If we collected the whole evolutionary history of animals in a book, we’d start with the original common ancestor and before we knew it, even if we managed to fit a million years on each page, we’d still end up with over 600 pages!

There’s a lot of important stuff spread throughout The Grand Evolutionary History of Animals. But one of the most foundational chapters would come fairly early on.

For the animals we know today, with very few exceptions seeing means being an animal. In fact, we wouldn’t even make it 10% of the way through the book before seeing, well, seeing! The earliest known fossil with eyes is Spriggina, a bilaterian from over 550 million years ago.

Seeing or vision is the ability to interpret your environment based on how light interacts with it. And about 96% of all animal species have eyes, the organ or collection of tissue and cells that make vision happen. Even many animals that have evolved to be “eyeless” or have eyes that don’t function to their full capacity still have some ability to sense light.

In fact, vision is so evolutionarily advantageous, it’s evolved multiple times and in multiple ways across all Metazoans, and it took eyes a surprisingly short time, evolutionarily speaking, to be seen all over the animal tree of life. I’m Rae Wynn-Grant, and this is Crash Course Zoology. INTRO.

Eyes can be intricate structures or simple collections of cells, but basically eyes detect light and process that information into signals that their nervous system understands. And there are two minimum requirements for an eye to work. Photoreceptors, which are cells that react to light by sending electrical signals, and a nervous system made of special cells that can process those signals into colors, brightness, darkness, and other visual information.

Everything else you’ve heard eyes have -- like lenses, pupils, corneas -- is extra to make the signal from the photoreceptors and its interpretation by the nervous system more effective. So there are lots of different types of eyes. But we can split animal eyes up into two big categories: eyepatches and image-forming eyes.

Animals with eyepatches can have something as basic as a flat cluster of a few photoreceptors. They can perceive light and dark, and if that patch is in a small ‘cup’, tell vaguely where the light is coming from. But they can’t see shapes, details or patterns at all.

To really take advantage of the sense of vision, animals need an image-forming eye, which not only detects light, but, thanks to some extra eye-machinery, uses it to make an image in the back of the eye. The most basic image-forming eyes form dim, blurry images. To see fine and faraway details, animals need a lens, or a transparent, crystal-like structure that focuses incoming light onto the retina, the specialized layer of photoreceptors located in the back of image-forming eyes.

But while it's convenient to divide animal eyes into eyepatches and image-forming, it doesn’t truly reflect the diversity of animal vision. Like box jellyfish, octopuses, fireworms, horseshoe crabs, and parrots all have image-forming eyes. But even if they were all looking at the same object, they’d all see very different things because some have stronger lenses or more sensitive retinas than others, and some build a picture of the world in a fundamentally different way.

So it’s not so easy to compare eyes, and it’s hard to not be biased by how we humans see. Animal eyes are specialized to see in a way that’s evolved to match their lifestyle and environment, not necessarily ours. So we have to be more specific.

Like by looking at how animals perceive fine details, called acuity or resolution. Visual acuity depends on lots of factors like how many photoreceptors are packed into the retina, eye size, and how the structures of the eye focus light into a picture. The compound eyes of crustaceans, insects, and some molluscs are made of multiple, sometimes even thousands, of light-sensitive eye-units, called ommatidia, that combine their information to form an image.

These eyes generally have poorer visual acuity than camera-type eyes, which focus light onto a single, much larger, retina. BUT because they require less space, compound eyes can wrap around an animals’ head, giving them a wider field of view, or how much of their environment an animal can see without moving their eyes or head. And compound eyes are great at detecting movement!

So if resolution is the most important to us, we’d want camera-type eyes. But if we wanted to see a larger area, maybe to keep a look out for predators, we could go with compound eyes, especially if our animal is tiny. This trade-off between seeing a small area in lots of detail, and seeing a larger area in less detail, is really common in animal eyes.

Some animals take it to the extreme. Like jumping spiders have evolved telescope eyes. Two of their eight camera-type eyes have two lenses instead of one, which magnifies the image and really bumps up the resolution⁠!

But their field of view is tiny, so the spider’s six other eyes see a wide area to compensate. We can also compare eyes based on how much light they need to work, called sensitivity. Most animals see in different lighting by having two types of photoreceptors -- rods, which are so sensitive to light that they get overwhelmed in daylight, and cones, which pick up finer details and color.

Animals can also change how much light gets in their eyes with other adaptations, like pupils, the hole in the center of some eyes that grows and shrinks to let more or less light in. Or with tapetum lucidum, a layer of reflective tissue at the very back of the eye that bounces light so it passes the retina twice, giving the photoreceptors a second chance to catch the light. So if we’re working in a low light environment, we’d want wide-open eyes with a tapetum lucidum to maximize light sensitivity.

