Volcanic eruptions had previously been blamed for the sudden loss of 90% of all species on Earth at the end of the Permian era.
But new research suggests volcanoes only played a bit part in the catastrophe.
The chief perpetrators were a microscopic methane-producing archaea life-form called Methanosarcina that bloomed explosively in the oceans.
Enormous quantities of methane, a potent greenhouse gas, generated by Methanosarcina are thought to have sent temperatures soaring and acidified the seas.
Unable to adapt in time, countless species perished and vanished from the Earth.
Alarmingly, the same effects are starting to happen today as a result of global warming caused by man-made carbon emissions.
Analysis of geological carbon deposits reveals a significant boost in levels of carbon-containing gases - either carbon dioxide or methane - at the time of the mass extinction.
But volcanic eruptions alone could never have produced the amount of carbon laid down in rock sediments during this period, the researchers claim.
"A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease," said US scientist Dr Gregory Fournier, from the Massachusetts Institute of Technology (MIT). "Instead, we see the opposite: a rapid, continuing increase.
"That suggests a microbial expansion. The growth of microbial populations is among the few phenomena capable of increasing carbon production exponentially, or even faster."
A timely combination of two factors may have sent Methanosarcina into overdrive, according to the findings reported in the journal Proceedings of the National Academy of Sciences.
First, a genetic change allowed it to become a major producer of methane from accumulations of carbon dioxide in the oceans.
Second, a surge in volcanic activity led to a sudden influx of nickel, a vital nutrient that helped the microbes proliferate.