Volcanoes and asteroids are sometimes blamed for wiping out nearly all life on Earth 252 million years ago, but US research Monday suggested a more small-time criminal: microbes.

These microbes, known as Methanosarcina, bloomed in the ocean on a massive and sudden scale, spewing methane into the atmosphere and causing dramatic changes in the chemistry of the oceans and the Earth's climate, according to the new theory put forth by scientists at the Massachusetts Institute of Technology and colleagues in China.

Scientists studied sediments in rock formations in south China, seeking to explain why the end Permian extinction happened and what caused the largest of five major death events in Earth's history to reap so much destruction over tens of thousands of years.

Volcanic eruptions on their own could not explain why the die-off happened so fast, but they may have released extra nickel into the environment, which fed the microbes, said MIT researcher Gregory Fournier.

"A rapid initial injection of carbon dioxide from a volcano would be followed by a gradual decrease," said Fournier.

"Instead, we see the opposite: a rapid, continuing increase," he added.

"That suggests a microbial expansion."

Microbes can increase carbon production exponentially, which might explain the speed and potency of the mass extinction, he said.

The research, funded by the US space agency NASA, the National Science Foundation, the Natural Science Foundation of China and the National Basic Research Program of China, appears in the Proceedings of the National Academy of Sciences, a peer reviewed US journal.