We address the chemical/biological history of HO back at the times of the Archean eon (2.5-3.9 billion years ago (Gya)). During the Archean eon the pO was million-fold lower than the present pO, starting to increase gradually from 2.3 until 0.6 Gya, when it reached ca. 0.2 bar. The observation that some anaerobic organisms can defend themselves against O has led to the view that early organisms could do the same before oxygenic photosynthesis had developed at about 3 Gya. This would require the anaerobic generation of HO, and here we examine the various mechanisms which were suggested in the literature for this. Given the concentration of Fe at 20-200 μM in the Archean ocean, the estimated half-life of HO is ca. 0.7 s. The oceanic HO concentration was practically zero. We conclude that early organisms were not exposed to HO before the arrival of oxygenic photosynthesis.