Line up the brains of a fish, bird and a mammal, and something unexpected comes up. You do not see three different answers to the problem of making sense of the world. You see one answer, tilted three different ways. “You can really see it’s almost like a continuum,” says Emre Yaksi, a professor at the Kavli Institute for Systems Neuroscience in Trondheim.

Read across decades of anatomy, the same two ancient pathways carry the world into the forebrain of all these animals. What changes from one to the next is mainly which route does more of the work. Evolution built these brains from different parts, in creatures that parted ways hundreds of millions of years ago. It kept arriving at the same answer anyway.

That is the puzzle the Yaksi lab set out to chase. If animals this far apart on the tree of life keep landing on the same arrangement, perhaps the arrangement is no accident. Perhaps there are organizational rules deep enough that a fish and a person, for all the differences between them, are bound by the same ones.