The dual problem of explaining brain evolution and the way in which it has led to wide species differences in behaviour and physiology has often appeared intractable to scientists. The main limiting factor is that we do not understand enough about how brains work to appreciate why gross or fine morphological differences can lead to the considerable across- as well as within-species differences in behaviour. Even at a molecular level, while two-thirds of our genes are involved in regulating brain function, there is a high degree of homology within different phyla. In the context of quality of life (QoL), arguably the most important consideration is that the brain you have evolved is adapted to the environment you are living in and is capable of generating ‘conscious’ experience. When that environment is radically altered, issues arise regarding whether there is sufficient adaptability to cope and the extent to which mental as well as physical suffering might be experienced as a consequence. At the other end of the spectrum there is the question of how enriched social and physical environments might enhance QoL through promoting positive affect. Here I will discuss potential functional contributions of differences in brain size and organisation and the impact of experience. I will mainly focus on mental functioning and show particularly that capacities for consciousness, emotional experience, social interaction and cognition and behavioural flexibility are likely to be widespread in other animal species, even if less developed than in humans.