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It's MORNING EDITION from NPR News. I'm Steve Inskeep.
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And I'm David Greene. Good morning. The biggest extinction the Earth has ever seen took place 250 million years ago and it remains something of a mystery. Scientists suspected giant volcanoes or perhaps an asteroid caused it, but NPR's Christopher Joyce has seen new research suggesting the cause might not have been so cataclysmic - maybe something much more subtle.
CHRISTOPHER JOYCE, BYLINE: It's sometimes called the Great Dying. The Permian extinction, named after its geologic period, wiped out forests across the planet. Even insects died out. In the oceans, everything from giant sea-scorpions to crawling trilobites disappeared.
JON PAYNE: It was catastrophic - really, really awful. None of us would want to live in that world.
JOYCE: Jon Payne at Stanford University studies ancient life forms.
PAYNE: Clearly very large changes in climate, you know, warming of maybe as much as 5 or 10 degrees Celsius of the planet, major changes in vegetation.
JOYCE: The evidence for volcanoes or an asteroid doing this has not been convincing. There is a big clue, though: carbon. The chemistry of ancient rock and sediment shows that right about the time of the extinction there was a huge spike in the amount of carbon in the air and oceans. Really huge.
DANIEL ROTHMAN: If you added up all of the world's fossil fuels today, including coal, you wouldn't be able to still get that amount of carbon in the system.
JOYCE: Daniel Rothman is a geophysicist at MIT. He and his team there say they've got a novel idea about where that carbon came from: something very tiny at the bottom of the ocean - microorganisms. Here's how it might have happened. There was this common microbe that fed on organic junk in ocean sediments.
Then it mutated, possibly picking up a gene from another microbe. That increased its appetite incredibly fast. As it ate more, the basic mix of life in the ocean changed. Other microbes created yet more food for it.
ROTHMAN: You might think of it as a vicious cycle that feeds upon itself.
JOYCE: Now, that might not have been such a big deal, except this microbe created methane when it ate. And since there were uncountable numbers of them, that pumped lots of methane into the ocean and atmosphere. And that methane turned into carbon dioxide and that was bad. All that carbon dioxide warmed the planet and made the oceans acidic.
ROTHMAN: Greater than 90 percent of the species living at the time in the oceans went extinct.
JOYCE: Almost that many on land also are believed to have died out as well, probably from global warming. Rothman's team describes this hypothesis in the Proceedings of the National Academy of Sciences. They don't have direct evidence - say, actual remains of these super-microbes - but Rothman says it fits what they see in the fossil record. Looking back, though, there is an upside to this story. Stanford's Jon Payne points out that as bad as it was, the extinction kind of rebooted life on Earth.
PAYNE: In the very long run, these kinds of events may do as much to stimulate evolution as they do to damage biological diversity.
JOYCE: Easy enough to say, of course, if you're not a trilobite. Christopher Joyce, NPR News. Transcript provided by NPR, Copyright NPR.
