It's easy to think of mass extinctions as events that suddenly, instantly destroy most life on Earth. In reality, most mass extinctions take place over millions of years. Lots of plants, animals and microscopic organisms gradually die as a result of a massive stress on the ecosystem. This eventually leads to the extinction of many life forms.
When studying these extinctions, researchers look at life forms in groups using scientific classifications. These classifications organize life forms according to traits they have in common. From biggest to smallest, these groups are domain, kingdom, phylum, class, order, family, genus and species. Researchers usually examine how mass extinctions affected families and genera to determine the extent of the extinction.
Researchers disagree about exactly how many mass extinctions have struck the planet. However, most agree that there have been five primary mass extinctions:
- The Ordovician extinction took place about 490 million years ago. Glacier formation caused sea levels to drop, which led to the extinction of about half of all animal families.
- The cause of the late Devonian extinction is still under debate. Because of it, about a quarter of marine families and more than half of marine genera became extinct. The late Devonian extinction took place about 360 million years ago.
- The Permian-Triassic extinction was the biggest mass extinction of all. Nearly 85 percent of marine genera and 70 percent of all land species became extinct. This extinction happened 250 million years ago, and there are numerous theories about its cause.
- Volcanoes were the likely culprit in the end-Triassic extinction, which killed about 20 percent of marine families and half of marine genera 200 million years ago.
- The most famous mass extinction is the Cretaceous-Tertiary event, also known as the K-T event. This is the extinction that led to the end of the dinosaurs 65 million years ago. The current hypothesis is that an asteroid impact off the coast of what is now Mexico either caused or contributed heavily to the K-T event.
In each of these mass extinctions, some type of event placed extreme stress on the world's ecosystems. Large groups of animals died, making room for new life. After each mass extinction, new species arose. Survivors of the extinction also thrived, taking advantage of the newly available room and resources. It's because of these extinctions that life on Earth looks like it does today.
However, the Earth might not look this way for long. Next, we'll look at how extinctions are affecting the world today and whether humans are the cause of a sixth major mass extinction.