When we say things like "Years ago, life expectancy was forty years old," it doesn't mean what it sounds like. It means the life expectancy averaged out at forty years, because so many people died as small children from diseases we have stopped with hygiene and immunization.
500 years ago, people still lived into their 70's and 80's and beyond... the way they do now.
The real clue is in the transition to agriculture. Type in "diseases of civilization" into google and you'll find that dental caries and heart disease started around the same time people started eating more grains and sugars.