One of the most important processes in all of biology is the evolution of photosynthesis. The process that is essential to all complex life on Earth dates to around 3.8 billion years ago. Through photosynthesis, some specific species of single-celled Cyanobacteria forever changed the atmosphere of the early stage of Planet Earth by producing oxygen on a massive scale. This is the single event that allowed a huge expansion in terms of what life was possible on the barren lands and oxygen-deprived ocean of Planet Earth.
Photosynthesis is the only process that actively sustains complex life on Planet Earth – all of the oxygen on this planet originate from this process. And based on this process, every form of life is designed to survive and thrive. According to scientists, there are two kinds of photosynthesis – oxygenic and anoxygenic.
Oxygenic photosynthesis uses light energy to split water molecules, releasing oxygen, electrons, and protons as by-products. Anoxygenic photosynthesis utilizes compounds like minerals or hydrogen sulfides like iron or arsenic, alternatively water, and this process does not produce oxygen.
Previously for a long time, the scientific world believed or theorized that the anoxygenic photosynthetic evolved long before oxygenic photosynthesis, and for this reason, Planet Earth’s atmosphere contained no oxygen until about 2.4 to 3 billion years ago. However, two new studies that were published in 2018 in two different journals suggest that the origin of the oxygenic photosynthesis process may have been as much as a billion years earlier, which indicates that the oxygenic photosynthesis process likely originated concurrently with anoxygenic photosynthesis around 3.4–3.8 billion years ago which also suggests complex life would have been able to evolve earlier too.
A lifelong interest
According to Tanai Cardona, a research associate in the Imperial College London’s department of life sciences and the lead author of both of the studies, 3.8 billion years ago, cellular life had already evolved and possibly diversified into bacteria and archaea. Around that time, ancient forms of photosynthesis had already developed in ancestral forms of bacteria. Achaea are microorganisms that are similar to bacteria but differ somewhat in their chemical structure.
Earlier in his childhood, Cardona was deeply interested in biology, and when he was an undergraduate at the University of Los Andes in Colombia, he first became fascinated by the photosynthesis process. And for more than a decade, he has been studying photosynthesis. As he began to become more interested in the evolution of photosynthesis, he realized that the scientific world does not really know a lot about it and that some of the concepts the scientific world thought already knew about the origin of photosynthesis did not really make perfect sense.
Dr. Cardona wanted to uncover exactly when oxygenic photosynthesis originated. Instead of trying to detect oxygen in ancient rocks in the traditional way, he observed deep inside the molecular machines that accomplish photosynthesis. According to his observations, these are complex enzymes called photosystems, and find out that oxygenic and anoxygenic photosynthesis both use a common enzyme called “Photosystem I”.
The core of that enzyme resembles difference in the two types of photosynthesis, and by investigating how long ago the genes evolved to be different, Dr. Cardona could discover when exactly oxidative photosynthesis first occurred.
Dr. Cardona found that the differences in the genes may have occurred more than 3.4 billion years ago, long before oxygen was thought to have first been produced on Planet Earth. This also means that Cyanobacteria were thought to be the first organisms to produce oxygen or existed long before. This indicates that there must have been predecessors, such as early ancestral bacteria, that have since evolved to carry out anoxygenic photosynthesis instead.
According to Dr. Cardona, this is the first time that any scientist has tried to identify the time of the evolution of the photosystems. The result of this research also solved one of the big controversies in the field of biology and indicated the possibility that oxygenic photosynthesis actually began at a very early stage in the evolutionary history of life.
Another finding that surprised Dr. Cardona was that the evolution of the photosystem was not linear. According to previous studies, photosystems are known to evolve very slowly; they have done so since Cyanobacteria appeared at least 2.4 billion years ago. But when Dr. Cardona utilized that slow rate of evolution to calculate the origin of photosynthesis, he ended up with a date that was older than the Planet Earth itself. This indicates that the photosystem must have evolved much faster at the beginning. This is now something recent research hints that were due to the planet being hotter.
A rare bacteria hint at the ancient origin of photosynthesis
In 2019, a group of researchers from the Imperial College London led by Dr. Cardona, analyzing structures inside an ancient type of rare bacteria, suggested that a key step in oxygenic photosynthesis may have already been possible a billion years before which was already demonstrated by Dr. Cardona in his previous research. According to Dr. Cardona, the structures inside this rare bacteria are a lot similar to those that power photosynthesis in Cyanobacteria and Plants. The data Dr. Cardona and his team obtain about the structure and functioning of early bacterial photosynthesis systems are not supporting the already established story about the evolution of photosynthesis.
The bacteria they examined, Heliobacterium Modesticaldum, is found around hot springs, waterlogged fields, and soils, where it operates anoxygenic photosynthesis. It is very distantly related to Cyanobacteria, the bacteria that perform oxygenic photosynthesis even today.
According to Dr. Cardona, it is so distantly related that it last had a ‘common ancestor’ with Cyanobacteria billions of years ago. This means that any traits the two bacteria share are likely to also have been present in the ancient bacteria that gave rise to them both.
By analyzing the structures that both Heliobacterium Modesticaldum and modern Cyanobacteria use to operate their different types of the photosynthesis process, Dr. Cardona discovered striking similarities. Both structures contain a common site that Cyanobacteria and Plants exclusively utilize to split water, the very first crucial step in oxygenic photosynthesis.
The evolution of Cyanobacteria is usually assumed to also be the first presentation of oxygenic photosynthesis. Still, the fact that Heliobacterium Modesticaldum contains a similar site means that the building blocks for oxygenic photosynthesis are likely much more ancient than previously thought, as old as photosynthesis itself. It, therefore could have occurred much earlier in Planet Earth’s history. This also suggests that oxygenic photosynthesis was not the product of a billion years of evolution from anoxygenic photosynthesis.
Dr. Cardona indicated that this result helps explain in fantastic detail why the systems responsible for photosynthesis and oxygen production are the way they are today, but for it to make sense, it needs a change of perspective in the way the scientific world views the evolution of photosynthesis.
He also expressed that there is still a lot the scientific world does not know about why life is the way it is now and how most biological processes originated; sometimes, the best-educated guesses do not even come close to representing what really happened so long ago, but he hopes these findings may help scientists who are looking for life on other planets answer some of their biggest questions.
- Cardona, T. & Rutherford, A. W. (2019). Evolution of Photochemical Reaction Centres: More Twists? Trends in Plant Science, [online] Volume, 24(11), p. 1008-1021. Available at: https://doi.org/10.1016/j.tplants.2019.06.016 [Accessed 27th September 2021].
- Cardona, T. (2018). Early Archean origin of heterodimeric Photosystem I. Heliyon, [online] Volume, 4(3), Available at: https://doi.org/10.1016/j.heliyon.2018.e00548 [Accessed 27th September 2021].
- Cardona, T., et al. (2017). Molecular evidence for the early evolution of photosynthetic water oxidation. bioRxiv, [online] Volume, 5, Available at: https://doi.org/10.1101/109447 [Accessed 27th September 2021].