  Utilizing a new, high mass resolution hydrodynamic simulation we compute the
metallicity evolution of damped Lyman alpha systems (DLAs) and find a
reasonable agreement with observations. In particular, the observed slow
evolution of the DLA metallicity occurs naturally in the simulation due to the
combined effects of physical and observational selection. The slow metallicity
evolution is caused by the steady transformation, with increasing time, of the
highest metallicity systems to "galaxies", thus depleting this category, while
all the lower metallicity systems show, individually, an increase in
metallicity. Although the trend of DLA metallicity with redshift is in good
agreement with observations, it appears that the average metallicity of
simulated DLAs is higher than observed by 0.3-0.5 dex in the probed redshift
range (z=0-5). Our study indicates that this difference may be attributed to
observational selection effects due to dust obscuration. If we allow for a dust
obscuration effect, our model reproduces the observed metallicity evolution in
both amplitude and slope. We find that DLAs are not a simple population but
probe a range of different systems and the mix changes with redshift. About 50%
of all metals in the gaseous phase is in DLAs at all times from z=5 to z=1,
making a rapid downturn at z\le 1 to ~20% by z=0, as metals are swept into the
hotter components of the IGM as well as locked up in stars. While not the
primary focus of this study, we find that the model provides good matches to
observations with respect to column density distribution and evolution of
neutral gas content, if the same dust obscuration is taken into account. We
find Omega_{DLA,comp}=(1-3)E-3, depending on the effect of dust obscuration.
