Is radial symmetry considered to be at a lower evolutionary stage than bilateral symmetry?

Question

Are animals that have radial symmetry at a lower evolutionary stage than bilateral symmetry animals

Answer 1

It depends what you mean by lower evolutionary stage, but if you consider the levels of complexity of body plans as in the number of tissue layers, presence or absence of coelom and embryonic development then you will find radially symmetrical animals at both ends of the spectrum.

The echinoderms are radially symmetric and have complex bodies, they are deuterosomes which means that their embryonic development that is similar to chordates (which includes all vertebrates). There is nothing lowly about this group from the point of view of complexity and therefore I would say that radial symmetry is not a lower evolutionary stage, it is just 1 of 2 possible body plans that arose during evolution.

Answer 2

it would be easy to say yes, but if you think about it, they are actually MORE evolved than bilaterals. Radial symetry has been around for a lot longer than bilateral. Because of that, they have been evolving over a longer period of time. Don't forget, the word EVOLVE just means change. It doesn't signify intelegence, ability, or anything like that. It just means how long something has been changing over time. In that way, radial symetry organisms have been EVOLVING for millions of years longer than bilaterals and can be considered at a higher evolutionary stage.

It is also just chance that the higher intelegence lifeforms are bilateral. There is no reason that radially symetrical animals could not be the higher life form on other planets.

Answer 3

Both answers above mine are correct, but as is pointed out in the second one, we have to be careful with semantics. If you are asking whether organisms with radial symmetry tend to be less complex than those with bilateral symmetry, the answer is "yes". If you are asking whether organisms with bilateral symmetry tend to have a longer evolutionary history (i.e. have been evolving for a longer time), then the answer to that question is "no".

Answer 4

It could be supposed that this is the truth, as organisms with bilateral symmetry tend to be more complex. A fact which I suppose to hold more weight is that radial symmetry would seem to require a simpler structure in order to allow for symmetry to be experienced in any matter of cuts.