Spore-based probiotics exist naturally in dirt and would have been a source of exposure for ancestral humans.

“I think that the answer that question from my perspective is that our ancestors really didn't get into the problem in the first place because they weren't eating these processed foods and I think that most of these spore based probiotics exists naturally in nature and yes of course they're there in the dirt and that's that's what we would've been exposed to and so there's always this constant turnover of the microbiome in our guts there's a constant turnover of the microbiome in our mouths”