Symmetry and asymmetry in humans

Symmetry is mostly used to describe body shapes. In humans, symmetry can be investigated at microscopic and macroscopic level. At molecular level life is asymmetric. Most compounds are chiral, such as carbohydrates, amino acids, and nucleic acids. Since only one enantiomer of a chiral molecule predominates, this concept is referred to as homochirality. Amino acids appear almost exclusively in the left-handed form, whereas sugars appear in the right-handed form. At cellular level, some examples of symmetric structures are represented by the mitotic spindle, which allows cell division, and the primary organizer, which is responsible for the bilateral symmetry of the embryo. At macroscopic level, the sagittal plane divides an organism into roughly mirror image halves, with respect to external appearance only. Asymmetry is often an indication of unfitness, including defects during development or injuries throughout a lifetime. Facial symmetry influences human judgments of attractiveness. Nevertheless, nearly all organs of the thorax and abdomen are asymmetric. Although cerebral hemispheres look roughly symmetric, there are asymmetries, such as the planum temporale of the temporal cortex, which is larger in the left hemisphere. This difference correlates with right-handedness and language and opened the study on the functional asymmetry. Intriguing dilemmas occur when interrupting the connection of the two hemispheres, as in the split-brain syndrome: when patients are shown an image only in the left half of each visual field, they cannot verbally name what they have seen, because the language area is contralateral.