An article published in the magazine “Geology” describes the discovery of fossils of bacteria that date back to about 2.5 billion years ago. Those are exceptionally large sulfur-oxidizing bacteria compared to modern ones, of spherical shape with structures much larger than those of modern bacteria but similar to those of microorganisms that live today in sulfur-rich ocean depths. Above all, those are bacteria that lived before the Great Oxygenation Event.
The availability of fossils from the first billion years of the history of life on Earth is limited to say the least. Paleontologists are used to having to do with incomplete and limited fossil records but in this case we’re talking about a few dozen microbial fossils that cover a period from 3.5 to 2.5 billion years ago. The consequence is that we know very little of the forms of life that existed before the Great Oxygenation Event (GOE), which occurred between 2.2 and 2.4 billion years ago.
The appearance of the first life forms that used photosynthesis and as a consequence produced oxygen led to a gradual release of free molecular oxygen, meaning oxygen molecules that began to oxidize other elements they got in contact with. At a certain point, the oxidizable substances were oxidized and oxygen started spreading in the atmosphere and waters. Its concentration, although much lower than today, caused the first major mass extinction in history because most of the organisms that lived at the time were anaerobic and oxygen was toxic to them.
This event, called the Great Oxygenation Event or Oxygen Catastrophe, Oxygen Crisis, Oxygen Holocaust, Oxygen Revolution, or Great Oxidation, completely changed the history of life on Earth because only life forms that got adapted to the presence of oxygen survived except in some niches. Because we know very little of the life forms existing before that extinction, every discovery is precious.
Andrew Czaja, assistant professor at the University of Cincinnati (UC), together with his colleagues Nicolas Beukes of the University of Johannesburg and Jeffrey Osterhout of UC, discovered bacteria that thrived in the ancient oceans in Neoarchean, between 2.8 and 2, 5 billion years ago. To understand the times just think that animals and plants appeared about 450 million years ago.
These microfossils were discovered in a silica-rich layer of rock in the Kaapvaal Craton, one of the protocontinents that were part of the first supercontinent existed on Earth, called Vaalbara. This craton is named after the Kaapvaal region in South Africa.
This discovery shows that bacteria of that type were abundant. According to Andrew Czaja, they were feeding on hydrogen sulphide and then emit sulfate. Today there are also bacteria with that kind of metabolism in the ocean depths, in niches where there’s an abundance of sulfur and oxygen is almost absent.
Due to the very limited availability of fossils, there are only hypotheses about the appearance of those bacteria and their role in the evolution of life on Earth. This discovery shows that 2.5 billion years ago there were bacteria, already relatively complex life forms, and this is a new step in our knowledge of that era so remote.
