Extinction and the History of Life

A dinosaur skeleton. This one species of dinosaurs, like all dinosaur species that went extinct and paleontologists now agree that 65 million years ago, an asteroid collided on the earth's surface, triggering a series of catastrophic events that brought an end to all dinosaurs. In addition to evolution of new species, extinction is also part of the natural history of the earth.

A dinosaur skeleton. This one species of dinosaurs, like all dinosaur species that went extinct and paleontologists now agree that 65 million years ago, an asteroid collided on the earth’s surface, triggering a series of catastrophic events that brought an end to all dinosaurs. In addition to evolution of new species, extinction is also part of the natural history of the earth.(Nina Haghighi)

 

 

This planet is the one planet in our solar system that is capable of supporting life and like everything in the universe, life, like the planet earth also has a history that stretches back billions of years starting from its inevitable if still mysterious origin to all the current life that we observe today including human life which is also a product of the same evolution that produced all the variety of life. When you study the history of life, you cannot escape the conclusion that in the beginning there was simple life and from that simple life in the form of cells, came the evolution of multicellular organisms so it would seem that there is some sort of progress from simple to complex and this is mostly true for organisms that we are familiar with such as plants and animals but when you also study the history of life, it was not always progress from simple to complex and in fact there were ancient organisms that at one time thrived in one environment but are no more because they became extinct or ceased to exist.

 

From studies of the fossil record or that part of the earth that contains the remains of past life forms, and from methods of geology and paleontology, an almost complete record of the history of life can be reconstructed with some degree of certainty even though as Darwin (1859) was the first the first to point out, because of natural geological processes that range from erosion to plate tectonics, the entire history of the fossil record will be distorted and even destroyed to some degree but nonetheless we can gain enough information to reconstruct the patterns of evolution of life from the past to the present.

 

Through studying the fossil record, every species will have some time to live only to become rare and eventually extinct. Why would this be? Why would any species of organisms that has adapted so well in its habitat because natural selection, the mechanism that was first hypothesized by Darwin (1859) to explain the source of adaptation for every species results , because of competition within individuals of a population and those that are able to survive and prosper will outcompete those individuals that are unable to do so  one group of organisms in a population or group of organisms that share the same space and can interbreed with one another. For every competition there will be those that survive and those that will lose. Since the Modern Synthesis was established during the 1940’s which confirmed natural selection, it would seem at first that the reasons for extinction would then become clear but in reality it was not really so that is until the fossil record was thoroughly studied and it became clear that there would not be one main cause of extinction if one only relied on Darwinian struggle but a variety of causes of extinction with Darwinian struggle as just one example and since then, there has been a spectrum of extinctions that range from small scale to massive levels of extinctions, and through the history of life, there has been at least five major episodes of mass extinctions, one of the largest where about 90% of life almost went extinct which occurred between the end of a geological age, the Permian and the other extinction where a group of large, dominant reptiles, the dinosaurs went extinct at the end of the Cretaceous where we now know, or at least we think we know, the cause of which was the result of a asteroid or comet that crashed into the earth, setting of a chain of events that lead to the dinosaur’s demise.

 

It is worth studying extinctions or rather the causes of extinctions and there are many causes of extinctions and it is the task of science to identify each cause and also to try and reconstruct a plausible scenario based on the evidence available of how an extinction event occurred. From the patterns of extinction that is recorded in the fossil record, it seems that the death of species is an integral part of the evolution of life and from what the fossil record also reveals, whenever there is an extinction , some species will not survive simply because they were unable to adapt to the sudden changes in their environment but not every species will succumb so easily and in fact there will be species that, because either the possessed certain traits that confer survival or they may have been away when the extinction event happened or a combination of the two, that allowed survival but even so, the survivors will also not survive the next extinction event. The lesson then is that for every evolution of species, there will be a death of one or more species which allows survivals of other species and it all is part of a cycle of life and death for every species.

 

Still, it is worth investigating not just the emergence of species but their extinction and in the past 30 years or so, there has been plenty of research into the extinction of species and their causes and it is worth examining the various kinds of extinctions as well as the five major episodes of extinctions.

 

A Spectrum of Extinctions and their Causes

 

Throughout the history of life, for every evolutionary event there has been many extinctions or terminations of species or groups of related species and so the question is what causes extinctions? From studies of the fossil records, it is not only evident that there has been extinctions but finding the cause or causes of extinction has been undertaken and there is not one single kind of extinction but a variety of extinctions and these range from small scale extinctions which usually happens at the lowest level taxon or group of similar individuals, usually the species, all the way to mass extinctions where many species or even higher up the taxonomic scale becomes extinct.

