Evidence of life on Earth before the formation of oxygen in the atmosphere has been asserted in a new study.

The life before oxygen iteration has taken its cues from the fossils of bacteria etched in ancient rocks that are believed to be 2.5 billion years old and recovered from South Africa.

According to geologists, out of Earth's total age of 4.5 billion years, the first half mostly went into the evolution of early bacteria.

Neoarchean Eon

The paper carrying pre-oxygen life is titled "Sulfur-oxidizing bacteria prior to the Great Oxidation Event from the 2.52 Ga Gamohaan Formation of South Africa" and was published in the December issue of the journal Geology.

Authored by University of Cincinnati geologist Andrew Czaja, it has co-authors Nicolas Beukes from the University of Johannesburg and Jeffrey Osterhout, a student at UC's Department of Geology.

The researchers highlighted the evidence of bacteria found fossilized in the Northern Cape Province of South Africa as preserved in a layer of hard silica-rich rock called chert in the Kaapvaal craton of South Africa.

"These are the oldest reported fossil sulfur bacteria to date," said Czaja and added that these fossils represented the oldest organisms that lived in a dark, deep-water environment.

The paper reveals details of the bacteria which lived in deep water during the geologic time named Neoarchean Eon, which was 2.5 billion years ago.

The assumption is that since the early atmosphere contained less than 1 percent oxygen, these organisms must have lived in deep water in the mud as they do not need sunlight or oxygen.

According to Czaja, pre-oxygen bacteria flourished in sea beds of ocean water because the sulfur content was high and that came from the continental rock. At the same time, bacteria in shallow waters were spewing out large amounts of oxygen under the photosynthesis process.

Czaja said experts did not have any direct evidence that pre-oxygen life forms indeed existed.

Life Before The Great Oxidation Event

Czaja said the discovery was a milestone in helping to unravel the diversity of life and ecosystems before the Great Oxidation Event.

In terms of size, the sulfur-oxidizing bacteria were big, smooth-walled and sphere shaped with microscopic structures showing them as larger than modern bacteria.

Calling them look-alikes of modern single-celled organisms living in sulfur-rich oceans with no traces of oxygen, Czaja said isotope analysis exposed that the fossils were formed in ancient deep seabed of Vaalbara supercontinent.

It may be recalled that scientists have suggested South Africa and Western Australia were a single ancient supercontinent called Vaalbara before their tectonic plates split under the impact of a major event on the Earth's surface.

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