A mass extinction is a catastrophic loss of biodiversity in which at least 75% of all species on Earth disappear within a geologically short window of time. Five such events have occurred over the past 450 million years, each fundamentally reshaping life on the planet. The concept isn’t just historical: scientists actively debate whether human activity is pushing Earth toward a sixth.
What Qualifies as a Mass Extinction
Species go extinct all the time. There’s a natural “background rate” of extinction that ticks along as populations decline, habitats shift, and evolution moves forward. A mass extinction is something categorically different: a spike in species loss so severe and so rapid (in geological terms) that it stands out unmistakably in the fossil record.
The widely used threshold is the loss of 75% or more of all species globally. That number isn’t arbitrary. It reflects the scale of destruction visible in the five events paleontologists have identified in Earth’s history. What makes these events distinctive isn’t just the number of species lost but the speed. Background extinction unfolds over tens of millions of years. Mass extinctions compress that loss into timescales ranging from thousands to a few million years, a blink in geological time.
Paleontologists detect these events by reading rock layers like a timeline. In strata corresponding to a mass extinction, older layers contain a rich diversity of fossils, while the layer immediately above is dramatically emptier. Often the rock itself changes composition at the boundary, reflecting the environmental upheaval that drove the die-off. The most famous example is a thin clay layer found in more than 350 sites around the world that marks the extinction of the dinosaurs.
The Big Five Extinctions
Five mass extinctions dominate Earth’s history, each with its own combination of triggers.
The End-Ordovician extinction (roughly 445 million years ago) was driven by the onset of glacial cycles and major shifts in sea level. As the Appalachian mountains rose, they altered atmospheric and ocean chemistry, compounding the stress on marine life.
The End-Devonian extinction (around 375 million years ago) played out over a longer, more drawn-out period. Climate change, possibly linked to the rapid spread of land plants, combined with plummeting oxygen levels in deep ocean waters to devastate marine ecosystems.
The End-Permian extinction (252 million years ago) was the worst of all, sometimes called “the Great Dying.” Massive volcanic eruptions triggered cascading climate change, ocean oxygen loss, and shifts in ocean and atmospheric chemistry. More than half of all taxonomic diversity vanished. Marine ecosystems were so thoroughly dismantled that it took roughly 30 million years for terrestrial animal communities to fully recover their diversity, a span that eventually gave rise to dinosaurs, pterosaurs, crocodilians, and early mammals.
The End-Triassic extinction (about 201 million years ago) was again tied to volcanic activity on a massive scale, clearing ecological space that allowed dinosaurs to become the dominant land animals.
The End-Cretaceous extinction (66 million years ago) is the most well-known. An asteroid roughly 12 kilometers in diameter struck what is now Mexico’s Yucatán Peninsula, creating the Chicxulub crater. The impact launched microscopic dust and vaporized meteorite material across the globe. Scientists know this because a thin layer of iridium, a metal rare in Earth’s crust but abundant in primitive asteroids, has been found in rock layers at the extinction boundary in hundreds of sites worldwide. Iridium concentrations in this layer are up to 10,000 times higher than normal crustal levels. Volcanic activity and climate change were also underway, but the asteroid was the decisive blow. It ended the reign of non-avian dinosaurs and opened the door for mammals.
How Mass Extinctions Kill
The specific triggers vary, but the killing mechanisms tend to overlap across events. Most mass extinctions involve some combination of rapid climate change, loss of oxygen in the oceans, and shifts in ocean chemistry that make it harder for marine organisms to build shells and skeletons.
Ocean acidification is a recurring theme. When large amounts of carbon dioxide enter the atmosphere, whether from volcanic eruptions or other sources, the ocean absorbs much of it. This lowers the water’s pH, making it more acidic. Organisms that build shells or skeletons from calcium carbonate are especially vulnerable. Lab experiments and observations near naturally acidic underwater volcanic vents show that lower pH leads to reduced growth rates, smaller body sizes, increased shell porosity, visible deformities, and faster dissolution of shells. The worst projected effects hit warm tropical and subtropical waters, which are also where much of marine biodiversity concentrates.
Ocean deoxygenation works alongside acidification. When deep waters lose oxygen, vast stretches of seafloor become uninhabitable. During the End-Permian event, the combination of these stresses didn’t just kill individual species. It destabilized entire food webs. Research using food-web models of ancient marine communities shows that after the first wave of Permian extinctions, ecosystem stability held up reasonably well despite major species losses. It was the second wave that caused a collapse in overall community stability, suggesting that ecosystems can absorb a surprising amount of damage before reaching a tipping point.
What Helps Species Survive
Not every species dies in a mass extinction, and the survivors tend to share certain traits. The most consistent predictor of survival, both in the fossil record and among species at risk today, is having a broad habitat range. Species that can live in many different environments are far more likely to persist when conditions change drastically. Conversely, high habitat specificity, needing very particular conditions to survive, is a near-universal predictor of extinction risk across vertebrates, invertebrates, and plants alike.
Other traits that improve survival odds include faster reproduction (high fecundity, shorter generation times, smaller offspring size) and tolerance for a wide range of altitudes. Among invertebrates and plants, good dispersal ability also helps. Body size, diet, and position in the food chain matter too, but their effects are less consistent and depend more on the specific group of organisms involved.
Are We in a Sixth Mass Extinction?
This is one of the most debated questions in modern biology. Hundreds of plant and animal species have gone extinct due to human activity over the past 500 years, and current extinction rates clearly exceed the natural background rate. Some prominent studies have argued that Earth is already in a sixth mass extinction comparable to the Big Five.
But the numbers tell a more complicated story. Fewer than 0.1% of Earth’s known species have actually gone extinct in the last 500 years. Reaching the 75% threshold that defines a mass extinction, based on recent extinction rates and roughly 2 million described species, would take somewhere between 400,000 and 800,000 years. If you include the estimated 8.7 million total species (most undiscovered), the timeline stretches to 3.6 million years. No plausible scenario currently projects a path to 75% species loss.
The patterns of recent extinctions also differ from the ancient ones. Most documented extinctions over the past five centuries occurred on islands, driven primarily by invasive species. On continents, habitat loss is the dominant cause, with freshwater species hit hardest. Extinction rates have actually declined over the last century compared to earlier periods, though this may partly reflect how difficult it is to confirm a species has truly disappeared.
None of this means the situation isn’t serious. The biodiversity crisis is real and accelerating in many ecosystems. But calling it a mass extinction in the technical sense requires meeting a quantitative bar that current data don’t support. The more accurate framing may be that we’re in an extinction crisis with the potential to become far worse, rather than one that already mirrors the catastrophes of the geological past.