By the time babies are two months old, they start to smile.  By five months, they usually start picking up objects. By 12 months, they’ve probably said their first word. By 18 months, they’re walking. These milestones are familiar, but growing up isn’t just about moving, speaking and thinking.

It’s also about microbes.

When we are born, our mothers seed us with our first bacteria. As we grow up, these microbes—the microbiota—behave like an extra organ. They help us to digest our food, they shape the development of our gut and our nervous systems, and they train our immune systems to deal with threatening infections.

They also change. If you watch a newborn island, or perhaps a landscape scoured by fire