Evidence for the complex evolution of oxygenic photosynthesis is strong
Other examples of irreducible complexity abound, including…photosynthesis…. DBB. 160
I wrote a post discussing the importance of evidence for actual occurrence vs. mere possibility here, in part to lead into the evidence for the evolution of oxygenic (oxygen-releasing) photosynthesis. There is firm evidence for only one case of evolution of oxygenic photosynthesis, which uses both photosystem I (PSI) and photosystem II (PSII), and all of the evidence points to simple duplication of the photosynthetic genes of one photosystem (PSI-type, most think now) leading to the evolution of another photosystem (PSII-type), which was later coupled to produce oxygenic photosynthesis. At some point after that, algae and plants took in cyanobacteria in primary, secondary, or perhaps even tertiary and above, endosymbioses, to evolve plastids in which their own photosynthesis occurs.
I won’t bother trying to explain how the two photosystems interact in plants, algae, and cyanobacteria, since the process is rather involved and is explained in many places. For anyone wanting those basics, here is a diagram of the electron flow between the two photosystems, and here is a site that tells more about photosynthesis than most people wish to know. I am interested in the powerful evidence that oxygenic evolution evolved in part by the duplications that Behe continually claims are inadequate (and of course they are, by themselves) for evolution, and also in some of the details that Behe likes to demand–though I am rather less interested in the latter because these are not nearly so important to the question of whether or not oxygenic photosynthesis did evolve.
First off, it is interesting that the distribution of the PSI-type and PSII-type photosystems do not seem to follow any consistent “tree” in the various bacterial lines. Some modern bacteria utilize PSI-type photosynthesis, others the PSII-type, and, aside from the cyanobacteria, their photosynthesis is anoxygenic. But of course it is not very surprising if their photosynthetic systems do not produce a “tree of bacterial life” pattern, since bacteria rarely do have marks of consistent “vertical transmission” patterns, such as we mostly observe in eukaryotes, because the bacteria exchange a great deal of genetic material with species not closely related to themselves (Allen, John F., & Martin, William (2007). “Out of thin air.” Nature, 445, 610-612).
That the two photosystems are related (paralogous, in this instance) is clear from the evidence, however, and gene duplication is responsible:
Another striking example of gene duplication is found with the gene for the PSI-like RC core polypeptide. H. mobilis and C. limicola use a single RC core polypeptide (PshA and PscA, respectively) to form a homodimeric PSI-type RC complex. In contrast, oxygenic species synthesize a heterodimeric PSI RC with two highly related polypeptides, PsaA and PsaB. A composite phylogenetic tree (3E) of the PSI RC core polypeptides from a broad representation of oxygenic lineages with PscA of C. limicola as root indicates that PsaA and PsaB are divided into two distinct clades and are derived from an ancient gene duplication from a PscA/PshA-like common ancestor. Thus, the RC gene duplication seems to have occurred well before diversification of all oxygenic lineages. Interestingly, PSI RC polypeptides also share significant similarity to CP47 (PsbB) and CP43 (2), both of which are core antenna polypeptides of PSII. Our phylogenetic analysis (3E) indicates that these genes have undergone gene duplication from a pshA-like common ancestor that preceded the divergence of all oxygenic lineages.
In other words, oxygenic photosynthesis (in cyanobacteria, etc.) uses polypeptides in both its PSI and its PSII systems which phylogenetic analysis (see the references, to which I linked, or just use this to see the results of their phylogenetic analysis) indicates came from genetic duplication events.
I do not include these technical details because I think people here should pay close attention to them, but so that it is clear that peer-reviewed analysis is behind the assertions that gene duplications produced PSII from PSI.
