We propose that glycine was the first amino acid to be incorporated into the genetic code, followed by serine, aspartic and/or glutamic acid-small hydrophilic amino acids that all have codons in the bottom right-hand corner of the standard genetic code table. Because primordial ribosomal synthesis is presumed to have been rudimentary, this stage would have been characterized by the synthesis of short, water-soluble peptides, the first of which would have comprised polyglycine. Evolution of the code is proposed to have occurred by the duplication and mutation of tRNA sequences, which produced a radiation of codon assignment outwards from the bottom right-hand corner. As a result of this expansion, we propose a trend from small hydrophilic to hydrophobic amino acids, with selection for longer polypeptides requiring a hydrophobic core for folding and stability driving the incorporation of hydrophobic amino acids into the code.