Relative to our ancestors we: 1) Live in a much more sterile world. Where our immune systems used to get a workout, now it just sits on the (metaphoric) sofa a

Relative to our ancestors we: 1) Live in a much more sterile world. Where our immune systems used to get a workout, now it just sits on the (metaphoric) sofa all day, watching TV, while drinking soda and eating junkfood. Along the same lines, the cleanliness means less diversity in gut bacteria. 2) Live much more silo'ed. Hunter gathers had families that where together more often - hunting and/or gathering, eating and/or socializing around the fire. Less interaction means less exposure to bacteria and virus (read: immune system doesn't get stronger), as well as less exchange of what ultimately becomes gut bacteria.