  Observations show a tight correlation between the stellar mass of galaxies
and their gas-phase metallicity (MZR). This relation evolves with redshift,
with higher-redshift galaxies being characterized by lower metallicities.
Understanding the physical origin of the slope and redshift evolution of the
MZR may provide important insight into the physical processes underpinning it:
star formation, feedback, and cosmological inflows. While theoretical models
ascribe the shape of the MZR to the lower efficiency of galactic outflows in
more massive galaxies, what drives its evolution remains an open question. In
this letter, we analyze how the MZR evolves over $z=0-3$, combining results
from the FIREbox cosmological volume simulation with analytical models.
Contrary to a frequent assertion in the literature, we find that the evolution
of the gas fraction does not contribute significantly to the redshift evolution
of the MZR. Instead, we show that the latter is driven by the
redshift-dependence of the inflow metallicity, outflow metallicity, and mass
loading factor, whose relative importance depends on stellar mass. These
findings also suggest that the evolution of the MZR is not explained by
galaxies moving along a fixed surface in the space spanned by stellar mass, gas
phase metallicity, and star formation rate.
