  Infrared observations of high-z quasar (QSO) hosts indicate the presence of
large masses of dust in the early universe. When combined with other
observables, such as neutral gas masses and star formation rates, the dust
content of z~6 QSO hosts may help constraining their star formation history. We
have collected a database of 58 sources from the literature discovered by
various surveys and observed in the FIR. We have interpreted the available data
by means of chemical evolution models for forming proto-spheroids,
investigating the role of the major parameters regulating star formation and
dust production. For a few systems, given the derived small dynamical masses,
the observed dust content can be explained only assuming a top-heavy initial
mass function, an enhanced star formation efficiency and an increased rate of
dust accretion. However, the possibility that, for some systems, the dynamical
mass has been underestimated cannot be excluded. If this were the case, the
dust mass can be accounted for by standard model assumptions. We provide
predictions regarding the abundance of the descendants of QSO hosts; albeit
rare, such systems should be present and detectable by future deep surveys such
as Euclid already at z>4.
