Take solace in remembering that if you are here to read this, your progenators, in one form or another, survived all five events. You can also be consoled by the fact that these five events have taken place over a very long period of time. The rough speculation of when the earth was created is around 4. 5 billion years ago. However, undisputed scientific evidence dates the first life on earth at 3.5 billion years ago.
A mass extinction is considered as the event or period of time when an “overwhelming majority of the species living at that time is wiped out.” i.e. become extinct. https://www.thoughtco.com/the-5-major-mass-extinctions
That’s simple enough to understand, but pretty harsh. To grasp the full concept, first we need to put the five extinctions into perspective and, to do that, we need a simple timeline for the earth’s development.
Since there is indisputable fossil evidence of rudimentary life existing 3.5 billion years ago, it makes sense to begin measuring earth’s timeline from that point. Presumably giving life a billion years to develop, the measurement starts at 4,500 billion years ago. Most of the Eras are divided into smaller amounts of timer called Periods.
During the earliest era of earth’s evolution, the surface changed from molten to rock and the continental plates formed. The atmosphere was composed of 75% nitrogen & 15% carbon Dioxide, known as prokaryotic cells, which have free-floating DNA and no nucleus, and lack membrane-bound organelles -- any of a number of organized structures within a living cell -- associated with higher forms of life.
● Proterozoic Era [2500 to 542 million years ago]
Nearing the end of this era, soft bodied organisms appeared such as jellyfish.
● Paleozoic Era [542 to 151 million years ago]
Vertebrate animal forms, arachnids, and wingless insects developed. Breakup of supercontinents and reformation of continents may have caused the earth to be unstable and mass volcanoes appeared which destroyed the oxygen level balance.
● Mesozoic Era [3 Periods]
1. Triassic period [252 to 201 million years ago]
Many primitive dinosaurs and marine reptiles evolved. Some of the first species of dinosaurs were the Plateosaurus, Eoraptor, Herrerasaurus, Pisanosaurus, Saturnalia, Staurikosaurus.
2. Jurassic Period [201to 145 million years ago]
Dinosaurs such as Diplodocus and Apatosaurus, and carnivores such as Allosaurus flourished on the earth. The first reptiles with primitive abilities to attempt flight appeared.
3. Cretaceous Period [145 to 66 million years ago]
The Ceratopsidae family begin to form which included plated dinosaurs with defense body features such as spikes such as the Triceratops. At the end of this Period and Era, there is a large dinosaur extinction that completely wiped out all dinosaurs living on the earth.
● Cenozoic Era [65.5 million years ago to today]
The oldest known modern mammals diversify and radiate across the continents. The appearance of the first elephants, horses, and other mammals existed in this Era. Homo Sapiens evolved.
There are numerous theories about the causes for those periods when a large percentages of all living species went extinct, including climate changes, geologic catastrophes such as volcanic eruptions and movement of tectonic plates, even meteor strikes onto Earth’s surface.
Because so many species died off during these catastrophic events, there were opportunities for surviving species to spread out and fill in places in the environment that need to be filled: i.e. more space plus less competition for food and resources. These events were followed by a period of rapid speciation -- the formation of new and distinct species -- in the course of evolution.
As surviving species spread out and became adjusted to their new environmental conditions, they became reproductively isolated from their original populations, "and eventually became new species.” https://www.thoughtco.com/the-5-major-mass-extinctions
THE FIVE EVENTS…SO FAR
The five mass extinction events differed as to the cause and the impacts but each ravaged the biodiversity on the earth at the time. There are several explanation given by various sources, and some sources combine the first mass extinction with the second because it came so quickly in geologic and evolutionary time. The article treats them as separate events.
● Ordovician Mass Extinction [Paleozoic Era]
By the Ordovician period [roughly 485 to 444 million years ago] larger aquatic life forms existed and some scientists speculate that there were even some land species. Nonetheless, the earth was still in its formative stages and the land masses had not taken their current shapes. Still today, our land masses continue to move very slowly. The causes of this mass extinction are considered two.
1. Gamma Ray Burst
“Some scientists believe that the initial extinction resulted from the gamma-ray burst that originated from a hypernova [a very energetic supernova also called a collapsar]. The burst which lasted about 10 seconds may have caused the earth's atmosphere to immediately lose about one-third of its ozone, exposing the organisms to extreme ultraviolet radiation.”
2. Continential Drift and Glacial Period
Until the Ordovician period, there had been period of somewhat stable conditions and temperatures. Approximately 440 million years ago the supercontinent Gonwanda began to drift significantly which contributed to the onset of an ice age with spread rapidly over the earth. With the seawater captured in ice, the sea levels were significantly lowered.
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The specific causes that ended the ice age are unknown although volcanic action would make sense in light of the continental drift. What is certain is that it was back to the drawing board for life on earth.
Another 80% of surviving and new species, land and sea, were again wiped out in this subsequent mass extinction.
● Permian Mass Extinction
Approximately 250 million years ago, at the end of the Paleozoic Era, the Permian Mass Extinction killed off an estimated 96% of all species living then.
This is largest of all known mass extinctions and is dubbed “The Great Dying.” Aquatic and terrestrial life forms alike perished relatively quickly as the mystery event took place. In other words, scientists don’t know for certain what happened, but there are numerous hypotheses. The most likely was massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth.
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These could have caused a decrease in oxygen that suffocated life and brought about a quick change in the climate. Recent research points to a microbe from the Archaea domain that flourishes when methane is high. These extremophiles -- organisms able to live in extreme environments with conditions approaching or expanding the limits of what known life can adapt to -- may have “taken over” and choked out life in the oceans as well.
