Mass extinction theories

Mass extinctions—those periods in Earth's history in which large numbers of species rapidly become extinct—result from environmental changes caused by factors including volcanic activity, extraterrestrial impacts, shifting continents, and, most recently, destructive human activity. Although devastating to life on Earth, mass extinctions also lead to periods of evolutionary expansion, including major shifts in the types of animals on the planet.

Defining Mass Extinctions

Mass extinctions, also called major extinctions, are periods in the Earth's past in which large numbers of species became extinct in a relatively short period. These periods contrast with the background extinction rate, which is the rate at which species go extinct in the absence of any catastrophic environmental change. To be considered a mass extinction, according to paleontologists, the event must lead to the extinction of a relatively large number of species and must be a global or geographically widespread phenomenon, affecting species from distantly related evolutionary groups.

Paleontologists have identified five mass extinctions in Earth's history: Triassic-Jurassic, Permian-Triassic, Devonian-Carboniferous, Ordovician-Silurian, and Cretaceous-Tertiary. Each of the five was a major turning point for life on Earth, and each coincided with environmental catastrophes so severe that they have left clear signs in the geological record—signs that are used to demarcate the shift between geologic periods. In addition to the five major extinctions, there have been other periods (for example, the end of the Ice Age in North America) in which extinction rates rose far above the background level but where the resulting extinction affected a narrower geographical area or number of species.

The best-known and most-studied mass extinction resulted in the disappearance of most dinosaur species and marked the transition from the Cretaceous period to the Tertiary. This is also the most recent mass extinction, having occurred approximately 65 million years ago. Paleontologists estimate that the Cretaceous-Tertiary event led to the extinction of a minimum of 62 percent of Earth's species and 11 percent of the families of species. The extinction of entire families is significant because it means that there are no representatives of that larger group to repopulate the world with representative species in subsequent generations.

The Permian-Triassic extinction event, which occurred approximately 250 million years ago and marked the beginning of the age of dinosaurs, was the most massive extinction episode in Earth's history. Some scientists have estimated that the extinction resulted in the loss of more than 90 percent of the species on Earth and of more than one-half of Earth's families of organisms.

Evidence for Mass Extinctions

Evidence for mass extinctions is derived from the fossil record, which is the history of life on Earth as revealed from the fossilized remains of extinct organisms. The fossil record has been created by dating and organizing fossils into a temporal progression that can be used to measure the amount of time that a certain species appeared on Earth.

Extinction rates vary among organisms. Mammal species typically emerge and become extinct within approximately one million years, while there are examples of reptile species that have existed virtually unchanged for up to ten million years. Taking this into account, scientists estimate that between 10 and 100 million species become extinct each year. This is considered the background extinction level for the Earth.

In the nineteenth century, pioneering researchers discovered that, at certain levels of the geologic strata, many or most of the fossil species present at one level were missing from subsequent layers. At first, paleontologists thought that these “gaps” in the fossil record were caused by incomplete knowledge. Scientific leaders of the nineteenth century, such as Charles Darwin and Charles Lyell, believed that extinction was a gradual process brought about by local, immediate pressures; the two scientists could not conceive of conditions that would cause a mass extinction. However, paleontologists discovered that gaps in the fossil record occurred around the world at similar points in the geological strata. Standard evolutionary theory could not account for the seemingly global disappearance of species.

In the mid-twentieth century, paleontologists began using decayed radioactive isotopes to determine the age of geological samples. Radioactive isotopes occur naturally in the Earth's mineral surface and, because the substances are chemically unstable, they decay, giving rise to another substance called the decay product. When scientists find a fossil in the Earth, they can examine the proportion of isotopes to decay products in nearby sediment and are thereby able to determine the age of the sample.

Radioisotope dating strengthened the case for mass extinctions as paleontologists refined their temporal understandings of the fossil record. With this new evidence, paleontologists realized that extinction sometimes occurred relatively quickly. Paleontologist Norman D. Newell consolidated evidence for mass extinctions in his 1967 paper “Revolutions in the History of Life,” after which most paleontologists turned their attention toward the goal of discovering how and why mass extinctions occur.

