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Most peoples' knowledge of mass extinctions begins and ends with the K/T Extinction Event that killed the dinosaurs 65 million years ago. But, in fact, the earth has undergone numerous mass extinctions since the first bacterial life evolved about three billion years ago, and we are facing a potential 11th extinction as global warming threatens to disrupt our planet's ecosystems.01of 10
The Great Oxygenation Crisis (2.3 Billion Years Ago)A cyanobaterial bloom (green) of the type that caused the Great Oxidation Crisis.
Norman Kuring/Wikimedia Commons/Public Domain
A major turning point in the history of life occurred 2.5 billion years ago, when bacteria evolved the ability to photosynthesize, that is, to use sunlight to split carbon dioxide and release energy. Unfortunately, the major byproduct of photosynthesis is oxygen, which was toxic to the anaerobic (non-oxygen-breathing) organisms that appeared on earth as far back as 3.5 billion years ago. Two hundred million years after the evolution of photosynthesis, enough oxygen had built up in the atmosphere to render most of the earth's anaerobic life (with the exception of deep-sea-dwelling bacteria) extinct.
Snowball Earth (700 Million Years Ago)
Dirk Beyer/Wikimedia Commons/CC BY 3.0
More of a well-supported hypothesis than a proven fact, Snowball Earth posits that the entire surface of our planet froze solid anywhere from 700 to 650 million years ago, rendering most photosynthetic life extinct. While the geologic evidence for Snowball Earth is strong, its cause is hotly disputed, the possible candidates ranging from volcanic eruptions to solar flares to a mysterious fluctuation in the earth's orbit. Assuming it actually happened, Snowball Earth may be when life on our planet came closest to complete, irrecoverable extinction.
The End-Ediacaran Extinction (542 Million Years Ago)Dicksonia, a fossil organism from the Ediacaran period.
Verisimilus/Wikimedia Commons/CC BY 2.5
Not many people are familiar with the Ediacaran period, and for good reason: this expanse of geologic time (from 635 million years ago to the cusp of the Cambrian period) was only officially named by the scientific community in 2004. During the Ediacaran, we have fossil evidence of simple, soft-bodied multicellular organisms predating the hard-shelled animals of the later Paleozoic Era. However, in sediments dating to the end of the Ediacaran, these fossils disappear, and there's a gap of a few million years before new organisms once again appear in profusion.04of 10
The Cambrian-Ordovician Extinction Event (488 Million Years Ago)
Nobu Tamura/Wikimedia Commons/CC BY 3.0
You may be familiar with the Cambrian Explosion: the appearance in the fossil record, about 500 million years ago, of numerous bizarre organisms, most of them belonging to the arthropod family. But you're probably less familiar with the Cambrian-Ordovician Extinction Event, which witnessed the disappearance of a huge number of marine organisms, including trilobites and brachiopods. The most likely explanation is a sudden, unexplained reduction in the oxygen content of the world's oceans, at a time when life had yet to reach dry land.05of 10
The Ordovician Extinction (447-443 Million Years Ago)An Ordovician seascape.
Fritz Geller-Grimm/Wikimedia Commons/CC BY 2.5
The Ordovician Extinction actually comprised two separate extinctions: one occurring 447 million years ago, and the other 443 million years ago. By the time these two "pulses" were over, the world's population of marine invertebrates (including brachiopods, bivalves, and corals) had declined by a whopping 60 percent. The cause of the Ordovician Extinction is still a mystery; candidates range from a nearby supernova explosion (which would have exposed the earth to fatal gamma rays) to, more likely, the release of toxic metals from the sea floor.
The Late Devonian Extinction (375 Million Years Ago)
Zachi Evenor/Wikimedia Commons/CC BY 3.0
Like the Ordovician Extinction, the Late Devonian Extinction seems to have consisted of a series of "pulses," which may have stretched out for as long as 25 million years. By the time the silt had settled, about half of all the world's marine genera had gone extinct, including many of the ancient fish for which the Devonian period was famous. No one is quite sure what caused the Devonian Extinction; possibilities include a meteor impact or the severe environmental changes wrought by the world's first land-dwelling plants.
The Permian-Triassic Extinction Event (250 Million Years Ago)Dimetrodon, a victim of the Permian-Triassic Extinction Event.
H Zell/Wikimedia Commons/CC BY 3.0
The mother of all mass extinctions, the Permian-Triassic Extinction Event was a true global catastrophe, wiping out an unbelievable 95 percent of ocean-dwelling animals and 70 percent of terrestrial animals. (So extreme was the devastation that it took life 10 million years to recover, to judge by the early Triassic fossil record.) While it may seem like an event of this scale could only have been caused by a meteor impact, the more likely candidates include extreme volcanic activity and/or the sudden release of toxic amounts of methane from the sea floor.
The Triassic-Jurassic Extinction Event (200 Million Years Ago)The giant ampibian Cyclotosaurus was one of the victims of the Triassic-Jurassic extinction.
Nobu Tamura/Wikimedia Commons/CC By 3.0
The K/T Extinction Event brought the Age of Dinosaurs to an end, but it was the Triassic-Jurassic Extinction Event that made their long reign possible. By the end of this extinction (the exact cause of which is still debated), most large, land-dwelling amphibians were wiped off the face of the earth, along with the majority of archosaurs and therapsids. The way was cleared for dinosaurs to inhabit these vacant ecological niches (and evolve to truly gigantic sizes) during the succeeding Jurassic and Cretaceous periods.
The K/T Extinction Event (65 Million Years Ago)
Fredrilk/Wikimedia Commons/Public Domain
There's probably no need to recount the familiar story: 65 million years ago, a two-mile-wide meteor slammed into the Yucatan Peninsula, raising thick clouds of dust worldwide and setting off an ecological catastrophe that rendered dinosaurs, pterosaurs and marine reptiles extinct. Aside from the devastation it wrought, one lasting legacy of the K/T Extinction Event is that it caused many scientists to assume that mass extinctions could only be occasioned by meteor impacts-and if you've read this far, you know that simply isn't true.10of 10
The Quaternary Extinction Event (50,000-10,000 Years Ago)
Mauricio Anton/Wikimedia Commons/CC BY 2.5
The only mass extinction to have been caused (at least partially) by humans, the Quaternary Extinction Event wiped out most of the world's plus-sized mammals, including the Woolly Mammoth, the Saber-Toothed Tiger, and more comical genera like the Giant Wombat and the Giant Beaver. While it's tempting to conclude that these animals were hunted to extinction by early Homo sapiens, they also probably succumbed to gradual climate change and the inexorable destruction of their accustomed habitats (say, by early farmers clear-cutting forests for agriculture).
A Present-Day Extinction Crisis
Could we be entering yet another period of mass extinction right now? Scientists warn that this is indeed possible. The Holocene Extinction, also known as the Anthropocene Extinction, is an ongoing extinction event and the worse since the K/T extinction event that wiped out the dinosaurs. This time, the cause seems clear: human activity has contributed to the loss of biological diversity across the globe.