In the history of our planet there have been at least five major mass extinctions, and a number of minor extinctions (Elewa 2008a,b,c; Raup 1992; Raup and Sepkoski, 1982). These include the Ordovician Mass Extinction, the Devonian, Permian, Triassic, and Cretaceous Mass Extinction.

In addition to the big five, some scientists believe there have been additional major mass extinctions, including as many as 4 extinctions during Cambrian era. According to Joseph (2009a,b) these additional extinction events include the Paleoproterozoic (2.3 to 1.8 bya), the Sturtian (725 mya to 670 mya), the Marinoan/Gaskiers (640 to 580 mya), and the Ediacaran extinctions (540 mya).

Many scientists also believe we are now experiencing a sixth mass extinction, which is driven by Homo sapiens (Crutzen and Stoermer 2000; Elewa 2008a; Jones 2009; Ruddimann 2005; Steffen et al. 2007). There is considerable evidence that extinction has been accelerating over the last 500 years; and with the advent of weapons of mass destruction, and industrial poisons, pharmaceuticals, and other wastes which are dumped into the oceans and atmosphere (Levy and Sidel 2009; McKee 2009, Tonn 2009); , it could be said the human race is flirting with self-destruction and may trigger a world-wide mass extinction which could wipe humanity from the face of the Earth (Jones 2009).

Typically, numerous species may die off simultaneously, resulting in a mass extinction, or a few individual species may die out in isolation leaving the vast majority unscathed. Extinction is so common that it can be considered an integral and perhaps an essential feature of life on Earth (Bradshaw and Brook 2009; Elewa 2008a,b,c; Ward 2009). According to Prothero (1998) of the 5 to 50 billion species which have ever lived on this planet, only about fifty million are alive today. This means that 99.9% of all species that have ever lived are now extinct! In the future, will humans number among the exceptions?

However, the vast majority of these extinction events may have been of just a few species (Elewa 2008a,b,c). Mass extinctions may account for less than 5% of all species which have become extinct (Erwin 2001) .

2. Evolutionary Apoptosis and the Gai, Medea, Cronus Hypotheses

There are several competing explanations for why certain species become eradicated and for what causes mass extinctions. In what could be described as the battle of the metaphors (Glickson 2009): the Medea hypothesis (Ward 2009) and the Cronus hypothesis (Bradshaw and Brook 2009) focus on biological self-destruction, whereas the Gaia hypothesis (Lovelock and Margoulis, 1974) presents the Earth and life itself as an interacting organism which has the potential to live and grow, or die. Ward (2009) theorizes the mass extinctions of species is in part driven by life itself, and the interaction of biological processes forces species self-destruction. Bradshaw and Brook (2009) see mass extinctions as a natural consequence of the “ebb and flow of life on Earth along a thermodynamic spectrum” and that “the causes of extinction can be thought of as equivalent to the different processes that lead to individual deaths within a population" albeit in the context of Darwinian natural selection.

In what could be considered an integration of the Ward, Bradshaw, Brook, Lovelock, Magoulis hypotheses, Joseph (2009a,b) argues that the individual demise of a single species, and in some instances, mass species extinctions, are sometimes a consequence of genetically guided biological activity with "genes acting on the environment and the changing environment acting on gene selection;" interactions which result in cell and species death and the loss vs gain of genes which corresponds with the extinction of previous species and the evolution of new species which emerge from the old. Joseph (2000, 2009b) describes extinction as related to the same genetic mechanisms involved in embryogenesis and metamorphosis, where billions of cells die and others take their place, and where species shed one body, which dies, and replace it with another. Therefore, just as cells undergo programmed cell death during embryogenesis and metamorphosis, enabling, for example, a tadpole to shed its fish-like body to become a frog, or a crawling insect to destroy most of its body and then undergo a dramatic transformation to become a flying insect, that many species serve as genetic bridge to a subsequent species and die out after having served a biological purpose such as by altering the environment through the secretion of calcium or the excretion of oxygen. According to Joseph (2009b), extinction is sometimes a form of evolutionary apoptosis and is under genetic-environmental regulatory control. "As a form of evolutionary apoptosis, extinction is in part a direct consequence of the same cellular mechanisms which lead to cell death; albeit at the level of an entire species."

