Extinction

In 1796, a French naturalist, Georges Cuvier, demonstrated incontrovertibly that many species of once-living plants and animals had completely disappeared from the earth. Cuvier was also the first to recognize that many of the extinct species he identified had disappeared at approximately the same time—a mass extinction. He and his successors attributed extinctions to catastrophic events, such as the biblical Flood. The catastrophists held the field of scientific opinion concerning extinction until the publication of Charles Lyell’s Principles of Geology (1830–1833), which proclaimed the doctrine of uniformitarianism. Lyell maintained that no processes have affected the earth (including its flora and fauna) that are not presently observable. He denied that any spectacular cataclysms had occurred.

Climate, Evolution, and Extinctions

Lyell’s arguments convinced geologists, but the problem of extinctions remained: What could cause a species to die out? In 1859, Charles Darwin offered a biological explanation for extinctions that seemed to answer all questions. In On the Origin of Species, Darwin suggested that all life-forms engage in a perpetual struggle for survival. The best-adapted species, therefore, survive and perpetuate themselves. The less-adapted species are outcompeted and disappear. Darwin’s ideas seemed to fit well with the doctrine of uniformitarianism, but the problem of mass extinctions remained. He suggested no reasons why great numbers of very different species should disappear at approximately the same time.

Although his ideas were not accepted for many years after Darwin, another scientist proposed a possible solution to the puzzle of mass extinctions as early as 1837. Louis Agassiz produced evidence that the earth has periodically undergone periods of extreme cold, with much of its surface covered by glaciers. Agassiz’s glaciers are presently in existence, move very slowly, and thus fit well with the uniformitarian view. His glaciers might also explain mass extinction in evolutionary terms, since plants and animals that could not adapt to changing climate would be outcompeted by other species that could adapt, and, thus, the less-flexible species would disappear.

Uniformitarian views about extinction received a further boost in the twentieth century with the proposal of the theory of continental drift, which has evolved into the modern theory of plate tectonics. Scientists have demonstrated that the landmasses of the earth are in constant motion relative to one another and to the poles of the planet. Over millions of years, the present landmasses have occupied very different positions on the earth’s surface, often drifting quite near the polar regions, causing massive climatic changes. In addition, scientists have demonstrated that landmasses have subsided to be covered by the seas and have risen to create dry land from ocean floors. These massive changes would also have a profound effect on flora and fauna, creating constant competition and struggle for survival.

Genetic Factors in Extinctions

Many contemporary biologists believe that background extinctions may be the result of genetic rather than strictly climatic factors. In simple terms, extinction results from an excess of deaths over births in a given species. This excess represents, in biological terms, a reduction in the characteristics of a species that allows it to adapt to its environment. Those characteristics, biologists argue, are brought about through natural selection, a process that favors the genes that give the organism an advantage in the struggle for survival. Since natural selection has produced not only the species itself but its ancestors as well, any change in environment must work against the existing organisms. In the new process of changed environment, new gene combinations will result that will produce an organism better adapted to the changed environment. If, however, these genetic changes do not occur rapidly enough, adaptation will not take place and the species will become extinct.

Another possibility results if an organism is too well adapted to its environment. The gene pool of a species usually combines those genes that work well with one another to produce a well-adapted individual. Some biologists have observed a tendency within the gene pools of some species to resist genetic adaptation when environmental changes occur. Thus, genetic changes that could impart greater chances for survival of the individual in times of environmental turbulence do not occur. This effect also occurs when groups of genes are linked together (usually by a chromosomal inversion) in such a way as to prevent new gene combinations that might impart the ability to adjust to changed environmental conditions. This phenomenon inhibits the ability of a species to survive and may lead to extinctions.

Genetics may also explain mass extinctions. One biologist has theorized that species which live in environments that change very little over long periods of time probably have low genetic variability (a gene pool in which nonutilitarian genes have disappeared). Conversely, species in changeable environments should have much more genetic diversity, allowing them to cope with rapidly altering living conditions. If that is true, then mass extinctions might result from a rapid environmental change after a very long period of stability. When the change came, the theory goes, widespread extinctions of many species resulted. Most geneticists, however, reject this theory of mass extinctions. The species in unstable environments may actually have less genetic variability than those in stable environments. Biologists, therefore, have not been able to advance a plausible genetic explanation for mass extinctions.

