It can all begin to seem impossibly complicated, and in some ways itis impossibly complicated. But there is an underlying simplicity in all this, too, owing to an equally elemental underlying unity in the way life works. All the tiny, deft chemical processes that animate cells — the cooperative efforts of nucleotides, the transcription of DNA into RNA — evolved just once and have stayed pretty well fixed ever since across the whole of nature. As the late French geneticist Jacques Monod put it, only half in jest: "Anything that is true of E. coli must be true of elephants, except more so."

Every living thing is an elaboration on a single original plan. As humans we are mere increments — each of us a musty archive of adjustments, adaptations, modifications, and providential tinkerings stretching back 3.8 billion years. Remarkably, we are even quite closely related to fruit and vegetables. About half the chemical functions that take place in a banana are fundamentally the same as the chemical functions that take place in you.