Mass extinctions

Mass extinctions demonstrate that the evolution and extinction of life are not smooth but rather are interrupted by mass dyings as a result of as yet poorly understood causes. Mass extinctions seem to be an integral part of the overall pattern of life processes on the Earth.

Patterns of Extinction

The extinction of animal species appears to be as much a part of the pattern of life on Earth as is the evolution of new species. Indeed, the two processes go hand in hand as new species fill the environments vacated by dying ones. Extinction is so common that more than 99 percent of the species that have ever lived are now extinct. At periods throughout Earth's history, extinctions of numerous and widely varied species have occurred within a relatively narrow range of time. Climatic changes, worldwide catastrophic events, and changing sea levels lead the list of possible causes of these mass extinctions. A species cannot be considered separately from the environment in which it lives. A change in the environment may occur faster than the organism can adapt and evolve along with it. This species may become extinct, while another species will develop to fill the ecological niche left behind. Thus, it appears that the ability of a species to survive may depend on its ability to adapt quickly to changing conditions.

The pattern holds true for both plant and animal life. Mass extinctions of plant and animal life, however, do not seem to coincide. There have been three revolutions in principal plant forms. Each evolved quickly, and once a major group of plants was established, it continued for millions of years. Animal life is marked by frequent minor extinctions, and mass extinctions of animal life appear successively throughout the geologic record. In fact, it is these very extinctions that have, in many cases, provided the reference points that separate one geologic period from another. The consensus among scientists is that Earth has experienced five mass extinction events. Hundreds of minor extinctions have occurred as well.

Known Mass Extinction Events

The earliest known mass extinction occurred some 450 million years ago and is known as the Late Ordovician mass extinction. Scientists believe the Late Ordovician mass extinction to be the second-largest mass extinction event in the history of the Earth, with an estimated global loss of 85 percent of oceanic species of life.

The second mass extinction event is known as the Late Devonian extinction. Occurring near the end of the Devonian period, 370 million years ago, the Late Devonian extinction consisted of a collection of extinction events that resulted in the loss of 97 percent of all vertebrate life on Earth.

The third and largest mass extinction in Earth's history is known as the Permian–Triassic extinction event and happened some 250 million years ago at the end of the Permian period. This extinction also marked the end of the Paleozoic era. Half of all families of animal life were exterminated, including 75 percent of all the amphibian families and 80 percent of the reptiles. Marine life was hit hardest; it has been estimated that 95 percent of shallow marine organisms became extinct. It took 15 million to 20 million years for animal life to recover to anywhere near the previous variety.

Known as the Triassic–Jurassic extinction event, the fourth mass extinction took place at the end of the Triassic period, some 200 million years ago. Up until this time, primitive amphibians and reptiles had dominated the land. By the end of the event, they had died out and were replaced by the early dinosaurs. It could appear that competition with the dinosaurs led to the demise of these creatures, but other extinctions took place at this time, particularly in the oceans, which point to a more general cause.

The fifth extinction event, known as the Cretaceous–Paleogene extinction event, is the most studied and best-known mass extinction event. The fifth mass extinction occurred at the end of the Cretaceous period some 65 million years ago, at a point called the Cretaceous–Paleogene (K–Pg) boundary. It is known mostly for the disappearance of the dinosaurs, but it also marks the extinction of many other forms of life, including many ocean reptiles, shelled creatures called ammonites, and forms of microscopic plankton. In all, 25 percent of all the known families and 75 percent of the known species of animals were eliminated. In this extinction, as was the case in previous extinctions, there was a gradual decline in diversity among animal life for tens of millions of years before the final extinction at the end of the period.

The latter part of the Cretaceous–Paleogene extinction event, some ten thousand years ago, saw the extinction of the giant land mammals. Well-known mammalian life forms that went extinct include the woolly mammoth, the mastodon, and the saber-toothed cat. There is some evidence that implicates the early forms of humankind in the extinction of these giant mammals. With organized hunting and the use of fire, it is conceivable that early humans played a role in the decimation of animal populations. Remains at ancient kill sites show that thousands of animals were killed in a single hunt. Even in historical times, it has been seen that with the arrival of primitive people to a new area, extinctions of indigenous life-forms followed.