Another source using somewhat different evidence states:
The arrangement of these helices [in photosystem II] is remarkably similar to that of the helices in the reaction centres of purple bacteria and of plant photosystem I, indicating a common evolutionary origin for these assemblies. “Three-dimensional structure of the plant photosystem II reaction centre at 8 Å resolution”
And a more recent source states:
Like others, they suggest that photosystem I was the ancestral prototype, from which an evolutionary precursor of photosystem II arose. The reaction-centre cores of the photosystems are similar in stucture, and their divergence probably began with a simple duplication of the associated gene cluster. Allen, John F., & Martin, William (2007). “Out of thin air.” Nature, 445, 610-612
There seems to be little reason to belabor this particular point further. If we simply pay attention to the evidence of what actually happened, normal observable processes gave bacteria the two photosystems, namely, gene duplications caused two photosystems to appear where there had been one. As appears to have often been the case, almost certainly many or all of the genes for the PSI-type system would have been duplicated in one generation, producing a second copy.
Behe demands the details of this 2.3 billion year old event–which left only bits and pieces of information behind–because he knows that the firm details are in many cases impossible to obtain. Well, that hardly matters, yet the details are of interest to scientists, so speculation and plausible scenarios are discussed. A good source for possible pathways of subsequent evolution of photosynthesis is the one used in the last quote, “Out of thin air.”
One supposes that the duplicate photosystem might have supplied redundancy to the bacterium for a while, perhaps also boosting peak photosynthetic capacity of the cell. Later, however, one would likely change to fit certain situations better than other environmental conditions. Eventually it would become like the authors of “Out of thin air” state, the bacterium would switch between photosystems to fit the conditions. Modern bacteria do this to some extent, as related here. According to some researchers, this putative bacterium which evolved to have two photosystems was in a now-extinct group, which laterally transferred photosynthesis genes to other groups, many of which ended up with only one system, while cyanobacteria obtained both photosystems. (Ibid.)
What is interesting in the article is how relatively smoothly the evolution of the highly coordinated oxygenic system might have evolved in an organism containing both systems. There is reason to believe that, in an environment enriched in manganese, ultraviolet light may well have jammed proto-photosystem II with electrons, meaning that it would have to shut down. Photosystem I (which paradoxically is second in line to photosystem II in the electron chain) would then have to be switched on, and when that happened it would be able (after a bit of evolutionary tinkering, at least) to take the electrons from photosystem II, relieving the jam. If the electron-jamming were too frequent, pressure for both photosystems to work concurrently, rather than successively, would exist, and then both photosystems would be linked and would readily evolve further adaptations to coordinate electron transfer properly (Ibid.).
Those are reasonable possibilities, unlike the mere supposition that a pluripotent, yet oddly constrained (very oddly, because the constraints appear similar to those of unguided evolution), “designer” steps in to “help evolution along.” No doubt Behe would not give much credit to such speculation, for the little that is worth. What is important to science having any kind of integrity, however, is that it builds upon the evidence of what actually occurred, and that is exactly what speculative hypotheses like the one sketched out above does. For we know that duplications set off the evolution of the two photosystems needed to split water (splitting water takes too much energy for any single photosystem that we find in life, but, fortunately, other molecules like hydrogen sulfide can be and are split by a single photosystem), and the question is simply how well we can narrow down the possible ways in which these evolutionary events may have happened.
I do not know for sure whether Behe would deny that oxygenic photosynthesis could evolve from anoxygenic photosynthesis in his DBB days. It would seem so, though, especially since he failed even to credit duplications as “causes” for the clotting cascade, as I discussed here. The fact is that this goes along well with discussions of the evolution of transport and of gene transfers with respect to chloroplasts and P. falciparum‘s apicoplast, for the documented evolution of photosynthesis (and of non-photosynthetic plastids descending from chloroplasts) from a single photosystem involves a great deal of complexity. Behe did well to ignore these biochemical pathways, for the evidence that these indeed evolved would be hard to ignore. I am sure that he would have ignored most or all of that evidence, however. The fact is that I will not be constrained to discuss only the pathways he prefers, since evolution is a whole, with none of the expected breaks coming from “designed interventions” being even slightly visible.
Furthermore, photosynthesis almost certainly has some role in the “Cambrian explosion,” which event Behe, like other creationists, likes to throw out there as a “problem,” without bothering to discuss the details of the selective pressures that might be behind it–like a dramatic rise in oxygen levels. I intend to get into that issue relatively soon.
This is part of a series of posts that I am combining into one long post, which may be found at Darwin’s Black Box.