You've got a good conversation going here. Thanks. Primatologist Richard Wranham has proposed that two related factors contributed to the diverging bonobo/chimp behavior: — far more plentiful food in the bonobo range than in the chimp range and, — chimps compete with gorillas for some of their food sources, while bonobos are isolated from gorillas (and chimps).

This hypothesis would seem to support our argument, in that we find that food supply was generally plentiful for prehistoric populations (with occasional crises), whereas for post-ag populations, food scarcity was a constant problem (as demonstrated by skeletal evidence).

Chris Ryan (co-author of Sex at Dawn)