Do ancient fish swim through your dreams?
I cupped a frog in my hand and looked closely at a colorless little fleck in the middle of its head between two protruding eyes – the brow spot. This speck is a remnant of an ancient, functional third eye, and we (and reptiles, birds, and other mammals) have something related to it deep within our heads.
The story starts with primitive fish 450 million years ago. They weren’t good swimmers and spent much of the time feeding on the bottom. They had two eyes facing sideways and another that looked straight up, which probably was an early warning device telling when something dangerous was approaching overhead.
Those jawless fishes are today represented by hagfishes and lampreys, each with a pair of sideways-looking eyes and a definite, but questionably useful, third eye in the center of the head. I have examined with great interest the median eye of a lamprey and its optic nerve, which descends to a portion of the brain entirely separate from the familiar optic lobes that serve the paired eyes.
As fish evolved into amphibians, the remnant third eye was not entirely lost. Some modern amphibians have a tiny hole in their skulls where once an optic nerve passed through to the brain. This suggests that the ancestors of modern frogs, toads, and salamanders had working eyes in their foreheads, just like their jawless fish predecessors.
The brain of a tadpole, as it develops into that of a frog, produces an upper middle portion that grows forward, then detaches from the brain to form a little knot of cells, some of them becoming the brow spot.
In all vertebrates today, this cellular knob is called the pineal body, an isolated bit of tissue with one or two distinct functions. It secretes hormones in some species, while in others it contains special sensory cells whose function is still uncertain. In a tadpole whose paired eyes have been surgically removed, the pineal organ develops into a fully formed median eye, complete with lens and retina. Instructions for doing so have lain dormant for millions of years.
The odd thing is that, despite the pineal body’s modification in “higher” vertebrates, a recognizable middle eye remains in a few lizards. We can still just see it in anoles, those little green and brown lizards incorrectly called chameleons, where it is almost certainly without function as an organ of sight.
If a light-sensitive organ, or genuine third eye, is still present in certain fishes, a few amphibians, and at least one reptile (New Zealand’s tuatara), what form does it take in birds and mammals? Neither have the least sign of a median eye in their heads, but they do have a well-developed pineal organ attached by a stalk to the rear of a cavity deep inside the brain. Close examination shows it is served by nerves and by blood vessels, suggesting an active function.
In birds, the pineal body, even though it is not an eye, is affected by light diffusing through the skull. It detects shortening or lengthening days preparatory to migration. Such sensitivity establishes a biorhythm, or biological clock. Biological clocks are also evident in us humans, but how they are adjusted isn’t certain.
The human pineal body has sufficient secretory function to be called a gland. In 1958, A. B. Lerner discovered that pineal glands secrete a hormone, melatonin, whose abundance increases in darkness and decreases in light. Melatonin has since been found to be involved in various functions affected by the photoperiod, including reproductive cycles in some animals. It is used as a human sleep aid, although the medical profession has not yet fully embraced its effectiveness or safety.
Nerves produce neurotransmitters, one of which, serotonin, is responsible for producing melatonin in the pineal gland. This tiny organ, once part of a functioning third eye in primitive vertebrates, has developed a new and vitally important role in the well-being of higher vertebrates.
The pineal body in birds is tied to their migratory patterns, and it has also been found to play a role in the seasonal and day-night behavior of other vertebrates. It may have short term effects like sleep cycles, or very long term ones, such as the delayed onset of puberty in humans.
Our own middle eye may be long gone, but nothing is more remarkable than the refashioning of a tiny part of an ancient fish brain, once responsible for detecting light and movement, into part of a chemical system that affects our moods, eases us into sleep, and even leads our children through adolescence into adulthood.