Extinction — the complete disappearance of a species — is a normal part of life on Earth. Scientists estimate that 99% of all species that have ever existed are now extinct. The average lifespan of a species in the fossil record is about 1–10 million years before it either evolves into something different or disappears entirely.
Natural extinction
Species go extinct naturally when their environment changes and they can't adapt quickly enough. Climate shifts, sea level changes, disease, competition from new species, or just bad luck with a small population. Extinction is the other side of the evolutionary coin — the species that fail to adapt die out, clearing space for new ones.
Think of evolution as a slow, continuous tournament. Species are constantly competing, adapting, and being replaced. Most players eventually get knocked out — either eaten by better competitors, outlasted in resource competition, or wiped out by environmental change. Being knocked out is the expected outcome for most players over long enough time. The remarkable thing is that life as a whole keeps going, with new players constantly entering the tournament.
Mass extinctions
Five times in Earth's history, a catastrophic event wiped out the majority of species in a geologically short period. The most famous was 66 million years ago — a 10km asteroid struck the Yucatán Peninsula in Mexico, triggering fires, debris clouds that blocked sunlight, and cooling that killed about 75% of species, including all non-avian dinosaurs. The most severe was the "Great Dying" 252 million years ago, which wiped out 96% of marine species and 70% of terrestrial vertebrate species, probably caused by massive volcanic eruptions in Siberia.
What's happening now?
We are in a sixth mass extinction, driven this time not by asteroids or volcanoes but by one species: us. Species are currently going extinct at an estimated 100–1,000 times the natural background rate. The main causes are habitat destruction (particularly deforestation), overexploitation (hunting, fishing), invasive species, pollution, and climate change. Unlike the asteroid event, this one has a knowable cause — and therefore, in principle, a solution.
Extinction means when all animals of one type die forever. This is normal on Earth. Scientists think 99% of all animals that ever lived are now extinct. Most animal types live for 1 to 10 million years. Then they either change into something new or disappear completely.
Natural extinction
Animals go extinct naturally when their home changes too fast. They cannot change quickly enough to survive. The weather might get too hot or cold. The sea might rise or fall. New diseases might spread. New animals might move in and take their food. Sometimes small groups of animals just have bad luck. Extinction is like the other side of a coin from evolution. Animals that cannot change fast enough die out. This makes room for new animals.
Think of evolution like a long school sports tournament. Animal types are always competing and changing. Most teams eventually get knocked out of the tournament. Better teams might beat them. Other teams might be better at finding food. Big changes to the playing field might knock them out. Getting knocked out is normal for most teams over a very long time. The amazing thing is that the tournament keeps going. New teams keep joining all the time.
Mass extinctions
Five times in Earth's history, something terrible killed most animals very quickly. The most famous happened 66 million years ago. A huge space rock hit Mexico. It was 10 kilometres wide! This started massive fires. Dust blocked out the sun. The Earth got very cold. About 75% of all animals died. This included all the big dinosaurs. The worst extinction was 252 million years ago. It was called the "Great Dying". It killed 96% of sea animals and 70% of land animals with backbones. Massive volcanoes in Russia probably caused this.
What's happening now?
We are in a sixth mass extinction right now. This time it's not space rocks or volcanoes causing it. It's one animal type: humans. Animals are dying 100 to 1,000 times faster than normal. The main reasons are destroying animal homes like forests. We also hunt and fish too much. We bring new animals to places where they don't belong. We make pollution and change the climate. Unlike the space rock, we know what's causing this extinction. This means we might be able to fix it.