[Vrondi]

Not just antibiotics to consider along this line of thought. We historically had a higher load of parasites. Far more of the population had some amount of parasites more of the time. Things like sewer systems/sanitation/clean drinking water/bathing and personal hygiene/wearing shoes/not having piles of animal feces all over the streets. That all changed the amount of exposure to parasites for the common person. We know it affected our immune systems (overall rates of allergies increased). We do not know how it affected our brains. Makes intuitive sense that it must apply to bacteria as well. Before foods were pasteurized (and before refrigeration), for example, we were exposed to more dietary sources for bacteria, both beneficial and non-beneficial.