Modelling the Evolution of the Complex Eye
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I'm wading through a few books at the moment and came upon something I thought folks might be interested in - a description of a mathematical model of the evolution of the eye by Dan-Eric Nilsson and Susanne Pelger of Lund University in Sweden. The pair decided to model the evolution of the eye to see if a precursor of the eye found in flatworms could develop into a complex eye in a sensible amount of time.
They started wth a patch of light sensitive cells backed by a layer of pigment which forms the most basic retina through its light absorbent properties. Next, they allowed the tissues around the simple structure to deform randomly with potential single changes limited to one per cent of size or thickness in each step. Their model accepted only those beneficial mututions that would enhance the capability of an organism to survive and reproduce thanks to improved visual performance. The model was then set in motion.
First the basic eye folded inward to make a cup. Then the eye cup became capped with a transparent surface. Next, the interior of the cup gelled to form a lens with dimensions that produced the best possible image. The number of incremental steps taken for progression from a flatworm's cell patch to a complex eye was 1829. Using conservative numbers the researchers estimated the entire process of evolution from cell patch to complex eye took less than 400,000 years.
Estimates suggests the complex eye has evolved at least 1500 times in the history of life on earth - the true number is likely to be many more.
Credit goes to Nilsson and Pelger, and to Jerry Coyne. I am paraphrasing his Why Evolution is True here.
"Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination." Max Planck
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I saw a TV special that had examples of all the intermediate stages as found in currently living species. From flatworm to various sea creatures to the entire eye. I don't know which came first, this modeling or the compilation of various versions of eyes. Interesting that the model apparently parallels evolution.
It's late, I'm tired, I'm not sure this paragraph makes sense. Let me know.
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those stages relate directly to the evolution of fossils - the model supports the fossil record. It all makes perfect sense.
"Experiments are the only means of knowledge at our disposal. The rest is poetry, imagination." Max Planck
Well, the octopus has an eye that is so close to our own that it just can't not be the same thing. The only problem being that the nearest common ancestor between us did not have anything like either of our eyes. Still, the basic structure of both of our eyes, is so good, that it would be profoundly unreasonable to think that evolution would not develop the same structure independently in many lineages.
On another note, Octopus have a nervous system that is pretty much fully decentralized. Completely different from us. However, they have been shown to have the ability to learn from observing the environment which they live in, in a way that is not unlike what hominids (even distantly related ones such as lemurs) are capable of. So in that respect, there seems to be more than one way to set up an adaptive nervous system.
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Another interesting fact about evolution is that all critters that are even close to us have a nerve that connects from the brain stem to the lower jaw. Actually, that nerve evolved before hard skeletons did. When it was a new thing, it ran through the gills.
Well, it is still routed almost the same way as it was a couple of billion years ago. The only thing being that the blood vessel that it used to go behind is now the aortic arch. So there is one cranial nerve that routs down your neck into your chest, under your aorta and back up your neck to your jaw.
I suppose that would be interesting enough as is. However, it is the same routing in all land animals, including the giraffe. In fact, in his his most recent book, Dawkins spends a couple of pages talking about the time that he was able to participate in a dissection of a giraffe and he saw exactly that for himself.
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