Earth has experienced five confirmed mass extinctions over the past 440 million years, and many scientists argue we are now living through a sixth. The “Big Five” were first identified in 1982 through analysis of more than 36,000 marine invertebrate fossils. The proposed sixth extinction is driven not by asteroids or volcanic eruptions but by human activity. Together, these six events frame the story of life on Earth: how it was nearly destroyed and how it rebuilt itself each time.
What Counts as a Mass Extinction
A mass extinction is defined by two criteria: magnitude and rate. At least 75% of all species must disappear within a geologically short window, typically less than two million years. That threshold matters because species go extinct all the time at a slow, steady “background rate.” Mass extinctions are fundamentally different. They represent a collapse of global ecosystems, not the normal turnover of life.
Interestingly, a 75% drop in species doesn’t always require a spike in die-offs. If the creation of new species slows dramatically while normal extinction continues, biodiversity can drain away just as effectively. Scientists sometimes call these events “mass depletions,” but they’re treated the same way.
1. The Ordovician-Silurian Extinction (443 Million Years Ago)
The oldest of the Big Five wiped out roughly 85% of marine organisms. Life at this point was almost entirely in the oceans, and the trigger was a massive glaciation across the ancient supercontinent Gondwana. As ice sheets expanded, global temperatures plummeted, sea levels dropped, and the shallow coastal habitats where most marine life thrived simply vanished.
The extinction unfolded in at least two pulses. The first wave was tied to the glaciation and habitat loss. The second appears connected to the opposite problem: as glaciers eventually retreated and sea levels rose again, oxygen levels in the water dropped sharply. Recent research points to volcanic activity as a contributing factor in this second wave, adding toxic conditions to an ocean already under stress.
2. The Late Devonian Extinction (375–359 Million Years Ago)
The Late Devonian extinction killed 70–82% of marine invertebrate species, but it wasn’t a single catastrophe. It played out over roughly 16 million years in two major pulses. The first struck around 374 million years ago at the boundary between two geologic stages. The second, called the Hangenberg Event, hit near the transition from the Devonian to the Carboniferous period around 359 million years ago.
Widespread oxygen depletion in the oceans was a central feature of both crises. The seas became suffocating for most complex life. The causes likely involved a combination of factors: changes in sea level, shifts in ocean chemistry, and possibly the rapid spread of land plants, whose roots accelerated rock weathering and flushed nutrients into the water. That nutrient overload could have triggered massive algal blooms that consumed the ocean’s oxygen. Reef ecosystems were devastated and took millions of years to recover.
3. The Permian-Triassic Extinction (252 Million Years Ago)
Known as the “Great Dying,” this was the worst catastrophe life has ever endured. Roughly 96% of marine species and 70% of land vertebrate species were eliminated. The primary culprit was the Siberian Traps, one of the largest volcanic events in Earth’s history. Over hundreds of thousands of years, eruptions poured lava across millions of square kilometers of what is now Siberia.
The eruptions didn’t kill directly so much as they poisoned the planet’s climate systems. Massive releases of carbon dioxide drove intense global warming, while sulfur emissions caused wild temperature swings, disrupted ocean circulation, and altered the water cycle. The oceans warmed, lost oxygen, and became acidic. Climate model simulations show that sulfur and carbon gases from the Siberian Traps combined to create a cascading series of environmental changes, pushing both land and sea ecosystems past their breaking point. Recovery from the Great Dying took longer than any other extinction event.
4. The Triassic-Jurassic Extinction (201 Million Years Ago)
About 76% of all species disappeared at the end of the Triassic, clearing the ecological stage for dinosaurs to dominate the Jurassic. The cause was another massive volcanic episode: the Central Atlantic Magmatic Province, or CAMP, which erupted as the supercontinent Pangaea began splitting apart to form the Atlantic Ocean.
At least 3 million cubic kilometers of basaltic magma were erupted or forced into the crust across an area of 10 million square kilometers. The eruptions came in rapid pulses lasting only a few centuries each, and research on gas bubbles trapped in the ancient basalt confirms that enormous quantities of CO2 were released, much of it originating from deep in the Earth’s crust or mantle. Each individual pulse may have injected CO2 into the atmosphere comparable to the total anthropogenic emissions projected for the 21st century. The result was severe global warming and ocean acidification that collapsed food webs on land and in the sea.
5. The Cretaceous-Paleogene Extinction (66 Million Years Ago)
This is the one most people know: an asteroid roughly 10 kilometers wide slammed into what is now Mexico’s Yucatán Peninsula, forming the Chicxulub crater. It killed the non-avian dinosaurs along with about 76% of all species. The extinction played out in stages over a geologically brief period.
In the first hours, the impact triggered earthquakes, tsunamis, and a “fireball stage” as ejected debris rained back through the atmosphere, generating an intense heat pulse that ignited wildfires across the globe. Then came the longer nightmare. Dust and sulfur aerosols from the vaporized rock, combined with soot from burning organic matter, blocked sunlight so severely that only about 20% of normal solar radiation reached the surface. This “impact winter” shut down photosynthesis, collapsing food chains from the bottom up. Plants died, herbivores starved, and predators followed. Small mammals, birds, crocodilians, and many insects survived, likely because they could shelter, hibernate, or subsist on seeds and detritus. Their survival set the stage for the age of mammals.
6. The Holocene Extinction (Now)
The proposed sixth mass extinction is happening in real time. Unlike the previous five, it is driven almost entirely by one species: us. The average rate of vertebrate species loss over the past century is up to 100 times higher than the natural background rate. Using conservative estimates, the number of species that have gone extinct since 1900 would have taken between 800 and 10,000 years to disappear under normal conditions. Without human influence, only about nine vertebrate extinctions would have been expected in that time.
The primary drivers are habitat destruction and conversion, the introduction of invasive species, pollution, overexploitation, and climate change. Habitat loss is the most immediate threat: as forests are cleared, wetlands drained, and land converted for agriculture and development, species with small ranges and specialized needs lose the places they depend on. Invasive species, often transported by global trade, outcompete or prey on native wildlife. Climate change amplifies all of these pressures, pushing species toward the poles or up mountainsides, and those that can’t move fast enough face elimination.
Amphibians illustrate the crisis clearly. They are hit by habitat modification, pesticides, pollution, introduced predators, and an emerging fungal disease, all compounded by warming temperatures. Tropical mountain species are at special risk because they have nowhere higher to go. The pattern repeats across mammals, birds, reptiles, and freshwater fish.
How Long Recovery Takes
After each of the Big Five, life eventually bounced back, but not quickly by any human measure. Research on marine fossil records shows it takes at least 10 million years for biodiversity to fully recover after a mass extinction. This appears to be a consistent “speed limit” imposed by the pace of evolution itself. New species simply cannot arise fast enough to replace what was lost in less time. Studies of single-celled marine organisms after the Cretaceous-Paleogene extinction confirmed this pattern: 10 million years after the asteroid impact, overall diversity had nearly returned to pre-extinction levels.
That timeline puts the current crisis in sharp perspective. Whatever species are lost in the coming centuries will not be replaced on any timescale meaningful to human civilization. The previous five extinctions were driven by forces beyond any organism’s control. The sixth is the first where the cause has the capacity to change course.