Earth has experienced five major mass extinctions, events in which an unusually large proportion of species disappeared over a relatively short period in geological time. Known as the Big Five, these crises occurred during the past 500 million years and profoundly reshaped life on the planet. Each eliminated entire groups of organisms, disrupted ecosystems, and created opportunities for surviving species to evolve into new forms.
The five major mass extinctions were the Ordovician–Silurian extinction, the Late Devonian extinction, the Permian–Triassic extinction, the Triassic–Jurassic extinction, and the Cretaceous–Paleogene extinction. Their causes varied, ranging from dramatic changes in climate and ocean chemistry to massive volcanic eruptions and an asteroid impact.
Although these events destroyed enormous amounts of biological diversity, they did not end life on Earth. Instead, surviving organisms inherited altered environments, and over millions of years, evolution produced new ecosystems and new dominant groups of species.
What makes an extinction event a mass extinction?
Extinction is a natural part of evolution. Species disappear when environmental conditions change, habitats vanish, competitors emerge, or populations become unable to survive and reproduce. Most extinctions occur at a relatively low background rate, but a mass extinction happens when the rate rises sharply across many different groups of organisms worldwide.
Scientists commonly use the loss of roughly 75 percent of species over a geologically short interval as a benchmark for identifying a mass extinction. This threshold is a useful guide rather than an absolute rule, particularly because the fossil record is incomplete and some extinction events unfolded in several phases.
Mass extinctions differ from localized disasters. A volcanic eruption might destroy an island’s ecosystem, and a drought might eliminate many species from a region, but a global mass extinction affects organisms across broad geographic areas and often disrupts multiple habitats simultaneously.
The fossil record provides the main evidence for these events. Scientists examine changes in the diversity and abundance of fossil organisms in sedimentary rocks, looking for intervals when many species disappear from geological layers. Chemical evidence preserved in rocks can reveal changes in ocean oxygen levels, carbon cycling, temperature, and other environmental conditions.
The Big Five occurred at different points in the history of complex animal life. Their effects were especially visible in marine ecosystems, where fossils are often preserved more readily than those of land organisms. Each event had its own combination of triggers and environmental consequences.
1. The Ordovician–Silurian extinction
When: Approximately 444 million years ago.
The Ordovician–Silurian extinction was the first of the Big Five. During the Ordovician Period, marine life diversified extensively, filling oceans with brachiopods, trilobites, mollusks, corals, and other organisms. Life on land was still relatively limited, and complex terrestrial ecosystems had not yet developed.
The extinction primarily affected marine environments. Many species of brachiopods, bryozoans, trilobites, and other ocean-dwelling organisms disappeared. Estimates commonly suggest that roughly 85 percent of marine species were lost, making this one of the most severe biological crises in Earth’s history.
The leading explanation involves a major period of global cooling and glaciation. As large ice sheets formed on the ancient southern supercontinent Gondwana, substantial amounts of water became locked up in ice. Global sea levels fell, exposing shallow marine habitats that had supported diverse communities.
This loss of habitat placed enormous pressure on organisms adapted to warm, shallow seas. Cooling also changed ocean circulation and affected the distribution of marine species.
The crisis did not end with cooling alone. As the climate subsequently warmed and glaciers melted, sea levels rose again. These shifts altered ocean conditions, and oxygen-poor waters likely contributed to additional losses during parts of the extinction interval.
The precise sequence of environmental changes remains an area of scientific investigation. However, the combination of glaciation, sea-level fluctuations, and changes in ocean chemistry provides a strong explanation for the event.
The extinction reshaped marine ecosystems at a time when most complex life was still concentrated in the oceans. During the following Silurian Period, surviving groups diversified, and marine communities gradually recovered.
2. The Late Devonian extinction
When: Approximately 372–359 million years ago.
The Devonian Period is often called the Age of Fishes because fish groups diversified dramatically during this time. Oceans contained extensive coral reefs and a variety of marine invertebrates, while plants were spreading across land and early forests were developing.
Unlike some other mass extinctions, the Late Devonian extinction was not a single, sharply defined catastrophe. It consisted of several extinction pulses spread over millions of years. Two particularly important intervals were the Kellwasser events, around 372 million years ago, and the Hangenberg event, near the end of the Devonian, around 359 million years ago.
Marine life suffered especially severe losses. Reef-building organisms were devastated, and many brachiopods, trilobites, ammonites, and other marine groups declined or disappeared. Estimates of the total loss vary, but roughly three-quarters of species may have been eliminated across the broader crisis.
