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home : features : science and nature December 17, 2014

3/11/2014
Bill Amos : Old Three-Eyes
The New Zealand tuatara has a tiny vestigial eye in the middle of its forehead.
+ click to enlarge

The New Zealand tuatara has a tiny vestigial eye in the middle of its forehead.


Old Three-Eyes

Bill Amos

Last summer I cupped a handsome frog in my hands. During this time of their worldwide decline, we should leave frogs alone, but I held this one briefly (a male, judging by his large ear drum), marveling at its historical antiquity. Before releasing it, I looked closely at a colorless little fleck in the middle of its head between two protruding eyes. The mark is aptly named the brow spot, although that tells us nothing. The story of this speck is both ancient and contemporary: ancient because 450 million years ago it undoubtedly was functional; contemporary because we (and reptiles, birds, and other mammals) have something related to it deep within our heads.

The story started with very primitive fish 450 million years ago. They weren't good swimmers and spent much of the time resting or feeding on the bottom. They had two eyes facing sideways, and another that looked straight up. We have no idea how good this third eye was, how much of an image it formed, but the chances are it was an early warning device that told the fish when something dangerous was approaching overhead. Since the fishes were jaw-less and slurped soft food from sandy and muddy bottoms, it is unlikely the eye served notice of possible dinner.

Those jaw-less fishes are all gone except for a couple of varieties still found in the sea. Hagfishes and lampreys (the latter are not eels as often called) have a pair of pretty good sideways-looking eyes and a definite, but questionably useful, third eye in the center of the head. I have closely examined the median eye of a lamprey with its optic nerve leading to a portion of the brain entirely separate from the familiar optic lobes that serve the paired eyes, the same lobes that over millions of years developed into our own visual centers of the brain..

By following the development of a frog's brain, it is possible to hypothesize what must have occurred as fish evolved into amphibians. The brain of a tadpole, as it changes into that of a frog, has an upper middle portion that grows forward, then detaches from the brain to form a little knot of isolated cells that finally becomes the brow spot in many amphibians. (A couple of our Northeast Kingdom salamanders still retain a vestigial pigmented retina in this third "eye").

Some modern amphibians have a tiny hole in the top center of their skulls where once an optic nerve passed through to the central brain. All this evidence suggests that ancestors of modern frogs, toads, and salamanders had working eyes in their foreheads, as did their own more distant ancestors, those ancient jaw-less fishes.



LTK

Because the small knob of cells in vertebrate brains is today separated from the brain and no longer has a clear sensory function, it is known as the pineal body, an isolated bit of tissue with one or two distinct functions. It secretes hormones in some species, while close examination of others shows it contains special sensory cells whose function is still uncertain.

The odd thing is that despite the pineal body's modification in most "higher" animals, such as reptiles, a recognizable middle eye remains in a few lizards. We can still see it -- just barely -- in common anoles, those popular little green and brown lizards incorrectly called chameleons often sold in pet shops. It takes a magnifier to see the median eye in an anole, for it is almost featureless and almost certainly without function as an organ of sight.

There is, however, a pretty good centrally-placed eye on the head of a rare, primitive lizard, the tuatara (Sphenodon), that survives only on certain small islands off New Zealand. The tuatara's median eye is well-developed and easily recognizable, although covered by translucent scales. Within the eye itself clear evidence of a retina remains and the animal responds to light, if not to images.

If a light-sensitive organ, or third eye, is present in certain fishes, frogs, and a few reptiles, what form does its descendant pineal body take in our feathered friends and in mammals, all of which evolved from reptiles? Neither birds nor mammals have the least sign of a median eye in their heads, but they do have a well-developed pineal body attached by a stalk to the rear of a cavity deep inside the brain. Close examination shows it is served by nerves and nourished by blood vessels, evidence that suggests a vital function.

Experimentation reveals that in birds the pineal body is affected by light, even though it is not any sort of eye. It seems to be responsible for detecting (without visual means) the shortening or lengthening of days preparatory to migration. Such sensitivity establishes a biorhythm, or a biological clock. Biological clocks of one sort or another are evident even in us.

The 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 duration of light, including reproductive cycles in some animals. It is often in the news as a human sleep aid, although the medical profession has not yet fully embraced either its effectiveness or its safety when it is purchased without prescription over the counter.

All nerves produce substances called neurotransmitters, one of which, serotonin, is responsible for producing melatonin in the pineal gland. It is evident that this tiny organ, once part of a functioning third eye in primitive vertebrates, has developed a new, different, and vitally important role in the well-being of higher vertebrates.

If the pineal body in birds is tied in with their seasonal migratory patterns, it has also been found to play a role in both the seasonal and the 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.

In the animal world there are many instances of one organ evolving into something different from its origins -- a recycling of swim bladders into lungs, fins into arms and legs, fish scales into teeth -- but nothing more remarkable than the refashioning of a tiny part of the ancient brain, once responsible for detecting light and moving objects, to become part of a chemical system that tells a bird to fly to inherited destinations, that eases us into sleep, or that leads a youth through adolescence into adulthood.

The middle eye is gone; the old structure has been lost; still the pineal remnant helps control our growing-up and our daily and nightly rhythm. When you tuck yourself into bed tonight, give a silent nod to an ancient fish and its frog descendants.







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