Mass Extinctions

Geologists have identified a number of what they call “mass extinction events.” The first such event known to paleontologists occurred 440 million years ago at the end of the Ordovician period, during which more than 22 percent of all families and 57 percent of all genera disappeared. Another mass extinction occurred during the Devonian period, 370 million years ago, during which more than 20 percent of all marine families disappeared. The greatest of all mass extinctions occurred 248 million years ago, at the end of the Permian period. During that event, 52 percent of all marine families, 83 percent of all genera, and a frightening 95 percent of all species became extinct. During this event, land animals and plants vanished, along with marine flora and fauna. Yet another “great dying” took place during the Triassic period, approximately 215 million years ago, when 20 percent of all marine families and 48 percent of all genera disappeared.

The mass extinction known most widely outside the scientific community occurred at the end of the Cretaceous period, about 65 million years ago. During this event, the dinosaurs vanished, along with 50 percent of all marine genera. A number of less-spectacular mass extinctions have taken place in Earth’s long history, including a relatively recent one at the end of the Pleistocene epoch, which included among its victims such well-known extinct animals as the woolly mammoth and the so-called saber-toothed tiger.

Some paleobiologists and archaeologists are convinced that many of the Pleistocene extinctions were caused by the activities of human beings. They point out that the extinction of most of the large North American land mammals coincided with one theoretical date for the appearance of humans in the Western Hemisphere, approximately 11,500 years ago. According to these scientists, especially efficient human hunters were responsible for those extinctions; however, this theory seems unlikely as an explanation for all the Pleistocene extinctions. Many species in areas other than North America disappeared at the same time, most of which would have had little or no value as game. In addition, there are huge “boneyards” containing fossils of the extinct species in areas as far separated as Alaska and Florida that apparently died at the same time from causes that seem to be related to some great natural cataclysm.

Asteroids as Agents of Mass Extinctions

The species that disappeared in each of these mass extinctions apparently died at approximately the same time, and scientists were at a loss to explain them until 1980. In that year, a scientific team led by Nobel laureate Luis Alvarez presented what seems to be irrefutable evidence that the extinction of the dinosaurs coincided with the collision of a huge asteroid or comet with the Earth. The evidence is based on a layer of clay that separates the rock formations associated with the dinosaurs from the overlying formations, which contain fossils from the era of mammals. The clay contains large amounts of iridium and other elements that are scarce in the crust of the earth but common in asteroids and comets.

Many scientists now contend that the collision between the Earth and a large extraterrestrial body would have thrown enormous quantities of dust into the atmosphere, sufficient to block out the sun’s radiation for an extended period. The resulting subzero weather would have had devastating effects on flora and fauna, including the seemingly invincible dinosaurs. Shortly after the evidence for the collision appeared, other scientists presented evidence for an even more frightening phenomenon—periodicity of extinction events.

A scientist charting the occurrence of mass extinction events showed that they seem to occur at regularly spaced intervals, approximately every 26 to 30 million years. Almost immediately after the presentation of the evidence for periodicity, new scientific studies demonstrated more evidence that several mass extinctions other than the one during which the dinosaurs disappeared are also associated with unusually high concentrations of iridium. Taken together, these data seem to indicate that most, perhaps all, mass extinctions are caused by extraterrestrial agents and recur on a regular basis. If that is indeed the case, the implications for all the sciences, including evolutionary biology, are profound.

An Interdisciplinary Study

Scientists from several different disciplines are currently studying extinctions, including mass extinctions, in a variety of ways. Many biologists believe that the most effective way to understand background extinctions is to examine those that have taken place in historic times. Geologists and paleontologists are subjecting the fossil record to a new and rigorous examination, armed with new, supersensitive techniques for ascertaining the ages of the rocks in which fossils occur in an attempt to understand mass extinctions better. These new techniques are the products of research in nuclear physics. Ecologists are particularly examining currently endangered flora and fauna, which may soon disappear. Even some astronomers are actively engaged in research into mass extinctions, scanning the heavens with powerful telescopes in search of an extraterrestrial agent that might explain the apparent periodicity of mass extinction events.

