have your planet evolve so that, until you reach the awakening stage, all multicellular life uses photosynthesis as a main energy source, and make the cambrian explosion happen by evolving a digestive system that only gives you the minerals and nutrients from organisms you eat, and nowhere near enough of the energy from them to let you become heterotrophic.
yummy hydrogen: evolve past the multicellular stage as a purely hydrogenotrophic organism