  The apparent absence of stars in the Milky Way halo with -5 ~< [Fe/H] ~< -4
suggests that the gas out of which the halo stars were born experienced a
period of low or delayed star formation after the local universe was lit up by
the first, metal-free generation of stars (Pop III). Negative feedback owed to
the Pop III stars could initially have prevented the pre-Galactic halo from
cooling, which thereby delayed the collapse and inhibited further star
formation. During this period, however, the nucleosynthesis products of the
first supernovae (SNe) had time to mix with the halo gas. As a result, the
initially primordial gas was already weakly enriched in heavy elements, in
particular iron, at the time of formation of the Galactic halo. The very high,
observed C/Fe ratios in the two recently discovered hyper metal-poor stars
([Fe/H]<-5) HE 0107-5240 and HE 1327-2326 as well as the diversity of C/Fe
ratios in the population of extremely metal-poor stars ([Fe/H]<-3) are then
naturally explained by a combination of pre-enrichment by Pop III stars and
local enrichment by subsequent generations of massive, rotating stars, for
which the most massive ones end their lives as black hole-forming SNe, only
ejecting their outer (carbon-rich) layers. The possible existence of
populations of mega metal-poor/iron-free stars ([Fe/H]<-6) is also discussed.
