Evolution of the first burrowing animals may have played a major role in stabilizing the Earth's oxygen reservoir, researchers hypothesize. The first burrowing animals significantly increased the extent to which oxygenated waters came into contact with ocean sediments. Exposure to oxygenated conditions caused the bacteria that inhabit such sediments to store phosphate in their cells. This caused an increase in phosphorus burial in sediments that had been mixed up by burrowing animals. This in turn triggered decreases in marine phosphate concentrations, productivity, organic carbon burial and ultimately oxygen....