  We consider the evolution of the sky-averaged 21 cm background during the
early phases of structure formation. Using simple analytic models, we calculate
the thermal and ionization histories, assuming that stellar photons dominate
the radiation background. The resulting 21 cm spectra can constrain the
properties of the first generations of stars and quasars. If Population II
stars dominate, Lyman-alpha coupling renders the IGM visible before it is
heated by X-rays and long before reionization. Thus the 21 cm background has a
strong absorption epoch followed by weaker emission that fades during
reionization. The harder spectra of very massive Population III stars compress
these transitions into a shorter time interval and decreases the signal
amplitude. However, the reionization epoch will remain visible except in
extreme cases. The global 21 cm signal will be challenging to observe because
of astronomical foregrounds, but it offers an exciting opportunity to study the
first sources of light. It also fixes the overall amplitude of the fluctuating
background whose detection is a major goal of several next-generation
low-frequency radio interferometers.
