In 2005, scientists analysed the chemical residues inside some ancient Chinese pottery jars and found traces of the alcoholic drink they once contained—a concoction fermented from rice, honey, and fruits. Fermentation means yeast. The jars were 9,000 years old, which means that humans have been using yeast to make drinks for at least that long. First, we brewed booze. Then, we baked bread. And all the while, the bacteria in our guts were adapting.

A team from Newcastle University, led by Harry Gilbert, have shown that a common and well-studied gut bacterium has become exceptionally good at eating the unique carbohydrates found in yeasts. Whenever we eat a sandwich or quaff some beer, this yeast-breaker—Bacteroides thetaiotamicron, or B-theta for