The last property of vision we’ll talk about to distinguish animals' eyes -- because there are others! -- is what types of light they see. First there’s the wavelength, which is based on how much energy the light has. We humans can only see light with wavelengths from about 380 to about 750 nanometers, so we named that the visible spectrum.

But most animals can see light outside that range ⁠— like infrared and ultraviolet light ⁠— or even polarization, which is the direction that light waves are vibrating in. By evolving to be more sensitive to specific wavelengths, animals can more easily notice things that are important to them, like camouflaged predators or the colors of tasty food. So the best set of eyes are...well, that depends on what you need them for.

But the wildest eyes, and one of the most complex visual systems discovered yet can be found in the ocean...punching things. Let’s live a day in the life of… a mantis shrimp! Allow me to introduce you to the peacock mantis shrimp.

This technicolor boxer has a pair of compound eyes that are set on stalks and can look two places at once. As he swims through shallow coasts of the Indo-Pacific, not only does he see both ultraviolet and polarized light, but while we have just three types of cones, our mantis shrimp has at least 12. But even with all that visual firepower, he doesn't distinguish colors nearly as well as we’d expect.

Which is weird, because he definitely lives a colorful life. He scares off other males of his species by flashing a bright patch of color on his clubs, called a meral patch. Though 12 cones still seems like overkill.

Lot’s of other animals put on colorful displays with only 3 or 4 cones. It might come down to what goes on in our crustacean’s brain when he sees color. Instead of comparing the signals from a few cones to perceive different colors, our shrimp uses a limited, 12-piece palette of finely-tuned photoreceptors that only respond to very specific colors.

One cone reacts to UV, one reacts to violet, one reacts to blue, another for turquoise, and so on, spanning a rainbow -- and more -- of wavelengths. With our 3 cones there’s a slight delay as our brains do the math to figure out the color we’re looking at. But instead, a mantis shrimp knows pretty quickly how much UV or violet or whatever light he’s seeing.

He isn’t very good at telling apart small color differences, but he doesn't need to wait long for his brain to process, so he can grab a snack or chase off an intruder at lighting speed. Have a great day, mantis shrimp! Eyes, especially ones as tricked out as what mantis shrimp have, might seem to be an overwhelmingly complicated structure to have evolved.

In fact, many animals that live exclusively in underground habitats like caves have evolved to have little or no sense of vision, likely because eyes are too expensive to maintain if they’re not being used. But even though evolution doesn’t always lead to more complexity, eyes are one of the best examples of how tiny changes in structure and function can build into sophisticated traits over time. In 1994, Swedish zoologists Dan-E Nilsson and Susanne Pelger proposed a sequence of events where a camera-type eye evolved from a light-sensitive patch of photoreceptors in… well, the blink of an eye (evolutionary history-wise).

Nilsson and Pelger used something similar to the maximum-likelihood approach. Assuming that eyes with sharper resolution always helped animals survive and reproduce, they calculated how many steps -- or mutations -- it would take to go from this eye to this eye. First, the flat patch of light-sensitive cells develops into a depression, then a cup, and finally a pinhole shape like in a Nautilus.

Eventually, the eye area evolves to be enclosed with a membrane and filled with fluid to keep junk out, and in more complex systems, take advantage of how fluids bend light to focus it more sharply on the retina. Next, the lens develops as another structure to help focus light on the photoreceptive patch. Then we get an iris and pupil for fine control of light going into the eye.

And what they found was that it might take 364,000 years or less to evolve all the complicated eyes out there today -- including the ones mantis shrimp have. Which sounds like a long time, but it took tens of million years for feathers to go from this to this. And once eyes did evolve, they were an evolutionary game changer, which might explain why they seem to have become wildly popular almost overnight.

Instead of relying mostly on close-proximity senses like touch, animals could literally see other animals coming! Today, most known animal species have eyes, but like other senses, eyes are fine tuned to an animal's lifestyle, so they all see very different things. Next episode, we’ll cover another sense that’s evolved many times -- hearing!

Before you go, you should feast those super awesome image-forming eyes of yours on a new. PBS show! Otherwords!

Otherwords on Storied digs into the quintessential human trait of language and finds the fascinating, thought-provoking, and funny stories behind the words and sounds we all take for granted. Incorporating the fıelds of biology, history, cultural studies, literature, and more, Otherwords has something for everyone and offers a unique perspective into what it means to be human. Give it a peak and let them know that Crash Course sent you.

Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE. It’s shot on the Team Sandoval Pierce stage and made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.