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The evolution of the eye
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True or false? All of the human eye's components were formed at the same time during one single mutation.
This is a human eye. When light reaches the eye, the light is refracted by the cornea, and the lens. When the light rays hit the rear part, a clear image is projected on the retina. Optic nerves send the image to the brain that receives and interprets the image. The eye is an advanced organ.
Remove any single one of the eye’s components, and it ceases to function. We become blind. All of the eye’s components must be there for it to function. How does that fit with evolution? If the eye needs all its parts in order to work, how can it have arisen through small, gradual changes and adjustments?
Can all the eye’s components have come to be at the same time, in one single mutation? And what advantage can an animal get from just a cornea? Or just an optic nerve? Nothing at all. Can the eye really have been created by random mutations?
This is one of our ancestors, in the ocean, several hundred million years ago. A single celled organism. A mutation has changed the cell slightly. It has a tiny special dot on its surface. The dot is a protein that reacts to light.
Light stimulates the cell, so it starts to move. It moves just a little, towards the light, to a place where it can get more nutrition. More nutrition makes it easier for the organism to survive. And whatever increases the chances of survival will be spread through natural selection. Cells with such a light sensitive dot will become more common, by the mechanisms of evolution.
This animal looks a bit like a worm. By mutations and natural selection, the light sensitive dot has developed, into a larger spot that is sensitive to light. The spot has the shape of a cup. The cup shape helps the animal to sense the angle of the light. The deeper the cup, the more accurately it can sense the light’s angle.
A really deep cup, with a small opening at the top, helps the animal to distinguish shapes. This animal can do that. It’s a nautilus. The image it sees is simple and blurry but a nautilus can get a feeling for the shape of a predator and seek shelter. The nautilus has a simple eye, which has evolved step by step, by mutations.
Each step of the development has been valuable for the survival of the individual. And characteristics with survival value are favoured in natural selection. The eye keeps developing, generation after generation. Over time more mutations occur. Some individual happens to get transparent proteins in front of the opening.
They form a cornea that protects the eye. Mutations in some other individuals form transparent proteins behind the opening. They form a lens. The lens refracts the light, and a sharp image reaches the rear part of the eye. And someone else happens to get muscles able to pull and stretch the lens.
The muscles can change the shape of the lens, so that the eye can focus and have sharp vision over both short and long distances. All this has been favoured by natural selection. Like with these monkeys. They can see food nearby and scout dangers far away. Every mutation happens at random.
If the change helps the individual to survive the traits will persist and be spread. This way, the eye has evolved, step by step, from a light sensitive dot on a single cell, to a complex eye. Let’s go back to that worm-like animal. It had no eyes. During evolution it became different species that developed differently.
Eventually some developed into cats, others into octopuses. Animals in different environments with bodies that look very different from each other. Except for the eyes. The eyes are built the same way. Round organs, with a retina, a lens, and a cornea.
Their common ancestor did not have eyes. But the cat and the octopus have eyes, which have developed in different environments. They have developed independently. Yet they look quite similar. So evolution is not only a matter of chance.
Mutations, changes in the DNA and traits, occur randomly. But the traits that succeed during the process of natural selection are not random. Therefore, animals which are very different can have very similar eyes. Natural selection makes sure of that.