 

To begin with and if you have read some of my blogs on evolutionary biology, and as you probably know, natural selection allows individuals in a population to survive any changes in the environment and the environment acts as the agent for selecting those individuals with the traits for survival while those without such survival conferring traits will be more likely to die and this has gone on ever since the origin of life about 3.8 billion years. With such a long time span for a changing earth, there has been a variety of environments that range from warm and dry, warm and wet, cold and dry and so on that has put any population through the filter of natural selection, favoring the winners or those that can tolerate temperature changes, the kinds of food that can be digestible, and the ability to find mates over those that can do such things and because of natural selection, there has been a diversity of many life forms but throughout the history of the earth, for every successful species, there are those that have gone extinct and biological evolution with its emphasis on natural selection is just one of the factors that are responsible for extinction but other causes can range from habitat size where a large roaming animal species that once inhabited vast areas but then slowly the habitat becomes smaller and smaller and if the animal species cannot adapt to the changes in sizes then it will likely go extinct for the reasons.

 

In addition, if one species goes extinct and if that species is part of an ecological network where it ends up supporting other species such as a species of tree that only some animals can live on and if it went extinct, then it is also likely that those other animals that can only depend on it for food as well as shelter will also likely go extinct.

 

For the mass extinctions, many species go extinct and the reason would have to do with sudden changes in the environment and there is plenty of evidence to support when there is such a change in the quality of the environment where so many species have successfully adapted for so long but with the oncoming changes, many of those species will likely go extinct and what the fossil record says is that for mass extinction to occurs both the earth as well as the sky can bring about mass extinctions. In the case of the earth this can come as a result of plate tectonics where movement of the plates that define the earth’s continents and oceans can either change sea currents leading to long terms patterns of cooling , for example, can bring about the deaths of species of animals and plants that were previously adapted only to warm environments, or also when there is also a flow of oxygenated water being delivered to a marine community that can only thrive in oxygen rich waters but because of the shifting plates, the flow of oxygen rich waters has been cut off and as a result , the entire marine community collapses.

 

Also astronomy can play a role in species extinction and ever since 1980 when the father and son team Luis and Walter Alvarez present the then controversial theory that all species of dinosaurs became extinct 65 million years ago when an a asteroid or comet, or probably both crashed into the earth’s surface creating a massive cloud of dust which blocked out the sun and killing most of the vegetation which then started a collapse in the food chain which brought about the dinosaur’s demise. Ever since the paper was published, there has been plenty of evidence for the support of the collision hypothesis and paleontologists are now in unaminous agreement that asteroid impacts can bring about mass extinctions so not only geology but astronomy can bring about the deaths of species that are unable to adapt to the sudden changes.

 

At first when it was discovered that an asteroid colliding on the earth’s surface was responsible for the dinosaur’s demise, it seemed at first that it was only that an asteroid collision was responsible for the extinction of the dinosaurs while other mass extinctions were the results of geological forces such as supervolcano activity. But, as Becker (2002) revealed, it is likely that it was not just one event of asteroid impact but several impacts that may have been responsible for mass extinctions during the 600 millions years of prehistoric life and if it is proven that is beyond the case then astronomy would play a larger role in earth’s extinction much more than geology and climatology.

 

If five major extinctions events were documented in the fossil record, is it possible that there is a recognizable pattern? Since 1982, two paleontologists at the University of Chicago, David Raup and Jack Sepkowsky Jr., did a study of genera and families from the Cambrian to the Cretaceous and noticed a pattern. It seems that extinctions do have a pattern that on average there is a 26 million year gap where there are fewer extinctions and in between the 26 million year gap, there are the mass extinction events. What this suggests, from a statistical point of view, is that there is a pattern where for every small scale extinction event there are fewer major extinctions where many species die in a sudden short period of geological time ( by this it is meant that mass extinction would occur less than a million or even a thousand compared to the long drawn out period of geological time which can be hundreds of millions of years). If true, and also considering the fact that asteroid collision can bring about extinction and since the evidence for mass extinction caused by asteroid impact is now more convincing, it was even suggested that out in space, our sun is orbited by a star, aptly called Nemesis, and beyond the orbit of pluto there is a region of icy object called the Oort cloud which is the source of comets and as the hypothesis goes the orbit of Nemesis would perturb the orbit of these comets, and some of them would send them flinging into the direction of earth, increasing the chance of collision and hence extinctions. As for the evidence of a second star orbiting the sun, there is hardly no evidence that there is a second star and not others think that the 26 million year interval in between is genuine considering that although more and more fossils are being discovered , it is still far from complete and the fossil record is very imperfect so it will not contain every and all species of life forms that have ever existed so any pattern discerned from such a meager record may not be indicative of anything genuine.

 

I cannot tell the story of the five major extinctions without considering the history of life for extinctions are interwoven into the story of the evolution of life on earth and for every extinction there will not just be those which did not survive but the survivors only to later become extinct. This is the apparently the theme in the history of life; a cycle of winners and losers.

