We measured growth under 24 different conditions for 85 accessions of Psychrobacter in tandem with whole genome sequencing and animal studies. We conducted comparative (phylo)genomics for these 85 Psychrobacter strains, as well as family-level comparative phylo-genomics. Our results indicate that growth at 37C is likely ancestral for the genus and is likely necessary for growth within mammalian bodies. Furthermore, Psychrobacter's closest relatives are pathobionts, and Psychrobacter likely evolved from a pathobiont ancestor.