  Galaxies represent the main form of organization of matter in our universe.
Therefore, they are of obvious interest for the new multidisciplinary field of
astrobiology. In particular, to study habitability of galaxies represents one
of the main emerging challenges of theoretical and numerical astrobiology. Its
theoretical underpinnings are, however, often confused and vague. Here we
present a systematic attempt to list and categorize major causal factors
playing a role in emergent habitability of galaxies. Furthermore, we argue that
the methodology of cosmological merger trees is particularly useful in
delineating what are systematic and lawful astrobiological properties of
galaxies at present epoch vs. those which are product of historical contingency
and, in particular, interaction with wider extragalactic environment. Employing
merger trees extracted from cosmological N-body simulations as a new and
promising research method for astrobiology has been pioneered by Stanway et al.
(2018). We analyse the general issue of applicability of merger trees and
present preliminary results on a set of trees extracted from the Illustris
Project. In a sense, this approach is directly complementary to using
large-scale cosmological simulations to study habitable zones of individual
galaxies with high mass/spatial resolution; taken together, they usher a new
era of synergy and synthesis between cosmology and astrobiology.
