How can evolution explain body symmetry?

Interesting idea, but in need of major revision, I think.First, it would seem to be that the position of the stomach relative to the mouth would be more important, since we aren't significantly assisted by gravity for passage of (ex)food from our stomach to our anus - in fact a significant portion of our GI tract is orientated so that the fecal matter is moving against gravity (hence the "ascending colon"). Also, we retain the ability to pass food from mouth-to-stomach standing on our heads.Also, this rule assumes that organisms maintain a standard orientation relative to gravity. There are plenty of organisms that have no such orientation - worms, many insects and aquatic creatures.Additionally, there are many organisms that spend feeding time with their stomachs or anuses higher than their mouths - many bats feed while hanging upside down.Perhaps a more testable (and less general) idea is that organisms' potential esophageal peristaltic force will correlate in a meaningful way with their standard mouth-stomach orientation during feeding. In other words, a bat should have greater peristaltic force than a rat.I think this sort of analysis would test your essential concept of evolution of gastrointestinal characters relative to orientation to gravity - definitely much better than counting the heights of mouths and anuses...

Sorry - I thought you were arguing that gravity aided the passage of food from mouth to anus; now I'm not sure what you mean. Perhaps that gravity aids food falling into our mouths and feces falling out of our asses? Then all that should matter is that most mouths/inputs face upwards, and most anuses/outputs face downwards, which is not the case. This is quite off - without peristalsis the GI tract fills and expands until it ruptures, killing the organism (as in Hirschsprung disease).Gravity has a significant effect on internal organs other objects/fluids within our bodies. It seems like you are stating that the opposite is true - "suspension by the body tissues", whatever that means, doesn't negate gravity.

Hi Mick-This is a fascinating example of asymmetrical development that I wish I would have remembered to bring up - though you haven't quite got the details correct if I remember correctly. The apparent purpose of the rotating cilia is not to circulate blood, but rather to move small signaling molecules, resulting in an asymmetrical gradient of 'signal'.The findings discussed are exciting because they explain how asymmetry can arise from a symmetrical biological environment: In the case you cite a triangular field of cells protrude cilia into a small fluid filled area; the cilia all rotate in the same direction. At this point the field of cells (including cell biology and gene expression) is uniformly symmetrical. However, the beating of the cilia causes small molecules to build up at one end of the fluid filled space, given the cilia force combined with the shape of that space. Signaling molecules concentrate at one side of the space, inducing gene expression and cellular biology changes in the cells at the side of the space. Thus, asymmetrical gene expression has arisen from a symmetrical biological structure. Further development can now build upon this initial asymmetry to produce the asymmetry of the adult organism.If nothing else, you've reiterated a very important point to this thread - all mammals are developmentally asymmetrical. This doesn't mean one eye is slightly lower than the other, but that entire organ systems must develop asymmetrically to allow basic function.When symmetry makes an organism more fit, it is selected for, as in most external aspects of the human body (testicles, for one, are an exception).When asymmetry makes an organism more fit, it is selected for, as in many internal organ systems of the body, including the circulatory and digestive systems.These results are exactly what evolution theory would predict. Thus the theory of evolution can explain both symmetry and asymmetry quite well.