Abstract
AbstractGeneralizing Landau's spontaneous symmetry breaking arguments using the standard groupoid approach to stereochemistry allows reconsideration of the origin of biological homochirality. On Earth, limited metabolic free energy density may have served as a low temperature analog to 'freeze' the system into the set of simplest homochiral transitive groupoids representing reproductive chemistries. These engaged in Darwinian competition until a single configuration survived. Subsequent path dependent evolutionary process licked in this initial condition. Astrobiological outcomes, in the presence of higher initial metabolic free energy densities, could well be considerably richer, perhaps of mixed chirality. One result would be a complicated distribution of biological chirality across a statistically large sample of extraterrestrial stereochemistry, in contrast with a recent prediction of a racemic average.
Highlights
Amino acids and the backbone of DNA/RNA in living things on Earth are found in only one of the two possible mirrorimage states available to them
The L-forms of amino acids primarily serve as the building blocks of proteins, and D-sugars form the DNA/RNA backbone [1]
As a consequence of groupoid symmetries associated with stereochemical structure, a statistically large sampling of extraterrestrial stereochemistries could well be far more complex than [2] proposes, i.e., not racemic on average
Summary
Amino acids and the backbone of DNA/RNA in living things on Earth are found in only one of the two possible mirrorimage states available to them. Attempts to replicate early conditions on Earth – Miller/Urey experiments – always produce ‘racemic’ mixtures having equal amounts of both possible amino acid symmetry forms. This fundamental conundrum was recently addressed by Gleiser et al [2], who, in a computationally intensive study adapting Sandars’ ‘toy model’ of polymerization [3], conclude that other planetary platforms in this solar system and elsewhere could have developed an opposite chiral bias to that of Earth. 3. Groupoid symmetries and available metabolic free energy are, as a consequence of the Darwinian individuality of coding schemes, in the sense of Gould [17], contexts for, rather than determinants of, the resulting evolutionary processes, including punctuated equilibrium. Depending on the degree of noise, either high or low reproductive source uncertainty can have selective advantage, a kind of stochastic resonance related to the mesoscale resonance arguments of [16]
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