  I present a new method to unveil the history of cosmic accretion and the
build-up of Supermassive Black Holes (SMBH) in the nuclei of galaxies, based on
observations of the evolving radio and (hard) X-ray luminosity functions of
AGN. The fundamental plane of black hole activity discovered by Merloni, Heinz
& Di Matteo (2003) is used as a mass and accretion rate estimator. I adopt the
local BH mass function as a boundary condition to integrate backwards in time
the continuity equation for the SMBH evolution, neglecting the role of mergers.
Under the most general assumption that accretion proceeds in a radiatively
efficient way above a certain rate, and in a radiatively inefficient way below,
the redshift evolution of the mass and accretion rate functions are calculated
self-consistently. The only tunable parameters are the accretion efficiency and
the critical ratio of the X-ray to Eddington luminosity at which the transition
between accretion modes takes place. The evolution of the BH mass function
between z=0 and z ~ 3 shows clear signs of an anti-hierarchical behaviour:
while the majority of the most massive objects (M > 10^9) were already in place
at z ~ 3, lower mass ones mainly grew at progressively lower redshift. As an
example, I will discuss the consequences of these results for the lifetimes of
accreting black holes.