3. Natural Selection, Catastrophes, and Bad Luck: The Causes of Extinction

Traditional explanations typically do not take genetic or cellular mechanisms into account, and instead rely on Darwinian mechanisms emphasizing the survival of the fit and natural selection (Elewa 2008a,b,c). Therefore, changing environmental conditions, predation, disease, and so on, weed out the weak in favor of the strong. However, as pointed out by Raup (1992) natural selection often has little to do with the outcome, for in many instances, survival vs extinction is a matter of luck. Be it luck, natural selection, or evolutionary-apoptosis, there is general agreement that the following factors have at one time or another played a central role in the extinction of species:

Global Cooling and Warming

Major glaciation

Fluctuations in Sea Level

Global Anoxia

Volcanic Eruptions

Asteroid, Comets, and Meteor impacts

Plate Tectonics

Gamma Rays

Disease

These factors, often in combination with other forces, can be linked to almost every major extinction event which has brought mass death to this planet; except, perhaps the hypothetical extinction event which some believe brought life to Earth. Each of these factors, and the history or mass extinctions, will be briefly detailed in the following sections.

4. The First Mass Extinction: Snow Ball Earth.

Most scientists do not view the mass extinction of microbial life as of sufficient significance to be counted among the great extinction events which have taken place on Earth. Yet, the events surrounded the first mass extinction may have doomed billions of life forms to an early death during the Archaean era and its frigid aftermath (Joseph 2009a,b).

The Archaean era began with a 600 my episode of global warming associated with high level of methane and carbon dioxide and a greenhouse effect. This was followed by a rise in oxygen, a reduction in methane, and global freezing (2.3 bya) and then a rise in methane and a global meltdown (1.8 bya) as our planet again began to warm (Barleya et al. 2005; Brocks et al. 2005; Buick 2008; Canfield 2005; Holland 2006; Nisbett and Nisbett 2008; Olson 2006). Therefore, numerous species were likely driven to extinction by these global climatic extremes (Joseph 2009a,b).

Central to this first mass extinction was the first global ice age, known as the Paleoproterozoic "Great Oxidation Event" and the first "snow ball Earth." The surface of the entire planet is believed to have been nearly frozen solid, beginning around 2.3 bya (Kasting and Ono, 2006; Kirschvink, et al. 2000), killing off all prokaryotes and eukaryotes who were not adapted to freezing temperatures, low levels of methane, and the presence of increased oxygen.

However, since all life at this time was microscopic, and as eukaryotes are believed to have consisted of less than 2 cell types (Hedges et al. 2004), most scientists do not rank the death of these species as worthy of a "mass extinction" designation. Nevertheless, the Paleoproterozoic extinction should be counted as among the great extinction events.

5. The Second Snow Ball Earth.

The first "Snow Ball Earth" may have been primarily the result of biological activity and the release and breakdown of various gasses which affected the climate (Joseph 2009a,b). The second Snow Ball Earth appears to have had multiple causes, with consequences which may have been even more deadly, impacting a wider and more complex variety of life.

By 1.5 BYA, eukaryotes consisted of approximately 10 cell types (Hedges et al. 2004). Between 1.6 to 1.2 bya a varied assemblage of complex multi-cellular eukaryotes diverged and proliferated, including green and red algae, dinoflagellates, ciliates, amoebae, and a diverse array of unornamented organic-walled acritarchs (Butterfield 2000; Porter and Knoll 2000; Wang et al. 1999; Xiao and Knoll, 1999; Zhou et al. 2001).

As early as 800 mya, Acritarchs, as well as plankton, coccoid and filamentous cyanobacteria, protozoa, fungi, amoebozoans, cercozoans, and eukaryotic and marine algae proliferated throughout the oceans and inland seas (Butterfield 2005a,b). Many were engaging in photosynthesis, reducing the CO₂, and releasing so much oxygen it rose to present levels (e.g. Holland 2006).

Beginning around 850 to 820 MA, Rodinia, the pre-Pangean supercontinent which occupied the tropical equatorial regions, began to slowly break apart; a consequence of plate tectonics, mantle subduction, and extensive volcanism coupled with magma super plumes (Druschke et al. 2006; Li et al. 2003; Sung et al. 2006; Wang and Li 2002; Zhou et al. 2002).

