The first generation of stars: first steps towards chemical evolution of galaxies

We argue that extreme metal-poor stars show a high dispersion in metallicity, because their abundances are the outcome of very few supernova events. Abundance anomalies should appear because of the discrete range of progenitor masses. There is a natural metallicity threshold of $Z/Z_\odot\sim 10^{-4}$ below which one would expect to find very few, if any, halo stars. Similar reasoning is applied to lower mass systems, such as metal-poor compact blue galaxies and Lyman alpha absorption line clouds seen towards high redshift quasars, where a somewhat higher threshold is inferred.