Index to Creationist Claims,  edited by Mark Isaak,    Copyright © 2007

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Claim CB301:

Source:

Response:

1. This is the quintessential example of the argument from incredulity. The source making the claim usually quotes Darwin saying that the evolution of the eye seems "absurd in the highest degree". However, Darwin follows that statement with a three-and-a-half-page proposal of intermediate stages through which eyes might have evolved via gradual steps (Darwin 1872).
   
   
   • photosensitive cell
   • aggregates of pigment cells without a nerve
   • an optic nerve surrounded by pigment cells and covered by translucent skin
   • pigment cells forming a small depression
   • pigment cells forming a deeper depression
   • the skin over the depression taking a lens shape
   • muscles allowing the lens to adjust
   
   All of these steps are known to be viable because all exist in animals living today. The increments between these steps are slight and may be broken down into even smaller increments. Natural selection should, under many circumstances, favor the increments. Since eyes do not fossilize well, we do not know that the development of the eye followed exactly that path, but we certainly cannot claim that no path exists.
   
   Evidence for one step in the evolution of the vertebrate eye comes from comparative anatomy and genetics. The vertebrate βγ-crystallin genes, which code for several proteins crucial for the lens, are very similar to the Ciona βγ-crystallin gene. Ciona is an urochordate, a distant relative of vertebrates. Ciona's single βγ-crystallin gene is expressed in its otolith, a pigmented sister cell of the light-sensing ocellus. The origin of the lens appears to be based on co-optation of previously existing elements in a lensless system.
   
   Nilsson and Pelger (1994) calculated that if each step were a 1 percent change, the evolution of the eye would take 1,829 steps, which could happen in 364,000 generations.

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