Eye evolution

The eye is one of the best inventions of blind nature.

US, WASHINGTON (ORDO NEWS) — When asked how animals use their eyes, most of us will answer: just like humans. But this is not true. Absolutely. So says Dan-Eric Nielson. In the laboratory of Lund University (Sweden), while studying the organs of vision of other creatures, as they say, he looks into both: a pair of gray-blue eyes of a scientist against 24 brown eyes of cubomedusa.

The visual organs of the jellyfish are symmetrically located in four ropalia – shortened and thickened tentacles on the edge of her umbrella. “When I first saw all this, I could not believe my eyes,” recalls Nilson. Four of the six organs of vision in each ropalia are simple photosensitive spots, but the remaining two are surprisingly complicated. These are chamber eyes in which there is a thin transparent integumentary layer (cornea), a light-focusing lens (lens) and a vitreous body with an underlying layer of photosensitive cells (retina). So jellyfish, as a person, is able to see the image, but not very sharp.

Nielsen collects data on the diversity of the structure and functions of the visual organs in animals. Let’s say a person uses his eyes to study the world around him. And why do we need eyes … cube jellyfish? After all, this is one of the simplest creatures on the planet – a pulsating lump of mucus, dragging behind itself several tows with hundreds of stinging stinging cells. She doesn’t even have a brain – the nerve ring located in the bell of the jellyfish plays its role. What data does such a creature collect?

In 2007, a team of researchers led by Nielson found that the Cubedusa Tripedalia cystophora uses downward-looking chamber eyes to navigate underwater mangroves where it spends most of its life. It took another four years to figure out why the jellyfish had the same eyes looking up. Here a small mineral weight, rolling in the lower part of ropalia, helped – statolite. Thanks to the statolith, which always rolls down, these eyes always – even when the jellyfish floats “upside down” – look up. When a shadow falls on them, the jellyfish concludes that it is under the cover of the mangrove, where it can get food – tiny crustaceans. If they see only bright light, then it was carried away to the open sea, where there is nothing. So an animal devoid of a central nervous system

The eyes of cubomedusa are only one of the various visual organs. Someone is able to see only a black and white image, while someone perceives all the colors of the rainbow and even spectra invisible to humans. Some are not able to figure out where the light source is, while others are able to track prey from a distance of several kilometers.

The smallest eyes crown the head of the rider Gonatocerus ashmeadi from the hymenopteran insect order: they are slightly larger than the tiny amoeba. And the largest, with a diameter of a plate, among the inhabitants of the sea depths are giant squids. Their camera-type visual apparatus is similar to the human eye and is arranged like a camera: the only lens lens focuses light on the retina, which consists of photoreceptors – photosensitive cells that absorb photon energy and convert it into an electrical impulse that is transmitted to the brain via the optic nerve.

The flies have facet eyes – they consist of thousands of independent units, facets (each has its own lens and photoreceptors). In humans, in flies and squids, eyes are arranged in pairs on the head, while in scallops, for example, the eyes dot the skin folds surrounding the body and mantle in several rows; starfish – are at the tips of the rays. There are eyes with bifocal lenses, and with a reflective pigment layer (glow in the dark, like a cat or a crocodile), and even eyes that can look up, down and to the side at the same time.

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Such diversity is perplexing. All organs of vision react to light, the distribution of which is described by fairly simple laws of optics. However, the light signal itself can be used in very different ways: to determine the time of day, the depth of water, the appearance in the field of view of the contour of a predator or possible partner. Cubomedusa, focusing on the light, looks for a safe place for feeding, and we use our eyes to examine the area, perceive the changes in the facial expression of the interlocutor and read these lines.

To understand how the evolution of the eyes took place, it is not enough for scientists to simply study their structure. They need to go the way of Dan-Eric Nielson: to find out why animals use sight. About 540 million years ago, the distant ancestors of most animals appeared almost simultaneously in the ocean, and a rapid speciation process called the Cambrian explosion began. Some of the Cambrian animals have been preserved as fossils. By examining these fossils using microscopy methods, scientists can find out the internal structure of long-extinct organisms, including their organs of vision. And even look at the world through their eyes. “This is amazing! – Rejoices Brigitte Schönemann from the University of Cologne. “We can even calculate the number of photons once captured by their retina!”

