How did the eye evolve?

Question

To me it seems that the organ of sight in its incipient form must have been inseparable from the neural network of the organism in which it developed.

The first organism to develop light-sensitive cells would have no use for them whatsoever if it didn't also have a region of its brain that was conditioned to interpret the data received. That implies that at least two very rare and complementary mutations must have randomly coincided in the same organism for its new apparatus to be of any benefit to its survival.

This seems very unlikely to me, so would you say sight in a very primitive form was somehow originally inseparable from that which processed the data, and from that point gradually evolved into eyeballs and the visual cortex etc?

Answer 1

You just totally forget the exploratory processes of neurons. How do you think the eye wires itself up now? How did you think " conditioned " regions of the brain come about?

It may seem unlikely to you, but the experimental data rather outlines your ignorance in sharp relief. Educate yourself a bit deeper into the evolutionary developmental process next time and save yourself the embarrassment.

Answer 2

You're right, it probably wasn't two separate mutations, but one structure that diverged into two and developed over countless generations to it's current form.

If you look at a photoreceptor cell, it's actually a type of neuron. It has photoreceptive pigments that, when exposed to light, set off a cascade that results in the depolarization of the photoreceptor and the release of neurotransmitters. The signal is then relayed to the brain, where it is processed. These photoreactive pigments likely evolved in single-celled organisms that clustered in patches on the membrane and set off signal cascades within the cell, resulting in very simple responses to light.

Very primitive organisms, such as planaria, have eyespots that sense light. This allows them to go towards or away from light (i.e. towards the surface or away from the shadow of a predator). They have no brain to speak of, only a pair of ganglia and some nerves. The nerves allow them to move in a coordinated manner, but it is still very primitive and mostly input/response oriented, lacking the processing abilities of higher organisms.

Over time, ganglia probably became more complex, allowing for more complex arrays of photoreceptors. Once a high level of complexity had been achieved, image processing became possible. From there, it would have been advantageous to enhance these images, and thus the eye evolved around the retina. Most existing animals have either primitive or complex eyes, but there are exceptions. Planaria have very simple photoreceptors. The nautilus has a much more complex eye, but lacks a cornea or lens, operating on a principle similar to a "pinhole camera" (this is direct evidence that "irreducible complexity" in the sense of the eye is a fallacy).

You're right, that the retina probably evolved as an extension of the nervous system (even now, it's still considered part of the nervous system), and the eyes evolved as later enhancements to it.

Answer 3

It's likely that eyeballs, in their complexity, developed gradually from very simple light sensitive cells.

From a simple light detector (that could have had the simple advantage of knowing when the sun was out) to an concave group of light detectors to determine the direction of light to increasingly more focused interpretation of reflected light.

If you can't see the survival benefit of knowing what's around you then it's no wonder you don't think it's plausible.

Additionally, skin cells can work similarly to the eyes, UV light is interpretted by pigment cells as a threat and additional pigment is produced to help protect tissues below the skin. This is an example of a simply precursor to actual sight.

Answer 4

The whole process of light being converted to image can be fundamentally broken down into the actions of a single cell. On the plasma membrane of a cell are numerous numbers of receptors. On some cells, there are photoreceptors. These receptors are specifically designed to react to a certain wavelength or intensity of light. They trigger the creation of certain signals within the cell, which then move on to cause a certain reaction or function. For example a cell might be exposed to a bright light and thus might trigger a response involving the creation of certain proteins to protect the cell from the energy it is being bombarded with (because light is a form of energy).

This is the origin of the eye, which is just a super complex photo receptor. Now lets move forward into evolution, where the previous cell is now living in a complex of other cells. Cells working together allow more productivity and specialization. This particular cell has evolved to grow on the outer layer of the complex, and thus when it is hit by light it triggers a response not only to itself but to other cells as well.

Jumping further in evolution, there is even more specialization and growth. Now this cell does not just release the signal to be diffused, but is connected to a cell which is a ancient version of neurons, whose key function is the ability to rapidly transport signals across larger distances than simple diffusion could do. This neuron connects to other cells or neurons, and they in turn release the response to different areas.

As you can see, even farther down in evolution, these pre eye and pre brain cells will specialize even further, creating an even more advanced way of responding to and detecting light wavelengths and intensities. This is in essence the eye. What you are considering is known as the theory of irreducible complexity, in which it is impossible for something like an eye to simply evolve. It has been largely disproven.

Answer 5

The first organism to develop a light sensitive area was a prokaryote, which used it to move away from sunlight and into mud where more food could be found.

It's likely some later prokaryotes which were photosynthetic used their light sensitive areas to move toward greater illumination. *shrug*

No brain necessary.

Answer 6

Well first I'm not a fan of evolution. Nothing evolved everything that exist was created by God. So God formed the eye and made it pretty well.

Answer 7

I will give you a star, although I don't know the answer.

Answer 8

from mono to stereo

Answer 9

"I ve got a bridge for sale"

Answer 10

It didn't it was Created. Intelligent design is the only thing that can explain such complexities.