Ancient Bacteria Were Breathing Long Before Oxygen Became Abundant

Today, oxygen makes up about 21% of our atmosphere, but it wasn’t always so plentiful. Around 2.7 billion years ago, cyanobacteria—aquatic bacteria that generate energy through photosynthesis—evolved and began releasing oxygen into the oceans. This oxygen gradually accumulated in the atmosphere in a process called the Great Oxidation Event (GOE), which took place between 2.4 and 2.1 billion years ago. New research, however, suggests that aerobic (oxygen-dependent) bacteria may have emerged long before the GOE.

An international team of researchers has reconstructed the evolutionary tree of one of Earth’s earliest life forms, revealing that bacteria may have adapted to the presence of oxygen long before it was plentiful in our atmosphere. Their work, detailed in a study published today in the journal Science, challenges the previous assumption that most life prior to the GOE was anaerobic, that is, organisms that don’t need oxygen to survive.

The researchers used a multidisciplinary approach to reconstruct an evolutionary tree for bacteria and trace when they adapted to oxygen. This included analyzing geological records, fossil evidence, and over 1,000 diverse bacterial genomes; applying phylogenetic reconciliation (comparing the history of two closely intertwined life forms); and computer modeling. According to their evolutionary tree, the last common ancestor of modern bacteria likely existed sometime between 4.4 and 3.9 billion years ago.

“This combined approach of using genomic data, fossils, and Earth’s geochemical history brings new clarity to evolutionary timelines, especially for microbial groups that don’t have a fossil record,” Gergely Szöllősi, a co-author on the study and an evolutionary biologist from the Okinawa Institute of Science and Technology Graduate University, said in a university statement.

Their results suggest that some aerobic bacteria emerged before the GOE, around 3.22 to 3.25 billion years ago. It’s likely that these bacterial lineages were the ancestors of cyanobacteria, meaning they evolved the ability to metabolize small amounts of oxygen before developing photosynthesis. In fact, the research suggests that oxygen adaptation may have played a crucial role in the evolution of cyanobacteria’s photosynthetic abilities—and, as a consequence, the changes in Earth’s atmosphere during the GOE.

The team’s approach “works well for studying the spread of aerobic metabolisms and might also be a useful approach for exploring how other traits emerged and interacted with the planet’s shifting environment across geological time,” said Tom Williams, a computational evolutionary biologist from the University of Bristol and also a co-author on the study.

The study is also a reminder of the fact that the atmosphere we enjoy today was shaped by billions of years of microbial activity.