Single-cell organism may have caused Earth's most massive extinction: scientists have evidence
If you ask a random person about an extinction event on Earth, they will probably mention the asteroid that hit the planet and killed all the dinosaurs. The so-called Cretaceous extinction, which occurred about 66 million years ago, was indeed large-scale, but not the largest in history. The Permian mass extinction is also known as the "largest mass extinction of all time" for a reason, and it was most likely caused by a tiny organism.
This is stated in the theory of the Massachusetts Institute of Technology (MIT), the essence of which is retold by IFLScience. The Permian mass extinction wiped out about 70% of all terrestrial species and 96% of all marine species on Earth.
There are various theories about the causes of this event: from a catastrophic release of methane from the ocean floor to an asteroid impact.
Researchers know that at the end of the Permian period, there was a lack of oxygen in the oceans and shallow waters. This played a significant role in the extinction of species and also caused a domino effect.
The lack of oxygen probably led to the flourishing of sulfate-reducing microorganisms that use sulfates rather than oxygen for respiration. In the process, these microorganisms produced a byproduct, hydrogen sulfide, and caused sulfidization of the oceans. This, in turn, affected the atmosphere, causing plant poisoning and damage to the ozone layer, exposing life to the sun's deadly ultraviolet rays, and heating the planet in the process. As the oceans warmed, this led to the release of frozen methane into the atmosphere and further compounded the problem.
However, scientists at MIT have a different explanation for those events and their theory is troubling. Probably, the root cause of the largest extinction on Earth was microbes.
Daniel Rothman, a professor of geophysics at MIT, said that he and his team discovered an increase in the number of a certain microbe around the same time as the extinction.
The single-celled organism Methanosarcina became capable of digesting organic matter, producing methane as a byproduct. It acquired this ability through a single gene transfer from the bacterium Clostridia.
The scientists' hypothesis suggests that Methanosarcina thrived by releasing methane into the atmosphere, causing destruction or disrupting the carbon cycle and ultimately leading to species extinction.
As proof of their theory, the scientists said that the chemical process involved in the methane-producing microbes involves the metal nickel. When the scientists examined the most studied sediments in South China, they found it.
The authors of the study suggested that a single gene transfer "triggered biogeochemical changes, massive volcanism acted as a catalyst, and the expansion of acetoclastic methanosarcina led to increased levels of CO2 and O2.
"As a result, biogeochemical disturbances would likely have been widespread. For example, anaerobic methane oxidation could have led to increased sulfide levels, which could have resulted in toxic release of hydrogen sulfide into the atmosphere, causing species extinctions on land," the scientists said.
They acknowledge that while more evidence is needed to confirm this theory, the study has shown how sensitive the Earth is to the evolution of microbial life.
"The implications for today are that there are many ways in which natural fluctuations can occur in the Earth's carbon cycle. By studying the changes happening to the carbon cycle now, we should try to account for as many of them as possible to make future predictions," Rothman said.
The study is still far from a definitive conclusion, as it is possible that the mass extinction was caused by several factors at once, and it is also impossible to determine exactly when Methanosarcina evolved to start producing methane.
However, if the hypothesis is correct, it is quite possible that up to 90% of the species on the planet were partially wiped out by a single gene transfer in a single microorganism, and this is a thought-provoking idea given the huge number of microbes on Earth.