 

 

A History of Life on Earth and the Big Five Extinction Events

 

Life has been and still is the most dominant feature on the surface of the earth for about 3.85 billion years and beginning from a single origin or multiple origins, whatever the transition from life from nonlife, even though how life originated from nonliving matter is still far from solved but it did occur judging from the fact that life is present on this earth. At first life was simple or more likely prokaryotic judging from the earliest fossils which revealed that life was microscopic and single cell and also from molecular sequencing of eukaryotic cells along with two kinds of prokaryotic cells, eubacteria or “true bacteria” and archaea or those kind of bacteria adapted to extreme environmental conditions, it is likely that the ancestor was not only prokaryotic but may have also been adapted to extreme environments. Eventually, the ancestor of today’s life began to diversify into many kinds of environments, even changing the environment in a way that may have favored the survival for some but the death of others. One dramatic example of this is the evolution of photosynthesis where sunlight is used to convert carbon dioxide and water to form carbohydrates while releasing oxygen as a waste gas.

 

Previously there was no oxygen in the atmosphere and so the prokaryotes or cells without nuclei such as bacteria were likely anaerobic and that is bacteria that can survive without oxygen but when oxygen became available after the evolution in some cells the ability to carry out photosynthesis, oxygen was being released in large amounts by those cells that could photosynthesize, surrounded by sunlight and water and with so much oxygen, that excessive amount of this reactive gas would have been the first pollutant as Sagan and Margulis (1986) called oxygen and in another blog I argued that oxygen, as vital to life is also something of a poison. The bacteria that have previously adapted to an anaerobic lifestyle were unable to survive  this gas and so were destroyed but some cells that could use oxygen for their metabolism were at an advantage and for one thing, in the process of metabolism, a molecule which is present in all of life is used for every biological function and that molecule is called adenosine triphosphate or ATP for short. In the beginning, the first life forms capable of Darwinian evolution also used ATP but prior to an oxygenated atmosphere, the first form of metabolism was likely based on fermentation which also takes place without oxygen. In fermentation, ATP is made but only up to two to three ATP molecules are made but in respiration or metabolism that uses oxygen, a total of 36 molecules of ATP are made!

 

With the ability to use oxygen in making that many ATP molecules, these cells then had an advantage and it was because of an oxygen atmosphere that biologists believe triggered the further evolution of eukaryotic cells or cells with nuclei and these would include fungi, plants, and animals. As far as the evolution of eukaryotic cells go, it is also believed that the eukaryotic cells were the result of a merger of two different kinds of cells, known as endosymbiosis, and the evidence is that in both animal and plant cells, there are a kind of organelle or cellular structure that perform a specific function, called a mitochondrion and these are organelles that produce ATP from food. What is interesting about mitochondria is that they have both DNA and ribosomes and ribosomes are structures that make protein molecules. In addition, molecular sequence comparisons of mitochondria reveal that they are more similar to a particular kind of bacteria called purple nonsulfur bacteria which are aerobic or use oxygen. This suggests that at one time, the ancestors of mitochondria were once free living organisms that were taken up by larger cells and if the cells that captured the aerobic bacteria did not digest their meal completely but benefited their captors by provided an efficient way of providing biochemical energy, then that cell with the bacteria were also at advantage.

 

Also in plant cells, there are the chloroplasts or the organelles that carry out photosynthesis and like mitochondria, chloroplasts also have DNA and ribosomes and molecular sequencing also reveal that there are more similar to a kind of prokaryote called cyanobacteria which also photosynthesis and like the cells that captured the mitochondria, it would have been the same story: the cells with chloroplasts that did not digest them but if helped by their guests, were also at an advantage.

 

From the cells with bacteria that can carry out respiration and photosynthesis, they would be the ancestors of today’s plants and animals. With an atmosphere of oxygen, not only drove the extinction of some prokaryotes but also favored the survival of those that could use oxygen, and endosymbiosis was likely helped because of an oxygenated atmosphere. With the rise of eukaryotic cells together with the invention of sex which promoted further genetic diversity, the stage was set not just for the evolution of life but with changing environments of many kinds, not just the evolution of new life forms but also the extinction as well.

 

The Cambrian Explosion, Silurian Ordovician Devonian Extinction, and the Great Dying of the Permian

 

A fossil of a trilobite. Trilobites are one example of the many species present during the Cambrian explosion where there was an evolution of many life forms and as there were many species of trilobites, some went extinct but all species became extinct in one of largest mass extinction of all time, the Permian extinction where up to 90% of marine and land animals as well as plants went extinct. (Steve. L. Martin)

A fossil of a trilobite. Trilobites are one example of the many species present during the Cambrian explosion where there was an evolution of many life forms and as there were many species of trilobites, some went extinct but all species became extinct in one of largest mass extinction of all time, the Permian extinction where up to 90% of marine and land animals as well as plants went extinct. (Steve. L. Martin)

 

Around 600 millions years ago, most of the surface of the earth was covered with water with an atmosphere that was full of oxygen and ozone and the forms of continents that would completely be alien to today’s obsevers but were scattered here and there and were slowly being carried about by tectonic forces. It was in the waters where life began and also life continued on its evolution of becoming different species but there were animals evolving into strange forms that would like the ancient continents, be unfamiliar to today’s observers yet 600 millions years ago there was an explosion of many varieties of marine life forms that are so strange and so bizarre it is likely that these animals belonged to phyla or higher taxon that are no longer here today but out of these strange evolutionary experiments came a handful of invertebrate phylum that are still around such as the arthropods which includes today’s species of insects and spiders and of course the chordate phylum or the animals with a chordate or backbone.

