It is thought that a small excess of L-amino acids was “rained” onto the ancient Earth by meteorite bombardment, and scientists have found that a small excess of L-amino acids can catalyse formation of small excesses of D-nucleotide precursors. This, we proposed, led to a marginal excess of D-polynucleotides over L-polynucleotides, and a bias to D-chains of longer mean length than L-chains in the RNA world.
In the primordial soup, local excesses of one or other hydrogen peroxide enantiomer would have occurred. Specific interactions with polynucleotides destabilise the shorter L-chains more than the longer, more robust, D-chains.
With a greater fraction of L-chains over D-chains destabilised, hydrogen peroxide can then “go in for the kill”, with one enantiomer (let us say M) preferentially oxidising L-chains.
But could there be mirror-image life made of L-nucleic acids elsewhere in the universe? Shutterstock
Overall, this process works in favour of increasing the fraction and average length of D-chains at the expense of L-species.
But the hydrogen peroxide itself remains a racemic mixture, on average, meaning that over time and space it has a balance of M and P enantiomers. So we have a subtle reinforcement effect: the fraction D/P increases while the fraction L/M decreases over time.
Thus, the emergence of homochirality in itself confers a significant advantage on replicating RNA species.
But could there be mirror-image life made of L-nucleic acids elsewhere in the universe?
Well, all I can say at this stage is that when one reflects on it (or attempts to, so to speak), in a sense we are all vampires, made of molecules that have no natural mirror images on this world, and forever searching the universe for our lost reflections.