Extinctions and evolutionary explosions

Extinction as a Process

Living organisms on the Earth are classified under a taxonomy, or a biological system of classification, first devised by the Swedish biologist and physician Carolus Linnaeus (1707–1778). The primary groupings include the domain, kingdom, phylum/division, class, order, family, genus, and species. The most encompassing ranks begin with the three domains—Bacteria, Archaea, and Eukarya—groupings containing the largest number of organisms. There are between five and seven kingdoms depending on the system used, but genetic research in the twenty-first century sparked debate among scientists concerning these groups. Some researchers proposed abandoning the traditional kingdom classification, and others suggested revisions. A genus (plural is genera) is a classification for organisms with far more similar characteristics. The most basic and specific groupings are species and subspecies. Research indicates that between 2 million and 3 trillion species exist on Earth, and scientists estimate that between 85 and 90 percent of species have yet to be described.

The extinction of species is a continuous process, and evidence of its occurrence abounds in the fossil record. Scientists estimate that marine species persist for about four million years, which translates into an overall loss of about two or three species each year. This is the “background” extinction rate, and it is balanced by speciation events that result in the development of new species. However, some research indicates that modern rates of extinction are 100 and 1,000 times higher than the pre-human background extinction rate. A more catastrophic occurrence is called a mass extinction event. These periods are characterized by the loss of 70 to 95 percent of all species of animals, plants, and microorganisms. It is estimated that there have been five mass extinction events on Earth in the previous 450 million years. A number of mass extinction events have been recognized in the Phanerozoic era, the era during which life on Earth became visible. Using statistical methods, researchers estimated that at least 65 percent of species became extinct at each of these events, with 77 percent being eliminated at the event at the end of the Cretaceous period and 95 percent at the event at the end of the Permian period. These mass extinctions were balanced by periods of explosive development that often followed as organisms moved into vacant adaptive zones during periods of adaptive radiation. The most important of these was at the base of the Cambrian period, 544 million years ago, when all the major groups in existence originated, but other stages occurred in the Early Triassic period and at the start of the Tertiary period.

Causes of Mass Extinctions

Attempts to explain the causes of mass extinctions have centered on terrestrial phenomena such as sea level changes, climatic changes, or volcanic activity. Volcanic activity contributes to extinction by increasing the carbon dioxide in the atmosphere and causing the oceans to become toxic, leading to toxic metal poisoning, acid rain, and ozone damage. Global sea levels experienced regular fluctuations during the Phanerozoic era, likely related to the melting or formation of polar ice caps or to major tectonic events such as continental splits or the collision and uplift or subsidence of ocean ridges. Several mass extinction events, including the Ordovician, Permian, and Cretaceous, are correlated with periods of marine regression. During such a regression, the withdrawal of the ocean leaves a much smaller habitat for shallow marine organisms. This leads to increased crowding and competition and, ultimately, to an increased extinction rate. The reduction of large terrestrial vertebrates during these regressions, as happened during the events at the end of the Permian and Cretaceous periods, may be related to increased seasonality caused by the loss of the ameliorating influence of the shallow epicontinental seas.

Some extinctions are related to transgressive events, which are the spread of the sea over land areas. They may also result from the spread of anoxic or oxygen-poor waters across epicontinental areas. Climatic changes seem to be correlated with eustatic events, such as worldwide changes in sea levels. The evidence implicating temperature as the main cause of extinctions seems weak. For example, the most important extinction event at the end of the Permian period occurred at a time of climatic amelioration marked by the disappearance of the Gondwanaland ice sheet. Volcanic activity has been presented as a possible cause of the extinctions that occurred at the end of the Cretaceous period. The Deccan Traps of northern India were erupting at that time and would have produced large quantities of volatile emissions that could have resulted in global cooling, ozone-layer depletion, and changes in ocean chemistry. However, no evidence exists as yet for the involvement of volcanic activity in other extinction events.

