  The reionization of cosmic hydrogen, left over from the big bang, increased
its temperature to >~ 1.e4 K. This photo-heating resulted in an increase of the
minimum mass of galaxies and hence a suppression of the cosmic star formation
rate. The affected population of dwarf galaxies included the progenitors of
massive galaxies that formed later. We show that a massive galaxy at a redshift
z >~ 6 should show a double-peaked star formation history marked by a clear
break. This break reflects the suppression signature from reionization of the
region in which the galaxy was assembled. Since massive galaxies originate in
overdense regions where cosmic evolution is accelerated, their environment
reionizes earlier than the rest of the universe. For a galaxy of ~ 1.e12
M_solar in stars at a redshift of z ~ 6.5, the star formation rate should
typically be suppressed at a redshift z >~ 10 since the rest of the universe is
known to have reionized by z >~ 6.5. Indeed, this is inferred to be the case
for HUDF-JD2, a massive galaxy which is potentially at z ~ 6.5 but is inferred
to have formed the bulk of its 3.e11 M_solar in stars at z >~ 9.
