  We critically review the basic assumptions of the standard model for the
synthesis of the XRB in the light of new data from ultradeep surveys by Chandra
and XMM, resolving major parts of it. Important constraints come in particular
from the observed z-distributions of faint hard X-ray sources -- showing large
excesses at redshifts (z~0.8) much lower than expected by the synthesis models
-- and from their X/optical/IR SEDs combined with the IR counts of type-II
AGNs. Our analysis, although supporting the general scheme which interprets the
XRB as due to absorbed AGNs with broad N_H distributions, requires major
revision of the other postulate of the XRB synthesis models: the AGN
unification. We argue that the unification scheme based on a simple orientation
effect fails at high redshifts, where galaxy activity is induced by strong
interactions and mergers among gas-rich systems. This helps explaining the
observational evidence that type-I and II AGNs follow different evolutionary
patterns, with type-I quasars providing a very biased trace of this activity.
Combined deep X-ray and IR surveys consistently find that the universe has
experienced a violent phase of galaxy activity around z~1, probably related
with the assembly of massive galaxies, involving both SF and obscured AGN
fueling. Roughly 10 to 20% of this activity has involved substantial AGN
emission, this fraction likely reflecting the AGN/starburst duty cycle during
the activation phase.