However, all the fossil eyes that scientists were able to investigate were already quite complex, since very simply arranged organs of vision have practically no chance to petrify, due to their small size and the absence of hard shells. The study of fossils does not allow us to understand how blind animals gained the opportunity to see. This mystery haunted Charles Darwin. “The assumption that the eye, with all its unsurpassed adaptations … was formed through natural selection, frankly, seems to be extremely absurd,” the famous Origin of Species says. Creationists prefer to break off a quote in mid-sentence, giving the impression that the great scientist doubted his own conclusions. However, in the next sentence, Darwin resolves the dilemma: “But reason tells me that, if the existence of innumerable gradations from the ideal and complex eyes to the eyes of simple and imperfect, each of which was useful to its owner … could be shown, then the difficulties arising from the idea that the ideal and complex eye could be formed by natural selection, although insurmountable for our imagination, they could hardly be considered significant. ” And the gradations Darwin reflected on actually exist.

Living beings exhibit a wide range of diversity, from simple photosensitive spots on the body of an earthworm to the perfect eyes of birds of prey with sharp eyesight. Nielson was able to create a model for the development of the eye and show how a simple organ in the form of a tiny flat disk from pigmented photosensitive cells was transformed in a relatively short period of time. In each subsequent generation, he became a little thicker. Then gradually began to bend, turning into a glass. Then he acquired a coarse lens that was “polished” step by step. Let us try to assume that the organ of vision improved by 5 thousandths of a percent in each generation. Then, according to model calculations, it took only 364 thousand years to turn a photosensitive spot into a perfect camera-type eye – by evolutionary standards,

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You should not consider simple visual organs as just one of the intermediate stages of evolution on the way to complex devices: all of them regularly served their owners. In starfish, the eyes located at the tips of the rays do not distinguish between color and small details and cannot notice fast-moving objects. With such vision, a flying eagle would crash into the first tree it came across. But starfish do not track a hare running down the field from the height of a skyscraper, they just need nothing: figure out where the coral reef is – a huge unshakable underwater structure – in order to leisurely get to the house. For this, the stars have enough of those eyes that are, and they do not need anything better. For a starfish, an eagle eye is a waste of life resources.

“Eyes have not evolved from bad to perfect,” explains Nielson. “At first, they met some of the simplest visual needs in order to begin to perform numerous and incredibly complex functions as they develop.”

In his model, Nielsen identified four stages of eye evolution. The characteristic features of each of the stages, he chose not the physical parameters of the visual organs, but those new opportunities that appeared among their owners. At the first stage, vision was used to measure the intensity of the incident light, determine the time of day, or estimate the depth of the water at which the animal is located. Here one photoreceptor is enough. Hydra, a small sedentary relative of the jellyfish, has no eyes at all, but there are photoreceptors on the tentacles. As Tod Oakley and David Plachetsky of the University of California (Santa Barbara) showed, these photoreceptors are connected with poisonous stinging cells, which, perhaps, helps the hydra to react to the shadows of potential victims passing by (and shoot poisonous harpoons at them) or wait for the night to come, when the prey does not see the hydra, and itself floats in the tentacles. In the second stage, animals get the ability to determine where the light comes from – their photoreceptors acquire a shielding lining (a layer of pigment cells) that blocks part of the light rays. Receptors allow their master to see a single-pixel picture of the world, but this is enough to choose the direction of movement to the light source or, conversely, into the shadow of the shelter. So the larvae of many marine animals are oriented. At the third stage, photoreceptors with a pigment lining are grouped into “eyes”, each of which looks in its own direction. Owners of such eyes are able to distinguish information coming from different directions, and reduce it into a single “picture” – quite blurry, but still allowing you to make an idea of ​​the world around us. This is a turning point when animals do not just pick up light signals, but begin to perceive visual images, the moment of the appearance of real eyes. Creatures with this vision can find refuge (like a starfish) or avoid a collision with an obstacle (like cubo-jellyfish). But the real evolution of the eyes begins in the fourth stage. With the advent of a crystalline lens that can focus light rays, vision gains sharpness and sharpness. “At this stage, the list of visual functions grows indefinitely,” Nilson concludes. able to focus light rays, vision gains sharpness and sharpness. “At this stage, the list of visual functions grows indefinitely,” Nilson concludes. able to focus light rays, vision gains sharpness and sharpness. “At this stage, the list of visual functions grows indefinitely,” Nilson concludes.

