Extinction event

An extinction event, also called a mass extinction or a biotic crisis, is a large and fast drop in the biodiversity of Earth. Scientists know an extinction event has happened when there is a sharp fall in the variety and number of multicellular organisms, which are living things made of many cells.

An extinction event happens when many species die out much faster than usual. This happens when extinction speeds up compared with the background extinction rate, and when new species appear more slowly. New species appearing is called speciation.

Scientists do not all agree on how many major mass extinctions have happened in the last 540 million years. Estimates range from as few as five to more than twenty. The different answers come from disagreement about what counts as a "major" extinction event, and from the information used to measure past diversity.

The "Big Five" mass extinction events

In 1982, Jack Sepkoski and David M. Raup identified five periods in Earth's history when very large numbers of living things died out. These "Big Five" events happened during the Phanerozoic Eon. Later, stronger statistical tests showed that animal life in this eon has had at least five major mass extinctions, along with many smaller ones. The "Big Five" are not clearly separate from each other. They seem to be the largest events in a fairly smooth range of extinctions. Earlier, microbial life suffered a much bigger loss during the Great Oxidation Event (also called the Oxygen Catastrophe) early in the Proterozoic Eon. The Late Ordovician mass extinction happened in two pulses, just before and during the Ordovician–Silurian change. It removed 27% of families, 57% of genera, and 85% of species. Many scientists rank it as the second-largest of the five major extinctions by the percentage of genera lost. The Late Devonian had two major extinction events, both linked to anoxic events, when seawater runs out of oxygen. The larger was the Kellwasser Event at the Frasnian–Famennian boundary. It destroyed coral reefs and many sea-floor animals, such as brachiopods and trilobites. The Hangenberg Event at the Devonian–Carboniferous boundary ended the Devonian. Together, these two events removed at least 70% of all species.

The End Permian extinction, also called the "Great Dying," was the largest extinction in the Phanerozoic Eon. About 81% of sea species died, and an estimated 70% of land backboned animal species also died out. It was also the largest known extinction for insects. The "Great Dying" ended many synapsids (mammal relatives) on land. Backboned animals took 30 million years to recover. The empty niches gave archosaurs (dinosaur and crocodile relatives) a chance to spread. The End Triassic extinction marks the Triassic–Jurassic change. About 70% to 75% of species died, and many large amphibians and therapsids were lost. This left dinosaurs with little competition on land. Pterosaurs and crocodylomorphs were the only other surviving archosaurs.

The End Cretaceous extinction, also called the K–Pg extinction (formerly K–T extinction), happened at the Cretaceous–Paleogene change. About 75% of species died. In the seas, all ammonites, plesiosaurs, and mosasaurs disappeared. All known dinosaurs that were not birds also became extinct. Mammals, which descended from synapsids, and birds, a side branch of theropod dinosaurs, became the two main land groups.

Although these five events are often presented as the main ones, no measure shows a clear line between them and many other extinctions that were only slightly smaller. Older fossils are harder to study because they are buried deeper and are harder to date. Some scientists suggest that changes in ocean life may be partly caused by how much rock is available to sample. However, statistics show this explains only half of the pattern. Other evidence, such as fungal spikes, supports most of the widely accepted extinction events. Many minor events are better explained by sampling bias, which means the rocks we have studied are not an even sample of the past.

Sixth mass extinction

Main articles: Holocene extinction and Biodiversity loss

Scientists studying this topic after a important 1982 paper by Sepkoski and Raup have concluded that humans are causing a sixth mass extinction right now. Since 1900, species have been dying out at over 1,000 times the normal rate, and the rate is getting faster. This loss of living things is caused by human activity, which some people call an ecocide. It is driven by more people living on Earth, by economic growth, and by people using up the planet's natural resources, a problem called overconsumption.

A 2019 global biodiversity assessment by IPBES estimated that there are about 8 million species. It said that about 1 million plant and animal species are at risk of dying out. In late 2021, the WWF warned that over a million species could disappear within ten years. A 2023 study in PNAS found that at least 73 kinds of animals have died out since 1500. Some people also use the term "sixth mass extinction" for much older events that were not well known, such as the Capitanian mass extinction, which happened about 260 million years ago.

Extinctions by severity

Main article: List of extinction events

Scientists can study extinction events in several ways. They look at changes in rock layers and at how ecosystems were affected. They also compare the number of species that disappeared with the number of new species that formed (speciation). Most often, they count how much diversity was lost among taxonomic groups. Early studies mostly counted families, using lists of marine animal families made by Sepkoski. Later studies by Sepkoski and other scientists counted genera instead. Genera are more precise than families and are less likely to be affected by mistakes in sorting or gaps in the fossil record.

