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 A microbe that spewed humongous amounts of methane into Earth's
atmosphere triggered a global catastrophe 252 million years ago that
wiped out upwards of 90 percent of marine species and 70 percent of
land vertebrates.
That's the hypothesis offered on Monday by researchers 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.
The implicated microbe, Methanosarcina, is a member of a kingdom of
single-celled organisms distinct from bacteria called archaea that
lack a nucleus and other usual cell structures.
"I would say that the end-Permian extinction is the closest animal
life has ever come to being totally wiped out, and it may have come
pretty close," said Massachusetts Institute of Technology biologist
Greg Fournier, one of the researchers.
"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. But these researchers suggest a
microscope would be needed to find the actual culprit.
Methanosarcina grew in a frenzy in the seas, disgorging huge
quantities of methane into Earth's atmosphere, they said.
This dramatically heated up the climate and fundamentally altered
the chemistry of the oceans by driving up acid levels, causing
unlivable conditions for many species, they added.
The horseshoe crab-like trilobites and the sea scorpions — denizens
of the seas for hundreds of millions of years — simply vanished.
Other marine groups barely avoided oblivion including common
creatures called ammonites with tentacles and a shell.
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 including people.
"RADICALLY CHANGED"
"Land vertebrates took as long as 30 million years to reach the same
levels of biodiversity as before the extinction, and afterwards life
in the oceans and on land was radically changed, dominated by very
different groups of animals," Fournier said.
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.
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The best example of that, Rothman said, was the advent about 2.5
billion years ago of bacteria engaging in photosynthesis, which
paved the way for the later appearance of animals by belching
fantastic amounts of oxygen into Earth's atmosphere.
Methanosarcina is still found today in places like oil wells, trash
dumps and the guts of animals like cows.
It already existed before the Permian crisis. But genetic evidence
indicates it acquired a unique new quality at that time through a
process known as "gene transfer" from another microbe, the
researchers said.
It suddenly became a major producer of methane through the
consumption of accumulated organic carbon in ocean sediments.
The microbe would have been unable to proliferate so wildly without
proper mineral nutrients. 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.
The Permian mass extinction unfolded during tens of thousands of
years and was not the sudden die-off that an asteroid impact might
cause, the researchers said.
The most famous of Earth's mass extinctions occurred 65 million
years ago when an asteroid impact wiped out the dinosaurs that ruled
the land and many marine species. There also were huge die-offs 440
million years ago, 365 million years ago and 200 million years ago.
(Reporting by Will Dunham; editing by James Dalgleish)
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