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Mass Extinctions

Extinction is the complete disappearance of a species or higher taxon of a living organism. Fossils offer testimony not only to light in the geologic past, but also to a gradual death that extinguished many life-forms in a very short time, geologically speaking. In these episodes, called mass extinctions, species of living organisms disappeared at a rate at least twice that of normal evolutionary processes. Some paleontologists describe evolution as a gradual process occasionally but rarely interrupted by cataclysmic changes (sudden and dramatic changes in Earth’s surface).

extinctions_passenger_pigeon.jpg

The passenger pigeon was made extinct by humans.

Scientists estimate that at least 99 percent of all species that ever existed are now extinct. Perhaps the best-known large-scale extinction was that of the dinosaurs at the end of the Cretaceous period of the Mesozoic era. In recent time, humans have had a dramatic impact on the number of species that are disappearing. The eventual survivors of the cataclysm then manage to regain a steady evolutionary rate. These cataclysms, or mass extinctions, are often referred to as the punctuation in evolution. Extinction is the eventual fate of all life-forms. Paleontologists say that at least 99 percent of all species that ever lived are now extinct. Different species dominated in different geologic times. The dinosaurs ruled Earth for some 100 million years before being ousted by mammals, which have ruled for the last 65 million years.

At certain points in Earth's history, vast numbers of different species of living organisms around the world have died out within a relatively short period of time. (In geologic time, a short period of time can be 10 million years.) Scientists refer to this phenomenon as a mass extinction. Such an extinction could not possibly be due to genetic changes in the species involved. From studies of the fossil record, scientists have identified a number of mass extinctions, including five in the last 500 million years. Species are continually becoming extinct, but in each of these mass extinctions, species suddenly started dropping much more rapidly than usual before eventually tapering off to a normal rate.

The earliest known mass extinction occurred about 650 million years ago, during the Precambrian period. At this time life was still in its early stages of evolution and consisted of large quantities of algae floating in Earth's primitive ocean. The fossil record reveals that about 70 percent of all algal species suddenly died. Some researchers suggest that the algae may not have been able to survive an intensive ice age that began at that time.

The most devastating mass extinction in Earth's history occurred between 255 million and 250 million years ago, at the end of the Permian period. This extinction, which stretched over some 10 million years (more than 100 times longer than humans have lived on Earth), killed 75 to 90 percent of all marine creatures and a large number of land species. Scientists have not agreed on a cause for this extinction, although some suspect that climate changes may have been to blame. Perhaps the bestknown mass extinction occurred about 65 million years ago, at the end of the Cretaceous period, which also marked the end of the geological era called the Mesozoic and the start of the Cenozoic era. It was during this mass extinction that the dinosaurs died out; in addition, a wide variety of other ocean and land animals and plants died.

In 1980 U.S. researchers Luis (1911–1988) and Walter Alvarez (b. 1940) discovered that, at several different areas around the globe, the fossil record of this mass extinction reveals a layer of minerals containing the rare element iridium. The Alvarezes used this information as the basis for a dramatic theory. They suggested that an iridiumrich asteroid may have hit Earth at that time, throwing a vast cloud of dust into the atmosphere. The crash of the asteroid itself would not have caused a worldwide mass extinction, but the dense cloud produced by the impact would have cooled the atmosphere by blotting out sunlight for several years or warmed the atmosphere by trapping heat. Either effect could have killed huge numbers of plants and the animals that depended on them for food. Since the theory was first proposed, other researchers have uncovered signs that the crater the asteroid made may lie off the coast of what is now Mexico's Yucatan peninsula.

This theory remains controversial. Some scientists have proposed other explanations for the iridium layer, such as the possibility that it was spewed into the air by volcanoes. Also, it is unclear whether an asteroid could have been responsible for the earlier mass extinctions because no extra iridium has been found in the fossil record for those times. Other researchers think that mass extinction caused by asteroids might be quite common. Scientists have statistically analyzed recent mass extinctions and note that they have occurred in a regular pattern. Some scientists believe that this pattern suggests a periodic bombardment from space, by comets or asteroids.