Around 730 MYA, silicate weathering secondary to the continued breakup of Rodinia, coupled with increase levels of O₂ began to significantly effect the climate (Joseph 2009a). Methane and CO₂ levels began to to drop as O₂ levels rose (Cavalier-Smith 2006), temperatures fell and the Earth began to freeze.

The Sturtian global ice age had its onset around 725 mya. It is believed that the surface of the oceans and the planet, from the poles to the equatorial latitudes, froze and became glaciated, leaving perhaps only islands of open-water refuges on the surface deep beneath the sea (e.g. Harland 2007; Hoffmann et al. 1998; Hyde et al. 2000). A vast number of species were doomed to extinction. This period of world wide glaciation is known as the "Sturtian" and may have lasted until 670 mya (Fanning and Link 2004).

6. The Third Snow Ball Earth.

Between 640 to 580 mya, the planet underwent yet another global ice age, the "Marinoan" (Bowring et al. 2003; Condon et al. 2005; Hoffmann et al. 1998, 2004; Hyde et al. 2000) followed by a less extreme period of cooling referred to as Gaskiers, which came to a close around 580 Ma (Eyles & Eyles 1989), These global ice ages were likely triggered by a combination of oxygen buildup and the spewing of volcanic ash into the atmosphere (Condon et al. 2005).

Innumerable creatures died and many species became extinct during the Marinoan glaciation (Joseph 2009a). However, as these life forms were microscopic, the number which perished is unknown. This mass loss of life could best be described as the Marinoan/Gaskiers extinction.

7. The Ediacaran Mass Extinction

Following the close of the Marinoan/Gaskiers glaciation and the warming of the planet, an explosion of life ensued (Condon et al. 2005; Peterson and Butterfield 2005) including the evolution of megascopic Ediacarans (Narbonne 2005; Narbonne and Gehling 2003). However, by 540 mya, and the onset of the Cambrian Era, the Ediacaran age would come to a close and the Ediacaran would become extinct.

Joseph (2009b) argues that the Ediacaran extinction, the Marinoan/Gaskiers extinction, the Sturtian extinction, and the Paleoproterozoic extinction, are examples of biologically induced and genetically controlled evolutionary apoptosis; and that these species were "shed from the tree of life" after having served an an evolutionary bridge to subsequent species including those which flourished during the Cambrian Explosion.

8. The Cambrian Extinction Explosion

Beginning around 540 mya and within 5 my to 10 million years there was an explosion of life and over 32 phyla suddenly evolved many with the body plans seen in modern animals (Budd and Jensen 2000; Fortey et al. 1997; Conway and Morris 2000; Peterson and Butterfield 2005; Valentine et al. 1999). Many of these creatures were very complex and bizarre in appearance and immediately died out (Cloud 1948; Mooi and Bruno 1999; Whittington 1979) and therefore may represent phyla that became extinct. Others propose that some of these species actually served as the stem groups of the extant phyla, (Budd & Jensen 2000).

Nevertheless, there were four major extinctions during the Cambrian era (from 540 to 510 mya), the most famous of which is the demise of the trilobites, whose extinction was accompanied by archaeocyathids which were reef-building organisms. Some species of trilobite recovered. However, their days were numbered, as they were finally wiped out along with several other species, including brachiopods and conodonts.

The proposed causes are many, including predation, as well as global cooling and reductions in sea level and oxygen leading to anoxia (Zhuravlev and Wood 1996) and changes in ocean chemistry (Saltman et al. 1995). However, the question then arises: why did some species become extinct and others survive? Could selective survival be yet another example of evolutionary apoptosis? Or evidence favoring the Cronus, Medea, or Gaia hypotheses? Or did they die because they were not "fit"?

9. The Ordovician Extinction

The Ordovician era lasted from 510 to 440 mya, and ended with yet another mass extinction, the second most devastating (in terms of animal life) in the history of our planet. Over one hundred families of marine invertebrates perished and others were driven to near extinction during a two-pulse extinction episode (Sheehan 2001). The victims included nearly half of all brachiopod and bryozoan families, as well as conodonts and graptolites and many species of reef builders (Brenchley et al. 2001).

Like so many other mass extinctions, the primary cause is believed to be global cooling and glaciation. However, rather than having a biological source, contributing factors included plate tectonics and continental drift; i.e. the passage of the giant supercontinent, Gondwana, over the north pole (Brenchley et al. 2001). The Earth froze, sea levels dropped, shorelines disappeared, and so did numerous species (Sheehan 2001).

