Human digestive systems are relatively short and simple, suited for fruit and easily digested proteins. Yet, we conquered the globe by eating grass seeds like wheat, barley, rye; which are, in their raw state, bio-unavailable, gut-irritating, and indigestible.

So, how did a primate with a fruit-eater’s gut survive on a grass-eater’s diet? We didn't evolve a new organ to do it. We built one.

This is the story of the ancient biological pact between humans and microbes, that enabled us to build an “external stomach” in a sourdough starter jar.