What Causes a Mass Extinction?

Evidence of five mass extinctions during Phanerozoic time (that period of time in which sediments were deposited that contain abundant plant and animal remains—from the Cambrian to the Quaternary; can be seen from five sudden breaks in the fossil record. Each of these mass extinctions drastically changed the nature of life on Earth. Alter a mass extinction, life does recover and may change and surpass its previous diversity. These changes take millions of years.

Studying the data left behind in fossils and rock strata, paleontologists attempt to discover what triggered the five major mass extinctions in the geologic past. A better understanding of plate tectonics and increased resolution of paleoclimates (the climates of past ages) are beginning to suggest reasons for mass extinctions, although each extinction event may have had a different cause. The fossil evidence of ancient life is testimony to more than the life-forms themselves. It also offers evidence of past environments influenced by climate and geographical location.

Seafloor sediments, soil, and rock samples attest to previous global glaciation—which affected much of Earth’s surface—ending only 10,000 years ago and also to extraterrestrial inputs to Earth’s environment. Concentrations of iridium space dust in seafloor and nonmarine sediments vary over time, and, in at least some cases, the highest dust concentrations correspond with mass extinctions. This finding supports the idea that a large meteor precipitated the mass extinctions. Also, at least for the recent geologic past, the high rate of iridium dust accumulation corresponds with times when ice covered much of Earth.

Is Climate Change Responsible?

Climate change is a primary candidate for the cause of mass extinctions. However, many factors can cause climate change. For example, even a small change in how much energy the Sun provides could drastically alter the climate on Earth. Climate change, both local and global, has also been caused by continental drift. As the relative location of the different landmasses changes, their climates also change. As the elevation of the land changes, the local climate and the nature, size, and position of the seas are altered. With each of these changes, life-forms have to adapt or die.

Even before the shelly fossils of Cambrian time, there was at least one mass extinction. Paleontologists theorize that the first known mass extinction was caused by a sudden climate change, an early ice age in Precambrian time. Some 650 million years ago, this first mass extinction killed 70 percent of algal species.

Five Mass Extinctions in the Fossil Record

Three mass extinctions marked the Paleozoic era, the first at the end of the Ordovician period, 438 million years ago. The trilobites that had dominated the marine environment since the start of the Cambrian were devastated. Paleontologists estimate that 57 percent of marine genera disappeared in this first mass extinction of Phanerozoic time.

The next extinction occurred in the late Devonian period, some 360 million years ago. This was a time when fish evolved in waters edged by swampy forests. Sharks, insects, and the first amphibians appeared. When the extinction began, marine life changed drastically. As many as 20 percent of the families of life-forms died out. A few families of trilobites survived this event. Sudden in terms of geologic time, the Devonian extinction was well over, with new faunas having mostly filled the niches by the end of the Devonian period.

The last mass extinction of the Paleozoic era occurred toward the end of the Permian period, approximately 245 million years ago (see PERMIAN PERIOD). In terms of the number of species wiped out, this is the largest known mass extinction. Some 95 percent of the species disappeared.

It took more than 10 million years for most of the life-forms in the seas, which were marine invertebrates, as well as a number of creatures on land to become extinct. Many species and groups that had successfully survived the previous mass extinctions finally succumbed, among them the trilobites. This was the time of the supercontinent Pangaea (see PANGAEA). As most of Earth’s landmass formed into a giant continent, the shallow coastal waters were drastically diminished. Inland seas drained as sea level fell. These changes would nave affected life by drastically altering the climate; with no surface waters to improve climate, a dry cold set in over newly formed deserts. Geochemical evidence confirms that the interior of Pangaea was extremely arid.

In the Mesozoic era (see MESOZOIC ERA), the first mass extinction occurred near the end of the Triassic period, around 208 million years ago (see TRIASSIC PERIOD). Some 48 percent of marine genera disappeared. Land animals and plant species also died.

