  We explore the mass-assembly and chemical enrichment histories of star
forming galaxies by applying a population synthesis method to a sample of 84828
galaxies from the Sloan Digital Sky Survey Data Release 5. Our method
decomposes the entire observed spectrum in terms of a sum of simple stellar
populations spanning a wide range of ages and metallicities, thus allowing the
reconstruction of galaxy histories. A comparative study of galaxy evolution is
presented, where galaxies are grouped onto bins of nebular abundances or mass.
We find that galaxies whose warm interstellar medium is poor in heavy elements
are slow in forming stars. Their stellar metallicities also rise slowly with
time, reaching their current values ($Z_\star \sim 1/3 Z_\odot$) in the last
$\sim 100$ Myr of evolution. Systems with metal rich nebulae, on the other
hand, assembled most of their mass and completed their chemical evolution long
ago, reaching $Z_\star \sim Z_\odot$ already at lookback times of several Gyr.
These same trends, which are ultimately a consequence of galaxy downsizing,
appear when galaxies are grouped according to their stellar mass. The
reconstruction of galaxy histories to this level of detail out of integrated
spectra offers promising prospects in the field of galaxy evolution theories.
