(A) Schematic representation of the proposed structural evolution of extant tRNAs (Right) from the association through sequence complementarity and subsequent fusion of ancestral minihelices (Left). Red dots indicate positions proposed to act as identity elements in ancestral minihelices that would eventually generate anticodons in extant tRNAs and remain as vestigial signatures in the acceptor stem. (B) Pipeline used to identify tRNAs presenting a structure-independent enrichment of cognate anticodon-like triplets in their acceptor stems.
A new study published in PNAS brings that question into the realm of testable molecular evidence. Researchers examined 77,335 transfer-RNA gene sequences and found patterns consistent with remnants of an earlier coding system. These possible “molecular fossils” are not ancient molecules preserved unchanged: they are features of living organisms’ molecules that may retain information about their evolutionary ancestry.
Transfer RNAs, or tRNAs, connect genetic instructions with the amino acids used to build proteins. An anticodon at one part of each tRNA recognises a corresponding three-letter codon in messenger RNA; the amino acid is attached at the other end. One hypothesis proposes that these molecules arose from simpler RNA hairpins, or minihelices, in which elements involved in amino-acid recognition lay close to the attachment site. The researchers found an excess of matching anticodon-like triplets in modern tRNA acceptor stems, with the strongest signals surviving controls for RNA structure and base composition.
According to the institutional account, the signals generally fit proposed histories in which the code acquired amino acids progressively. However, some matching triplets also occur elsewhere in tRNA, and the precise evolutionary route remains unresolved. These findings support an ancestral system; they do not prove that one particular reconstruction is correct.
That distinction is central to the contrast with creationism. A scientific explanation makes predictions, tests them against observations and leaves room for competing accounts. Declaring the genetic code too complicated to have evolved does none of those things. The interesting question is how chemistry became organised into an inherited coding system, and how that system subsequently expanded. Present-day complexity is the starting point for that investigation, not evidence that there was never anything simpler.
How could the triplet genetic code have evolved? The genetic code is the correspondence between RNA codons and amino acids. RNA uses four bases, represented by A, U, G and C. Two positions allow 4 × 4 = 16 combinations; three allow 4 × 4 × 4 = 64. In the standard modern code, 61 codons specify amino acids and three signal a stop. Most amino acids have more than one codon, a property called degeneracy or redundancy.The research is explained in a news release from Institut de Recerca Biomèdica (IRB) Barcelona:
- Chemical associations before a complete translation system
One possibility is that some assignments began with chemical affinities between amino acids and particular RNA structures. Experiments have found that RNA sequences selected for binding certain amino acids are enriched in corresponding codons or anticodons. This supports a stereochemical hypothesis for some coding relationships, although it does not explain every assignment. An RNA capable of binding an amino acid is also not, by itself, a complete protein-making system.
Relevant research: RNA–ligand chemistry: a testable source for the genetic code.
- Small RNA adaptors before modern tRNAs
Another proposal starts with short RNA structures capable of carrying amino acids. Association, duplication and fusion of such structures could have produced larger adaptors, eventually linking amino-acid attachment to template reading. In this context, an operational code means RNA features that determine which amino acid attaches, rather than the complete modern codon table. The new tRNA study concerns possible surviving traces of this earlier relationship.
- A two-letter stage: two bases carrying the information
An early system need not have used today’s full set of 20 standard amino acids. A smaller repertoire could have been specified by fewer distinctions between codons. In one family of hypotheses, the first two bases determined the amino acid while the third carried little or no identifying information.
“Two-letter code” therefore need not mean that the message was physically read two bases at a time. Some models retain triplets throughout, with an initially uninformative third position. This avoids assuming a wholesale change in reading frame. Other proposals discuss genuine two-base associations preceding the modern system; these are related ideas, not interchangeable claims.
A surviving illustration of two-position specificity is the alanine family: GCU, GCC, GCA and GCG all specify alanine. The first two letters, GC, suffice to identify it in the standard code. This pattern is compatible with some ancestral-code models, but does not establish their historical truth.
Relevant models: a proposed association mechanism and doublet code; error minimisation in putative primordial codes.
- Redundancy, expansion and protection against errors
If four triplets differing only at the third position specified the same amino acid, a substitution at that position would leave the protein’s amino-acid sequence unchanged. For example, changing GCU to GCC still specifies alanine. Such redundancy could have buffered some copying and translation errors in an early system.
However, adding a third base would not automatically create this protection: it depends on how the resulting codons are assigned and read. Nor must error buffering have been the original reason for triplet reading. It could have been a useful consequence of an initially less discriminating decoding mechanism.
As the amino-acid repertoire expanded, some codon families could have been subdivided by distinctions at the third position. Today, CAU and CAC specify histidine, whereas CAA and CAG specify glutamine. The third letter therefore contributes both redundancy and additional coding capacity. It does not simply serve as a spare letter throughout the code.
Redundancy is also incomplete protection. A third-position substitution can change the amino acid or produce a stop signal, and even a synonymous change can affect gene expression. The code neither prevents mutations nor guarantees that they are harmless.
- Several processes could have contributed
Chemical affinities, expansion of amino-acid biosynthesis, duplication and divergence of RNA adaptors, and selection for less damaging errors could have acted at different stages. Once many proteins depended on established assignments, major changes would become increasingly disruptive, helping to conserve the code. This is the idea behind a partly “frozen accident”: historical assignments become difficult to replace, without having been deliberately chosen.
