I’ve always wondered how eyes evolved.
You need an optic nerve to transmit the info to the brain, but why would we evolve eyes without that, or an optic nerve without eyes. What purpose would it serve?
Maybe a random mutation led to eyes that were useless till then a nerve evolved like a million years later?
Or they both happened at the same time over a million years or so?
In: Biology
That’s not how evolution works. Evolution is a change in pattern, under reproductive pressure. The current anatomy distinctions, like eye and nerve, are retrospective analysis of the pattern, and don’t have anything to do with the pattern’s emergence.
Vision likely started with a light sensitive cell connected through a nerve into the brain. This is a “day vs night” detector pattern, which might sometimes provide reproductive advantage.
To use an incorrect camera analogy, because it’s ELI5 friendly, this likely changed from one-pixel daytime detection to many-pixel moving object detection to mega-pixel vision.
They evolved alongside each other, but essentially all you need is a nerve if the light detection is simple. Imagine a simple organism that has a single cell cluster that detects light. The output of this cluster is basically a binary “yes” or “no” to the question “is there light touching this cluster?” That would be the proto eye. That is whaf develops into a more complex eye. Insect eyes are kind of like this where instead of a single eye with an optical nerve it’s a bunch of simple photo receptors with a bunch of nerves that send signals to the brain.
I would guess it’s the optic nerve, which would have been a way to sense light through the skin. As time passed I would guess the eye developed as an evolving improvement on the light sensor. I doubt there’s good evidence of the early evolution process because of how soft the tissue is. Once the skull developed recessions for the eye, then we would start seeing it in the fossil records.
You have the nerve for lot of other usages than to see that go to the brain.
Then the question is what is an eye and what is an optic vs other nerves? Single-celled organisms can detect light but it is not an eye. Multicellular life has never sent a lot of different signals like for touch and nerve like that can be adapted to transmit information of a cell detect light.
Look at [https://en.wikipedia.org/wiki/Evolution_of_the_eye#/media/File:Diagram_of_eye_evolution.svg](https://en.wikipedia.org/wiki/Evolution_of_the_eye#/media/File:Diagram_of_eye_evolution.svg) that is how eyes evolved in vertebrates. There are nerve-to-light cells with nerve connections that are not eye but at the same time, we do not call it an optic nerve. It is clear the nerve connection is there before the eye because the eye is an advanced form of light light-sensitive body part.
If you require it to be an optic nerve the answer might be the same time if we define an optic nerve as the nerve connection to the eye. It is alos quite a meaningless answer because it is only one about of a word change
IIRC, the most primitive eye was basically a photo receptive nerve bundle, so I guess the answer is both. It’s not like an eyeball or nerve evolved in place without the other and then the other evolved to complete the system.
Eyes and other sensory organs have actually tended to evolve before the brain, rather than vice versa. If there was no sensory information to process, you wouldn’t need the brain.
In any case, the basic most part of photoreception is to have a protein that reacts to light. After that it’s quite similar evolution to other senses and organs.
This should clear it up. Also single celled organisms have eyespots that can sense light vs dark. Not sure if you count that though
What came first? the car or the road
Well, before there was optics that wouldn’t have been the optic nerve, but yes the nerve was there.
Eyes started as photosensitive cells, no pictures, no definition, just a cell that can sense if there is UV and heat or not. Very simple. Add more cells that can do this and you can tell better which direction that light and warmth is coming from. Create a cavity or “socket” for these cells and they can tell with even more detail where the light is coming from or if something is blocking it, perhaps even detect movement based on the blocking of that light.
Something that would count as “sight” as we think of it took a long time to come along. But there were always nerves present for all of these precursor steps. At what point did that become what we would call an “optic nerve”? that’s probably impossible to say.
It’s like the ole “what came first the chicken or the egg”? Well there were eggs first, but the things hatching out of those eggs weren’t chickens. What was the very first egg out of which something we would call a chicken emerged, therefore making it a “chicken egg”? Well that’s impossible to pinpoint.
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