There seems to be agreement that there have been 5 mass extinctions of mobile forms of life on Earth. A mass extinction is one in which an extinction rate of at last 75% has been estimated. I wonder how these figures (and the dates of occurrence) are calculated.
The dates of extinction are: 434 million years ago (mya) with a loss exceeding 60%; 360 mya (% loss?); 251 mya – 80 to 95%; 205 mya – 80% loss; 65 mya – no large animal survived. Noticeably, an extinction period can take a few hundred thousand years to a couple of million years. This I interpret as placing us and other life forms in a situation akin to that frog whose watery habitat (say, a beaker over a Bunsen burner on very low heat) is subject to slow heating, to boiling point and death. That is, the expected sixth extinction on Earth will not be observable by us and our descendants until we are beyond recovery.
Will extinction and revival be cyclical? Apparently there is a cycle of about 62 million years of high rates of extinction. This cycle is said to coincide with “periodic ‘excursions’ of the solar system outside the central plane of the Milky Way galaxy,” (whatever that means) when Earth is bombarded with high levels of cosmic radiation. In the past 542 years, the number of animals in the ocean have also been known to have fallen and risen every 62 million years.
Glaciation (asserted by some researchers to be challengeable as a relevant process) is the principal speculative causal agent. Meteoric bombardment with associated vulcanism (and its consequences) is a more probable cause.
In the land of the unknowable, we seem to remain unknowing.