The Great Oxidation Event (GOE) some 2.4 billion years ago established the oxygen-rich atmosphere that many living things depend on today.
Curiously, when it came to evolving ways to actively use the element for respiration, some strains of bacteria had a head start.
Using a combination of bacterial genome data, geological markers, and machine learning techniques designed to spot genetic patterns, an international team of researchers searched for evidence describing the earliest aerobic (oxygen-breathing) bacteria.
While most strains took up the ability to tolerate and use oxygen after the GOE, the researchers did spot some outliers – certain bacteria that were aerobic approximately 900 million years before oxygen levels rapidly rose in Earth's atmosphere.
Aerobic chart
The researchers found some aerobic bacteria pre-dated the Great Oxidation Event. (Davin et al., Science, 2025)
Among 1007 species of related bacteria, the researchers identified more than 80 genetic transitions from metabolisms that couldn't use oxygen to metabolisms that could. Estimates based on the rate at which mutations accumulate suggest at least a few of these happened before oxygen levels rose to significant levels in Earth's atmosphere.
"At least three transitions predated [the GOE], suggesting that aerobic respiration evolved before widespread atmospheric oxygenation and may have facilitated the evolution of oxygenic photosynthesis in cyanobacteria," write the researchers in their published paper.
In other words, these early oxygen-breathers may have laid the foundations for their descendents to use water and carbon dioxide to capture sunlight, freeing up stored oxygen in what would become the Great Oxidation Event.
We've got a lot to thank these little microbes for. As oxygen levels increased, bacteria already using the element could tolerate its effects to diversify more quickly than their anaerobic cousins. The game of survival had changed, and ultimately led us to where we are now.
There are some assumptions being made here, about how genes in modern bacteria might link back to genes in ancient bacteria – and do the same oxygen processing job – but the researchers are confident that they included enough bacteria species and corroborating evidence to confirm the link.
"This allowed us to calibrate bacterial evolution to the record of biospheric oxygenation, greatly augmenting the limited fossil record of early life and bringing a new level of resolution to the study of evolution in deep time," write the researchers.
As well as validating the idea that aerobic bacteria have a history that goes way, way back, these findings also give us more evidence that cyanobacteria evolution happened relatively slowly, with its roots streching back far before the GOE.
And the researchers are hopeful that the combination of techniques used here – pulling together different tools to fill in gaps in our knowledge – could help in looking at the development of other traits in Earth's earliest lifeforms.
"The approach developed here provides a framework for linking microbial traits to Earth's geochemical history, offering a pathway for exploring the evolution of other phenotypes in the context of Earth's history," write the researchers.
The research is published in Science.