The fossil record contains the unambiguous evidence of five separate occasions on which most of the species alive at the time died, in geological terms, very quickly. According to the Natural History Museum’s reference on mass extinctions, a mass extinction is conventionally defined as an event in which roughly 75 percent of the world’s species are lost over a short period of geological time — less than 2.8 million years. The Big Five mass extinctions are the End-Ordovician (~445 million years ago, glaciation and sea-level collapse), the Late Devonian (~360-375 million years ago, marine anoxia), the End-Permian “Great Dying” (~252 million years ago, the worst single event in life’s history), the End-Triassic (~201 million years ago, large-scale volcanism), and the End-Cretaceous K-Pg event (~66 million years ago, the asteroid impact that ended the non-avian dinosaurs).
Each of these events removed most of the multicellular biosphere over a span ranging from a few tens of thousands of years to perhaps a million years. Each was caused by some external disruption to the planet’s climate, atmosphere, or oceans: glaciation, marine de-oxygenation, runaway volcanic CO₂, or a large extraterrestrial impact. Each was followed by a long recovery interval, on the order of millions to tens of millions of years, during which surviving lineages diversified into the ecological roles left empty by the extinct species. The current biosphere is largely a product of the most recent of these recoveries. Mammals, including humans, occupy ecological space that became available when the dinosaurs died.
The case for a sixth extinction
The argument that the planet is now in the early stages of a sixth mass extinction has accumulated substantial peer-reviewed support over the past several decades, although it remains contested. The case is built on three observations.
The first is the extinction rate. A 2015 paper in Science Advances by Gerardo Ceballos of the National Autonomous University of Mexico, Paul Ehrlich of Stanford and colleagues looked at the documented loss of vertebrate species since 1900. The team identified 477 vertebrate extinctions in the modern record, against a background rate that would predict approximately 9. Even using deliberately conservative assumptions, including a background rate twice as high as previous estimates, the team concluded that current vertebrate extinction rates are between 8 and 100 times higher than the rate that would prevail in the absence of human activity. Under the background rate, the number of species lost in the past century would have taken between 800 and 10,000 years to disappear, depending on the taxonomic group.
The second is the cumulative loss. A 2022 review published in Biological Reviews by Robert Cowie of the University of Hawai’i at Mānoa and colleagues at the Muséum National d’Histoire Naturelle in Paris extrapolated from invertebrate extinction data, particularly from land snails and slugs, to estimate that Earth may already have lost between 7.5 and 13 percent of its known two million species since the year 1500, or between 150,000 and 260,000 species. The Cowie team argued that previous estimates focusing on charismatic vertebrates had substantially understated the scale of loss, because most of life’s diversity is invertebrate and most invertebrate extinctions are not officially documented.
The third is the cause. Where the previous five mass extinctions were driven by external geological forces, the current acceleration of extinction is the consequence of a single species. Human activity has converted approximately half of the planet’s habitable land surface to agricultural and urban use, raised atmospheric CO₂ concentrations by about 50 percent above pre-industrial levels, acidified the surface oceans by roughly 30 percent, and introduced invasive species, novel chemical pollutants, and direct hunting pressure across nearly every habitat on Earth. The Cretaceous-Paleogene extinction was caused by an event lasting hours. The Permian extinction was caused by a million-year volcanic episode. The sixth extinction, if it is occurring, is being caused by a few centuries of industrial expansion.
The case against
The framing has been challenged in the peer-reviewed literature, most recently by John Wiens of the University of Arizona and Kristen Saban of Harvard. A 2025 paper by Wiens and Saban in PLOS Biology, summarised in CNN’s coverage, examined extinctions at the genus level — a higher taxonomic rank than the species — using data from over 163,000 plant and animal species in the International Union for Conservation of Nature’s database. The team found that 102 genera had gone extinct over the past 500 years, representing less than 2 percent of mammal genera and less than 0.5 percent of all assessed genera. By the 75-percent threshold that defines a mass extinction in the geological record, the current crisis is nowhere close. Most documented extinctions, the Wiens and Saban paper noted, have involved species confined to islands rather than continental ecosystems.
The Wiens and Saban critique does not deny that biodiversity is declining sharply or that human activity is the cause. The point is more specifically about whether the current decline meets the technical definition of a mass extinction. Wiens has argued that the term should be reserved for events that demonstrate a 75-percent species loss, and that calling the current crisis a “sixth mass extinction” overstates what the data so far supports. The dispute is over framing rather than over the underlying observations of accelerating biodiversity loss.
What both sides agree on
The disagreement among biologists about whether a sixth mass extinction has begun should not obscure what is not in dispute. Current extinction rates are far above geological background. The cause is overwhelmingly anthropogenic, including habitat loss, climate change, pollution, invasive species, and direct exploitation. The trajectory of biodiversity loss is accelerating rather than slowing. Many species are functionally extinct in their natural ranges, even when small captive populations or remnant wild populations technically meet the criteria for not-yet-extinct status. The disagreement is about whether what is happening now is best categorised alongside the End-Permian or as a serious but historically distinct biodiversity crisis.
The difference matters for how the situation is communicated and what kinds of policy responses are warranted, but it does not materially change the underlying picture. By either framing, a substantial fraction of Earth’s species are being driven to extinction or to near-extinction over a time interval that is unprecedented in human history and brief by geological standards. The previous five mass extinctions were caused by climate change driven by volcanism, by ocean chemistry shifts driven by tectonic processes, and in one case by a six-mile-wide asteroid hitting the Yucatán. The current crisis is the first in Earth’s history caused by the deliberate and incidental activities of a species that emerged less than half a million years ago and that, on most accounts, has the option of choosing differently.