Although various extraterrestrial causes for mass extinction events have been suggested in the past, these ideas have gained greater credence since the 1980s work of Luis and Walter Alvarez. They ascribe the end-Cretaceous extinction event to the effects of the impact of a large bolide, or an asteroid, perhaps ten kilometers in diameter. The depression formed by the impact of the asteroid is called the Chicxulub crater and is located off the Yucatán Peninsula in Mexico. The impact of such a large object would have resulted in months of darkness because of the global dust clouds generated. This would have halted photosynthesis and resulted in the collapse of both terrestrial and marine food chains. Although cold would initially have accompanied the darkness, greenhouse effects and global warming would have followed as atmospheric gases and water vapor trapped infrared energy radiating from Earth. Other physical evidence for the impact rests on the presence in the period boundary layers of high concentrations of iridium and other elements generally rare at the Earth’s surface but abundant in asteroids. In addition, these layers often contain shocked quartz grains, otherwise found only in impact craters and at nuclear test sites, as well as microtectites, glassy droplets formed by impact. Although the evidence for extraterrestrial impacts having caused the other major extinction events is slight, this causal factor has been linked with the apparently regular 26-million-year periodicity exhibited by extinctions. Scientists suggest that the regular passage of an unidentified planetary body by the Oort Cloud of comets and the subsequent perturbation could result in increased asteroid impacts and extinction events on Earth.

Historical Mass Extinctions

The mass extinction event at the end of the Permian period was the most severe of the Phanerozoic era and resulted in the extinction of up to 95 percent of all marine invertebrate species. On land, amphibians and mammal-like reptiles were badly affected and plant diversity fell by 50 percent. No iridium anomaly was found, and the most likely explanation is climatic instability caused by continental amalgamation and the simultaneous occurrence of marine regressions. These occurrences would have disrupted food webs on a major scale. The event that occurred at the end of the Triassic period was much less severe but still involved extensive reductions in marine invertebrates and reptiles. On land, a major faunal turnover took place. Primitive amphibians, early reptile groups, and mammal-like reptiles died out and were replaced by advanced reptiles and mammals. No evidence of an impact event has been found, and the extinctions are generally correlated with widespread marine regressions. The extinction that took place at the end of the Cretaceous period has become the most hotly debated, in large part because of the bolide impact hypothesis. Although the broad pattern of extinction among marine organisms is known, the detailed picture only encompasses microorganisms such as planktonic foraminifera and calcareous nannoplankton. Study of the ranges of these microorganisms shows that the extinctions occurred over an extended period, starting well before and finishing well after the boundary. Although much has been made of the extinction of ammonites at the end of the Cretaceous period, there are too few ammonite-bearing sections to show whether it was gradual or abrupt. On land, evidence of an increase in the population of ferns just above the boundary suggests the presence of wildfires, as ferns are usually the first plants to recolonize an area devastated by fire. However, in many sections, a return of the Cretaceous vegetation is seen above the fern increase, indicating little extinction.

The mass extinction event at the end of the Permian period was the most severe of the Phanerozoic era and resulted in the extinction of up to 95 percent of all marine invertebrate species. On land, amphibians and mammal-like reptiles were both badly affected, and plant diversity fell by 50 percent. No iridium anomaly was found, and the most likely explanation is climatic instability caused by continental amalgamation and the simultaneous occurrence of marine regressions. These occurrences would have disrupted food webs on a major scale. The event that occurred at the end of the Triassic period was much less severe but still involved extensive reductions in marine invertebrates and reptiles. On land, a major faunal turnover took place. Primitive amphibians, early reptile groups, and mammal-like reptiles died out and were replaced by advanced reptiles and mammals. No evidence of an impact event has been found, and the extinctions are generally correlated with widespread marine regressions.

The extinction that took place at the end of the Cretaceous period has become the most hotly debated, in large part because of the bolide impact hypothesis. Although the broad pattern of extinction among marine organisms is known, the detailed picture only encompasses microorganisms such as planktonic foraminifera and calcareous nannoplankton. Study of the ranges of these microorganisms shows that the extinctions occurred over an extended period, starting well before and finishing well after the boundary. Although much has been made of the extinction of ammonites at the end of the Cretaceous period, there are too few ammonite-bearing sections to show whether it was gradual or abrupt. On land, evidence of an increase in the population of ferns just above the boundary suggests the presence of wildfires, as ferns are usually the first plants to recolonize an area devastated by fire. However, in many sections, a return of the Cretaceous vegetation is seen above the fern increase, indicating little extinction.

