The scientific legacy of Paul MacLean provides important insights into the neural substrate of adaptive social behavior in mammals. Through his research and visionary conceptualizations, current investigators can legitimately study social behavior from a neurobiological perspective. His research and writings provided three important contributions. First, he emphasized the importance of evolution as an organizing principle that shaped both the structure of the nervous system and the adaptive social behavior. Second, by defining the limbic system, he legitimized the biological perspective in the study of emotion. Third, he recognized the important role of the vagal afferents in the regulation of higher brain structures. The paper will focus on the Polyvagal Theory. The Polyvagal Theory is a new conceptualization of the role of vagus and employs several features that MacLean emphasized including the importance of evolution, limbic structures and vagal afferents. The Polyvagal Theory builds on these early findings by MacLean and focuses on the link between phylogenetic changes in the autonomic nervous system and social behavior. By focusing on the phylogenetic changes in the structure of the vagus and the role that the vagus plays in the neural regulation of visceral state, new insights regarding social behavior emerge. Moreover, by articulating the phylogenetically organized hierarchy of neural circuits, insights into benefits of social behavior become evident as do an understanding of the behavioral and physiological features associated with stress and psychiatric disorders.