Did a Comet Strike Down the Dinosaurs?

The next and best-known mass extinction separates the Cretaceous period of the Mesozoic era from the beginning of the Tertiary period of the Cenozoic era. This extinction happened 65 million years ago at the Cretaceous-Tertiary boundary (called the K-T boundary) and wiped out the dinosaurs. Large reptiles perished on land, giant clams died in the seas, and many plant and animal species vanished from the fossil record in the space of less than a million years. Some 50 percent of all marine species died with the dinosaurs. This event cleared the way for mammals to evolve from their relatively small forms to eventually dominate Earth.

How long it might have taken to end the 100million-year-long age of reptiles depends on what caused the environmental crisis. If it was precipitated by an extraterrestrial trigger, such as a giant meteor or asteroid, the resulting crisis would have been swift (see the box on page 538). If, instead, it was the result of a gradual climatic change, the extinction process probably extended over a longer time.

More Recent Extinctions

Two more recent mass extinctions deserve particular attention because they are recent enough to have detailed rock records. They include the Eoccne Oligocene and the Pleistocene mass extinctions.

The Eocene-Oligocene extinction affected marine organisms more than terrestrial animals. It occurred at a time of major climate change. In a little over 15 million years, the mean global temperature dropped by more than 50°F (10°C). Earth changed from a warm greenhouse to an icehouse world, with large temperature differences between the Poles and the equator and with glaciers at the Poles.

The mass extinction at the end of the Pleistocene epoch (10,000 years ago) resulted in the loss of 33 animal genera (groups of related species) in the peiod of a few thousand years. Lost genera included mammoths, mastodons, ground sloths, giant beavers, and sabertoothed tigers. Nearly half of the large mammal species disappeared in North America, while smaller animals and birds survived.

Evidence from analyses of fossilized dung and pollen suggests that a global warming threatened many animal species. The geologic record of glaciation in this time is also clear. This evidence, combined with carbon dating, helps Pleistocene fossils tell a clear story of the sudden demise of a variety of land species. However, the Pleistocene extinctions did not occur at the same time around the world. Significant differences in the timing of large mammal extinctions on different continents led some scientists to seek reasons other than climate change. For example, some archaeologists suggest that humans suddenly flooded into North America, and slaughtered previously unhunted animals in large numbers. This so-called overkill theory, presented by Paul S. Martin at the University of Arizona, leads inevitably to comparisons with the present day. Physiologist Jarcd M. Diamond, at the University of California at Los Angeles (UCLA), likens modern-day extinctions to those of the Pleistocene. He and other scientists argue that Earth is currently in the midst of a sixth mass extinction—fed by modern civilization’s growth and pollution. Certainly the present time is beginning to rival past eras in the numbers of species that are disappearing. Of the myriad species that have disappeared as humans have expanded their dominion over the planet are the passenger pigeon, dodo, and Steller’s sea cow.

A dozen or more mass extinctions are evident in the record of the past 570 million years, which is the time period during which there are sufficient marine fossils to make such record-keeping meaningful. (Previous to 570 million years ago, few life forms had the hard shells or skeletons needed to leave prominent fossil remains.) Not all mass extinctions are equal, but four major ones are known in addition to the event that ended the Cretaceous period 65 million years ago. The greatest of these came 250 million years ago, at the end of the Paleozoic era, when it is estimated that three-quarters of all genera (and more than 95% of the individual species) in the Earth's oceans were destroyed. The event at the end of the Cretaceous resulted in the extinction of 40% of marine genera. The other three major events were 215 million years ago at the end of the Triassic period, 360 million years ago at the end of the Devonian period, and 435 million years ago at the end of the Ordovician period.

