Evolution of the First Code

Background/Objectives: tRNAs, tRNAomes, aminoacyl-tRNA synthetases (AARS), first proteins, the ribosome and the genetic code coevolved. We utilize sequence data to reconstruct key steps in establishing the first code on Earth. Methods: Networks were constructed to describe initial tRNAome and AARSome evolution. Results: tRNA-34 wobble modifications and tRNA-37 modifications were necessary to evolve the code, as were additional tRNA modifications, so diverse tRNA modification en-zymes (i.e., histidyl-tRNA -1 GTP synthase) are among first proteins. tRNA-linked chemistry brought asparagine, glutamine, cysteine and possibly additional amino acids into the code. tRNA, tRNA modifications and tRNA-linked chemistry were core founding innovations for code evolution. Coevolution of AARSomes was also essential. Class II and class I AARS have distinct folds but are nonetheless homologs by se-quence. Early AARS enzymes folded around Zn motifs. Networks were generated for tRNAomes and AARSomes in ancient Archaea, because Archaea are the closest living organisms to the last universal common ancestor. Conclusions: The first code on Earth was surprisingly ordered, and the few apparent deviations from regular order can yet be explained. Early in evolution of the code, innovation was more strongly selected than accuracy. The code froze, however, because of evolving fidelity mechanisms. A historical record was documented in tRNA and the genetic code and has been pre-served in living organism sequence.