protection for those crops (first versions of armies)
this isn't true. inter-tribe battle is something that existed long before agriculture. agriculture is what made it possible for dense walled cities to be built, but then they weren't protecting crops so much as the people who lived in them.
but regardless of that, to me this Diamond guy's stuff sounds like it's probably dumb theoretical bullshit not at all based upon science and only adhering to historical evidence when convenient. trying to REASON THROUGH LOGIC AND A LIMITED UNDERSTANDING OF SCIENCE AND HISTORY what's natural for humans is a complete waste of time/masturbation, depending on how much it entertains you. people almost invariably immediately think back to neo/paleolithic times, which stems from a poor understanding of evolution and is entirely erroneous. people were no more 'natural' back then than they were in the middle ages. the hunter-gatherers were people responding to their surroundings, as all people have in any 'unenlightened' age, arguably any age at all. humans are also extremely complex creatures, and what seemed natural back then is likely entirely different from what's thought to be natural today. really, if you follow the history of the human conception of nature, you'll find it has changed greatly through time (not in one linear direction, either) and varies strongly from culture to culture. interestingly, regardless of how the perception of it has changed, it has always been a subject of fascination. this relatively recent preoccupation with nature's therapeutic/wholesome/roots properties has not always existed, and is just another manifestation of the human fascination with nature. personally, I think this manifestation comes from the contemporary obsession with finding some nifty SECRET TO LIFE that will make everything wonderful and happy and full of life and energy--the same reason why people adopt all these absurd diets (raw food, atkins, paleolithic), watch doctor oz, join cults. really, all they're doing is responding to the stresses caused by modern capitalism
sorry I didn't break that up into paragraphs!! read it anyway!!
concerning sustainability, it's what velf said: really doesn't help us out at all. if someone could figure out a way that it does, that'd be neat.