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For the vast majority of animals on Earth, breath is synonymous with life. Yet during the first 2 billion years of our planet’s existence, oxygen was scarce.
This does not mean that the Earth was lifeless all this time, but this life was rarer and very different from what we know today.
It wasn’t until more complex bacteria capable of photosynthesis entered the picture that everything started to change, triggering what scientists are calling a great oxidation event. But when did all this happen? And how did it all happen?
A new technique of genetic analysis has provided clues for a new chronology. Estimates suggest that it took bacteria 400 million years to engulf sunlight and blow out oxygen before life could truly thrive.
In other words, there were probably organisms on our planet capable of photosynthesis long before the great oxidation event.
“In evolution, things always start small,” says geobiologist Greg Fournier of the Massachusetts Institute of Technology.
“Even though there is evidence for early oxygenated photosynthesis – which is the most important and amazing evolutionary innovation on Earth – it still took hundreds of millions of years for it to take off.”
Currently, there are two competing accounts to explain the evolution of photosynthesis in special bacteria known as cyanobacteria. Some believe that the natural process of transforming sunlight into energy arrived on the evolutionary scene early enough, but that it progressed with “a slow fuse.” Others believe that photosynthesis later evolved but “took off like wildfire”.
Much of the disagreement comes down to assumptions about how quickly bacteria evolve and different interpretations of the fossil record.
Fournier and his colleagues therefore added another form of analysis to the mix. In rare cases, a bacteria can sometimes inherit genes not from its parents, but from another distant species. This can happen when one cell “eats” another and incorporates the other’s genes into its genome.
Scientists can use this information to determine the relative age of different groups of bacteria; for example, those who stole genes must have pinched them from a species that existed at the same time as them.
Such relationships can then be compared to more specific dating attempts, such as molecular clock models, which use the genetic sequences of organisms to trace the history of genetic changes.
To this end, researchers combed through the genomes of thousands of bacterial species, including cyanobacteria. They were looking for cases of horizontal gene transfer.
In total, they identified 34 clear examples. By comparing these examples to six molecular clock models, the authors found one in particular that worked best. Choosing this model from the mix, the team made estimates to determine the actual age of photosynthetic bacteria.
The results suggest that all cyanobacteria species living today have a common ancestor that existed around 2.9 billion years ago. Meanwhile, the ancestors of those ancestors branched out from non-photosynthetic bacteria about 3.4 billion years ago.
Photosynthesis probably evolved somewhere between these two dates.
According to the team’s preferred evolutionary model, cyanobacteria likely photosynthesized at least 360 million years before GEO. If they are right, this further strengthens the “slow fuse” hypothesis.
“This new article sheds essential new light on the history of Earth’s oxygenation by linking, in an unprecedented way, the fossil record with genomic data, including horizontal gene transfers,” says biogeochemist Timothy Lyons of the University of California at Riverside.
“The results speak to the beginnings of biological oxygen production and its ecological importance, in a way that provides vital constraints on models and controls on the first oxygenation of the oceans and subsequent accumulations in the atmosphere.”
The authors hope to use similar gene analysis techniques to analyze organisms other than cyanobacteria in the future.
The study was published in Proceedings of the Royal Society B.
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