Something like 99.9% of every species that has ever lived is gone. Extinction is not the exception in the history of life — it is the norm, and the average species lasts somewhere between one and ten million years before it disappears. So the interesting question is not whether species die out, but why do species go extinct as quickly as they do, and whether the pace right now is unusual.
The Short Answer
Species go extinct when they cannot adapt fast enough to a changing environment, cannot move somewhere better, and run out of individuals. Three ways out, three ways to fail. The trigger can be anything: a volcano, an asteroid, a new predator, a shifting climate, or the loss of the one habitat the species needs.
The Big Five
| Event | When | Estimated species lost | Likely cause |
|---|---|---|---|
| End-Ordovician | ~445 million years ago | ~85% | Glaciation, sea level drop |
| Late Devonian | ~372 million years ago | ~75% | Poorly understood; possibly land-plant-driven anoxia |
| End-Permian (the Great Dying) | ~252 million years ago | ~81-96% | Siberian Traps volcanism, CO2 spike, ocean acidification |
| End-Triassic | ~201 million years ago | ~80% | Central Atlantic volcanism |
| End-Cretaceous | 66 million years ago | ~76% | Chicxulub impact, Deccan volcanism |
The End-Permian was the worst. The Siberian Traps eruptions released enough carbon dioxide to push global temperatures up by roughly 8-10°C and acidify the oceans. Recovery took an estimated 10 million years.
What the Fossil Record Can and Cannot Tell Us
The record is biased. Hard shells and bones fossilise; soft bodies mostly do not. Marine animals on shallow seafloors are well represented; animals in forests or on mountaintops barely appear. And the further back you go, the more the rock has been deformed or destroyed.
That is why the percentages above have ranges. The order of severity is solid; the exact numbers are still being revised.
What Makes a Species Vulnerable
Extinction risk is not evenly distributed. Traits that make a species fragile include:
- Small range. A species on one island can be wiped out by a single storm, disease, or invasive predator.
- Specialisation. Giant pandas depend almost entirely on bamboo; koalas feed almost entirely on a few eucalyptus species.
- Slow reproduction. Large mammals and sharks cannot replace losses quickly.
- Large body size. Needs more food and more space, and is often a target for hunters.
- Low genetic diversity. Less raw material for adaptation. Cheetahs are so genetically similar that unrelated individuals can accept skin grafts from each other.
The Rate Right Now
Here the numbers get contested, and it is worth being precise about why. Background extinction rate — the natural baseline — is estimated at roughly 0.1 to 1 species per million species per year, usually written as 0.1 to 1 E/MSY.
That is the context in which why do species go extinct stops being a historical question and becomes a current one. Recent documented extinctions for assessed groups run above that baseline. The IUCN Red List has evaluated roughly 160,000 species and classified more than 44,000 as threatened with extinction. But we have assessed only a fraction of described species, and only a fraction of species on Earth have been described at all — estimates of the total run from a few million to over a billion.
So the defensible statement is this: measured extinction rates for the groups we have assessed exceed the background rate, and habitat loss, invasive species, and climate change are increasing the pressure. The popular claim that we are in a sixth mass extinction comparable to the Big Five is a real scientific position with real supporters, but it is a projection and an argument, not an established measurement.
Can Anything Come Back?
De-extinction is mostly a branding problem dressed up as a biology problem. Cloning requires intact DNA, which degrades — you cannot recover a dinosaur from amber, whatever the films suggest. Candidates like the woolly mammoth and the passenger pigeon are closer, because well-preserved tissue exists, but what you would get is a cold-tolerant elephant with mammoth traits or a pigeon with edited genes, not the original animal in its original ecosystem.
Meanwhile the ordinary tools work. Habitat protection, invasive species removal, and captive breeding have pulled species back — the California condor went from 22 birds in 1982 to several hundred today. It is slower and less cinematic than resurrection, and it is what actually works.
Keep Exploring
- What is natural selection? — the mechanism behind adaptation and failure
- What is a food chain? — what disappears with each lost species
- What causes global warming? — the pressure most often cited today
