  The discovery of high redshift quasars represents a challenge to the origin
of supermassive black holes. Here, two evolutionary scenarios are considered.
The first one concerns massive black holes in the local universe, which in a
large majority have been formed by the growth of seeds as their host galaxies
are assembled in accordance with the hierarchical picture. In the second
scenario, seeds with masses around 100-150 M? grow by accretion of gas forming
a non-steady massive disk, whose existence is supported by the detection of
huge amounts of gas and dust in high-z quasars. These models of non-steady
self-gravitating disks explain quite well the observed "Luminosity-Mass"
relation of quasars at high-z, indicating also that these objects do not
radiate at the so-called Eddington limit.
