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The Five Biggest Endings on Earth

Updated: Dec 26, 2023



There have been five mass extinctions in the history of life on Earth. The Dinosaurs went extinct a long time ago, but their extinction was the most recent of the five. Life began on Earth around 3.7 Billion years ago, and between then and now, mass extinctions have led life to adapt and diversify, giving many different species the chance to rule the world. Common themes occur with these events, with dramatic changes  that pull the rug out of the base of ecosystems. So when did these mass extinctions occur? Who did they kill off? And who did they leave space for to grow into the new role as rulers of the Earth? Let’s get into it!


Ordovician-Silurian Extinction - 444 Million Years Ago

Interestingly, the first mass extinction was caused by the opposite conditions that we see today. During the Ordovician period, life was characterized by a dramatic increase in biodiversity in the oceans, especially with invertebrates like cephalopods and arthropods, and the emergence of plants on land. But it all ended around 444 million years ago. It was at this time as well that the Appalachian mountains were formed from volcanic eruptions. This released lots of carbon dioxide into the atmosphere millions of years before the extinction event began. But this wouldn’t last, because this increase of CO2 in the atmosphere led to acid rain, which when it fell on these mountains, eroded them down and trapped the CO2 into limestone deposits that washed off into the Nevada sea. 

Those emerging plants took advantage of the lack of competition for atmospheric carbon dioxide, sucking up the greenhouse gas, which caused less heat to be trapped in the atmosphere, leading to a global cooling. Eventually an ice age began which then trapped ocean water in glaciers, lowering the sea level and eliminating habitat for many species in low level seas. But this wasn’t the only problem for marine life, the plants clinging to the rocks along the shoreline released minerals like phosphorus into the ocean, which is an ingredient in photosynthesis. This led to intense algae blooms who thrived in these abundant conditions. When the algae die they are broken down by bacteria, which uses up a tremendous amount of oxygen in the water. 

The rapid cooling allowed organisms that preferred cold climates to move into the tropical regions, but unfortunately for them, this cooling was followed by a rapid warming once again from rapid plate tectonics releasing carbon dioxide. All together this extinction was the second largest in Earth’s history, killing 85% of the species. But it opened up a window for new species like fishes to develop and diversify. 


Late Devonian Extinction - 383-359 Million Years Ago

The Devonian period is sometimes called the age of fishes, because of many different species and variations of fish that evolved in this time. This was also when four legged animals called tetrapods evolved and were emerging onto land. And the plants, who had already been on land for millions of years, were developing vascular systems that allowed them to grow and diversify, creating the first terrestrial forests. 

But this increase of life on the lands is ironically one of the potential causes of the second mass extinction. This increase in biomass and nutrient production on land washed nutrients into the oceans, where similar to the Ordovician extinction, the phytoplankton proliferated under these favorable conditions. This led to algae blooms, and subsequently, a breakdown of biomass by bacteria, which used up the oxygen in the oceans making living conditions difficult for many species. This extinction is considered to be the least severe, eliminating around 70% of all marine species that existed at that time.

Although sharks were around during this period, they were not the rulers of the ocean. That title belonged to a fish called Dunkleosteus, who were one of the first apex predators with jaws. But they could not survive the changing conditions at the end of the Late Devonian period, opening a window for sharks to take the reins as the rulers of the ocean. 


Permian Triassic Extinction - 252 Million Years Ago

The next extinction is also called “The Great Dying”. This was the largest extinction event, killing off close to 90% of life on Earth. During this period, a group of animals called amniotes evolved, who had hard shells that protected them from drying out. This allowed animals to move away from the land and into the recently formed supercontinent called Pangea. The amniotes split into two major groups, who were the reptiles, and the synapsids, who were early ancestors to the mammals. 

This period came to a dramatic end due to volcanic activity at the Siberian traps over the course of two million years. This released clouds to carbon dioxide which heated up the atmosphere and oceans. The clouds also reduced the amount of sunlight reaching plants which in turn reduced the amount of photosynthesis which would add oxygen to the atmosphere. In fact, many forests and plant material were burned at this time, along with lots of coal released from the volcanos, adding even more carbon dioxide to the atmosphere. Atmospheric carbon can be absorbed by the ocean waters, which makes them acidic, and ocean acidification makes it uninhabitable for many species. Acidic oceans block shell formations for bivalves like clams and reef formation, which is important food and habitat for many species. And to add on to that, it's estimated that the ocean temperatures near the equator exceeded 104F (40C). 

Although this was the most catastrophic extinction in Earth’s history, it set the stage for an interesting evolutionary phenomenon called adaptive radiation. 


