The Sixth Extinction

By Elizabeth Kolbert

14 min read

Why does it matter? Because the extinction is already happening, and the cause is us.

You're probably waiting for the extinction to begin. Waiting for the moment — the meltdown, the tipping point, the sky-darkening impact — that announces the crisis has officially arrived. Here's what Elizabeth Kolbert wants you to understand: it started the moment modern humans first walked out of Africa, and the instrument of destruction isn't a comet or a warming atmosphere or a careless corporation. It's you. Specifically, the cognitive machinery that lets you read these words, cooperate with strangers, and imagine futures that don't exist yet. So what is it about that machinery — the restlessness that drove humans across open ocean to find Easter Island, the creativity that produces cathedrals and vaccines — that makes our species so uniquely lethal to every other one? The Sixth Extinction isn't coming. It's the story of what we already are.

A Species Goes Extinct While You Watch: The Panamanian Golden Frog

Edgardo Griffith tends to his frogs the way a sailor tends to a leaking ship. At the El Valle Amphibian Conservation Center in Panama, he moves between rows of glass tanks — tall ones for canopy species, short ones for ground-dwellers, a few dozen reserved for the Panamanian golden frog — checking seals, monitoring water filtered and chemically treated before it touches anything alive. No bag or piece of field equipment enters without disinfection. The frogs themselves had to be bathed in bleach solution just to get through the door. Griffith calls the place an ark mid-deluge, and that's exactly what it feels like: sealed, purposeful, and terrified.

The golden frog used to be everywhere in Panama. It appeared on lottery tickets. Souvenir shops sold ceramic figurines of it smoking cigarettes. Then, in the late 1990s, a wave of disappearances began moving west to east across the country. By 2004 it reached the forests around El Valle. By the time Kolbert arrived, the species was gone from the wild.

The killer is a fungus called Bd, which attacks amphibian skin — the organ frogs use to regulate their heartbeat through electrolyte exchange. Interfere with that, and the heart stops. What makes Bd so devastating is its patience: it doesn't need a living host to survive. It persists in the forest soil indefinitely, waiting. A lineage that had outlasted every mass extinction for 400 million years is now being erased at an estimated 45,000 times the background rate — not by a comet but by a fungus catching rides on global trade routes. Which means Griffith's frogs, swimming in their filtered tanks, cannot go back. Release them into the hills of El Valle and they die. You can disinfect a tank. You cannot disinfect a rainforest.

A herpetologist named Paul Crump, working on a project that might someday allow a reintroduction, described his own goal out loud and heard what it actually sounded like: keeping these animals alive until the forests are safe again, and then returning them so everything can be as it once was — "which now that I say it out loud," he admitted, "sounds kind of stupid."

Humans Spent Centuries Refusing to Believe Extinction Was Even Real

The idea that a species could simply cease to exist—permanently, irrevocably, gone—seems obvious now. It wasn't. For most of Western intellectual history, the possibility was literally unthinkable, ruled out in advance by theology and philosophy alike. Every creature occupied a link in the divine 'chain of being,' and a broken chain was a contradiction in terms. When naturalist Carl Linnaeus catalogued the living world in 1758, he made no category for species that no longer existed. In his framework, there was only one kind of animal: the kind that was alive.

The bones that broke this framework had been sitting in European collections for decades before anyone understood what they meant. The mastodon teeth that French troops hauled out of an Ohio riverbed in 1739—enormous, knobby molars as heavy as bricks—baffled every naturalist who examined them. The femur looked like an elephant's. The teeth looked like nothing on Earth. Thomas Jefferson, writing in the 1780s, decided the creature was simply still out there. Nature, he argued, was too economical to let an entire race perish. When he sent Lewis and Clark west in 1804, one of his private hopes was that they would encounter living specimens of the beast in the unexplored territories. He was not being credulous—he was expressing the consensus of his age.

Georges Cuvier ended that consensus through sheer anatomical precision. Arriving at the Paris natural history museum in 1795, the twenty-five-year-old examined the Ohio bones alongside elephant skulls from Ceylon and Africa. He noticed that the two elephant skulls were themselves distinct species—their teeth arranged in different patterns, one like festooned ribbons, one like diamonds. If the living world contained such distinctions invisible to the naked eye, the fossil record might too. Applying the same logic outward, he showed that the mammoth, the mastodon, and a twelve-foot skeleton from Argentina that he named Megatherium were not wandering somewhere undiscovered. They were gone. By 1800 he had assembled a 'fossil zoo' of twenty-three extinct species, and he concluded that life on Earth had been periodically shattered by catastrophes so extreme that entire branches of creation were cut short.

