  Candidates for the modest galaxies that formed most of the stars in the early
universe, at redshifts $z > 7$, have been found in large numbers with extremely
deep restframe-UV imaging. But it has proved difficult for existing
spectrographs to characterise them in the UV. The detailed properties of these
galaxies could be measured from dust and cool gas emission at far-infrared
wavelengths if the galaxies have become sufficiently enriched in dust and
metals. So far, however, the most distant UV-selected galaxy detected in dust
emission is only at $z = 3.25$, and recent results have cast doubt on whether
dust and molecules can be found in typical galaxies at this early epoch. Here
we report thermal dust emission from an archetypal early universe star-forming
galaxy, A1689-zD1. We detect its stellar continuum in spectroscopy and
determine its redshift to be $z = 7.5\pm0.2$ from a spectroscopic detection of
the Ly{\alpha} break. A1689-zD1 is representative of the star-forming
population during reionisation, with a total star-formation rate of about
12M$_\odot$ yr$^{-1}$. The galaxy is highly evolved: it has a large stellar
mass, and is heavily enriched in dust, with a dust-to-gas ratio close to that
of the Milky Way. Dusty, evolved galaxies are thus present among the fainter
star-forming population at $z > 7$, in spite of the very short time since they
first appeared.