When the time frame is compared to that of the breakup of the supercontinent Gonwanda and the formation of Pangaea and its mountains had a tremendous impact on global climate and sea levels, which resulted in glaciations and continent-wide sedimentation. These changes contributed to the Permian extinction event and left large deposits of hydro-carbons, coal, evaporate -- a natural salt or mineral deposit left after the evaporation of a body of water -- and metals.
The Permian Mass Extinction killed off an estimated 96% of all living species.
● TRIASSIC-JURASSIC MASS EXTINCTION
The fourth mass extinction occurred about 200 million years ago at the end of the Triassic Period of the Mesozoic Era, in which only half of all living species were obliterated.
In the seas, the entire class of conodonts and 23–34% of marine genus disappeared. Survivors were plants, the ancestors of crocodiles, dinosaurs, pterosaurs and mammals, which allowed these species to become the dominant land animals for the following 135 million years. Statistical analysis of marine losses suggests the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions.
during the Triassic-Jurassic Mass Extinction.
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Cause is attributed to gradual climate change, sea-level fluctuations, and changing environ-ments. Also, a pulse of oceanic acidification during the late Triassic may have also reached a tipping point.
How these possible processes affected the Triassic animal and plant groups is not well understood. Studies show a clear trend towards increased aridification towards the end of the Triassic. Although high-latitude areas like Greenland and Australia actually became wetter, most of the world experienced more drastic changes in climate as indicated by geological evidence. The world gradually got warmer over this time as well as mean annual temperatures rose by 7 to 9 °C.
Climate change was the only reason. Several Impacts from asteroids or comets are also serious possibilities contributing to the Jurassic extinction. These occurred in the late Triassic, including the second-largest confirmed impact in the Mesozoic. The Manicouagan Reservoir in Quebec is one of the most visible large impact craters on Earth, and at 62 miles in diameter it is tied with the event that created the fourth largest crater on earth located in Siberia.
The leading and best evidenced explanation for this mass extinction is massive volcanic activity in the central Atlantic, specifically from the Central Atlantic Magmatic Province, as indicated by dating with uranium-led and argon-argon dating processes, and paleomagnetism -- magnetism in rocks induced by the earth's magnetic field at the time of their formation.
Fortunately, this mass extinction only killed off 50 % of the living species.
● CRETACEOUS-TERTIARY (OR THE K-T) MASS EXTINCTION.
The K-T extinction is the most-studied and well-known extinction event in the history of the Earth, partly because it is the most recent. The K-T extinction is responsible for the elimination of at least 75% of all life forms on earth during the period.
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The extinction affected both marine and terrestrial organisms in all the continents. The extinction was greater among the animals that lived in the water column – an oceanogragphic concept to describe the temperature, salinity, light penetration, and chemical characteristics for a defined geographical point in the oceans -- than those that lived in the sea or seafloor. Among many species that died out, no amphibians became extinct.
The primary cause for this extinction is well known from the evidence of a large number of asteroids impacting the earth in many parts of the world.
Thus, the main cause of the event is well known. The evidence is the high number or large asteroid impacts found in many parts of the world. One such asteroid impact is the huge crater measuring 112 miles in diameters which was discovered buried under the sediments in the Yucatan Peninsula near Chucxulub in Mexico. Another smaller crater that predates the one found near Chucxulub was discovered in Boltysh, central Ukraine, in 2012.
The rock layers contain a high concentration of iridium, a rare element on the Earth’s crust but found in plenty in space debris. The residual effects of the rocks were devastating. A phenomenon known as “impact winter” was caused by the asteroids hitting the Earth. The amount of dust and debris released to the atmosphere blocked the sun for days. The plants began dying, leaving animals with no food, causing them to starve to death.
This forced into extinction 75% of all life forms on earth.
ARE WE LIVING DURING MASS EXTINCTION NUMBER SIX?
Fortunately, there hasn’t been a Mass Extinction since Homo Sapiens came on the scene, but we could be living in the era or period of the next one. While it is true that for human these extinctions happened a very long time ago, in evolutionary time they did not occur spread through the formation and evolution of the earth. All five took place in a span of 378 million years. Maybe it’s about time.
For the last 10,000 years, Earth has seemed to be in yet another extinction event that is rapidly removing animals from our planet. It’s slow, by human perception, so we don’t see it so much but the rate of extinction is increasing rapidly.
Scientists define a mass extinction as around three-quarters of all species dying out over a short geological time, which is anything less than 2.8 million years, according to The Conversation, and right now we are at the beginning of the next mass extinction. Since 1970, the populations of vertebrate species have declined by an average of 68%, and currently more than 35,000 species are considered to be threatened with extinction, according to the International Union for Conservation of Nature (IUCN). During the 20th century alone, as many as 543 land vertebrates became extinct, according to a research article in the journal PNAS. https://www.worldatlas.com/articles/the-ordovician-extinction.html
It is moving faster than any of the others, and the cause is no mystery as it is with some of the earliest events.
Posted by saveourstream.blogspot.com.au
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As cartoon character Pogo once said, “We have met the enemy, and he is us.”
We may possibly go down in Solar System History as the cause of the sixth Mass Extinction. Man had taken the habitat for farming and building, we have taken away food sources, we have polluted the environment, we have killed because the animals interfere with our way of life, and we have killed just for the fun of it.
No doubt humans will prevail, but it's not something to be proud of, and certainly something to think about.
JUST SAYIN’ !