 

From the fossil record that dates to around 600 million years ago, there were so many species of bizarre looking creatures that at this time, which in paleontology speak, is called the Cambrian and in the Cambrian, where so many fossils of so many bizarre forms with equally strange names such as Wixaxia, Hallucinegia, and Anomolacaris along with the well known but extinct invertebrate, the trilobites, that within this geological time period, is called the “Cambrian explosion” for it is likely that all these bizarre forms suddenly appeared quickly within the blink of a geological eye for reasons that are still unknown but it is likely that because previously prior to the Cambrian there were few animal species, mostly fossils of invertebrates that if taken at face values may have been few but after that, a massive radiation of species occupying every ecological niche from the top of the ocean to the bottom.

 

The fossils from the Cambrian can be found in one particular place in British Columbia, Canada, where north of the Rocky Mountains, there is an area called the Burgess shale which has the largest collections of these fossils and from geological evidence, the top of the mountains of northwest Canada was underwater but hundreds of millions of years later, the ancient seabed slowly lifted up to become the mountains of the Rockies where it has almost perfect record of ancient marine life. Originally these fossils were discovered by the American Paleontologist Charles Walcott of the Smithsonian Institute and although he knew that the fossils were marine and since he was working with fragments of marine fossils, he interpreted them as ancestors of modern arthropods such as crustaceans such as shrimps and lobsters and also vertebrate fossils that were ancestral of fish. That was at first the original interpretation until the late 1960’s and early 1970’s when University of Cambridge researchers Simon Conway Morris began investigations of the Cambrian fossils and came with a startling conclusions that many of the fossils were misidentified as ancestors to arthropods but seemed to belong to phyla that were actually extinct. For example, using refined methods of studying fossils that was unavailable in Walcott’s time, what looked like at first giant shrimp as interpreted by Walcott, turned out to actually be the mouthparts of a giant carnivorous creature that was so strange as to defy description and was aptly named Anomoclaris!

 

The consensus now is that these ancient marine life forms were likely the result of a strange experiment conducted by natural selection in adapting to various ecological niches that quickly become filled and from the Cambrian up to the Permian, many species lived for a time and then become rare and eventually extinct with the end of the Permian being the largest mass extinction where it is estimated that up to 90% of animal and plant life become extinct and we will look at the likely causes of this kind of extinction but what triggered the evolution of these animals and why?

 

Previously before the Cambrian there were not so many animal species and either this is because with few fossils it seemed that taken at face value, early animal evolution begin as simply as creatures resembling todays jelly fish but this may also be a mistaken conclusion considering that given an imperfect fossil record the kinds of fossils that make up the Precambrian called the Ediacaran Fauna after a locality in Australia, may not be the true representation of animal life before the Cambrian expolsion but species and hence phyla become more and more diverse and why the diversity? There would never be a single reason just many plausible reasons, For one thing, the eukaryotic ancestors of animal life had to adapt to using oxygen and it is currently accepted that the oxygen that was in the atmosphere may have been up to five to even ten times the amount than it is today where now it is just 21%. With a large amount of oxygen, this would have favored the survival of animals that could utilized a large amount of oxygen possibly leading up to the diversification of animal species. Another cause is the evolution of a set of genes only present in the animal kingdom called Hox genes and these are genes that determined the form of the animal body and that includes the head, thorax or where the appendages are attached, which appendages are used for walking as well as for feeding and even for copulation, and an abdomen where some of the vital organs are located. Hox genes are present in animals from worms to humans and it is likely that the evolution of Hox genes contributed to the various forms of animals. These and the fact that every species of animal will evolve to take advantage whatever niche is located since before it is likely that there were few niches for primitive animals to make a living but eventually as the evolution of marine life begin to increase, more and more niches began to be filled up. These are three of the reasons for the enigmatic Cambrian explosion and all three were likely the main reasons not to mention other reasons could also be put forth.

 

As life was evolving, surely there were extinction and three major mass extinctions occurred, one of the first mass extinction event was the Silurian extinction or rather the Ordovician-Silurian event and let’s see which species did not survive.

 

The Ordovician-Silurian extinction lasted from 445 MYA ( where “MYA” is short for Millions of Years Ago) to 430 MYA, a difference of 15 MYA, so in this first extinction event for about 15 million years, a large percentage of species became extinct. What where the victims that succumbed? From the fossil record since life was then evolving in the oceans, most of the extinct species were invertebrates or animals without a backbone and the animal species that died were various invertebrates called bivalves and the survivors would have been today’s clams and mussels, bryozoans which are sedentary colonial animals that specialize in filter feeding, and of ancient species of corals. Prior to the mass extinction, the waters would have been warm, supporting species of marine invertebrates that could thrive in warm waters but from 445 MYA, there is evidence of a prolonged cooling caused by the onset of glaciation where cooling temperatures would have begun the demise of many marine species adapted to warm waters. Because of glaciation, there would be periods of drop in sea levels because more and more water was being locked up in glaciers and in addition to cooling, there was also an increase in anoxia or lack of oxygen and this was enough to kill any animal species in areas of anoxia that already used oxygen for metabolism.

