The original version of this story appeared in Quanta Magazine.
Far from being solo operators, most single-celled microbes are in complex relationships. In the ocean, the soil, and your gut, they might battle and eat each other, exchange DNA, compete for nutrients, or feed on one another’s by-products. Sometimes they get even more intimate: One cell might slip inside another and make itself comfortable. If the conditions are just right, it might stay and be welcomed, sparking a relationship that could last for generations—or billions of years. This phenomenon of one cell living inside another, called endosymbiosis, has fueled the evolution of complex life.
Examples of endosymbiosis are everywhere. Mitochondria, the