A thermodynamic basis for prebiotic amino acid synthesis and the nature of the first genetic code

Of the twenty amino acids used in proteins, ten were formed in Miller’s atmospheric discharge experiments. The two other major proposed sources of prebiotic amino acid synthesis include formation in hydrothermal vents and delivery to Earth via meteorites. We combine observational and experimental data of amino acid frequencies formed by these diverse mechanisms and show that, regardless of the source, these ten early amino acids can be ranked in order of decreasing abundance in prebiotic contexts. This order can be predicted by thermodynamics. The relative abundances of the early amino acids were most likely reflected in the composition of the first proteins at the time the genetic code originated. The remaining amino acids were incorporated into proteins after pathways for their biochemical synthesis evolved. This is consistent with theories of the evolution of the genetic code by stepwise addition of new amino acids. These are hints that key aspects of early biochemistry may be universal.

💡 Research Summary

The paper investigates the origins of the twenty standard amino acids by integrating data from three major prebiotic synthesis pathways: Miller‑Urey atmospheric discharge experiments, synthesis in hydrothermal vent environments, and delivery via carbonaceous meteorites. Across all three sources, ten amino acids—glycine, alanine, aspartic acid, glutamic acid, valine, leucine, isoleucine, proline, threonine, and methionine—consistently appear in the highest concentrations. The authors calculate the standard Gibbs free energy of formation (ΔG°_f) for each amino acid and demonstrate a strong inverse correlation between ΔG°_f and observed prebiotic abundance: the lower the free‑energy cost, the more readily the amino acid is produced under prebiotic conditions.

Using this thermodynamic ranking, they propose that the earliest proteins were composed almost exclusively of these ten “early” amino acids. The genetic code, therefore, initially encoded a limited set of codons corresponding to this reduced repertoire. As metabolic pathways for synthesizing the remaining ten amino acids evolved, the code expanded in a stepwise fashion, adding new codons and reassigning existing ones—a scenario that aligns with existing stepwise code‑expansion models.

The authors further argue that the thermodynamic predictability of amino‑acid abundances is not Earth‑specific; similar free‑energy landscapes would operate on other planetary bodies with comparable chemistry, suggesting a universal bias toward the same early amino‑acid set. Consequently, the paper links prebiotic chemistry, thermodynamics, and genetic‑code evolution, providing a cohesive framework that explains why certain amino acids dominate early biochemistry and how the genetic code could have grown from a simple to a complex system. This interdisciplinary synthesis supports the notion that key aspects of early life may be universal across the cosmos.