On the evolution of sentience in Metazoa: an analysis in light of phylogenetic systematics
Animal sentience is a biological phenomenon that enables subjective sensations and experiences. Based on an interdisciplinary framework, every animal that has some capacity to feel, recognize stimuli and be affected by them in a negative or positive way, has a potential to be sentient. Research in this area has focused especially on groups of vertebrates and the possibility of animals distant from the genus Homo having sentience remains premature. Contemporary evidence points to some phylogenetic preservations of mechanisms responsible for reactions to noxious stimuli during the course of animal diversification. However, some authors emphasize that the phenomenon of sentience has evolved at least three times in different lineages. From this perspective, this work aimed to perform a comparative analysis of some attributes related to the sentience from the perspective of the phylogenetic systematics, using molecular, morphological and behavioral characters, thus trying to test a hypothesis of homology. Our results indicate the presence of several morphological and molecular characteristics already in the common ancestor of Eumetazoa with a greater diversification in Bilateria and also Chordata, and some of these were already present before the appearance of nerve cells. The behavioral data are also widely distributed among animal taxa, indicating the emergence of cognitive capabilities in a deep time. The emergence of sensory and nervous systems seem to be involved in evolutionary processes of bricolage, suggesting that the change from a life without subjective perception to the first steps of the consciousness may have been achieved by possible reorganizations of structures and mechanisms already existing, being then the consciousness a character distributed gradually between the animal taxa.