rationalskepticism.org

ABSTRACT

The evolution of the eye is a matter of debate ever since Darwin’s Origin of Species.
While morphological comparisons of eye anatomy and photoreceptor cell types led to the view that
animal eyes evolved multiple times independently, the molecular conservation of the pax6 eyespecifying
cascade has indicated the contrary - that animal eyes evolved from a common, simple
precursor, the proto-eye. Morphological and molecular comparative approaches are combined here
in a novel Evo-Devo approach, the molecular comparison of cell types ("comparative molecular cell
biology"). In the eye, the various types of photoreceptor cells, as well as pigment and lens cells, each
require distinct combinations of specifying transcription factors that control their particular
differentiation programmes, such as opsin expression in photoreceptors, specific neurotransmitter
metabolism, or axonal outgrowth. Comparing the molecular combinatorial codes of cell types of
animal extant eyes, their evolutionary histories can be reconstructed. This is exemplified here on
the evolution of ciliary and rhabdomeric photoreceptor cells in bilaterian eyes and on the evolution
of cell type diversity in the vertebrate retina. I propose that the retinal ganglion, amacrine and
horizontal cells are evolutionary sister cell types that evolved from a common rhabdomeric
photoreceptor cell precursor.