Something fishy happened in the evolution of the human eye. Many of us think that it is either a pinnacle of evolution or is irreducibly complex. In a previous post, I tackled how complex eyes, meaning eyes with a lens, iris, and cornea, could evolve and could in fact evolve quite simply.
But if we look across species at the different kinds of eye structures, we notice two types of complex eye structures:
In each diagram, light enters through the left, is focused by the lens, and the sensory cells pick up the light information and send it to the nerve fibers which transmit the signal to the brain via the optic nerve. The difference in these two structures is the layering of the sensory cells and nerve fibers. From an engineering standpoint, it would make sense for the light to hit the sensory cells first. But in the diagram on the left, light must first pass through the nerve fibers before reaching the sensory cells. Additionally, we see the nerve fibers branching out from the optic nerve, overlapping to create a space where no sensory cells can be. This creates a blind spot.
Now, you may have guessed what animals have the type of eyes on the left. That’s right, humans do, as well as other vertebrates. On the left, we see a diagram similar to that found in octopuses (yes, octopuses) and squids, where there is no blind spot due to the wiring being behind the sensory cells. From this, we can see that the complex eye evolved independently at least twice, in vertebrates and cephalopods.
Now which would you say is better designed? If these represented camera products, which one would you buy?
Additionally, we can note differences in the lenses. Vertebrate eye lenses are flexible and can be focused by special muscles that change the shape of the lens. Cephalopod lenses are inflexible but can be focused with muscles that move the entire lens closer to or father from the retina. There seems to be no advantage either way here, though.
So it all boils down to whether a blind spot would be advantageous. I find no such support for this, but please correct me if I am mistaken. Luckily enough, our blind spots do not overlap in our eyes, so one eye can fill in the missing information for the other. We never really notice the faulty wiring and were thus able to cope with the mistake.