 

The next extinction event that occurred  was the Devonian extinction and this extinction claimed about 50% of plants and animal genera. Aside from that, this was a period in earth’s history where some species of animals began colonizing the land and from the fossil evidence, first were the invertebrates which with the ability to live on land, they had a completely different world to conquer or rather take advantage of land based niches. Another set of animals, the amphibians, made their debut on the land and it is believed that amphibian evolved from fishes near shallow waters and with the ability to breathe oxygen, they had an obvious advantages but like their fish ancestors, they had to breed in the water. Also, there were primitive plants with roots which could anchor themselves in the ground and with roots they could absorb water and nutrients and these plants may have been the ancestors of todays mosses and liverworts. Slowly vegetation began to grow along shallow waters which slowly transformed the landscape becoming forests and with plenty of plants this exerted selective pressures for both land dwelling invertebrates and vertebrates for the vegetation would be food for some as well as shelter.

 

There was even changes in the distribution of the continents and from the arrangements of the continents can make a difference in promoting survival for some species as well as the death of other species. Separate groups of continents which would later be North America and Northwestern Europe were combined together while in the southern hemisphere India, Africa, and Australia were combined together, the former called Euramerica and the latter Gondwana and later they would combined together into one vast supercontinent called Pangaea. Iit is believed that the shifting continents played a role in the Devonian extinction and how continents can do this is by interfering with ocean currents where previously any sea currents that brought warm temperatures would now end up bringing cold temperatures and that would have been deleterious for any marine species that were adapted to warm temperatures.

 

As the Devonian began to come to a close, extinctions followed and the nonsurvivors included only marine life while animal and plant life survived. These included marine species of trilobites, corals and the Devonian included evolution of coral species which most did not survive, and brachiopods. In addition to marine life, freshwater species did not fare well and those that did not survived were the Sarcopterygii, a class of bony fishes with fleshy limbs which are distinct form the other classes of fishes both saltwater and freshwater the ray finned fishes, and most species of Sarcopterygii went extinct with some surviving, and one of the most famous of the surviving species of  Sarcopterygii, the coelacanth is still around so that would make it a living fossil.

 

The duration of the extinction lasted roughly about 25 million years and during that time, about 75% species went extinct and it was only both marine and freshwater species. What was the cause or causes of the extinction and why did it affect only marine and freshwater species while land based life was relatively spared? Since given the long history of the earth and because our planet is not immune to asteroid impacts, it would then seem logical to find a cause of extinction that is the result of an asteroid and so by studying sediment layers that correspond to the time of the Devonian evidence for asteroid impacts such as the presence of iridium and geological dating of any impact craters. As of now, now unambiguous evidence of asteroid impact has so far not been proven for the Devonian extinction and even though evidence of iridium was detected in sedimentary layers around the time of the Devonian, other causes for the source of iridium could also be found such as volcanism and even assigning the date of an impact crater has been problematic. Of course this failure of finding an extraterrestrial impact does not in itself prove that the Devonian extinction was not the result of an asteroid collision for it may possible that it could have been. But, if it is proven to be result of an asteroid, there would be problems accounting for why only freshwater species and marine species went extinct while there were many survivors on land.

 

If an asteroid did collided with the earth during the Devonian, not only marine life but land life as well would have been affected. It was the same when an asteroid collided on the earth 65 millions years bringing an end to dinosaurs on the land, sea, and air as well any ancient marine life but we will get to that later in this blog. Suffice it to say, it is possible that the evidence may also be misinterpreted and the reason is that the fossil record is anything but complete and it is likely it could be some sort of bias that is preventing from seeing the full account of the Devonian extinction since most fossil species that are well preserved tend to be marine species so there could be some sort of bias in analyzing the results by focusing only one species and genera that are from the seas.

 

Out of the five mass extinction events, there was one mass extinction event that was the most largest and well documented in the fossil record and the extent of the extinction was so large that about 90% of plants and animals species at the end of the Paleozoic era or the era in earth’s history that included the Precambrian up to the Permian and after that came the age of the dinosaurs whose ancestors did survive that catastrophic extinction event as well as the ancestor of today’s mammals which were small ratlike creatures with the dinosaurs being dominant on land, sea, and air.

 

If the Cambrian was the time of massive speciation of many phyla, some of which have survived to the present, then towards the end of the Paleozoic, there would have been an extinction of massive proportions. Because the Permian extinction as it is called, we will first see which species of animals and plants did not survive and what species did and shortly after, we will now try to find the cause or rather causes of that massive extinction in earth’s history.

