If you understand the structure of the human eye, you will be amazed by the subtlety of its design.
There is a wonderful similarity between the human eye and the camera:
The sclera is equivalent to the camera body; the pupil is like an aperture, and the size of the aperture is controlled by the iris;
The cornea and lens resemble a set of lenses; the retina is equivalent to a camera negative.
And the eye can adjust the focal length just like the camera, so that the image is clearly projected on the photosensitive cells.
The retinal in the photoreceptor cell then converts the received light signal into an electrical signal, and we can see things in the outside world.
(Retinal)
The structure of the eye is so complex and subtle, like a clock, that it seems to be the result of a careful design by a watchmaker, that many people think that the eye cannot be a product of random evolution, because the complex structure of the eye cannot be simplified casually, otherwise there will be dysfunction.
If you pull a clockwork out of a clock, it will cause the entire clock function to collapse, and the eyes will be the same.
So, how exactly did the complex and subtle eye evolve?
The evolutionary process of the eye once plagued Darwin.
He knew that if he could not use evolution to explain that the complex structure of the eye was the result of evolution, then creationists would use this as an example to attack evolution, saying that the eye was the result of God's creation.
In the view of creationists, the more elaborate the structure of the eye, the more likely it is to prove the existence of God. So, Darwin wrote to his friend: "Every time I think of the structure of the eye, I shudder.
At that time, people did not know much about the evolution of the eye, and similar confusion was certainly understandable. The available evidence suggests that the evolution of the eye is fully consistent with the general principles of natural selection, which is indeed a progressive process from simple to complex.
< h1 class= "pgc-h-arrow-right" > can see without eyes</h1>
Long before the eye appeared, life could perceive light, such as a salt-loving bacteria in seawater, which contains two photosensitive pigments in its body, which can feel blue light and orange light.
Due to the different penetration capabilities of different rays in seawater, blue light mainly appears in shallow sea areas, and orange light can be shot into the deep sea area, feeling different rays is equivalent to measuring different sea depths.
In this sense, salt-loving bacteria already have color vision. Although it has no eyes, it is able to perceive light, which we can call eyeless vision.
Hydra also has eyeless vision. In general, the hydra camp is fixed in life and should not need eyes, after all, it does not need to swim around to chase its prey. Still, the hydra's tentacles are sensitive to certain rays of light and can detect changes in nearby light in time, helping to trap floating worms.
The visual abilities of eyeworms are similar to those of hydra. From the outside, the eye worm seems to have a red eye, which is not actually an eye, but an eye spot.
Eye worms originally had only one cell, and of course it was impossible to evolve the eye. However, the role of the eye point is very similar to the role of the eye, its function is not to perceive light, but to block light.
True photoreceptor pigment at the root of its flagella. When the eyeworm moves, the eye spots inside the cell also move with it, constantly blocking foreign light.
Eyeworms can judge the direction of light based on the direction of the shadow, and thus decide whether to swim toward the light or avoid it. Eyeworms respond to light or avoid light according to such a simple visual system.
< h1 class = "pgc-h-arrow-right" > the evolution of the eye contributed to the Cambrian explosion of life</h1>
Simple photosensitive systems have been continuously evolved, molecular designs have become more and more exquisite, photosensitive structures have become more and more complex, and photosensitive capabilities have naturally become more and more powerful.
By half a billion years ago, the real eye suddenly appeared in the fossils, on the eve of the Cambrian explosion of life. Therefore, some scholars believe that it is the evolutionary development of the eye that promoted the explosion of life in the Cambrian Period, before the world was dark, and the world after that was colorful.
Driven by the eyes, Cambrian animals continued to evolve agile movements, and then with unprecedented athletic ability, they launched a predatory and anti-predation drama in the sea. For the sake of war, they were also draped in heavy armor – so they were the forerunners of crustaceans.
The simplest eye is just a flat surface with some light-sensitive cells evenly distributed on it. For example, the blind shrimp living near the crater of the deep sea have a bare retina on their backs, that is, a layer of photosensitive cells without a protective membrane. In primitive systems of life, this simple design was ubiquitous and could appear anywhere in the body. Earthworms are like that.
