The tuatara, a lizard-like reptile that lives only in New Zealand, has those three “eyes.” It belongs to a group of reptiles that once included many other creatures, but today, the tuatara is the only surviving member of that group.
To understand where the tuatara’s “third” eye came from, scientists studied a small growth attached to the front of the reptile’s brain, a gland my called the pineal body.
In some lizards, the part of the brain that is related to this pineal body is shaped much like an eye. It has a lens-shaped outer wall, a transparent covering, and a retina with pigment.
Scientists think that this “pineal eye” does not help most lizards see, but in the tuatara, the pineal eye is fully developed, giving the creature a “third eye” through which to see!