This Wikipedia article has a couple of speculations, with links, at the very end. One is that certain amino acids actually bind selectively to certain codons, even without tRNA. I'm interested, if rather skeptical. More likely is the second paper, whcih suggest that the number of amino acids was once significantly smaller, and that the codon - amino acid links branched. I find this much more plausible, since it gives one an evolutionary pathway from a simpler genome.