In general, it's hard to see where their eyes are, because their eyes can grow anywhere, and there's no obvious difference from other parts, just an extra layer of photosensitive cells.
If you'd like to call that eye, the folds around the jellyfish are also covered with "eyes," while starfish's "eyes" grow at the tip of the tentacles.
In this primitive visual system, because the photosensitive cells are tiled on the surface of the body, it is impossible to identify the direction in which the light is coming from, and only the intensity of the light can be perceived.
For earthworms, they only need to determine exactly whether they are exposed to sunlight, hiding under leaves, or burrowing in the dirt, and they are satisfied. Excess light information is a burden to them, and they don't have extra nerve cells to process these complex contents.
<h1 class= "pgc-h-arrow-right" > from bowl-shaped eyes to bottle-shaped eyes</h1>
Relatively speaking, snails are active on the ground all year round, and their eyes are more complex than earthworms, and they are more sensitive to the intensity of light.
The logic is simple, the snail must understand the strength of the light, a snail that is always exposed to the sun, will soon become a dead snail. In addition to distinguishing the intensity of the light, they also need to distinguish the direction in which the light is coming, so as to avoid the pursuit of the sun as quickly as possible.
To achieve this, the snail's eyes must be superior to the earthworm's, but not much higher, they just sink the light-sensitive cell layer down a little, like a shallow bowl, and the photosensitive cells are distributed at the bottom of the bowl.
In this way, light from different directions will hit different parts of the "bowl". For example, the light coming from the right side will only shine on the left inner wall of the "bowl".
As soon as the light-sensitive cells of the left inner wall capture the light stimulus, the snail knows that the light source is coming from the right side. In this way, the snail completes the basic positioning of the light source, so that the evasion behavior can be quickly made.
Of course, it doesn't need to be too fast, after all, the speed of light movement is limited, so the snail doesn't have to dodge too fast, it just needs to hide under the leaves before it is sunburned.
In addition, snails also do not need to collect too much optical information. It doesn't eat pollen, it doesn't have to discern the color of the flowers; it can't catch up with other insects, and its vision doesn't need to be sharp.
They only need a small bowl-shaped eye, although they can not be clearly imaged, but they can effectively avoid the sun, not to be sunburned into dried meat, and become the winner of natural selection.
More advanced than bowl eyes are bottle eyes, which are more pronounced downward depressions to the point where a bottle structure is formed. The light-sensitive cells inside the bottle are denser, leaving only a small bottle mouth for light to enter, and then a simple image is formed at the bottom of the bottle through the principle of small hole imaging, showing more external information.
Pearl oysters have such eyes, but their bottle mouth is open, there is no cap on it, that is, there is no lens, so it is difficult to get a clear image.
In order to solve this problem, the trilobite improved the design of the eye, adding a lid to the bottle mouth, which is not an ordinary lid, but a transparent calcite crystal, equivalent to the original lens, which mainly plays the role of a lens, which can focus the light and make the image at the bottom of the bottle clearer, so that the visual ability has been improved hundreds of times. The lens of the human eye has been greatly improved, which is rich in various lens proteins, and the imaging effect is of course unmatched by calcite crystals.
Although the structure of the eye is becoming more and more delicate, it only adds more and more components, such as iris and muscles, which can effectively adjust the amount of light ingested and ensure the imaging quality of the eye, but its basic optical imaging principle is not fundamentally different from that of trilobites.
It can be seen that the eye seems to be exquisite, but it is not mysterious, it is only the result of the cumulative evolution of a long river of time, and it is the most effective mechanism for responding to light and color.
Computer simulations show that complex eye structures can evolve in a very short period of time, from simple eyepoints to complex camera-like eyes, which only take about 360,000 years.
Compared to long geological years, 360,000 years is short. Or rather, there is plenty of time to test the structure of the eye in order to find the most efficient visual design. After all, about half a billion years have passed since the Cambrian explosion of life. For evolution, time is the most precious asset.
