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LOS ANGELES — Tiny microbes on the bottom of the ocean floor may have been responsible for the largest extinction event our planet has ever seen, according to a new study.

These microbes of death were so small that 1 billion of them could fit in a thimble full of ocean sediment, and yet they were almost responsible for killing off all the life on our planet, the scientists suggest.

The end-Permian extinction was the most catastrophic mass extinction the Earth has ever seen. It started roughly 252 million years ago — long before the dinosaurs — and it continued for 20,000 years. By the time it was over, nearly 90 percent of all life on Earth had been destroyed, the scientists say.

“It was not as dramatic as the impact that probably killed the dinosaurs, but it was worse,” said Gregory Fournier, an evolutionary biologist at MIT. “Things were very close to being over for good.”

Scientists have struggled to understand exactly what caused the long, slow, mass die-off in this dark era of our planet’s history. The geologic record tells us there was a sharp uptick in carbon dioxide levels at the time. That would have caused the oceans to acidify and the Earth to heat up, making the environment inhospitable for most forms of life. But what actually caused the C02 levels to rise has remained a mystery.

Some scientists have suggested an asteroid impact could be to blame; others have proposed that volcanic activity or coal fires might be the culprit.

Now, in a paper published this week in PNAS, researchers from the Massachusetts Institute of Technology and the Chinese Academy of Sciences in Nanjing, China, have fingered a new and unlikely suspect — a tiny methane-spewing microbe known as Methanosarcina.

The first clue that microscopic microbes could be involved in the greatest die-off the Earth has ever known came when MIT geophysicist Dan Rothman was looking at how carbon levels grew during this time. What he saw was not a straight line, but rather a rapid upward curve.

“The growth was like what you might see in a real estate bubble, or a financial bubble,” he explained. “If the C02 came from the sudden combustion of a coal field in Siberia, it wouldn’t behave this way. It has this special character that is consistent with microbial processes.”

It was the first time anyone had suggested microbes might be involved with the end-Permian extinction, but far from the first time that microbes have been accused of changing the chemistry of our planet. For example, photosynthetic microbes are responsible for creating the first oxygen in the Earth’s atmosphere.




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