  The field of astrobiology has made huge strides in understanding the
habitable zones around stars (Stellar Habitable Zones) where life can begin,
sustain its existence and evolve into complex forms. A few studies have
extended this idea by modelling galactic-scale habitable zones (Galactic
Habitable Zones) for our Milky Way and specific elliptical galaxies. However,
estimating the habitability for galaxies spanning a wide range of physical
properties has so far remained an outstanding issue. Here, we present a
"cosmobiological" framework that allows us to sift through the entire galaxy
population in the local Universe and answer the question "Which type of galaxy
is most likely to host complex life in the cosmos"? Interestingly, the three
key astrophysical criteria governing habitability (total mass in stars, total
metal mass and ongoing star formation rate) are found to be intricately linked
through the "fundamental metallicity relation" as shown by SDSS (Sloan Digital
Sky Survey) observations of more than a hundred thousand galaxies in the local
Universe. Using this relation we show that metal-rich, shapeless giant
elliptical galaxies at least twice as massive as the Milky Way (with a tenth of
its star formation rate) can potentially host ten thousand times as many
habitable (earth-like) planets, making them the most probable "cradles of life"
in the Universe.
