Eyes began to develop 550 million years ago. The simplest eyes were patches of photoreceptor protein in single-celled animals. The patches have subsequently been named eyespots and from these the 10 different forms of eye we have today evolved. Creatures such as snails and flatworms still have eyes that can only detect light and its direction. Others, such as the vent shrimp, which lives near boiling hydrothermal vents on the sea floor, have simplified compound eyes that can see infrared light, enabling them to avoid lethally hot water.
Box jellyfish have eyes with lenses, corneas and retinas similar to our own. Despite this, their eyes are permanently out of focus. This is because they don’t have a brain, just a ring of nerves around their mouth. Without central processing power, the blurry vision tells it all it needs to know: “How big is it? Can I eat it? Will it eat me?”
Eye colour is determined by the amount of melanin in the outermost part of the iris. Melanin is the main pigment found in the human body, responsible for hair colour and skin colour. A high concentration of melanin will create brown eyes. Lower concentrations will create green and blue eyes, because the next layer of the iris is made of collagen, which is blue. All people with blue eyes have a single common ancestor who lived approximately 10,000 years ago.
The size of the pupil controls the amount of light that goes into the eye. The word pupil is from the Latin pupillus/pupilla meaning a little child or doll, after the tiny reflection of yourself you see when you look into someone’s eye. Cuttlefish are the only animals in the world with w-shaped pupils. Goats have rectangular pupils.
In most eyes light enters and projects onto a retina, which converts the light into neural signals, which are sent to the brain by the optic nerve. In the late 19th century, it was thought that the last image seen by the eyes of a dying person would be fixed on the retina and this became a common theme in crime novels of the time. In fact, it is the brain that “sees” the image, turning the inverted retinal image the right way up.
In the 1890s, George Stratton, a psychologist at the University of California, wore “upside-down” glasses that reversed the images onto his retina. At first he was disorientated, but after eight days his brain adjusted enough to allow him to walk around. Stratton’s work proved that our brains correct our sensual perceptions to create our “normal” view of the world.
Mayans thought that crossed eyes were very attractive and would suspend a wax ball from the hair of newborn infants in order to make them cross-eyed. On encountering their descendants, Cortez was shocked that nearly all the high caste Indians he met in the Yucatan (the old Mayan lands) were cross-eyed.
The French philosopher René Descartes had a fetish for cross-eyed women and it was realising the source of this that allowed him to develop his defence of free will and the body/mind split. He wrote: “I loved a girl of my own age who was slightly cross-eyed… and for a long time afterwards, in seeing cross-eyed women, I felt more inclined to love them than others, because they had that defect; and I did not know that was the reason. Since I have recognised it as a defect, I have no longer been so moved.”
Eye of shrimp
Human eyes are not the most highly evolved. The eyes of the mantis shrimp have four times as many colour receptors as humans, some of which can see ultraviolet.