We investigate the effects of Population III stars on the sky-averaged 21-cm background radiation , which traces the collective emission from all sources of ultraviolet and X-ray photons before reionization is complete . While UV photons from PopIII stars can in principle shift the onset of radiative coupling of the 21-cm transition – and potentially reionization – to early times , we find that the remnants of PopIII stars are likely to have a more discernible impact on the 21-cm signal than PopIII stars themselves . The X-rays from such sources preferentially heat the IGM at early times , which elongates the epoch of reheating and results in a more gradual transition from an absorption signal to emission . This gradual heating gives rise to broad , asymmetric wings in the absorption signal , which stand in contrast to the relatively sharp , symmetric signals that arise in models treating PopII sources only . A stronger signature of PopIII , in which the position of the absorption minimum becomes inconsistent with PopII-only models , requires extreme star-forming events that may not be physically plausible , lending further credence to predictions of relatively high frequency absorption troughs , \nu _ { \min } \sim 100 MHz . As a result , though the trough location alone may not be enough to indicate the presence of PopIII , the asymmetric wings should arise even if only a few PopIII stars form in each halo before the transition to PopII star formation occurs , provided that the PopIII IMF is sufficiently top-heavy and at least some PopIII stars form in binaries .