Not a supervolcano. Not a mega-cyclone caused by a comet buzzing Earth. A germ is now thought to be responsible for killing 95% of the planet's species some 250 million years ago, by belching gargantuan quantities of methane, in what's known as the Permian Extinction.
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In the global catastrophe triggered by the primitive bacteria 252 million years ago, upwards of 90 percent of marine species and 70 percent of land animals were wiped out.
Thus suggested researchers on Monday, aiming to solve one of science's enduring mysteries: what happened at the end of the Permian period to cause the worst of the five mass extinctions in Earth's history.
The scale of this calamity made the one that doomed the dinosaurs 65 million years ago - a six-mile wide asteroid smacking the planet - seem like a picnic by comparison.
Actually quite advanced
The implicated microbe, Methanosarcina, is a member of a kingdom of single-celled organisms that are distinct from archaebacteria – which lack a nucleus and other usual cell structures. This bug had them and almost wiped out animal life on earth, says biologist Greg Fournier of the Massachusetts Institute of Technology.
"Many, if not most, of the surviving groups of organisms barely hung on, with only a few species making it through, many probably by chance," Fournier added.
Previous ideas proposed for the Permian extinction include an asteroid and large-scale volcanism.
Note that the bacteria theory remains just that. These researchers suggest a microscope will be needed to find the actual culprit. That said, they believe methanosarcina grew in a frenzy in the seas, disgorging huge quantities of methane into Earth's atmosphere.
The cloud of methane dramatically heated up the climate and fundamentally altered the chemistry of the oceans by driving up acid levels, mortally poisoning many species, they added.
This is what did for the trilobites and sea scorpions - denizens of the seas for hundreds of millions of years – which simply vanished.
It's still here...
On land, most of the dominant mammal-like reptiles died, with the exception of a handful of lineages - including the ones that were the ancestors of modern mammals. Yes, you had a reptilian ancestor way back when.
After the cataclysm, "Life in the oceans and on land was radically changed, dominated by very different groups of animals," Fournier said.
That also applies to land. The first dinosaurs appeared 20 million years after the Permian mass extinction.
"One important point is that the natural environment is sensitive to the evolution of microbial life," said Daniel Rothman, an MIT geophysics professor who led the study published in the journal Proceedings of the National Academy of Sciences.
The best example of that, Rothman said, was the advent about 2.5 billion years ago of bacteria engaging in photosynthesis. They paved the way for animals to develop by belching fantastic amounts of oxygen into Earth's atmosphere.
Methanosarcina lives on today, in places like oil wells, trash dumps and the guts of animals like cows.
The microbe would have been unable to proliferate so wildly without the proper food. The researchers found that cataclysmic volcanic eruptions that occurred at that time in Siberia drove up ocean concentrations of nickel, a metallic element that just happens to facilitate this microbe's growth.
Fournier called volcanism a catalyst instead of a cause of mass extinction - "the detonator rather than the bomb itself."
"As small as an individual microorganism is, their sheer abundance and ubiquity make for a huge cumulative impact. On a geochemical level, they really do run the planet," he said.