  We construct star formation histories at redshifts z > 5 for two physically
distinct populations of primordial, metal-free stars, motivated by theoretical
and observational arguments that have hinted towards the existence of an
intermediate stellar generation between Population III and Population I/II.
Taking into account the cosmological parameters as recently revised by WMAP
after three years of operation, we determine self-consistent reionization
histories and discuss the resulting chemical enrichment from these early
stellar generations. We find that the bulk of ionizing photons and heavy
elements produced at high redshifts must have originated in Population II.5
stars, which formed out of primordial gas in haloes with virial temperatures >=
10^4 K, and had typical masses >= 10 M_sun. Classical Population III stars,
formed in minihaloes and having masses >= 100 M_sun, on the other hand, had
only a minor impact on reionization and early metal enrichment. Specifically,
we find that only 10 per cent by mass of metal-free star formation went into
Population III.
