What is the best understood creature on Earth?
I once checked Biological Abstracts, a journal that indexes biological research. There were five times more references to Escherichia coli, a bean-shaped bacterium, than to any other species. Its genes have been exhaustively mapped. Its proteins cataloged. This microscopic blob of life has no secrets.
There are enough E. coli living in my digestive tract to make a line that would stretch from Boston to San Francisco. As house guests go, they are not unwelcome. They produce useful vitamins. They devour other microorganisms that cause disease. But mostly they just go about their business, happily sharing my interior space, doing me very little good or harm. The technical term for our relationship is commensal: literally, "eating at the same table."
I suppose it makes some folks squeamish to think of those zillions of troglodytes living in the dark cave of our guts. Not me. I sing their praises: root of life, commensal friends.