He was mocked for this. Then vindicated. The mastodon vanished roughly 13,000 years ago during a wave of megafauna extinctions that coincided precisely with the spread of modern humans across the continents. The 'catastrophe' Cuvier sensed lurking just before recorded history was not a flood. It was us—arriving, hunting, and leaving silence behind.

The Asteroid Proved Cuvier Right: Extinction Can Be Sudden, Total, and Random

The physical evidence of the rupture sits in a gorge outside Gubbio, Italy: a half-inch layer of clay wedged between two utterly different fossil worlds. Below the clay, the limestone is packed with large, diverse foraminifera — tiny shelled marine creatures that had thrived for millions of years. Above it, only a handful of miniature survivor species remain. When geologist Walter Alvarez and his physicist father Luis tested this clay layer in the late 1970s, they found iridium concentrations that were, in their words, off the charts. Iridium is vanishingly rare in Earth's crust but common in space rock. Their conclusion — that a six-mile-wide asteroid had struck the planet — was met with professional ridicule. Paleontologists trained in the Lyellian tradition, in which change is always slow and the fossil record just looks abrupt because it's incomplete, called the idea 'codswallop.' One editorial warned astronomers to leave earthly causes to earthly scientists. But shocked quartz, tsunami deposits, and finally a hundred-mile-wide crater buried under the Yucatán Peninsula eventually forced a reckoning. The bolide hit at roughly 45,000 miles per hour, blasted more than fifty times its own mass into the atmosphere, and generated enough radiant heat to broil the planet's surface. Picture something large and warm-blooded standing in what is now Alberta: it had about two minutes.

The ammonites are the clearest lesson in what survival actually meant after that day. They had inhabited the oceans for 300 million years, built shells that could withstand tremendous water pressure, and were still thriving right up to the boundary. By any Darwinian measure, they were fit. Their cousins the nautiluses were not obviously superior. But ammonites hatched from tiny eggs into larvae that drifted near the ocean surface — exactly where the post-impact chemistry turned lethal. Nautiluses laid large eggs in deep water, and their young emerged as miniature adults already swimming below the devastated surface layer. One group survived. One didn't. The difference had nothing to do with adaptation. It was a matter of where their babies happened to float.

We Are Acidifying the Ocean Faster Than Any Event in 300 Million Years

What if the bigger threat to ocean life isn't warming temperatures but something most people barely think about? Off the coast of Naples, there's a small rocky island called Castello Aragonese where volcanic vents push almost pure carbon dioxide up through the seafloor. Marine biologist Jason Hall-Spencer realized that the pH gradient those vents create — from normal seawater near the island's edges down to something far more acidic near the bubbles — is a time machine. Swim toward the vents and you swim into the ocean of 2100.

What he found there should reframe how you think about the coming century. In the normal-pH zones, the seafloor is dense with life: barnacles, mussels, limpets, sea urchins, corals, dozens of species of seaweed. Move into the zone where pH has dropped to 7.8 — the level that business-as-usual emissions are projected to deliver globally by the end of this century — and a third of those species simply aren't there. The barnacles are gone. So are the mussels. The limpets that do cling near the vents have shells worn nearly transparent, eaten away by the surrounding water. Hall-Spencer noted that the vents have been running for centuries. Any creature that could adapt to lower pH already would have. They haven't adapted. They're absent.

The chemistry is more literal than people expect. Ocean acidification doesn't make the water hostile in some vague sense — it steals the raw materials that shell-building creatures need. Calcifiers, any organism that constructs a shell or skeleton from calcium carbonate, must actively concentrate carbonate ions at the construction site to get them to bond at all. As CO2 dissolves into the ocean and forms carbonic acid, it consumes those carbonate ions, forcing calcifiers to spend more and more energy just to maintain the chemistry they need. Eventually the math goes negative: the water starts dissolving shells faster than animals can build them.

Since the Industrial Revolution, humans have dropped ocean surface pH from 8.2 to 8.1 — which sounds trivial until you remember the scale is logarithmic. That single decimal point represents a 30 percent increase in acidity. At current rates, pH hits 7.8 by 2100, a 150 percent increase from pre-industrial levels. To understand how anomalous that speed is, you need to go back 300 million years. Geologist Bärbel Hönisch reviewed that entire span of ocean chemistry and found nothing that matches what is happening right now. Even the volcanism that triggered the end-Permian extinction — the event that nearly ended multicellular life entirely — almost certainly released carbon more slowly than our cars and power plants do today. We are not replaying a past catastrophe. We are running one that has no precedent.

