  The analysis of the kinematics of solar neighbourhood stars shows that the
low and high metallicity tails of the thin disc are populated by objects which
orbital properties suggest an origin in the outer and inner galactic disc,
respectively. Signatures of radial migration are identified in various recent
samples, and are shown to be responsible for the high metallicity dispersion in
the age-metallicity distribution. Most importantly, it is shown that the
population of low metallicity wanderers of the thin disc (-0.7<[Fe/H]<-0.3 dex)
is also responsible for the apparent hiatus in metallicity with the thick disc
(which terminal metallicity is about -0.2 dex). It implies that the thin disc
at the solar circle has started to form stars at about this same metallicity.
This is also consistent with the fact that 'transition' objects, which have
alpha-element abundance intermediate between that of the thick and thin discs,
are found in the range [-0.4,-0.2] dex. Once the metal-poor thin disc stars are
recognised for what they are - wanderers from the outer thin disc - the
parenthood between the two discs can be identified on stars genuinely formed at
the solar circle through an evolutionary sequence in [alpha/Fe] and [Fe/H] .
Another consequence is that stars that can be considered as truly resulting of
the chemical evolution at the solar circle have a metallicity restricted to
about [-0.2,+0.2] dex, confirming an old idea that most chemical evolution in
the Milky Way have preceded the thin disc formation.
