Over the past five hundred and 40 million years, our planet has gone through five mass extinction events, each involving processes that altered the normal carbon cycle in the atmosphere and oceans.
All these changes in carbon, which are fatal on a global level, have played out over billions of years and coincide with the widespread extinction of marine species worldwide.
Scientists predicted of sixth mass extinction now. Research carried out in the last decade indicated that, during the 20th century, vertebrate species died up to 114 times faster than would occur without human activity, as animals that became extinct in 100 years would have taken 11,400 years to become extinct at the natural rates.
The study revealed that much of this is due to human activities that lead to an environmental imbalance, causing pollution, loss of habitat, the introduction of invasive species, and increased carbon emissions, which drive climate change.
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Mass Extinction and Their Reason
In the history of our planet Earth, there have been five mass extinctions – Ordovician-Silurian (450-440 million years ago), Devonian (359 million years ago), Permian- Triassic (253-251 million years ago), Triassic- Jurassic (208-200 million years ago) and Cretaceous -Paleogene (65.5 million years ago). It is almost impossible to establish the real reasons for these events, but it is possible to construct hypotheses based on data extracted from ancient rock deposits.
1. First mass extinction – Ordovician-Silurian
|445 million years ago
|85% of marine species disappeared
|brachiopods, trilobites, bivalves, and corals
|trilobites, brachiopods, corals, crinoids, and graptolites
|Oxygen Deficiency and a rapid glaciation
|General Global Cooling
The Late Ordovician or Ordovician-Silurian mass extinction, which took place 450-440 million years ago, became the first of the “big five” such events, allocated to a special list of global catastrophes in the history of the earth. The maximum number of species of animals and plants from several higher taxa at once disappeared for a very short time in geological terms.
At the end of the Ordovician, about 85% of marine species disappeared, most of which lived in shallow seas near continents. At the start of the late Ordovician period, the world was very different than it is today or even in the era of the dinosaurs. So, living organisms inhabited mainly the oceans, and plants were beginning to appear on land.
Most modern continents united into one supercontinent Gondwana, which included Africa, Antarctica, Australia, New Zealand, South America, Arabia, Madagascar, and Hindustan, formed at the end Precambrian period after the split of the supercontinent Rodinia.
Experts estimated that a rapid glaciation froze most of the planet’s water, causing sea levels to drop. Marine organisms such as sponges and algae were most affected, as were mollusks, primitive cephalopods, and jawless fish called ostracoderms. However, an international group of researchers tested this well-known hypothesis recently.
After studying the geochemical parameters of marine sediments and modeling the mechanism of reorganization of the global circulation of the ocean, scientists concluded that anoxia could not be the only cause of extinction and, most likely, as a consequence of more global processes associated with general cooling.
An article about this was published in the journal Nature Geoscience. The new study’s lead author, Alexandre Pohl of the American University of California, Riverside, and his co-authors recreated the ocean environment over millions of years, both before and during these events. They found that the tragedy played out, unlike other similar extinctions, rather slowly – from half a million to two million years.
Measuring the iodine concentration in carbonate rocks of that period made it possible to draw meaningful conclusions about oxygen levels at different ocean depths and their deficiency.
Combined with computer simulations, these data showed that oxygen deficiency did not correlate with extinctions in shallow water, where most organisms lived. The immediate cause of such extinctions was most likely the climate cooling that occurred at the end of the Ordovician, combined with several additional factors.
Oxygenation of the ocean’s upper layers was probably only a consequence of the cold snap since atmospheric oxygen tends to dissolve rapidly in cold water. Researchers associate marine anoxia primarily with the peculiarities of the global circulation of seawater in the World Ocean.
2. Second Mass Extinction: Devonian
|359 million years ago
|50% of all species of animals and plants and about 15% of families of living beings disappeared
|ammonites, ostracodes, and placoderm
|Oxygen Deficiency, asteroid strike, and Volcanic Eruption
The Devonian mass extinction occurred approximately 359 million years ago. Paleontologists find plant spores burned by ultraviolet radiation in sediments of this time. About 50% of all animals and plant species and about 15% of other living beings disappeared. Large creatures suffered the most from this, including giant trilobites, crustaceans, shellfish, and jawless fish.
Previously, scientists and researchers believed some cosmic events like a meteor or asteroid strike were behind this mass extinction. But a new study conducted by the scientists of the University of Illinois at Urbana-Champaign (USA) suggested that the Devonian extinction was due to a supernova explosion. This phenomenon could deprive the earth of its ozone shield and expose the planet to ultraviolet radiation from the Sun.
The conclusion of their research was published in the scientific journal Proceedings of the National Academy of Sciences.
“Exceptionally, terrestrial catastrophes like massive volcanic eruptions and global warming can also destroy the ozone layer. However, we have not been able to confirm this. We assume that it was destroyed by a supernova that occurred 65 light-years from earth.” said Brian Fields, one of the study’s authors professor at the University of Illinois at Urbana-Champaign (USA).
