  Disentangling distinct stellar populations along the red-giant branches
(RGBs) of Globular Clusters (GCs) is possible by using the pseudo two-color
diagram dubbed chromosome map (ChM). One of the most intriguing findings is
that the so-called first-generation (1G) stars, characterized by the same
chemical composition of their natal cloud, exhibit extended sequences in the
ChM. Unresolved binaries and internal variations in helium or metallicity have
been suggested to explain this phenomenon. Here, we derive high-precision
Hubble Space Telescope photometry of the GCs NGC6362 and NGC6838 and build
their ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit wider
ChM sequences than those of second-generation (2G). The evidence of this
feature even among unevolved 1G MS stars indicates that chemical
inhomogeneities are imprinted in the original gas. We introduce a pseudo
two-magnitude diagram to distinguish between helium and metallicity, and
demonstrate that star-to-star metallicity variations are responsible for the
extended 1G sequence. Conversely, binaries provide a minor contribution to the
phenomenon. We estimate that the metallicity variations within 1G stars of 55
GCs range from less than [Fe/H]~0.05 to ~0.30 and mildly correlate with cluster
mass. We exploit these findings to constrain the formation scenarios of
multiple populations showing that they are qualitatively consistent with the
occurrence of multiple generations. In contrast, the fact that 2G stars have
more homogeneous iron content than the 1G challenges the scenarios based on
accretion of material processed in massive 1G stars onto existing protostars.
