The origin of a light sensitive nerve was almost certainly an early step in the evolution of animal eyes, and this was thought to have happened only once, some 600 million years ago. But new open access research on the nematode C. elegans indicates a heretofore unknown instance where a nerve evolved light sensitivity.
One of the common, but incorrect, claims of anti-evolutionists is that the origin of complex features like eyes cannot be explained by evolutionary biology. As an example (although perhaps not written by an anti-evolutionist) see the comment on this post. For some history and a response to the incorrect anti-evolutionists claim, see this post.
It turns out that, as is often the case, the difficulty in understanding comes from trying to make a continuum into a dichotomy. An eye or even the genetic machinery for light sensitivity doesn't have to be all there or all not there. Instead, these features can build up gradually, by classical Darwinian evolution.
An early step in the evolution of complex eyes was almost certainly the origin of a light sensitive nerve. Darwin imagined that nerves might easily become light sensitive, but because of a lack of understanding of the molecular basis of photoreception, not to mention the molecular mechanisms of heredity, he could not even fathom a guess as to how nerves came to see the light. But just because Darwin didn't know how the machinery of light sensitivity could be built gradually by evolution doesn't mean it couldn't occur gradually.
Light sensitivity is a multi-step genetic process, where one protein signals to another. But all those proteins were not "turned on" all at once to suddenly allow for light sensitivity. Instead, light sensitivity evolved from existing "senses". Many signals are detected outside of a cell and they trigger proteins to send a signal inside a cell. Even yeast use proteins to sense pheromones outside their single cell and direct growth toward a potential mating partner. Many of these signaling genes are shared across all of our senses, and we can pinpoint by comparative study when a signaling gene became a light sensitive protein called opsin (again see this).
This brings us to the worm research. C. elegans has no visible eyes and its genome lacks light sensitive opsin genes. Yet it does display sensitivity to UV light!
Here is a link to a video showing light sensitivity.
How does it do this without opsin? It turns out that a different protein has become light sensitive. In this case, homologous proteins are known to be taste receptors (Gustatory receptors) in insects. Therefore, at some point in evolutionary history, the worm protein
gained light sensitivity, independently of the opsin "light switch"!
This is really fantastic work that suggests many new lines of research. For example, the mechanism of photoreception is unknown for the worm gustatory/light receptor. If this could be understood, we could get a specific idea about how a protein gained light sensitivity during evolution. Second, it is not known when light sensitivity was gained, which would require comparative study. These genes are present in insects - are the insect genes light sensitive? It would be great fun to put an insect gene in a worm to see if it could rescue the loss of light sensitivity due to a mutation.
Finally, I'll point out that this work is similar to another study published just last month . That workfocused not on the receptor, but on genes later in the cascade; especially on the gene responsible for "firing" (ie changing the concentration of the ions in) the light sensitive nerve. It turns out that that the "firing" gene is homologous to the "firing" gene used in vertebrate rod and cone cells - another example of how evolution often uses similar tools to produce parallel results.