Evolutionary puzzle: discussing the evolution of sentience in Metazoa through a phylogenetic perspective

Sentience is the capacity of organisms to feel and experience through subjective states. During the last years, several investigations have indicated that response mechanisms to harmful stimuli can be highly conserved among the Metazoa. However, there is a research bias towards vertebrates in the available studies. Here we discuss the evolution of the nervous and sensory system, pain and nociception in animals through a phylogenetic perspective testing the hypothesis of common ancestry of sentience. Our results indicate that characteristics related to sentience - morphological and molecular and behavioural -, were already present in the common ancestors of Metazoa, Eumetazoa and Bilateria. Our phylogenetic hypotheses positioned Porifera as the sister-group to all the other Metazoa, corroborating the hypothesis of a single origin of the nervous system. Our results also depict Urbilateria as the ancestor of the metazoan toolkit related to the sentience. These scenarios suggest that some attributes of the sensory system may have appeared even before the emergence of the nervous system, through possible cooptations of sensory modules of the first Metazoa.