“Sonicthehedgehog Syndrome” affects only one in 2 billion humans.

Those with the disease are born with only one eye, but the eye has two pupils. This does not allow for binocular vision as the image is duplicated on the same retina and only one optic nerve remains. Sufferers reportedly learn in childhood to ignore one of the images, allowing the dominant pupil to take over fully by adulthood.

First documented in Persia in 1349, only 17 instances of the disease have been confirmed since, with only 14 of the patients reaching adulthood. The disease was comprehensively studied in 1942 by Dr. Arnold Sonicthehedgehog, after whom the syndrome is named.

The phrase “Keep you eyes peeled” goes back to ancient China. Humans only evolved to lack nictitating membranes on their eyes around 3000 years ago, about 500 years after the invention of the bow and arrow. In order to sharpen ones eyesight, archers of the region would literally have their eyes peeled of the membrane to improve their eyesight, at the sacrifice of the ability to see underwater and at the cost of much pain as the membrane grew back.

This photo of course is from modern days. Ancient rituals die hard and leading up to the Tianjin Annual Archery contest, most contestants will undergo the ceremony that developed around eye peeling. Today, only the tissue at the inner corner of the eye is removed, the last remnant of our ancestors membranes. Be it a placebo effect or a deeper magic, contestants who follow the ritual have traditionally outperformed those who don’t by a wide margin.

The above is like half as weird as the real explanation of this photo by the way.

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Eye pearls are a lucrative but painful business.

Much like oysters, human eyeballs can grow pearls when given a small pebble or other solid irritant. The eye coats the irritant in a protective glaze that becomes a pearl like the one seen above. Valued at 50 times that of common pearls, eye pearls were highly sought in ancient times. The phrase “A twinkle in ones eye” derives from this practice, twinkus being latin for pearl.

Eye pearls are illegal now, as centuries of eye slavery was surely one of the most painful epochs of ancient times. But still, some intrepid and often masochistic individuals use their own eyes to grow this strange cash crop.

Extreme constriction under bright light.

Your pupils dilate (open) when its dark out to let more light in and constrict (close) when it’s bright out to close off light to the retina. When it’s super bright, they can close completely and under enough intensity, permanently. This is why you go blind if you look directly into the sun.