  Aims: We investigate the cosmological growth of dark halos and follow the
consequences of coeval growth for the accretion history of associated
supermassive black holes. Methods: The Press-Schechter approximation is used to
obtain an analytic expression for the mean rate of growth of dark matter halos.
Dark halo accretion rates are compared with numerical work and the consequences
for understanding AGN evolution are described. Results: The mean accretion rate
onto dark matter halos is shown to have a simple analytic form that agrees with
previous numerical work and that may easily be calculated for a wide range of
halo mass, redshift and cosmological parameters. The result offers a
significant improvement over published fitting formulae deduced from merger
trees. We then consider the growth of associated supermassive black holes, and
make a basic test of the simple hypothesis of `Pure Coeval Evolution' (PCE) in
which, on average, black hole growth tracks dark halo growth. We demonstrate
that both the absolute value of the integrated AGN bolometric luminosity
density and its cosmological evolution derived from hard X-ray surveys are
well-reproduced by PCE. Excellent agreement is found at z >~ 0.5, although the
observed luminosity density drops by a factor 2 compared with PCE by z=0: black
hole growth appears to decouple from halo growth at low redshifts, and this may
be related to the phenomenon of `cosmic downsizing'. Overall, AGN evolution
appears either to be caused by or to be closely linked to the slow-down in the
growth of cosmic structure. We also discuss the mean Eddington ratio averaged
over all galaxies, which is predicted to show strong evolution to higher values
with redshift.
