Interesting hypothesis that animals without bilateral strength would
evolve out of existence, but perhaps you didn't consider that humans
are 85-90% right dominant. This has been true for as far back as can
be studied, like from drawings of human cave dwellers which showed
approximately 90% were right-side dominant. I don't think from the
first Homo erectus that humans had predominantly unilateral stimuli.

Animals often develop a preferred limb, such as for digging or raking
food toward them, but throughout each lower species (including
primates, as I recall) the populations are about 50-50% for lateral
preference, as would be expected by chance.

So why haven't humans evolved out of existence? In fact, the opposite
appears to occur. I recall an extensive study within the past few
years which found human left-handers had a significantly shorter life
expectancy and were more prone to accidents than right-handers. And
left-handers tend to be much more ambidextrous than right-handers.
Ambidextrous literally means being able to use both hands with equal
facility (Latin ambi-, on both sides + Latin dexter, right-handed*).

*Excerpted from The American Heritage® Dictionary of the English
Language, Third Edition © 1996 by Houghton Mifflin Company.
Electronic version licensed from INSO Corporation; further
reproduction and distribution in accordance with the Copyright Law of
the United States. All rights reserved..

>Dan Barker
>Biomechanical Engineer
>Lund University Hospital
>As a fellow aussie, this is interesting. Does the tail act purely as an
>added balance mechanism or is there a more active role in the gait of the
>kangaroo. I think the question should be 'why is there little bilateral
>asymmetry in nature'. To me this is obvious. Animals evolve in an
>environment in which there will, in general, be equal stimulus on either
>side, L-R, of the body. An animal that had a weakness on the left or right
>side only would surely have a smaller chance of dominating a gene pool vs a
>balanced animal. Perhaps there are animals which do live in an environment
>where there are predominantly unilateral stimuli.

Bruce Etnyre, Ph.D., P.T.
Kinesiology Dept., Chair
Rice University
6100 Main MS 545
Houston, Texas 77005
Ph: (713)348-5936
FAX: (713)348-5329

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