Post-Extinction Recoveries

Among the vertebrates, a picture of gradual change is seen for mammals, with drastic reductions occurring only in the marsupials. The boundary also does not seem to have been a barrier for turtles, crocodiles, lizards, and snakes, all of which came through virtually unscathed. The dinosaurs did become extinct, and much argument has centered on whether this was abrupt or occurred after a slow decline. In this context, it must be noted that there is only one area where a dinosaur-bearing sedimentary transition across the boundary has been seen, and that is in Alberta, Canada, and the northwestern United States. Records of dinosaurs in this area during the upper part of the Cretaceous period show a gradual decline in diversity, with a drop from thirty to seven genera over the last eight million years.

The main period of evolutionary expansion in the Phanerozoic era was at the beginning of the Cambrian period, about 541 million years ago. Termed the “Cambrian explosion,” it marks the development of all the modern phyla of organisms, and as many as one hundred phyla may have existed during the Cambrian period. This period seems to have lasted only about 5 million years, and the subsequent history of animal life consists mainly of variations on the anatomical themes developed during this short period of intense creativity. This period is represented in the fossil record by the remarkably well-preserved Burgess Shale fauna of British Columbia, which has been extensively described, a well as saunas of similar age from China and Greenland. Why the Cambrian explosion could establish all major anatomical designs so quickly is not clear. Some scientists believe that the lack of complex organisms before the explosion had left large areas of ecological space open, and when experimentation took place, particularly with the advent of hard skeletons, any novelty could find a niche. Also, the earliest multicellular organisms may have maintained a genetic flexibility that became greatly reduced as organisms became locked into stable and successful designs. Why some of the innovations were successful in the long term, and others were not is unknown, as no recognized traits unite the successful taxa. It has even been suggested that success may be due to no more than chance.

In contrast, the recoveries after the major extinctions at the end of the Permian and Cretaceous periods did not result in the development of new phyla. The earliest Triassic ecosystems were more vacant than at any time since the Cambrian period, yet no new phyla or classes appeared in the Triassic period. This suggests that despite the overwhelming nature of the extinctions, the pattern was insufficient to permit major morphological innovations, in part probably because no adaptive zone was entirely vacant. Hence, despite the fact that the mass extinction at the end of the Permian period triggered an explosion in marine diversity described as the Mesozoic marine revolution, persisting species may have limited the success of broad evolutionary jumps.

Reading the Fossil Records

All understanding of extinction events or of evolutionary explosions depends on the fossil record. The study of the diversity of organisms through time—the number of different types of organisms that occur at a particular time and place—is therefore very important. The basic data consists of compilations of extinctions of taxa plotted against similar compilations of originations of taxa. Periods when either extinction or origination was unusually high show as peaks or troughs on a graph. Unfortunately, biases in the preservation, collection, and study of fossils have conspired to obscure patterns of change in diversity.

Geological history of patterns of diversity is obscured by a variety of filters, many of which are sampling biases that cause the observed fossil record to differ from the actual history of the biosphere. The most severe bias is the loss of sedimentary rock volume and area as the age of the record increases because the volume and area correlate strongly with the diversity of organisms described from a stratigraphic interval. The quality of the record also tends to fall with increasing age because the rocks are exposed to changes that may destroy the fossils they contain. The differences in levels of representation among the paleoenvironments in the stratigraphic record also influence the composition of the fossil record; for example, shallow-marine faunas are much better represented than terrestrial faunas.

Diversity patterns are studied at a variety of levels, from the species upward, that vary in their quality and inclusiveness. The basic problem is that many of the processes of interest occur at the species level or below it, but the biases of the fossil record mean that data are best at higher levels. The diversity of shallow-marine organisms for the Phanerozoic era cannot be read directly at the species level because the record is too fragmentary. The record at the family level is much more complete because the preservation of one species in a family allows the family to be recorded. For this reason, paleodiversity studies are often conducted at the family level. However, higher taxon diversity is a poor predictor of species diversity. For example, an analysis of the mass extinction at the end of the Permian period indicated that the 17 percent reduction in marine orders and 52 percent reduction in marine families probably represented a 95 percent reduction in the number of species. Another problem with the study of fossils is that soft-bodied and poorly skeletonized groups may leave little or no record. It has generally been assumed that the ratio of heavily skeletonized to non-skeletonized species has remained approximately constant through the Phanerozoic era; however, there is no data to support this, and there is some evidence that skeletons have become more robust through time in response to newly evolving predators. The net result of these biases is severe. Only 10 percent of the skeletonized marine species of the geologic past and far fewer of the soft-bodied species are known.

