(CN) — One major event in Earth’s history paved the way for the existence of life as we know it: oxygenic photosynthesis. By converting light and water into energy, cyanobacteria released oxygen as a byproduct, leading to the Great Oxidation Event which opened the door for every organism that followed.
If, assuming for a moment, life does not exist outside Earth’s immediate vicinity, one could make a case that oxygenic photosynthesis is the most significant development since the Big Bang — a deus ex machina event heralding the start of something entirely unforeseeable.
In a study published Tuesday in the journal Proceedings of the Royal Society B, a team of MIT scientists describe a novel gene analysis technique they developed which pairs molecular clock dating with horizontal gene transfer to arrive at an improved estimate for the origins of cyanobacteria and oxygenic photosynthesis.
The researchers successfully traced back all modern cyanobacteria to a common ancestor living around 2.9 billion years ago, which itself branched off from other bacteria species around 3.4 billion years ago. During the intervening half-billion years, the cyanobacteria evolved the ability for oxygenic photosynthesis. The question is: Did the Great Oxidation Event occur shortly thereafter, or was the accumulation of oxygen in Earth’s atmosphere a prolonged event?
“It is important to nail down this timeline, because an oxygenated atmosphere is a major indicator of planetary habitability, and how large a biosphere can be,” said lead author Greg Fournier, associate professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences, in an email. “Detecting such a large gap between the origin of oxygenic photosynthesis and the accumulation of oxygen therefore tells us that oxygen production, in itself, is not sufficient to tell us about habitability. We also need to know where that oxygen goes. Is it locally consumed by other organisms? Is it absorbed by other minerals and dissolved compounds? Or were the first cyanobacteria so rare that they did not produce a lot of oxygen to begin with? Having a timeline is the first step to answering all of these questions.”
Molecular clock dating is often used to trace genetic changes in an organism back through time, but it has its drawbacks. The method relies on ancient fossils and rate models which can produce a diverse range of estimates based on the assumed rate of fossil concentrations used by researchers.
Due to differing data sets and rates used, previous estimates vary wildly as to when oxygenic photosynthesis first began on Earth. Some studies claim it began with a whimper, causing oxygen to slowly accumulate in the biosphere. Others say it kicked off with a bang, almost immediately triggering the Great Oxidation Event, wherein Earth’s lower atmosphere filled with breathable air.
“In order for us to understand the history of habitability on Earth, it’s important for us to distinguish between these hypotheses,” Fournier said in a statement accompanying the study. The authors of this study believe their estimates are more precise thanks to the inclusion of horizontal gene transfer data, which relies far less on fossils and uncertain rates.
Horizontal gene transfer typically occurs when an organism inherits a gene from another organism, sometimes distantly related, rather than directly from a parent. By tracing back these gene transfers, researchers can determine who the elder organisms are, and map out a type of family tree accordingly. Fournier and his colleagues found this process could be used to assess age gaps between groups of bacteria, which could then be compared against various molecular clock model predictions to determine the most accurate model. The winning model is then used to more accurately predict the age of bacteria species going forward.
During the course of their research the authors studied the genomes of thousands of species of bacteria and identified 34 instances of horizontal gene transfer. They compared their results with existing molecular clock models and found that one in six models correctly predicted the relative ages for a key group of cyanobacteria. That group turned out to be around 2.9 billion years old, having branched out from other bacteria species around 3.4 billion years ago, suggesting that oxygenic photosynthesis was occurring for 500 million years before the Great Oxidation Event.
“In performing this work, we also recovered several other interesting ages,” Fournier said in the email. “We see the last common ancestor of all extant bacteria diversifying around 3.7 billion years ago. We see major groups of cyanobacteria diversifying at the time of the [Great Oxidation Event], suggesting that this is a likely cause of that event; and, we see the most abundant marine cyanobacteria, Synechococcus and Prochlorococcus, appearing following the major 'Snowball Earth' event 715-630 million years ago, suggesting this event likely caused a mass extinction of marine algae.”
Follow @dmanduff
Subscribe to Closing Arguments
Sign up for new weekly newsletter Closing Arguments to get the latest about ongoing trials, major litigation and hot cases and rulings in courthouses around the U.S. and the world.
