Why else would an animal that was perfectly suited to life on this planet go extinct? His theory became known as "catastrophism. That paradigm persisted until the s and s. That was when Walter Alvarez and his father Luis Alvarez came up with the theory that an asteroid impact had done in the dinosaurs. But then it was proven.
And so now the prevailing view of change on planet Earth, as one paleontologist put it, is that the history of life consists of long periods of boredom interrupted occasionally by panic. It usually changes slowly, but sometimes it changes fast, and when it does, it's very hard for organisms to keep up. Nowadays, scientists are aware of five mass extinction events in the past, starting with the End-Ordovician Extinction million years ago and up to the End-Cretaceous Extinction that killed off the dinosaurs 66 million years ago see chart.
Is there a lot we still don't know about what caused these events?
Yes, absolutely, although it depends. So I think with the dinosaurs, [the asteroid theory] is quite widely accepted at this point. There was a big paper in Science on this subject last year, although there are still a couple of holdouts. The worst mass extinction of all time came about million years ago [the Permian-Triassic extinction event ].
There's a pretty good consensus there that this was caused by a huge volcanic event that went on for a long time and released a lot of carbon-dioxide into the atmosphere. That is pretty ominous considering that we are releasing a lot of CO2 into the atmosphere and people increasingly are drawing parallels between the two events.
The very first extinction event [the end-Ordovician ], seems to have been caused by some kind of sudden cold snap, but no one's exactly sure how that happened. But then, with the other two, the causes of those are pretty murky and people have tried to come up with a unified theory for these extinctions, but that hasn't worked at all.
The causes seem to be pretty disparate. Though precise estimates are tricky because measuring that background rate turns out to be very difficult. How did they realize this? I think a point that's important to make is that, normally, you shouldn't be able to see anything go extinct in the course of a human lifetime.
The normal background rate of extinction is very slow, and speciation and extinction should more or less equal out. But that's clearly not what is happening right now. Any naturalist out in the field has watched something go extinct or come perilously close. Even children can name things that have gone extinct.
So as soon as this concept of background vs.
Now, whether you make the jump to say that a major mass extinction is going on or just an elevated extinction rate, that's up for debate. But if you are looking at this in a rigorous way, you can see that something unusual is going on. One thing your book explores is that no single factor will drive current and future extinctions. There's hunting and poaching. There are invasive species. There's climate change and the acidification of the oceans. Which of these stands out as most significant? To me, what really stood out And I always say, look, I'm not a scientist, I'm relying on what scientists tell me.
And I think many scientists would say that what we're doing to the chemistry of the oceans could end up being the most significant. One-third of the carbon-dioxide that we pump into the air ends up in the oceans almost right away, and when CO2 dissolves in water, it forms an acid, that's just an unfortunate fact. The chemistry of the oceans tends to be very stable, and to overwhelm those forces is really hard.
But we are managing to do it. When people try to reconstruct the history of the ocean, the best estimate is that what we're doing to the oceans or have the potential to do is a magnitude of change that hasn't been seen in million years. And changes of ocean chemistry are associated with some of the worst extinction crises in history.
Are there lessons we can learn from past extinctions that provide clues for what the current changes hold? A lot of people are trying to tease out what survived previous extinctions and ask what are the characteristics of those that survived. It's called the selectivity of extinction events. Why did some groups survive and others didn't? It turns out to be, 65 million years after the fact, very, very difficult. But speaking very broadly, the species that tend to survive mass extinction events often tend to be very widely distributed, or groups that have a lot of species.
I'm not sure whom that's going to help today, but that seems to be the pattern. In your book you talk about this quasi-experiment in Brazil dating back to the s, where ranchers had down swaths of rain forest at random and scientists could study the effects on species. What did we learn about deforestation and extinction from that?
It's in the Amazon rain forest north of the city of Manaus. What happened there was that this area was already being converted into ranches, so in collaboration with some American scientists, they deforested it in an interesting way. They left these square patches surrounded by ranch. You can see it from the air, it's quite striking. And what you find are variations on this theme of loss. First most of the primate species don't survive in these smaller patches or even in the bigger patches of forest. Then you lose a lot of your bird species.
In some cases species leave, and in some cases, when you maroon them in small patches of habitat, their populations shrink, and very small populations are just more vulnerable to chance. So when people talk about the dangers of habitat fragmentation, on the one hand, a big animal that needs a large range can't survive in a small patch. But it's also smaller animals that don't need that much space become vulnerable to the dynamics of small populations. Did that Brazil project yield any lessons for protecting rain forest habitats? But also in the s there was this battle about protecting forests, and whether it was better to do it in lots of little patches or in one big patch.