Despite these problems, scientists have demonstrated that biodiversity varied in several ways in the Phanerozoic era. Tabulations of classes, orders, and families indicate significant periods of increased extinction or increased evolutionary rates. One of the most important uses of these data is the tabulation at the family level, which shows a regular periodicity of about 26 million years for extinction events, and that has been used to support ideas about periodic extraterrestrial events. However, although fluctuations occurred, it has also been possible to show that the number of marine orders increased rapidly to the Late Ordovician period and has remained approximately constant since then.

The Ebb and Flow of Life on Earth

Mass extinctions and evolutionary explosions are the opposite faces of the pattern of diversity of organisms through time. During periods of mass extinction, the diversity of organisms on Earth has dropped drastically, and in some cases, entire lineages have been wiped out. Evolutionary explosions, on the other hand, resulted in enormous innovation, particularly at the beginning of the Cambrian period and the development of new variations on established morphotypes (animal and plant forms and structures) later in the geologic record. Understanding the processes that caused these events is of major importance because people have reached the point where they are capable of influencing their environment in drastic ways.

Studies of extinction events have shown that there are a variety of causes. Some are environmental changes brought about by natural processes, while others may result from extraterrestrial forces. The most severe of these extinction events occurred at the end of the Permian period, 252 million years ago, and resulted in the loss of up to 95 percent of marine invertebrate species. The cause of this extinction is that continents were amalgamating and oceans were retreating, which resulted in a major reduction in the habitat of shallow-marine organisms. Terrestrial habitats were also affected as the increase in continental area and loss of the ameliorating effect of extensive areas of shallow ocean brought about climatic changes. While climatic changes are the main culprit in most extinction events, some scientists believe that large bolides, or extraterrestrial bodies, struck the Earth with such force as to create major environmental changes that significantly reduced diversity. This theory has enjoyed the most popularity as the explanation for the event at the end of the Cretaceous period 65 million years ago, during which the dinosaurs became extinct. Still, evidence for an extraterrestrial body’s involvement in other events is slight.

Whatever the cause, environmental change that results in habitat reduction is the main reason for species decline. As humans have risen to dominance over other species, the extinction rate has accelerated, and in the last half-century, this rate has climbed considerably above natural attrition as populations have increased and habitats have been altered or destroyed. Approximately 75 percent of Earth's marine areas have been altered by humans, as well as around 70 percent of land. Although the levels of extinction have not yet reached those recorded during major extinction events of the past, some scientists believe Earth is approaching its sixth ecological disaster that could end in mass extinction. Evidence supporting this assertion is pervasive in the twenty-first century—extreme weather changes, flooding, drought, wildfires, melting ice caps, rising temperatures, and marine life depletion, among others. A better understanding of the processes surrounding past extinction events and the rebounds that followed them will help people prepare for and deal with the future.

Principal Terms

Adaptive Radiation: the rapid production of a new species following invasion of a new geographic region or exploitation of a new ecological opportunity

Bolide: an extraterrestrial object (for example, a meteorite) that hits the Earth

Diversity: the number of fossil taxa (classification groups) associated with a particular place and time

Lazarus taxa: groups that apparently disappear during a mass extinction only to appear again later

Mass extinction: an event in which a large number of organisms in many different taxa are eliminated

Periodicity hypothesis: the proposal that mass extinctions have occurred approximately every 26 million years over the past 250 million years

Phanerozoic: an era of geologic time beginning approximately 541 million years ago at the start of the Cambrian period, when animals with mineralized skeletons became common

Regression: the migration of the shoreline and associated environments toward the sea

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