  We propose the concept of a "Galactic Habitable Zone" (GHZ). Analogous to the
Circumstellar Habitable Zone (CHZ), the GHZ is that region in the Milky Way
where an Earth-like planet can retain liquid water on its surface and provide a
long-term habitat for animal-like aerobic life. In this paper we examine the
dependence of the GHZ on Galactic chemical evolution. The single most important
factor is likely the dependence of terrestrial planet mass on the metallicity
of its birth cloud. We estimate, very approximately, that a metallicity at
least half that of the Sun is required to build a habitable terrestrial planet.
The mass of a terrestrial planet has important consequences for interior heat
loss, volatile inventory, and loss of atmosphere. A key issue is the production
of planets that sustain plate tectonics, a critical recycling process that
provides feedback to stabilize atmospheric temperatures on planets with oceans
and atmospheres. Due to the more recent decline from the early intense star
formation activity in the Milky Way, the concentration in the interstellar
medium of the geophysically important radioisotopes, $^{40}$K, $^{235, 238}$U,
$^{232}$Th, has been declining relative to Fe, an abundant element in the
Earth. Also likely important are the relative abundances of Si and Mg to Fe,
which affects the mass of the core relative to the mantle in a terrestrial
planet. All these elements and isotopes vary with time and location in the
Milky Way; thus, planetary systems forming in other locations and times in the
Milky Way with the same metallicity as the Sun will not necessarily form
habitable Earth-like planets.
