The eye is too complex to have evolved.
Brown, Walt, 1995. In the Beginning: Compelling evidence for creation
and the Flood. Phoenix, AZ: Center for Scientific Creation, p. 7.
Hitching, Francis, 1982. The Neck of the Giraffe, New York:
- This is the quintessential example of the argument
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
- 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.
Lindsay, Don, 1998. How long would the fish eye take to evolve?
- Darwin, C., 1872. The Origin of Species, 1st Edition. Senate,
London, chpt. 6,
- Nilsson, D.-E. and S. Pelger, 1994. A pessimistic estimate of the time
required for an eye to evolve. Proceedings of the Royal Society of
London, Biological Sciences, 256: 53-58.
- Shimeld, Sebastian M. et al. 2005. Urochordate
βγ-crystallin and the evolutionary origin of the vertebrate
eye lens. Current Biology 15: 1684-1689.
Dawkins, Richard, 1996. Climbing Mount Improbable, New York:
W.W. Norton, chpt. 5.
Land, M. F. and D.-E. Nilsson, 2002. Animal Eyes. Oxford University
Fernald, Russell D. 2006. Casting a genetic light on the evolution of
eyes. Science 313: 1914-1918.
created 2001-2-17, modified 2007-11-10