Fragmentation Turns Forests into Islands — and Islands Always Lose Species

Habitat destruction doesn't kill species outright. It puts them on a clock — one that runs for decades or centuries before the bill comes due.

Ecologists call this extinction debt, and it explains why the crisis is simultaneously urgent and nearly invisible. When a forest gets fragmented into isolated patches, the species inside don't vanish immediately. They linger. But the math of small populations is ruthless. A fragment that once held twenty breeding pairs of a particular bird now holds two. Two pairs can survive a bad season. They cannot survive ten bad seasons in a row, and given enough time, ten bad seasons in a row will happen. A coin that comes up heads ten times is unlikely — but flip it long enough and it becomes inevitable. In a continuous forest, local extinction gets erased by recolonization: survivors from neighboring populations drift in and fill the gap. In a fragment surrounded by cleared land or roads, there are no neighboring populations. The gap stays empty. What looks like stability is actually a slow countdown.

A 2004 study by ecologist Chris Thomas and colleagues in Nature put a number on the debt being accumulated right now by climate change, which creates fragments not of land but of suitable climate. His team modeled two scenarios: one where species are essentially immobile as temperatures rise, and one where they can track shifting conditions freely. In the pessimistic case, between 38 and 52 percent of species would be committed to extinction by 2050. In the optimistic case, 9 to 13 percent. Splitting the difference across plausible warming, the number settles around 24 percent. A paleontologist summarizing the finding put it simply: look around you and kill a quarter of what you see. That's the commitment already being made, even if the deaths don't arrive for generations.

The timescale is the vertiginous part. Species currently reproducing, currently filling their ecological roles, currently invisible to any census of crisis, are already gone in any meaningful sense. The debt exists. The collection just hasn't started.

Humans Have Reassembled the Continents — and Unleashed Biological Chaos

Al Hicks first noticed the problem in March 2007, when bats near Albany, New York started appearing with white powder caked around their muzzles — as if someone had dipped their faces in flour. The substance was a cold-loving fungus called Geomyces destructans, and it was killing little brown bats at a rate that should have been statistically impossible. When Kolbert accompanied Hicks to Aeolus Cave in Vermont — once the largest bat hibernaculum in New England, home to nearly 300,000 animals — she squeezed through iron slats in the entrance and immediately slipped on a pile of dead bat corpses. Ice formations studded the floor, and bats were frozen into them. A researcher collecting specimens for the American Museum of Natural History explained why he was doing it: bats had wintered in that cave since the last ice age, unbroken generation after generation, and this might be the last chance to record what had lived there. 'That's what makes this so dramatic,' he said. 'It's breaking the evolutionary chain.'

The fungus didn't evolve alongside North American bats. It was introduced from Europe, almost certainly on someone's gear, and American bats had no coevolved defenses against it. That's the recurring signature of what biologists are calling a New Pangaea. For hundreds of millions of years, the physical separation of the continents — oceans, mountain ranges, climate barriers — drove the evolution of distinct faunas and floras on every landmass. That separation is the reason a bat in Vermont and a bat in Germany can face the same pathogen so differently. Those geographic barriers are also the engine of biological richness: erasing them doesn't just move species around, it erases the conditions that made species different in the first place. Humans have dismantled them. Ten thousand species move daily in ballast water alone, before you count the freight, the luggage, the soil on hiking boots.

Most introductions fail. But out of every hundred species transplanted into a new environment, roughly one finds conditions it was built for — and finds them without any of the predators or parasites that kept it in check back home. The Aeolus bats were that unlucky one in a hundred, on the receiving end. Ecologist John Alroy has estimated that a fully homogenized world — one where every barrier has been erased — would sustain 66 percent fewer mammalian species than exist today. The war isn't being fought on one front. It's everywhere at once.

The Sixth Extinction Began 13,000 Years Ago — With Us

Picture a crime committed so slowly that no single generation could witness it. The perpetrator kills once every few weeks. After a thousand years, the giants are gone, and the forest has already forgotten they were there. No one remembers the before.