< h1 class= "pgc-h-arrow-right" > the structure of the eye may have evolved only once</h1>
Although life has plenty of time to design and perfect the eye, it is speculated that the structure of the eye may have evolved only once, which is the single origin theory of eye evolution. Corresponding to this is the theory of multiple origins, that is, the belief that the eye has originated independently several times, thus forming several completely different eye types on Earth.
To test which theory is more correct, the researchers compared anatomical features such as the eye structure, photoreceptor types, the embryogenesis process of the eye, and the location of the photoreceptor nerves of different organisms, and the result of the comprehensive analysis was that the eye had at least forty ways of origin, or had evolved independently forty times.
If this is the case, then the theory of origin is correct, but it is difficult to make sense in evolutionary logic, because there is bound to be fierce competition between each other in such different eye structures, and in the end there must be one of the most efficient eye structures to prevail, that is, only one evolutionary model of the eye can be favored by natural selection. This is the main idea of single origin, and they do not believe that the eye has such a complex evolutionary source.
The problem is that the structure of the eye is difficult to leave fossils, and researchers can only find another way to find clues from genes, and they really find it. This gene is the Pax6 gene, Chinese meaning "sixth paired homologous box gene", which controls many traits during biological development, one of which is responsible for the formation of the eye. For the sake of simplicity, we might as well call it the eye gene.
Eye genes are quite conserved. When we say that a gene is conserved, we mean that it rarely mutates so much that it maintains a similar sequence and similar function in different species.
Eye genes are just like this, which can cross species and induce the formation of eyes. The researchers first obtained the eye gene in mice, and then cloned this gene into the fruit fly, and the result was to induce the fruit fly to grow eyes in many parts, which proved that the eye gene can be used in different organisms.
What does this mean? This means that the eye may have evolved only once, everyone has adopted the same design scheme, and the superficial differences cannot erase the essence of the gene. Whether it is the compound eye of a fly or the single eye of an octopus, it is only a local adjustment to cope with the visual needs of different environments.
For hundreds of millions of years, the basic sequence of eye genes has not changed drastically, which can be seen as important evidence to support the single-origin theory.
Another important piece of evidence is that the photoreceptor system of all eyes is centered on opsins, and although different animals have different opsins, they all come from the same ancestor. In the face of two heavyweight evidences, eye genes and opsins, we are certainly more inclined to believe in the single-origin theory.
Now that the single-origin theory holds, we can conclude that no matter how complex the eye, it originates from the simplest eyepoint. Now researchers have constructed a roadmap for the evolution of the eye from simple to complex. If you want, it is entirely possible to think of the trilobite's eyes as half an eye, or even 0.3 eyes, but such eyes are still indispensable to the trilobites. That said, a minimalist eye can still give animals a clear survival advantage.
<h1 class= "pgc-h-arrow-right" > the simplification and degradation of the eye</h1>
The eye can not only evolve from simplicity to complexity, but can also appear simplified or even degenerate, which is fully consistent with the general principles of evolution.
Complication is not the ultimate direction of evolution, but only a side effect. Many animals that live in desert rivers eventually lose their eyes, because in underground rivers, the eyes are not stimulated by light, thus losing their place.
Maintaining vision requires a lot of energy, so losing your eyes when you don't have to, it is equivalent to getting rid of the useless burden, and it will become a survival advantage. Underground cave animals such as blind eels are basically victims of this mechanism, or victors.
Curiously, though, it was only a superficial change, not a genetic change, and many blind animals still retained the eye gene, and there was no problem in the sequence, but the dna methylation blocked the gene activity, so it would not express the external eye.
The above is from "The Colors of Life: Why We Don't Have Green Hair"
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The Colors of Life: Why We Don't Have Green Hair
Author: Shi Jun
Publisher: Chongqing Publishing House
Publication date: January 2021
Price: 45 yuan
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The charm of this world is everywhere, and we need to retain more exploration of this world. Every exploration is an exploration of humans and even the animal kingdom.
The author's logical and clear scientific knowledge in the book not only broadens our scientific horizons, but also makes us aware of the difficulties behind science. The evolutionary process and selection mechanism that began to enter this colorful world step by step from the discovery of light waves is the result of the efforts of countless scientists.
If you also like creatures, don't miss this encyclopedia of life and color.