  The ages of globular clusters in our own Milky Way are known with precision
of about \pm 1 Gyr, hence their formation at redshifts z>~3 and their role in
hierarchical cosmology and the reionization of the intergalactic medium remain
relatively undetermined. Here we analyze the effect of globular cluster
formation on the observed rest-frame UV luminosity functions (LFs) and UV
continuum slopes of high redshift galaxies in the Hubble Ultra Deep Fields. We
find that the majority of present day globular clusters have formed during two
distinct epochs: at redshifts z ~ 2-3 and at redshifts z>~6. The birth of
proto-GC systems produce the steep, faint-end slopes of the galaxy LFs, and
because the brightness of proto-GCs fades 5 Myrs after their formation, their
blue colors are in excellent agreement with observations.
  Our results suggest that: i) the bulk of the old globular cluster population
with estimated ages >~12 Gyr (about 50% of the total population), formed in the
relatively massive dwarf galaxies at redshifts z>~6; ii) proto-GC formation was
an important mode of star formation in those dwarf galaxies, and likely
dominated the reionization process. Another consequence of this scenario is
that some of the most massive Milky Way satellites may be faint and yet
undiscovered because tidal stripping of a dominant GC population precedes
significant stripping of the dark matter halos of these satellites. This
scenario may alleviate some remaining tensions between CDM simulations and
observations.
