At what point did the Earth become habitable?

A new gene analysis technique shows that all species of cyanobacteria living today can be traced back to a common ancestor that evolved around 2.9 billion years ago.

It was then that this group of enterprising microbes developed oxygenated photosynthesis, the ability to convert light and water into energy, releasing oxygen in the process.

Scientists at the Massachusetts Institute of Technology (MIT) also found that the ancestors of cyanobacteria branched out from other bacteria about 3.4 billion years ago, and that oxygenic photosynthesis probably intermediate, during the Archean Aeon.

Interestingly, this estimate places the onset of oxygenated photosynthesis at least 400 million years before the Great Oxidation Event, a period when the Earth’s atmosphere and oceans first experienced an increase in oxygen. This suggests that cyanobacteria may have developed the ability to, but that it took a while for this oxygen to really take hold in the environment.

“In evolution, things always start small”, says lead author Greg Fournier, a professor asof Geobiology in the Department of Earth, Atmospheric and Planetary Sciences at MIT. “Even though there is evidence for early oxygenic photosynthesis, which is the most important and truly amazing evolutionary innovation on Earth, we still

To accurately date the origin of cyanobacteria and oxygenated photosynthesis, Fournier and his colleagues paired molecular clock dating with horizontal gene transfer, an independent method that does not rely entirely on fossils or rate assumptions.

Normally, an organism inherits a gene “vertically”, when it is passed down from the organism’s parent. In rare cases, a gene can also jump from one species to another, distantly related species. For example, one cell can eat another and, in the process,

When such a horizontal gene transfer history is found, it becomes clear that the group of organisms that acquired the gene is evolutionarily younger than the group from which the gene originated. Fournier reasoned that such cases could be used to determine the relative ages among certain bacterial groups. The ages of these groups could be compared to the ages predicted by various molecular clock models. The model that comes closest is likely to be the most accurate and could then be used to accurately estimate the age of other bacterial species, specifically cyanobacteria.

Following this reasoning, the team looked for cases of horizontal gene transfer through the genomes of thousands of bacterial species, including cyanobacteria. Also taken by Bosak and Moore, to more accurately use fossil cyanobacteria as calibrations. In the end, they identified 34 clear cases of horizontal gene transfer. They then found that one in six molecular clock models consistently matched the relative ages identified in the team’s horizontal gene transfer analysis.

Fournier ran this model to estimate the age of the “crown” group of cyanobacteria, which encompasses all species that live today and are known to exhibit oxygenated photosynthesis. They found that, during the Archaic eon, the crown group originated about 2.9 billion years ago, while cyanobacteria as a whole branched out from other bacteria about 3.4 billion years ago. This strongly suggests that oxygenated photosynthesis was already occurring (GOE), and that cyanobacteria were producing oxygen for quite some time before it accumulated in the atmosphere.

The analysis also revealed that, shortly before the GOE, about 2.4 billion years ago, cyanobacteria underwent a burst of diversification. This implies that a rapid expansion of cyanobacteria may have tilted the Earth towards the GOE and released oxygen into the atmosphere.

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