How can evolution explain body symmetry?

But the aero/hydro dynamics argument is environmental pressure. The environment selects for shapes that move through the air/water better. As CF pointed out with his plane/submarine it is an advantage to be symetrical and therefore it is selected for in living things.I think it's worth noting that you are actually quite asymetrical. The outer organs (legs arms, face etc) are, more or less, the same on both sides, but your inner organs are not in the least bit symetrical. Imagine the problems if we were to have a gut that ran straight down the middle of our body, or a heart with two identical sides. For relatively simple things like fruit flies, the inner workings of the body can be set out symetrically but for vertebrates like us it is a distinct advantage to to have things on one side or the other, and it is therefore selected for. This is a real pity, as the basis for symmetry/asymmetry is set out in the development of the embryo and is therefore vitally important for this discussion. I don't mean to be mean, or snipe at you in any way but I would really suggest you start to get at least a passing knowledge of such processes. If there are any questions you have about what Mick has said then feel free to ask them here. I'm sure Mick (or a number of others for that matter) will be only to happy to answer them.This message has been edited by Ooook!, 15-03-2005 09:58 AM

OK, let’s try and take a step back towards the embryo again .I’m going to stick my neck out here and say that it isn’t symmetry that is needed to be explained in multicellular organisms, it’s asymmetry. If you think about dividing cells in an embyro, symmetry is produced naturally without need for magic or ‘design’ — just by the process of cell division.When one cell divides into two identical cells, symmetry is produced. Another plane of symmetry is formed when two cells become four and so on. Something different has to happen in order for asymmetry to occur: different signals have to be received on one side and not the other. I think this was what Mick was getting at in his first post.If the cells in the bottom half of an embryo start to develop differently to those at the top then you’ve lost symmetry. Likewise, if those at the front receive different signals to those at the back then you’re left with only bilateral symmetry. There are even examples of the last plane of symmetry being lost (like the internal organs) that involve different proteins being produced solely on the left or the right side.So, it’s no longer a question of:"Why did animals evolve symmetry?"It is more like:"Why did animals retain a certain amount of symmetry?""and why do plants not display this particular type of symmetry?"The answer is of course because the selective pressures made it an advantage. Animals and plants have different selective pressure (including things like mobility) and therefore have evolved different types of body pattern.This message has been edited by Ooook!, 17-03-2005 04:44 PM

Mick,I've not got too much more than a passing knowledge of the mechanisms for producing asymetry, but it does seem that it is a bit of a hot topic at the moment, and this is how I understand it (I'm sure someone like PS or Wounded will step in to correct me though ):As it says in the article that you posted, the important protein is 'Nodal': a signalling molecule that is transiently expressed on one side of the embryo during development. This is conserved throughout vertebrates. What doesn't seem to be conserved is the mechanism by which Nodal expression is induced. I may be wrong (my understanding of this is basically gained from a talk I attended recently), but I think that while in Mice the cilia play an important role in the process they don't seem to in Xenopus and Chick . So the master switch gene is the same but the trigger seems to have diverged - definitely a case of watch this space.I'll try and dig up some more on this when I have some time free after work (I don't have full journal access at home). Hope this is interesting.Deleted by edit: Nodal is not a transcription factor.This message has been edited by Ooook!, 18-03-2005 10:25 AM