We all started out as a fertilized egg: a solitary cell about as wide as a shaft of hair. That primordial sphere produced the ten trillion cells that make up each of our bodies. We are not merely sacs of identical cells, of course. A couple hundred types of cells arise as we develop. We’re encased in skin, inside of which bone cells form a skeleton; inside the skull are neurons woven into a brain.

What made this alchemy possible? The answer, in part, is viruses.

Viruses are constantly swarming into our bodies. Sometimes they make us sick; sometimes our immune systems vanquish them; and sometimes they become a part of ourselves. A type of virus called a retrovirus makes copies of