III. Ecosmos: A Revolutionary Fertile, Habitable, Solar-Bioplanet Lifescape

Brandenburg, Axel and David Hochberg. Introduction to Origins of Biological Homochirality. Origins of Life and Evolution of Biospheres. 52/1-3, 2022. Stockholm University and Center for Astrobiology, Madrid editors open a special issue for the latest research on life’s deep tendency to branch into relative left or right topological phases See, for example, Symmetry Breaking by Consecutive Amplification by Laura Huber and Oliver Trapp (view his website), Biological Homochirality and the Search for Extraterrestrial Biosignatures by Marcelo Gleiser, et al, Molecular Self-Assembly as a Trigger of Life Origin and Development by Dmitry Zlenko, et al, and Frontiers in Prebiotic Chemistry and Early Earth Environments by Ulrich Muller, et al.

The chemistry of terrestrial life is based on a spatial molecular asymmetry whose three dimensional geometrical structure or conformation is not identical to that of their mirror image or spatial reflections though a mirror. The parity P, or space inversion, is a discrete symmetry transformation in physics, and is broken at the molecular level. Such molecules thus possess homochirality or handedness. Amino acids, the building blocks of proteins, and the sugar backbones present in DNA and RNA are chiral molecules. Chirality constitutes a unifying feature of the living world and is a prime driving force for genetic evolution and biologic selection. (First paragraph)

Most amino acids and sugar molecules occur in mirror, or chiral, images of each other, knowns as enantiomers. However, life on Earth is mostly homochiral: proteins contain almost exclusively L-amino acids, while only D-sugars appear in RNA and DNA. We review potential mechanisms for biological homochirality on primal Earth an their implications for astrobiology: that it is a stochastic process driven by local environmental fluctuations; second, that it is driven by circularly-polarized ultraviolet radiation in star-forming regions; and that it is driven by parity violation at the elementary particle level. (Gleiser, et al)

Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their own mirror images, so they are chiral. In biology, 19 of the 20 natural amino acids are homochiral, being L-chiral (left-handed), while sugars are D-chiral (right-handed). (Wikipedia)