  We investigate the effect of variation in the value of the fine structure
constant at high redshifts (recombination > z > 30) on the absorption of the
cosmic microwave background (CMB) at 21 cm hyperfine transition of the neutral
atomic hydrogen. We find that the 21 cm signal is very sensitive to the
variations in the fine structure constant and it is so far the only probe of
the fine structure constant in this redshift range. A change in the value of
the fine structure constant by 1% changes the mean brightness temperature
decrement of the CMB due to 21 cm absorption by > 5% over the redshift range z
< 50. There is an effect of similar magnitude on the amplitude of the
fluctuations in the brightness temperature. The redshift of maximum absorption
also changes by ~ 5%.

Astrophysics