Thursday, July 07, 2011

Richard Dawkins explains the evolution of the eye

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It’s a favorite Creationist canard that the modern eye is too complex to have evolved (and therefore, evolution is false!). In yet another attempt at driving basic scientific facts through through their overly thick skulls, here’s Richard Dawkins making an appearance on the BBC production Bang Goes The Theory and explaining, in wonderfully clear and simple detail, how the ocular globe started as a mere light-sensitive dot on a cell and ended up as the incredibly complex device it is now:

My transcript: (click the [+/-] to expand/collapse →) []

[summarized for brevity]

Presenter Liz Bonnin visits an ophthalmologist to learn more about the eye, which functions essentially as does a camera with light receptors behind a lens. Liz then meets with Dr. Richard Dawkins, who explains how the very first eye was no more than a dot at the front of single-celled organisms, called a flat-eye, which only responded to the presence or absence of light. He uses a strip of paper coated in a substance that glows under ultraviolet light as a representation of this earliest stage of eyeball development, and demonstrates that under a light source at any angle, the surface glows equally, thus making it rather useless in determining anything such as direction and orientation.

Dawkins then shows what happens if the paper is cupped slightly: different lighting angles allow for one side of the strip to be exposed to more light than the other, which in organisms having undergone a similar evolutionary step, would grant them a massive advantage in allowing them to determine which direction a potential predator’s shadow is coming from.

Dawkins then shows what happens if the curve is completed to form a pinhole camera with light-sensitive (photosensitive) cells on all sides. With this new development, the organism can now form an extremely rudimentary and very blurry picture of whatever is directly in front of them.

Dawkins then uses a small bag of water in front of an actual pinhole camera to demonstrate what happens if one uses a rudimentary lens: the image becomes much clearer. This bag of water would take the form of jelly inside organisms’ eyeballs, which over time and with more mutations, would gradually harden and reshape itself to form more of an actual lens, providing a clearer, crisper and wider picture of the creature’s surroundings.

Finally, Liz explains that scientists estimate that it only took about 400,000 generations to go from the first rudimentary flat-eye to a fully developed eyeball, which corresponds to roughly half-a-million years.

(via Roger Ebert's Journal)