Several major studies estimate the losses or ecological effects from fifteen well-known extinction events. The "Big Five" mass extinctions are shown in bold.

^a Graphed but not discussed by Sepkoski (1996), considered continuous with the Late Devonian mass extinction
^b At the time considered continuous with the end-Permian mass extinction
^c Includes late Norian time slices
^d Diversity loss of both pulses calculated together
^e Pulses extend over adjacent time slices, calculated separately
^f Considered ecologically significant, but not analyzed directly
^g Excluded due to a lack of consensus on Late Triassic chronology

The study of major extinction events

For much of the 20th century, scientists did not have enough information to study mass extinctions well. They seemed like mysterious exceptions to the gradualistic view of prehistory, in which slow changes shaped animal life. In 1980, a team led by Luis Alvarez found traces of metal that pointed to an asteroid impact at the end of the Cretaceous period. The Alvarez hypothesis for the end-Cretaceous extinction brought new attention to mass extinctions and catastrophic explanations. In 1982, David M. Raup and Jack Sepkoski published a paper in the journal Science. It was based on a list of extinct sea animal families that Sepkoski had made. The paper found five peaks of extinctions. Four were statistically significant: the Ashgillian (end-Ordovician), Late Permian, Norian, and Maastrichtian (end-Cretaceous). The fifth was a long period of high extinction in the later Devonian, peaking in the Frasnian stage. In 1984, Raup and Sepkoski also suggested a debated pattern that repeated every 26 million years. Two teams of astronomers linked it to a possible brown dwarf far away in the Solar System, called the "Nemesis hypothesis". Other astronomers have strongly disagreed with this idea. Their work also produced a biodiversity curve (the "Sepkoski curve") and evolutionary faunas.

Sepkoski also made a list of sea animal genera to study extinction in more detail. He died in 1999, and the list was published in 2002, which led to new studies. In 1995, Michael Benton found 22 extinction intervals and no repeating pattern. A. Hallam wrote summary books with others. Bambach (2006) identified 18 mass extinction intervals in Sepkoski's data, including 4 large ones in the Cambrian. Bambach, Knoll & Wang (2004) found a different pattern for each of the "Big Five" extinctions. Foote (2005) found that sudden bursts of extinction fit the history of diversity better than a steady rate. Stanley (2007) found support for an overall growth in biodiversity over time. Researchers also worked to correct sampling biases. In 1982, Jere H. Lipps and Phillip W. Signor noted that gaps in the fossil record made sharp extinctions look less sudden. This idea is now called the Signor-Lipps effect. Methods such as rarefaction help account for biases like the "Pull of the recent". Alroy (2010) developed "shareholder quorum subsampling", which compares the number of taxon in samples. Stanley (2016) tried to fix two common errors. He used the Santonian and Campanian stages to estimate changes in the Maastrichtian before the K-Pg mass extinction.

Uncertainty in the Proterozoic and earlier eons

Most of the variety and biomass on Earth is microbial, meaning it is made of tiny living things that are hard to study from fossils. Because of this, the extinction events we know about mostly affect the easy-to-see, complex part of the biosphere, not all life. For this reason, well-documented extinction events are mainly found in the Phanerozoic eon. The main exception is the Oxygen Catastrophe in the Proterozoic. Before the Phanerozoic, all living things were either microbial or, if they had many cells, soft-bodied. Mass extinctions may only seem to be mostly a Phanerozoic event, because the extinction rates we can see look low before large complex organisms with hard body parts appeared.

Extinction does not happen at an even 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. The Oxygen Catastrophe, which happened around 2.45 billion years ago in the Paleoproterozoic, may be the first major extinction event. It may also have been the worst. However, Earth's ecology just before that time is poorly understood, and the groups of prokaryote life are very different from groups of complex life. So it would be hard to compare it to any of the "Big Five," even if Paleoproterozoic life were better known.

Evolutionary importance

See also: Evolutionary radiation and Macroevolution

Mass extinctions have sometimes helped speed up the evolution of life on Earth. When one group of living things takes over the way of life of another, it is usually not because the new group is "better." Instead, an extinction event clears away the old main group and opens space for new ones. This process is known as adaptive radiation. For example, mammaliaformes ("almost mammals") and then mammals lived alongside the dinosaurs but could not compete for the large land animal roles the dinosaurs had. The end-Cretaceous mass extinction removed the non-bird dinosaurs, which let mammals spread into those roles. The dinosaurs had themselves benefited from the earlier end-Triassic mass extinction, which removed most of their main rivals, the crurotarsans.