 

What species of organisms went extinct and which survived? The species of organisms that fared no better were of course the marine invertebrates, most notably the trilobites and during that catastrophic extinction, all species of trilobites were gone as the Permian came to a close. Other marine invertebrates that went extinct included Euryptids or “sea scorpions” or large scorpionlike marine arthropods, orthids, which are an taxonomic order of the phyla, brachiopoda or brachiopods or shelled invertebrates that open with hinged valves and are attached by a fleshy stalk on a solid substrate, about 79% of species of a colonial marine invertebrate called Bryozoan, and a species of chordates, the Ancanthodians or a group of bony and cartilaginous fishes. Other Permian marine life forms included many species of echinoderms which includes today’s species of starfish, Cnidara which includes today’s species of corals, and Mollusca which includes that ancient species of ammonites, where about 97% of ammonites died out.

 

Many marine invertebrates and vertebrates suffered great losses. The same is true for the terrestrial life forms and those that died out included the group of insects with the imposing name of paleodictyopterids which were insects with unusual mouthparts. Other terrestrial life forms that succumbed to the catastrophe were the earwig like insects, the protelytropterans and whose surviving descendants include today’s species of thrips, earwigs, and my least favorite insects, the cockroaches. From the fossil record in addition to marine life, terrestrial invertebrates also went extinct in large numbers.

 

As for terrestrial vertebrates, the losers were the labyrinthodontia or an ancient group of amphibian, along with species of sauropsids which included the dinosaurs, and therapsids or reptiles with mammal like characteristics. Plant species that died out were the seed ferns or Glossopteris. Unlike animal species, there are not that many species of plants that fossilize easily and so pollen studies need to be used to estimate which species of plants died out.

 

When all species are accounted for, the total percentage comes out to be 96%, a very large percentage of extinction and from this, this extinction event counts as the largest of all extinction event.

 

It is then natural to ask, what are the causes of this extinction? Since this event occurred 250 million years ago, and being an ancient event, in terms of geology, finding the cause or causes has proven, not surprisingly, to be very difficult, simply due to the fact that earth has active plate tectonics, and any evidence will be destroyed through the slow recycling of earth’s crust , through metamorphosis of rocks and fossil up to melting into magma. Many possible mechanisms have been proposed and the most well known cause is to assume an asteroid impact but what is the evidence for an asteroid impact?

 

One evidence for an ancient asteroid collision would be the presence of shocked quartz, a type of quartz where it is compressed quickly under the force of shock by a brief but powerful external event such as the shock waves released when an asteroid hits the earth’s surface. There has been evidence of the presence of shocked quartz at the Permian-Triassic boundary which is the boundary where the extinction occurred. In addition fullerenes or carbon atoms that form under brief but intenses sources of energy such as a meteorite impact have also been found. However, recent studies of shocked quartz from another Permian-Triassic boundary from Antarctica reveals that the shocked quartz was more likely formed from tectonic stresses rather than asteroid impact. If true, then perhaps an asteroid collision may not have been the cause of the Permian extinction.

 

Even if shocked quartz may not be a reliable indicator of an asteroid impact, can there be the presence of an impact crater that was around during the Permian extinction? Assuming that an asteroid did collide with the earth around 250 million years ago, where would it be found?

 

About 70% percent of the earth’s surface is covered with water so an colliding asteroid or comet is likely to crash into the oceans more so than land but finding an ancient impact crater that is more than 200 million years on the ocean floor is problematic and recall that the ocean crust is constantly shifting in a time span over 200 million years and ocean crust is likely to encounter continental crusts and when ocean and continent collides , the ocean floor will be subducted under the continent and after 200 million years much of the minerals, rocks, and fossils will be destroyed and so will evidence for a impact crater, thus finding evidence for a 250 million year old crater very problematic.

 

Despite this, there has been some evidence of craters that date to about 250 million years, suggesting a possible link between their presence and the Permian extinction. One such evidence is the Wilkes Land Crater and the other is the suspect Araguainha crater in Brazil that has been dated to about 250 million years.

 

In order for an asteroid to cause an extinction, it must be of sufficient size to produce a large blanket of dust to block out sunlight and in addition to blocking out sunlight, it would interact with the carbon dioxide and nitrogen to produce acid rain, and also with so much fiery debris released from the initial impact, global wildfires would result, and the massive impact could even trigger volcanic activity.

 

From studies of the size, it seems unlikely to cause global wildfires, and produce a global blanket of dust thick enough to block out sunlight but even if the dating of the impact event is true, then it may have been strong enough to trigger volcanic activity so in a way an asteroid impact would have brought the end of marine and terrestrial life but not so much as the impact that ended all species of dinosaurs.

 

As for the Wilkes Crater, even dating of the crater to Permian times is also problematic so if true then it may be unlikely but additional research needs to be continue to prove whether or not asteroid impacts may have triggered the extinction at the end of the Permian.

 

It is more likely that geology more so than astronomy played a role in the extinction and around 200 million years ago, all of today’s continent were combined in one landmass called Pangaea surrounded by an even larger ocean called Panthalassa. Around 250 million years, which was still in the time of Pangaea was the Great Dying. Was it a coincidence that there was a Great Dying during the time of Pangaea or was the presence of Pangaea may have been responsible for the Great Dying?

