Because one human lifetime may encompass a million bacterial generations, individual species and the microbiome itself can evolve within a single host.

In one striking demonstration of the potency of the so-called "microbiome-gut-brain axis," published in Gastroenterology in 2011, Bercik and colleagues gave BALB/c mice, a strain of mice that are typically timid and shy, a cocktail of antibiotics, dramatically changing the composition of their gut bacteria. "Their behavior completely changed," Bercik says. "They became bold and adventurous."