Somewhere in the mud, in your gut, in a handful of ocean water, there is a machine. It is made of proteins. It self-assembles from nothing. It spins faster than the flywheel in a race car engine. It senses its environment and can reverse direction in milliseconds. It is half a billion years old, give or take, and it has barely changed because it was already perfect.

It’s called the bacterial flagellar motor. And after 50 years of research, we finally know how it works.