Zeroing in on the origins of Earth's 'single most important evolutionary innovation'

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Earth's first history saw a turning point toward habitability. A group of microbes called cyanobacteria developed oxygenic photosynthesis, which is the ability to convert light and water into energy. They also released oxygen.

This evolutionary moment allowed oxygen to eventually accumulate within the atmosphere and oceans. It set off a domino effect and shaped the unique habitable planet that we now know.

Now scientists at MIT have a precise estimation of when cyanobacteria and oxygenic photosyntheses first appeared. The Proceedings of the Royal Society B contains their results.

A new method of gene analysis was developed by the researchers. It shows that all species of cyanobacteria today can be traced back around 2.9 billion years ago to a common ancestor. They discovered that cyanobacteria's ancestors branched from other bacteria about 3.4 billion years back. Oxygenic photosynthesis probably evolved during the Archean Eon, which was approximately half a billion years later.

This estimate puts the appearance of oxygenic photosynthesis at least 400million years before the Great Oxidation Event. This is the time when the Earth's atmosphere, oceans, and atmosphere first saw an increase in oxygen. This suggests that cyanobacteria might have been able to produce oxygen in a relatively early time, but it took some time for the oxygen to take root in the environment.

Greg Fournier, an associate professor of geobiology at MIT's Department of Earth, Atmospheric and Planetary Sciences, said that "In evolution, everything always starts small." "Even though evidence exists for early oxygenic photosynthesis, which is the single most important and amazing evolutionary innovation on Earth, it took hundreds of millions to get off the ground."

Fournier's MIT coauthors are Kelsey Moore and Jack Payette. Tanja Bosak is Tanja Momper.

Or slow fuse?

There are many estimates of the origin of oxygenic photosyntheses, as well as methods to track its evolution.

Geochemical tools can be used by scientists to search for trace amounts of oxidized elements within ancient rocks. These methods showed that oxygen existed as far back as 3.5 billion years ago. This suggests that oxygenic photosynthesis was the source. However, other sources of oxygen are possible.

Researchers also use molecular clock-dating, which is based on the genetic sequences from microbes to track back evolutionary changes. These sequences are used by researchers to create models that estimate the rate of genetic changes, and trace when organisms evolved. However, molecular clock dating is constrained by the quality and age of fossils.

Fournier states that different age estimates could lead to conflicting evolutionary narratives. Some analyses indicate that oxygenic photosynthesis developed very early and was "like a slow fuse." Others suggest it emerged later and then "took off like a wildfire" to trigger "the Great Oxidation Event" and the accumulation and destruction of oxygen in the biosphere.

He says, "To understand the history and evolution of habitability on Earth it is important to distinguish these hypotheses."

Horizontal genes

Fournier and his coworkers paired molecular clock and horizontal gene transfer to date the origins of cyanobacteria, oxygenic photosynthesis. This independent method doesn't rely on fossils nor rate assumptions.

Normaly, an organism inherits a gene from its parent "vertically". Rarely, a gene may also be transferred from one species to another distantly related species. One cell might eat another and incorporate new genes into its DNA.

It is clear from such a horizontal gene transfer history that the organisms that have acquired the gene are evolutionarily younger then the original group. Fournier argued that these instances could be used for determining the relative ages of certain bacterial groups. These ages could then be compared to the predicted ages of various molecular clocks. The closest model would be most accurate and could be used to accurately estimate the age of other bacteria species, specifically cyanobacteria.

The team used this logic to search for horizontal gene transfer between the genomes of thousands upon thousands of bacterial species. To calibrate fossil cyanobacteria more accurately, they used modern cyanobacteria cultures taken by Bosak & Moore. They found 34 instances of horizontal gene transfers. The team found that six of the six molecular clock models matched the relative ages in their horizontal gene transfer analysis.

Fournier used this model to calculate the age of the "crown group" of cyanobacteria. This includes all species that are known to have oxygenic photosynthesis. The Archean eon was when the crown group formed. Cyanobacteria, as a whole, branched from other bacteria around 3.4 billion years ago. This strongly suggests that oxygenic photosynthetic was already taking place 500 million years ago, and that cyanobacteria had been producing oxygen for a very long time before it accumulated.

Analyses also showed that cyanobacteria experienced rapid diversification just before the GOE (around 2.4 billion years ago). This suggests that the Earth may have been pushed into the GOE by rapid growth of cyanobacteria, which could have also launched oxygen into the atmosphere.

Fournier will use horizontal gene transfer to identify the origins of other species, beyond cyanobacteria.

Fournier states, "This work shows how molecular clocks incorporating horizontal genes transfers (HGTs), promise to reliably provide ages for groups across the entire tree-of-life, even for ancient microbes which have not left any fossil record something that was previously impossible."

This research was partially supported by the Simons Foundation, and the National Science Foundation.

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More information: The Archean Origin of Oxygenic Photosynthesis and Extant Cyanobacterial Lineages, Proceedings of the Royal Society B, rspb.royalsocietypublishing.or .1098/rspb.2021.0675 Journal information: Proceedings of the Royal Society B The Archean Origin of Oxygenic Photosynthesis and Extant Cyanobacterial Lineages,