In 2017, a news story about goats in India refreshed people's minds.
This is a strange-looking goat, with only one large eye on its head, although the eye is large, the vision is very poor, the ear is also incomplete, only a complete ear, no complete nose, no eyelashes and eyelids, the chin is also small, and the teeth are not complete, scientists believe that it suffers from cyclopteria.
As soon as the news came out, many netizens who ate melons said that this is a spectacle in the world.
Why is it that one-eyed is considered a deformity, but having two eyes like humans is considered normal?
An individual with two eyes, from the day a person is born, is considered normal, because all along, people have been like this.
The rationality of the eyes should start from the symmetry of human development.
Everyone knows it, but a common denominator in all chordate phylum is symmetry on both sides, which means that one side of the body is a mirror image of the other side (not absolutely symmetrical).
That's why we have even numbers of eyes, teeth, nostrils, kidneys, nipples, arms, legs, fingers, toes, lobes, ribs, etc.; and we only have one nose, and that's because it's on the axis of symmetry, which is actually two noses squeezed together.
Many scientists believe that a large part of the reason why living things are symmetrical is for balance, especially animals that need to move, and plants, because they are rooted in the ground, do not need to move, so there is no symmetry.
Therefore, the body is symmetrical by default, as was the case throughout the evolution of multicellular animals. In this case, only one eye will be difficult to develop.
So why don't we have an eye in the middle of the brain, like the one-eyed sheep at the beginning?
Obviously, two eyes can give us more three-dimensional vision, while one eye can't.
One-eyed animals are not non-existent, a copepod creature, only the size of a grain of rice, this creature has only 1 eye, and it is located on the top of the head, but such eyes can only be used to sense light.
While the eye function of most animals is so complex that one eye cannot do it.
Most animals have control over half of the body's vision in each eye, and those two eyes gain double vision.
In humans, for example, when both eyes are facing forward, the visual fields overlap and the eye sockets can rotate over a wide field of view, which makes 3D stereoscopic vision possible. Owls also have frontal eyes, which have stereoscopic vision that facilitate hunting.
If there is only one eye, there will be a large blind spot, resulting in the field of view can not overlap, can not get a wider field of view.
In general, herbivores have a wide field of view, they can see the arrival of predators, two eyes, allowing them to focus more on their prey, and use three-dimensional depth perception to correctly estimate the time and depth of the attack.
Why don't we grow 4 eyes, or more?
Vision is so precious to humans, one more pair of eyes seems to be very useful, if the back of the head grows two more, is it really 360 degrees without dead angles, and the pair of eyes also meet the principle of symmetry, so why are 4-eyed creatures or 8-eyed creatures so rare?
In fact, multi-eyed creatures do exist around us, such as some spiders with 8 eyes, while some jellyfish have 24 eyes.
But most organisms still follow the principle of two eyes, due to the limitations of embryonic development and evolution.
At least 500 million years ago, almost all vertebrates had two eyes, and it was difficult to evolve more eyes over a long period of time.
First of all, the eye is a very complex and special structure, so the function of the two eyes from a genetic sequence mutation to four, can not be completed through gradual evolution, on the contrary, the appearance of excess ribs, spine, fingers and toes is easier.
Second, if there are really extra eyes in mammals, it is also because the entire face or part of the tissue is repetitive, the survival rate is very low, and the one-eyed sheep only survive for 8 days.
In addition, to achieve all the functions of the eye, the nerves on the eye are extremely complex, if you really add additional eyes, then you need to complete a lot of additional nerve circuit laying, which will cause a certain waste of energy.
In other words, when the embryo develops, we need more energy to support the development of additional eyes, given the efficiency of evolution, two eyes have been able to meet the daily needs of human beings, with more energy to develop more eyes, but it is superfluous, a waste.