https://www.sciencedaily.com/releases/2014/08/140815192240.htm

∞ “Our diets have a huge impact on microbial populations in the gut,” Maley said. “It’s a whole ecosystem, and it’s evolving on the time scale of minutes.”

Note that last statement. If its evolutionary heartbeat is the minute… and ours is the millenium… how ‘old’ are the organisms that comprise your commensal microbiota… remembering that you inherited »all of them post-natally?

They comprise ~50% of the cells of your body. Yet ‘your DNA’ does not include them.

By the way, the nerve trunk that connects the brain and the gut carries crucial signalling traffic, but approximately 80% of that traffic originates in the gut, not, as we would expect, the brain.
The brain is a relatively recent evolutionary flower.

The original organs of knowing were the gut and digestive organs, and their forward and lateral senses. Nose. Tongue. Anus.