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 trilobites, starfish and this weird lookin' thing came on the scene. They're thought to have independently evolved at least 50 different times which makes sense: even a simple eye is 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. [theme music] 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 photopigment whereas 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 shrimps 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 twelve clear colors That works fine for picking up potential mates or prey but it means that they don't get
the rich depth of tints and 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 visible 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 its 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 about 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 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 the tapetum lucidum It reflects light that the rods didn't catch the 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 that make up light go in all different directions: vertical, horizontal, diagonal, whatever. That's unpolarized light. But in some cases, light if the light bounces
off 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 sometimes
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
and that helps the animal figure out if it has a clear shot. The effect works 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 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 to be kinda pear-shaped to help focus light from both air and water. Number eight: compund 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 details 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 toward 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 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 a face for. Take the West Indian fuzzy chiton.
This unassuming marine mollusk 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 that 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 it 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 it's only been in the last five years or so that 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 They 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! Thanks for using *your* eyeballs to watch
this SciShow List Show. And thanks especially to all of our patrons on Patreon who make this show possible. If you want to help us keep making videos like this,
just go to patreon.com/scishow and don't forget to go to youtube.com/scishow
and subscribe!
Cool! So I'll add eyes to my "list of features it's reasonably safe to assume an alien species has".