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Survival is a relentless challenge. Animals face constant pressure to secure enough food, striving to adapt as best they can to their environments. Those less suited to their surroundings risk starvation, failure to reproduce, and eventual extinction. Life on Earth has continuously evolved, with every form tested by survival challenges. When the environment or climate changes dramatically, species that can't adapt often face extinction. Mass extinctions occur when a large proportion of Earth's life disappears entirely, leaving no descendants or fossil record. These cataclysmic events have been part of Earth's history since life first appeared. The animals alive today are the lucky descendants of those whose ancestors met the ever-changing demands of survival. Here, we explore ten of the most significant extinction events in Earth's history.
10. End-Ediacaran Extinction
The Ediacaran period saw the emergence of complex life on Earth for the first time. Simple bacteria evolved into more complex and specialized Eukaryotes, some of which formed groups for better chances of survival. Many of these strange organisms left no fossils, as they lacked skeletons and would decompose upon death. Fossils could only form under rare conditions, like when a creature's body left an imprint on soft mud that eventually hardened. These few fossils reveal seas filled with alien creatures resembling modern worms, sponges, and jellies. However, these life forms, like us, relied on oxygen. As oxygen levels began to drop, a global extinction event occurred 542 million years ago, wiping out over 50% of all species. The decomposing remains of these organisms contributed to today's fossil fuels. While the cause of the oxygen decline remains unclear, this extinction event paved the way for the Cambrian explosion, a period of rapid diversification of complex life forms far beyond just worms.
9. Cambrian-Ordovician Extinction
The Cambrian period was a time of incredible growth for life on Earth. After millions of years of relative stability, the life forms from the Ediacaran period had changed little. However, during the Cambrian, life suddenly exploded into a diverse array of forms. Crustaceans and trilobites took center stage, becoming the dominant creatures with their remarkable variety and vast numbers. Giant aquatic arthropods, similar to insects, and shellfish filled the oceans. Their rigid exoskeletons left an abundance of fossils for us to examine. Life thrived until, in a geological blink of an eye, over 40% of all species vanished around 488 million years ago. Those that survived struggled to adapt to an abrupt environmental change, the exact cause of which remains a mystery. One theory suggests that a glaciation, the coldest phase of an ice age, triggered the mass extinction. This would have drastically shifted temperatures, leading to the collapse of numerous species. This event marks the boundary between the Cambrian and Ordovician periods.
8. Ordovician-Silurian Extinction
Life began to thrive again during the Ordovician period. The seas teemed with nautiloids (early octopuses), trilobites, corals, starfish, eels, and jawed fish. On land, plants struggled to establish themselves. Life was slowly becoming more intricate. 443 million years ago, over 60% of life perished in what is considered the second-largest extinction event. This catastrophe was triggered by a sudden ice age caused by a sharp decrease in carbon dioxide levels. As water became locked in ice caps and glaciers, it diminished the habitats of many species and led to a drop in oxygen levels. One theory suggests that a burst of gamma radiation from space destroyed the ozone layer, allowing harmful ultraviolet rays from the Sun to wipe out much of the plant life, causing the initial reduction in carbon dioxide. Though some life forms survived, it would take more than 300 million years for the number of species to recover from this extinction event.
7. The Lau Event
After the Ordovician extinction, the Silurian period began, marked by the recovery of life. This era saw the emergence of true sharks and bony fish, many of which were strikingly similar to modern species. On land, moss and small plants began to grow more freely along coastlines, and arthropods evolved into spiders and millipedes, adapted to the dry conditions and coexisting with land plants. Enormous sea scorpions thrived, while trilobites continued to dominate the seas. However, around 420 million years ago, a sudden climate shift led to the extinction of around 30% of all species. The atmospheric gases shifted in such a way that many creatures found the conditions inhospitable or even toxic. The cause of this atmospheric change remains unclear. Life persevered, and by the end of the Silurian period, the Devonian period began, bringing with it new life forms that flourished.
6. Late Devonian Extinction
During the Devonian period, certain fish evolved sturdy fins, enabling them to crawl onto dry land, eventually giving rise to reptiles and amphibians. The seas flourished with vast coral reefs inhabited by fish and sharks, some of which preyed on trilobites. For the first time in over 100 million years, trilobites lost their dominance in the ocean. Sharks of this time were so successful that their descendants have barely changed over the millennia, with some modern sharks looking nearly identical to their ancient relatives. On land, plants began developing seeds and diversifying, leading to the appearance of complex land plants and soil for the first time in Earth's history. Strange forests made up of 8-meter-tall fungi sprouted but have since disappeared. However, around 374 million years ago, 75% of this flourishing life was wiped out, likely caused by shifts in atmospheric gases, possibly due to massive volcanic activity or a meteorite impact.