There is no agreed, complete reconstruction of these events. In particular, the new tRNA findings do not demonstrate a two-letter stage or show that a third position evolved specifically to reduce mutation damage. They add evidence relevant to the broader proposition that modern translation emerged through earlier, simpler coding relationships.
- Origin and evolution of the genetic code: the universal enigma.
- Creationism In Crisis - The Evolution Of Life And The Genetic Code.
- Wang, Minglei; et al Tracing the Origin of the Genetic Code and Thermostability to Dipeptide Sequences in Proteomes Journal of Molecular Biology (2025)
- Lane, Nick. Life Ascending (pp. 46-49)
Remnants of an ancestral genetic code identified
- Researchers at IRB Barcelona, in collaboration with MIT and Scripps Research, identify signals in transfer RNAs that provide insights into the origin of these molecules and the genetic code.
- The work, published in PNAS, analyzed more than 70,000 sequences and strengthens the hypothesis that present-day transfer RNAs evolved from simpler molecules.
The genetic code determines the correspondence between genetic instructions and amino acids—the building blocks that form proteins. How this correspondence arose is one of the major open questions in evolutionary biology. A study led by Dr. Lluís Ribas de Pouplana, ICREA researcher at IRB Barcelona, and Dr. Paul Schimmel, with the participation of researchers from MIT and Scripps Research, has identified conserved signals in transfer RNAs that could help reconstruct the early stages of this system.
Proposed model of transfer RNA evolution from simpler RNA molecules called mini-helices (IRB Barcelona)
The paper, published in the journal PNAS, features two undergraduate students as first authors who completed placements in IRB Barcelona's Gene Translation Laboratory, demonstrating the Institute’s commitment to training young talent.
Transfer RNAs, or tRNAs, act as a bridge between genetic instructions and amino acids during protein synthesis. In one region, they contain the anticodon, a group of three RNA "letters" that recognizes a genetic instruction. At another end, the corresponding amino acid attaches. These two points are physically far apart in the present-day molecule, raising the question of how the connection between them was established during evolution.
In current tRNAs we have identified signals compatible with a simpler system that linked RNA sequences to amino acids. These results provide new insights into how the genetic code developed.
Dr. Lluís Ribas de Pouplana, Senior author.
Institute for Research in Biomedicine
The Barcelona Institute of Science and Technology
Barcelona, Catalonia, Spain.
A prediction about the origin of tRNAs
One of the main hypotheses proposes that tRNAs evolved from small RNA structures folded into a hairpin shape, called mini-helices. According to this notion, the three-letter groups that make up the anticodon today would have already been present in these primitive RNAs, near the amino acid attachment site. The model therefore predicts that present-day tRNAs could retain copies of these triplets in that same area, the acceptor stem.
To test this prediction, the team analyzed more than 70,000 tRNA gene sequences from nearly 5,000 genomes across bacteria, archaea, and eukaryotes. The dataset was obtained after filtering nearly half a million sequences and is the largest used to date in this type of analysis.
To check whether the signals could be explained solely by the structure of the RNA, the team compared the real sequences with simulated versions of equivalent structure. In several groups of tRNAs, the triplets appear more frequently than predicted by these simulations, which further supports the possibility that they are remnants of an ancestral system.
The key was to verify that the signal persisted when taking into account the structural constraints of the molecule. This control supports the interpretation that the observed patterns may contain information about their evolutionary history.
Dr. Lluís Ribas de Pouplana.
Insights into the expansion of the genetic code
The team compared the intensity of these signals with the proposed chronology for the incorporation of amino acids into the genetic code. In general, the patterns are stronger in tRNAs associated with amino acids that, according to this timeline, were incorporated earlier into the code—such as glycine, alanine, or valine—and weaker in those associated with late incorporations, such as tyrosine and tryptophan. This agreement provides additional support for a progressive development of the genetic code.
The study also detected triplets in other arms of the tRNA, an observation that the mini-helix hypothesis does not explain on its own. The results offer insights into an ancestral system, but the exact pathway that led to present-day molecules remains an open question.
Publication:
The significance of this research is not that scientists have finally solved the origin of the genetic code. They have not, and the authors do not claim otherwise. It is that a hypothesis about a simpler ancestral system makes predictions that can be tested against modern molecules. The observed patterns are consistent with that evolutionary history, even though they do not uniquely establish it. That is a substantial advance over declaring the problem insoluble and inserting a supernatural designer into the gap.
For creationism, the difficulty is the assumption that the elaborate machinery found in living cells must always have existed in its present form. An earlier coding system did not have to manufacture the full range of modern proteins, use all 20 standard amino acids, or achieve today’s accuracy. It needed only to function well enough in its own circumstances for useful variations to persist. Simpler RNA–amino-acid relationships could then have provided the foundations on which more elaborate translation systems developed. The possible traces of those relationships in modern tRNAs give researchers something concrete to investigate.
The proposed two-letter stage illustrates how such development might have proceeded, but it must remain a hypothesis rather than a conclusion attributed to this study. A code with fewer informative distinctions could have accommodated a smaller amino-acid repertoire, while redundancy could have buffered some errors. Later refinements could have expanded the repertoire without eliminating all that redundancy. Neither this particular sequence of events nor the precise origin of triplet reading has been demonstrated here. Scientific uncertainty requires that qualification; it does not turn an unevidenced designer into an explanation.
What emerges is a picture of biological complexity as something with a history: components recruited, relationships modified and earlier features potentially retained within later systems. Science advances by asking which parts of that history can be recovered, which explanations survive testing and which must be revised. Creationism contributes no comparable account of how the genetic code arose. Calling it “designed” merely gives the unanswered question a theological label.
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