  Long-duration gamma-ray bursts (GRBs) are associated with the death of
metal-poor massive stars. Even though they are highly transient events very
hard to localize, they are so bright that they can be detected in the most
difficult environments. GRB observations are unveiling a surprising view of the
chemical state of the distant universe (redshifts z > 2). Contrary to what is
expected for a high-z metal-poor star, the neutral interstellar medium (ISM)
around GRBs is not metal poor (metallicities vary from ~1/10 solar at z = 6.3
to about solar at z = 2) and is enriched with dust (90-99% of iron is in solid
form). If these metallicities are combined with those measured in the warm ISM
of GRB host galaxies at z < 1, a redshift evolution is observed. Such an
evolution predicts that the stellar masses of the hosts are in the range M* =
10^(8.6-9.8) Msun. This prediction makes use of the mass-metallicity relation
(and its redshift evolution) observed in normal star-forming galaxies.
Independent measurements coming from the optical-NIR photometry of GRB hosts
indicate the same range of stellar masses, with a typical value similar to that
of the Large Magellanic Cloud. This newly detected population of
intermediate-mass galaxies is very hard to find at high redshift using
conventional astronomy. However, it offers a compelling and relatively
inexpensive opportunity to explore galaxy formation and cosmic chemical
evolution beyond known borders, from the primordial universe to the present.
