  Relative elemental abundances, and in particular the alpha/Fe ratio, are an
important diagnostic tool of the chemical evolution of damped Ly alpha systems
(DLAs). The S/Zn ratio is not affected by differential dust depletion and is an
excellent estimator of the alpha/Fe ratio. We report 6 new determinations of
sulphur abundance in DLAs at zabs greater than or equal to 2 with already known
zinc abundances. The combination with extant data from the literature provides
a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA
systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe]
enhancement observed in metal-poor stars of the Milky Way at comparable level
of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a
general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H].
This would be the first evidence of the expected decrease of the alpha/Fe ratio
in the course of chemical evolution of DLA systems. However, in contrast to
what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar
values when the metallicity is still low ([Zn/H] < -1) and to decrease below
solar values at higher metallicities. The behaviour of the alpha/Fe ratio
challenges the frequently adopted hypothesis that high redshift DLAs are
progenitors of spiral galaxies and favours instead an origin in galaxies
characterized by low star formation rates, in agreement with the results from
imaging studies of low redshift DLAs, where the candidate DLA galaxies show a
variety of morphological types including dwarfs and LSBs and only a minority of
spirals.