Among Synapsida, groups from the early Pennsylvanian and Cisuralian periods, often still called "pelycosaurs" (though this is a paraphyletic group), were replaced by therapsids around the Kungurian/Roadian transition. This is often called Olson's extinction. It may have been a slow decline over 20 million years rather than a sudden event. The Escalation hypothesis suggests that species living in places with more fighting between living things are less likely to survive extinctions. Many groups that survive do not recover and slowly decline. This is sometimes called "Dead Clades Walking." Groups that last a long time after an extinction, having been reduced to only a few species, may bounce back in an effect called the "push of the past." Darwin believed that competition between living things mattered more for evolution and extinction than changes in the physical environment. He wrote about this in The Origin of Species.

Patterns in frequency

Some authors have suggested that extinction events happen regularly, about every 26 to 30 million years. Others think that the number of different kinds of living things rises and falls in cycles of about 62 million years. Most ideas that try to explain this pattern involve astronomical influences. These include a hidden companion star to the Sun, movement up and down in the galaxy, or passing through the Milky Way's spiral arms.

Other authors disagree. They say the data on sea extinctions does not fit a regular pattern, and many of the proposed links are doubtful. Still others argue that there is strong evidence for regular patterns. They point to matching patterns in non-living features of Earth, such as strontium isotopes and lava floods. One possible explanation for the cycle is that the ocean floor stores and releases carbon, moving it between the air and the deep Earth.

Mass extinctions are thought to happen when a long-term stress is combined with a short-term shock. Over the Phanerozoic, individual groups of living things seem less likely to die out. This may be due to stronger food webs, fewer species prone to extinction, and other factors, such as where continents are located. Even so, there appears to be a gradual decline in both extinction and the appearance of new species. It has also been suggested that the oceans have become more friendly to life over the last 500 million years. This would make them less likely to suffer mass extinctions. However, the chance of a group dying out does not seem to make mass extinctions more or less likely.

Causes

Scientists still do not agree on what causes mass extinctions. A large extinction may happen when life that has been under long-term stress is hit by a sudden shock. A good explanation should account for all the losses, explain why some groups died out while others lived on, and be based on events that can be shown to have happened. Several causes may work together. For example, the ocean part of the end-Cretaceous extinction seems to have had several causes that happened at about the same time. Researchers Arens and West (2006) suggested that mass extinctions usually need long-term pressure on ecosystems, followed by a sudden disaster near the end of that pressure. Common suspects include flood basalt events, which are giant volcanic eruptions. These can release particulate dust, sulfur that falls as acid rain, and carbon dioxide that can cause sustained global warming. All of these can harm food chains. Falling sea levels and asteroid impacts are also linked to some extinctions. The Cretaceous–Paleogene extinction event is the best-known example of an extinction linked to an asteroid, and the Cretaceous Chicxulub asteroid impact is thought to have been a key reason for it. Other possible causes include a nearby gamma-ray burst that could damage the ozone layer and leave living things exposed to ultraviolet radiation. Other ideas include long periods of global cooling and methane clathrates that could release methane into the air.

Problems in the ocean have also been linked to extinctions. Anoxic events are times when the deeper layers of the ocean have little or no oxygen, and they may have contributed to several extinctions. Changes in the movement of continents can also affect the climate. The creation of the super-continent Pangaea has been suggested as a cause of the End-Permian mass extinction. Some scientists are worried that human activities, such as hunting, fishing, and destroying natural homes, could cause more plants and animals to die out than at any time in the past. A 2017 study argued that a "biological annihilation" similar to a sixth mass extinction event is already happening. Many other ideas have been proposed, but most have been rejected because there is no evidence for them or they do not fit the evidence. Looking far into the future, the growing Sun could eventually wipe out nearly all life, leaving only simple prokaryotes behind.

Main article: Extinction risk from global warming

Main article: Clathrate gun hypothesis

See also: Future of Earth and Medea hypothesis

Effects and recovery

Mass extinctions affected life in different ways. After a major extinction, usually only weedy species survive. These are plants and animals that can live in many different places. Later, other species spread out and fill the empty spaces in nature. Recovery is slow. It often takes millions of years for biodiversity to return, and after the most severe mass extinctions it may take 15 to 30 million years.

The worst Phanerozoic event was the Permian–Triassic extinction. It destroyed much of life on Earth and wiped out more than 90% of species. Life seemed to bounce back quickly, but this was mostly thanks to disaster taxa, such as the hardy Lystrosaurus. New research shows that the animals with specialized roles, which had formed complex ecosystems, took much longer to recover. This may have been because extinctions kept happening in waves and the environment stayed under stress for a long time. Plants were affected differently. Some mass extinctions, such as the end-Permian, harmed plants just as much. Others, such as the end-Devonian, did not affect plant life very much.

In media

The term extinction-level event (ELE) has been used in the media. The 1998 film Deep Impact shows a comet hitting Earth as an ELE.