The Common Ancestry Thread (page 14)

cantleave

Birds like woodcock have eyes on the side of their head so they can see behind as well as in front but only in 2D, enough to detect predators

Another detrimental effect of the back to front configuration of the retina is our susceptibility to detached retina. This occurs when the photoreceptor layer comes away from the pigmentment epithelium (cell layer) beneath. This results in a loss of nourishment and causes blindness in the part of the retina which has become attached. Retinal detachment can be caused by even minor trauma, since a small rupture will allow fluid to leak and build up between the retina and the pigment epithelium, thus separating the layers. By contrast t he cephalopod eye does not suffer from retinal detachment because the axons from the photoreceptors anchor them to the layers beneath.

What the hell was our Intelligent designer thinking of?

cantleave

One of the most common causes of blindness is a condition known as macular degeneration, although the cause MD is often unknown, its severe effects on vision are a consequence of the need for a macula as a partial fix for the poor retinal design.

The macula is that part of the retina that has the densest concentration of rods and cones for detailed vision. Within the macula is the smaller area already mentioned by Cofty called the fovea which contains only cones and has the highest density of these receptors. As stated above the macular has evolved to compensate for this “backward” arrangement of retinal layers with the nerve and blood vessels between the receptors and the direction of light. This limits the density of rods and cones, and so the partial fix is to have one small area cleared of nerves and blood vessels where rods and cones can be denser. If the human retina were designed like that of the squid and other cephalopods, this would not be necessary.

The dependence of the human eye on the macular for sharp vision creates a vulnerability, for any problem with that small area will have a dramatic effect on visual acuity. The rest of the retina will not be able to adequately compensate for the loss or compromise of the macula because the density of rods and cones is just too diffuse.

Phizzy

More thanks are due to Cantleave and the other contributors to this excellent thread, I am sure it is of great help to many.

So much I did not know, I never cease to be amazed at how very, very uneducated and ignorant we were as JW's, kept in darkness and fed s**t, just like mushrooms.

Comatose

I have just downloaded "Life Ascending" by Nick Lane

jgnat that 8 minute video was one of the coolest things I have seen.

cofty

I have just downloaded "Life Ascending" by Nick Lane

Fantastic book

PSacramento

The human eye is an excellent example of an organ that appears to be designed but upon close investigation shows a great deal of suboptimal design resulting from the constraints of its evolutionary legacy. This can be demonstrated not only by the obvious flaws in the design but also in a number of medical disorders. I will discuss the design flaws first and then go onto the medical conditions that result because of the flaws.

The most obvious design flaw of the retina is that the cellular layers are backwards. Light has to travel through multiple layers in order to get to the rods and cones that act as the photoreceptors. There is no functional reason for this arrangement. Even in a healthy and normally functioning eye, this arrangement causes problems. Because the nerve fibers coming from the rods and cones need to come together as the optic nerve, which then has to travel back to the brain, there needs to be a hole in the retina through which the optic nerve can travel. This hole creates a blind spot in each eye. Our brains compensate for this blind spot so that we normally do not perceive it—but it is there. If you search “blind spot” on Google there are many fascinating tests that show the effect of this.

Practically, this is a minor compromise to visual function, but it is completely unnecessary and can only be explained by evolutionary adaptation. If the rods and cones were simply turned around so that their cell bodies and axons were behind them (oriented to the direction of light), then there would be no need for a blind spot at all.

Design flaw?

If it was as big a flaw as you imply, with the humanoid eye being around as long as it has, wouldn't evolution have fixed that?

cofty

If it was as big a flaw as you imply, with the humanoid eye being around as long as it has, wouldn't evolution have fixed that?

No Psac that's the point. Every step has to add to the fitness of its host or it will not survive natural selection.

What intermediate steps would be needed to reverse the structure of the retina that would not involve designs that were less efficient than preceding steps?

Its like starting with a biplane and ending up with a fighter jet but every single change you make to the plane must result in a functional plane that is better than the previous version.

Nature does not have the luxury of starting from a clean slate - it has to work with modifying existing systems.

That's why we see so many examples of poor design.

PSacramento

So eventually evolution WILL address the issue?

The Common Ancestry Thread

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