While Jad was on paternity leave, Carl Zimmer told Robert and producer Soren Wheeler about the ecosystem inside each and every one of us. According to Carl, when we're in the womb, we have no bacteria in us at all, but as soon as we're born we start gathering up the bugs. And by the time we're grown, we have a three-pound organ of others inside our guts.

With the help of some bacteria, swimming mice, and neuroscientist named John Cryan, we learn that this organ does much more than just help us digest food. And finally, Jonah Lehrer joins Carl to help us understand what the bugs in our gut have to say about the feelings in our heads.

Correction: An earlier version of this piece implied that subjects in the human probiotics study were given a larger dose of probiotics than they actually were. We also compared the results of the study to the effect of giving people anti-anxiety medication. The subjects did experience a significant positive mood change, but the researchers did not compare that result to the effect of anti-anxiety medication. The audio has been adjusted to reflect these facts.