  We calculate the redshift evolution of the global 21cm signal in the first
billion years using a semi-analytic galaxy formation model, DELPHI, that
jointly tracks the assembly of dark matter halos and their constituent baryons
including the impact of supernova feedback and dust enrichment. Employing only
two redshift- and mass-independent free parameters, our model predicts galaxy
populations in accord with data from both the James Webb Space Telescope (JWST)
and the Atacama Large Millimetre Array (ALMA) at $z \sim 5-12$. 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\% 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 {\it 21cm} parameters that include the impact of
X-ray heating ($f_{\rm X,h} =0.02-2.0$) and the escape fraction of Lyman Alpha
photons ($f_\alpha = 0.01-1.0$). Our key findings are: (i) the fiducial model
predicts a global 21cm signal which reaches a minimum brightness temperature of
$ T_{\rm b, min}\sim -215$ mK at a redshift $z_{\rm min} \sim 14$; (ii) since
the impact of dust on galaxy properties (such as the star formation rate
density) only becomes relevant at $z <= 8$, dust does not have a sensible
impact on the global 21cm signal; (iii) the ``maximal" model predicts $T_{\rm
b, min}= -210$ mK as early as $z_{\rm min} \sim 18$; (iv) galaxy formation and
21cm parameters have a degenerate impact on the global 21cm signal. A
combination of the minimum temperature and its redshift will therefore be
crucial in constraining galaxy formation parameters and their coupling to the
21cm signal at these early epochs.
