  The many-multiplet method applied to high redshift quasar absorption spectra
has indicated a possible time variation of the fine structure constant.
Alternatively, a constant value of alpha is consistent with the observational
analysis if a non-solar isotopic ratio of Mg24,25,26 occurs at high redshift.
In particular, a higher abundance of the heavier isotopes Mg25,26 are required
to explain the observed multiplet splitting. We show that the synthesis of
Mg25,26 at the base of the convective envelope in low-metallicity asymptotic
giant branch stars, combined with a simple model of galactic chemical
evolution, can produce the required isotopic ratios and is supported by recent
observations of high abundances of the neutron-rich Mg isotopes in metal-poor
globular-cluster stars. We conclude that the present data based on high
redshift quasar absorption spectra may be providing interesting information on
the nucleosynthetic history of such systems, rather than a time variation of
fundamental constants.