Causes of Mass Extinctions

Biologists sometimes identify extinctions as having either external or internal causes. Internal causes arise from within living organisms, such as the development of a fatal bacterium or virus that causes the extinction of one or more species. Alternatively, the origin of one species can, in some cases, lead to the extinction of other species. The evolution of the human species has led to the extinction of hundreds of other species relatively rapidly, and these extinctions would be considered internally caused. However, most internal extinction events affect only a few species and, therefore, have little potential to cause mass extinctions on their own.

Paleontologists believe that the five major extinctions were largely the result of external causes, which are changes in the physical environment brought about by geochemical or extraterrestrial events. These environmental catastrophes can lead to widespread extinction, as many species are left without the evolutionary adaptations that would enable them to adjust to changing conditions.

Volcanism has been one of the most important forces of environmental change, and scientists have discovered evidence of increased volcanic activity closely associated with mass extinctions. Lava emanating from volcanoes leaves a layer of material in the sediment that geologists refer to as flood basalt, and this characteristic combination of minerals has been found in samples preceding the Cretaceous-Tertiary, Triassic-Jurassic, and Permian-Triassic extinctions.

In addition to devastating lava flows, volcanic activity can bring about global changes in temperature, as massive amounts of ash and gas are ejected by this activity into the atmosphere. Volcanoes are therefore naturally occurring sources of both global warming and global cooling. In addition, geologists have discovered that there can be “supervolcanoes,” which grow much larger than standard volcanoes and have a correspondingly greater potential to wreak environmental havoc. Few of these supervolcanoes exist on the modern Earth, but geologists have found evidence suggesting that supervolcanoes may have been more common in Earth's distant past.

Researcher Henrik Svenson of the University of Oslo in Norway has discovered evidence of a supervolcano that erupted in the area that is now Siberia, and that may have been a major factor in the Permian-Triassic extinction. Svenson's research, published in 2009, indicates that this supervolcano may have been active for more than 200,000 years, resulting in numerous large eruptions that covered the surrounding environment with lava and that spewed toxic gasses into the air. In the longer term, this volcano might have led to widespread changes in temperature, thereby bringing about environmental collapse.

Volcanic activity also is related to the movement of continents, a phenomenon that scientists refer to as plate tectonics. The continents move when pressure is exerted from deep within the Earth's surface, driven by activity in the Earth's core. The continents have, in the distant past, been joined into “supercontinents,” which are defined as the combination of two or more continents. At the end of the Permian period, when the largest mass extinction in Earth's history took place, most of the Earth's continents were united in a landmass known as Pangaea. The joining of continents necessitates a wide range of environmental changes, including increased volcanic activity, reduced coastal habitat for colonization, and climatic changes caused by shifting ocean and wind currents.

Species living on supercontinents may be more vulnerable to internal extinction pressures, such as the spread of disease or the introduction of a new predator. As the continents join, land bridges are created between them, and species begin to move between both areas. In these conditions, a predator from one continent can prove devastating to prey animals on another continent because the prey species lack the evolutionary adaptations to avoid predation. Species diversity is also lower on supercontinents because the same species can occupy available niches across a larger area. Paleontologists believe that the formation of Pangaea led to a decline in the number of species and, therefore, left the biota more vulnerable to extinction when the environment changed because of external factors.

Climate change is one of the most important causes of mass extinction, and paleontologists have found evidence of major climate changes associated with each of the five major extinctions. Geologists now know that the planet alternates between periods of relatively mild climate and periods in which large portions of the Earth are covered in glaciers, sometimes called ice ages. Paleontologists have found evidence of a massive, global ice age that occurred around the time of the Ordovician-Silurian extinction, which occurred more than 440 million years ago. Geologists have also noted that global warming was occurring during other extinction events and most likely played a leading role in the extinction event. Global warming and global cooling can be affected by the shifting of continents and by more rapid catastrophic events such as volcanism.

