Bilateral Symmetry

The Evolution of Bilateral Symmetry and Understanding the Consequences of Asymmetrical Limb Function

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Bilateral symmetry is thought to have evolved very early in animal history. Throughout the evolutionary process, the overwhelming majority of living organisms on this earth have developed bilaterally symmetrical ‘body plans’ as the result of the selective advantages afforded by this superior body design or body plan.

Bilateral symmetry refers to the body plan wherein a plane or axis divides the entire body into two equal halves. Bilateral symmetry is when an animal’s body parts on opposite sides of a center axis are the same in size, shape, and relative position, and are also the same in the way that they work or function. This body symmetry is important because it allows animals to move in a purposeful, streamlined, and directional way, allowing for efficient body movement going forward and in other directions as well (backward and side to side), and also to stop and turn quickly. In evolutionary terms, bilateral symmetry likely helped animals move more efficiently and quickly, which made them more successful and better able to compete for resources and escape predators. Bilaterally symmetrical animals can respond quickly to external stimuli and move quickly towards or away from them.

Animals with bilateral symmetry have both a head and tail end, and this helps maintain body balance and aids in more mobile and directional movement forward in a streamlined manner—a clear advantage over those organisms with asymmetry or radial symmetry. Symmetry also helps the brain recognize and coordinate different body parts, making visual perception easier and causing better movement coordination. Asymmetrical animals tend to swerve more and move randomly in one direction or another because their unbalanced shape propels them asymmetrically.

Higher organisms, like plants and animals, all exhibit bilateral symmetry. In fact, ninety-nine percent of animals exhibit bilateral symmetry. Even lower invertebrates and microorganisms, like fungi and bacteria, all exhibit bilateral symmetry as well. Biologists believe that symmetry (or the absence of asymmetry) is an indicator of health (good genes) since only a healthy organism can maintain a symmetrical plan throughout its development and in the face of environmental stresses, such as illness or lack of food. Therefore, symmetrical animals are usually healthy animals.

Asymmetrical limb function is often the result of injury, as is the case when a dog tears their cranial cruciate ligament (CrCL). An observant pet owner can usually sense a potential underlying injury or issue with their dog, often noticing a lack of symmetry—possibly a subtle hitch in an otherwise normal gait or an awkward standing position when still. They can sense something is “off”.

NAHAH’s MFLS surgical services are about preserving the body’s bilateral symmetry: Preserving a type of body symmetry that allows the animal to move straight forward in a coordinated and harmonious manner. Once a dog has torn their cranial cruciate ligament (CrCL) in one hind limb, it is important to surgically restore the same way of functioning—to re-establish the same biomechanics—to the injured stifle joint. The surgery avoids asymmetrical hind limb function after surgery and restores natural, balanced, and symmetrical limb function.

When the same biomechanics are restored, the original bilaterally symmetrical limb function is restored, and in turn, the natural bilateral symmetry of the patient’s body as a whole is preserved. Bilateral symmetry is a body plan that has evolved over billions of years and has been a proven advantage for high functionality, successful existence, and survival for the overwhelming majority of living organisms on Earth.

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