Scientists believe that human interference has had enough of an impact on the Earth to initiate a sixth mass extinction event. Referred to as the Holocene extinction, this differs from prior mass extinctions due to the fact that the sixth mass extinction is mainly a result of human causes. Though Earth, in 2022, was not nearing the loss of species diversity seen in the K-Pg extinction, the planet was experiencing alarming rates of biodiversity loss. While concerning, scientists were also aware of their ability to study a mass extinction event in the making.

A study published in Biological Reviews in 2022 supported the idea of the Holocene. In the study, researchers from the University of Hawaii examined fossil records of mollusks dating back to 1500 AD to determine that an estimated 7.5 to 13 percent of all known species of invertebrates (which make up 95% of all known species of life) have gone extinct at a rate higher than that of the prehuman era. This is due to a number of possible reasons. While human demand for space increases, available habitats for other animals diminish. Remote areas that were once the havens of wildlife are now being destroyed by hunters, farmers, loggers, fishers, and developers. Those areas devoid of humans are visited by the products of civilization—acid rain, air and water pollution, and destruction of the ozone layer. The study concluded with the assertion that unless drastic environmental and conservation efforts are taken, the sixth mass extinction will continue to occur.

Causes of Mass Extinction

Current extinctions aside, why should a large percentage of life on Earth die out suddenly? The question can be misleading. For example, “suddenly” is a relative term when applied to geologic events. Changes happen over thousands of millions of years. An abrupt change in layers of sediment may indicate a change that took place over a period of a few days or a few million years. It is difficult to determine from the geologic evidence how sudden “suddenly” was.

Although it was long known that mass extinctions occurred from time to time throughout Earth's history, the causes were thought to be unknowable for lack of definitive evidence until high levels of the element iridium were found at the boundary marking the end of the Cretaceous period. With the possibility of testable evidence, the study of mass extinctions was opened again in earnest, and new relationships among available data began to be formed. As mass extinctions were documented exhaustively, what appeared to be a cyclic pattern emerged. According to one study, exterminations of life occurred about every 26,000 years. The periodicity remains in question, and theories range from collisions from an orbiting death star to the fact that random extinction events create an intrinsic periodicity.

Mass extinctions have a profound impact on the environment. Many theories begin, then, by determining the changing conditions at the time of the extinction. Although valid, a weakness in this method is that Earth's environment is constantly in a state of flux. In whatever time period one examines, changes are bound to be found. It is, therefore, merely speculation whether any particular change or set of changes was sufficient to cause a mass dying. In addition, it seems that mass extinctions cannot be studied from the perspective of a single species or even a single extinction event. Only by studying them as a whole can scientists determine patterns that may explain them. In general, mass extinctions kill species on the land and in the oceans at the same time. They strike hardest at large animals on the land. Freshwater animals are generally left unscathed. Plants are not affected as severely as animals. Any theory of mass extinction must therefore address these issues.

Mass Extinction Theories

Two theories of mass extinction have emerged that seem to handle the criteria. Neither theory is new, but each has received renewed interest with emerging evidence. One popular theory invokes a global catastrophe with a celestial origin. Impacts by meteorites or comets are proposed to be responsible for at least the mass extinction that killed the dinosaurs, if not others. A 1979 study by Walter Alvarez and Luis Alvarez showed that the presence of glass microspherules, shocked quartz, and the rare element iridium at the K–Pg boundary provides evidence for the impact of a bolide (a meteorite or comet that explodes upon striking the Earth). However, the effect of this on the extinction event has been hotly debated.

The other theory has to do with the changing sea levels of the Earth, a theory from the 1920s that was revived during the 1960s and 1970s in the light of the theory of plate tectonics. As the sea level drops, warm, shallow seas recede. The habitats of some marine animals are eliminated, while chemical changes in the deeper ocean water cause other extinctions. Land bridges appear, connecting areas that were once separated by water, and the migration of animals follows. Competition for available habitats is generated among species that were previously separated, also leading to extinctions. The drifting together and breaking apart of continents would dramatically affect habitats. It has been shown that, in general, the arrangement of continents affects the variability of climate, the variety of species on land, and the nature of habitats along the continental shelves in the ocean. Changes in these factors over long periods of time would lead to extinctions of animal species.

