D. E. Nilsson and S. Pelger  used computer modeling of the evolution of eyes to find out if it was possible for there to be a smooth gradient of change from a pigmented eye spot to an eye with a lens and cornea, and how long such a transformation would take. They used highly conservative figures for the possible change in each generation. Many human traits are over 50% heritable, and they gave their model a lower figure. They also chose very conservative values for the amount of possible variation in a population. Using these figures, they calculated that through the mechanism of evolution an eyespot could evolve into an eye with a lens and a cornea in less than a half a million years.
This scenario is contested by Young Earth Creationists, who point out that the scenario begins with a highly-complex structure, and doesn't include the supporting neural structures, without which the eye would be useless.
- Wile, Dr. Jay L. Exploring Creation With Biology. Apologia Educational Ministries, Inc. 1998
- D. E. Nilsson and S. Pelger, "A pessimistic estimate of the time required for an eye to evolve" (Proceedings of the Royal Society B, 1994, v.. 256, pp. 53-58)
- Sarfati, Jonathan , with Matthews, Michael, Refuting Evolution 2, chapter 10.