Triassic-Jurassic Extinction - 201 Million Years Ago

The Triassic Period was not a long period of time in Earth’s history, only 50 million years. But due to the huge gaps in ecological niches left open from the largest extinction in history, evolutionary diversification was able to ramp up at a fast pace. The rate of mutation doesn’t change, but the availability of niches to be filled allowed for these mutations to take hold. This is called adaptive radiation. This created new species who look similar to the species we see inhabiting those niches today, even though they are not always related, which is called convergent evolution. For example, Drepanosaurs were lizards that looked a lot like chameleons with variations of bird-like beaks, even though they are not related to either species. There were also Drepanosaurs, who looked similar to crocodiles and inhabited similar environments even though they were not related, and ichthyosaurs, who look a lot like dolphins, but they are reptiles.

The Triassic period was ended by volcanic eruptions and the splitting of the supercontinent Pangea, which released up to 100,000 gigatons of carbon dioxide into the air, again increasing the temperatures and acidifying the oceans. This split was the birth of the Atlantic Ocean. Some scientists have debated whether an asteroid strike could have also caused this extinction, with the Manicouagan Crater in Quebec being dated to that time, but it's estimated that this collision was a few million years too early to be associated with this extinction event.  This period is the second least severe of the 5 mass extinctions, but it set the stage for one of the most famous groups of animals to arise, the Dinosaurs. 


Cretaceous-Paleogene Extinction - 66 million Years Ago

The Cretaceous period was filled with some of the most famous organisms in the history of life on Earth. The land was ruled by Tyrannosaurus Rex, the skies were ruled by Pterosaurs, and the oceans were ruled by Mosasaurs. In the plant world, this period is when flowering plants evolved, also known as angiosperms. This period was also very warm and the sea levels were much higher than they are today, which flooded the two supercontinents, Gondwana and Laurasia. This created lots of shallow seas that covered what was previously land. This included the Western Interior Seaway which covered what is now the center of North America, and Northern Africa which is now the Saharan Desert. 

The Cretaceous-Paleogene Extinction is one of the most famous events in geological history. 66 million years ago an asteroid the size of Mt. Everest slammed into the Earth in the Yucatan Peninsula in the southern Caribbean sea, triggering earthquakes, forming massive tsunamis, and launching debris into the atmosphere. As the larger debris fell back down to the ground, the friction caused them to burn up, super heating the atmosphere, sparking mass forest fires. But that was just the beginning. The lighter debris and dust didn’t settle as quickly and instead spread out across the globe, covering the planet in a dust blanket, which blocked the sun. This prevented plants from photosynthesizing, which killed off the base of the food web, and it also blocked sun radiation from entering the earth’s atmosphere, causing the temperatures to plummet. Which may have lasted for many years. Once that cleared, the sun’s radiation was able to enter the atmosphere again, but the impact also launched a lot of debris with carbon into the air which increased the amount of atmospheric CO2. So an accelerated greenhouse effect increased the global temperatures, peaking in temperature around 55 million years ago. 

Some scientists think the impact wasn’t the only variable that caused this extinction. Around this time, large volcanic eruptions in what is now India called the Deccan Traps were erupting, launching CO2 into the atmosphere. This was leading to the detrimental effects that increased CO2 causes like increased atmospheric temperatures and ocean acidification. And scientists think that the dinosaur species may have already been in decline, which could be why many of them disappeared whereas other species were able to make it through. 

It is commonly said that this extinction event killed off all the dinosaurs, but that is not exactly correct. Avian dinosaurs which eventually gave rise to birds did make it through, along with crocodiles, turtles, frogs and salamanders, and mammals. But not without immense losses of their own. How did species like mammals survive? It is not fully known but some hypotheses are that the mammals that did survive were generalists when it came to food preferences, which means they were less picky in a time of great food scarcity, and also that their smaller size gave them an advantage over the larger dinosaurs. But not all dinosaurs were massive. But nonetheless, this extinction cleared the role for mammals to rise and become the rulers of the next age of life.


Today

Life has gone through many challenges. These extinction events drove species adaptation, diversification, and gave different groups the opportunity to rule their environments for a period of time. In all of these extinctions, the environment was changed so dramatically that it disrupted the foundation of food webs and ecosystems, leading to immense biodiversity loss. Rising CO2 levels in the atmosphere and ocean acidification are two environmental factors that played a recurring role in the largest extinctions in history, and we are seeing that again. But this time, we are aware of what is happening and can make adjustments to minimize the effects. Otherwise perhaps the holocene extinction will be the next on this list. 



Sources:


Ordovician-Silurian Extinction - 444 Million Years Ago



Late Devonian Extinction - 383-359 Million Years Ago


Permian Triassic Extinction - 252 Million Years Ago


Triassic-Jurassic Extinction - 201 Million Years Ago


Cretaceous-Paleogene Extinction - 66 million Years Ago

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