If you think of an extinction event, the asteroid that killed the dinosaurs probably comes to mind – but Earth has been through five mass extinction events and may be going through a sixth.

Of these extinction events, the Cretaceous one that ended the dinosaurs is not even the biggest. That honor goes to the Permian mass extinction, also known as the Dead Sea, which wiped out about 90 percent of all terrestrial species and 70 percent of all marine species on Earth.

In addition, a strange 10 million year gap in charcoal created around the time of extinction suggests that a large number of charcoal-forming trees disappeared during this event, taking millions of years to recover.

Finding a period of time in the fossil record where there is a sharp decline in the number of species is apparently the easy part. Scientists have proposed a number of explanations for the extinction and its underlying causes – from a catastrophic release of methane from the ocean floor to an asteroid impact.

Earth has gone through five mass extinction events

By studying the rocks that formed at the time of the extinction, we know that the oceans and shallow waters were devoid of oxygen at the end of the Permian period.

Lack of oxygen (also known as anoxia) certainly seems to have played a role in the extinction event, as well as having a domino effect.

Sulfate-reducing microorganisms, which can perform anaerobic respiration using sulfate instead of the old and reliable O2, likely thrived in these low-oxygen environments. The hydrogen sulfide byproduct they produce, as well as the oceans turning to sulfide as a secondary consequence of the lack of oxygen, could have been released into the atmosphere.

It may have poisoned plants and damaged the ozone layer, exposing life to lethal levels of UV rays some 250 million years before it developed sunscreen, plus warming the planet in the process. Ocean warming, in turn, could have caused frozen ocean methane to be released into the atmosphere, exacerbating the problem.

Lack of oxygen appears to have played a role in the extinction event

An alternative explanation for the extinction, proposed by a team at MIT in 2014, is perhaps the most troubling. Could it be the biggest extinction event the world has ever seen caused by microbes?

Daniel Rothman, professor of geophysics at MIT, and his team observed the growth of a particular microbe around the time of the extinction. Methanosarcina, a single-celled organism, became capable of digesting organic matter, producing methane as a byproduct, thanks to a single gene transfer from the bacterium Clostridia.

The hypothesis is that Methanosarcina thrived during this time, spewing methane into the atmosphere and disrupting the carbon cycle, ultimately fueling the extinction event.

The chemical process involved in the process of creating methane by microbes involves nickel – meaning that if the team couldn’t find more nickel during the extinction, the hypothesis could effectively be rejected.

The study shows how sensitive Earth is to the evolution of microbial life

However, the team analyzed the most studied sediments in southern China and found high levels of nickel, which could confirm the theory, they write IFL Science.

“A single horizontal gene transfer instigated the biogeochemical change, massive volcanism acted as a catalyst, and the resulting expansion of acetoclastic Methanosarcina acted to disrupt CO2 and O2 levels,” the team concluded in study.

“The ensuing biogeochemical disturbance would likely have been widespread. For example, anaerobic oxidation of methane could have increased sulfide levels, which could have led to a toxic release of hydrogen sulfide into the atmosphere, causing extinctions on land.”

The team pointed out that while more evidence is needed for this theory, the study could show how sensitive Earth is to the evolution of microbial life.

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