The study group discovered that during the Devonian extinction, the ozone content of the earth’s atmosphere dropped sharply, as a result of which more ultraviolet radiation from the Sun began to fall on the surface of our planet. Fields and his colleagues tried to determine the cause of this cataclysm by examining possible scenarios for how the ozone layer might have disappeared.
Scientists suggest that traces of this cosmic cataclysm may remain in the earth’s rocks. In particular, scientists can confirm its existence if they find a vast number of two radioactive isotopes – samarium-146 and plutonium-244, which occur only during supernova explosions.
These isotopes have a reasonably long half-life, about 103 and 80 million years. Therefore, they must “survive” to our time if they were brought to earth by a supernova explosion. Astronomers hope that their fellow geologists will soon find traces of these isotopes and thus confirm their theory.
3. Third Mass Extinction: Permian
|251 million years ago
|96% of all marine species and 73% of terrestrial vertebrates disappeared
|amphibian and reptile species and nearly one-third of insect species with corals, brachiopods, bryozoans, ammonoids, fusulinids, and Trilobites
|Therapsids, which were mammal-like reptiles, and the more reptilian archosaurs
|Siberian Trap Eruption
|Volcanic Eruption in China and atmospheric aerosols of sulfuric acid accelerated global cooling and warming
The Great Permian Extinction happened about 251 million years ago. It is the largest of five Phanerozoic mass extinctions and has resulted in the extinction of 96% of all marine species and 73% of terrestrial vertebrates. In addition, the extinction was the only one in the history of insects: 57 percent of genera and 83 percent of species disappeared.
The main reason for the global climatic changes, which led to the mass extinction at the end of the Permian, is considered to be the incredibly high volcanic activity in the territory of present-day Siberia, which led to the formation of the largest trap province in the world – the Siberian Traps.
However, new research has established another probable cause for the massive extinction. Researchers and scientists from the Chinese Academy of Sciences, Nanjing University (China), the National Museum of Natural History, and Yale and New York Universities (USA) analyzed the composition of fossil rocks from southern China. They published the conclusion of their work in Science Advances.
For a long time, scientists believed that the eruption of the Siberian traps led to extinction, which caused a global ecological catastrophe, including strong warming of the climate. After analyzing the composition of fossil rocks from China, scientists noticed anomalies probably associated with sulfur-rich emissions from nearby volcanoes. The rocks were covered with petrified ash.
Thus, scientists concluded that volcanoes erupted in southern China 250 million years ago in addition to the Siberian traps. Atmospheric aerosols of sulfuric acid, formed due to these eruptions, could cause accelerated global cooling and, equally, sharp warming observed at the end of the Permian period.
Most scientists today are confident that these outpourings of lava were implicated in the extinction of animals. However, the exact mechanism of their action on the earth’s climate and ecosystems is still a matter of controversy.
4. Fourth Mass Extinction: Triassic-Jurassic
|201.3 million years ago
|50% of all living species disappeared, including 20% of marine families and 34% of marine genera.
|Giant Amphibians, erect-headed archosaurs, and mammal-like reptiles
|Survived Species: mammals, crocodiles, dinosaurs, fishes, and turtles
|climate change, asteroid strike
|Volcanic Eruption and excessive carbon dioxide
The Triassic-Jurassic mass extinction happened 201.3 million years ago, one of the main extinction events in the Phanerozoic eon. It deeply affected life on land and in the oceans. A whole class of conodonts and 34% of marine genera disappeared into the seas.
At least half of the species known to have lived on earth at that time became extinct. Several terrestrial ecological niches were vacated, which allowed dinosaurs to assume a dominant role in the Jurassic period.
The victims of the Triassic-Jurassic vertebrates were all giant amphibians, erect-headed archosaurs that rivaled dinosaurs (thecodonts and crurotarsans ), and many prominent families of therapsids (descendants of mammals, formerly known as “mammal-like reptiles,” although they are not reptiles).
Among the causes put forward, the hypotheses of volcanoes and asteroids are particularly popular. A study published in Nature Communications has just further strengthened that of volcanoes. However, the two assumptions are not mutually exclusive.
The extinction of the Triassic-Jurassic corresponds to the moment when Pangea fractured, gradually giving shape to the continents as we know them. This fracture was accompanied by the birth of a new ocean: the Atlantic. The northern part of the Atlantic Ocean opened up first, probably thanks to tectonic and magmatic phenomena, at the origin of a vast area resting on magmatic rocks.
There are several areas like this. In this case, it is the central Atlantic magmatic province (Camp). By studying the Camp, researchers have reinforced the hypothesis of volcanoes. Indeed, they found evidence of a large amount of CO 2 bubbles trapped in its rocks. It suggests a role for the Camp” in triggering extreme climate change,” a statement read. The Camp would have had volcanic activity causing a rapid increase in atmospheric CO2.
