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What's the Difference?
Topic Started: May 25 2009, 03:16 AM (279 Views)
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It's argued below humans are not unique only more advanced. I would stick with different degree, amount, of for example intelligence, emotion, etc. Perhaps just different. I would not argue more advanced. Progress would imply purpose and design.

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ngc1514
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Don't forget that the human eye is built inside out and a definite example of non-intelligent design. The blind spot that fascinates kids is the result of this inside out construction. Why do we have this second rate optic?

Two reasons:

1. It's the same optical design as found in other vertebrates, so it's a legacy of our evolutionary path and
2. It works ok. No reason for evolution to change it.

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Just to recap, vertebrates (like ourselves), and the invertebrates Squid and Octopi have “camera eyes”. They differ in how the photoreceptors in the retina, the part of the eye that receives the image, is wired up to the brain. The vertebrate wiring system is often cited as an example of “bad”, or at least quirky, design that is explainable by evolution.

The vertebrate retina is wired “backwards”. That is the photoreceptors point to back of the retina, away from incoming light, and the nerves and blood vessels are on the side of the incoming light, this means that any image formed on the vertebrate retina has to pass though layers of blood vessels and ganglion cells, absorbing and distorting the image.

To get decent visual acuity, vertebrates must focus light on a small patch of retina where the blood vessels and nerves have been pushed aside, the fovea. This patch must be small because of the nutrient requirements of the retina. Also, the construction of the vertebrate retina means that blood vessels and nerves must pass through the retina, creating a “blind spot”, where no image is formed. Finally, the “backwards” retina means that vertebrates have a high risk of retinal detachment. Altogether this shows that having the nerves and blood vessels in front of the photoreceptors is less than optimal design.

Imagine taking a pane of glass, then smearing it thickly with vaseline, then wiping a tiny hole in the vaseline. That is what the vertebrate retina is like.

Now consider the eye of squids, cuttlefish and octopi. Their retinas are “rightway round”, that is the photoreceptors face the light, and the wiring and the blood vessels facing the back (1). Squid and octopi have no blind spot; they can also have high visual acuity. The octopus also has a fovea-equivalent structure, which it makes by packing more (or longer) photoreceptors into a given area (1). Because it doesn’t have to create a hole in the supporting tissue it can have arbitrarily large “fovea”, and greater visual acuity. Cuttlefish have better visual acuity than cats (2) and because of their “rightway round” retinas; this level of acuity covers nearly the entire retina (1,2) unlike vertebrates where it is confined to the small spot of the fovea.

The vertebrate retina is a prime example of historically quirky “design”. The vertebrate retina is backwards because the development of the retina was first elaborated in rather small chordates, where issues of acuity and blind spots were non-existent; all subsequent vertebrates got stuck with this “design”. Vertebrates do very well with the limitations of the design of the eye, but it is clear that this is no system a competent designer would make. Naturally, this annoys the proponents of an Intelligent Designer, and they have been looking for ways to put a better spin on the kludged design of the vertebrate eye.

http://pandasthumb.org/archives/2006/11/denton-vs-squid.html
Edited by ngc1514, May 25 2009, 06:57 AM.
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"It works ok."

That, I think is what evolution is all about. Not improving or progressing to some perfection, but what's OK, what just good enough to survive.

We can think of evolution as following a sort of hill climbing model towards best fitness, but in a landscape that keeps changing.
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ngc1514
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The whole idea of evolution being aimed towards some goal or perfection is a common mistake and a flaw in many of the creationist arguments against it.

It's all about reproductive success or that "Selfish gene" Dawkins writes about. Of course, science brought a lot of that on themselves with the evolutionary "tree" with the tall, straight trunk that usually had man sitting at the top. I expect religion had a lot to do with that representation; the desire to fit science to the preconceived notion that man was, in some fashion, a pinnacle of evolutionary pressure.

Gould would frequently say that evolution is not a tree, but a bush with thousands of twigs and man sits at the end of one of those twigs. Just as every other species alive today sits at the end of its own evolutionary path - not higher or lower, just different.
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