The biosphere is a network of interacting species that connects organisms across all scales, from microbes to mammals. Knowledge of the mechanisms underlying these relationships, and the evolutionary forces that shape them, is fragmentary. My lab has pioneered the study of rove beetles (Staphylinidae) as a model clade to break open basic problems in organismal interactions. Most of the >66,000 known rove beetle species are free-living predators, found in leaf litter and soil habitats spanning the globe. From this ancestral lifestyle, however, hundreds of lineages have transformed into remarkable symbiotic organisms, specialized for life as impostors inside the complex societies of ants. The transition from free-living to symbiotic embodies evolution in the extreme, with dramatic changes in social behavior and chemical communication that enable the beetles to assimilate into the social fabric of host colonies. The widespread, convergent evolution of this form of symbiosis, combined with the experimental tractability of both free-living and symbiotic rove beetles, provides a virtuoso system for understanding both how and why novel ecological relationships are forged by evolution. Work in our lab strives to understand all facets of rove beetles and their interactions with ants, from both mechanistic and evolutionary perspectives. We focus on the brains and behavior of these beetles to comprehend how their remarkable social interactions with ants are controlled and have evolved at the level of sensorimotor circuitry. In conjunction, we investigate the evolution of specialized biosynthetic pathways and cell types that these beetles possess, which have enabled them to chemically communicate with ants and integrate inside their colonies. In this talk, I will discuss how our work is creating an integrated picture of how species recognize and interact with each other, illuminating the conditions that predispose such interactions to emerge, and pinpointing forces that shape the evolutionary path towards obligate and highly intimate relationships between species.