Researchers have uncovered insights into Earth’s atmospheric history, highlighting that oxygen, which comprises approximately 21% of the atmosphere currently, was not always abundant. Roughly 2.7 billion years ago, cyanobacteria—aquatic bacteria capable of photosynthesis—began producing oxygen, which subsequently accumulated in the atmosphere during a period known as the Great Oxidation Event (GOE), occurring between 2.4 and 2.1 billion years ago. However, recent findings indicate that aerobic bacteria, which rely on oxygen, might have existed well before the GOE.
An international research team has reconstructed an evolutionary timeline for one of Earth’s earliest life forms, suggesting bacteria adapted to oxygen long before it was widely available. Their study, published in the journal Science, challenges the prior belief that most life forms before the GOE were anaerobic, meaning they did not require oxygen.
In their study, researchers utilized a multidisciplinary approach to trace bacterial adaptation to oxygen, incorporating geological records, fossil evidence, analyses of over 1,000 diverse bacterial genomes, phylogenetic reconciliation, which compares the history of intertwined species, and computer modeling. Their findings indicate that the last common ancestor of modern bacteria might have existed between 4.4 and 3.9 billion years ago.
Gergely Szöllősi, a co-author and evolutionary biologist from the Okinawa Institute of Science and Technology Graduate University, stated in a university release that this combination of genomic data, fossils, and Earth’s geochemical history offers clearer evolutionary timelines, particularly for microbial groups lacking a fossil record.
The study suggests that certain aerobic bacteria appeared prior to the GOE, around 3.22 to 3.25 billion years ago. These bacterial lineages likely predated cyanobacteria, evolving the ability to metabolize small quantities of oxygen before developing photosynthesis. The adaptation to oxygen may have significantly influenced the evolution of cyanobacteria’s photosynthetic capabilities and consequently, the atmospheric shifts during the GOE.
Tom Williams, a computational evolutionary biologist from the University of Bristol and co-author of the study, noted that the team’s approach is effective for studying the development of aerobic metabolisms and may be useful for exploring the evolution of other traits in response to Earth’s changing environment over geological time.
The study serves as a reminder of the pivotal role microbial activity has played over billions of years in shaping the atmosphere present today.