  We quantify the gas-phase abundance of deuterium and fractional contribution
of stellar mass loss to the gas in cosmological zoom-in simulations from the
Feedback In Realistic Environments project. At low metallicity, our simulations
confirm that the deuterium abundance is very close to the primordial value. The
chemical evolution of the deuterium abundance that we derive here agrees
quantitatively with analytical chemical evolution models. We furthermore find
that the relation between the deuterium and oxygen abundance exhibits very
little scatter. We compare our simulations to existing high-redshift
observations in order to determine a primordial deuterium fraction of 2.549 +/-
0.033 x 10^-5 and stress that future observations at higher metallicity can
also be used to constrain this value. At fixed metallicity, the deuterium
fraction decreases slightly with decreasing redshift, due to the increased
importance of mass loss from intermediate-mass stars. We find that the
evolution of the average deuterium fraction in a galaxy correlates with its
star formation history. Our simulations are consistent with observations of the
Milky Way's interstellar medium: the deuterium fraction at the solar circle is
85-92 per cent of the primordial deuterium fraction. We use our simulations to
make predictions for future observations. In particular, the deuterium
abundance is lower at smaller galactocentric radii and in higher mass galaxies,
showing that stellar mass loss is more important for fuelling star formation in
these regimes (and can even dominate). Gas accreting onto galaxies has a
deuterium fraction above that of the galaxies' interstellar medium, but below
the primordial fraction, because it is a mix of gas accreting from the
intergalactic medium and gas previously ejected or stripped from galaxies.
