In the Archean era (3.8–2.5 Ga ago) the Earth probablylacked a protective ozone column. Using data obtained inthe Earth’s orbit on the inactivation of Bacillus subtilisspores we quantitatively estimate the potential biologicaleffects of such an environment. We combine this practicaldata with theoretical calculations to propose a historyof the potential UV stress on the surface of the Earthover time. The data suggest that an effective ozone columnwas established at a pO2 of ;5 3 1023 present atmosphericlevel. The improvement in the UV environmenton the early Proterozoic Earth might have been amuch more rapid event than has previously been supposed,with DNA damage rates dropping by two ordersof magnitude in the space of just a few tens of millionsof years. We postulate that a coupling between reducedUV stress and increased pO2 production could have contributed toward a positive feedback in the production ofozone in the early Proterozoic atmosphere. This wouldcontribute to the apparent rapidity of the oxidationevent. The data provide an evolutionary perspective onpresent-day Antarctic ozone depletion.