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  • Symmetry


    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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