The most common belief is that RNA came first -- it has both the information-carrying capacity and enzymatic activity. As for why are there 20 encoded amino ac

The most common belief is that RNA came first -- it has both the information-carrying capacity and enzymatic activity. As for why are there 20 encoded amino acids (+ selenocysteine and pyrrolysine)? I've read that codon similarities hint at an original more compact alphabet of two base codons (allowing for up to 15 distinct amino acids and a stop). I've also read that the "more recent" amino acids, like cysteine, are more readily oxidized and thus provided some advantage as atmospheric oxygen levels increased. But all of these are guesses about the world before LUCA, which is really lost to us.