  We compare the evolution with cosmic time of the star-formation rate per
comoving volume in galaxies and of the volume emissivity by Active Galactic
Nuclei, as a clue to understand the relationship between black hole accretion
and the formation of the surrounding structure. We find an interesting
similarity between the evolution rates for the total populations of galaxies
and AGNs, which indicates that, on average, the history of BH accretion tracks
that of stellar formation in the hosting galaxies. Similarly, the evolution of
luminous quasars parallels that of the stellar populations in massive
spheroidal galaxies, in keeping with the locally established association of
supermassive BHs and galactic bulges. We finally comment on our finding that
high-luminosity, high-mass systems evolve on a faster cosmic timescale than the
lower mass ones: to explain this, the theories of structure formation based on
the gravitational collapse of dark matter halos have to be complemented with a
detailed description of the dynamical processes in the baryonic component,
which dominate the formation and evolution in high-density environments.