 

According to this one scenario, with the presence of Pangaea, oceanic circulation were affected in a way that there were seasonal monsoons but with so much land in the interior, there would have been vast desert areas and any animal life forms such as amphibians that were adapted to humid conditions could not have tolerated dry climates in the interior and so would have gone extinct.

 

Although extinction was high for marine life forms but it was not even catastrophic after the formation of Pangaea and also there was an increase in the number of species of therapsids during Pangaea so even though, there were ongoing extinctions, it seemed that Pangaea wasn’t enough to cause the Great Dying. If it wasn’t Pangaea then what else could have been the causes?

 

It is hypothesized that towards the end of the Permian, there was not one but two major volcanic events, and the two are the Emeishan traps of China and the Siberian traps of Russia. The presence of these traps which formed towards the end of the Permian were not like any regular volcanic events and what sets them apart is that these were likely the most powerful volcanic eruptions and these were so powerful that these volcanoes released so much ash resulting in global warming and acid rain that set of a series of extinction events affecting both land and sea.

 

It seems that both of these supervolcanoes may have been largely responsible for the Great Dying and if true, other resulting causes that would have greatly accelerated the mass extinction would have been the increase of anoxia or areas where no oxygen was available along with a proposed mechanism of hydrogen sulfide  which would have been also caused, somehow, by the supervolcanic activities present.

 

As the extinction event came to close, some species of animals and marine life did survive and it is estimated that for each extinction event and on average, it takes about 30 million years for any surviving species to recover and dominate after the extinction of those that did not and during the beginning of the Mesozoic era, came the dinosaurs which dominated the land, seas, and air, and although mammals did coexist with the dinosaurs, there were not that many species of mammals since the dinosaurs dominated every ecological niche.

 

The Triassic Jurassic Extinction

 

The Mesozoic era was well known for those various animals called dinosaurs that still grip the imagination, young and old, and during this time in the history of the earth every ecological niche was inhabited by every kind of dinosaur. From the Triassic up until the Jurassic, there was another mass extinction and this mass extinction affected about 34 % of marine species while certain dinosaur species and amphibian species went extinct.

 

Recall that for every extinction, there will be species that have survived and the survivors will take over any ecological niche that is empty and eventually will diversify into new species until another round of extinction. It was the same for the Devonian and Permian extinction and it is also the same for the Triassic Jurassic extinction.

 

As for the causes of the extinction during the Triassic, several theories had been proposed and one such convincing cause is evidence for massive vulcanisms which would have resulted in emissions of carbon dioxide triggering global warming as well as emissions of sulfur dioxide aerosols which would result in global cooling. As for an extraterrestrial cause, this has not been ruled out although evidence for an impact crater during the Triassic period has not yet been found so whether or not the extinction was triggered by and asteroid has yet to be revealed.

 

The surviving species would later become the dinosaurs which would be dominant from the Jurassic up until the Cretaceous with many species of dinosaurs, occupying niches on land, air, and in the oceans and many species of dinosaurs were present up until 65 million years when their time on earth came to an end.

 

 

The K-T Event and the End of the Dinosaurs

 

 

A skeleton of T-rex. This dinosaur was one of many species of dinosaurs that went extinct towards the end of the Cretaceous. What caused the extinction of the dinosaurs? Prior to 1980, there was no satisfactory explanation until it was discovered that the cause of the extinction of the dinosaurs and other life forms was the result of an asteroid collision.

A skeleton of T-rex. This dinosaur was one of many species of dinosaurs that went extinct towards the end of the Cretaceous. What caused the extinction of the dinosaurs? Prior to 1980, there was no satisfactory explanation until it was discovered that the cause of the extinction of the dinosaurs and other life forms was the result of an asteroid collision.(InSapphoWeTrust)

 

 

At the boundary between the end of the Cretaceous and the beginning of the Cenozoic, the fossil record indicates that there was an extinction of all species of dinosaurs followed by the evolution of the mammals and this happened around 65 million years ago when after all the dinosaurs went extinct, plenty of ecological niches were soon available for mammals to take over and diversify into many species but what caused the extinction of the dinosaurs?

 

Various kinds of theories were proposed to explain the extinction of the dinosaurs at the end of the Cretaceous period and none were all satisfactory until around 1978 when the father and son team Luis and Walter Alvarez, a physicist and geologist respectively noticed something unusual at the boundary separating the cretaceous and tertiary event, and recall that at this particular boundary, it not only marked the end of the dinosaurs but there was also a unusually high concentration of iridium, which is a metal that is rare in terrestrial rock but common in extraterrestrial rocks such as meteorites. Also, many K-T boundaries showed the same concentration of iridium worldwide and since iridium is found in extraterrestrial rocks, it occurred to the Alvarez’s that it was not a coincidence that the presence of iridium was somehow linked to the extinction of the dinosaurs. In addition to iridium, there was also the presence of an unusual kind of quartz called shocked quartz and this kind of quartz can only form under intense but brief periods of pressure such as meteor impact. With both the anomalous concentration of iridium and shocked quartz found in K-T boundaries, the Alvarez’s and their colleagues were forced to conclude that the extinction of the dinosaurs was caused by a large meteor that impacted the earth 65 million years ago.

