symmetry and asymmetry

symmetry and asymmetry Exhibiting external bilateral symmetry about a vertical midline, the human body consists of two enantiomorphs — the right and left sides. The overwhelming preponderance of bilateral symmetry in the animal kingdom suggests that it provides an enormous evolutionary advantage. A close inspection of many individuals, however, reveals that small differences between left and right structures exist. These variations are called fluctuating asymmetries because neither side differs in a consistent manner when compared to the other.

What is the origin of bilateral symmetry? In the Symposium (190–1), Plato had Aristophanes explain that the original human beings were globular wholes with two faces on a single head, eight limbs, and double of every other structure; to punish them for their insolence and power, Zeus bisected each spherical human, and Apollo rotated each face toward the cut edge, pulled skin over the gash and tied it into a tight bundle at the navel.

Wilhelm Ludwig (1816–95) believed that bilateral symmetry was the basic body plan of all vertebrates, and that all asymmetries were superimposed upon the ground plan consequent to the lengthening and folding of the intestinal tract during evolution. Jacques Monod (1910–76) took a stance opposite to Ludwig's and, because the fundamental molecules — proteins and nucleic acids — responsible for organic structure are not symmetrical, considered asymmetry to represent the true order of living beings and symmetry to be a superficial phenomenon acquired during evolution.

Bilateral symmetry appears mainly in structures such as the brain, nervous system, skin, hair, and nails, and in parts of the eye and ear — all of which arise from the ectoderm (outer germ layer) of the embryo — and in some structures, including the skeleton and skeletal muscles, tendons, glands, and reproductive organs, which develop from the mesoderm (middle germ layer). The heart, originating in the mesoderm, and the liver, stomach, pancreas, and intestines, which arise from endoderm (the inner germ layer) appear singly to one side of the midline.

Struck by the general doubling of organs, Aristotle rescued symmetry by asserting that the spleen was a bastard liver and by pointing out that the heart has two halves. The bilateral symmetry of ectodermal and some mesodermal structures appears to have resulted from the interactions of the organism with its environment where left–right bias does not exist. Directional locomotion almost always requires bilateral symmetry and precludes the front — rear symmetry of Dr Dolittle's pushmi-pullyu (and Plato's original spherical human). The development of limbs for locomotion requires concomitant development of symmetry in the sense organs and in the central nervous system.

Certain anatomically symmetrical structures, notably the human hands and brain, do not exhibit functional symmetry. The evolution of bipedalism freed the hands from participation in locomotion and allowed them to evolve new functions, such as feeding, food gathering, tool using, and manipulating rather than simply reacting to the environment. The hands came to perform different functions, for example, in tool-making, for which one hand holds the material while the dominant hand shapes it. The bias of right-handedness in 88–90% of humans seems to be unique among animals. Some stone-flake evidence suggests that even the earliest human ancestors, the Australopithecines, were predominantly right-handed. As the human hand adapted, the mouth, freed from grasping functions, could specialize partly for communication, controlled by the brain.

Functions such as speech, reading and facial recognition are asymmetrically located in the brain. During the evolution of handedness, duplication of functions in the brain might have wasted precious space and caused cognitive confusion. Such asymmetry of function correlates with the necessity of the association of the control of linked bodily movements and sensations. Some controversial evidence suggests that asymmetry of brain function may be more pronounced in males than in females, though it is an open question whether such differences are innate or environmentally determined.

External symmetry is a measure of health and provides a criterion of reproductive fitness in many species. Some research provides evidence that body symmetry also enhances human reproductive success. One study found that college males exhibiting greater body symmetry, as determined by measurements of ankles, feet width, wrists, hand, elbows, and ears, begin sexual intercourse several years earlier and have women partners who achieve orgasm more frequently. Another study indicates that slight asymmetries in women's fingers, ears, and breasts tend to diminish around ovulation, when women are fertile.

Critics of this research point out that much of the evidence linking symmetry with reproductive success rests on studies of non-humans and that the asymmetries discussed require fine calipers for detection. Furthermore, there is an alternate explanation for the greater approval of symmetry. A Swedish study suggests that the preference for symmetrical mates may be a by-product of a general preference for symmetry that evolved because it facilitated recognition of signals and objects varying in position and orientation in the field of vision.

Whatever the cause, it is certain that humans, including infants, find the symmetrical face — and body — more beautiful. In early representations, such as Mesopotamian and Etruscan sculpture, the human body was rigidly symmetrical. The Roman architect Vitruvius (first century bce) wrote that sacred buildings should be scaled according to human proportions because the human body, with legs and arms extended, fits into both the circle and the square. The 1480 drawing of a Vetruvian man by Leonardo da Vinci (1452–1519) illustrates how mathematical proportion became the foundation of an aesthetic philosophy in the Renaissance. Dürer (1471–1528) also applied Vetruvius' measurement to the human form, which he represented in numerous studies of the male and female figures, many of which were bisected vertically, with numerous horizontal lines to assure perfect symmetry and proportion. Varying from slender to obese, many of these figures are unattractive. The conscious artistic preference for symmetry tends to resurface, as suggested by a German physician's assessment that the Venus de Milo, discovered in 1820, had too many flaws and asymmetries to be considered beautiful. He lived too early to see works such as Picasso's Guernica (1937).

Kristen L. Zacharias

Bibliography

Corballis, M. C. (1991). The lopsided ape. Evolution of the generative mind. Oxford University Press, New York.
Weyl, H. (1952). Symmetry. Princeton University Press, Princeton, New Jersey.

See also beauty; handedness; language and the brain.