One of the leading explanations is widespread oxygen depletion in the oceans, a condition called anoxia. Marine animals require dissolved oxygen to survive, so when oxygen levels fall sufficiently, entire communities can collapse. In some environments, oxygen-poor conditions may also have been accompanied by the accumulation of toxic substances.
Changes in climate and ocean circulation likely contributed to the crisis. Evidence also points to disruptions in the global carbon cycle, including changes associated with organic matter being buried in marine sediments.
The expansion of land plants may have played a role. As roots penetrated deeper into soils and vegetation spread, increased chemical weathering could have carried more nutrients into rivers and oceans. Excess nutrients can stimulate algal growth, and the subsequent decomposition of this organic matter consumes oxygen. This is a plausible contributing mechanism, although its importance relative to other factors remains uncertain.
Volcanic activity and climate fluctuations have also been investigated as possible contributors. The event’s prolonged and episodic nature makes it difficult to attribute all the losses to a single cause.
The Late Devonian extinction profoundly altered marine ecosystems and contributed to the collapse of many ancient reef systems. Some surviving fish lineages continued to diversify, while the ecological landscape of the oceans changed substantially.
3. The Permian–Triassic extinction
When: Approximately 252 million years ago.
The Permian–Triassic extinction was the most severe known mass extinction in Earth’s history. It is often called the Great Dying because of the extraordinary scale of biological loss. Estimates suggest that about 90 percent of marine species and a substantial proportion of terrestrial vertebrate species disappeared, although exact percentages depend on the group studied and how the fossil record is interpreted.
At the time, most of Earth’s land was joined into the supercontinent Pangaea. Marine ecosystems supported a wide range of invertebrates, including brachiopods, trilobites, and reef-building organisms. On land, diverse plants, insects, amphibians, and early relatives of mammals and reptiles inhabited terrestrial environments.
The principal trigger is strongly associated with enormous volcanic eruptions in what is now Siberia. These eruptions formed the Siberian Traps, an extensive region of volcanic rock. The activity released large quantities of carbon dioxide and other gases, contributing to rapid global warming and major disruptions to the atmosphere and oceans.
Carbon dioxide accumulation warmed the planet and intensified the greenhouse effect. As the oceans warmed, their ability to hold dissolved oxygen decreased. Changes in ocean circulation and biological processes likely further reduced oxygen availability, creating widespread oxygen-poor conditions.
Some marine environments may also have become euxinic, meaning they contained both little or no oxygen and elevated levels of toxic hydrogen sulfide. Such conditions would have been devastating to organisms that depended on oxygenated water.
Ocean acidification was another likely consequence of increased atmospheric carbon dioxide. When carbon dioxide dissolves in seawater, it changes the water’s chemistry and reduces the availability of carbonate ions needed by many organisms to build shells and skeletons.
On land, extreme heat, drought, disrupted rainfall patterns, and damage to vegetation placed additional pressure on ecosystems. Some regions may have experienced toxic atmospheric conditions and other environmental stresses associated with volcanism.
The extinction probably resulted from several interacting processes rather than one isolated mechanism. The timing and relative importance of individual effects remain subjects of research, but the connection between Siberian Traps volcanism and the crisis is well established.
Recovery was exceptionally difficult. Ecosystems remained unstable for a prolonged period, and some marine communities took millions of years to regain substantial diversity. Many familiar Paleozoic groups disappeared permanently, including the trilobites, which had existed for hundreds of millions of years.
Surviving lineages eventually diversified into new ecological roles. Among terrestrial vertebrates, some reptile groups and the ancestors of later mammal lineages persisted, while archosaur relatives became increasingly important in subsequent ecosystems.
4. The Triassic–Jurassic extinction
When: Approximately 201 million years ago.
The Triassic–Jurassic extinction occurred near the end of the Triassic Period, when Earth’s landmasses were still joined within Pangaea. Reptiles and other terrestrial vertebrates occupied a wide range of ecological roles, while the earliest dinosaurs had already appeared but had not yet become the dominant large land animals everywhere.
The extinction eliminated many marine organisms and numerous terrestrial vertebrate groups. Estimates commonly suggest that around three-quarters of species were lost, although the magnitude varied among environments and biological groups.
The leading explanation involves massive volcanic activity associated with the breakup of Pangaea. As the supercontinent began to split apart, extensive eruptions occurred in the region now represented by the Central Atlantic Magmatic Province. This volcanic province contains an enormous volume of ancient igneous rock and records one of the largest episodes of volcanism in Earth’s history.
