How can evolution explain body symmetry?

Hello everyone,
Been lurking around this forum for a day or so and thought I might leap gamely into this conversation. Hi.I should probably make my position clear from the outset; I am an Evolutionary Psychologist; I study human behaviour from an evolutionary standpoint. I am a scientist, as are all Ev Psych'ers, and I am an atheist. I have no issue at all with scientists who are not atheists. I personally find the principles of evolution incompatible with belief in a deity or deities but that is my own position, reached after careful thought, and I have no illusions that it is a position that is or should be shared by everyone. Having said that, I have engaged, from time to time, in lively debate with creationists and have discovered that scientists and creationists occupy totally incompatible platforms. There is no common language in which to conduct a discussion; science is based on reason, logic and rationality, while creationism (by it's own admission)is based on faith, not reason. Neither group is capable of convincing the other... but that doesn't mean we can't have some fun trying.So... to the subject at hand. Symmetry.CrackerJack seems to have gone walkabout recently, which is a shame, but just in case he's still about (or if anyone else feels like taking up his position); I am a little confused by his argument. Could you explain exactly WHY you would expect terrestrial animals to have evolved away from a symmetrical bodyplan? As far as I can see, you base this on the idea that in the amount of time since we left the sea SURELY some deviation from symmetry should have evolved. Could you explain this position?Secondly, regarding the Gangestad and Thornhill scent of symmetry article (back in message 49), CrackerJack is right in principle. The subjects could indeed be detecting one of a number of things. Healthy people have been shown, in seperate studies, to smell nicer. Men prefer the scent of ovulating women too. However, the study referred to is the result of rigorous scientific analysis in which other effects were controlled for. That women prefer the scent of symmetrical men (especially when at their most fertile) is a statistically significant effect.

Indeed. Many studies have linked symmetry with the quality of an animal's immune system. If bilateral symmetry is the fundamental body plan, as you suggest, it can be referred to as the "target phenotype". Most of the factors that can subvert the body's attempt to produce this target occur during development of the foetus. A developing foetus is under constant stress from environmental factors, including parasites, pathogens and the mother's own immune syste. These factors are called "developmental noise". Individuals who can withstand developmental noise and develop the target phenotype are adept at defending themselves from developmental noise, presumably due to their immune system. Deviation from the target phenotype, then, becomes a marker for developmental instability and poor immunocompetence. This, presumably, is the basis for symmetrical individuals being more attractive.I also see no reason why symmetry should be related to an aquatic environment. I agree that it seems more efficient to have a symmetrical organism that would require the doubling of a single set of genes to construct rather than one set for each side of the body. Our earliest multicellular ancestors were probably symmetrical. There has been no reason to abandon it. I believe the flatfish, that was posted earlier, developed from a symmetrical ancestor.