The photoreceptive molecules in animal eyespots are similar to
G-Protein coupled receptors, that would provide a suitable starting point.
As one of the many papers published on the evolution of the eye, see the reviews:
Goldsmith, T.H. (1990) Optimization, Constraint, and History in the Evolution of Eyes.
The Quarterly Review of Biology Vol. 65, No. 3 (Sep., 1990), pp. 281-322 (Stable URL:
JSTOR: Access Check)
or
Land M F, Fernald R D (1992), The Evolution of Eyes,
Annual Review of Neuroscience, March 1992, Vol. 15, Pages 1-29