Mass extinction also known as “biotic crisis” is a widespread and rapid decrease in the amount of life on Earth. Such an event is identified by a sharp change in the diversity and abundance of macroscopic life. It occurs when the rate of extinction increases with respect to the rate of speciation. Because the majority of 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.
Over 98% of documented species are now extinct, but 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 invertebrates and vertebrates every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land organisms.
Since life began on Earth, several major mass extinctions have significantly exceeded the background extinction rate. The most recent, 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 the past 540 million years there have been five major events when over 50% of animal species died.
In a landmark paper published in 1982, Jack Sepkoski and David M. Raup identified five mass extinctions. They are:
1. Cretaceous–Paleogene extinction event (End Cretaceous, K-T extinction, or K-Pg extinction):
About 17% of all families, 50% of all genera and 75% of all species became extinct. In the seas it reduced the percentage of sessile animals to about 33%. All non-avian dinosaurs became extinct during that time. The boundary event was severe with a significant amount of variability in the rate of extinction between and among different clades. Mammals and birds emerged as dominant land vertebrates in the age of new life.
2. Triassic–Jurassic extinction event (End Triassic):
About 23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct. Most non-dinosaurian archosaurs, most therapsids and most of the large amphibians were eliminated, leaving dinosaurs with little terrestrial competition. Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g., Koolasuchus ).
3. Permian–Triassic extinction event (End Permian):
About 57% of all families, 83% of all genera and 90% to 96% of all species (53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species, including insects). The evidence of plants is less clear, but new taxa became dominant after the extinction. The “Great Dying” had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles. The recovery of vertebrates took 30 million years, but the vacant niches created the opportunity for archosaurs to become ascendant. In the seas, the percentage of animals that were sessile dropped from 67% to 50%. The whole late Permian was a difficult time for at least marine life, even before the “Great Dying”.
4. Late Devonian extinction:
About 19% of all families, 50% of all genera and 70% of all species. This extinction event lasted perhaps as long as 20 Ma, and there is evidence for a series of extinction pulses within this period.
5. Ordovician–Silurian extinction event (End Ordovician or O-S):
About 27% of all families, 57% of all genera and 60% to 70% of all species. Ranked by many scientists as the second largest of the five major extinctions in Earth’s history in terms of percentage of genera that went extinct.
Should there be another mass extinction, it could be the human extinction, typically a precedent of which exists in the Permian-Triassic extinction event. Humans are very widespread on the Earth and live in communities that (while interconnected) are capable of some level of basic survival in isolation. Therefore, pandemics and deliberate killing aside, to achieve human extinction the entire planet would have to be rendered uninhabitable, with no opportunity provided or possible for humans to establish a foothold beyond Earth.
Possible anthropogenic extinction scenarios exist such as global nuclear annihilation, total global war, dysgenics, overpopulation, global accidental pandemic, ecological collapse and global warming; besides natural ones: Meteor impact and large scale volcanism; and anthropogenic-natural hybrid events like global warming and catastrophic climate change. Naturally caused extinction scenarios have occurred multiple times in the geologic past although the probability of reoccurrence within the human timescale of the near future is negligibly small.
As technology develops, there is a theoretical possibility that humans may be deliberately destroyed by the actions of a nation state, corporation or individual in a form of global suicide attack. There is also a theoretical possibility that technological advancement may resolve or prevent potential extinction scenarios. The emergence of a pandemic of such virulence and infectiousness that very few humans survive the disease is a credible scenario.
While not necessarily a human extinction event, this may leave only very small, very scattered human populations that would then evolve in isolation. It is important to differentiate between human extinction and the extinction of all life on Earth. Of possible extinction events, only a pandemic is selective enough to eliminate humanity while leaving the rest of complex life on earth relatively unscathed.