Extinction is a word commonly associated with undesirable, catastrophic loss of entire populations or species. However, extinction is as much a part of the cycle of life on Earth as is evolution. Indeed, extinction and evolution together form the cycle responsible for the ever-increasing complexity of life on Earth. From the earliest evidence in the fossil record, we see populations and species disappear, to be replaced by other populations and species. In most cases, this occurs as the more complex organism replaces the simpler one. However, there have been several times in the Earth’s history when mass extinctions occurred that wiped out the complex forms but allowed simpler forms to survive.
It is human nature to seek ways to forestall or avoid the inevitable concerning human extinction, and thus the study of extinction has earned high priority in scientific circles. This heightened interest has led to significant research on the topic. A true understanding of the processes of extinction requires a detailed understanding of most facets of the Earth’s history, including climate shifts, tectonic movements, atmospheric conditions and content, sea level changes, and the flora and fauna itself. The geologic and fossil records are notoriously incomplete, making this research difficult. Even the tidbits that have been discovered, however, provide tantalizing glimpses into the intricate and complex patterns and processes of extinction.
Extinction can best be defined as the condition that exists when the last remaining individual of a population or species dies. Since this death can be caused by any number of factors and since extinction is inevitable for every species on Earth, understanding the causes, patterns, and processes involved can be daunting.
Types of Extinction
Extinction events can generally be divided into two types: background extinction and catastrophic, or mass, extinction. Background extinction follows the concept of Darwin’s “survival of the fittest,” perhaps better expressed as “survival of the luckiest.” For one reason or another, groups of living organisms regularly pass into oblivion. These reasons include lack of nutrients, disease, genetic anomalies, unusual weather conditions, and being out-competed for existing resources. Changes in climate seem to play the largest role in this type of extinction. Very minor changes, as small as having a drought in a region for a few years, can cause an extinction event.
Background extinction seems to be part of a cycle most species of organisms follow. Initially, a species comes into existence from organisms occupying a new environment or following a new life style compared with their ancestors’. These organisms typically exhibit few specializations, and they are generally not very well suited to the new environment or lifestyle. Through successive generations, the organisms become more specialized and better able to exist in the new environment, but this comes at the cost of being less able to cope with changes to the new environment. Finally, the organism faces the problem of a change in its new environment, and because it no longer has the ability to adapt and change rapidly, it dies out. If the species is not an evolutionary dead end, somewhere in this process, some of the offspring were either born defective, forcing them to move into a different environment to survive, or perhaps a small group was merely trapped in an area where the environment was somewhat different. In any case, the demise of the parent species leaves behind one or more daughter species, and the cycle continues. Throughout the history of the Earth, this pattern has produced a gradual increase in both the number of different species and the complexity of organisms on Earth.
Catastrophic extinction often has the opposite effect on the number and complexity of Earth’s species. Catastrophic extinctions, as far as is currently known, are always related to a massive, rapid change in the Earth’s global environment. Such extinctions tend to affect the most complex and specialized organisms more rapidly and seriously. Simpler, generalized forms are usually better able to survive catastrophic events. There have been numerous catastrophic extinction events throughout Earth’s history. Although there are many references to the “Big 5” and the current extinction as the sixth, there have actually been more than 20 times when mass extinction has occurred. Catastrophic extinction is defined as the extinction of a large number of species in a short time interval. It is easy to see that extinction events may be included or excluded from this category based simply on what one decides is a “large number of species” and a “short time interval.”
There have been at least 5 major catastrophic extinction events and at least 14 more events that saw losses of more than 20% of species of organisms on the Earth. In addition, there may be as many as a dozen more extinction periods in the fossil record that are potentially this severe. However, due to the incomplete nature of the rock and fossil records, these events may or may not have occurred, and the actual number of Earth’s catastrophic extinction events may never be known. Also, the classification of extinction events is arbitrary; there are no sharp separations in severity of extinction events. Extinctions of virtually every resolvable magnitude have occurred, from one species to virtually all species wiped out in a single event or time period.