There is no single or simple answer to the cause of mass extinctions. The pivotal question seems to be whether they occurred instantaneously, requiring a single catastrophic event, or over an extended period. In a period of several million years, many changes will occur in the environment, some of them catastrophic. Volcanoes erupt, meteorites strike, climates change, and continents drift. There is no doubt that catastrophes occur, but how they affect life on Earth is as yet poorly understood.

All things considered, it is most likely that mass extinctions occur as a result of a combination of many factors, including catastrophic ones that happen over long periods of time. Perhaps a cyclic pattern exists, or perhaps species survival is a matter of playing the odds on a changing planet. Yet, if the Earth is considered to be a living organism, it is no wonder that as the planet ages and evolves, the life it supports also changes. In 2019, the United Nations published a comprehensive assessment compiled by hundreds of experts that concluded that the direct interference by humans had transformed natural landscapes so drastically that about one million plant and animal species have become at risk for extinction. Specifically, activities like farming, poaching, mining, overfishing, and deforestation have impacted the environment so much that up to 40 percent of amphibian species, 33 percent of marine mammals, and more than 500,000 land species face extinction due to the destruction or repurposing of their natural habitats. In addition, about 5 percent of species worldwide face extinction if the global average temperatures rise 2 degrees Celsius above preindustrial levels.

In 2020, several studies suggested the global warming related to volcanism and anoxia could have been a cause of mass extinction.

Study of the Geologic Record

The major clues to unraveling the pattern of life on Earth come from the study of the geologic record. The sequential layering of rocks, along with their deformation and erosion, allows scientists to construct a history of the processes the rocks have undergone. By analyzing the chemical composition of layers of rock and sediment and studying the nature of the fossil remains in them, scientists can begin to understand which forms of life existed during which time periods and when (and sometimes how) they became extinct. It was discovered that the layers could be dated by the decay of radioactive elements and that, in this way, the ages and periods of geologic time could be dated accurately.

Of all the fossils in the geologic record, fossils of tiny marine animals (microfossils) give the clearest pictures of mass extinctions. Produced prolifically in the world's oceans, they settled among the sediment on the ocean floor and fossilized in layers. These layers tell the story of how the creatures developed, diversified, flourished, and finally ceased to exist. Found in diverse areas of the world, from desert to mountainous regions, these fossils also tell the story of the movement of continents. Some of them form such complete records that their forms and types can be used to date sediments all over the world. While fossil remains of land animals can also be dated and related to specific time periods, these larger fossils are much scarcer and therefore do not present as complete a picture.

By studying traces of magnetism preserved in rock along with fossil deposits, geologists can chart the drifting of the continents in the distant past. The emergence of the theory of plate tectonics provided scientists with a framework for understanding why the Earth's environment and all its features are in a constant state of change. This changing environment is a key clue to periodic mass extinctions.

Besides the rise and fall of fossil species, other characteristics of the geologic layers can be measured. The relative abundances of certain elements indicate the processes that may have been occurring at the time the deposits were laid down. An example is the iridium found at the site of the extinction of the dinosaurs. Iridium is uncommon on Earth but is found in certain types of meteorites, which has led many scientists to believe that the extinction of the dinosaurs was caused by the impact of a comet or meteorite. Particles of soot were also found in the layers from that time period, suggesting that a large part of the Earth may have been burned. Shocked quartz, indicating violent activity, also supported the theory.

The story of life found in the geologic record leaves scientists with myriad clues but few answers. It is as if they had millions of minute pieces to an enormous puzzle; collecting the pieces does not solve the puzzle. The various clues must be interpreted, seen in the light of their prehistorical significance, and put together in fresh ways. As patterns emerge in the study of mass extinctions, the clues are beginning to fall into place, and an understanding of the dynamics of life on the Earth is emerging.

Principal Terms

ecology: the study of the relations between organisms and their environments

environment: the conditions affecting the existence, growth, and overall status of an organism

era: one of the major divisions of geologic time, including one or more periods

geologic record: the history of the Earth and its life as recorded in successive layers of sediment and the fossil specimens they contain

niche: in an ecological environment, a position particularly suited for its inhabitant

period: the fundamental unit of the geologic time scale

plate tectonics: the theory and study of the formation and movement of the sections that make up the outer portion of the Earth, which move independently over the interior

stratigraphic time scale: the history of the evolution of life on the Earth broken down into time periods based on changes in fossil life in the sequence of rock layers; the time periods were named for the localities in which they were studied or from their characteristics

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