 

The consequence of an impact is that the impact would release so much dust as to block out the sunlight, which would kill a lot of vegetation and with the death of vegetation, there would be no food for herbivorous dinosaurs and following the death of herbivorous dinosaurs, carnivorous dinosaurs such as T-rex would finally succumb. In addition to blocking out sunlight all the dust would react with all the atmospheric moisture resulting in acid rain and all the fiery fallout from the impact would start massive wildfires.

 

From this grim scenario of  global darkness, corrosive rain, and global conflagrations, along with massive vulcanisms caused by the powerful impact and also tsunamis inundating coastal areas, and it is believed that this started the chain of events that brought all species of dinosaurs to their end. This not only brought about the end of the dinosaurs but the extinction of the dinosaurs paved the way for the evolution of mammals and even though mammals coexisted with dinosaurs, the dinosaurs occupied far too many ecological niches, leaving very little room for mammalian evolution but as the dinosaurs died, the mammals had plenty of opportunity to occupy various niches on land, then water, and even air. Every extinction event creates opportunities for the survivors.

 

Based on the then hypothesis that the iridium present in higher concentrations at the K-T as caused by an asteroid impact, there should logically be a large crater, large enough to be found and indeed such a crater was found and identified as being formed around 66 million years ago, and that is the Chixculub crater near the Yucatan peninsula of southern Mexico.

 

Conclusion

 

It is undeniable that evolution of life has begun from the simple to the complex, and as Darwin (1859) has beautifully summed up the whole of evolution in the last paragraph describing the whole of earth’s diversity evolving as ” from so simple a beginning, endless forms most beautiful have and most wonderful have been and are being evolved.” but the pathway doesn’t show an unbroken lineage undergoing a progression towards “advanced forms” and not without extinctions from small scale to large scale and extinctions are the rule rather than the exception. In fact, without extinction, no further evolution of life could occur and there would be no room for additional surviving species, and the fossil record does show that after every extinction, life takes over those places vacated by unsuccessful species.

 

 

 

References

 

Alvarez, L., Alvarez.W, Asaro,F., Michel, H,V (1980). Extraterrestial Cause for the Cretaceous-Tertiary Extinction” Science, 208, 1095-1108, doi 10.1126/science.208.4448.1095

 

Becker, L. (March, 2002). Repeated Blows. Scientific American, 286(3), 76-83

 

Benton, M (2009). Paleontology and the History of Life. In M. Ruse and J. Travis (Eds). Evolution: The First Four Billion Years (pp. 80-105). Cambridge MA: Harvard Belknap Press

 

Cretaceous-Paleogene Extinction Event. (n.d) Retrieved June 2nd 2016 https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event

 

Darwin, C.  (1859) On the Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life.  London, England: Murray London

 

Extinction. (n.d) Retrieved April 4th 2016 https://en.wikipedia.org/wiki/Extinction

 

Extinction Event. (n.d) Retrieved April 21 2016 https://en.wikipedia.org/wiki/Extinction_event

 

Gore, R. (June 1989). March Towards Extinction . National Geographic, pg. 662-699

 

Gould, J.S (1993) The Book of Life : An Illustrated History of the Evolution of Life on Earth New York, NY: W.W Norton & Company

 

Gould, J.S (1994) The Evolution of Life on Earth. Evolution: A Scientific American Reader. pp 234-250. Chicago, IL: The University of Chicago Press.

 

Ordovician-Silurian Extinction Event. (n.d) Retrieved April 22 2016 https://en.wikipedia.org/wiki/Ordovician%E2%80%93Silurian_extinction_events

 

Permian-Triassic Extinction Event. (n.d) Retrieved May 2016 https://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event

 

Raup, D.M (1991) Extinction: Bad Genes or Bad Luck? , New York, NY: W.W. Norton and Company

 

Wilson, E.O (1992) The Diversity of Life, New York, NY: W.W. Norton and Company

 

 

Photo Credits

 

Nina Haghighi Dinosaur skeletons 1 https://www.flickr.com/photos/nina_pix/6653280049/in/photolist-b8VNmZ-btwdYB-btwfd2-btw9UM-bByin8 CC BY-ND 2.0

 

Steve. L. Martin Trilobite https://www.flickr.com/photos/64476710@N00/19989491/in/photolist-2Lsbp-6JSPyu-9cgx5R-9cma1i-gzyHU4-fXDMq8-fXEb6M CC BY 2.0

 

InSapphoWeTrust  Tyrannosaurus Rex, American Museum of Natural History https://www.flickr.com/photos/skinnylawyer/7356570166/in/photolist-fLU8CY-6rULPK-67bj8F CC BY-SA 2.0

 

 

 

 

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