Introduction
Living organisms have inhabited the Earth for millions of years. From the single-celled prokaryotes to the modern humans with complex tissues, organs, and organ systems, life on Earth has evolved almost in the same way as a tree spreads its branches. But within these millions of years, various extinctions of certain organisms occurred, which changed the direction of evolution. Among the extinctions, there were five “mass extinctions” that impacted almost every family, genus, and species on earth. This blog tries to explain these five mass extinctions in a simpler and more accessible way. So, let’s begin.
Chapter 1: Ordovician-Silurian Extinction (~443 Million Years Ago)
The Ordovician-Silurian Extinction occurred during the late Ordovician period, when life was almost entirely marine with minimal land ecosystems. During that period, the biodiversity was high in shallow seas. The extinction began with sudden global cooling triggered by continental drift as Gondwana moved over the South Pole, causing massive glaciation. This resulted in the dropping of sea levels as water was locked by massive ice caps. The second trigger happened when rapid warming followed the global cooling, resulting in the melting of glaciers and ice caps. This further resulted in a decrease in oxygen (anoxia) in oceans, which caused further loss of life. In this event, about 85% of marine species went extinct, with some trilobites, early jawless fish, and deep-water organisms surviving the extinction. The recovery from this event took about ~5-10 million years, which set the stage for later Silurian marine expansion. This extinction shows how climate change alone can trigger mass extinction without any asteroids or volcanism.
Chapter 2: Late Devonian Extinction (~375-360 Million Years Ago)
This occurred during the age “Age of Fishes” as well as when the first forests and early land vertebrates began emerging. The extinction was not a single event, but a prolonged one that continued for millions of years. This occurred separately in oceans and on land for separate reasons. On the ocean surface, a sudden algal bloom occurred, which cut off sunlight from aquatic life. Further, there was a decrease in the oxygen level, which further accelerated the extinction. On land, the roots of the large soiled penetrated through the soil for the first time, thereby altering soil chemistry. This further triggered volcanism, resulting in massive extinctions on land. The entire event caused the extinction of 75% of species on Earth, including Armored fish (placoderms) and most of the reef ecosystems. The event also altered the entire food web as the older predators died out and new predators arrived who were preyed upon earlier, like sharks and ray-finned fishes. On land, the tall trees survived, with many earlier plants going extinct. This event shows how a single form of life explosion (in this case, algae on the ocean surface and large trees on land) can trigger mass extinctions. Life itself can destabilize ecosystems.
Chapter 3: Permian-Triassic Extinction (~252 Million Years Ago)
Also dubbed the Great Dying, this is the largest extinction in Earth’s history. The event occurred due to the Siberian Traps volcanism, causing millions of years of massive eruptions. This caused an enormous release of carbon dioxide and methane in the atmosphere, affecting the animal kingdom on a drastic scale. The released gases caused extreme global warming as well as acidification of the oceans, wiping out ~90-96% of marine species and ~70% of terrestrial vertebrates from Earth. Large forests, insects, and apex predators went extinct, giving way to small, hardy animals such as burrowers. The recovery took about 10-30 million years, resulting in the rise of reptiles and archosaurs. This event shows how rapid carbon dioxide increase is lethal, as it can damage nearly all the life systems on Earth.
Chapter 4: Triassic-Jurassic Extinction (~201 Million Years Ago)
This extinction event occurred when the massive continent Pangaea began breaking apart as life was still recovering from the Permian trauma mentioned in the previous chapter. The primary cause of the extinction is the Central Atlantic Magmatic Province (CAMP) volcanism, leading to massive carbon dioxide release, which further led to rapid global warming, ocean acidification, and ecosystem fragmentation. This event eliminated about 80% species on Earth, leading to the rise of the dinosaurs and early mammals. This was similar to the Permian extinction but on a relatively smaller scale. This event gave rise to the ages of Dinosaurs for the next 135 million years.
Chapter 5: Cetaceous-Paleogene Extinction (~66 Million Years Ago)
This is the most famous extinction as it ended the age of non-avian dinosaurs. The primary cause for the extinction is said to be the Chicxulub asteroid impact, a massive space rock about 10 km wide, which struck the Yucatan Peninsula. This collision, combined with the Deccan Traps volcanism, caused firestorms, darkness, global cooling, and photosynthesis collapse. This event eliminated about 75% of species, including most of the gigantic life forms. This event slowly gave rise to the age of mammals – small, furry creatures with flexible diets, who survived through burrowing. This event took ~5-10 million years to recover, slowly leading to the birth of the first ancestors of Homo sapiens. This event shows how sudden shocks can reset evolution instantly, including a massive extinction.
Summary Table
| Extinction Event | Approx Time (mya) | Estimated Species Loss | Primary Cause(s) | Long Term Outcome |
| Ordovician-Silurian | ~443 | ~85% | Ice age, sea-level fall, ocean anoxia | Marine ecosystems reorganized; new Silurian radiations |
| Late Devonian | ~375-360 | ~75% | Ocean anoxia, climate change, nutrient runoff | Reef systems collapsed; shift in marine dominance |
| Permian-Triassic | ~253 | ~90-96% | Massive lolcanism, carbon dioxide, spike, anoxia, acidification | Near-total biosphere reset; rise of reptiles |
| Triassic-Jurassic | ~201 | ~80% | Volcanism, rapid climate warming | Dinosaurs rise as dominant terrestrial vertebrates |
| Cretaceous-Paleogene | ~66 | ~75% | Asteroid impact + volcanism | Mammals diversify; modern ecosystems emerge |
Conclusion
The life forms on Earth were altered at least five times in their history. Whether due to climate change, population explosion, volcanism, release of harmful gases, or external asteroid impacts, the lives we see around us can be eliminated suddenly when triggered at a certain level. As the proverb goes, “History repeats itself,” the sixth mass extinction might be right around the corner. Aside from the external asteroid causes, all the other factors can be controlled today by human technologies. So it is our responsibility to see that the triggering causes are checked and controlled regularly, if we want to avoid a sixth extinction. Sorry for being a bit preachy (I myself dislike preachy posts), but that was just a short reminder and not any type of activism at all. Anyway, thanks for reading the blog. Please like, share, and comment if you find this blog useful. Also, subscribe to my newsletter for future updates. This will motivate me to research and write more such posts. Thank you again and goodbye.
Suggested Readings
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