to California and mentioned the mystery to his father. Luis Alvarez, a physicist at the Lawrence Berkeley Laboratory, had won the Nobel Prize in 1968 for discovering (in his son’s apposite description) ‘a whole zoo of subatomic particles’. He liked interesting challenges. He had just probed one of the pyramids at Giza with cosmic rays in the hope of finding secret chambers. (There were none.) Why not determine how much time was compressed into the clay layer by using another radiometric technique? Fine meteorite dust, which could be identified by its high concentration of iridium, settles on the Earth at a steady rate. A layer with more iridium would be a layer that had accumulated over a longer period. Detecting such infinitesimal traces wouldn’t be easy, but Luis had a former student who could do it. They sent the samples out for testing and the results were startling. The clay contained much more iridium than anyone expected: much more than would ordinarily be found anywhere.
Now father and son were really interested. They studied other samples from the K-Pg boundary, from Denmark and New Zealand. The results were the same. A hypothesis occurred to them: asteroid impact. They wrote it up for Science, which published ‘Extraterrestrial Cause for the Cretaceous-Tertiary Extinction’ in 1980. The world was interested, but unconvinced. More data needed. The Alvarezes realised that an impact would leave a kind of fingerprint, in the form of ‘shocked quartz’, a pressure deformation that geologists had first noticed around the sites of underground nuclear tests. They looked for that fingerprint in the records of thousands of core samples from around the world, and were able to zero in on a possible centre of impact on the Yucatan Peninsula, where geologists had previously discovered, then forgotten, a hundred-mile-wide crater hidden under a half-mile of sediment: Chicxulub. This time scientists were more persuaded. The last to get on board was the press. ‘Astronomers should leave to astrologers the task of seeking the cause of earthly events in the stars,’ the editors of the New York Times wrote. ‘Complex events seldom have simple explanations.’ Walter Alvarez wrote back and told them their claim was contradicted by the entire history of physics.
The evidence of the sixth extinction has been more direct. In the 1960s, David Wake studied the toads that densely populated the Sierra Nevada in Panama. ‘You’d be walking through meadows, and you’d inadvertently step on them,’ he told Kolbert. In the 1980s, his students in the field began to report that toads were nowhere to be found. Wake assumed they were looking in the wrong places. He went down to see for himself and ‘found like two toads’. Other herpetologists were reporting similar amphibian crashes: the golden toad of Costa Rica, the southern day frog of Australia, even the blue poison-dart frogs raised in captivity at the National Zoo in Washington DC. What was happening? The culprit, veterinary pathologists discovered, was Batrachochytrium dendrobatidis. The fungus, which makes it difficult for amphibians to soak up the electrolytes they need to prevent their hearts from stopping, was spreading by way of the ships that connect all the watery parts of the world. On any given day, the ballast water of the global fleet may contain as many as ten thousand different species, any one of which might, once blasted from the bilge, make war on some new world.
The fifth extinction was caused by an asteroid, the sixth by man. The comparison is unflattering. One doesn’t wish to be revealed as unthinkingly, irredeemably murderous. Balzac suggested that Cuvier’s discoveries made him the greatest poet of the century, because he had ‘reconstructed worlds from a whitened bone; rebuilt, like Cadmus, cities from a tooth’. But Cuvier’s greater achievement, perhaps, was simply to recognise so subtle a disruption in the pattern of existence. We have always been very conscious of our own mortality, and now we’re waking up to the existence of another, slower type of death. It’s an important recognition. Palaeontologists have found Neanderthal bones everywhere from Israel to Wales, and agree that the species died out suddenly, about thirty thousand years ago, which is suspiciously close to the time that Homo sapiens began its expansion from Africa. One theory is that clever man simply murdered his stronger cousin. But there are other theories. Maybe we simply outhunted our cousins, or carried a disease that was novel to them. Or maybe our contribution to their demise was even more indirect; animals with a long reproductive cycle are vulnerable to even the slightest of disruptions. John Alroy, an American palaeobiologist, has run computer simulations that suggest it would take just a tiny bit of interference with the Neanderthal birth rate, over the course of a few thousand years, to drive it to extinction. Alroy called this a ‘geologically instantaneous ecological catastrophe too gradual to be perceived by the people who unleashed it’. Such imperception is no longer possible.