ABSTRACT We calculate the redshift evolution of the global 21 cm signal in the first billion years using an advanced semi-analytical galaxy formation model delphi. Employing only two redshift- and mass-independent free parameters, our model predicts galaxy populations in accord with data from both the JWST and the Atacama Large Millimetre Array (ALMA) at z ∼ 5–12. In addition to this ‘fiducial’ model, which fully incorporates the impact of dust attenuation, we also explore an unphysical ‘maximal’ model wherein galaxies can convert a 100 per cent of their gas into stars instantaneously (and supernova feedback is ignored) required to explain JWST data at z > =13. We also explore a wide range of values for our 21 cm parameters that include the impact of X-ray heating (fX,h = 0.02–2.0) and the escape fraction of Lyman Alpha photons (fα = 0.01–1.0). Our key findings are (i) the fiducial model predicts a global 21 cm signal, which reaches a minimum brightness temperature of Tb, min ∼ −215 mK at a redshift zmin ∼ 14; (ii) since the impact of dust on galaxy properties only becomes relevant at z < = 8, dust does not have a sensible impact on the global 21 cm signal; (iii) the ‘maximal’ model predicts Tb, min = −210 mK as early as zmin ∼ 18; and (iv) galaxy formation and 21 cm parameters have a degenerate impact on the global 21 cm signal. A combination of the minimum temperature and its redshift will therefore be crucial in constraining galaxy formation parameters and their coupling to the 21 cm signal at these early epochs.