And this project has resolved that. You need big areas if you want to preserve biodiversity, for the reasons I just mentioned. You discuss global warming in your book. And the big concern here seems to be that a lot of species are adapted to particular climate ranges, and if those heat up, some species may not be able to move or relocate fast enough to more suitable climates.
How much do we really know about these dynamics? What people are finding, what the scientists that I was out in the Peruvian cloud forest with are finding, is that things move at very different rates. But some of the other plants weren't moving at all, and others were moving but not nearly fast enough.
So the lesson is that all those pretty complicated relationships, which in the tropics have been been pretty stable for a long time, are going to break up. And we just don't know what the fall-out from that is going to be. So you end up with pretty wide estimates for how many species could go extinct if the planet heats up this much. Other scientists think those estimates are flawed. There's still a lot we don't know here. You often hear that what we're doing is a planetary experiment, but we only have one planet, and we can only run this experiment once.
So some of these modeling efforts get pretty complicated. Just because a species lives in a certain climate under a certain set of conditions, could it live under different conditions? Or is this just where it's maximally competitive? What happens if some of your competitors are disadvantaged? We just don't know. Life turns out to be incredibly complicated.
The great majority of species ever to have lived on Earth are now extinct. In recent years recognition of a biodiversity crisis, and the development of new. What caused Earth's first five mass extinctions? Graptolites, like most Ordovician life, were sea creatures. Credit: Paul Taylor / Natural History Museum.
Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Because most diversity and biomass on Earth is microbial , and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.
Extinction occurs at an uneven rate. Based on the fossil record , the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years. Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals. The Great Oxygenation Event was probably the first major extinction event. The most recent and arguably best-known, the Cretaceous—Paleogene extinction event , which occurred approximately 66 million years ago Ma , was a large-scale mass extinction of animal and plant species in a geologically short period of time.
Estimates of the number of major mass extinctions in the last million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity. In a landmark paper published in , Jack Sepkoski and David M. Raup identified five mass extinctions.
They were originally identified as outliers to a general trend of decreasing extinction rates during the Phanerozoic,  but as more stringent statistical tests have been applied to the accumulating data, it has been established that multicellular animal life has experienced five major and many minor mass extinctions.
Despite the popularization of these five events, there is no definite line separating them from other extinction events; using different methods of calculating an extinction's impact can lead to other events featuring in the top five. It has been suggested that the apparent variations in marine biodiversity may actually be an artifact, with abundance estimates directly related to quantity of rock available for sampling from different time periods. A quantification of the rock exposure of Western Europe indicates that many of the minor events for which a biological explanation has been sought are most readily explained by sampling bias.
Research completed after the seminal paper has concluded that a sixth mass extinction event is ongoing:. More recent research has indicated that the End-Capitanian extinction event likely constitutes a separate extinction event from the Permian—Triassic extinction event; if so, it would be larger than many of the "Big Five" extinction events.
This is a list of extinction events: Mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one. For example, mammaliformes "almost mammals" and then mammals existed throughout the reign of the dinosaurs , but could not compete for the large terrestrial vertebrate niches which dinosaurs monopolized.
The end-Cretaceous mass extinction removed the non-avian dinosaurs and made it possible for mammals to expand into the large terrestrial vertebrate niches. Ironically, the dinosaurs themselves had been beneficiaries of a previous mass extinction, the end-Triassic , which eliminated most of their chief rivals, the crurotarsans. Another point of view put forward in the Escalation hypothesis predicts that species in ecological niches with more organism-to-organism conflict will be less likely to survive extinctions. This is because the very traits that keep a species numerous and viable under fairly static conditions become a burden once population levels fall among competing organisms during the dynamics of an extinction event.
Furthermore, many groups which survive mass extinctions do not recover in numbers or diversity, and many of these go into long-term decline, and these are often referred to as " Dead Clades Walking ". He expressed this in The Origin of Species: Mass extinctions are thought to result when a long-term stress is compounded by a short term shock.
It has also been suggested that the oceans have gradually become more hospitable to life over the last million years, and thus less vulnerable to mass extinctions, [note 1]   but susceptibility to extinction at a taxonomic level does not appear to make mass extinctions more or less probable. There is still debate about the causes of all mass extinctions. In general, large extinctions may result when a biosphere under long-term stress undergoes a short-term shock. High diversity leads to a persistent increase in extinction rate; low diversity to a persistent increase in origination rate.
