Redshift evolution of key quantities affecting the 21 cm signal. The green, blue, and magenta curves correspond to contributions from PBH-only, SF-only, and SF plus PBH galaxies, respectively. Leftmost panel: Intrinsic X-ray emissivity as a function of redshift. In this case, PBH-only dominates over SF-only for 30 < z < 15. Middle panel: Evolution of the Ly-α background flux. While dominated by PBH-seeded sources for the very early redshift around z = 30 − 20, the SF-only galaxy completely dominates the redshift z = 20 − 7. Rightmost panel: Ionising photon emissivity, which mimics the Ly-α background trends.