The Cambrian Explosion is one of the most significant events in the history of life;essentially all easily fossilizable animal body plans first evolved during this event.Although many theories have been proposed to explain this event,its cause remains unresolved.Here,we propose that the elevated level of oxygen,in combination with the increased mobility and food intake of metazoans,led to increased cellular levels of reactive oxygen species (ROS),which drove evolution by enhancing mutation rates and providing new regulatory mechanisms.Our hypothesis may provide a unified explanation for the Cambrian Explosion as it incorporates both environmental and developmental factors and is also consistent with ecological explanations for animal radiation.Future studies should focus on testing this hypothesis,and may lead to important insights into evolution.