Eyes have been around for at least half a billion years. First appearing in the fossil record during what's known as the Cambrian Explosion, when all kinds of different animals, like trilobite, starfish, and this weird looking thing, came on the scene. They're thought to have independently evolved at least 50 different times. Which makes sense, even simple eyes are a useful tool giving you the evolutionary advantage over the sightless species around you.

With your new visionary superpower you can scan your surroundings, looking out for predators, prey or somewhere to live. And over the years, eyes have diversified into all kinds of weird and wonderful forms. This is SciShow List Show.

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Number one: Extra cones.

We can see color thanks to the cone cells at the back of our eyes. Most people have three types of cone cells, each with a different pigment that's sensitive to light of different wavelengths. We see these as red, green or blue, our three primary colors. This is known as trichromacy. 

Most other mammals have just two types of photopigments, or some birds and fish have four. And then there's the mantis shrimp, which has 12. Each is sensitive to a different wavelength and therefore color, of light. Yet despite this, and despite what you may have heard online, according to a 2014 paper published in the journal Science, mantis shrimps probably don't live in a dazzling world of super-mega-technicolor. 

Researchers tested mantis shrimp, and they gave a "surprisingly poor performance" at telling apart light of slightly different colors, differences that would be obvious to us. So, it's possible that mantis shrimps process color in an entirely different way from us. Their world might be simplified to just 12 clear colors. That works fine for picking out potential mates, or prey, but it means that they don't get the rich depth of tints or shades that we get to enjoy. 

Number two: Ultraviolet light. 

We may think flowers are pretty, but honestly they couldn't care less about our feelings. Flowers evolved alongside pollinators, like bees, and that's what matters most to them. And bees, like many insects, see into the ultraviolet range. They have three types of cone cells, like we do, but one of them peaks at 350 nanometers, way out of our own range, which is between 400 and 700 nanometers. So what a bee sees, as it flies through meadows or gardens, looks very different. 

Flowers like marigolds, for example, gain a striking layer of complexity. When you take ultraviolet into account they look much more like a landing pad awaiting a hungry bee. This way the bees know exactly where to go for their nectar, and they pick up some pollen along the way.

Number three: Infrared light.

Many snakes have what's essentially a built-in thermal imaging camera. They can use infrared light to track down even the quietest prey in the dark. Take the pit vipers, for example. They're named after the indentations in the sides of their faces, between their eyes and mouths. These pits can look a little bit like nostrils, but they're actually wired up to the parts of the snake's brain that deal with vision, and their job is to pick up light rays, just ones we humans can't see. 

Warm bodies radiate infrared energy all the time, which is useful for a pit viper looking for, say, a warm, juicy mouse for dinner. The pits are sensitive enough to detect the slightest shift in temperature, which lets the snake build a picture of it prey's location and movement. Heat vision, the perfect tool for a cold-blooded killer.

Number four: Night vision. 

When it's dark, the cone cells in our eyes are practically useless, but that's when our rod cells come into their own. Rods only see in black and white. But they're very sensitive in low levels of light. Now, you may pride yourself on your ability to find your way to the kitchen for midnight snacks without switching the lights on, but there are a lot of animals that put our night vision to shame. 

Light levels can be measured in lux. A bright sunny day is around 10,000 lux, and we can comfortably see down to one lux, a moonlit night. Cats do somewhat better, seeing and hunting at just an eighth of a lux. Their retinas are densely packed with rods, which help capture more of the action. And if you've ever seen that eerie, green, shiny eye thing they do, you'll have spotted another one of their tricks.

Eyeshine comes from an iridescent layer called tapetum lucidum. It reflects light that the rods didn't catch first time around, giving it a second chance at coming back and being absorbed. Actually most mammals have this layer, only a few day dwelling primates don't, including us.

But the night vision of some insects is even better. For cockroaches to navigate, their cells require less than one photon per second, and researchers still haven't figured out how they do it. Right, because cockroaches really needed another superpower. 

Number five: Polarized light.

