Of all the species that have ever lived, more than 99% are now extinct. Most of them quietly disappeared during periods of “background extinction”, whereby a handful of species become extinct every 100,000 years or so.
But there were also occasions when extinction rates increased rapidly in short periods of time and wiped out a significant proportion of all life on Earth. These are known as mass extinctions. They have profoundly influenced the history of life – and many scientists now argue that we are in the midst of another one. To see if they’re right, we can look at previous occasions when large numbers of species went extinct.
Traditionally, scientists have referred to the “Big Five” mass extinctions, including perhaps the most famous mass extinction that brought about the end of the dinosaurs. This was triggered by a meteorite impact at the end of the Cretaceous period, but the other major mass extinctions were caused by phenomena originating entirely on Earth. While they are less well known, we may learn something from exploring them that could shed light on our current environmental crises.
1. The Late Ordovician
This ancient crisis around 445m years ago saw two major waves of extinction, both caused by climate change associated with the advance and retreat of ice sheets in the southern hemisphere. This makes it the only major extinction to be linked to global cooling.
2. The Late Devonian
This period is now regarded as a number of “pulses” of extinction spread over 20m years, beginning 380m years ago. It saw the extinction of around 50% of marine genera; among the species killed off were many corals, trilobites, sponges and the heavily armoured fish known as placoderms. This extinction has been linked to major climate change, possibly caused by an eruption of the volcanic Viluy Traps area in modern-day Siberia. A major eruption might have caused rapid fluctations in sea levels and reduced oxygen levels in the oceans.
3. The Middle Permian
Scientists have recently discovered another event 262m years ago that rivals the “Big Five” in size. This event coincided with the Emeishan eruption in what’s now China, and is known to have caused simultaneous extinctions in the tropics and higher latitudes. In particular, there were exceptionally high extinction rates: more than 80% of species were wiped out, among them brachiopods and single-celled benthic foraminifera.