Biologists studying recent background extinctions conclude that virtually all of them are the results of the activities of humankind. The great auk, the last known specimens of which were killed by Icelandic fishermen in 1844; the Tasmanian tiger cat, the last known specimen of which died in captivity in 1934; and many other species are examples of human-caused extinctions. These studies lend validity to the theory that at least some of the many extinctions during the Pleistocene resulted from the hunting activities of prehistoric peoples. More disturbing are studies which show that such modern phenomena as acid rain and ozone depletion, both results of industrialization, may be doing irreparable damage to the environment, which could result in another mass extinction event in the very near future. Indeed, some biologists and ecologists believe that such an event has already begun.

A number of geologists and paleontologists, using new dating techniques based on the rate of radioactive decay in rocks, are reassessing the ages traditionally assigned to fossils by less sophisticated techniques in the past. These studies should eventually reveal whether the mass extinction events of the remote past occurred in a very short or over a relatively longer period of time. Other geologists are searching for impact craters, to lend further credence to the theory that at least some and perhaps all mass extinctions resulted from periodic collisions between the Earth and large celestial bodies.

A team of physicists (including astrophysicists), engaged in an ongoing search for a hypothetical dark companion to our sun, has postulated that the orbit of this presently undiscovered body may periodically disrupt the comet cluster on the outer fringes of the solar system, resulting in many comets being diverted into an intersection with the Earth’s orbit. If their search is successful, it will provide powerful substantiating evidence for the collision theory, with sobering implications. Some physicists have even proposed ways to prevent future collisions between the Earth and large heavenly bodies.

The implications of background and mass extinctions are profound. If, as an overwhelming mass of evidence seems to indicate, the activities of humankind are a major cause of recent background extinctions, then those activities may soon lead to an ecological disaster of gigantic proportions. At present rates of disappearance, as many as two million species currently in existence will disappear by the middle of the twenty-first century. Unless immediate steps are taken, plant extinctions on the scale envisioned by many botanists may also cause massive climatic changes. Some ecologists and geophysicists warn that if the tropical forests disappear, the result will be the greenhouse effect. Of less immediate, but nevertheless great, concern is the theory that mass extinction events in the fossil record have resulted from collisions between the Earth and asteroids or comets. The implications of the collision theory for evolutionary biology, however, are far-reaching. If the theory is correct, then the struggle for survival is not the most important feature of evolution. No matter how well adapted to its environment a species may be, survival of a collision would be largely a matter of chance. If the theory proves to be correct, biologists will need to rewrite the textbooks on evolution.

In 2019, the United Nations published a comprehensive assessment compiled by hundreds of experts that concluded that the direct interference of 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. Human impact on the environment not only affects the lives of a multitude of species, but it also impacts human livelihood. Land degradation and the decline of wild bee populations put annual crop production at risk; and coral reef and mangrove loss along coasts cause an increased risk of flooding that could affect up to three million people. Experts predict that radical changes to strengthen and create environmental protection laws to protect both endangered species and to expand protected nature preserves across the globe are needed to slow the decline of biodiversity. It was announced in 2019 that diplomats from around the world would meet under the Convention on Biological Diversity global treaty to discuss how best to improve efforts at conservation.

In 2021, however, the US Fish and Wildlife Service proposed to move twenty-two animals and one plant species off of the endangered species list after they were shown to be extinct. The species joined an extinction list of over 650 US species. By the close of 2022, scientists counted approximately 900 known species that had gone extinct since 1500. Among those lost in 2022 include—Chinese paddlefish, mountain mist frog, Coote’s tree snail, ivory-billed woodpecker, and dugong, among others. Over 9,000 species remained on the critically endangered list. In 2023, twenty-one more species were declared extinct, inlcuding eight of Hawai'i's honeycreeper birds.

Principal Terms

Catastrophism: A scientific theory which postulates that the geological features of the earth and life thereon have been drastically affected by natural disasters of huge proportions in past ages

Fossil: A remnant, impression, or trace of an animal or plant of a past geological age that has been preserved in the earth’s crust

Gene: An element of the germ plasm that controls transmission of a hereditary characteristic by specifying the structure of a particular protein or by controlling the function of other genetic material

Gene Pool: The whole body of genes in an interbreeding population that includes each gene at a certain frequency in relation to other genes

Species: A category of biological classification ranking immediately below the genus or subgenus, comprising related organisms or populations potentially capable of interbreeding

Uniformitarianism: A scientific theory that all processes which have affected the earth and living creatures thereon in the past are presently at work and observable by scientists

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