Recent research about the human microbiome—the ecology of the microorganisms that live in our bodies—indicates that the many species of flora and fauna inhabiting our digestive tracts originated with our prehuman ancestors and evolved in parallel with them:Primates and Gut Microbes
A genomics researcher in Bethesda, Maryland, suggests that "this mutualistic symbiosis helped the human species evolve." We inherit not only our genes but our internal symbionts.
Another article I read about this discovery mentions that the cumulative mass of microbes in our intestinal tract typically outweighs our brain. It's boggling and humbling to contemplate how much of what we call our own body consists of alien organisms, most of them friendly or harmless.
This topic reminds me of Madeleine L'Engle's A WIND IN THE DOOR (sequel to her classic A WRINKLE IN TIME). Young heroine Meg becomes miniaturized in order to travel inside the body of her gravely ill little brother, Charles Wallace. She meets submicroscopic creatures who live in Charles Wallace's mitochondria. To these beings, a cell is their entire world, and Charles's body is a galaxy. They don't even realize their "galaxy" is sentient until Meg enlightens them. They and she become aware of the vital interconnectedness and inestimable value of all parts of creation, no matter how tiny or vast.
Margaret L. CarterCarter's Crypt