2024-01-06 06:15:00
Oxygen. Without it, organisms of more than one cell would not have emerged on Earth. No plants, no animals, no complex life. 2.4 billion years ago, what is more correctly called dioxygen (O₂, the one we breathe), spread into the atmosphere. This episode is called the Great Oxygenation Event.
What caused it? We believe that cyanobacteria, so called because of their bluish hues, played a large part in releasing the molecule which is essential to us through photosynthesis. But what were those that led to this primordial event and to which we owe our existence? Did cyanobacteria already exist with internal structures dedicated to this task? “It’s vague,” admits Emmanuelle Javaux.
With several of her colleagues, this biologist from the University of Liège (Belgium) nevertheless offers us, today, a big leap in time in the quest for our origins. In an article published Wednesday January 3 by the journal Naturethe team presents a cyanobacteria fossil dating back 1.75 billion years and equipped with thylakoids, complex membranes which are the matrix of oxygenic photosynthesis, of which it becomes the oldest direct trace.
A “major breakthrough”
“We push back the age of these fossil thylakoids by 1.2 billion years. This makes it possible to give a minimum age to the cyanobacteria which have these membranes,” rejoices the researcher. It is now a fact: they appeared more than 1.75 billion years ago. As this still does not tell us whether or not they were present before the Great Oxygenation Event, the team from the University of Liège is looking for older fossils.
For Éric Maréchal, this work nonetheless represents a “major advance”: “These authors have established methods that make it possible to search for thylakoids inside fossilized cells in other rocks, which could allow future discoveries,” enthuses this research director at the CNRS.
“Searching for this type of structure takes time and is not easy,” observes Emmanuelle Javaux. The microfossil in question was discovered in clay rocks collected in Australia. “The cells are preserved, compressed in mud that has become rock, namely clay shales. With the weight of the sediments, there was a little transformation but not too much, which allowed exceptional preservation of these membranes in the fossil cells,” continues the scientist.
The disrupted atmosphere until today
Where to look now? On Earth, rocks have been greatly transformed by plate tectonics and traces of ancient life have often been damaged or even erased. “We are going to target very ancient places where there was no formation of mountain ranges, for example,” explains Emmanuelle Javaux. Areas that can be found in Scotland, South Africa, China, Russia or India…
That the oxygen we breathe was produced very early thanks to the famous membranes is an idea seriously studied. In the past, “proto-cyanobacteria produced so much O₂ that the atmosphere was modified until today,” notes Éric Maréchal, who recalls that the molecule represents around 20%.
To explain the scale of the Great Oxygenation Event, the director of the Laboratory of Cellular & Plant Physiology, in Grenoble (Isère), formulated last year the hypothesis of a “multiplier element”, citing these thylakoids as the most likely option. “To date, we do not have fossils allowing us to evaluate this possibility, or in any case to date the appearance of thylakoids inside archaic proto-cyanobacteria. » That’s it.
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