Class 1: control of circadian rhythms, shadow detection to avoid predators, and surface detection for burrowing can be performed at very low light intensities requiring only opsin and a signalling system e.g. coral.
Class 2: nematode need to know the direction of light to move away from it effectively - requires the addition of a screening pigment/photoreceptor shielding to introduce directional selectivity.
Class 3: low spatial resolution needed for some behaviours e.g. anti-collision mechanisms, selecting a new habitat, and monitoring light changes in different directions - pigment cups/pits. (e.g. flatworms).
Class 4: High spatial resolution is needed for detecting/pursuing prey, evading predators, visual communications, etc. - requires a lens/focusing system and integration times need to be short to avoid motion blur.
Each higher level of task requires faster integration, narrower angular selectivity and higher contrast sensitivity, therefore, the system must capture more photons per unit time - aided by membrane stacking and focusing optics.
Nillson and Pelger (1994)
Assuming a continuous selection for improved spatial resolution, a patch of light-sensitive epithelium can be transformed into a fully focused camera-type eye in less than 400,000 generations.
Nillson and Pelger (2004)
If selection constantly favours an increase in the amount of detectable spatial info., a light-sensitive patch will gradually turn into a focused eye lens.
Arendt et al. (2004)
Started out needing something to harvest light energy coupled to a signalling system, therefore, photoreceptors evolved - rhabdomeric in insects and ciliary in vertebrates. These transduce light energy signals by different phototransductory cascades. Both photoreceptor types existed in Urbelateria, the last common ancestor of insects and vertebrates.
Arendt, Hausen, and Purschke (2009)
Original cell type had at least 3 different functions: light detection by photoreceptive organelles, light shading by pigment granules, and steering through locomotor cilia - mediate phototaxis in the absence of an NS. This diversified in sister cells specialised for different sub-functions - photoreceptors, shading pigment cells, and ciliated locomotor cells - led to more elaborates photosensory motor axonal circuits w/interneurons needed for visually guided behaviour seen today.
Mammalian brains don't follow the same basic plan - primates tend to have an increased % of brain neurons with an increased % of brain mass, compared to rodents and insectivores. Primate brains show isometric brain size increase as a function of cell numbers, but rodent brains increase faster in size than they do in number of neurons.