Permian mass extinction triggered by humble microbe

AROUND 251 million years ago, over 90 per cent of the species on Earth suddenly went extinct. Their killer may not have been a devastating meteorite or a catastrophic volcanic eruption, but a humble microbe.

The prevailing theory is that the mass extinction at the end of the Permian period was triggered by volcanic eruptions over a vast area of what is now Siberia. This led, among other things, to a dramatic rise in greenhouse gas emissions.

But the scenario just doesn't fit the facts, says Daniel Rothman of the Massachusetts Institute of Technology. From his analysis of an end-Permian sediment sample from China, Rothman says carbon levels surged much too quickly for geological processes to be at work.

Microbes can generate carbon compounds that fast, though. When Rothman's group analysed the genome of Methanosarcina - a methanogen responsible for most of Earth's biogenic methane today - they discovered that the microbe gained this ability about 231 million years ago. The date was close to that of the mass extinction, but not close enough to suggest a link.

But Methanosarcina needs large amounts of nickel to produce methane quickly. When the team went back to their sediment cores, they discovered that nickel levels spiked almost exactly 251 million years ago - probably because the Siberian lavas were rich in the metal. That suggests Methanosarcina did trigger the extinction, Rothman told the American Geophysical Union meeting in San Francisco last week.

Other geologists remain to be convinced. "[But] it's a fascinating idea that the evolution of a new life form led to an extinction," says Anthony Barnosky of the University of California, Berkeley. Today's mass extinction of biodiversity is similar, says Barnosky, because it is largely driven by our species.

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