  The Gaia hypothesis considers the life-environment coupled system as a single
entity that acts to regulate and maintain habitable conditions on Earth. In
this paper we discuss three mechanisms which could potentially lead to Gaia:
Selection by Survival, Sequential Selection and Entropic Hierarchy. We use the
Tangled Nature Model of co-evolution as a common framework for investigating
all three, using an extended version of the standard model to elaborate on Gaia
as an example of an entropic hierarchy. This idea, which combines sequential
selection together with a reservoir of diversity that acts as a 'memory',
implies a tendency towards growth and increasing resilience of the Gaian system
over time. We then discuss how Gaian memory could be realised in practice via
the microbial seed bank, climate refugia and lateral gene transfer and conclude
by discussing testable implications of an entropic hierarchy for the study of
Earth history and the search for life in the universe. This paper adds to the
existing taxonomy of Gaia hypotheses to suggest an "Entropic Gaia" where we
argue that increasing biomass, complexity and enhanced habitability over time
is a statistically likely feature of a co-evolving system.
