As an anthropology major in my undergraduate studies many years ago I was facinated by this very subject. After Man and Dog (a dangerous combo!) had managed to kill off all of the big game animals that sustained human life for hundreds of thousands of years, there was no alternative but to find something else...fast. Hence, monoculture agriculture developed rather quickly and took over as Man's main food source in many parts of the world. Population quickly grew, astoundingly fast as men and women were grouped into small communities that gradually became larger communities as property suddenly became a commodity. The average grain eater...even today...produces offspring about once every 11-14 months...wonder where the population problems are coming from? The average hunter-gatherer produces offspring about once every five to seven years. Wonder where they all went? Young grain-eating girls mature into first menses..now, on the average around 11-12 years old...some are starting menses at 7-8 years old in the last 50 years! The average hunter-gatherer girl doesn't even start her menses until she's between 17-21! GRAINS! They have properties that are causing our children to mature before their time and it's getting worse as new strains are being produced with more starch and more of evrything else. No wonder the population explosion happened! Life expectancy went way down soon after the advent of agricutlure and didn't start to go on the rise like you said, after there were some better public health practices. After taking into account the dangers inherent in the lifestyle of a hunter-gatherer (ie. being eaten by a large cat, injuries, etc.) we find that they had a life expectancy of about 70 years...even back then. But the agricultural folks int eh city-states went down to about 37 years. If you want more, I could go on and on...probably not, huh?