Today, about 2 million species of plants and animals have been identified on the Earth. Large as this number is, it represents only a small fraction of all of the species that have lived in the past, since the first documented life-forms of 3.6 billion years ago. In fact, it is estimated that for every species in existence today, there were several hundred others that thrived at some time in the past and then became extinct. Even at higher levels in the hierarchy of life, most genera and families from the past have also perished. At any given time, some species are dying out, while new ones are evolving. This is by no means a continuous process, however. Following the lead of Stephen Jay Gould of Harvard University and others, most students of the history of life today recognize that the fossil record is characterized by periods of relative stability alternating with short bursts of evolutionary divergence. Often the flowering of new species follows upon the sudden extinction of preexisting organisms. This stop-and-go progression of life is called “punctuated equilibrium.” In this view of biological evolution, mass extinctions play a critical role. If cosmic catastrophes repeatedly disrupted our planet and precipitated paroxysms of dying, they probably also made possible the development of most new species. It is almost as if the course of evolution were dependent on an occasional kick or shake from the outside. Once the Cretaceous event was identified with a cosmic impact, it was natural to inquire whether iridium concentrations or other evidence could be found that might link other extinctions with impacts.

So far no major iridium anomalies have been found in association with the four older great extinctions discussed above, although uncertainties remain. None of these discontinuities has a well-defined boundary layer like that at the end of the Cretaceous, and rocks this old are more difficult to locate and work with than the relatively young sediments from 65 million years ago. These extinctions could instead have been caused by impacting objects that were depleted in iridium or by higher velocity impacts that resulted in ejection of the iridium-rich projectile material back out into space. Scientists have had partial success in finding iridium associated with two smaller mass extinctions that took place since the Cretaceous, both of which are classed as second-rank, which means that less than 50% of marine species perished. The most recent iridium- enhanced boundary was deposited, in at least some places, just 12 million years ago, at the end of the middle Miocene age. The second enhancement recorded at some sites is close in time to the mass extinction at the Eocene—Oligocene boundary, 38 million years ago, where there are layers of glassy spherules ejected from impact events. But it seems that the iridium was deposited several million years after the Eocene extinction, which casts doubt either on Eocene dating measurements or on the association of iridium with the extinction.

One intriguing aspect of these three well-documented recent mass extinctions is their spacing. They took place 12, 38, and 65 million years ago, at intervals of about 26 million years. Is this just a coincidence, or might we be seeing evidence of regularly spaced, periodic impacts and their associated mass extinctions? No one had previously imagined that impacts on the Earth or Moon might have followed such a regular pattern, but here is a possible indication of such an effect. If impacts and mass extinctions are actually regular and periodic rather than random, our view of these events, and of the cratering history of the solar system, would be greatly modified.

Let us look at the statistics of mass extinctions to see if there is additional evidence that they are evenly spaced in time. The standard intervals into which we divide geological time were established more than a century ago, although the assignment of chronological ages to these “periods” and “eras” is more recent. Some (but not all) of the divisions between these time intervals correspond to mass extinctions, and some (but perhaps not all) such mass extinctions could well have been caused by impacts. It is apparent from a cursory examination of the geological time scale that many of these divisions are between 20 and 40 million years in length. Most scientists have assumed, however, that this spacing was an artifact of the naming process itself, representing an interval that was convenient for classification and nothing more. The suggestion that these divisions are not artifacts, and that mass extinctions are truly spaced at regular intervals, was first made in the early 1980s by David Raup of the University of Chicago, a past president of the Paleontological Society and a member of the National Academy of Sciences, and by his younger Chicago colleague Jack Sepkoski, who had completed his doctoral work under Steven Jay Gould at Harvard.

Both Raup and Sepkoski are experts in statistics and have earned reputations for their careful analysis of extinction rates in the geological record. Their research utilized a new computerized data base that listed the times of first and last occurrence in the geological record of thousands of individual families of marine organisms. In 1983, they began to see a pattern in the recorded times of extinction of the families in their data base. The mass extinctions seemed to come at intervals of 25 to 30 million years. They were aware, however, of the pitfalls in verifying such regularity, since even in a random set of data, patterns often seem to emerge, just by the luck of the draw. They needed a sophisticated computer analysis to determine if the apparent regularity represented statistically sound evidence of periodicity that was capable of standing up to informed criticism by other scientists.

