The problem
Your left and right hands are mirror images, but rotating one does not turn it into the other. Many molecular building blocks also come in two such forms. Life mostly uses one form of amino acids to build proteins and a particular form of sugars in DNA and RNA. How did these strong preferences arise from early chemistry that could supply both forms?[2]
Many biological building blocks have mirror-image forms; proteins predominantly contain L-amino acids, and DNA and RNA contain D-sugars. How did early systems select and amplify these preferences? This is biological homochirality. One proposed route concerns attaching an amino acid to RNA before protein synthesis, a step called aminoacylation. A 2018 simulation of a simplified RNA attachment system found that L-alanine more often adopted a geometry suitable for the reaction than D-alanine. The model uses already handed RNA: it can help explain a preference for amino acids without explaining the origin of the RNA’s sugar preference.[1] [2] [3]
Scope
The historical selection and amplification of predominantly L-amino acids in proteins and D-sugars in nucleic acids.
What remains open
The 2025 paper proposes a translation-related route. Neither favorable geometry in a model nor amplification under selected laboratory conditions reconstructs early Earth’s sequence of events. The 2026 review retains the question of whether strong handedness preceded living systems or developed with them.[1] [2] [3]
Why it matters
Molecular shape affects how building blocks join and how the resulting chains fold. A history of life’s ingredients must therefore explain their shapes as well as their chemical identities.[2]
The RNA attachment case shows why separate ingredient histories may depend on each other. A handed RNA scaffold could favor particular amino acids, linking the emergence of genetic molecules to protein synthesis. Testing those dependencies constrains which proposed origin pathways can fit together.[2] [3]
Definitions
2 terms
homochirality
A strong preference for one mirror-image form of a molecular building block. Biological preferences are predominant and have exceptions.[2]
aminoacylation
Attaching an amino acid to RNA. Modern protein synthesis uses amino acids attached to transfer RNA; simplified attachment systems are studied as possible early precursors.[3]
Sources
- Origin of homochirality in terrestrial biology
Notes on Origin of homochirality in terrestrial biology
Author proposes a translation-related route to amino-acid preference. The fresh read is the publisher Abstract and metadata; a proposed pathway is distinct from a reconstructed historical sequence.
- Progress and persistent questions in understanding the origin of life on Earth
Notes on Progress and persistent questions in understanding the origin of life on Earth
Section 6.1 distinguishes molecular handedness, proposed biases and amplification from the unresolved historical timing. Laboratory possibilities do not identify which sequence supplied early life.
- Principles of chemical geometry underlying chiral selectivity in RNA minihelix aminoacylation
Notes on Principles of chemical geometry underlying chiral selectivity in RNA minihelix aminoacylation
Simulation examines the geometry of amino-acid attachment to an already handed RNA model, finding a preference for L-alanine-compatible geometry. It illustrates a possible dependence between RNA and amino-acid preferences, without explaining the original sugar preference or establishing an ancient pathway.