5. Carboniferous Rainforest Collapse
Following the Devonian period came the Carboniferous period, during which land animals developed terrestrial eggs, allowing them to live almost anywhere on land rather than being restricted to the shores as modern-day turtles still are. Winged insects emerged and thrived, while sharks experienced a golden age, and the few remaining trilobites from the previous extinction event became increasingly rare. Gigantic trees and vast rainforests covered much of the land, boosting the oxygen content of the atmosphere to 35%, compared to today's 21%. Conifers from the Carboniferous period remain remarkably similar to those of today. However, 305 million years ago, a sudden ice age dramatically lowered carbon dioxide levels to the lowest point in Earth's history. The great forests perished, taking many land animals with them. Nearly 10% of all species vanished during this time. The decayed trees eventually formed what is now our main source of carbon fuels, after which this period earned its name.
4. Permian-Triassic Extinction
After the fall of the rainforests, the most successful animals that remained on land were those that laid eggs. These creatures quickly took over and, before other species had time to recover, they diversified, producing a vast range of reptiles and synapsids, which were mammal-like reptiles and the ancestors of mammals. However, 252 million years ago, Earth faced a catastrophe unlike any it had ever seen, nor has it seen since. This event was likely caused by either a meteorite impact or massive volcanic activity, radically altering the atmosphere. Between 90% and 99% of all life perished in what is considered the largest mass extinction event in history, known as the 'Great Dying.' To put this into perspective, in terms of animal extinction caused by humans, estimates suggest we've driven nearly 1000 species to extinction. With about 8 million species alive today, even in the worst-case scenario, that accounts for only 0.01% of all animal life. While this is certainly a tragedy, it is insignificant compared to the colossal extinctions that have occurred naturally.
3. Cretaceous-Tertiary Extinction
This is the most renowned extinction event. Following the end of the Jurassic period, dinosaurs continued to thrive and evolve throughout the subsequent Cretaceous period. They evolved into the familiar forms we recognize today. More notably, the Cretaceous period marked the recovery of life from the much earlier Ordovician-Silurian extinction. For the first time, the number of species surpassed the numbers from the Ordovician period, over 300 million years earlier. During this period, synapsids evolved into small, rodent-like creatures that became the first true mammals. 65 million years ago, a massive meteorite struck Earth at Chicxulub in present-day Mexico, disrupting the atmosphere and triggering intense global warming, which led to the extinction of 75% of all species. This meteorite was rich in iridium, an element rare on Earth, and rocks from the same time period globally contain a thin layer of iridium, a remnant of the impact. A handful of small reptiles and mammals survived this cataclysmic event. Mammals would eventually go on to replace dinosaurs as the dominant land animals.
2. End Jurassic Extinction
During the Jurassic period, massive marine reptiles like the iconic plesiosaur ruled the oceans. Pterosaurs dominated the skies, and dinosaurs were the supreme land creatures. Species such as Stegosaurus, the long-necked Diplodocus, and the mighty predator Allosaurus became abundant. Lush forests were filled with conifers, cycads, ginkgoes, and ferns. Some smaller dinosaurs developed feathers, and birds began to appear. However, 200 million years ago, 20% of life disappeared suddenly from the fossil record, mainly among marine species. Once widespread, shellfish and corals nearly vanished. The few survivors of this extinction gradually repopulated the seas over the course of millions of years. This event had little effect on land animals, and only a few dinosaur species were lost. The cause of this extinction, which primarily affected marine life, remains debated. One theory is that tectonic activity caused the oceanic plates to sink slightly, deepening the oceans, where many marine species adapted to shallow waters and perished as they were pushed further from the surface.
1. Triassic-Jurassic Extinction
Following the devastation of the Permian extinction, reptiles once again rose to dominance, paving the way for the appearance of dinosaurs. However, dinosaurs were not yet the towering giants we often associate them with today. At this stage, they were not much larger than horses and did not dominate the landscape over other reptiles. It was the descendants of these early dinosaurs that would later evolve into the famous, fearsome creatures we recognize. The massive dinosaurs such as the Tyrannosaurus, Stegosaurus, Triceratops, and the immense sauropods would emerge during the Jurassic and Cretaceous periods. 205 million years ago, 65% of Triassic life was wiped out, including all the larger land animals. Smaller dinosaurs managed to survive due to their modest size. Unlike most mass extinctions, which unfold over a million years, this event occurred in just ten thousand years. The likely culprit for this swift extinction was massive volcanic eruptions that released vast amounts of carbon dioxide or sulfur dioxide, leading to a rapid and extreme climate shift.
