The Congo describes a vast arc, and the bonobo's range sits inside it. No wild bonobo population has ever been confirmed on the northern bank, and no chimpanzee population on the southern one. Great apes are poor swimmers, lacking the buoyancy and body proportions that make swimming easy, so a river a few kilometres wide is a real wall.
Genetic estimates put the split between the two Pan species at roughly one to two million years ago, which is broadly consistent with the modern lower Congo taking its present course. Genome studies have since detected limited gene flow after the split, so the barrier was probably not absolute from the first day.
The common name pygmy chimpanzee misleads. Adult bonobos weigh about the same as chimpanzees, roughly thirty to forty-five kilograms for males, and the differences are in proportion and posture rather than size.