A Banded Iron Formation (BIF) from Australia’s Hamersley in the Pilbara. Image: Mark Barley
Scientists may have resolved the debate about when oxygen was first produced on Earth.
Life first appeared on Earth during the Great Oxidisation Event, when Earth’s oxygen levels increased significantly. This increase was caused by the presence of cyanobacteria, whose ability to perform oxygenic photosynthesis is thought to have converted the early oxygen-poor atmosphere into an oxidizing one.
However, there has been an ongoing debate about when cyanobacteria began producing oxygen and how long it took for oxygen levels to increase enough to support more complex life. There is possible evidence that cyanobacteria may have evolved 2.7 billion years ago, but the oldest unambiguous fossils that have been found were produced long after the atmosphere became oxygen rich.
“Some people think there is evidence for them at least 2.7 billion years ago, but others have debated this and it has been impossible to find completely unambiguous evidence before oxygen had rose significantly- mainly because many of the ancient rocks are altered by late oxidation,” said University of Western Australia researcher Mark Barley, co-author of the study published in Nature.
To determine when the cyanobacteria first began producing oxygen, Barley and his colleagues from the University of Alberta, Canada, carried out a detailed study of chromium abundance in banded iron formations (BIFs) on seabeds. Chromium would have been added to the ocean by oxidation of the mineral pyrite by chemolithautorobic bacteria, which would in turn indicate an increase of oxygen in the atmosphere.
The results from the BIFs prior to 2.48 billion years ago indicated that chromium was low in the ocean. Then at 2.48 billion years the chromium levels increased in both the in the oceans and the BIFs.
“We produced the first clear evidence that aerobic respiring chemolithautrobic bacteria, the most primitive form of aerobic respiring life on land, came into existence 2.48 billion years ago as they are the only things that could have added chromium to the oceans by oxidation of the mineral pyrite,” Professor Barley said.
And the presence of respiring bacteria suggests that cyanobacteria would also have been present. “Because chemolithautrobic bacteria require oxygen produced by cyanobacteria to oxidise pyrite and would not be able to do this if they did not coexist with cyanobacteria, this is the first unambiguous evidence that cyanobacteria existed before or started producing oxygen 2.48 billion years ago.”
The results also indicate that the oxygen levels rose significantly between 2.48 and 2.32 billion years ago, determining the timing of the Great Oxidisation Event and the rise of more advanced life.
