It has long been known that Earth’s biochemistry is overwhelmingly dissymmetric or chiral.1–4In model chemical systems5–7that spontaneously evolve to a state dominated by either theLor the D emantiomer parity violation in &bgr;‐decay and that attributable to weak neutral currents (WNC) in molecules8,9is thought to be too small to have any significant influence on the emergent chirality.10,11Other conceivable systematic chiral influences are generally even weaker.12–14We show here that there is a simple and extremely sensitive mechanism by which a minute but systematic chiral interaction, no stronger than the WNC interaction is amino acids, can, over a period of ∼15,000 yr, determine which emantiomer will dominate. Such a mechanism is especially interesting when considering the origins of terrestrial biochemistry, particularly in view of the work by Mason and Tranter,15who found that it is the terrestrially dominant L amino acids that are favoured by the WNC interaction. ©1996 American Institute of Physics.