Additionally, the !Kung in Botswana retain 40% of their calories from hunting and this percentage varies from 20% to 90% depending on the season. For physical evidence Buss first looks to the guts of humans and apes. The human gut consists mainly of the small intestines, which are responsible for the rapid breakdown of proteins and absorption of nutrients. The ape's gut is primarily colon, which indicates a vegetarian diet. This structural difference supports the hunting hypothesis in being an evolutionary branching point between modern humans and modern primates. Buss also cites human teeth in that fossilized human teeth have a thin enamel coating with very little heavy wear and tear that would result from a plant diet. The absence of thick enamel also indicates that historically humans have maintained a meat-heavy diet. Further, Buss looks to Vitamins A and B12, which the body is unable to produce, but are found in meat. The absence of these vitamins in the human body also implies a human dependence upon meat to obtain such vitamins. Finally, Buss notes that the bones of animals human ancestors killed found at Olduvai Gorge have cut marks at strategic points on the bones that indicate tool usage and provide evidence for ancestral butchers