That's the calculation John Alroy, a paleobiologist at Macquarie University, ran on the disappearance of North America's Pleistocene megafauna. His mathematical model showed that a founding band of a hundred hunters — stone tools, no agriculture — could drive an entire continent's large mammals to extinction simply by killing one slow-reproducing giant every few weeks. The math is merciless. Large mammals survive by outgrowing predation, not by outbreeding it. An elephant gestates for twenty-two months. It doesn't twin. It doesn't reproduce until its teens. These constraints, irrelevant for millions of years because nothing could take down an adult elephant anyway, became catastrophic the moment a predator arrived that wasn't constrained by the same biology. Humans don't get slower as prey gets larger. The size that made a mammoth safe from everything else made it maximally vulnerable to us.

The sediment record at Lynch's Crater in Australia sealed the case. Scientists drilling down through the lake bottom found a layer where spores from a fungus called Sporormiella — which grows exclusively in the dung of large herbivores — simply vanish. Shortly after that disappearance, the vegetation shifts: browse plants decline, fire-adapted scrub takes over. The animals didn't die because the environment changed. The environment changed because the animals were killed. The sequence is there in the mud, dated and ordered.

This reframes everything. The Sixth Extinction didn't begin with the Industrial Revolution or the chainsaw. It began roughly 13,000 years ago when modern humans first stepped onto continents whose megafauna had never encountered a tool-using, cooperative hunter. Industrial civilization isn't the origin of the crisis — it's an acceleration of a pattern as old as our species. We have always been, wherever we arrive, the thing the giants had no answer for.

The 'Madness Gene': Our Greatest Strength Is the Planet's Biggest Threat

What if the cause of the Sixth Extinction turned out to be not the worst of us but the best?

Svante Pääbo, the geneticist who spent years coaxing readable DNA from thirty-thousand-year-old Neanderthal bone fragments, keeps returning to a particular puzzle. Neanderthals were not our inferiors in any obvious sense. They were physically tougher, made sophisticated stone tools, cared for their injured, and lived in Europe for over a hundred thousand years without meaningfully changing it. Modern humans arrived and, within a geological eyeblink, the Neanderthals were gone, the megafauna were gone, and the remodeling of the entire planet was underway. Something had shifted. Pääbo believes it was genetic — a mutation, or a cluster of mutations, that produced what he calls human madness. His favorite illustration: how many people must have launched themselves into the open Pacific before someone found Easter Island? Dozens of canoes. Hundreds of sailors. Swallowed by the ocean, no trace. And still the departures continued. Pääbo doesn't know whether that restlessness was driven by curiosity or glory-seeking or something else entirely, but he's certain it was there in the DNA — and that it changed every ecosystem the species ever touched.

The same architecture that makes this madness possible is language. The paleontologist Michael Benton observed that human language works like the genetic code: information gets stored, modified, and passed down across generations. The difference is speed. Biological evolution operates over thousands of generations. Cultural transmission operates over one. A hunter who discovers that mammoths can be driven off a cliff doesn't have to wait for that knowledge to be encoded in her descendants' neurons — she tells someone. That compression of learning is what let a small band of anatomically modern humans reorder an entire continent's fauna in a few centuries.

Kolbert ends not by pointing at a poacher or a logger but at the reader. You, she suggests, holding a book, participating in the vast cooperative network of language and culture that is human civilization, are a more honest image of the cause than any villain. The Sixth Extinction isn't driven by ignorance or malice. It's driven by restlessness and creativity and the ability to transmit ideas across time — the very traits that make us recognizably human. The cave painters at Grotte des Combarelles who crawled through darkness to engrave aurochs and woolly rhinos on the walls — those rhinos are now in the fossil record too. The capacity to represent the world turned out to contain, all along, the capacity to end it.

The Asteroid That Drives Itself to Work

Here is the uncomfortable place Kolbert leaves you: somewhere in the world right now, Joel Sartore is photographing the last of a species against a white backdrop, and Edgardo Griffith is checking the seals on his frog tanks in El Valle, and you are holding a book about it. The problem isn't that we forgot to care, or that we didn't know. The problem is the thing that makes you capable of reading this sentence — the accumulated, transmitted, restless intelligence that let one species talk its way across every continent and ocean on the planet. You cannot excise that from the species without losing the species. Which means the Sixth Extinction isn't a problem civilization created while doing something wrong. It's the shadow civilization casts while doing what it does. What remains — and what Kolbert refuses to resolve for you — is whether knowing that changes anything at all.