A News & Views essay in Nature addresses a very significant question. The authors write: "Biologists agree that cyanobacteria invented the art of making oxygen, but when and how this came about remain uncertain." The measure of the problem is here: "Oxygenetic photosynthesis involves about 100 proteins that are highly ordered within the photosynthetic membranes of the cell. The main players are two molecular machines, photosystem I and photosystem II, that act as electrochemical solar cells. With the help of chlorophyll (...), they transform sunlight into electrical current."
Did the precursurs specialise in anoxygenic photosynthesis? There are bacteria that do this, but none have the two photosystems: it is a case of either photosystem I or photosystem II. "No tree of bacterial life can readily account for the observed distributions of the two sets of photosystem genes among the species. This has left biologists with little alternative but to suggest that genes encoding the photosystems have moved across species boundaries during evolution, a process called lateral gene transfer."
With the field apparently wide open for hypotheses, the authors comment approvingly on a recent paper by Mulkidjanian and colleagues suggesting that both photosystems arose in "a precursor of cyanobacteria - a 'protocyanobacterium' - later to be exported to other lineages by lateral transfer." The protocyanobacterium is hypothesised to have a regulatory switch, allowing it to go from photosystem I to photosystem II depending on the environment. The model appears to be one of degeneration: the anoxygenic photosynthesisers were derived by genetic system loss from the precursor.
The cyanobacteria also are associated with genetic loss: in their case the switch. "It would have been only a small step away from the cyanobacterial state of oxygenic photosynthesis provided that it underwent the right mutation - disabling the regulatory switch - and provided that this happened in the right environmental setting and at the right time."
The authors say: "The best evidence for this evolutionary scheme would be the discovery of a modern-day protocyanobacterium." The Editor's summary refers to this hypothetical organism as a "missing link".
However, instead of a gradual increase in complexity (which is most people's perception of a "missing link"), the authors favour a scenario that starts with all the complex machinery and changes thereafter either involve genetic loss or some "fine-tuning" of the existing system. This illustrates the problems of evolving irreducible complexity: if you want to produce an IC system like photosynthesis, you must first start with an IC system! Complexity has to be front loaded. Neo-darwinism cannot deliver this. Intelligent design can.
Out of thin air
John F. Allen and William Martin
Nature 445, 610-612 (8 February 2007) | doi:10.1038/445610a
The invention of oxygenic photosynthesis was a small step for a bacterium, but a giant leap for biology and geochemistry. So when and how did cells first learn to split water to make oxygen gas?
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