  Observations of the most luminous quasars at redshift z>6 reveal the
existence of numerous supermasssive black holes (>10^9 Msun) already in place
about twelve billion years ago. In addition, the interstellar medium of the
galaxies hosting these black holes are observed to be chemically mature
systems, with metallicities (Z>Zsun) and dust masses (>10^8 Msun) similar to
that of more evolved, local galaxies. The connection between the rapid growth
of the first supermassive black holes and the fast chemical evolution of the
host galaxy is one of the most puzzling issues for theoretical models. Here we
review state-of-the-art theoretical models that focus on this problem with
particular emphasis on the conditions that lead to the formation of quasar
seeds and their subsequent evolution at z>6.
