If I had a hundred eyes, what would I see? - The evolution of eyes from scallops

The winning works of the 2023 "China Science Popularization Star Creation Competition"

Author: Jie Yan

Summer is here, and it's time to eat barbecue again. The sizzling garlic oil roasts the sweet and tender adductor muscles of scallops. Take a bite and the fresh and fragrant juice will overflow. Then take a bite of the vermicelli soaked in the soup. It can be said to be a must-order item at the night snack stalls. When you chew the scallop meat and vermicelli, have you ever thought that the more than 100 eyes of the scallop are hiding under the garlic vermicelli and watching you?

Eye Position: Where should my one hundred or so eyes be placed?

We will inevitably lose our SAN if we continue with this topic... Let's change our way of thinking for now: if you also have more than a hundred eyes, what kind of world will you see?

First, we need to find a place to put these eyes.

According to conventional thinking, eyes should be on the head. Whether it is humans, mammals, birds, fish, amphibians, reptiles and other vertebrates that we come into contact with on a daily basis, or even insects that are driven out of the room, they all seem to put their eyes on their heads. However, there is no shortage of "odd species" in nature that break through human cognition. For example, the familiar scallops (actually a general term for many types of scallops such as Chlamys farreri, Bay scallops and Yezo scallops) do not have specialized heads, so they choose to put their eyes on their mantle (covering the dorsal side of the mollusk, the membrane formed by the skin folds extending downward, the vegetable market owner calls it scallop edge), and its eyes are typical non-head eyes.

Another example is the blind shrimp (Rimicaris exoculata) living near the hydrothermal vents. Their eyes are two shiny retina-like structures on their backs, not on the eye stalks of their heads like the familiar "shrimp". Therefore, the discoverers of the blind shrimp mistakenly believed that they had no eyes. It was not until the first female pilot of the deep-sea submarine Alvin, a marine biologist, Cindy Van Dover, who became interested in the thin film on the back of the blind shrimp that could reflect the submarine's searchlight when visiting the hydrothermal vents, that she sent the dorsal tissue to scientists studying invertebrate eyes for identification, which cleared up the misunderstanding for the first time.

If you still remember your high school biology textbook, maybe we can review the lancelet. The cephalochordate lancelet (collectively referred to as the genus Amphioxus) is a transitional type between vertebrates and invertebrates, and can be called a living fossil in the history of biological evolution. Its eyes are not even on the surface of the body, but buried deep in the front end of the neural tube in the body, in a small concave shape, which can sense the light on the back of the body, called the frontal eye. Because the body of the lancelet is transparent, this frontal eye in the body can still help it sense light.

Therefore, given the wildness and freedom of nature's demands, you can freely choose a place you like to place your eyes, wherever you need them.

Types of eyes: Two cells make up an eye?

When you have just gained more than a hundred eyes and think of the compound eyes of dragonflies, flies, butterflies and other creatures, you can't help but wonder: Does having a hundred eyes mean I have compound eyes?

First of all, the answer is not equal.

Simple eyes and compound eyes refer to the type of eyes, not the number. Eyes in nature are varied, and biologists generally divide them into two categories: simple eyes and compound eyes based on their structure.

Among them, monocular can be roughly divided into three types.

The first type is the most primitive cup-shaped eye and small hole eye, which has a simple structure, no focusing function, cannot form an image, and can only observe the position of the object.

Imagine an eye with just two cells. The simplest cup eye is made up of just two cells, one for light sensing and one for pigment (which acts as a protective layer). This type of two-cell eye is found in organisms such as the Japanese worm Polycelis auricularia.

The small holes are not actually small. The radius of the eyes of the nautilus with small holes can reach 1 cm. The small holes mean that its eyes are no longer a slightly concave plane, but more inward, so that the species with it can adjust the resolution by adjusting the size of the small holes.

The second type is concave lens eyes, which can produce blurred images. Our protagonist, the scallop, has dozens to more than 200 such eyes. Because the outer surface of the scallop's eyes is covered with a pigment epidermis, it looks like a wonderful color between blue, cyan and purple, which can be called a filter with a mysterious atmosphere.

Just like we have only two eyes, there are also some asymmetries. Scallops have different sizes of eyes, and the smaller eyes are often located between the larger eyes. As early as the 18th century, people became interested in scallop eyes and conducted anatomical research. I wonder what it felt like for the first person to look a scallop in the eye...

The third type is the camera eye, which is the main type of eye in vertebrates. Among the animals we are familiar with in our daily lives, invertebrates such as squids, cuttlefish and octopuses also have this advanced eye.

