The evolution of ribonucleotide reduction.

Ribonucleotide reduction was essential for the transition from RNA to DNA by supplying deoxyribonucleotide precursors. The reaction requires free radical chemistry. Three quite different classes of ribonucleotide reductases are known today. All three are proteins containing a stable free radical amino acid, but each uses a different mechanism for its generation. Did they evolve from a common ancestor, with the arrival of atmospheric oxygen providing the driving force for their divergence, or was each a separate evolutionary invention?