Yet other species evolved, including those with dense skeletal support, and this was made possible, in part, by the near extinction of reef builders, and then the subsequent evaporation of these reefs which released tremendous amount of calcium into the oceans; calcium which was incorporated to create bones and new species (Joseph 2009b). Hence, a new era was born: the Devonian.

10. The Devonian Extinction

The Devonian age (410 mya to 360 mya) is marked by the evolution of bony fish who were evolving lungs and legs, as well as amphibians, insects, and a new generation of reef builders.

The Devonian age also ended with yet another mass extinction, and over 70% of all taxes disappeared including most reef builders, i.e. stromatoporoids, and the rugose, and tabulate corals (Raup 1992).

Like the mass extinctions of the past, the Devonian extinction event has also been attributed to a global ice age, perhaps triggered by bolide impact (Joachimski and Buggisch 2002). The consequences were a complete restructuring of many components of the marine ecosystem.

11. The Permian Extinction

The Permian era ranged from 290 mya to 248 mya, and ended with yet another spectacular mass extinction; the most devastating mass extinction in the history of our planet. Amphibians, reptiles, and repto-mammals were impacted. Nearly 95% of all species of marine animal were nearly destroyed (Raup 1992).

Numerous theories abound as to the causes, which may have been multiple. These include bolide impact, volcanic eruptions (Benton and Twitchett 2003), and glaciation followed by global warming (Kidder and Worsley 2004).

12. The Triassic Extinction

The Triassic period extends from about 250 to 200 Ma and is marked by the evolution of the mammal-like therapsids, the first flying vertebrates, the pterosaurs, and the concentration of all the world's land masses into one supercontinent, Pangaea, located in the temperate and tropical regions of Earth. The Triassic-Pangaean climate is believed to have been generally hot and dry.

The closing of the Triassic was slammed shut by yet another mass extinction which killed off 22% of marine families and possibly about half of marine genera and most marine reptiles except ichthyosaurs and plesiosaurs (Tanner et al. 2004). Some studies suggest there may have been at least two periods of extinction perhaps 12 million years apart.

The possible causes are many, including asteroid impacts which may have contributed to the breaking up of the Pangaea supercontinent (Joseph 2000). The planet was also plagued by huge volcanic eruptions (Hautmann 2004) which coincided with the fracturing of Pangea (Tanner et al. 2004). Associated factors include continental flood and flood basalts (Marzoli et al. 1999), increases in carbon dioxide and depletion of calcium carbonate levels (Hartman 2004), depletion of oxygen levels and increased anoxia (Ward et al. 2004), and global cooling perhaps secondary to sunlight blocking debris tossed into the atmosphere following bolide impact or widespread volcanic emissions.

With the near extinction of the therapsids, and so many other species, tiny dinosaurs were given a competitive advantage and quickly grew in size. However, why were the dinosaurs spared?

Ward (2006) argues that dinosaurs survived this mass extinction because they developed respiratory systems far more efficient than other terrestrial creatures. However, the days of the dinosaurs were also numbered, and although they became extinct, yet other species survived, including mammals which gave rise to primates, leading, finally to humans.

13. The Cretaceous Extinction

The Cretaceous period began around 135 mya and came to a sudden and catastrophic end, 65 mya. Although the Cretaceous Extinction is associated with elimination of the dinosaurs, there are estimates that up to 85% of all species were nearly destroyed by what has been called the KT (Cretaceous/Tertiary) extinction event (Raup 1992). The victims included a variety of plants, marine reptiles, diatoms, dinoflagellates, and nannoplankton.

Two major catastrophes may have caused K/T extinction: asteroid impact and volcanic eruptions. For example sediment dated to the extinction event contains unusually high concentrations of Iridium, shocked quartz, and basalt, all of which are associated with meteor impact (Alvarez 2008). Iridium has two major sources: the earth's mantle, and meteors and asteroids. Therefore, the widespread distribution of Iridium, the presence of shocked quartz which is indicative of high pressure of impact, coupled with small droplets of basalt which usually result when the Earth's crust has been melted, indicate the Earth was struck by an asteroid, probably in the area which today is known as Yucatan Peninsula of Mexico (Alvarez 2008).

