This kitten was born with two faces

In 2012, a kitten was born in Oregon with two faces, four eyes, two noses, and one brain.

The condition is called diprosopus - Greek for "two-faced". It's not the same as conjoined twins. The body is single. The skull is single. But somewhere early in development, a signalling protein called Sonic Hedgehog - yes, that's its real name - is released in the wrong amount, and the face duplicates while the rest of the embryo continues normally.

It happens across mammals: cats, calves, pigs, sheep, the occasional human infant. The animal usually lives only hours. The two mouths often share a single throat that can't coordinate swallowing, and the brain stem misfires when both faces try to breathe at the same time.

But a cat called Frank and Louie lived to be fifteen. Born in Massachusetts in 1999, he had one body, one brain, two faces, and one functional mouth. Only one mouth was connected to a working oesophagus, so he ate from the left. The other face simply sat there, breathing, blinking, and occasionally purring out of sync. He held the Guinness record for longest-lived Janus cat until his death in 2014.

Sonic Hedgehog is also what tells a developing embryo where to put its eyes, its limbs, its teeth, and its spinal cord. Too little of it in the wrong place and the embryo grows a single eye in the centre of the face - a condition called cyclopia. Too much, and you get diprosopus.

Our faces, in other words, are held together by a chemical gradient so delicate that a small chemical hiccup can produce a creature that looks like a myth.