  Evolution of the 7Li abundance in the convection zone of the Sun during
different stages of its life time is considered to explain its low photospheric
value in comparison with that of the solar system meteorites. Lithium is
intensively and transiently burned in the early stages of evolution (pre-main
sequence, pMS) when the radiative core arises, and then the Li abundance only
slowly decreases during the main sequence (MS). We study the rates of lithium
burning during these two stages. In a model of the Sun, computed ignoring pMS
and without extra-convective mixing (overshooting) at the base of the
convection zone, the lithium abundance does not decrease significantly during
the MS life time of 4.6 Gyr. Analysis of helioseismic inversions together with
post-model computations of chemical composition indicates the presence of the
overshooting region and restricts its thickness. It is estimated to be
approximately half of the local pressure scale height 0.5Hp) which corresponds
to 3.8 % of the solar radius. Introducing this extra region does not noticeably
deplete lithium during the MS stage. In contrast, at the pMS stage, an
overshooting region with a value of approximately 0.18Hp is enough to produce
the observed lithium depletion. If we conclude that the dominant lithium
burning takes place during the pMS stage, the dispersion of the lithium
abundance in solar twins is explained by different physical conditions,
primarily during the early stage of evolution before the MS.
