Life on our planet has suffered many mass extinctions over its 4.5 billion years. Scientists see evidence of at least five major episodes that wiped out creatures large and small. And many experts say that the Anthropocene era, the current period of human activity that is changing the Earth, is causing the sixth.
Paleontologists track mass extinctions by analyzing rock samples from different eras. An extinction event has been discovered when many species that were once abundant in the fossil record reach a point where they become scarce. The five great extinctions wiped out more than 70 percent of life on Earth during that time, and the deadliest of them, the Permian-Triassic extinction about 252 million years ago, probably wiped out more than 90 percent.
The origin of these extinctions is still debated, but scientists believe that most of them were the result of great volcanism: giant fissures opened up in the Earth’s surface and spewed lava over thousands or even millions of years. Volcanism likely caused many other changes leading directly to the extinction, including anoxia, or lack of oxygen, in the oceans due to ocean warming; depletion of ozone, due to volatiles released by volcanism; and forest fires caused by a hot climate.
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Studying our geological past can also teach us about the present, says David Bond, a mass extinction expert at the University of Hull in England. “We need to draw lessons from these events and use them to try to understand our potential diversity crisis.”
Triggers vs. killing mechanisms
Many of the causes listed here—including volcanism and sea-level change—are not necessarily the primary mechanisms of death per se. For example, an asteroid is not a real cause of death beyond the people who are directly hit. Rather it causes widespread lethal conditions such as anoxia and acidification.
Rick Simonson; David Bond/University of Hull (scientific reviewer)
Magnitude and victims
Each of the five mass extinction events is the disappearance of 70 percent or more of the species that were alive at the time. Some groups disappeared completely and others had a significant decline.
Rick Simonson; David Bond/University of Hull (scientific reviewer)
