You probably thought that the best way to position the retina
is with the rhodopsin ends facing into the light. That is the best way to design the eye in order to get the most
By orienting the layer such that the rhodopsin faces the incoming
light you have designed the eye to be a more sensitive system.
But the retina is NOT positioned that way. It's positioned backward!
|The rhodopsin portion is at the back of the retina and light has to pass through the nucleus, mitochondria and axon before striking the rhodopsin. Also, the nerve cells that connect to the rod's axon are in the path of the light so they too block some light from reaching the rhodopsin. As if that weren't bad enough, the layer of nerve cells that carry the signal from the rod's axon is embedded in a matrix (material) that I have not bothered to include in these diagrams (because it makes the drawing difficult to interpret).|
So, the path that light must travel to reach the photosensitive rhodospin is crisscrossed with nerves and supporting materials plus the rod's own organelles like mitochondria and nucleus!
Who would design an eye like this?
Certainly not me. Certainly not an engineer or anyone who had given it a little thought. This back to front arrangement causes the eye to be less sensitive to light but that is the "plan" that nature has used.
In fact there is another problem with this design.
All those millions of nerve cells conducting signals from the
rods to the brain converge into a signal bundle called the optic
nerve. This bundle acts as the main "cable" that carries
all the millions of nerve paths to the brain. As the nerves along
the top of the retina converge onto this single point their density
becomes so great that no light gets pass them.
If the eye had been designed correctly, with the rodopsin in the front and the axons and nerves in the back, the nerves would converge behind everything and there would be no blind spot.
Here's a demonstration of the blind spot.
Close your right eye and place your face about a metre from the screen while looking directly at this black cross. Notice that you can still see the black circle while staring directly at the cross. Now, while maintaining your gaze on the cross, slowly move your face closer to the screen. (Keep your head level and slowly move forward. It helps if you are sitting in a chair with wheels.) As you move forward you will see the black circle disappear! Keep moving closer to the screen and it will reappear again! The circle appears and disappears as the image moves out of and into your blind spot.
You can demonstrate the blind spot in your right eye by closing your left eye and focusing on the circle with your right eye. Now as you move towards the screen, while maintaining your stare on the circle with your right eye, the cross will disappear and reappear. (The effect isn't as striking as before because the cross isn't the same shape as your blind spot.)
We rarely notice our blind spot for two reasons.
1) We (usually) have two working eyes so objects lost in the blind spots of one eye are seen by the other eye because the two blind spots are positioned so they don't overlap. The brain puts the two images together to make what we interpret as a single seamless image with no blind spot.
2) We constantly move our eyes, often without thinking about it, and that helps us to visualize the entire scene. Once again, our brain interprets it all for us and fills in the missing details.
All vertebrates (from fish to humans) have the retina built the
wrong way around and therefore we all have blind spots.
But why design the eye this way? Well, it wasn't designed at all! (Sorry Paley.)
Off hand, I don't know why the retina ended up backwards. My guess is that when the retina was first evolving it was laid down wrong because even a slight amount of photosensitivity was enough for our very ancient ancestors who lived in the seas. It was "good enough" to distinguish between sunshine and darkness so a sea creature could tell if it were day or night (so it knew when to feed). Even a badly positioned retina system would allow a fish to tell if it were entering a cave or if a potential predator moved over it (blocking out the sun). As the millions of years went by evolution selected the eye to perform more complex tasks and true vision came about probably with the evolution of a good lens. But by that time the backward facing retina was a well-established "tradition" inherited from the parents and there was no turning back.
The blind spot and other mistakes (many more than we have time
to discuss) are good evidence that evolution is not perfect. That's
because natural selection doesn't plan for the future nor can
it easily command a major redesign in order to correct an error.
Of course, all this talk about homologous structures, vestigial organs and mistakes are evidence for Darwinian evolution (Natural Selection). But there's one more line of evidence we have only mention briefly.
Can you think of a large body of evidence for evolution we have yet to discuss in detail?