Tummy bugs

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.

-- from Microbes R Us by Olivia Judson.