On the other hand, since Iridium is also associated with the Earth's mantle, and given the extensive volcanic deposits dated to the K/T boundary, it has been argued that volcanic eruptions ejected tremendous amounts of ash into the atmosphere, blocking sunlight, and causing temperatures to fall and altering the chemistry of the Earth's ocean's and atmosphere.

Of course, it may have been a confluence of catastrophes, asteroid impact and volcanoes which triggered the K/T extinction event. However, neither event can explain why a wide variety of species were spared, or quickly recovered, such as amphibians, birds, crocodiles, ferns, insects, lizards, seed-producing plant, snakes, turtles, and and most mammals which quickly diversified and became the dominant land animal.

Selective extinctions, such as typified by the K/T extinction event, must be due to additional factors, such as disease. According to Poinar and Poinar (2008), dinosaurs were plagued by disease-causing parasites and swarms of biting insects infected with malaria and other pathogens and were so weakened by disease and oozing infected wounds that when calamity struck, they were the most vulnerable.

14. Disease

Species-destroying-diseases induced by bacteria, fungi, and viruses (Casadevall 2005; Devaraj 2000; Emiliani 1993; Gong et al. 2008; Poinar and Poinar 2008), can act selectively to kill one species while preserving others. For example, alterations in the host-genome or the genome of the pathogen, enable pathogens to selectively target and kill off a specific host long after it has evolved (Flint et al. 2009; Norkin 2009; Joseph 2009a). Not uncommonly, diseases which have extinction-potential, are transmitted vertically, from genome to genome (Engelstädter and Hurst 2007) and will selectively sicken or kill specific species in response to as yet unknown biological and environmental triggers. Male-killing bacteria, for example, can hitchhike over thousands of generations, from mother to offspring, embedded within the mitochondrial genome (Jiggins 2003) and may begin destroying a species only after a related species has evolved. Genes which encode for retroviruses can be passed down for millions of years, embedded in the eukaryotic nuclear genome, and when expressed not only promote speciation and the evolution of new species but simultaneously eradicate others.

In the last 500 years, infectious diseases have accounted for or contributed to approximately 12% of all species extinctions (Smith et al. 2006), and in the last decade has been linked to amphibian mass mortality (Rachowicz et al. 2006). Changes in climate and global warming have been blamed for unleashing these pathogens.

Many pathogens are sensitive to temperature, rainfall, and humidity. Climate warming, for example, can trigger then increase pathogen development, disease transmission, and host susceptibility and survivability (Harvel et al. 2002).

Global warming and the spread of disease may have also played a role in the demise of the dinosaurs. Specifically, about 90 mya, a "super-greenhouse" had developed (Bornemann et al. 2008), possibly secondary to biological activity. Alligators flourished in the Arctic and the surface temperatures of the ocean average 100 degrees F. It was during and after this time period that dinosaurs began to be infected with malaria and leishmania (Poinar and Poinar 2008).

Poinar and Poinar (2008) have amassed an impressive body of evidence indicating that dinosaurs may have been infected by disease-carrying insects, blood-sucking flies, and internal pathogens including intestinal worms and protozoa. Thus, they were so weakened by disease that their numbers began to decline, such that when the Earth was struck by a meteor, and/or beset by erupting volcanoes, dinosaurs were selectively eradicated.

However, yet another factor in the demise of the dinosaurs may have been "hot house glaciers." Bornemann et al. (2008) suggests that glaciers stretched across 50- to 60-percent of Antarctica some 91.2 million years ago, and that this global, hot-house ice age lasted almost 200,000 years.

15. Global Cooling and Glaciation

Prior to the onset of the Cambrian Explosion, the Earth had already suffered 4 major ice ages. The Ordovician marked the fifth global ice age and innumerable life forms and species died out and suffered total extinction.

A cold planet is not the same as a frozen planet. Nevertheless, a variety of scientists have proposed that global cooling also contributed to the major mass extinctions during the Late Ordovician, the Late Devonian, Late Permian, Late Triassic, and the Late Cretaceous. These authors suppose that global cooling triggered glaciation and significant lowering of the sea level. Therefore deep marine organisms as well as organisms favoring warm conditions died.

Sheehan (2001), for example, has provided considerable evidence indicating that the Ordovician mass extinction occurred during a brief glacial interval that produced two pulses of extinction. Saltzman et al. (1995) argued that volcanic weathering led to draw down pCO₂ and resulted in a cooling episode, which produced the well known End-Ordovician (Hirnantian) glaciation. Likewise, rapid warming and glaciation might have occurred during the Permian.

