There have been five big mass extinctions in Earth’s history – these are called the “Big Five”. Understanding the reasons and timelines of these events is important to understand the speed and scale of species extinctions today.

When and why did these mass extinction events happen?

What is a mass extinction?

First, we must be clear on what we mean by “mass extinction”. Extinctions are a normal part of evolution: they occur naturally and periodically over time.¹

There’s a natural background rate to the timing and frequency of extinctions: 10% of species are lost every million years, 30% every 10 million years, and 65% every 100 million years.² It would be wrong to assume that species going extinct is out of line with what we would expect. Evolution occurs through the balance of extinction – the end of species – and speciation – the creation of new ones.

Extinctions occur periodically at what we would call the “background rate”. We can therefore identify periods of history when extinctions were happening much faster than this background rate – this would tell us that there was an additional environmental or ecological pressure creating more extinctions than we would expect.

However, mass extinctions are periods with much higher extinction rates than normal. They are defined by both magnitude and rate. Magnitude is the percentage of species that are lost. Rate is how quickly this happens. These metrics are inevitably linked, but we need both to qualify as a mass extinction.

In a mass extinction, at least 75% of species go extinct within a relatively (by geological standard) short period of time.³ Typically less than two million years.