Such a variety of new opportunities for perceiving the environment could well be the impetus for violent speciation, that is, for the Cambrian explosion. Suddenly, a new element appears in the “predator-prey” system – and, instead of sniffing, tasting or looking for possible prey by touch, the predators gained the ability to track it from a distance. The “arms race” began, as a result of which the animals sharply increased in size, became more mobile and acquired protective shells and spikes.

The development of the organs of vision, meanwhile, continued. All the main types of eyes found today have appeared in the Cambrian period, albeit in a primitive form. Of course, since then they have acquired many new features. Let’s say, the eyes of male mayfly insects look as if they had “glued” another small, faceted eye, a huge one, the main task of which is to constantly look into the sky in search of females. Four-eyed fish, it is not difficult to guess, each eye is divided into two parts, one of which is above the water surface and monitors what is happening in the sky, and the other looks down, looking for prey and tracking predators. The human eye is perfectly adapted to perform various tasks, it quickly perceives information, perfectly captures contrasts.

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Thus, the evolution of the visual organ does not at all contradict the theory of natural selection, but, on the contrary, serves as its excellent confirmation. “There is greatness in this view …” Darwin wrote on the last page of the main book of his life. Man in all its splendor could not be seen by man if he did not possess the chamber eyes of the fourth evolutionary stage.

The Nielsen model allows you to take a fresh look at a long-standing debate: did the eye appear once or repeatedly? According to the hypothesis of the famous German evolutionary biologist Ernst Mayr, the eyes independently developed from 40 to 65 times – how else to explain the variety of naturally occurring and completely different forms? Walter Goering, a Swiss specialist in developmental biology, on the contrary, believed that the eyes appeared only once, since the formation of the eye in almost all animals is controlled by the same Pax6 gene. And both of them turned out to be right.

At the third stage, the development of the eyes in different animals occurs independently, based on the visual organs of the second stage. For example, cubomedusa acquired vision independently of mollusks, vertebrates and arthropods. But the eyes of each of these creatures were formed on the basis of primitive photoreceptor cells. This is confirmed by the fact that all eyes are made of the same basic “bricks”. To see, any body needs proteins of opsin. Opsins are associated with chromophores – molecules that absorb the energy of photons and as a result change shape, which, in turn, affects the spatial structure of the opsin. These transformations trigger a series of chemical reactions that produce an electrical signal at the output.

There are thousands of different opsins in nature, but they are all alike. A few years ago, Megan Porter from the University of Hawaii in Manoa compared the nucleotide sequences of 900 genes encoding opsins in a wide variety of animal species, and concluded that they all had one precursor protein. And this “ancestor” did not arise from scratch. Evolution created the first opsins from proteins, the main task of which was to measure time, and not to respond to light. These source proteins are associated with the signaling hormone melatonin, which controls the circadian rhythms of many organisms. Melatonin is destroyed in the light, which serves as a signal to awaken the body with the first rays of the sun. However, a one-time mechanism requires constant synthesis of the hormone. Chromophores associated with opsins, when light is absorbed, they only change shape and can easily return to their original state. In other words, the evolutionary changes in proteins previously used in conjunction with melatonin have turned these proteins into reusable light sensors. So the opsins appeared, and this acquisition was so successful that nature did not create a worthy alternative.

However, other components of the eye developed differently. For example, most lens lenses are formed from crystalline proteins, which improve vision by focusing the transmitted light on photoreceptors. But the genealogy of crystallins is not as straightforward as that of opsins. Crystallins are just the general name for a group of proteins, but in humans, squid and flies these proteins are different. During the evolution, many organisms acquired their own crystallins independently of each other, selecting precursor proteins from those that were sometimes not at all associated with vision. Some participated in the processing of alcohol, while others were produced by the body in stressful situations. But all of them were stable and easily acquired the necessary spatial structure, and most importantly, they could refract the rays of light – just what you need to create a lens.

The most unusual lenses do not contain crystallins at all. It is these eyes that are found in tunics. Chitons are mollusks whose body is decorated with several calcareous protective plates. The plates are dotted with hundreds of tiny eyes with mineral lenses – a living creature has managed to improve eyesight with pieces of rock! Moreover, as lenses wear, chitons grow new ones to replace them.