Therefore, there is considerable evidence that the numerous mass extinctions have occurred during periods of global cooling leading to global ice ages. In some instances, biological factors contributed to these global ice ages (Joseph 2009b), whereas in others a confluences of catastrophic events invovling volcanism, plate tectonics, bolide impact (Raup 1992), and even an exploding comet (Firestone 2009) appear to be the responsible interacting agents.

16. Sea Levels, Glaciation & Global Warming

Glaciation binds water and lowers sea levels. By contrast, global warming can cause sea levels to markedly rise as glaciers melt and melt water returns to the sea. Global warming, the end of an ice age, and even bolide impact, can effect water levels and trigger flood basalts and mass extinctions (Alvarez 2003).

The history of the Earth includes a history of temperature extremes, from the hot Hadean era to the warm Archaean age, to the first snow ball Earth, 2.4 billion years ago, and with the warm/cold/warm cycle repeating yet again with global ice ages 750 mya, and again 640 mya, and again 590 mya, with a period of warmth continuing throughout the Cambrian era (Joseph 2009ab).

Naturally, life adapted to a warm environment and high sea levels may be driven to near extinction during prolonged periods of global glaciation and a lowering of the water table, with many species dying out and yet others recovering and new species emerging (Elewa 2008a,b,c). The pattern would repeat itself with the melting of the glaciers and subsequent warming of the planet.

Further, as glaciers melt, chemicals, metals, ions, and gasses would be released, flooding the oceans and killing those who could not adapt (Joseph 2009a,b). However, those living on land, vs lakes and streams, vs the surface layers of the ocean vs the deeper layers of the ocean, may be differentially effected (Elewa 2008a,b,c).

Lantzy et al. (1977) for example, argues that the Permian extinction of marine invertebrates was due to a reduction in oceanic salinity. However, marine invertebrates living in shallow seas were not as severely effected.

Jَzsef Pلlfy and others (2000) declared that the end-Triassic biotic crisis on land appears to have preceded similar extinction events in the sea by at least several hundred thousand years. This suggests that extinction began on land accelerated and spread to ocean waters.

17. Gamma-Rays

Gamma rays are electromagnetic radiation of high frequency and are believed to be released during a supernova event as the star rapidly collapses to form a black hole. Yet others may result from the merger of binary neutron stars. A 10 second Gamma-ray bursts contains more energy than the Sun will emit during its entire lifetime! They are extremely dangerous as they have the shortest wavelength of all waves in the electromagnetic spectrum, and consist of highly penetrating, highly energetic ionizing radiation which can easily penetrate clothes, skin, bone, cells and DNA, destroying DNA and cell alike (Melott et al. 2004).

It has been estimated that the Earth has been struck by gamma rays between 8 to 10 times in its 4.6 billion year history. Gamma rays, therefore may be responsible for several of the mass extinctions which have plagued this planet. Melott and colleagues (2004) have proposed, for example, that the late Ordovician mass extinction, approximately 440 million years ago, may be at least partly the result of a gamma ray burst which may have caused a severe depletion of the ozone layer, thereby also allowing elevated levels of UV radiation to strike the planet. Gamma rays could easily cause mass death and mass extinctions.

According to Melott et al. (2004) this gamma ray burst may have triggered episodes of rapid global cooling and glaciation, thereby triggering repeated mass extinctions over the ages.

18. Global Anoxia

It is believed that significant reductions in oxygen leading to global anoxia may be one of the decisive causes of Devonian mass extinction. According to Wignall and Twitchett (1996) the world's oceans became anoxic at both low and high paleolatitudes in the Late Permian and may have been responsible for the mass extinction at that time. Samar Abbas and colleagues (2000) argues that the evidence quite convincingly indicates that the Late Permian biotic crisis was in fact a binary extinction and both were linked to low oxygen levels and anoxia.

Some authors have also attributed the Cenomanian/Turonian extinction to global anoxia. For example, Bing Shen (2008) presents evidence suggesting that during Cenomanian/Turonian times an enhanced flux of volcanic ash resulted in micro-nutrient iron flooding into the oceans, fertilizing surface waters. A global phytoplanktonic bloom resulted, which in turn depleted oxygen in deep oceans.

