At the molecular level vision begins with a complex signal transduction cascade. As photons enter your eye they interact with light-sensitive chromophore molecules in the photoreceptor cells. The interaction causes the chromophore to change configuration and this, in turn, influences the large, trans-membrane rhodopsin protein to which the chromophore is attached.
The chromophore photoisomerization is the beginning of a remarkable cascade that causes action potentials to be triggered in the optic nerve. In response to the chromophore photoisomerization, the rhodopsin causes the activation of hundreds of transducin molecules. These, in turn, cause the activation of cGMP phosphodiesterase (by removing its inhibitory subunit), an enzyme that degrades the cyclic nucleotide, cGMP.
A single photon can result in the activation of hundreds of transducins, leading to the degradation of hundreds of thousands of cGMP molecules. cGMP molecules serve to open non selective ion channels in the membrane, so reduction in cGMP concentration serves to close these channels. This means that millions of sodium ions per second are shut out of the cell, causing a voltage change across the membrane. This hyperpolarization of the cell membrane causes a reduction in the release of neurotransmitter, the chemical that interacts with the nearby nerve cell, in the synaptic region of the cell. This reduction in neurotransmitter release ultimately causes an action potential to arise in the nerve cell.
This is the beginning of vision. And while there are variations on this remarkable sequence, it is found throughout the wide variety of vision systems found in biology. It is even found in the relatively simple, non image forming, third-eye. You can read more about this here.
Because this signal transduction cascade is so widespread, evolutionists must envision it to be present in the earliest organisms with vision capabilities. This need for evolution to have created unimaginable complexity early on is a consistent theme in evolutionary theory. Over and over, the fascinating designs found in biology must have, according to evolution, appeared early on, even before any need for such marvels.
For vision, this theme of early complexity is repeated at the cellular level, where two distinct photoreceptor cell morphologies—rhabdomeric and ciliary—are found. These two morphologies have different membrane folding strategies as well as biochemical pathways. But their widespread presence in organisms forces evolutionists to conclude they both must have been present in the last common bilaterian ancestor. Rhabdomeric photoreceptor cells are often associated with invertebrates and ciliary photoreceptor cells with vertebrates, but both invertebrates and vertebrates have cells with both morphologies.
And so both morphologies must trace back to that last common bilaterian ancestor. While it may stretch common sense for early evolution to create such complexity in duplicate, if you believe it can perform the feat once, then why not twice?
But there is more to the story. In invertebrates the ciliary morphology plays a lesser role. It is not found to provide directional light detection, but in more rudimentary light detection roles. So how then does it emerge as the chief architecture in vertebrate vision systems? The evolutionary narrative calls for a migration of the ciliary photoreceptor cells to the retina where they overtake the rhabdomeric photoreceptor cells while attaining new visionary skills. Why (and how) this would happen is anyone’s guess.
Only a few years ago evolutionists were confident of this narrative. It was, according to evolutionists, a compelling story that reaffirmed the truth of evolution. These amazing claims were yet another demonstration of how evolutionists interpret unlikely data into a favorable apologetic.
But now the story has become even more unlikely. The new research has indeed found ciliary photoreceptor cells providing directional light detection in Terebratalia transversa, an invertebrate. The narrative of ciliary photoreceptor cells migrating to the retina in vertebrates suddenly makes little sense. Evolution needs a new narrative, and as usual it is more complex:
The presence of ciliary photoreceptor-based eyes in protostomes suggests that the transition between non-visual and visual functions of photoreceptors has been more evolutionarily labile than previously recognized, and that co-option of ciliary and rhabdomeric photoreceptor cell types for directional light detection has occurred multiple times during animal evolution.
In other words, yes that evolutionary scenario we were so confident of must be discarded, but so what? We can always add more drama to the plot line. Whatever biology reveals, it must have evolved—theory respectability is not important. Religion drives science, and it matters.