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Opsins, lenses and other elements of the visual system once again confirm that evolution is mosaic: it combines different complex structures from the same simple fragments and, in fact, acts by trial and error, adapting existing materials to new tasks. At the same time, alas, evolution is blind, and its creations are sometimes far from perfect. Say, Nielson is surprised by the structure of faceted eyes, consisting of many identical elements – facets. Such an arrangement of the visual organ significantly limits the resolution: a fly would need eyes the size of a soccer ball to see as clearly as a person. “Insects and crustaceans managed to achieve fantastic success, despite their faceted eyes, and not at all thanks to them,” Nielson is sure. – It would be much easier for them to live with chamber eyes, like mammals. But nature decreed otherwise. Still, evolution is not smart. ”

Eric Warrant, who works in a nearby laboratory at Lund University, is not so categorical in his assessments: “The advantage of insects is that their eyes are arranged like a camera for slow motion. Therefore, two flies can chase each other in flight at high speed, and their eyes will capture up to 300 “frames” per second. The human eye is capable of catching 50 at best. ” (It seems to us that we very quickly slam the fly with a newspaper, and she sees a clumsy creature slowly and slowly pulls her hand away, and then slowly and slowly reaches for it, and, of course, manages to calculate the trajectory of the blow and hide.) The dragonfly’s viewing angle reaches almost 360 degrees, we are only able to see what is happening in front of us. And the wine-hawk butterfly, Warrant’s favorite subject of study, distinguishes colors even when the stars flicker dimly. “Of course, in some ways our eyes are better, but in most respects, human vision is worse, ”says Warrant. “Nature has failed to create the perfect eye for all occasions.”

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The human eye has its own flaws. For example, the retina with photoreceptors is located behind the “web” of nerve fibers. This is like exposing the camera wires directly in front of its lens. Nerve endings, interwoven into the optic nerve to reach the brain, pass through an opening in the retina. Therefore, we have a blind spot – the area of ​​the retina insensitive to light. True, the brain eliminates some gaps and completes the picture. But not all problems were avoided. The retina can exfoliate, and then a person becomes blind. Detachment would not have been possible if the photoreceptors were soldered to neurons, like octopuses and squids (in these creatures, photoreceptors are in front of the neurons, and they have no blind spots). Here is one more confirmation that the evolutionary path is by no means straight.

The complexity of the structure of the eyes is determined by the needs of their owner: if the needs are limited, there is no need to work on creating a complex visual apparatus. Most birds and reptiles perceive colors using four types of cone photoreceptors, each of which contains its own opsin, configured to perceive a certain part of the color spectrum. However, mammals, presumably, descended from a common ancestor who led a nocturnal lifestyle and lost two types of cones (these photoreceptors are most effective in daylight). To this day, most mammals have remained with only two types of cones, so some colors of the surrounding world are simply inaccessible to them. In the course of evolution, the Old World monkeys managed to restore the opsin sensitive to red, apparently in order to distinguish ripe fruits from unripe ones, and sometimes poisonous greens. Marine mammals evolved differently: they got rid of the “blue” cones as the development of the aquatic environment. Many whale species have retained only stick photoreceptors, suitable for low light levels in the deep ocean.

In nature, there are eyes, the possibilities of which it is even difficult to imagine, due to the limitations of our own organs of vision. Tom Cronin of the University of Maryland in Baltimore County is carefully examining something in a laboratory aquarium. From behind the glass a couple of bulging eyes stared at him. “Loupe-eye” – as Tom affectionately calls his ward – a beautiful creature, painted in all colors of the rainbow. Before us is a mantis shrimp – a marine inhabitant, named for its resemblance to namesake insects with agile grabbing limbs.

The ends of the Lupoglazka’s jaws are crowned with imposing hammers of impressive size, which in a blink of an eye can fall on the victim with a force sufficient to crush its carapace (or, even worse, an aquarium glass). Tom found out that the retina of the Lupoglazka contains 12 different color receptors (we have only 3), and the eyes of the cancer are able to see the ultraviolet part of the spectrum and polarized light. Why does he need all this? Dan-Eric Nielson asks similar questions. One thing is certain: we can understand the evolution of the eyes of animals only when we learn to see the world, as they say, with their eyes.


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