When did bacteria acquire the ability to generate energy from light and release oxygen in the process? Depending on the dating method, the answer to this question differs significantly. Using a combination of genome and fossil data, researchers have now come to the conclusion: The oxygen-producing cyanobacteria developed 3.4 to 2.7 billion years ago – at least 300 million years before oxygen was increasingly concentrated in the atmosphere.
The development of photosynthesis laid the foundation for life as we know it today. At a time when oxygen was still toxic to most living things, cyanobacteria began to release this very toxin as a product of their energy production from sunlight, water and carbon dioxide. As a result, 2.4 billion years ago, the so-called Great Oxygen Disaster occurred: The oxygen content of the atmosphere rose massively, which not only led to a mass extinction of the anaerobic bacteria living at the time, but also enabled the development of today’s oxygen-based life. When exactly the cyanobacteria developed, however, was so far unclear.
Complement to the molecular clock
Previous dating was usually based on the method of creating a “molecular clock” based on mutation-related changes in the genome of the bacteria, which estimates when different bacterial lines have split off from one another. Depending on the model used, however, these estimates fluctuate considerably, since without external reference points it is often unclear how quickly the changes took place. A team led by Greg Fournier from the Massachusetts Institute of Technology (MIT) in Cambridge has therefore combined the molecular clock with another method: The researchers also analyzed what are known as horizontal gene transfers. These are rare cases in which a bacterium takes on genes from a different species. This can happen, for example, when one cell eats another and in the process incorporates some of their genes into its own genome.
In these cases it is clear that the group of organisms that took over the gene must be evolutionarily younger than the group from which the gene originated. The method therefore does not allow absolute information, but enables the relative age of different bacterial groups to be determined in comparison. Fournier and his colleagues took advantage of this. First they created various molecular clock models on the origin of the cyanobacteria based on genome and fossil data. “In order to decide on an empirical basis which of the models best depicts reality, we then used the relative age, which can be derived from events of horizontal gene transfer,” the researchers explain.
Slow start of photosynthesis
The team found 34 clear cases of horizontal gene transfer in the genomes of thousands of bacterial species. The relative age dating derived from this confirmed one of six previously established molecular clock models. According to this, the last common ancestor of all cyanobacteria living today arose around 2.7 billion years ago. According to the model, the cyanobacteria as a whole split off from other bacteria about 3.4 billion years ago. “On the basis of the genetic data known to date, it is still unclear exactly when photosynthesis with oxygen took place during this period, since the corresponding bacteria that lived in the transition period are either extinct or have not yet been discovered,” explain the researchers.
However, the oxygen-producing bacteria emerged no later than 2.7 billion years ago – and thus at least 300 million years before the Great Oxygen Disaster. This supports theories that the cyanobacteria developed the ability to produce oxygen early on, but it took a while for oxygen to gain influence in the environment. “In evolution, things always start small,” says Fournier. “Even if there was oxygen-producing photosynthesis early on – the most important and truly amazing evolutionary innovation on earth – it took hundreds of millions of years to get going.”
Increasing influence 2.4 billion years ago
The finding fits in with earlier evidence from rock analyzes, according to which elements were oxidized at least three billion years ago. One source of the oxygen required for this could have been photosynthesis, whereby the oxygen was bound in the rock instead of being enriched in the atmosphere. “The age estimates suggest that the early cyanobacteria had little impact on the global atmosphere,” the authors write. However, according to genetic analyzes, the cyanobacteria experienced a surge in diversification 2.4 billion years ago. “The most common and diverse groups of cyanobacteria appear to be spreading around the time of the Great Oxygen Disaster,” say the researchers.
From their point of view, this indicates that the spread of the cyanobacteria, among other factors, led to the Great Oxygen Catastrophe – with all the consequences for further life on earth. The authors also intend to use the dating method with the help of information on horizontal gene transfers in the future to explore the origins of other species. “This work shows that molecular clocks that involve horizontal gene transfers (HGTs) could reliably determine the age of groups across the entire tree of life, even for ancient microbes that have left no fossil record – something that was previously impossible,” says Fournier.
Source: Greg Fournier (Massachusetts Institute of Technology, Cambridge) et al., Proceedings of the Royal Society B, doi: 10.1098 / rspb.2021.0675