<P> An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth . Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms . It occurs when the rate of extinction increases with respect to the rate of speciation . Because most diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life . </P> <P> Extinction occurs at an uneven rate . Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years . Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals . </P> <P> The Great Oxygenation Event was probably the first major extinction event . Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate . The most recent and arguably best - known, the Cretaceous--Paleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large - scale mass extinction of animal and plant species in a geologically short period of time . In addition to the five major mass extinctions, there are numerous minor ones as well, and the ongoing mass extinction caused by human activity is sometimes called the sixth extinction . Mass extinctions seem to be a mainly Phanerozoic phenomenon, with extinction rates low before large complex organisms arose . </P> <P> Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty . These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity . </P>

Over what time periods have the mass extinctions taken place