These presumably ecologically controlled relationships likely amplify smaller perturbations asteroid impacts, etc. A good theory for a particular mass extinction should: It may be necessary to consider combinations of causes. For example, the marine aspect of the end-Cretaceous extinction appears to have been caused by several processes which partially overlapped in time and may have had different levels of significance in different parts of the world. Macleod  summarized the relationship between mass extinctions and events which are most often cited as causes of mass extinctions, using data from Courtillot et al.
The formation of large igneous provinces by flood basalt events could have:. Flood basalt events occur as pulses of activity punctuated by dormant periods. As a result, they are likely to cause the climate to oscillate between cooling and warming, but with an overall trend towards warming as the carbon dioxide they emit can stay in the atmosphere for hundreds of years.
It is speculated that massive volcanism caused or contributed to the End-Permian , End-Triassic and End-Cretaceous extinctions. These are often clearly marked by worldwide sequences of contemporaneous sediments which show all or part of a transition from sea-bed to tidal zone to beach to dry land — and where there is no evidence that the rocks in the relevant areas were raised by geological processes such as orogeny.
Sea-level falls could reduce the continental shelf area the most productive part of the oceans sufficiently to cause a marine mass extinction, and could disrupt weather patterns enough to cause extinctions on land. But sea-level falls are very probably the result of other events, such as sustained global cooling or the sinking of the mid-ocean ridges. A study, published in the journal Nature online June 15, established a relationship between the speed of mass extinction events and changes in sea level and sediment.
The impact of a sufficiently large asteroid or comet could have caused food chains to collapse both on land and at sea by producing dust and particulate aerosols and thus inhibiting photosynthesis. Sustained and significant global cooling could kill many polar and temperate species and force others to migrate towards the equator ; reduce the area available for tropical species; often make the Earth's climate more arid on average, mainly by locking up more of the planet's water in ice and snow.
The glaciation cycles of the current ice age are believed to have had only a very mild impact on biodiversity, so the mere existence of a significant cooling is not sufficient on its own to explain a mass extinction. It has been suggested that global cooling caused or contributed to the End-Ordovician , Permian—Triassic , Late Devonian extinctions, and possibly others. Sustained global cooling is distinguished from the temporary climatic effects of flood basalt events or impacts. This would have the opposite effects: It might also cause anoxic events in the oceans see below.
Global warming as a cause of mass extinction is supported by several recent studies. The most dramatic example of sustained warming is the Paleocene—Eocene Thermal Maximum , which was associated with one of the smaller mass extinctions. Furthermore, the Permian—Triassic extinction event has been suggested to have been caused by warming.
Clathrates are composites in which a lattice of one substance forms a cage around another. Methane clathrates in which water molecules are the cage form on continental shelves. These clathrates are likely to break up rapidly and release the methane if the temperature rises quickly or the pressure on them drops quickly—for example in response to sudden global warming or a sudden drop in sea level or even earthquakes. Methane is a much more powerful greenhouse gas than carbon dioxide, so a methane eruption "clathrate gun" could cause rapid global warming or make it much more severe if the eruption was itself caused by global warming.
The most likely signature of such a methane eruption would be a sudden decrease in the ratio of carbon to carbon in sediments, since methane clathrates are low in carbon; but the change would have to be very large, as other events can also reduce the percentage of carbon It has been suggested that "clathrate gun" methane eruptions were involved in the end-Permian extinction "the Great Dying" and in the Paleocene—Eocene Thermal Maximum , which was associated with one of the smaller mass extinctions. Anoxic events are situations in which the middle and even the upper layers of the ocean become deficient or totally lacking in oxygen.
Their causes are complex and controversial, but all known instances are associated with severe and sustained global warming, mostly caused by sustained massive volcanism. It has been suggested that anoxic events caused or contributed to the Ordovician—Silurian , late Devonian , Permian—Triassic and Triassic—Jurassic extinctions, as well as a number of lesser extinctions such as the Ireviken , Mulde , Lau , Toarcian and Cenomanian—Turonian events.
On the other hand, there are widespread black shale beds from the mid-Cretaceous which indicate anoxic events but are not associated with mass extinctions. The bio-availability of essential trace elements in particular selenium to potentially lethal lows has been shown to coincide with, and likely have contributed to, at least three mass extinction events in the oceans, i. Kump, Pavlov and Arthur have proposed that during the Permian—Triassic extinction event the warming also upset the oceanic balance between photosynthesising plankton and deep-water sulfate-reducing bacteria , causing massive emissions of hydrogen sulfide which poisoned life on both land and sea and severely weakened the ozone layer , exposing much of the life that still remained to fatal levels of UV radiation.