Benefits of Extinction
Most of the research concerning extinction is concerned with the harmful effects to living species and the best means of preventing their extinction. A major force in human politics today is maintaining the current “status quo.” Humans do not like to see change, especially when it concerns the loss of species that humans value. However, the history of hominids would be quite different were it not for the driving forces of extinction and adaptation. The last few million years of Earth’s climate history have been unusually variable. The Earth has gone from very warm to very cool several times during this period. We are currently experiencing a moderate global temperature that is unusual from a geologic perspective. In fact, scientists are divided over whether we are on the brink of another ice age or a severe greenhouse event.
Such fluctuations of global climate have undoubtedly been responsible for the “weeding out” of hominid species such as Australopithecus in favor of the more intelligent Homo habilis. Had the climate not changed and the forests of the Pliocene given way to savannah grasslands, the australopithecines would likely still abound. The global cooling of the climate during the early Pleistocene caused the loss of the forest habitat of Australopithecus, and the species most likely died out as a result of this loss of habitat. However, the ability to use tools and the addition of meat to the diet allowed at least some members of Homo to survive and thrive long after the extinction of Australopithecus.
Similar climatic variation throughout Earth’s history is likely the single most important mechanism behind the evolution of increasingly more complex species. An event that spells disaster for one population or species is likely to provide an opportunity for another group to change, adapt, and grow. From the current data, it seems that only the most cataclysmic events in Earth’s history reverse this trend and cause the loss of the more complex species.
Major Extinction Events
The earliest-known, well-documented catastrophic extinction occurred near the end of the Precambrian, approximately 670 million years ago. This event wiped out approximately 70% of Earth’s species; stromatolites and acritarchs were especially hard-hit. This extinction likely produced the conditions needed for the Vendian fauna, a series of soft-bodied animals, to appear and evolve. In turn, the Vendian fauna were largely wiped out approximately 520 million years ago, and the Cambrian hard-shelled fauna appeared. Other events documented by major faunal turnovers include the end-Cambrian (488 mya), end-Ordovician (438 mya), late Devonian (360 mya), end-Permian (251 mya), end-Triassic (200 mya), end-Cretaceous (65 mya), and end-Pleistocene (the last 10,000 years).
Five of these extinction events have been recognized as the “Big 5”: the late Ordovician event, when 84% of animal species went extinct; the late Devonian extinction, when 79% of existing species were lost; the end Permian extinction, when over 95% of all species died out; the late Triassic extinction, when 79% of species vanished (this actually turned out to be two closely timed but separate events; however, it is still considered one of the “Big 5”); and the Cretaceous Tertiary (K/T) extinction, which ended the reign of the dinosaurs and over 70% of animal species disappeared. It is likely that more taxa were lost from the Vendian and Cambrian extinctions than from some of the “Big 5” events, but the paucity of the fossil record prevents truly accurate determinations. Also, please note that the numbers of missing species supplied here are calculations and estimates from studies mostly concerned with hard-shelled invertebrates. Other organisms either preserve poorly (small vertebrates, soft-bodied invertebrates, unicellular and colonial organisms) or are primarily nonmarine (plants, higher vertebrates) and thus are preserved only under fortuitous circumstances. These factors severely limit the accuracy of the numbers listed above but should not significantly affect the relative severity comparisons.
The most severe extinction event was the Permo/ Triassic (P/T) extinction. It is estimated that the Earth was virtually sterilized at that time, with over 99% of all life being wiped out. Because it takes only a small surviving population of any taxon to preserve that taxon, the extinction percentages are largest at the lowest taxonomic levels. For instance, over 99% of individuals were lost during the Permo/Triassic event, but only 95% of species, 70% of genera, and 50% of family-level taxa were lost. It should also be noted that such a massive disruption to global ecology is also seen in the rocks themselves. The Paleozoic rocks of the world are noted for their abundance of marine, coral reef structures, and even amateur geologists easily recognize the red sandstones of the early Triassic in outcrops around the world. Only one family of coral survived. There has not been one fossil reef structure found in rocks dating to the first 10 million years after the Permo/ Triassic extinction event, and it took 100 million years for corals to again create significant reefs worldwide.
