The distribution of enriched material in the stars and gas of our Galaxy contains information pertaining to the chemical evolution of the Milky Way from its formation epoch to the present, providing general constraints on theories of galaxy formation. Detailed studies of the metallicities of well-defined samples of long-lived G-dwarf stars in the solar neighbourhood have ruled out the ‘simple closed-box’ model of galactic chemical evolution—too few very metal-poor stars are observed1–3. The weakest assumption inherent in this model is that the galactic disk formed and evolved as a closed system. Allowing accretion of gas, either metal-enriched or primordial (the latter requiring a particular dependence of star formation on gas density), can yield an improved fit to the observations3. However, secondary infall from the galactic extreme spheroid provides only a slight alleviation of this ‘G-dwarf problem’4. Here we show that consideration of the chemical properties of the thick-disk population of the Galaxy5,6 results in a self-consistent model for galactic chemical evolution.