  Deuterium plays a crucial role in testing big-bang nucleosynthesis. Its
chemical evolution, while simple (it is burned to $^3$He), is intertwined with
the more complicated evolution of $^3$He. Gloeckler \& Geiss' new measurement
of the $^3$He abundance and the HST measurement of D, both in the local ISM
today, can be compared to the pre-solar nebula abundances of D and $^3$He.
Within the uncertainties, the sum of D + $^3$He relative to hydrogen is
unchanged. This provides some validation of the cosmological utility of D +
$^3$He, first suggested by Yang et al (1984), and further, indicates that over
the past 4.5 Gyr there has been at most modest stellar production of $^3$He, in
contradiction with stellar modeling, or modest stellar destruction of $^3$He,
in contradiction with some ``solar spoons.'' While the earlier Galactic
evolution of D + $^3$He cannot be constrained directly, it is expected to be
dominated by massive stars, which deplete their $^3$He and produce metals.
Based on the Galactic metallicity and the constancy of D + $^3$He over the past
4.5 Gyr, we derive a more empirically based lower bound to the cosmological
baryon density; while not dramatically different from the original bound of
Yang et al (1984) based on D + $^3$He, it alleviates some of the cosmic tension
between the big-bang $^4$He abundance and those of D and $^3$He.
