  According to Pei, Fall & Hauser (1999), the global metallicity evolution of
the Universe can be represented by the ratio of the total metal content to the
total gas content measured in Damped Lyman-alpha (DLA) systems (the ``column
density weighted metallicity'' `a la Pettini). To minimize dust obscuration
effects, a DLA sample with negligible dust content is considered, namely, 50
DLAs with log N(HI) < 20.8. The global metallicity found shows clear evidence
of redshift evolution that goes from \sim1/30 solar at z\sim4.1 to solar at z
\sim 0.4. More generally, DLAs with measured heavy elements probe the ISM of
high redshift galaxies. The whole sample collected from the literature contains
75 DLAs. The metallicity is calculated adopting for the dust correction the
most general method used so far, based on models of the ISM dust depletions in
the Galaxy. The intrinsic metallicity evolution of DLA galaxies is dlog
Z(DLA)/dz \propto -0.33 +/- 0.06.
