The Cambrian explosion was, however, constrained by the partial pressure of oxygen in the hydrosphere (and, indirectly, the atmosphere). And free O2 couldn't be

The Cambrian explosion was, however, constrained by the partial pressure of oxygen in the hydrosphere (and, indirectly, the atmosphere). And free O2 couldn't begin to build up in seawater until after the Great Oxygen Catastrophe when the crust and upper mantle was finally fully oxidized, mopping up all the unoxidized iron and other elements. (Which in turn killed off almost all previous life forms, which were methanogenic and to which oxygen was a deadly poison/metabolic waste product.) The speed with which this happened was in turn constrained by tectonic subduction ... it all turns out to be a knotty ball of string that took at least a couple of billion years to unwind.