Symmetry breaking mechanisms: an evolutionary enigma
Looking at both hands allows one to get an intuitive grasp of what “chirality” means. Indeed, a (admittedly, somewhat simplified) definition states that an object is “chiral” if it is distinguishable from its mirror image and cannot be super-imposed onto it. Exactly as the left hand is a non super-imposable mirror image of the right hand; no matter how often and in which direction they are rotated: it is not possible for both hands to completely coincide across all axes. Chirality is widespread across the animal world and can be recognized at the molecular level or at the anatomical level, in features as diverse as the clockwise or anti-clockwise coiling of gastropod shells, the structure and position of internal organs, or the species-specific left or right turning of marine flatfish, prior to their roaming existence at the bottom of the ocean.
Given the fact that genetic mutations resulting in inconsistency of each organ's chirality in humans - e.g. manifested through the discordant internal positioning of the heart, spleen and/or lungs - are often connected to health problems, it is tempting to speculate that left-right asymmetry is a crucial aspect of the developmental program of metazoans. As to the reason why biological structures demonstrate a genetically determined left-right asymmetry - that is to say, involving an energy consuming decision making process, based upon an elaborate machinery of gene interplay under evolutionary pressure - one can only speculate at present.