for National Geographic News
As far as Earth-shattering events go, nothing comes close to the mass extinction that punctuated the Permian period some 250 million years ago.
Around 95 percent of marine species and 70 percent of land species were abruptly wiped out (related photos).
So sharp is the break in the fossil record at this geologic boundary that scientists in the 1800s believed they were dealing with two separate, unrelated starts to life on Earth.
But what caused the Permian extinction is one of science's greatest mysteries.
In his newly published book Extinction: How Life on Earth Nearly Ended 250 Million Years Ago, Douglas Erwin explores the many theories put forth to explain the phenomenon, from plate tectonics to meteor strikes.
Erwin, a paleobiologist at the Smithsonian Institution in Washington, D.C., doesn't pinpoint a definite culprit in the quarter-billion-year-old whodunitbut he has a suspect.
The extinction coincided with the million-year-long eruption of Siberian flood basalts. A flood basalt is a giant volcanic eruption that coats vast stretches of land with basalt lava. (Related feature: Build your own volcano.)
The Siberian event was one of the most massive volcanic events in the last 600 million years.
Erwin suggests the eruptions may have produced everything from acid rain to global warming, which helped kill the majority of life on Earth.
"The most likely explanation at this point is that the effects of the Siberian flood basalts were responsible," Erwin said.
Until a decade ago scientists thought the Permian extinction was a continuous event that lasted for up to ten million years. But experts have now concluded that the die-offs occurred in two waves separated by eight million years or so.
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