  Using MOPED we determine non-parametrically the star-formation and
metallicity history of over 37,000 high-quality galaxy spectra from the Sloan
Digital Sky Survey (SDSS) early data release. We use the entire spectral range,
rather than concentrating on specific features, and we estimate the complete
star formation history without prior assumptions about its form (by
constructing so-called `population boxes'). The main results of this initial
study are that the star formation rate in SDSS galaxies has been in decline for
~6 Gyr; a metallicity distribution for star-forming gas which is peaked ~3 Gyr
ago at about solar metallicity, inconsistent with closed-box models, but
consistent with infall models. We also determine the infall rate of gas in SDSS
and show that it has been significant for the last 3 Gyr. We investigate errors
using a Monte-Carlo Markov Chain algorithm. Further, we demonstrate that
recovering star formation and metallicity histories for such a large sample
becomes intractable without data compression methods, particularly the
exploration of the likelihood surface. By exploring the whole likelihood
surface we show that age-metallicity degeneracies are not as severe as by using
only a few spectral features. We find that 65% of galaxies contain a
significant old population (with an age of at least 8 Gyr), including recent
starburst galaxies, and that over 97% have some stars older than 2 Gyr. It is
the first time that the past star formation history has been determined from
the fossil record of the present-day spectra of galaxies.
