The community of microbes living in your gut—your so-called microbiota—is like another organ of your body. It’s a seething alien living inside of you, fermenting your food and jealously protecting you against interlopers. It’s a pretty unusual organ by any measure, but even more so in that its composition changes with every meal.

Millions of years ago, bacteria and animals struck up a deal. In return for a moist bed and a warm buffet, beneficial bacteria took up the job of defending us against the madly proliferating pathogens in the world. It takes a germ to fight a germ.

Microbes can mutate every 20 minutes, while humans try to counterpunch with genetic evolutionary updates every 10,000 years or so. They are genetic dynamos, running circles around us.

When everything is running smoothly, you pay no attention to your gut. Like your heart or your liver, it’s best if these things are on autopilot. Your conscious mind is too busy looking for your keys to be trusted with running these critical organs.

Only one percent of your genes are human, and those genes are fairly stable, but your microbial genes—the other 99 percent—are in constant flux. Measured by your genes, you’re a different creature each and every morning.

It may be a shock to the ego, but you are not alone in your body, and your microbiota is right now making plans for your future. By manipulating your cravings and mood, it gains control over your behavior.