holes shown in figure 1. This means that the evolutionary path in which the BH mass grows as the host galaxy is assembled, which is probably the origin of the $M - \sigma$ relation, is unable to explain the existence of very massive BHs in the early universe or SMBHs present today in bright galaxies like NGC 5252 or Cygnus A. In the next sections an alternative evolutionary path will be examined.

The early formation of SMBHs
============================

As it was shown in the previous section, the coeval evolution of seeds and host galaxies is not able to explain the existence of bright QSOs at high redshift and the fact that in the local universe some objects have masses higher than that expected from the simulated $M - \sigma$ relation.

Could a unique accretion disk form a SMBH in a timescale of about 1 Gyr? The answer to this question implies the solution of different related problems. The disk must be quite massive in order to provide enough gas to form a $10^9 ~M_\odot$ black hole and the angular momentum transfer must be very efficient in order to maintain a high accretion rate necessary to provide the observed luminosities as 