Along with the common chimpanzee, the bonobo is the closest extant relative to humans. Because the two species are not proficient swimmers, the formation of the Congo River 1.5–2 million years ago possibly led to the speciation of the bonobo.lthough large male chimpanzees can exceed any bonobo in bulk and weight, the two species actually broadly overlap in body size. Adult female bonobos are somewhat smaller than adult males. Body mass in males ranges from 34 to 60 kg (75 to 132 lb), against an average of 30 kg (66 lb) in females.