Gut microorganisms live organisms have decreased in number during the last 2,000 years. That is one of the preliminary results from studying fossilized human feces extracted from rock shelters using a genomic approach. Since the atmosphere in this may be a ton, researchers assume it could be effective for microbes to grow and change. There are more antibiotic-resistant bacteria in the modern gut microbiota. The findings may be helpful to us in understanding the implications.
Mass Extinction in The Human Gut Revealed by Fossil Remains of 2,000-Year-Old Feces. https://www.sciencealert.com/2-000-year-old-feces-fossils-reveal-mass-extinction-in-the-human-gut