Evolution and diversification of Group 1 [NiFe] hydrogenases. Is there a phylogenetic marker for O2-tolerance?

► O2-tolerant hydrogenases harbor a novel [FeS] cofactor with 6 cysteine (6C) coordination. ► Through genomic screenings we map the 6C trait onto the most recent phylogenetic tree. ► Structural/functional characteristics correlate with the planet's geochemical history. ► 6C hydrogenases are omnipresent in recent microaerophilic and aerobic bacterial phyla. ► A link between 6C enzymes and increasing oxidative atmospheric conditions is proposed.