Bacteria have resided on our planet for over 3.7 billion years, spanning approximately 85% of our planet’s history. By contrast, human life only represents 0.001%. They created oxygen from light, water, and carbon dioxide, witnessed the shift of Earth’s biosphere, and ushered in the beautiful possibility of life. In fact, thanks to a stunningly improbable union between a bacterium and an archaeon 2 billion years ago, they are the reason we exist at all. This moment represents the emergence of eukaryotes, our oldest ancestors, on the tree of life.

They live in the deepest depths of our oceans, the acid-laden hotsprings of Yosemite, and the frigid climate of our polar caps. Over time and generations, they adapted and evolved, passed from mother to child, finding homes in and on our bodies. That means, like a coral reef or a rainforest, we are all living ecosystems—multitudes of minute interactions amongst countless species.