The Sixth Extinction: Are We Living Through One?

On a quiet stretch of coast in 1844, three fishermen on the Icelandic island of Eldey clubbed the last confirmed breeding pair of great auks and crushed the single egg they were guarding. The great auk had once gathered in colonies of hundreds of thousands, a flightless seabird that swam like a penguin across the cold North Atlantic. Within a human lifetime, demand for its feathers, meat, and eggs erased it completely. There would be no more great auks anywhere on Earth, and there never will be again.

That small, brutal scene captures the question scientists now ask on a planetary scale. Extinction is normal: nearly all species that have ever existed are gone. But every so often, the geological record shows a moment when life itself nearly unravels, when extinctions pile up so fast and so widely that the rules of survival break down. Five such episodes are written into the rocks. The pressing question is whether a sixth is happening right now, and whether we are the cause.

What Counts as a Mass Extinction

Extinction happens constantly at a slow, steady pace that paleontologists call the background rate. Species appear, persist for a while, and disappear as climates shift and competitors evolve. A mass extinction is something else entirely: a relatively brief interval in which a large fraction of the planet's species vanish, far faster than new ones can arise to replace them.

Scientists generally reserve the term for events that wiped out roughly three-quarters or more of species across many different groups of organisms, on land and in the sea, more or less at the same time. The signature in the fossil record is unmistakable. Whole categories of life that flourished for tens of millions of years simply stop appearing in younger rock layers. The boundaries between geological periods are often drawn at exactly these catastrophes, because the cast of characters before and after is so different.

Only five events in the last 540 million years clear that high bar. Paleontologists call them the "Big Five," and each tells a story of how fragile even a thriving biosphere can be.

The Big Five, in Brief

First, the Late Ordovician (around 444 million years ago). Life was still almost entirely confined to the oceans. A pulse of intense global cooling and falling sea levels, followed by rapid warming, devastated marine communities of trilobites, brachiopods, and early reef builders. It is usually ranked as one of the most severe of all five in terms of the share of species lost.

Second, the Late Devonian (around 372 million years ago). This was less a single blow than a drawn-out crisis spread over millions of years, marked by widespread loss of oxygen in the seas. Reef ecosystems collapsed so thoroughly that nothing on the same scale would rebuild for a very long time.

Third, the end-Permian (around 252 million years ago). This is the catastrophe scientists call "the Great Dying," and it remains the most severe extinction event known. Estimates suggest something on the order of nine in ten marine species disappeared, along with the majority of land vertebrates and even many insects, which rarely suffer mass losses. The leading explanation points to colossal volcanic eruptions in what is now Siberia, which released enormous volumes of carbon dioxide and other gases, driving runaway warming, ocean acidification, and oxygen loss. Life took millions of years to recover its former diversity.

Fourth, the end-Triassic (around 201 million years ago). Another burst of massive volcanism, linked to the breakup of the supercontinent Pangaea, cleared the stage of many competitors. In the aftermath, the dinosaurs rose to dominate the land for the next 135 million years.

Fifth, the end-Cretaceous (around 66 million years ago). This is the famous one. A roughly ten-kilometer asteroid struck near the present-day Yucatan Peninsula in Mexico, leaving the buried Chicxulub crater as evidence. The impact, combined with its global aftermath of darkness, cooling, and disrupted food chains, ended the reign of the non-bird dinosaurs and killed an estimated three-quarters of all species. The survivors included the small mammals whose descendants, eventually, became us.

What "Now" Looks Like

Set against that deep history, the present moment looks unsettling. The International Union for Conservation of Nature, which maintains the Red List, the world's most comprehensive inventory of species' status, has assessed well over 150,000 species. More than 40,000 of them are currently classified as threatened with extinction. Amphibians are especially hard-hit, with a large share of frogs, toads, and salamanders at risk; sharks and rays, reef-building corals, and many freshwater species are also in steep decline.

Documented extinctions in recent centuries are real and sobering. The dodo of Mauritius, the passenger pigeon (which once darkened North American skies in flocks of billions before the last individual died in a Cincinnati zoo in 1914), the Tasmanian tiger, and the great auk are only the most famous names on a long list. Beyond outright extinction, scientists emphasize a quieter crisis: sharp drops in the abundance of animals that still technically survive. Wild populations of many vertebrate species have fallen dramatically over the past half century, a process researchers sometimes call defaunation, the emptying-out of ecosystems even where the species' name still appears on the books.

