 It is worth noting that almost all of the work, both theoretical and empirical, on animal emotions has limited its scope to mammals, or at least amniotes (For a notable recent exception, see Mendl et al. 2011). This work has exploited certain deep homologies of, e.g. brain structure and molecular neurophysiology (focusing especially on hormones and neurotransmitters) in making arguments that animals in these taxanomic groups share our emotions because they share the mechanisms of our own emotions (as well as behaviors that tend to indicate emotions in us, and bear the same relations to the underlying physiological mechanisms). This approach is not available to animals which are very distantly related to us, i.e. invertebrates. The ancestors of vertebrates split off from the rest of the animal phyla at a time when all animals were still relatively simple, in terms of structure, tissue types, number of neurons, bodily capacities for locomotion and other forms of behavior. The elaboration of complex physiological systems occurred independently in the various phyla. Therefore the physiological systems underlying emotions in other phyla — if these exist — may be very different, and hence difficult to identify, either in terms of direct physiological observations or in terms of observations of behavioral expressions of emotion. It is also possible that the repertoire of emotions in other phyla might be different, further problematizing the task of individuating non-vertebrate emotions. (Further discussion and additional scientific references for the topic of emotions and empathy in animals can be found in the  section on emotions and empathy in the entry on   animal cognition.)