The cosmic evolution of active galaxies is influenced by their tiny central regions where the most energetic steady outflows of matter and energy in the known universe are produced. The paradigm that has emerged for these powerful engines involves an interaction between magnetized accretion flows and rotating supermassive black holes. I describe recent work according to which the power produced in such systems depends tightly on the spin parameter of the black hole. The consequence of this scenario is that the cosmological evolution of galaxies is intimately linked to the evolution of black hole spin, implying in turn, that the properties of the general relativistic spacetime that dominate the region very close to the center of galaxies are ultimately responsible for determining the properties of galaxies as a whole.