The eruptions released carbon dioxide into the atmosphere, likely producing substantial global warming. Rapid environmental changes may also have involved ocean acidification, shifts in rainfall, and disruptions to the carbon cycle.
In the oceans, warming and changes in water chemistry would have placed stress on marine organisms. Ocean acidification was particularly threatening to species that built shells or skeletons from calcium carbonate. Oxygen depletion may also have contributed to marine losses, although the precise extent and timing of these conditions varied.
On land, changing temperatures and precipitation patterns altered habitats and affected plants and animals. Species that could not tolerate the pace or scale of these changes were more likely to disappear.
The extinction created ecological opportunities for surviving groups. Many competitors of dinosaurs vanished, helping dinosaurs become the dominant large terrestrial vertebrates during the Jurassic Period. This does not mean that the extinction directly produced dinosaurs; they had evolved earlier. Rather, the disappearance of competing groups helped reshape the ecosystems in which dinosaurs subsequently diversified.
Other surviving reptiles, early mammals, and marine organisms also benefited from the gradual rebuilding of ecological communities. Recovery took time, and the resulting ecosystems differed considerably from those that had existed before the crisis.
5. The Cretaceous–Paleogene extinction
When: Approximately 66 million years ago.
The Cretaceous–Paleogene extinction, often abbreviated as the K–Pg extinction, is the most familiar of the Big Five. It marked the end of the Cretaceous Period and the beginning of the Paleogene Period. About 75 percent of species disappeared, including all non-avian dinosaurs.
The extinction affected marine and terrestrial ecosystems. Many ammonites, marine reptiles, plankton groups, and other organisms vanished. On land, numerous plant and animal communities were disrupted. However, not every group disappeared: birds, mammals, crocodilians, turtles, and many other lineages survived.
The principal trigger was the impact of a large asteroid near what is now the Yucatán Peninsula in Mexico. The collision formed the Chicxulub crater, a vast impact structure buried beneath the region and the adjacent Gulf of Mexico.
The impact released an enormous amount of energy, generating intense heat, powerful shock waves, earthquakes, and destructive waves in nearby oceans. It also blasted material into the atmosphere, including dust, rock debris, and sulfur-bearing compounds derived from the impact region.
These materials interfered with incoming sunlight, causing a sharp reduction in the light reaching Earth’s surface. This period of impact-driven darkness and cooling is often described as an impact winter. Reduced sunlight disrupted photosynthesis, weakening plants and microscopic organisms that formed the foundation of many food webs.
As primary producers declined, the effects spread through ecosystems. Herbivores lost food sources, and predators were affected by the disappearance of their prey. Marine food webs were also severely disrupted as photosynthetic plankton declined.
The impact’s effects extended beyond darkness and cooling. Sulfur released into the atmosphere contributed to acid rain and environmental stress, while fires and other regional disturbances may have added to the destruction. The precise contribution of each process varied across the planet.
Volcanism in the Deccan Traps of present-day India was another major environmental factor around this time. These eruptions released gases that affected climate and ocean chemistry before, during, and after the impact. Their role in the extinction remains debated, but the asteroid impact is widely accepted as the primary trigger of the abrupt global biological crisis.
The extinction ended the reign of non-avian dinosaurs, but birds survived as the only living dinosaur lineage. Mammals also survived and later diversified into a wide variety of ecological roles, eventually giving rise to the major mammalian groups found today, including primates.
The K–Pg extinction demonstrates how a sudden external catastrophe can rapidly destabilize ecosystems that evolved over millions of years. It also illustrates that survival depends not only on an organism’s physical resilience but on its ecological needs, geographic distribution, and ability to withstand the collapse of its food sources.
Why did the five mass extinctions happen?
Although the Big Five differed in timing and severity, they share a broad pattern: environmental conditions changed faster or more extensively than many organisms could tolerate. Several recurring mechanisms help explain how these crises unfolded.
Rapid climate change was a major factor in multiple events. Sudden or sustained warming can shift suitable habitats, alter rainfall, increase heat stress, and change ocean circulation. Cooling and glaciation can also cause severe disruption by lowering sea levels and eliminating shallow marine habitats.
Changes in ocean chemistry were especially important because the oceans supported much of the diversity recorded in the fossil record. Acidification makes it more difficult for many organisms to build carbonate shells and skeletons. Deoxygenation reduces the amount of habitat available to animals that require oxygen, while severe oxygen depletion can transform entire marine ecosystems.