However, just like the square-pupiled goat eyes will bring a little shock from the "ancient gods" to those who look into their eyes for the first time, the eyes of the squid and octopus do not look so serious. The squid's eyes are shaped like a well-known emoticon "ww", while the octopus's eyes look like "- -".

The compound eye is composed of varying numbers of small eyes. If each of these small eyes can form an image independently, it is called a combined eye. If they form an image on the retina together, it is called an overlapping eye.

The advantage of compound eyes over our camera eyes is that they have a wider field of view and are extremely sensitive to movement so the organism can react (so it's not our fault if we're slow to swat a mosquito!).

The disadvantages of compound eyes are also obvious, because the lens of the small eyes is small and easily affected by diffraction, so the image seen will be blurry (imagine that the larger the lens, the clearer the picture). According to calculations by researchers, if you want to have the same acuity as human vision, the diameter of the compound eyes must reach an astonishing one meter, which can be said to be full of legs below the eyes...

The structure of the eye: Why do I have calcite in my eye?

If you could choose, would you rather have a hundred cat eyes that can see at night, or a hundred mantis shrimp eyes that have thirteen more types of cones than humans?

If we could really see the world through the eyes of other species, we would definitely see a new and perhaps bizarre world. And these visual differences ultimately come from the differences in the eye structures of different species.

The most primitive eye structure may be just a piece of skin covered with photosensitive cells, like an exposed retina, such as the blind shrimp we introduced earlier, and the earthworms we are familiar with. For these animals, "eyes" are everywhere, and any part of the body can be their "eyes."

These photosensitive cells on the surface of the body can help them sense light, but they cannot determine the specific direction of the light. So more advanced eyes appeared. They made this piece of skin slightly concave so that they could determine the general direction (for example, light coming from the left will shine on the right wall of the depression).

Then another creature increased the size of the depression, turning it from a plate into a soda bottle. Light must enter from the mouth of the bottle and pass through the small holes to form an image at the bottom of the bottle covered with photosensitive cells, forming a simple, blurry image.

However, this is not enough. Biological evolution is like a never-ending equipment competition between predators and prey. In order to survive, everyone has to become more and more "competent". The structure of the eye has also become more and more sophisticated to follow the pace of biological evolution. Finally, the most critical part of the eye appeared in evolution. A transparent crystal appeared at the mouth of the bottle. It is the original form of the lens, which can focus light and make the image clearer.

Gradually, more complex regulatory mechanisms emerged. The cornea protects the eyeball behind it, the iris adjusts the intensity of light entering the eye, the muscles around the lens give the eye the ability to focus, and the retina is no longer a simple piece of skin covered with photosensitive cells...

If you ask what was the first animal that truly had vision, from an imaging perspective, based on existing archaeological discoveries, it was our familiar old friend the trilobite.

540 million years ago, as the first organism to have a lens, it saw the world we live in from a completely new perspective for the first time.

However, their eyes are also quite special. Specifically, their eyes are veritable "stone eyes" composed of a mineral crystal that we call calcite today.

One of the properties of this crystal is that only light at a specific angle can penetrate the crystal, while light at other angles will be deflected. Trilobites use this property to arrange the calcite crystals in each small eye so that the light they receive goes directly into the retina below.

To this day, we can still find these "stone eyes" in the ocean. Today, they belong to a class of creatures called brittle stars. They have five soft and delicate arms, most of which are indifferent to changes in light and swing freely in the sea water. The skeletons supporting these arms are connected calcite plates, and calcite can also form the hunting spines on their arms (by the way, the thorns on the sea urchins we are familiar with are also made of calcite).

For a long time, scientists thought that was all calcite did in brittle stars (it was already very useful), until researchers discovered that Ophiocoma wendtii seemed to be able to sense the presence of predators. Since there were no eyes on the surface of this beautiful creature, researchers associated the calcite on its arms with the lenses of ancient trilobites, and then proved that the calcite crystals on these arms can actually focus light to the photoreceptors below! In other words, these brittle stars don’t have brains, but they do have eyes!

In addition to calcite, even mitochondria can fuse to form lenses. Yes, that's the mitochondria in your high school textbooks that are responsible for respiration. This magical lens belongs to a flatworm, Entobdella soleae.

There is also something you might not imagine in the eyes of scallops, the protagonist of this article. Guanine. (Humans first encountered it in bird droppings, hence the name guano purine, but fortunately, it is generally referred to as guanine.)

