No end in sight

Vision includes the frontier of perception, and involves cortical interpretation; thus requiring an “eye” and the neurological machinery for processing. But photoreception is a far different matter and is much more common.

There are many creatures that have extraocular photoreception, even beyond the parietal eye, discussed in the BJO March 2005 essay. Most plants have photosynthesis and, in some sense, photoreception. But, there is more to it than that.

Living creatures are divided into three great domains, bacteria, Archaea (including the extremophiles), and eucarya or eucaryotes. The bacteria and Archaea are both prokaryotes, meaning they lack a nucleus. The eucaryotes include all single celled nucleated organisms, such as the protists, and all metazoa, or multicellular organisms. All three domains contain creatures that use photoreceptive compounds that include vitamin A aldehyde, or retinal. The opsins, however, are different. Bacteria and Archaea have opsins that resemble those in eucaryotes, but are not identical. All metazoan opsins, however, are very similar suggesting that these proteins must be very old.

The sun represents the ultimate source for most, …