The parity-violating effect of the weak force is much too small to be statistically significant for the origin of biological homochirality. Other physical interactions such as with circularly polarized light (CPL) are larger and are discussed in the literature as an effect on small molecules such as amino acids or sugars. To enable their polymerization, they must have existed in large quantities, so that statistical fluctuations may have been smaller than the CPL effect. However, the subsequent formation of the first polymer with good self-reproducing ability was very improbable and may indeed have happened only once. (Such a polymer is isotactic, containing only homochiral constituents.) In this case, the enantiomer excess was 100%, the sign emerging by pure chance without any deterministic influence. The single-event hypothesis can also easily explain why many natural sugars and related compounds (e.g. ascorbic acid) belong exclusively to the l-series, while with others both enantiomers are represented, although Nature uses only d-glucose and d-ribose. It is also pointed out that in the sugar series the sign of physical effects not only varies from species to species but even in the easily equilibrating anomers. It is, however, not excluded that a chemical deterministic effect may have supported the formation of a self-reproducing polymer: enantiomer enrichment by adsorption on a mineral surface.