By 1984, Raup and Sepkoski were sufficiently confident of their conclusions to publish a paper in the Proceedings of the National Academy of Sciences entitled “Periodicity of Extinctions in the Geologic Past.” Their paper asserted that 12 mass extinctions (8 substantial events and 4 smaller ones of marginal significance) have taken place in the past 250 million years, and that these events repeat in a 26-million- year cycle. The illustration shows how well the 8 major extinctions match the 26-year interval. Two of the expected events, number 5 (120 million years ago) and number 7 (170 million years ago), seem to be missing, but the others are quite evenly spaced. The most recent 3 extinctions are those mentioned before as possibly associated with iridium enhancements, and thus likely involved extraterrestrial impacts.

Since the original publication of these results, many scientists have joined the debate about the reality of the reported periodicity. Critics noted that the mass extinctions were based on the time history of just 567 families of fossil organisms. By 1988, Sepkoski had extended the analysis to a larger data base of 11,000 genera, which demonstrated an even more impressive periodicity, but only since the missing event 170 million years ago. Another criticism concerns uncertainties in the exact chronological dates of the extinctions. The geological time scale is defined by the fossil-bearing layers in marine sediments, but it is not simple to date these well-defined boundaries as having specific ages, which must be measured from the residual radioactivity of volcanic ash or other igneous rocks buried in the largely sedimentary geological column. The uncertainty in timing the end of the Cretaceous is probably only a million years either way, but the older mass extinctions could be misdated by as much as 10 million years, and there are also uncertainties about the more recent Eocene. If the dates of the extinctions are uncertain, it is difficult to establish the reality of their periodicity. Raup and Sepkoski believe, however, that errors in the dates of extinctions are unlikely to lead to a false appearance of periodicity. Large errors might mask a truly periodic effect, but it is very difficult to see how random errors could introduce the illusion of regular spacing if none were really present.

In 1983, as word of the Raup and Sepkoski analysis circulated within the scientific community, several groups of geologists and astronomers were inspired to examine the growing data on the ages of terrestrial impact craters for possible indications of periodicity. By this time about a hundred astroblemes had been identified on our planet. Is there direct evidence, from the large craters themselves, of a regular 26-million-year spacing? A first analysis, carried out by Raup and Gene Shoemaker, turned up no indication of regular spacing from craters with known ages. But in April 1984 two groups published papers reporting positive results. Geologists at Berkeley (including Walter Alvarez) found evidence of a 28-million-year periodicity for terrestrial impact craters, while two NASA astronomers found a period of 31 million years, working from a somewhat different selection of crater age data. Although the differences between these two derived periods and the 26-million-year period found by Raup and Sepkoski may seem significant, they probably are not, given the uncertainties in the measured ages of the craters.

The new results seemed to confirm both the association between impacts and extinctions and the periodicity of these events, but, as usual in science, objections were raised. Only about 20 ancient craters have been dated, and their ages are even more uncertain than those of the boundaries between geological periods. The estimated uncertainties in many of the crater ages amount to 10 million years or more. With such large uncertainties in the basic data, the statistical analysis is not very convincing. It thus remains an open question whether the evidence from terrestrial impact craters supports the suggestion of periodicity or not.

One of the most surprising new aspects to this puzzle is the possibility that large impacts are multiple. There are at least two iridium-rich layers dating from 35 million years ago, near the Eocene- Oligocene boundary, separated by intervals of at least tens of thousands of years. Also, about seven glassy ejecta layers have been recognized within a 2-million-year period around the end of the Eocene. It would seem to be a remarkable coincidence for numerous separate large impacts to have occurred within so short a span of geological time. Perhaps this is no coincidence, and the 25- to 30-million-year periodicity actually involves many projectiles. Maybe the Earth was subjected to periodic comet or asteroid “showers,” rather than single impacts.

References


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