Oceanic overturn is a disruption of thermo-haline circulation which lets surface water which is more saline than deep water because of evaporation sink straight down, bringing anoxic deep water to the surface and therefore killing most of the oxygen-breathing organisms which inhabit the surface and middle depths. It may occur either at the beginning or the end of a glaciation , although an overturn at the start of a glaciation is more dangerous because the preceding warm period will have created a larger volume of anoxic water.
Unlike other oceanic catastrophes such as regressions sea-level falls and anoxic events, overturns do not leave easily identified "signatures" in rocks and are theoretical consequences of researchers' conclusions about other climatic and marine events. It has been suggested that oceanic overturn caused or contributed to the late Devonian and Permian—Triassic extinctions.
A nearby gamma-ray burst less than light-years away would be powerful enough to destroy the Earth's ozone layer , leaving organisms vulnerable to ultraviolet radiation from the Sun. One theory is that periods of increased geomagnetic reversals will weaken Earth's magnetic field long enough to expose the atmosphere to the solar winds , causing oxygen ions to escape the atmosphere in a rate increased by 3—4 orders, resulting in a disastrous decrease in oxygen.
Movement of the continents into some configurations can cause or contribute to extinctions in several ways: Occasionally continental drift creates a super-continent which includes the vast majority of Earth's land area, which in addition to the effects listed above is likely to reduce the total area of continental shelf the most species-rich part of the ocean and produce a vast, arid continental interior which may have extreme seasonal variations. Another theory is that the creation of the super-continent Pangaea contributed to the End-Permian mass extinction.
Pangaea was almost fully formed at the transition from mid-Permian to late-Permian, and the "Marine genus diversity" diagram at the top of this article shows a level of extinction starting at that time which might have qualified for inclusion in the "Big Five" if it were not overshadowed by the "Great Dying" at the end of the Permian. Many other hypotheses have been proposed, such as the spread of a new disease, or simple out-competition following an especially successful biological innovation. But all have been rejected, usually for one of the following reasons: Scientists have been concerned that human activities could cause more plants and animals to become extinct than any point in the past.
Along with human-made changes in climate see above , some of these extinctions could be caused by overhunting, overfishing, invasive species, or habitat loss. The eventual warming and expanding of the Sun, combined with the eventual decline of atmospheric carbon dioxide could actually cause an even greater mass extinction, having the potential to wipe out even microbes in other words, the Earth is completely sterilized , where rising global temperatures caused by the expanding Sun will gradually increase the rate of weathering, which in turn removes more and more carbon dioxide from the atmosphere.
When carbon dioxide levels get too low perhaps at 50 ppm , all plant life will die out, although simpler plants like grasses and mosses can survive much longer, until CO 2 levels drop to 10 ppm. With all photosynthetic organisms gone, atmospheric oxygen can no longer be replenished, and is eventually removed by chemical reactions in the atmosphere, perhaps from volcanic eruptions.
Eventually the loss of oxygen will cause all remaining aerobic life to die out via asphyxiation, leaving behind only simple anaerobic prokaryotes. This is the most extreme instance of a climate-caused extinction event. Since this will only happen late in the Sun's life, such will cause the final mass extinction in Earth's history albeit a very long extinction event.
The impact of mass extinction events varied widely.
After a major extinction event, usually only weedy species survive due to their ability to live in diverse habitats. Generally, biodiversity recovers 5 to 10 million years after the extinction event. In the most severe mass extinctions it may take 15 to 30 million years. Life seemed to recover quickly after the P-T extinction, but this was mostly in the form of disaster taxa , such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems, with high biodiversity, complex food webs and a variety of niches, took much longer to recover.
It is thought that this long recovery was due to successive waves of extinction which inhibited recovery, as well as prolonged environmental stress which continued into the Early Triassic.
Recent research indicates that recovery did not begin until the start of the mid-Triassic, 4M to 6M years after the extinction;  and some writers estimate that the recovery was not complete until 30M years after the P-T extinction, i. The effects of mass extinctions on plants are somewhat harder to quantify, given the biases inherent in the plant fossil record.
Some mass extinctions such as the end-Permian were equally catastrophic for plants, whereas others, such as the end-Devonian, did not affect the flora. From Wikipedia, the free encyclopedia.