The structure formed by guanine in the eyes of scallops is very similar to that of cats we are familiar with. If you have ever encountered a group of cats at night, you must be impressed by their sparkling eyes, but cat eyes cannot actually emit light by themselves. The structure that makes them sparkle comes from the choroid tapetum behind the retina (just like the "red light" behind a bicycle that lights up when it is illuminated by light). This layer of choroid tapetum can reflect light a second time, helping cats see the world clearly at night.

Scallops living in the dimly lit sea also need this ability very much. The desire for evolution drove the scallops to rummage through their bodies, and finally they chose guanine, one of the four purines that make up DNA.

When it exists in the form of natural crystals, guanine is one of the most superior optical materials in nature. Scallops "control" these guanine crystals to grow into small squares, which are closely arranged like tiles (20-30 layers) to form a mirror-like structured silver film (also called a reflective mirror) behind the retina. Light passes through the cornea, enters the crystal, passes through the retina, and then comes to the reflective mirror, where it is reflected and returns to the retina to form an image.

The reason why scallops are said to "control" guanine is that in theory, the growth shape of guanine crystals should be prismatic. However, when researchers used a low-temperature scanning electron microscope to observe the ultrastructure of scallop silver films, they were surprised to find that the crystal plates formed by these guanines were almost perfect squares. You should know that in Euclid, if you want to form a plane through regular congruent polygons, you can only use equilateral triangles, hexagons or squares, just like scallops have also learned geometry.

In addition, the arrangement of the guanine crystal plates is also very sophisticated, so that the high refractive index side is kept facing the direction of the incident light, thereby minimizing the surface defects of the crystal plates and forming a highly reflective surface. This tiling method is very similar to the segmented mirrors in space telescopes. The hexagonal mirror array of the James Webb Space Telescope (the successor to the well-known Hubble Space Telescope) is almost what the scalloped silver film looks like when it is magnified to a macroscopic level.

Some of our familiar friends (dinner guests), some crustaceans (such as shrimps) also use mirror structures formed by guanine in their compound eyes to help focus. The colorful tropical fish in the aquarium also use guanine crystals to form the silvery and colorful luster on their scales. The same mechanism also accounts for the white color on some spiders and the colorful colors of some planktonic crustaceans.

The evolution of the eye: What can a half-evolved eye see?

Before the discovery of calcite lenses in the eyes of trilobite fossils and guanine lenses in the eyes of animals such as scallops, people have always had a question (similar to whether the chicken came first and the egg, or the egg came first and the chicken): that is, based on the high specialization of lens proteins in human eyes, humans should have had eyes first and then evolved such highly specialized proteins according to the needs of the eyes. But if lens proteins had not evolved, how could humans have eyes?

Furthermore, some scientists who oppose Darwin's theory of evolution believe that it is impossible for an organ as sophisticated and complex as the eye to be formed bit by bit through a long process of evolution. This is because animals cannot form vision when the eye has just evolved halfway (or even a little more than half). Then, according to the theory of evolution, such a useless organ will soon be eliminated and will not have the chance to evolve into a complete eye.

It is not surprising that scientists would doubt this. Let us first look at the human lens, which is a component of the highest-level camera eye. It is highly specialized, colorless, transparent, and flexible, like a lens truly born for optics. It has no blood vessels, and the cells are all nourished by the aqueous humor behind them. Even these cells are not like regular cells, and almost all functions are simplified, leaving only full and concentrated proteins. These proteins are delicately arranged at the microscopic level to form a liquid crystal array, which conscientiously assumes the function of focusing and focusing.

Doesn’t it look like a lens that was designed and then processed according to the drawings?

What a coincidence! Scientists at the time also thought so. So who was the designer? Of course, it was God.

However, with the development of molecular biology, scientists finally had the opportunity to further study the lens proteins that seemed to be specially designed for the eyes. At this time, they were surprised to find that these proteins were not born specifically for "making lenses". In fact, they also have many other functions in the body (for example, many lens proteins are enzymes). Instead, "making lenses" is like a small episode in their work life. They are like being temporarily transferred to the eyes to gather together and form the human lens.

Then it is not surprising to see calcite in the eyes of trilobites, guanine in the eyes of scallops, and even mitochondrial lenses in the eyes of flatworms. Since we are conscripting able-bodied men, we can capture any one we can. As long as they can work normally, it doesn't matter what they are!

Since it has been proven that the eye was not designed by God using CAD and then manufactured according to the blueprint, it is clear that the situation where the eye evolved only half has existed in the evolutionary process. So is this half eye the front half of the eye or the back half of the eye? What can a creature with half an eye see?

Perhaps we should start with a drop of pigment.

