Bilateral Symmetry Quotes

Bilateral symmetry can have grave consequences for a police officer in a tense situation where he is holding a pistol on a suspect.

SCHISMATRIX is a creeping sea-urchin of a book—spikey and odd. It isn’t very elegant, and it lacks bilateral symmetry, but pieces of it break off inside people and stick with them for years.

Today biologists believe that during the “Cambrian explosion,” about half a billion years ago, nature experimented with a vast array of shapes and forms for tiny, emerging multicellular creatures. Some had spinal cords shaped like an X, Y, or Z. Some had radial symmetry like a starfish. By accident one had a spinal cord shaped like an I, with bilateral symmetry, and it was the ancestor of most mammals on Earth. So in principle the humanoid shape with bilateral symmetry, the same shape that Hollywood uses to depict aliens in space, does not necessarily have to apply to all intelligent life.

The evidence of cheetah genetic monotony would only grow. Bob Wayne, a talented postdoctoral fellow in our lab, examined cranial measurements and the bilateral symmetry of cheetah skulls. Although no one is certain why, in most livestock, asymmetry in skeletal characteristics (the difference between right and left measures of a trait) increases with inbreeding. Bob measured sixteen bilateral traits in thirty-three cheetah skulls held in natural history museums in Washington, Chicago, and New York. The study was not perfect because several of the skulls were incomplete due to a bullet hole in the skull. Nonetheless, in nearly every case, cheetah skulls were more asymmetric compared to the skulls of leopards, ocelots, or margays. When I explained these skull results in a television interview, the correspondent asked, "Dr. O'Brien, are you telling me that these cheetahs are lopsided?" Not exactly, but the cheetahs certainly looked very inbred.

Suppose we had some kind of device with particles moving with a certain definite symmetry, and suppose their movements were bilaterally symmetrical (fig. 20). Then, following the laws of physics, with all the movements and collisions, you could expect, and rightly, that if you look at the same picture later on it will still be bilaterally symmetrical. So there is a kind of conservation, the conservation of the symmetry character. This should be in the table, but it is not like a number that you measure, and we will discuss it in much more detail in the next lecture. The reason this is not very interesting in classical physics is because the times when there are such nicely symmetrical initial conditions are very rare, and it is therefore a not very important or practical conservation law. But

Supergravity is a version of Einstein's theory of general relativity, dressed up with supersymmetry-a symmetry that draws a connection between bosons and fermions, matching one boson for every fermion into "superpartners." Bosons are the entities responsible for transmitting forces, such as the photon that transmits the electromagnetic force. On the other hand, fermions are both matter particles, such as electrons and quarks, and the antiparticles of all these subatomic entities. Looking in the mirror is an example of how symmetry works. The image you see closely resembles how you appear to others because of your bilateral symmetry, even though the mirror switches your left side for your right. Think of supersymmetry as a mirror that switches Bosons for fermions without changing the behavior of the physical system.

Duality is particularly widespread within biological order, from the ‘base-pairs’ of (RNA and) DNA code, through the binary fission of bacterial propagation, the (binary) sexual difference of meiotic reproduction, to the bilateral symmetry of the typical vertebrate organism with consequent pairing of limbs (arms, legs), sense-organs (eyes, ears), lungs, brain-hemispheres, etc. ‘Dual-organization’ provides a basic model for primordial human kinship structure.

Even in surgery there have been some encouraging developments. For instance, operating on the wrong knee or foot or other body part of a patient has been a recurrent, if rare, mistake. A typical response has been to fire the surgeon. Recently, however, hospitals and surgeons have begun to recognize that the body’s bilateral symmetry makes these errors predictable. In 1998, the American Academy of Orthopedic Surgeons endorsed a simple way of preventing them: make it standard practice for surgeons to initial, with a marker, the body part to be cut before the patient comes to surgery.

Among invertebrates, only sponges are not built on any symmetrical plan; all cnidarians (polyps, jellyfish, sea anemones) have radial symmetry; molluscs, arthropods (insects, crustaceans, centipedes, etc.), and all vertebrates have bilateral symmetry, with only some subtle, or hidden asymmetries. Differences in facial features, which hand we prefer to use (very few people, no more than about 1 percent, are ambidextrous), and the placement of internal organs are the most obvious examples of asymmetry in humans.