Photosynthesis facilitated the “Cambrian explosion”

With the discovery of the biological Big Bang [Cambrian radiation], however, the window of time for life to go from simple to complex has shrunk to much less than nineteenth-century estimates of the earth’s age. DBB 28

First, one should note that in the quote above that Behe does not use the already hyperbolic “Cambrian explosion” to designate the rapid radiation occurring at the start of the Cambrian, he uses an even more hyperventilating term for what is in fact a considerable period for evolution (rather more than the time in which humans evolved from walking apes), “the biological Big Bang.”

Secondly, of course, he’d need to show that the animals that gave rise to “complex life” were not themselves complex, if not large. This is quite unlikely, because there seems to have been no vast genetic change at the start of the Cambrian. Indeed, the preceding Ediacaran fauna were evidently quite complex, though probably were less so than the Cambrian fauna.

And third, while the causes of the radiation of the Cambrian fauna remains controversial, the previously-discussed complex evolution of photosynthesis almost certainly played a role in it, by releasing oxygen to the atmosphere. As one highly credible source states:

Geochemical and physical evidence suggests that a stepwise increase in oxygen occurred around 1.1 to 0.54 billion years ago (Ga) and was a necessary precondition to support the physiological needs of large metazoans. Martin Kennedy, et al. “Late Precambrian Oxygenation; Inception of the Clay Mineral Factory.” 10 March 2006 Science 311:1446-1449

Would it be at all suprising if, in the low oxygen conditions of the later Precambrian, the ability to produce complex body types for the sake of advantage in predation would occur? And then, when oxygen levels rose, the very ability to develop larger bodies becomes possible for the first time, and a predatory arms race leads to an “explosive” evolution of body types, armor, and the like. This seems to be virtually certain, given the extreme conservatism in development across the phyla, so that virtually the same Hox genes pattern insect bodies as pattern human bodies.

This does not in the least mean that rising oxygen levels were the “trigger” for the “Cambrian explosion.” Some have argued that the Ediacaran fauna were large, hence, oxygen levels apparently were not holding those animals back evolutionarily. However, that does not seem persuasive, for it seems that Edicaran fauna were relatively flat in shape (presenting a relatively large surface area to the water), and may have had relatively low metabolic rates in any case. More important than that, though, is that Ediacaran fauna were only slightly earlier (on the geological scale), and there is recent evidence that the extinction of the Ediacaran fauna may have led to the rise of the Cambrian fauna. The same paper which provides evidence of a likely extinction event also credits the rise of Ediacaran fauna to oxygenation (due to photosynthesis):

Increasing oxygenation of the upper ocean and atmosphere following the Marinoan glaciation is thought to have triggered the development o fthe Ediacaran fauna, and to have caused destabilization of the oceanic chemocline. As a consequence, upwelling euxinic bottom water is likely to have rapidly poisoned the Edicaran fauna. Martin Wille, et al. “Hyrogen sulphide release to surface waters at the Precambrian/Cambrian boundary.” 5 June 2008 Nature 453:767-769.

The upshot being that increased oxygen levels appear to have allowed Ediacaran fauna to evolve, while hydrogen sulfide upwellings to the surface waters ended up killing them off, at least according their interpretation of the molybdenum isotope record discussed in the aforementioned paper. Then the photosynthetically-caused oxygen levels were able to support the evolution of Cambrian fauna, after the Ediacaran extinction.

The fact is that the entire scenario of the evolution of photosynthesis is what happens to explain (in part) the “explosive” evolutions of both the Ediacaran and the Cambrian fauna, insofar as we have any kind of explanation worthy of the name. We have the DNA evidence of gene doublings being causal in the evolution of oxygenic photosynthesis, which led to the endosymbiosis and some rather complex evolution of eukaryotic photosynthesis, which led in time to rising oxygen levels which then supported not one, but two, evolutions of metazoan organisms.

Above all, perhaps, we have the use of human intelligence where we are pursuing identifiable causes. Not that obscuring and ignoring science, in the manner that Behe does, is completely devoid of intelligence, yet it is not a very impressive kind of intelligence used in that endeavor. Evolution setting the stage for further evolutionary development is the fascinating story of deep time, far more interesting and useful than any “poof” tale that Behe can conjure up.

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.

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