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Thomas Stoffregen
05-15-2001, 12:15 PM
Some comments on the origins and utility of bi-lateral symmetry
(i.e., even numbers of limbs) in animals.

>My solution: Bilateral symmetry in the animal body plan produces the even
>number of limbs, but those animals that develop with radial symmetry (like
>the starfish) can have odd numbers of limbs. Why does bilateral symmetry
>predominate in the animal kingdom would perhaps be a better question. My
>answer to that question is: pure dumb luck.

Dumb luck is possible, but before we accept this non-functional
explanation, we should consider whether bi-lateral symmetry might
have a functional role that lead to its being selected for (rather
than merely tolerated) in the evolutionary history of animals.

Bi-lateral symmetry is locomotor appendages is not universal, cf.
star fish, and snails (the latter have only one foot, an odd number).
But it is very common (including fish, which don't have legs, but do
have bi-laterally symmetric fins, and even have bi-laterally
symmetric muscles attached to their otherwise unitary tails). One
thing that distinguishes bi-laterally symmetric animals from
asymmetric ones is that the former move faster (relative to the
viscosity of their media). Bi-lateral asymmetry in quick-moving
animals is useful simply because it makes for more efficient
locomotion when speed matters. Star fish are in no hurry, but if
they had evolved in the direction of faster locomotion, their
asymmetric bodies would have imposed rather severe constraints on how
the increase in speed could be accomplished (i.e., consider what odd
interlimb coordination patterns would be required for a starfish to
"run"). Bi-laterally symmetric animals, such as fish or humans, have
a great advantage when it comes to organizing and achieving
locomotion.

I don't think the "choice" was dumb luck.

*****************************************
Thomas A. Stoffregen, Associate Professor
Department of Psychology
University of Cincinnati
P. O. Box 210376
Cincinnati, OH 45221-0376
USA

Office: 513-556-5535; Lab: 513-556-5682; Fax: 513-556-1904

URL: http://ucaswww.mcm.uc.edu/psychology/Faculty/Stoffreg.htm
Lab: http://homepages.uc.edu/~stoffrta/psl/index.html

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