While some bacteria employed the precursor of photosystem I, and others used the precursor of photosystem II, there may also have been bacteria that possessed the genetic coding necessary to build both photosystems. This would allow them to switch between them, depending on environmental conditions and the availability of food. This is a relatively common thing for bacteria to do today; their genes can be switched on and off, allowing them to make hay while the sun shines – or at least, in this case, to use sunshine to make sugar or ATP, depending on whether the imperative is to reproduce or simply to survive. The possibility of an ingenious evolutionary adaptation now presents itself. What if it were possible to run these two machines at once, connecting the electron circuit from photosystem II into photosystem I, which would dutifully dispose of the cascade of electrons by pushing them onto carbon dioxide to form sugar? This would confer a great advantage on the organism in question, allowing it to make both food and ATP at the same time using sunlight as an energy source. This is certainly a plausible explanation for the separate evolution and then recombination of the two photosystems, but it leaves one remaining question: where does this machinery get its electrons? Here is where the Oxygen Evolving Complex enters the story and, with it, one of the most important evolutionary steps in the history of life on Earth.
The Oxygen Evolving Complex is an odd structure: more mineral than biological. It consists of four manganese atoms and a single calcium atom, held together in a lattice of oxygen. Manganese is locked away in vast mineral deposits on the ocean floor today, but in the early history of our oceans it would have been available in seawater for organisms to use. Bacteria use manganese to protect them from UV light, in much the same way as we use melanin – manganese is easily ‘photo-oxidised’, absorbing the potentially harmful UV photon and releasing an electron in the process. This may have been one of the ways in which electrons made their way into the primitive photosystem II in early bacteria. So manganese, at least, was already an important component of living things from the earliest of times. Today, manganese performs a different task. It sits at the heart of the Oxygen Evolving Complex, whose job is to grab water molecules and hold them ready for electrons to be ripped off and used as input into photosystem II. As a result, water molecules are split apart and, just as in the electrolysis of water so beloved of Mr Bell (see here), oxygen is released as a gas.
Bacteria genes can be switched on and off, allowing them to make hay while the Sun shines – or at least to use sunshine to make sugar or ATP.
This theory is a piece of cutting-edge research. The structure of the Oxygen Evolving Complex was determined only in 2006, and it is only in the last few years that the locations of each of the 46,630 atoms in photosystem II have been mapped. There are therefore many details in this story yet to be uncovered, but the broad sweep we have outlined here is certainly a strong candidate for an explanation of how the complexity of the Z scheme arose.
There is one last quite wonderful sting in the tail of this story, however, and it is something we know for certain: oxygenic photosynthesis evolved only once.
The evidence for this rather definite statement is clear when we look down a microscope at the structures inside plants and algae that carry our photosynthesis. They are called chloroplasts, and they are all self-evidently related to each other because they are so similar. But there is more than this, because they look for all the world as though they were cyanobacteria living inside the leaves, just like those found today in the blooms on Lake Atitlan. This is because that is exactly what they are. They even maintain their own independent rings of DNA, just as free-living bacteria do today.
But how does one cell end up inside another? At some point in the history of life on Earth, a cyanobacterium cell must have been engulfed by another cell and, instead of being digested, it survived to perform a useful purpose. This process, called endosymbiosis, has happened more than once in the history of life on Earth; indeed, it is thought to have been fundamental in the evolution of complex life. Endosymbiosis allows for great leaps in the capability of living things – a merger of fully formed skills to produce a result greater than the sum of the parts. In the case of oxygenic photosynthesis, this particular example of endosymbiosis led to the evolution of two of the great kingdoms of life – the algae and the plants – by allowing machinery evolved over billions of years inside cyanobacteria to be co-opted into more complex multi-cellular organisms.
A coloured electron micrograph of a leaf of Zinnia elegans, showing chloroplasts (green), starch granules (pink), the nucleus (red), and a large vacuole (white). The large air spaces allow for gas exchange during photosynthesis.
This coloured electron micrograph shows two chloroplasts in the leaf of a pea plant (Pisum sativum). Chloroplasts convert light and carbon dioxide into carbohydrates.
The quite dizzying conclusion is that, because everything that carries out oxygenic photosynthesis today does so in precisely the same way, we owe the beauty of life on Earth – with its hues, colours and seemingly limitless diversity – to a cyanobacterium whose ancestors, somehow, found their way inside another cell. The descendants of that cell are still present on Earth today, inside every leaf, every blade of grass and every algal bloom, and they have filled our atmosphere with oxygen.
A coloured electron micrograph of the inside of a chloroplast’s thylakoid membrane, containing the green pigment chlorophyll.
As levels of atmospheric oxygen rose, the Earth began to rust. Evidence of this rusting can be seen at Rockham beach, North Devon, where deposits of iron oxide appear as orange patches.
For almost half of Earth’s history, one of the most important ingredients for complex life was absent from the Earth’s atmosphere. Oxygen is an unstable, reactive gas that must be constantly replenished. The first rush of oxygen released from water by the cyanobacteria did what oxygen does best, reacting with the myriad elements present on Earth’s primordial surface to form oxides. In our planet’s infancy, large amounts of iron could be found in the oceans and, to a lesser extent, on land. Left over from the Earth’s formation, this dissolved iron remained stable for billions of years, but as the levels of atmospheric oxygen began to rise, a very familiar reaction began to take place. The Earth began to rust. Today, across the planet the evidence of this global rusting can be found in deposits of iron oxides known as banded iron formations.
Oxygenic photosynthesis doesn’t automatically fill the atmosphere with oxygen, however. It is necessary, but not sufficient, because both rusting and respiration act to undo all the good works of the plants, algae and cyanobacteria, and remove oxygen from the atmosphere. While photosynthesis takes carbon dioxide out of the atmosphere and turns it into organic matter, aerobic respiration takes organic matter and burns it using oxygen, releasing carbon dioxide and water. These processes will naturally reach a balance, which is why oxygen levels today have been stable at around 21 per cent for many millions of years. In order to change oxygen levels, something has to happen. It is known that oxygen levels first increased on Earth around 2.4 billion years ago, a time when many of the great banded iron formations were laid down. This rise may have been triggered by the complete oxidation of the Earth’s iron and other elements, which until that time acted