  The chemical evolution history of a galaxy hides clues about how it formed
and has been changing through time. We have studied the chemical evolution
history of the Milky Way (MW) and Andromeda (M31) to find which are common
features in the chemical evolution of disc galaxies as well as which are
galaxy-dependent. We use a semi-analytic multi-zone chemical evolution model.
Such models have succeeded in explaining the mean trends of the observed
chemical properties in these two Local Group spiral galaxies with similar mass
and morphology. Our results suggest that while the evolution of the MW and M31
shares general similarities, differences in the formation history are required
to explain the observations in detail. In particular, we found that the
observed higher metallicity in the M31 halo can be explained by either a) a
higher halo star formation efficiency or b) a larger reservoir of infalling
halo gas with a longer halo formation phase. These two different pictures would
lead to a) a higher [O/Fe] at low metallicities or b) younger stellar
populations in the M31 halo, respectively. Both pictures result in a more
massive stellar halo in M31, which suggests a possible correlation between the
halo metallicity and its stellar mass.
