||Atmospheric O2 levels have played a key role in the evolution of vertebrates (Canfield, Poulton and Narbonne 2007). Leinfelder and coworkers (Leinfelder et al. 1988) have suggested that the highly oxidizable Sec (Jacob et al. 2003) is counter selected (substituted by Cys) in response to rising O2 levels, a hypothesis later embraced by Jukes (1990). Although this adaptive factor was suggested to be important 2.4 billion years ago, examples of molecular adaptations to variable O2 concentrations have been described in animals (Bargelloni, Marcato and Patarnello 1998). A great increase in O2 levels in late Proterozoic (~600 Myr ago) preceded the appearance of the first animals, and wide variations in atmospheric O2 concentrations followed in the Phanerozoic (~550 Myr ago to present). Vertebrates have evolved for half a billion years with a maximum O2 concentration around 300 Myr ago (~31% O2), a minimum about 200 Myr ago (~13% O2), followed by a steady rise to present times (21% O2) (Berner 2006 Berner, VandenBrooks and Ward 2007). O2 levels have been recently proposed to drive non-neutral evolution of eukaryotic selenoproteomes (Lobanov et al. 2007 Lobanov, Hatfield and Gladyshev 2008).