Mass Extinctions in Earth's History
Five mass extinctions have reshaped life on Earth over the past 540 million years. Each eliminated at least 70% of marine species and triggered cascading reorganisation of terrestrial ecosystems. A sixth extinction, driven by human activity, is underway.
The big five (plus the Holocene)
Ordovician–Silurian extinction
Late Devonian extinction
Permian–Triassic extinction
Triassic–Jurassic extinction
Cretaceous–Paleogene extinction
Holocene (ongoing) extinction
Sources: Sepkoski compendium of marine genera; Raup & Sepkoski (1982) for the original big-five definition; Barnosky et al. (2011) on the Holocene extinction; Schulte et al. (2010) on the K–Pg impact. Species-loss percentages are for marine genera/species from the primary compendium.
What makes an extinction "mass"
Species extinction is continuous: at any time, lineages end and new ones emerge. A "mass" extinction is distinguished by rate — species vanishing faster than speciation can replace them — and by breadth, affecting multiple ecosystems and multiple taxonomic groups simultaneously.
The five events identified by Raup and Sepkoski in 1982 each eliminated between 70% and 96% of marine species within a geologically brief interval. Four of the five are linked to large igneous province volcanism (Siberian Traps, Deccan Traps, CAMP, and possibly the Viluy Traps), which released enough CO₂ and sulphur to destabilise climate and ocean chemistry. The fifth — the end-Cretaceous — was triggered or amplified by the Chicxulub asteroid impact.
The Holocene extinction, by contrast, is attributed primarily to human activity: overhunting of megafauna, habitat conversion, introduced species, and more recently climate change. Recent studies place current vertebrate extinction rates 100 to 1000 times above geological background — a pace consistent with a sixth mass extinction if sustained.