Now we have a type of vision that's way beyond anything we can interpret, polarization. Light travels in waves, and most of the time the waves make up light go in all different directions -- vertical, horizontal, diagonal, whatever -- that's unpolarized light. But in some cases, like if the light bounces off of a flat surface, like a lake, all the light waves will be oriented the same way, that's polarized light. When it's particularly strong, we humans see polarized light as a harsh glare. But some animals, like cuttlefish, use polarization to add a whole extra layer of information to their world, that might be as clear and intense as colors are to us. 

Cuttlefish have monochromatic vision. They see the world in shades of gray. But they can also see polarized light, and use it to communicate. Cuttlefish have special flat surfaces around their eyes and tentacles that polarize the light bouncing off of them. This works as sort of a secret signal, flashing brightly, but only to those who can see it.

Number six: Vertical pupils. 

An eye's structure, including the shape of the pupil, can tell you a lot about the animal that owns that eye. Some animals have pupils that are vertical slits, usually predators that attack by sneaking up on their prey, like cats, crocodiles, and vipers.

The vertical slits are useful for these animals, because they give what's called a defocus blur to the horizontal parts of the animal's vision. Now, blurriness might sound like a bad thing, but it's actually pretty useful here. It tells the hunter how far away other objects are from the target it's focusing on. That helps the animal figure out if it has a clear shot.

The effect works for best for animals lower to the ground, which may explain why big cats and tigers don't have slit pupils.

Number seven: Eyes split in half.

Teasingly being called four-eyes for having glasses is no fun, but anableps fish have it even worse. They're known as four-eyed fish, and for good reason. Four-eyed fish spend a lot of time at the water's surface, and they do have two eyes, but each one is split horizontally, with one half above the water and the other below. 

Each half has its own pupil opening, so without moving its head, the fish can watch for flying predators above them, and swimming predators below them. They are paranoid little guys. Their eyes are incredibly complex. Light travels at different speeds through air and water and changes direction when it moves between the two. So each half of the eye needs its own adaptations. 

The top part of the eye has a thicker, tougher front coating that protects it from drying out and from UV damage from the sunlight. It's also more tightly curved to help direct the bending of the light from the air onto the lens inside. And the lens itself is a weird compromise. Fish generally have spherical lenses, while land animals have thinner, more oval-shaped ones. But in the four-eyed fish, the lens evolved kind of pear-shaped, to help focus light from both air and water. 

Number eight: Compound eyes.

When cartoons draw a fly's view of the world they tend to show the same image repeated over and over like some kind of crazy kaleidoscope, and actually that's pretty accurate. Flies, like other insects, have compound eyes, which are divided up into thin, column-like units called ommatidia.

Each unit is capped with a small lens that focuses light onto the long receptor cells underneath. Then, the fly's brain stitches the data from the ommatidia together, compiling it into a mosaic. These small lenses can't resolve the fine detail too well, so the image might appear to be somewhat pixelated. But if you're a fly, you don't really care if that thing swooping towards you is a frog's tongue or a rolled up newspaper, as long as you see it coming with enough time to respond. 

The dome shape of the compound eyes helps with that, providing a very wide viewing angle for detecting an oncoming threat.

Number nine: Rock eyes.

Getting information from light isn't something you necessarily need an eye for. Take the West Indian fuzzy chiton. This unassuming marine mollusc might not look like it's staring back at you, but recent research shows it's seeing something, at least. There are hundreds of neat little bumps, each about the width of a human hair, embedded into the chiton's shell.

It turns out, these bumps are tiny lenses, made from a thin, translucent layer of a mineral called aragonite, one of the main components of limestone. Scientists have known about these lens structures for a while, and the fuzzy chiton certainly reacts like he can see. 

If it's suddenly in shadow, for example, it'll clamp down tightly, protecting itself from what might be a looming predator. But in the last five years or so, people have started researching whether these eyes can detect an actual image as opposed to just light and dark. 

Researchers isolated some of these rocky eyes and shone light into them and saw that the aragonite lens was focusing that light onto receptor cells below. Next, they projected an image of a fish over the eyes and recorded what came out the other side, the same way those receptor cells would. What they got was an image -- a blurry pixelated image, sure, but an image. From eyes that are made of rock. 

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