In the 1970s, father and son researchers Luis and Walter Alvarez found evidence suggesting that the Cretaceous-Tertiary extinction may have been partially caused by the impact on Earth of a large meteor originating from elsewhere in the galaxy. The team found a layer of iridium and other rare elements deposited in the soil at what paleontologists have named the K-T boundary, which is the layer that marks the end of the Cretaceous. Whereas iridium is rare on Earth, evidence suggests that it frequently occurs in meteors originating elsewhere in the solar system. With the publication of the Alvarezes' theory in 1980 came controversy and much debate in the paleontological community. The Alvarezes based part of their claim on the existence of the Chicxulub Crater in the Yucatán Peninsula of Mexico. Paleontologists believe that the crater resulted from an impact that occurred approximately 250,000 to 300,000 years before the Cretaceous-Tertiary extinction.

The Chicxulub impact would have created an explosion thousands of times more powerful than the detonation of the global military arsenal, immediately vaporizing plants and animals for miles surrounding the impact site. In addition, the aftereffects would have included earthquakes, volcanic eruptions, and massive storms reaching miles inland from the coast. Finally, the impact may have spewed a dense cloud of particulate matter into the atmosphere, eventually leading to long-lasting climate change that may have led to the loss of many plant species and a collapse of the food chain for many other species.

Paleontologists have found evidence linking extraterrestrial impact events with four of the five major extinctions. However, geologists also have found significant evidence for impact events that did not result in a mass extinction, leading to questions regarding exactly which conditions must be present for a meteor impact to facilitate a mass extinction.

Combination of Causes

Paleontologists now believe that major extinctions result from a special set of circumstances in which environmental conditions shift rapidly because of many interconnected, causal events in a relatively short time. As geological research continues, paleontologists recognize that each of the extinctions in Earth's history seems to be connected with more than one potential agent of environmental change.

In a 2006 article in Geological Society of America, researchers presented the press/pulse model of extinction, which proposes that mass extinctions result from a combination of gradual factors (press) exacerbated by some major event (pulse) that results in a chain reaction of environmental collapse. The “press” part of the model can involve any variety of environmental changes, slowly putting pressure on the biosphere. During the Permian-Triassic extinction, the press may have involved environmental changes resulting from the unification of the Earth's continents. This gradual pressure then received a “pulse” from volcanism or an impact event, or both, thereby accelerating the mass extinction event.

In 2008, geologist Shanan Peters published the results of a comprehensive research study indicating that, in each of the five major extinction events, changing sea level was a major determining factor. For example, during the Cretaceous-Tertiary extinction event, it appears that the global sea level fell by a significant margin. This sea level “regression” may have had a devastating effect on marine life and would also have led to global warming because of changes in ocean and wind currents. Peters believes that all the major extinction events were, at minimum, partially caused by falling sea levels, like those that occurred in the Cretaceous, setting the stage for a mass extinction event with influence from a major environmental pulse.

In each case, it appears that mass extinctions can occur only when the biota of the Earth has been experiencing a steady increase in gradual extinction pressures, facilitating lower species diversity and greater vulnerability to environmental change. Meteor impacts, volcanic eruptions, and other major catastrophes then push environmental change into overdrive. The oceans swell or shrink, the temperature rises or falls, and the food chains collapse.

Luis and Walter Alvarez, and a number of other researchers, have theorized that extinction events may occur on a periodic cycle, perhaps linked with cosmic cycles in the solar system that leads to periods in which Earth is more vulnerable to meteor impacts. Attempts to develop a cyclic theory of extinction have met with some success, but the exact timeline of extinction events is still a matter of considerable debate among paleontologists. The periodicity of extinctions remains an area of speculation as paleontologists attempt to understand the factors that might contribute to an extinction cycle.

The Sixth Extinction

Some scientists have speculated that the human species has had sufficient impact on the Earth to usher in a mass extinction event, sometimes referred to as the sixth extinction or the Holocene extinction. The sixth extinction is believed to be the first known mass extinction caused by internal, rather than external, factors (in this instance, human activity). Human activity is directly responsible for the increasingly devastating biodiversity loss the Earth has begun experiencing in the twenty-first century.

