I learned this today. There have been six mass extinctions on Earth that experts know about. The Great Oxidation Event about 2.4 billion years ago. The Ordovician-Silurian Extinction 440 million years ago. The Devonian Extinction 365 million years ago. The Permian-Triassic Extinction 250 million years ago. The Triassic-Jurassic Extinction 210 million years ago. The Cretaceous-Tertiary Extinction 65 million years ago.
There were probably extinctions before these, but it is impossible to know. The earliest fossils have been found from about 3.42 billion years ago and life potentially started on Earth up to 4.28 billion years ago. The oceans are speculated to have formed around about that time and life is speculated to have begun pretty much straight after that. How it began is something I can research another day. The problem with fossils from these very early days of the Earth is that those living creatures were very small and usually very soft. There has been a lot of movement in the crust of the Earth that could have moved or destroyed any fossils. So, here is a breakdown of the extinction events that the experts do have evidence for.
The Great Oxidation event happened about 2.4 billion years ago. At that time, the oceans were shallow and the atmosphere was mostly made of dinitrogen and carbon dioxide. Most of the life on Earth were bacteria that had the ability to produce energy from photosynthesis. Photosynthesis works by transforming water and carbon dioxide into oxygen and glucose. Now, when plants photosynthesize, there are other beings that make use of the oxygen they produce and keep the balance of gas in the atmosphere, but at that time there were only the bacteria producing oxygen. The level of oxygen in the atmosphere slowly built up until it was too high for the bacteria themselves to survive and they mostly died out.
This is proved because there are fossil soils and rock from this time that contain minerals that are only stable in low oxygen conditions.
The next extinction event happened 440 million years ago. It happened at the end of the Ordovician Period and killed up to 85% of all living species.
By the Ordovician Period, marine life had evolved into plants, shellfish, and some fish. Some plants had evolved to live on land. The extinction event decimated all of them.
It happened in two stages. The first stage was caused by a sudden ice age, which was caused by a combination of volcanoes, and the position of the continent. The supercontinent Gondwana was over the North Pole and combined with increased volcanic activity there was a global freeze. Glaciers advanced and a lot of ocean was trapped in ice. Then, just had life adapted, the ice age finished, the seas rose, and the amount of oxygen in the water dropped so low that most of the life in the oceans died.
The Devonian Mass Extinction happened 375 million years ago. About 80% of all life died. After the last extinction, photosynthesizing plants had replenished oxygen levels and life was flourishing again. There were corals, lots of shellfish, and some fish in the seas. There were plants and insects on land.
This extinction was also caused by a sudden drop in oxygen levels that killed the majority of ocean life. There are three theories as to why. Firstly, a lot of marine plants adapted to live on land and the amount of oxygen producing plants in the oceans dropped, leading to low oxygen levels and mass deaths. Secondly, a huge increase in biomass meant less carbon dioxide in the air, which caused global temperatures to plummet. Thirdly, Earth was hit by far more meteors than it is now. There is also the possibility that a supernova caused a drop in ozone, allowing massive ultraviolet damage to life on Earth.
The Permian Extinction happened 250 million years ago. This is the largest known mass extinction and 96% of life was killed. Marine life was flourishing and on land insects were increasing dramatically. There were also some small lizard like vertebrates, some as large as pigs.
The Permian Extinction appears to have been caused by massive volcanic eruptions. The Siberian traps erupted and put so much carbon dioxide into the atmosphere that the temperature rose drastically, and the oceans were acidified. Most of life was wiped out. The Siberian traps covered 2 million square kilometers with lava.
The Triassic Extinction happened 200 million years ago. We have a lot of early dinosaurs in this period and the seas are teeming with life. The extinction didn’t happen at once but was caused by gradual climate change, and massive volcanic eruptions that slowly wiped out about half of all life on Earth.
The final extinction is the Cretaceous Extinction that we all know about. There are many theories, but the most commonly accepted one is the meteor that wiped out the dinosaurs. It happened 66 million years ago and was caused by a comet about 15km wide that hit Earth and wiped out about 75% of all plants and animals. It can be seen by looking at the K-Pg boundary in the earth, which is a line of sediment worldwide that has high levels of iridium, a metal found more commonly on asteroids than in the earth’s crust.
So, there have been numerous extinction events throughout the life of the Earth. We should be grateful, I suppose, because the changes they caused in the evolutionary tree mean that we are here today. And, we can assume that there will be more of these events in the future. Looking at the time scale, they seem to come once every hundred million years. When the next one comes, will we have the technology to avoid it? Will we even be here? And that is what I learned today.
Image By Fredrik at English Wikipedia. https://commons.wikimedia.org/w/index.php?curid=5786440
Sources:
https://www.livescience.com/mass-extinction-events-that-shaped-Earth.html
https://www.amnh.org/exhibitions/dinosaurs-ancient-fossils/extinction/mass-extinction
https://www.thoughtco.com/the-5-major-mass-extinctions-4018102
https://en.wikipedia.org/wiki/Extinction_event
https://en.wikipedia.org/wiki/Late_Ordovician_mass_extinction
https://en.wikipedia.org/wiki/Late_Devonian_extinction
https://en.wikipedia.org/wiki/Permian%E2%80%93Triassic_extinction_event
https://en.wikipedia.org/wiki/Triassic%E2%80%93Jurassic_extinction_event
https://en.wikipedia.org/wiki/Cretaceous%E2%80%93Paleogene_extinction_event
https://www.discovermagazine.com/the-sciences/mass-extinctions
https://en.wikipedia.org/wiki/Earliest_known_life_forms
https://en.wikipedia.org/wiki/Origin_of_water_on_Earth#History_of_water_on_Earth
https://www.discovermagazine.com/the-sciences/what-are-the-oldest-fossils-in-the-world
https://en.wikipedia.org/wiki/Great_Oxidation_Event