The speciation process that led to the separation of bonobos (Pan paniscus) from chimpanzees (P. troglodytes) is estimated to have begun between 0.8 and 2.6 million years ago (Takemoto et al. 2017), most likely between 1.5 and two million years ago (Fontsere et al. 2022). Since then, the two species have accumulated substantial differences in morphology, physiology, behavior and cognition. Of particular importance for social behavior, bonobos tend to occur in more stable parties and to have stronger associations among females (Kano 1982, 1992; Furuichi 2009, 2011, 2023). In this chapter, I consider potential explanations for the evolution of these differences. I conclude that bonobos have likely evolved from a chimpanzee-like ancestor, and have been subject to selection against male aggressive behavior, or “self-domestication.” I then consider ecological explanations for this process. I focus on the “No gorillas” hypothesis, which proposes that bonobo evolution was strongly influenced by their experiencing reduced scramble competition for food compared to chimpanzees.