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Jenkyn, Thomas
05-16-2001, 12:42 AM
Dear List-members,

I welcome the introduction of these summer science quiz questions. They are
definitely encouraging a fruitful discussion.

I will state up front that I fall into the "dumb luck" camp on this
discussion.

In this discussion we need to be careful to distinguish between what
architectures of animals actually exist and what architectures are possible.
There is a big difference between the two and by no means are all the
possible modes of locomotion represented in the fauna of this planet.

Therefore making the conclusion that bi-laterally symmetrical animals are
faster than assymetric and radially symmetric animals because that is what
we see in the animal kingdom is valid (with perhaps the exception of the
squid).

But to draw the further conclusion that therefore bi-laterally symmetric
architecture always yields faster locomotion does not follow. One can
imagine a rabbit-like animal which possesses a single front limb instead of
two (a three legged rabbit!). This front limb may be placed along the
midline of the animal and the locomotion would be much the same as the
common four-legged rabbit. With the same maneouvrability.

One could also image an scorpion-like animal with 3 legs on one side and 2
on the other. The timing of the legs would be somewhat different perhaps,
but no biomechanical restrictions would stop this animal from being as fast
or maneouvrable as its 6 legged cousin.

These thought experiement animals aren't even particularly different from
the existing fauna. And I would suggest that there are plenty of other
odd-legged schemes that the bioengineer could dream up that would hold their
own just fine in a foot race with even-legged competitors.

Therefore, as unsatisfying as it is to an engineer, it is probably just dumb
luck that the even-legged animals on this planet are the fastest. And also
dumb luck that we even-leggeders are in the oppressive majority.

To paraphrase George Orwell: "Even legs good, odd legs bad".

Tom Jenkyn

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Thomas Jenkyn, PhD (Bioengineering)
Post-doctoral Research Fellow
Orthopaedic Biomechanics Lab
Mayo Clinic/ Mayo Foundation
200 First St. SW
Rochester, MN 55905

Tel: 507-284-2262
Fax: 507-266-2227
Email:
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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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