Much thanks to my brother for the excellent title.
First, let’s talk a bit about how eyes evolved in general.
First, there was the eyespot–a cell, or even part of a single celled organism, that has chemicals that react to light. It can’t really see beyond “there is light” vs “there isn’t light”, but even that can be useful.
Then you have cup eyes, like in Planarians. Basically, an eyespot that’s in a little divot of skin. It still can’t see images, but it can give an organism at least crude directionality, which can tell an animal which *way* to go to approach or avoid light.
As the cup gets deeper and deeper, you get a better sense of directionality to incoming light, until eventually you end up with a pinhole camera eye like the Nautilus–basically, a cup eye that’s surrounding an imaginary ball, with only a small opening, so that you can get crude images, because you can get more or less exact directionality of any light.
From there, the narrow opening of the pinhole camera eye got covered by clear skin. Initially, this probably just protected the eye from parasites, infections, and dirt, but it allowed for the development of the vitreous body (eye goo) and the lens.
The vitreous body allowed for control over the exact optic properties of the space between the outside of the eye and the retina, such as filtering out UV light or changing the refractive index. And the lens allowed the eye to let in more light while having an even more focused image than with a pinhole camera eye.
From there, various accessory features–lens-focusing muscles, specialized layers, and so forth–were able to gradually accumulate, giving us (and cephalopods) the complex eyes we have today.
Now back to my favorite aquatic aliens.
Cephalopod eyes (other than those of the Nautilus) are conceptually better than vertebrate eyes. That’s because vertebrate eyes effectively evolved from the inside out, while cephalopod eyes evolved from the outside in.
Vertebrate eyes probably evolved from a photosensitive patch of nerve cells (we know that modern vertebrate eyes are essentially an outgrowth of the brain). So, when they got to more complex forms, the nerve running from the cells to the brain was still running through the middle of the light collecting bits, because evolution doesn’t plan ahead. This gives our eyes an inherent, unavoidable, and remarkably large blind spot. The only reason we never notice it under normal circumstances is that our brains are really good at compensating.
But cephalopod eyes probably evolved from a photosensitive patch of skin cells (and now develop from a dip in the skin). So, the nerves were always behind the bit actually collecting the light. So, they lack the blind spot that all vertebrate eyes inherently have.
Further, while it is thought that cephalopods generally lack color vision (making their various camouflage shenanigans extra impressive, more on those in another article), they frequently can sense the polarization angle of light, while we vertebrates can’t. It’s probably because their photoreceptors are laid out in neat rows, while ours are sort of randomly placed.
Just one more way that cephalopods are absolutely amazing creatures.
Science is Really Weird 