Scientists have made many speculations about the origin of eyes, and many people believe that the ancestor of eyes should be some algae. Algae have many photosensitive pigments, which they put in their eyespots to "see" the surrounding light, so that they can always stay in the sunny place for photosynthesis.

One of these pigments is called rhodopsin, a protein composed of retinal and opsin, which is found throughout our eyes and is responsible for sensing light. It can even be said that for the ancestors of all of us, the birth of the first ray of light was not due to the rising of the sun, but rather that the ancient creatures finally synthesized this protein during the long process of evolution, thus ending the life of groping in the dark and ushering in a new era of light.

So how does this protein accomplish this exciting job of sensing light?

As mentioned earlier, rhodopsin is a protein composed of retinal and opsin. In fact, retinal is the only warrior in the eye that faces the light directly. It exists in our body in two forms: cis structure and trans structure (you can imagine it as our left and right hands. The structures are similar, but because of the different spatial conformations, your left hand will never become your right hand no matter how you turn it upside down.) The trans-structured retinal firmly stays on the opsin until a ray of light. After being magnified tens of millions of times, from the perspective of retinal, we will see countless photons pouring in. One of the photons hits it, and it becomes a retinal with a formal structure and leaves the opsin. The photon retires, and the opsin passes the news of the arrival of the photon down step by step until we see the light.

So where do the colors we see come from?

The lens materials mentioned above that can be moved wherever needed may have made you realize the "stinginess" of biological evolution. After all, energy is limited, and organisms are well aware of the consumption concept of saving where they can. The production of color vision does not require the purchase of a new system in the eye. Retinal and visual protein are still used, but a small change is required.

To be more precise, it is "distortion". Although visual proteins do not have the ability to receive photons, different types of visual proteins can use different structures to distort retinal, allowing retinal to receive photons of different wavelengths. Through the arrangement and combination of visual proteins and retinal, the world in the eyes of organisms suddenly appears in many colors. Therefore, even if we only have half an eye, we can still perceive light and even color. Perhaps our vision is not as exquisite as usual, just a colorful mosaic, but half an eye can still help us escape from the darkness.

Now we can answer the question: what kind of world would we see if we had a hundred eyes?

First, we will have a scallop-like 250° field of vision (360° if you like); second, just as the optic nerves of scallops converge on the outside of a parieto-visceral ganglion (PVG), our brains will also have a place dedicated to processing visual information from more than a hundred eyes and processing them into a perfectly clear picture; finally, the quality of this image will depend on the type of each of our eyes. If you are not satisfied with our three-color vision based on red, blue and green (corresponding to three visual protein complexes that sense different light), then the fourth color vision gene discovered in organisms 400 million years ago, ultraviolet vision, will bring you a more magical visual experience...

Given the high similarity of rhodopsin found in vertebrates and invertebrates, and the fact that you, me, scallops, cats, and even cockroaches all use this system to see things, researchers reasonably suspect that we inherited this photoreceptor cell containing rhodopsin from the same ancestor, and then gradually evolved into completely different species during the long evolutionary process.

However, there are still some significant differences between rhodopsin in vertebrates and invertebrates, which make the hypothesis of common ancestor shake like a tumbler from time to time.

The theory that solidifies this set of conjectures comes from an ancient lucky guy, the sandworm. This tiny creature has lived from the Cambrian period to today with almost no change in its morphology. It is a living fossil of ancient bilaterally symmetrical animals. Researchers have always known that the rhodopsin in their eyes is closer to that of invertebrates. Until 2004, researchers found another type of photoreceptor cell in their brains that functions as a biological clock. The rhodopsin in this cell is surprisingly similar to that of vertebrates. Later, researchers found retinal ganglion cells in human eyes. This cell functions as a biological clock in humans, but is very similar to the photoreceptor cells of invertebrates.

These two discoveries, which echo each other, slowly reveal the true face of the evolutionary path that has been shrouded in fog. The eyes of all living things on Earth today do indeed come from the same ancestor.

During the long darkness when day and night were meaningless, as the first creature to possess rhodopsin, it "saw" the light and led all its descendants out of the darkness.

After a period of time, the photoreceptor cells containing rhodopsin differentiated into two types by chance. One continued to perform visual functions, while the other found a new job of controlling the biological clock by virtue of its ability to sense light. There is not much difference between these two types of cells in essence, it is just that invertebrates and vertebrates happened to choose different cells. Then the structure of the eye began to become more and more complex from a piece of skin covered with photoreceptor cells. Organisms have realized the wonders of the eyes and taken materials from all over the body to make the functions of the eyes more and more abundant. Some of them choose to increase the quantity, and some choose to improve the quality. All kinds of strange eyes have appeared...