The NY Times had a good opinion piece
about the bacteria and microbes that prevade our bodies. Although mostly general and review, I found the points raised at the end of the article to be interesting ones:
Bacteria evolve quickly: they can go through many thousands of generations for every human one.
This has two potential consequences. First, during your lifetime, your bacteria can change their genes even though you cannot change yours. (You do have some flexibility: your immune system has a built-in capacity to change.) It may be that gut bacteria evolve in response to short-term changes in the environment, especially exposure to food-borne diseases. They may thus act as an evolving supplement to the immune system.
The second potential consequence is further reaching. Because bacteria can evolve so fast, it may be that some of what we think of as human evolution — like the ability to digest new diets that accompanied the invention of agriculture — is actually bacterial
evolution. We know that hostile bacteria — those that cause diseases in ourselves and our domestic plants and animals — have undergone dramatic genetic changes in the last 10,000 years. Perhaps our friendly bacteria have, too.