Are We Really in a Sixth Extinction?

Here is where careful language matters. By the strict geological definition, a mass extinction means losing roughly three-quarters of species in a geologically short window. We have not crossed that threshold. Most assessed species are not yet extinct, and the great auks and dodos, however tragic, number in the hundreds rather than the millions.

What concerns scientists is not the total reached so far but the rate and the trajectory. Comparing modern extinctions to the background rate inferred from the fossil record is difficult, because the two are measured in very different ways, and researchers debate the exact figures. Even so, multiple independent studies conclude that species are currently disappearing far faster than the long-term background pace, by a wide margin. If those elevated rates continue or accelerate, many biologists argue, the cumulative loss over the coming centuries could indeed reach mass-extinction levels. In that framing, we may be at the early stages of a sixth event rather than its peak.

So the honest answer is conditional. We are not yet living through a completed sixth mass extinction in the technical sense. We do appear to be living through a period of unusually rapid, human-driven biodiversity loss that has the potential to become one if current pressures are not eased. Scientists still debate precise numbers and timescales, but the direction of travel is widely agreed upon.

The Difference This Time

The five ancient catastrophes had non-human triggers: asteroid strikes, vast volcanic provinces, swings in sea level and ocean chemistry. The current decline has a different driver, and it is us. Conservation biologists often summarize the main pressures with a handful of categories.

Habitat loss is the largest. Forests cleared for agriculture, wetlands drained, grasslands plowed, and coral reefs degraded leave species with nowhere to live. Overexploitation through hunting, fishing, and harvesting pushed the great auk and the passenger pigeon over the edge and still threatens many large animals today. Invasive species, carried around the world by human trade and travel, devastate native wildlife that evolved without those predators or competitors, as happened to flightless island birds. Pollution, including agricultural runoff and plastics, poisons habitats. And climate change, driven by the same carbon dioxide buildup that played a role in ancient extinctions, shifts the conditions species depend on faster than many can adapt or migrate.

There is a sobering parallel here. In the Great Dying and several other past events, a rapid rise in atmospheric carbon dioxide, ocean acidification, and oxygen loss were central to the destruction. Today's carbon release, though from a different source, alters the atmosphere and oceans on a timescale that is extremely fast by geological standards.

Why It Matters, and What Can Be Done

It would be easy to treat extinction as an abstract loss, a thinning of the catalog of life. But biodiversity is the scaffolding of the systems that sustain us. Pollinating insects support a large share of the crops we eat. Healthy forests and oceans absorb carbon and regulate climate. Wetlands filter water; diverse soils grow food; coral reefs shelter fisheries that feed hundreds of millions of people. When species and populations vanish, those services fray.

The encouraging part is that this crisis, unlike an asteroid, has a cause we can influence. Conservation has genuine successes to point to. The American bison was reduced to a few hundred animals and brought back to tens of thousands. The bald eagle recovered after harmful pesticides were banned. The southern white rhino, the giant panda, and various whale species have rebounded from the brink through protection and managed recovery. Protected areas, restored habitats, hunting and trade restrictions, and the removal of invasive species have all demonstrably pulled species back from collapse. None of this reverses what is already lost, and the great auk is gone for good, but it shows that the trajectory is not fixed.

Key Takeaways

Earth's fossil record holds five mass extinctions, each wiping out roughly three-quarters or more of species, from the deep-ocean crises of the Ordovician and Devonian to the volcanic Great Dying of the Permian and the asteroid that ended the dinosaurs 66 million years ago. By that strict standard, we are not yet living through a completed sixth extinction: most species survive, and documented losses, though real and tragic, remain far below catastrophe levels. What alarms scientists is the rate and direction of change. Species today appear to be vanishing far faster than the long-term background pace, with more than 40,000 currently assessed as threatened and wild populations shrinking across the globe, all under pressures we created: habitat loss, overexploitation, invasive species, pollution, and climate change. Whether the present moment becomes a true sixth mass extinction depends largely on what we do next, and the documented recoveries of bison, eagles, and rhinos prove that the outcome, unlike an asteroid, is still partly in our hands.