Scientists have discovered 2.52 billion-year-old bacteria fossils which show that some early life forms existed on Earth without any oxygen.
While researchers proclaim the first half of our 4.5 billion-year-old planet's life as an important time for the development and evolution of early bacteria, evidence for these life forms remains sparse including how they survived at a time when oxygen levels in the atmosphere were less than one-thousandth of one per cent of what they are today.
Researchers from the University of Cincinnati (UC) in the US found fossilised bacteria in two separate locations in the Northern Cape Province of South Africa.
"These are the oldest reported fossil sulphur bacteria to date," said Andrew Czaja, UC assistant professor of geology. "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," said Czaja.
The 2.52 billion-year-old sulphur-oxidising bacteria are described by Czaja as exceptionally large, spherical-shaped, smooth-walled microscopic structures much larger than most modern bacteria, but similar to some modern single-celled organisms that live in deepwater sulphur-rich ocean settings today, where even now there are almost no traces of oxygen.
Researchers unveiled 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," said Czaja.
"These bacteria existed two billion years before plants and trees, which evolved about 450 million years ago," he said.
"We discovered these microfossils preserved in a layer of hard silica-rich rock called chert located within the Kaapvaal craton of South Africa," said Czaja.
With an atmosphere of much less than one per cent 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.
Finding rocks this old is rare, so the understanding of the Neoarchean Eon are based on samples from only a handful of geographic areas, such as this region of South Africa and another in Western Australia, researchers said.
According to Czaja, scientists through the years have theorised that South Africa and Western Australia were once part of an ancient supercontinent called Vaalbara, before a shifting and upending of tectonic plates split them during a major change in the Earth's surface.
Based on radiometric dating and geochemical isotope analysis, Czaja characterises his fossils as having formed in this early Vaalbara supercontinent in an ancient deep seabed containing sulfate from continental rock.
According to this dating, Czaja's fossil bacteria were also thriving just before the era when other shallow-water bacteria began creating more and more oxygen as a byproduct of photosynthesis.
The study was published in the journal Geology.