Castle and Rodgers (2009) propose that toxin-producing algae were an important factor in Phanerozoic mass extinctions. They suggested that the large mass of organic material produced by algal blooms led to oxygen depletion during decay, leading to suffocation and the death of some biota.

Yet others have proposed that the Permian-Triassic mass extinction was also due, in part, to oceanic anoxia. It is believed that oxygen levels were depleted secondary to enrichment of the oceans in methane, hydrogen sulfide and other compounds, associated with large scale basaltic volcanism and a large asteroid impact (Glikson 2009).

19. Volcanic Eruptions

Some scientists consider volcanism as one of the most important causes of mass extinctions in the fossil record (Courtillot 1999; MacLeod, 2000, 2001). Ash and dust resulting from these eruptions would block sunlight causing a drop in temperature, both of which, in combination, would kill most plants and many species. There is also evidence suggesting that volcanism can contribute to global anoxia (Glickson 2009).

Some scientists have argued that volcanic activity was the prime cause of the extinction of the dinosaurs. For example, iridium is common in rocks from deep within the Earth. This has led to the suggestion that the presence of iridium in the dinosaurs-bearing beds is due to volcanic eruptions, and this is what drove the dinosaurs to extinction.

Norman MacLeod (2000, 2001) has emphasized tectonic factors - giving rise to flood-basalt volcanism and causing sea-levels to fall - as a major factor in large-scale extinction events over the course of the last 600 million years.

Widespread volcanic eruptions, therefore, can be secondary to global tectonic movement. They are also linked to asteroid impact.

20. Asteroid Impact

Alvarez et al. (1980) have provided convincing evidence indicating that asteroid impacts are a major factor in some mass extinction events including the ‘death of the dinosaurs.’ Further, as summarized by Alvarez et al. (1980): "impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record" (see also Thompson and Crutzen 1988).

Yet another impact, 12.9 mya, may have led to the extinction of the mammoths and other megafauna and the abrupt environmental changes that contributed to Younger Dryas (YD) cooling and the demise of the Clovis culture of North America (Firestone 2009; Firestone et al. 2007). According to Firestone (2009) and others, a comet exploded over North America, melting the Laurentide glaciers of Canada some of which acted as natural ice barrier which had dammed up a giant inland sea and a thousand mile wide lake of meltwater which scientists today call Lake Agassiz. When the glaciers and this natural ice dam melted and broke apart, the huge inland sea rushed into the North Atlantic shutting down the ocean's natural heating/cooling cycle and triggering an instant ice age, tsunamis and an avalanche of water causing sea levels to rise (Firestone 2009).

Like volcanic eruptions, impact by a sufficiently large asteroid or comet would eject massive amounts of debris into the atmosphere. Sunlight would be blocked, temperatures would drop, plants and animals would freeze and die (Thompson and Crutzen 1988). Therefore, in cases of collision involving the Earth and an asteroid or comet, the resulting impact is just one of the contributing causes of the resulting mass extinction event which are often due to multiple factors (see Molina et al. 1996; Twitchett 2006; Elewa, 2008a,b; Elewa and Dakrory 2008a), including stratospheric ozone depletion, nitric acid formation, anoxia, fire, and destruction of the food chain; interlinked events and a multi-causal scenario which can lead to the abrupt extinction of some species and the slow death of yet others(Arens and West 2008; Elewa and Dakrory 2008b).

21. Current mass extinction

Are we going towards a sixth mass extinction? Many prominent scientists answer this question with a "yes" and point to human activities as responsible (Crutzen and Stoermer 2000; Elewa 2008c; Ruddimann 2005; Steffen et al. 2007; Wilcove et al. 1998). It has been argued that species are programmed to die out, and that some forms of extinction are due to genetically controlled evolutionary-apoptosis, where species are "pruned" from the tree of life after serving some biological purposes or acting as a bridge to a subsequent species. Cell and species death are a normal part of life and integral to evolution, such that all organisms contain the genetic seeds of their own self-destruction. Are humans genetically programmed to self-destruct?

Humans are biological organisms and we have created poisons, wastes, and weapons of mass destruction which can destroy much of animal life on this planet. Perhaps, we too, despite all our wisdom, have sown the seeds for not only the sixth mass extinction, but the eradication of humans from the face of the Earth.

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