Controlled research making this link seems to be fairly recent, but I think one of the reasons farmers were so happy to embrace the habitual use of antibiotics on their livestock was because it made them grow so much bigger. Considering how often they’re prescribed to humans, and what a big problem obesity is, it’s pretty crazy there hasn’t been any research on this so far. Weight problems are only the tip of the iceberg too, as I’m sure a lot of people here are fully aware.
The research into ancient microbes sounds promising though. I read something in the book Good Germs, Bad Germs about the miraculous benefits of bacteria found in soil in South America. One researcher was able to shrink cancer tumours by injecting dead microbes from these sources into people’s blood.