An article published in the journal “Proceedings of the Royal Society B: Biological Sciences” reports the results of a study on the origin of oxygenic photosynthesis. A team of researchers developed a new gene analysis technique to estimate the origin of cyanobacteria capable of oxygenic photosynthesis. The conclusion is that all the species of these cyanobacteria existing today have a common ancestor that dates back to about 2.9 billion years ago and that the ancestors of cyanobacteria branched off from other bacteria about 3.4 billion years ago. Oxygenic photosynthesis probably evolved during that time interval.
Many scientists used various methods to try to reconstruct the timeline of oxygenic photosynthesis’ origin but the results are still far from certain. The ability to produce oxygen and release it into the atmosphere had exceptional consequences for life on Earth. Even today, cyanobacteria such as Prochlorococcus marinus (photo ©Luke Thompson and Nikki Watson) are important for oxygen production. Greg Fournier, a professor of geobiology at MIT and lead author of this study, called it the single most important evolutionary innovation on Earth. For this reason, understanding that event means obtaining new information on the evolution of all life forms that use that oxygen.
To try to estimate when cyanobacteria emerged, Greg Fournier and his colleagues used a combination of different methods. One method is based on a kind of biological clock that uses genetic sequences of organisms existing today to track their past changes. Another method uses horizontal gene transfer, tracing the history of genes that, in various ways, have been passed from one organism species to another. In the case of cyanobacteria, the researchers tracked down 34 clear cases of horizontal gene transfer to be used for their estimation.
The results provide some indications on the era in which oxygenic photosynthesis was born. One result is that the cyanobacteria species existing today have a common ancestor that dates back to about 2.9 billion years ago. Another finding is that the ancestors of cyanobacteria branched off from other bacteria about 3.4 billion years ago. Oxygen photosynthesis likely evolved during that time interval, during the Archean eon.
This study also offers some possible answers on the diffusion rate of cyanobacteria. According to Greg Fournier and his colleagues, about 2.4 billion years ago, just before the so-called Great Oxidation Event that led to the mass extinction of primitive anaerobic life forms, cyanobacteria underwent considerable diversification. This suggests that they may have caused the Great Oxidation Event by injecting such amounts of oxygen into the atmosphere that killed life forms that couldn’t tolerate it.
These conclusions seem consistent with available fossil records and genetic research findings. There’s still a lot to study to understand what happened between 3.4 and 2.9 billion years ago if cyanobacteria really developed oxygen photosynthesis in that period. These are events really distant in time but they’re so important that the present Earth is their consequence.
