Oxygen. Without them, no organisms with more than one cell would have emerged on Earth. No plants, no animals, no complex life. 2.4 billion years ago, what is actually called dioxygen (O₂, what we breathe) spread through the atmosphere. This episode is called the “Great Oxygenation Event.”
What triggered it? We believe that cyanobacteria, so called because of their bluish color, played a large role in releasing the molecule vital to our survival through photosynthesis. But what was it that led to this primal event and to which we owe our existence? Were there already cyanobacteria with internal structures dedicated to this task? “It’s vague,” admits Emmanuelle Javaux.
Today, together with several of her colleagues, the biologist from the University of Liège (Belgium) offers us a major leap in time in the search for our origins. In a paper published Wednesday, January 3, in the journal Nature, the team presents a cyanobacteria fossil that is 1.75 billion years old and is equipped with thylakoids, complex membranes that form the matrix of oxygen-containing photosynthesis form, of which it is the oldest direct trace.
A “big breakthrough”
“We are pushing back the age of these fossil thylakoids by 1.2 billion years. This makes it possible to give the cyanobacteria that have these membranes a minimum age,” says the researcher happily. It is now a fact: they appeared more than 1.75 billion years ago. Since this still doesn't tell us whether or not they were present before the Great Oxygenation Event, the team at the University of Liège is looking for older fossils.
For Éric Maréchal, this work still represents a “major advance”: “These authors have developed methods that make it possible to search for thylakoids in fossilized cells in other rocks, which could enable future discoveries,” enthuses the research director at the CNRS.
“The search for such a structure takes time and is not easy,” notes 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 shale. “The weight of the sediments caused a small but not too large transformation, which allowed an exceptional preservation of these membranes in the fossil cells,” the scientist continues.
The disturbed atmosphere to this day
Where to look now? On Earth, rocks have been greatly altered by plate tectonics and traces of ancient life have often been damaged or even erased. “For example, we will target very old places where there were no mountain ranges,” explains Emmanuelle Javaux. Areas found in Scotland, South Africa, China, Russia or India…
The fact that the oxygen we breathe was produced very early thanks to the famous membranes is an idea that is being seriously studied. “Proto-cyanobacteria produced so much O₂ in the past that it changed the atmosphere to this day,” notes Éric Maréchal, who recalls that the molecule accounts for about 20%.
To explain the magnitude of the major oxygenation event, the director of the Laboratory of Cell and Plant Physiology in Grenoble (Isère) last year formulated the hypothesis of a “multiplier element” and named these thylakoids as the most likely option. “To date, we do not have fossils that would allow us to evaluate this possibility or at least to date the appearance of thylakoids in archaic proto-cyanobacteria. ” That's it.