So apparently some think cavities (caries) are contagious via mouth fluid exchange in kissing... if so, then perhaps an ancestor became an unwitting host through some unknown mechanics. Perhaps poorly cooked jowls of some sort.
You have a source for that? It's my understanding that it is -very- hard to change another human's oral biome, the bacteria that are currently there have a huge advantage of out surviving foreign bacteria.
Could have been hand to mouth contact from any number of sources. Bacterial biofilms are pretty easy to transfer and as we recently learned from eating bats it only has to happen once.
People will, even today, grab fruit off of trees, wipe it on their shirt, and eat it.
Same goes for eating berries, which are much closer to the ground.
Go back in time a bit, imagine 20k years ago, someone pulling a carrot, wiping it off, and eating it. Or even washing the dirt off, but unless cooked, or soap is used (a relatively new discovery), or a knife to remove the outside?
A little water isn't going to destroy all bacteria on a carrot.
Even today, I'll pick carrots, throw them in sink, wash them a bit and peel. Then eat raw.
Bacteria is everywhere, and we're eating the same thing bateria eats, eg that sugary carrot plant...
And...
* milk
* honey (literally bee spit sorta)
* cheese (pig stomach juices thrown in with milk for a few days)
By no means do we isolated ourselves baterialogically.