Causes of Extinction
Causes of catastrophic extinction, except in the current instance, are speculative at best. One observation that has troubled researchers for a long while now is the periodicity of extinctions and how this periodicity may correlate with astronomical events. There seems to be statistically significant regularity in the timing of extinctions, and this observation has spawned several hypotheses concerning causes for increased large meteor, asteroid, or comet impacts during those periods. As of this time, there is no conclusive evidence of this being the case, but the hypothesis does coincide with the K/T extinction being caused by asteroid impact. Unfortunately, the cycle is 26 or 27 million years in duration, and most evidence of impacts vanishes in only a few thousand years due to erosion and redeposition. Most recently, this line of investigation has led to the suggestion of a hypothetical binary sun (Shiva or Nemesis) or 10th planet (Planet X) that has an orbit that passes through the Oort Cloud, disturbing orbits and sending an increased amount of comets and other objects hurtling toward the Earth. It is well documented that a magnetic anomaly affects Uranus, Neptune, and Pluto, and this is the primary evidence that some large celestial body exists in the distant reaches of our solar system. Without precise measurements of this object and its orbit (its very existence is conjectural at this point), its effect on other objects in our solar system is impossible to accurately predict.
There are, however, several well-documented causes of worldwide catastrophe that don’t involve a hypothetical planetary body. Every major extinction event and most of the minor ones have been associated with one or more known climate-altering events. In fact, of the “Big 5,” all but the late Ordovician extinction are time equivalent, with massive extrusive volcanics and large meteor craters. Also, all of the “Big 5” are associated with large-scale glaciation except the K/T event. Trying to sort out the relative importance of these factors contributing to Earth’s extinctions has been the focus of much research, speculation, argument, and frustration. It is likely that every major extinction event involved a variety of factors that combined to create inhospitable, often uninhabitable, environments over large portions of the Earth’s surface.
It is now known that many of the recorded extinction events are correlated with changes in sea level. When glaciers cover significant portions of the Earth, virtually all continental shelf environments are brought to the surface, severely reducing the amount of available primary marine habitat. Likewise, when sea levels rise, major portions of the most habitable continental lowlands are inundated. This rise and fall of sea level is largely responsible for most documented extinctions, because the preserved sediments are primarily marine and show the loss of these types of life. Sessile organisms abound in continental shelf environments, but they are very susceptible to extinction through loss of habitat. It is possible that current research is skewed in over reporting (or underreporting) the seriousness of an event because most of the data gathered to date are from these marine environments.
The Current and Future Extinction
Compared with past extinction events, the current rate of species loss is very high. Should this rate continue for more than a few thousand years, the current event will easily rival the “Big 5” when it comes to number of species lost. However, it is beyond human experience to be able to recognize if this loss is comparable to other minor events or if it is truly a major catastrophe for life on Earth. The fossil and geologic records provide us with glimpses in time, but the records are of instants that are usually hundreds or thousands of years apart. If we were looking at the fossil record of this time from the vantage point of several million years in the future, we would not be able to separate those species lost during the last ice age from those lost last year.
The one comparison that can be made is how, in general terms, the current conditions on Earth match up with conditions during, or just prior to, any other extinction event. The Earth is recovering from an ice age, which matches the conditions found at the onset of at least four major extinctions. This is significant in that every documented ice age event is observed to coincide with an extinction event, and ours is no exception. When combined with a dramatic rise in temperature, as in a virtually instantaneous global-warming event, the matching situations found in the geologic record indicate a severe extinction. This, in fact, was the situation that was thought to have existed at the time of the P/T extinction event.
Unlike the P/T event, however, the Earth is not currently experiencing massive volcanic eruptions, nor is the Earth experiencing sizable asteroid impacts. Should both of these events happen, the Earth would undoubtedly see a repeat of the P/T extinction and most life would cease to exist. On the other hand, the P/T event was mitigated somewhat by the amount of carbon removed from the system through the reef-building processes of the Paleozoic. The Earth is currently experiencing a reintroduction of massive amounts of carbon previously removed from the system, and this is creating a carbon overload and atmospheric carbon dioxide spike.
On the whole, many scientists see many similarities between the Earth’s final minutes before the P/T extinction and the present. The two questions, “Do we need to stop this pattern?” and “Can we stop this pattern?” are likely to plague scientists for the foreseeable future. Unfortunately, the answers to these questions may determine the cause and severity of the next extinction event on Earth.
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