Molecular clock: bacteria used oxygen long before widespread photosynthesis

Summary

Molecular clock analysis header Image (OIST). Credit: Kaori Serakaki (OIST) Microbial organisms dominate life on Earth, but tracing their early history and evolution is difficult because they rarely fossilize. Determining when exactly a particular group of microbes first appeared is especially hard. However, ancient sediments and rocks hold chemical clues of available nutrients that could support the growth of bacteria. A key turning point was when oxygen accumulated in the atmosphere around 2.3 billion years ago. Scientists have used this oxygen surge and how microbes adapted to it to map out bacterial evolution. In a study published in Science, researchers from the Model-Based Evolutionary Genomics Unit at the Okinawa Institute of Science and Technology (OIST) and their international collaborators have constructed a detailed timeline for bacterial evolution and oxygen adaptation. Their findings suggest some bacteria could use trace oxygen long before evolving the ability to produce it through photosynthesis. The researchers focused on how microorganisms responded to the Great Oxygenation Event (GOE) some 2.3 billion years ago. This event, triggered in large part by the development of oxygenic (oxygen-generating) photosynthesis in cyanobacteria and carbon deposition, fundamentally changed Earth's atmosphere from one mostly devoid of oxygen to one where oxygen became relatively abundant, as it is today. Until now, establishing accurate timescales for how bacteria evolved before, during, and after this pivotal transition has been difficult due to incomplete fossil evidence and the challenge of determining the maximum possible ages for microbial groups—given that the only reliable maximum limit for the vast majority of lineages is the moon-forming impact 4.5 billion years ago, which likely sterilized the planet. The researchers addressed these gaps by concurrently analyzing geological and genomic records. Their key innovation was to use the GOE itself as a time bounda...