Around 252 million years ago, 90 percent of all species on Earth were wiped out in an extinction event commonly called The Great Dying. Now, a team of MIT researchers from the U.S. and China might have the answer for the largest mass extinction our planet has ever seen.

It wasn’t asteroids or volcanoes, but methane-producing microbes called Methanosarcina having sex — or rather, passing genetic material in a strange microbial form of sex.

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Scientists believe that these microbes acquired a set of genes that allowed them to feed off the rich organic carbon deposits that have developed in the oceans. In doing so, the microbes spewed “prodigious amounts” of methane into the atmosphere, dramatically changing the climate and chemistry of the oceans — literally suffocating almost all other life forms on the planet.

The researchers’ case against the potential culprit Methanosarcina rests on three pieces of evidence. There was an exponential increase in carbon dioxide in the oceans at the time; genetic evidence for a change in Methanosarcina to make it produce methane; and sediments showing an increased amount of nickel deposits.

Previously, scientists attributed the increase in carbon dioxide and methane to volcanic eruptions. But calculations by the MIT team proved that the volcanic activity at the time couldn’t have produced enough to account for the carbon in the sediments. Similarly, the way in which volcanoes produce gas — rapidly at first, followed by a steady decrease — couldn’t account for the steady increase in carbon dioxide.

“That suggests a microbial expansion,” Gregory Fournier, a MIT postdoc and researcher on the team, said in a statement. “The growth of microbial populations is among the few phenomena capable of increasing carbon production exponentially, or even faster."

The team tested 50 different genomes of methane-producing bacteria, and discovered that they acquired the ability to “eat” organic carbon around 252 million years ago.

The researchers believe that Methanosarcina acquired this ability — and in the process, produced extreme levels of methane — through gene transfer, aka sex, with a different microbe. The Methanosarcina needed the right nutrient to proliferate so quickly — namely nickel.

The MIT team’s findings were published in the Proceedings of the National Academy of Science on Monday.