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The biggest mass extinction in Earth history some 251 million years ago was preceded by elevated extinction rates before the main event and was followed by a delayed recovery that lasted for millions of years. New research by two University of Washington scientists suggests that a sharp decline in atmospheric oxygen levels was likely a major reason for both the elevated extinction rates and the very slow recovery.

Earth's land at the time was still massed in a supercontinent called Pangea, and most of the land above sea level became uninhabitable because low oxygen made breathing too difficult for most organisms to survive, said Raymond Huey, a UW biology professor.

What's more, in many cases nearby populations of the same species were cut off from each other because even low-altitude passes had insufficient oxygen to allow animals to cross from one valley to the next. That population fragmentation likely increased the extinction rate and slowed recovery following the mass extinction, Huey said.

"Biologists have previously thought about the physiological consequences of low oxygen levels during the late Permian period, but not about these biogeographical ones," he said.

Atmospheric oxygen content, about 21 percent today, was a very rich 30 percent in the early Permian period. However, previous carbon-cycle modeling by Robert Berner at Yale University has calculated that atmospheric oxygen began plummeting soon after, reaching about 16 percent at the end of the Permian and bottoming out at less than 12 percent about 10 million years into the Triassic period.

"Oxygen dropped from its highest level to its lowest level ever in only 20 million years, which is quite rapid, and animals that once were able to cross mountain passes quite easily suddenly had their movements severely restricted," Huey said.

He calculated that when the oxygen level hit 16 percent, breathing at sea level would have been like trying to breathe at the
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