Human activity poses a threat to life on Earth in several different ways. First, humans kill animals directly, either for food or for sport, as well as unintentionally, as occurs in collisions between automobiles and wildlife. Additionally, humans remove vast amounts of vegetation, leading to loss of habitat and a collapse of local food chains. Scientists also believe that humans are causing significant global climate change, both through industrial activity and through the removal of vegetation that acts to regulate climate. Though estimates vary wildly, some biologists think that human activity, taken as a whole, has increased the extinction rate to seven times the background level, resulting in the loss of between seventy and seven hundred species each year.

In January 2022, researchers from the University of Hawaii published a study in Biological Reviews that supported the idea that humanity is currently experiencing the sixth mass extinction. The study, which focused on mollusks (invertebrates account for an estimated 95 percent of all known species), examined extinction rates and determined that since 1500 CE, between 7.5 and 13 percent of all known species have gone extinct at a rate higher than that prior to human involvement. The paper concludes that the sixth mass extinction event will continue to occur unless drastic conservation and environmental measures are taken. According to the World Economic Forum in 2023, these measures included building technologies and processes to help humans secure animal DNA to reverse the damage humans have caused. Also important was assisted breeding, cloning, genome editing, and sythnetic genomics to prevent extinctions.

Mass Extinctions in Evolution

Each of the Earth's major extinction events resulted in the loss of a minimum of 60 percent of species around the world. In thousands of years, the environmental conditions contributing to the event inevitably begin to dissipate, however, and the world is left with a relatively small number of species occupying a vast area. Large and highly specialized species are typically the first to succumb to extinction pressures, followed by species with restrictive needs for certain types of habitats. In the wake of the event, most of the species that survive are small, opportunistic species that are able to survive in a variety of habitats. The relatively hardy “pest” species that remain after the event proceed to populate the globe, giving rise to new species as they radiate to fill available niches in the surrounding environment. As a result, mass extinction events also represent periods in which the dominant species of the Earth shift from one group to another.

To give one of many examples, the Cretaceous-Tertiary extinction resulted in the loss of the terrestrial dinosaurs, but the direct ancestors of the dinosaurs, the birds, survived the extinction and spread around the world, occupying many of the environmental niches that were formerly occupied by prehistoric reptiles and dinosaurs. Similarly, the only mammal species that lived during the age of the dinosaurs were small, scavenger species. In the wake of the dinosaurs, mammals were free to radiate, growing into a variety of species and occupying all habitats on Earth. While extinctions bring an end to many types of organisms, they also enable new types of life to emerge and flourish in the new environment.

Principal Terms

background extinction rate: the rate at which species become extinct given the absence of any extraordinary environmental phenomena

biodiversity: a measure of the diversity of life forms in a certain area, which may be limited to a certain biome or may include the entire biosphere of the planet

extinction: the disappearance of a species from the biota of the Earth, as occurs when the last representative of a species dies without leaving offspring

fossil record: the history of life on Earth as interpreted from the fossilized remains of extinct life forms

global warming/cooling: the increase or decrease in the average temperature of Earth, as caused by either naturally occurring geochemical processes or by the activity of life forms on Earth

K-T boundary: the layer of sediment deposited at the end of the Cretaceous period that contains minerals that may have arrived on Earth from a meteorite impact

mass extinction: a period in which the extinction rate for species rises above the background extinction rate predicted for species under relatively constant conditions

press/pulse model: the theory that extinctions result from the slow and constant buildup of environmental pressures set into motion by major environmental changes

radioisotope dating: the process of using the decay of radioactive isotopes found in mineral deposits to obtain the date at which nearby geological items were deposited in the surrounding soil

sixth extinction: a theoretical extinction predicted to occur sometime in the future and potentially caused at least partially by human activity on Earth's surface

supercontinents: large landmasses containing two or more of Earth's continents in a single landmass

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