Neutral Gas and Metals From z = 4 to z = 0.5

A complementary method to the emission selection of high-redshift galaxies consists in the observation of absorbers along the line of sight toward a background quasar. This selection technique has a constant sensitivity at all redshifts up to z = 6 (i.e. no redshift desert) and allow to select all types of galaxies regardless of their luminosity or star formation rate. The highest column density absorbers, the Damped Lyman-α (DLAs) systems, in particular, can be used to determine the cosmic evolution of HI gas in the Universe, Ω_HI, and the global metallicity in the gas phase. Since stars are known to form from HI gas, Ω_HI provides an indirect tracer of the history of star formation. Recent results from several parallel VLT programmes aiming at determining the cosmological evolution of the metallicity in the neutral gas phase are presented.