Researchers have discovered fossils of 2.5 billion-year-old sulfur-oxidising bacteria that existed just fine without any oxygen.
The ancient life forms were found fossilised in two separate locations in the Northern Cape Province of South Africa.
"These are the oldest reported fossil sulfur bacteria to date," said Andrew Czaja, Assistant Professor of Geology at University of Cincinnati in the US.
"And this discovery is helping us reveal a diversity of life and ecosystems that existed just prior to the Great Oxidation Event, a time of major atmospheric evolution," Czaja noted.
These bacteria were thriving just before the era when other shallow water bacteria began creating more and more oxygen as a by-product of photosynthesis.
"We refer to this period as the Great Oxidation Event that took place 2.4 to 2.2 billion years ago," Czaja said.
The 2.52 billion-year-old sulfur-oxidising bacteria are described by Czaja as exceptionally large, spherical-shaped, smooth-walled microscopic structures much larger than most modern bacteria.
In the study published in journal Geology, Czaja and his colleagues revealed samples of bacteria that were abundant in deep water areas of the ocean in a geologic time known as the Neoarchean Eon (2.8 to 2.5 billion years ago).
"These fossils represent the oldest known organisms that lived in a very dark, deep water environment," Czaja said.
"We discovered these microfossils preserved in a layer of hard silica-rich rock called chert located within the Kaapvaal craton of South Africa," Czaja noted.
With an atmosphere of much less than one percent oxygen, scientists have presumed that there were things living in deep water in the mud that did not need sunlight or oxygen, but experts did not have any direct evidence for them until now, Czaja said.
"These early bacteria likely consumed the molecules dissolved from sulfur-rich minerals that came from land rocks that had eroded and washed out to sea, or from the volcanic remains on the ocean's floor," he added.
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