It is no exaggeration to say that the most important event on Earth is the evolution of photosynthesis. The ability to harvest energy from light freed life from the need to harvest energy from the environment. This new ability allowed life to grow in complexity, invade new environments, and ultimately reshape Earth.
For such a pivotal event, we know surprisingly little about it. Tracking the presence of oxygen in the atmosphere suggests that photosynthesis evolved at least 2.4 billion years ago, but increasing oxygen levels has turned out to be surprisingly complex. Tracing current genetic mutations, the origins of photosynthesis date back approximately 3 billion years. The timing is similar to the origin of photosynthetic cyanobacteria, both of which continue to live independently and are incorporated into plant cells as chloroplasts.
What we don’t have is clear evidence of photosynthetic cells of similar age. Although some microfossils resembling cyanobacteria have been identified, it is impossible to determine whether they made proteins that facilitate photosynthesis. Now, new fossils described by a team from the University of Liege extend unequivocal evidence of photosynthesis from more than a billion years ago to 1.7 billion years ago.
What is a thylakoid?
This research relies on the identification of a structure called . thylakoid membrane. These are disc-shaped stacks of membranes that increase the surface area within the cell that serves as a host for photosynthetic protein complexes. Thylakoid membranes are present in the chloroplasts of plant cells, although not all modern cyanobacteria have thylakoid membranes.
To search for thylakoids, the researchers obtained tiny cell-like bodies from sedimentary rocks at several locations. They made ultra-thin sections of these rocks and performed electron microscopy to reveal some of the details inside the cells. This makes it possible to detect features just a few tens of nanometers in diameter.
Two of the sites had cells with multilayered inner membranes typical of thylakoids. These are the McDermott Formation of Australia and the Grassy Bay Formation of the Canadian Arctic. The latter is more than a billion years old, making it much older than previous evidence of thylakoids. However, the McDermott Formation is more than 1.7 billion years old, meaning that fossil evidence of these structures dates back 1.2 billion years earlier than that.
At the same time, cyanobacteria fossils found in the Democratic Republic of the Congo that are thought to be 1 billion years old show no trace of thylakoid membranes. As mentioned above, it appears that these lineages have been separated for quite some time, as species of cyanobacteria that do not have these structures still exist today.
go back to the past
While important in their own right, most of the findings are important in their own way. Molecular data suggest that the split between her two groups of cyanobacteria, those with and without thylakoids, goes back even further. There are also some proposals that the evolution of thylakoid membranes provided the photosynthetic boost needed to trigger the Great Oxygenation Event, the first time that atmospheric oxygen levels rose significantly.
By showing that it is possible to identify thylakoid membranes despite their vast ages, the researchers behind this study provide strong evidence that they confirm the existence of thylakoid membranes before and after important evolutionary events. This will provide a great driving force. Fossil evidence may eventually catch up with genetic and chemical evidence when it comes to the evolution of photosynthesis.
Nature, 2024. DOI: 10.1038/s41586-023-06896-7 (About DOI).