Submitted by TheSonOfKayra t3_125nk1x in askscience
As far as I know, very special conditions are required for such a formation to occur in nature. At this point, how did the lens in the eye take its current shape? What were the factors causing this?
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Something to keep in mind: in the creatures eyes began evolving in, you aren't talking about generations spanning 20 years or something. Generations were on the order of a few weeks, and evolution was rather rapid fire by comparison to today. We have a nice stable planet, with stable weather and stable chemistry. When life was evolving basic forms of biology, that was not quite the case.
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Beyond that, there is more than 1 path to creating an eye.
It really is a matter of photoreceptor cells that evolved in a pit rather than flat on the surface. The "pit" allowed for a directional sensation.
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Here are a couple of links to articles that you might find interesting. Reference #1 posits that the evolutionary precusor to crystallins (a transparent protein that comprises the lens) occurred in sea squirts (Ciona). Reference #2 provides a review of how the crystallins evolved into two super families are kinds that we see today.
Your species relationship with food dictates a lot about how your eyes evolved.
In many land animals who eat primarily with their faces in their food they have an inner eye lid tied to the jaw muscle that closes during eating to keep food debris out. You can see the vestige of ours in the pink muscle stub in the inner corner of your eye and compare it to the full version found in your dog, cat or bird.
The placement of your eyes is also related to what you eat. Herbivore animals often have horizontal pupils, their day predator animals often have circular pupils and their night predator animals will frequently have vertical pupils. https://physicsworld.com/a/eye-shape-reveals-whether-animal-is-predator-or-prey/
In humans our ability to see in color was in part due to our evolution with gauging the effort to eat ripe fruit. This also improved our fore arms as more than legs and improved our hand eye coordination and thumb evolution.
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This is a great response, it's beautifully visualised which makes it easy to understand
The eye actually evolved four separate occasions (at least). Because at every point in making an eye, you gain an advantage. And that is how the lens also evolves.
IIRC the book "Selfish Gene" goes into this. Great audio book that's worth listening to a few times.
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btw thank you everyone for the serious answers.
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> and evolution was rather rapid fire by comparison to today
Important to note that we are in the middle of a mass extinction period.
After the extinction period is over, a lot of unfilled niche means an possible explosion of new species, but not during the extinction period.
Stacksmchenry t1_je54o5m wrote
There is a fantastic visualization of this on the show Cosmos (the Neil Degrasse Tyson version) on the evolution episode (how molecules work was the name of the episode if I recall)
Sorry I can't give you a great scientific answer but if you're looking for a surface level explanation it should help.