  Studies of the formation of the first stars have established that they formed
in small halos of $\sim 10^5 - 10^6 M_{\odot}$ via molecular hydrogen cooling.
Since a low level of ultraviolet radiation from stars suffices to dissociate
molecular hydrogen, under the usually-assumed scenario this primordial mode of
star formation ended by redshift $z \sim 15$ and much more massive halos came
to dominate star formation. However, metal enrichment from the first stars may
have allowed the smaller halos to continue to form stars. In this Letter we
explore the possible effect of star formation in metal-rich low-mass halos on
the redshifted 21-cm signal of neutral hydrogen from $z = 6-40$. These halos
are significantly affected by the supersonic streaming velocity, with its
characteristic baryon acoustic oscillation (BAO) signature. Thus, enrichment of
low-mass galaxies can produce a strong signature in the 21-cm power spectrum
over a wide range of redshifts, especially if star formation in the small halos
was more efficient than suggested by current simulations. We show that upcoming
radio telescopes can easily distinguish among various possible scenarios.
