Yes ...but there were many decades BEFORE the governments starting adding fluoride to water - where natural fluoride in water was drunk by unsuspecting people.
Areas in the Southwest - including areas of Texas, has groundwater used for drinking water that has much higher concentrations of fluoride than is usually added in cities. I had an uncle who grew up in such an area. The higher fluoride in the water actually caused his teeth to have brown spots - but he never had any cavities!! (And BTW - it was this sort of long-term experience that caused people to consider that fluoridation of water would reduce cavities. And the people who proposed adding fluoride to water were able to show that the people in the regions where they were regularly consuming higher quantities of fluoride had no abnormal problems (or any problems had the same statistical chance of occurring as in areas without fluoride in water.)
So ....the studies should be evaluated as to whether or not there is a degree of hysteria in the study. Recall that there are many studies showing dangers of amalgam fillings or using Thimerisol as a preservative in medicines - yet most studies declare that the amalgam fillings are very safe, and Thimerisol is a safe preservative - and the other studies are based on junk science. Buyer beware!! Are the studies reliable - or are they the equivalent of the Anthropomorphic Global Warming hyped studies?
Naturally occur fluoride is generally calcium fluoride. We now know that calcium is required for healthy Teeth. Back in the day, researchers ignored calcium and other tooth-building ingreadients of water and focused on fluoride because it’s what discolored the decay resistent teeth. So they theorized that fluoride was an essential nutrient required for healthy teeth and they were wrong.
Modern science indicates that ingesting fluoride does not reduce tooth decay. Fluoride hardens teeth topically only.
Natural fluoride in water supplies has crippled and killed many people in the world