In a recent study (Boyle et al. 2014), we show that the animal invasion of the seafloor at the Ediacaran/Cambrian boundary (~542 Ma) affected the global marine phosphorous and carbon cycles through bioturbation. Ultimately, this resulted in lowering of atmospheric O2 levels on a million year time scale. As O2 levels decreased, bioturbation decreased as well. This mechanism explains why O2 levels have remained stable after the emergence of motile animals. To put it shortly, early animal ecosystems took stewardship over O2 regulation on Earth!
Boyle et al. Nature Geoscience (2014): Stabilization of the coupled oxygen and phosphorus cycles by the evolution of bioturbation
Filip Meysman News & Views Nature: Oxygen burrowed away