Despite eons of mingling inside our cells, gene networks we’ve inherited from primitive, singled-celled ancestors have stayed separate. Our cells remain chimeras, a hybrid fusion of unrelated creatures.

The genes date from an event 1.5 billion years ago, when two kinds of simple cells, neither having a nucleus or cellular membrane, shacked up and created an entirely new form of life: eukaryotes.

While the two distinct communities of genes work together to keep cell machinery ticking, they otherwise stay out of each other’s hair, report biologists from the National University of Ireland.

“We h...