  We present a new method to determine the star formation and metal enrichment
histories of any resolved stellar system. This method is based on the fact that
any observed star in a colour-magnitude diagram will have a certain probability
of being associated with an isochrone characterised by an age t and metallicity
[Fe/H] (i.e. to have formed at the time and with the metallicity of that
isochrone). We formulate this as a maximum likelihood problem that is then
solved with a genetic algorithm. We test the method with synthetic simple and
complex stellar populations. We also present tests using real data for open and
globular clusters. We are able to determine parameters for the clusters (t,
[Fe/H]) that agree well with results found in the literature. Our tests on
complex stellar populations show that we can recover the star formation history
and age-metallicity relation very accurately. Finally, we look at the history
of the Carina dwarf galaxy using deep BVI data. Our results compare well with
what we know about the history of Carina.
