Here is the data on tRNA evolution
Pathways of standard genetic code evolution remain conserved and apparent, particularly upon analysis of aminoacyl-tRNA synthetase (aaRS) lineages. Despite having incompatible active site folds, class I and class II aaRS are homologs by sequence. Specifically, structural class IA aaRS enzymes...
pubmed.ncbi.nlm.nih.gov
Here is data on the relationship between tRNA and LUCA.
Life on Earth and the genetic code evolved around tRNA and the tRNA anticodon. We posit that the genetic code initially evolved to synthesize polyglycine as a cross-linking agent to stabilize protocells. We posit that the initial amino acids to enter ...
www.ncbi.nlm.nih.gov
polyglycine was evolution's first version of a cytoskeleton, to stabilize cell structure and shape. These days it's done with microtubules.
The hypothesis is that tRNA evolved to synthesize polyglycine.
Figure 1 in the above link shows the early evolution.
"Mini-helices" are very short strands of RNA, only 3-5 bases long. In this case they would be GCC, UCC, and CCC.
To make polyglicine, we use an aminoacyl-tRNA synthetase enzyme, which is itself s fascinating molecule. The synthetase (also called ligase) attaches the amino acid to the tRNA. Once the tRNA is charged, it can be used to build a protein. The synthetase enzymes use Mg++ as a catalyst. There are two classes of synthetase/ligase with different ATP binding mechanisms, one called "backbone brackets" that uses hydrogen bonds, another called "arginine tweezers" that uses salt bridges.
In some of the aminoacyl tRNA synthetases, the cavity that holds the amino acid can be mutated and modified to carry unnatural amino acids synthesized in the lab, and to attach them to specific tRNAs. This expands the genetic code, beyond the twenty canonical amino acids found in nature, to include an unnatural amino acid as well. The unnatural amino acid is coded by a nonsense (TAG, TGA, TAA) triplet, a quadruplet codon, or in some cases a redundant rare codon. The organism that expresses the mutant synthetase can then be genetically programmed to incorporate the unnatural amino acid into any desired position in any protein of interest, allowing biochemists or structural biologists to probe or change the protein's function. For instance, one can start with the gene for a protein that binds a certain sequence of DNA, and, by directing an unnatural amino acid with a reactive side-chain into the binding site, create a new protein that cuts the DNA at the target-sequence, rather than binding it.
Then:
the evolution of tRNA is a known and solved problem [
4]. Specifically, tRNA evolved from ligation of three 31-nt minihelices followed by 9-nt internal deletion(s). A single internal 9-nt deletion generated a type II tRNA (initially 84-nt) with an expanded variable loop. Two internal 9-nt deletions generated a type I tRNA (initially 75-nt).
Type 1 is the bracket, type 2 is the tweezers.
Finally (my favorite part):
By mutating aminoacyl tRNA synthetases, chemists have expanded the genetic codes of various organisms to include lab-synthesized amino acids with all kinds of useful properties: photoreactive, metal-chelating, xenon-chelating, crosslinking, spin-resonant, fluorescent, biotinylated, and redox-active amino acids. Another use is introducing amino acids bearing reactive functional groups for chemically modifying the target protein
Turns out, the 3 nucleotide code "demands" a 7 nt loop. 6 or 8 will not work, they won't bend into the proper shape. A 4 nucleotide code is possible with engineering but doesn't occur naturally. So if life evolves on another planet, chances are good it will use a 3 nucleotide tRNA.
