Natural selection is stingy in its edits. It takes only three letters to go from “Hamlet” to “ham” and hardly more differences in DNA nucleotides to span the distance back between rodent and Rodin. The entire diversity of mammals reflects a modest tinkering with the original mammal plan. We share 99 percent of our genes with chimps, but we also share 95 percent of our genes with pigs and rats. And so most of the story of humans, our biology and deep history, is told equally well by the body of any mammal — even a lowly rat. Especially a rat.

We imagine the rat as despicable, disease ridden, grotesque. In tandem with fleas and plague, rats nearly did us in. But they may also have saved us. It is from the rat that we have learned most of what we know about ourselves. Look into a lab rat’s black eyes and you will find, if not your sister, an increasingly good mirror. Rats have immune systems like our immune systems. They have brains like our brains, fat like our fat. They have no language. They don’t dream about the stars. But otherwise we are the same. And so we poke them to learn about ourselves. We have long known that rats are like us. What is new is the realization that our similarities to each other are greater each year. More and more, I am a laboratory rat, and so are you.

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