Massive volcanic eruptions contributed to several of the major extinctions. Volcanism can release carbon dioxide, sulfur gases, and other substances that alter climate and atmospheric chemistry. The consequences depend on the scale, duration, and geological setting of the eruptions. Carbon dioxide tends to produce warming over longer timescales, while sulfur aerosols can temporarily reflect sunlight and cause cooling.
Asteroid impact was the defining trigger of the K–Pg extinction. Unlike prolonged environmental changes, a large impact can produce immediate physical destruction and rapid atmospheric effects. Its consequences then spread through ecosystems as sunlight declines and food webs collapse.
These mechanisms often interacted. Warming can reduce oxygen levels in seawater, changes in precipitation can alter nutrient delivery to the oceans, and changes in ocean circulation can intensify oxygen depletion. Such interactions help explain why a single initial disturbance can produce widespread biological losses.
The evidence also shows that not every mass extinction followed the same pattern. The Ordovician–Silurian event was strongly associated with glaciation and sea-level change, the Late Devonian crisis unfolded through multiple episodes, and the Permian–Triassic extinction was closely linked to extreme environmental disruption associated with volcanism.
How did life recover after mass extinctions?
Mass extinctions did not erase life entirely. Even during the most severe events, some organisms survived in refuges where conditions remained tolerable. These survivors formed the starting point for the recovery of biological diversity.
Recovery was not immediate. In the aftermath of a major extinction, ecosystems may lack many of the species that previously filled important ecological roles. Food webs become simpler, habitats may remain damaged, and environmental conditions can continue to fluctuate.
Surviving organisms can eventually expand into newly available habitats and ecological niches. A niche is the role a species plays in its environment, including how it obtains food, uses resources, and interacts with other organisms. When competitors disappear, survivors may encounter opportunities to diversify into roles that were previously occupied by other species.
This process can produce evolutionary radiations, periods when a surviving group rapidly diversifies into many new species. Mammals after the K–Pg extinction and dinosaurs after the Triassic–Jurassic extinction provide important examples of groups that expanded following major ecological disruptions.
However, recovery does not restore the original ecosystem exactly. Evolution follows the opportunities and constraints created by the surviving organisms and changing environments. Extinct species do not return, and the new communities that develop may be structurally and functionally different from those that existed before.
The length of recovery varies. Some groups diversify relatively quickly, while rebuilding complex marine communities or restoring high levels of biodiversity can take millions of years. The most severe extinction events often leave effects that remain visible in the fossil record long after the initial crisis.
Are humans causing a sixth mass extinction?
Scientists have raised concerns that Earth may be entering a sixth mass extinction driven primarily by human activities. Unlike the Big Five, which were associated with major geological or astronomical disturbances, this emerging crisis is linked to the expansion of human populations, resource use, and the transformation of natural environments.
Habitat destruction is a major cause of biodiversity loss. Forest clearing, urban development, agriculture, and other land-use changes can eliminate habitats or divide them into smaller, isolated areas. Species may struggle to survive when their populations become too small or disconnected to reproduce successfully.
Other important pressures include overfishing and hunting, pollution, the spread of invasive species, and climate change. These pressures often reinforce one another. A species already weakened by habitat loss, for example, may be less able to withstand rising temperatures or a newly introduced predator.
Modern extinction rates are difficult to compare directly with ancient mass extinctions because the fossil record and contemporary biodiversity measurements capture different kinds of evidence. Many species have not yet been formally described, and the full extent of current losses remains uncertain. Nevertheless, documented extinctions, declining populations, and widespread habitat degradation provide strong evidence of a serious global biodiversity crisis.
The term sixth mass extinction is therefore used to describe the scale and trajectory of current human-driven losses, not to imply that the geological benchmark of roughly 75 percent of species lost has already been reached. Whether that threshold will ultimately be crossed depends partly on future environmental changes and human decisions.
There is an important difference between past mass extinctions and the present crisis: humans can identify many of the pressures driving biodiversity loss and can take steps to reduce them. Protecting habitats, restoring damaged ecosystems, limiting pollution, managing wildlife sustainably, controlling invasive species, and reducing greenhouse gas emissions can all help lessen the risks.
The five major mass extinctions reveal how vulnerable life can be when the physical and chemical conditions supporting ecosystems change dramatically. They also demonstrate life’s capacity to persist and diversify after catastrophe. Understanding these events helps explain not only the history of life on Earth but also the consequences of the environmental changes taking place today.
