  We present the metallicity distribution of a sample of 471 RR Lyrae (RRL)
stars in the Sculptor dSph, obtained from the $I$-band Period-Luminosity
relation. It is the first time that the early chemical evolution of a dwarf
galaxy is characterized in such a detailed and quantitative way, using
photometric data alone. We find a broad metallicity distribution (FWHM=0.8 dex)
that is peaked at [Fe/H]$\simeq$-1.90 dex, in excellent agreement with
literature values obtained from spectroscopic data. Moreover, we are able to
directly trace the metallicity gradient out to a radius of $\sim$55 arcmin. We
find that in the outer regions (r$>\sim$32 arcmin) the slope of the metallicity
gradient from the RRLs (-0.025 dex arcmin$^{-1}$) is comparable to the
literature values based on red giant (RG) stars. However, in the central part
of Sculptor we do not observe the latter gradients. This suggests that there is
a more metal-rich and/or younger population in Sculptor that does not produce
RRLs. This scenario is strengthened by the observation of a metal-rich peak in
the metallicity distribution of RG stars by other authors, which is not present
in the metallicity distribution of the RRLs within the same central area.
