  The evolution of the metallicity of damped Lyman alpha systems (DLAs) is
investigated in order to understand the nature of these systems. The
observational data on chemical abundances of DLAs are analysed with robust
statistical methods, and the abundances are corrected for dust depletion. The
results of this analysis are compared to predictions of several classes of
chemical evolution models: one-zone dwarf galaxy models, multizone disk models,
and chemodynamical models representing dwarf galaxies. We compare the
observational data on the [alpha/Fe] and [N/alpha] ratios to the predictions
from the models. In DLAs, these ratios are only partially reproduced by the
dwarf galaxy one-zone model and by the disk model. On the other hand, the
chemodynamical model for dwarf galaxies reproduces the properties of nearly all
DLAs. We derive the formation epoch of dwarf galaxies, and we find that dwarf
galaxies make a significant contribution to the total neutral gas density in
DLAs, and that this contribution is more important at high redshifts (z > 2-3).
We propose a scenario in which the DLA population is dominated by dwarf
galaxies at high redshifts and by disks at lower redshifts. We also find that
Lyman Break Galaxies (LBGs) may constitute a sequence rather than present a
sharp dichotomy between the two populations. We also arise the possibility that
we could be missing a whole population of high HI density column objects, with
metallicities intermediate between those of DLAs and LBGs. Finally, we discuss
the possibility that relying only on the observations of DLAs could lead to an
underestimate of the metal content of the high redshift Universe.
