For the first time, scientists have found direct evidence of microbial life that existed on Earth prior to the formation of oxygen – a 2.52 billion-year-old, sulfur-oxidizing bacteria discovered in two separate locations in South Africa by geologists from the University of Cincinnati.
UC assistant geology professor Andrew Czaja and his colleagues discovered the bacteria in the Northern Cape Province of South Africa, describing them as smooth-walled, spherically-shaped structures that are far larger than most current bacteria but similar to some modern, single-celled organisms that live deep in oxygen-sparse, sulfur-rich ocean environments.
“These are the oldest reported fossil sulfur bacteria to date,” Czaja, lead author of a new study detailing his team’s find published in the December 2016 issue of Geology, told UC Magazine. “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.”
While scientists had previously demonstrated that early life would have thrived during the period of time called the Neoarchean Eon (2.8-2.5 billion years ago), when oxygen levels were less than one-thousandth of one-percent their current levels, this marks the first time that they have located physical evidence that this was indeed the case.
Similarities to modern deep-water microbes identified
Czaja, recently graduated UC master’s student Jeffrey Osterhout and Nicolas Beukes from the University of Johannesburg reported that they discovered fossilized samples of bacteria dating back to the Neoarchean Eon preserved in a layer of hard, deep-water, silica-rich rock known as black chert in the Gamohaan Formation of the South Africa’s Kaapvaal craton.
Those fossils, he told UC Magazine, “represent the oldest known organisms that lived in a very dark, deep-water environment.” They existed two billion years before the evolution of plants and trees, and provide the first direct evidence confirming that there were some organisms which did not require oxygen or sunlight to survive living in deep water mud during the early Earth.
Based on his team’s radiometric dating and geochemical isotope analysis of the samples, Czaja believes that they likely formed in the early Vaalbara supercontinent (a landmass that included South Africa and Western Australia before splitting due to the shifting of tectonic plates) within an ancient seabed that was rich in sulfate due to continental rock. It likely lived just before other shallow-water microbes began producing more oxygen through photosynthesis.
According to United Press International (UPI), the study authors believe that the ancient bacteria behaved similarly to modern-day bacteria, recycling volcanic hydrogen sulfide and converting it into sulfate. While Czaja said that he and his colleagues “can’t claim that these early bacteria are the same ones we have today,” he noted that they “may have been doing the same thing.”
“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. While scientists are uncertain exactly how and why sulfur-oxidizing bacteria first evolved, “these fossils tell us that [they] were there 2.52 billion years ago, and they were doing something remarkable,” Czaja concluded.
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Image credit: Andrew Czaja, UC assistant professor of geology
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