The researchers detail the greenhouse effect contributing to global warming and acidification of the oceans at the end of the Triassic period. Their results also led to another hypothesis, more modern this time. “The environmental changes caused by CO 2 may have been similar to those provided shortly.” Don Baker, a researcher in the Department of Earth and Planetary Sciences at McGill University (Montreal) and co-author of the study, expresses his concern.
5. Fifth Mass Extinction: Cretaceous–Paleogene or K-Pg
|65 million years ago
|98% of dinosaurs, 40% of other reptiles, 70% of mammals, 60 % of insects, amphibians, and other reptiles disappeared
|Mammals like cimolestes placental and dinosaurs like vegavis and alamitornis
|Asteroid Hit and acidification of surface water
The great extinction of the Cretaceous-Paleogene affected dinosaurs and almost 70 percent of the planet’s species. It is believed to have lasted between one and three years. About 70% of biological genera disappeared, including 98% of dinosaurs, pterosaurs, marine reptiles (fewer turtles), 40% of other reptiles, 70% of mammals, and 60 % of insects, amphibians, and other reptiles.
However, some types of mammals survived, such as the cimolestes placental, of which fossils have been found in the Cretaceous and Tertiary. Some dinosaurs also survived, such as the vegavis or the alamitornis; A hadrosaur leg dating to 64 million years ago was discovered in mid-2002, more than a million years after the mass extinction ended.
Many geologists and paleontologists support the theory that a giant asteroid or comet that impacted earth generated a global catastrophe that led to this extinction. Chicxulub crater is the correct age (65 million years) and consistent with the impact of an asteroid 10 to 20 kilometers in diameter.
Many scientists think that the heat and dust from the asteroid impact could have caused the K-Pg extinction. The scorching heat would have destroyed life, and the dust thrown into the atmosphere would have caused a global dimming, halting photosynthesis and lowering temperatures.
However, this hypothesis did not explain the absence of significant extinctions at the bottom of the oceans. A study conducted by paleontologists from the University of Zaragoza found the possible cause. The results of their work, published in the Proceedings of the National Academy of Sciences, suggest that the acidification of ocean water caused most of the extinctions on the seabed.
After the passage of the meteorite that hit the Yucatan Peninsula (Mexico), the earth’s atmosphere warmed up and, as a consequence, its nitrogen oxidized and formed nitric acid. Furthermore, as the asteroid impact took place on gypsum-rich materials, it released sulfuric acid. All this contributed to the decrease in pH in ocean waters, mainly affecting surface waters. It would explain the extinction of many carbonate shell organisms that float in surface waters since these would dissolve when the pH decreases.
6. Sixth Mass Extinction: Holocene
|Started less than 10,000 years ago and still happening
|Reduction of Biodiversity by 58% in 40 years; 173 species are already extinct
|Formosan Clouded Leopard, Pinta Island Tortoise, Pyrenean Ibex, Po’ouli, West African Black Rhino
The sixth mass extinction is not a concern for the future. According to a study published, it’s happening now, much faster than expected, and it’s entirely our fault. Humans have already caused the death of hundreds of species and brought many more to the brink of extinction through the wildlife trade, pollution, habitat loss, and the use of toxic substances.
In addition, the sixth mass extinction of species is occurring in a substantially shorter time than the rest of the five previous extinctions since it began less than 10,000 years ago. Estimates suggest that today 40 amphibian species, 33 sharks and rays, 29 reptiles, 26 mammals, 14 birds, 64 cycads, and 34 conifers, among others, are in danger of extinction, according to the International Union for Conservation of Nature.
Also, findings published in the scientific journal Proceedings of the National Academy of Sciences (PNAS) show that the species extinction rate has increased in recent decades. A National Autonomous University of Mexico ecology professor and one of the study’s authors, Gerardo Ceballos González said that approximately 173 species became extinct between 2001 and 2014.
When one species disappears, it erodes the entire ecosystem and pushes other species towards annihilation. Researchers study amphibians as an example of this phenomenon. Hundreds of amphibians (frogs and toads) are experiencing population declines and extinctions due to chytrid fungal disease, sometimes spreading to new areas by humans. Climate change probably makes the problem worse.
“When humanity exterminates populations and species of other creatures, it is sawing the limb over the one sitting, destroying functional parts of our life support system,” said Paul Ehrlich, a well-known Stanford professor and co-author of the new study.
The researchers also said that the current coronavirus crisis shows how the recklessness with which people treat the natural world can backfire. The wildlife trade decimated many species in danger or on the verge of extinction.
Mass extinction is an ongoing process; one species disappear, giving place to another. It is not surprising that since the appearance of life on earth, living beings have had to experience events that put them on the brink of disappearance. However adapted we are to our world, we are nothing compared to the power of nature.
(Last Updated on May 25, 2022 by Sadrish Dabadi)